Cholesterol-Based Compounds: Recent Advances in Synthesis and Applications

Abstract

This review reports on the latest developments (since 2014) in the chemistry of cholesterol and its applications in different research fields. These applications range from drug delivery or bioimaging applications to cholesterol-based liquid crystals and gelators. A brief overview of the most recent synthetic procedures to obtain new cholesterol derivatives is also provided, as well as the latest anticancer, antimicrobial, and antioxidant new cholesterol-based derivatives. This review discusses not only the synthetic details of the preparation of new cholesterol derivatives or conjugates, but also gives a short summary concerning the specific application of such compounds.

Index

1. Introduction to Cholesterol-Based Compounds	1
2. Drug Delivery Applications……………………………………………………………………	3
3. Anticancer, Antimicrobial, and Antioxidant Compounds…………………………………	15
4. Cholesterol-Based Liquid Crystals……………………………………………………………	27
5. Cholesterol-Based Gelators…………………………………………………………………….	35
6. Bioimaging Applications……………………………………………………………………….	41
7. Synthetic Applications………………………………………………………………………….	48
8. Miscellaneous……………………………………………………………………………………	56
9. Conclusions………………………………………………………………………………………	59
Funding………………………………………………………………………………………………..	60
Conflicts of Interest………………………………………………………………………………….	60
Abbreviations List…………………………………………………………………………………….	60
References………………………………………………………………………………………………	62

1. Introduction to Cholesterol-Based Compounds

Cholesterol (cholest-5-en-3β-ol) is considered to be a lipid-type molecule, being one of the most important structural components of cell membranes. Chemically, cholesterol is a rigid and almost planar molecule with a steroid skeleton of four fused rings, three six-membered and one five-membered, conventionally lettered from A to D (1,2-cyclopentanoperhydrophenanthrene ring system) (Figure 1A). Therefore, the cholesterol molecule comprises four essential domains (Figure 1B). In domain I, the polarity of the 3-hydroxy group constitutes an active site for hydrogen bond interactions with a myriad of biological molecules (e.g., phospholipids in membranes) [1]. In domain II, the absence of methyl groups at C-4 and C-14 influences directly the planarity of the molecule, while in domain III, the natural (R) configuration at C-20 determines the “right-handed” conformation of the side chain. Finally, in domain IV, the conformation and length of the side chain is of prime relevance to intermolecular contacts [2]. The presence of a hydrophilic 3-hydroxy headgroup on the A-ring, together with a hydrophobic hydrocarbon body, give the molecule an amphiphilic nature, which makes cholesterol the most recognized sterol.

Cholesterol plays a vital role in life, particularly in cell membranes and as a precursor to the biosynthesis of several steroid hormones. In cell membranes, which are essentially constituted by a double layer of phospholipids, cholesterol has great influence on membrane fluidity, microdomain structure (lipid rafts), and permeability by interacting with both the hydrophilic headgroups and the hydrophobic tails of phospholipids. In addition, modifications of the stereochemistry and oxidation states of the fused rings, the side chain, as well as the functional groups of cholesterol, lead to a wide variety of biologically important molecules, such as bile acids, vitamin D, and several steroid hormones [1,2]. Interestingly, 13 Nobel Prizes have been awarded to scientists who studied the structure of cholesterol, its biosynthetic pathway, and metabolic regulation. Unfortunately, cholesterol has gained a bad reputation because it is increasingly associated with several cardiovascular and neurodegenerative diseases, among others [1,3].

Over the years, cholesterol has risen as an attractive starting material or a model system for organic synthesis due to its easily derivatized functional groups, availability, and low cost. Many useful chemical and enzymatic reactions are now widely used for multistep steroid transformations, leading to products of practical importance. The chemical transformations range from simple ones, such as manipulations of functional groups, to more complex ones, such as C­-H activation or C-C bond formation with organometallic reagents. In 2014, a purely synthetic chemistry review was published, dealing only with the advances in cholesterol chemistry since 2000, focusing on cholesterol oxidation reactions, substitution of the 3β-hydroxy group, addition to the C5=C6 double bond, C-H functionalization, and C-C bond forming reactions. However, this review paper excluded simple derivatization reactions of cholesterol such as the preparation of carboxylic and inorganic acid esters, aliphatic and aromatic ethers, simple acetals, or glycosides [4]. From our perspective, the simpler chemical transformations very often lead to the preparation of new cholesterol-based molecules with potential applications in several important research fields. Therefore, in this review, we focused our attention on publications from 2014 to date and described not only the synthesis of cholesterol-based new molecules, but also the application of these molecules in different fields, such as drug delivery; bioimaging; liquid crystals; gelators; anticancer, antimicrobial, and antioxidant applications; as well as purely synthetic applications. However, some interesting papers published before 2014 were included to fill some of the lacking papers from the 2014 review paper. Throughout the text, several reaction schemes will be depicted to describe the chemical reaction involved in the preparation of the cholesterol-based compounds. For simplification purposes, the structures of cholesterol will consistently be represented using the abbreviations depicted in Figure 2.

2. Drug Delivery Applications

Drug delivery is a method or process of administering a pharmaceutical compound to achieve a therapeutic effect in humans or animals. Drug delivery systems can in principle provide enhanced efficacy, reduced toxicity, or both for various types of drugs. Liposomes are the most common and well-investigated nanocarriers for targeted drug delivery because they have demonstrated efficiency in several biomedical applications by stabilizing therapeutic compounds, overcoming obstacles to cellular and tissue uptake, and improving the biodistribution of compounds to target sites in vivo [5].

In 2014, Vabbilisetty and Sun reported a study of terminal triphenylphosphine carrying anchor lipid effects on a liposome surface by postchemically selective functionalization via Staudinger ligation, using lactosyl azide as a model ligand. They synthesized two different anchor lipids, one of them based on the cholesterol molecule (Chol-PEG₂₀₀₀-thiphenylphosphine 3), which was synthesized through an amidation reaction of synthetic Chol-PEG₂₀₀₀-NH₂ 1 with 3-diphenylphosphino-4-methoxycarbonylbenzoic acid N-hydroxysuccinimide (NHS) active ester 2 (Scheme 1) [6].

The authors verified that the Staudinger ligation could be carried out under mild reaction conditions in aqueous buffers without a catalyst and in high yields. The encapsulation and releasing capacity of the glycosylated liposome based on cholesterol were evaluated, respectively, by entrapping 5,6-carboxyfluorescein (CF) dye and monitoring the fluorescence leakage. It was concluded that Chol-PEG₂₀₀₀-thiphenylphosphine 3 is particularly suitable for the ligation of water-soluble molecules and can accommodate many chemical functions, being potentially useful in the coupling of many other ligands onto liposomes for drug delivery purposes [6].

In 2015, a new method was reported for the deposition of a single lipid bilayer onto a hard polymer bead starting from discoidal bicelles and using chemoselective chemistry to hydrophobically anchor the lipid assemblies, using cholesterol bearing an oxyamine linker. The synthesis of oxyamine-terminated cholesterol 6 involved two steps, starting with a Mitsunobu reaction of compound 4, followed by a reaction of 5 with hydrazine hydrate (Scheme 2) [7].

The discoidal bicelles were prepared in water media upon mixing dimyristoylphosphatidylcholine (DMPC), dihexanoylphosphatidylcholine (DHPC), dimyristoyltrimethylammonium propane (DMTAP), and the oxyamine-terminated cholesterol derivative 6, in a specific molar ratio. These bicelles were exposed to aldehyde-bearing polystyrene (PS) beads and readily underwent a change to a stable single lipid bilayer coating at the bead surface. This approach may be advantageous in depositing membrane proteins at such surfaces for analytical, diagnostic, or therapeutic applications (namely drug delivery) [7].

Cholesterol chloroformate 7 was used as a lipid anchor for hydrophobization of arabinogalactan (AG), a liver-specific high galactose containing a branched polysaccharide, through a two-step reaction sequence that yielded a novel polysaccharide lipid, conjugated ligand 9 (Chol-AL-AG), with a bifunctional spacer β-alanine (AL) (Scheme 3) [8].

Ligand 9 was used to prepare conventional liposomes (CLs) and surface-modified liposomes (SMLs) through the reverse phase evaporation technique. These new liposomes were characterized by different techniques exhibiting the required particle size for targeting tumor and infectious cells. In vitro biological studies showed an enhanced binding affinity and cellular uptake of SMLs compared to CLs by HepG2 cells, making SMLs an interesting new approach for targeted drug delivery in liver cancer therapeutics [8].

Crucianelli et al. reported in 2014 a new delivery system based on liposomes containing dioleoylphosphatidilcholine (DOPC) and mannose 6-phosphate (M6P)-functionalized cholesterol 14. For this purpose, M6P cholesteryl conjugate 14 (Chol-M6P) was synthesized following a three-step route starting from cholesterol derivative 10, as depicted in Scheme 4 [9].

This novel vector system, designed to target lysosomes, was loaded with a model compound calcein to investigate intracellular trafficking in a 3T3-NIH cell line using a confocal and fluorescence microscopy technique. The affinity of the M6P group for the CI-M6PR receptor enabled these liposomes to carry calcein along the route leading to lysosomes, in opposition to calcein itself, which did not internalize into cells. These results suggest that liposomes containing Chol-M6P 14 appear to be promising vectors in the selective targeting of lysosomes for enzyme replacement therapy or anticancer therapy [9].

The importance of liposomes in drug delivery applications is well recognized. In this context, Silva and coworkers reported the synthesis of cholesterol-based neoglycoconjugates of 19 (galactose-Gal and N-acetylglucosamine-GlcNAc) for further incorporation into liposomes. The glycoconjugates were synthesized through a copper-catalyzed 1,3-dipolar cycloaddition (CuAAC) reaction of glycosyl azides (18) with cholesterol derivative 17 (Scheme 5) [10]. The authors carried out biodistribution in vivo studies to evaluate the targeting of these carbohydrate-coated liposomes, concluding that they showed high uptake by the liver, spleen, and kidneys and no significant accumulation into other organs. Furthermore, it was demonstrated that liposomes with galactose in the surface preferentially target the liver cells. The results suggest that this kind of liposome might be a promising delivery system for therapeutic agents in hepatic diseases [10].

To develop a drug delivery system for potential theranostic applications, Škorpilová et al. synthesized the fluorescent macrostructure 23 containing sesquiterpene lactone trilobolide (Tb), cholesterol, and a green-emitting boron dipyrromethene (BODIPY) dye. The synthesis of compound 23 involved a three-step sequence, starting from the CuAAC reaction of propargyl cholesterol 20 with BODIPY dye 21, followed by functionalization with sesquiterpene lactone trilobolide (Scheme 6) [11]. This fluorescent cholesterol conjugate 23 was successfully incorporated into liposome formulations, which showed promising immunomodulatory properties in primary rat macrophages and improved drug distribution in U-2 OS and HeLa cancer cells. The study of the intracellular trafficking pattern of liposomes revealed two populations: One localized on the cell membrane and the other inside the cell, this last one closely related to cell death. This new liposomal cholesterol conjugate 23 not only retains the biological properties of pure trilobolide, but also enhances bioavailability, and thus has potential for use in theranostic applications [11].

Recently, Lin et al. reported the synthesis of a fluorescent triple-responsive block-graft copolymer 27, bearing cholesteryl- and pyrenyl-side groups, with a disulfide (S-S) bridging point joining the hydrophilic and hydrophobic chains. The synthesis of such polymers relied on a typical click reaction between PNiPAAm₁₀-S-S-P(αN₃CL)₁₀ 26, pyrenylmethyl 4-pentynoate 25, and cholesteryl 4-pentynoate 24, affording PNiPAAm₁₀-S-S-P(αN₃CL₁₀-g-PyrePA₃/-CholPA₇) 27 (Scheme 7) [12]. Experimental results indicated that copolymer 27 could undergo self-assembly into polymeric micelles with excellent fluorescence performance in aqueous solution. The drug-loading capacity of cholesteryl-grafted copolymer 27 was evaluated using doxorubicin (DOX) as a template drug, and the results showed reasonable DOX-loading capacity. The authors also demonstrated that DOX-loaded micelles enter the cells at a substantially faster rate than their free-form counterparts, effectively inhibiting HeLa cell proliferation [12].

In 2014, the synthesis of a new dual-imaging and therapeutic agent for improved efficacy in Boron Neutron Capture Therapy (BNCT) in cancer treatment was reported [13]. The compound consists of a carborane unit (ten boron atoms) bearing a cholesterol unit on one side (to pursue incorporation into the liposome bilayer) and a Gd(III)/1,4,7,10-tetraazacyclododecane monoamide complex on the other side (as an magnetic resonance imaging (MRI) reporter to attain the quantification of the B/Gd concentration). The synthesis of the target compound Gd-B-AC01 (37) relied on an eight-step synthetic strategy, which ended with the complexation of 36 with Gd(III) in aqueous solution at pH 6.5 (Scheme 8). This dual probe 37 was functionalized with a polyethylene glycol (PEG)ylated phospholipid containing a folic acid residue at the end of the PEG chain. These liposomes presented interesting features such as the ability to selectively concentrate high amounts of boron in human ovarian cancer cells (IGROV-1), enough to perform efficient BNCT treatment with significantly reduced uptake by healthy cells in the surrounding regions. Furthermore, these liposomes, which can be used as nanoplatforms to deliver both Gd and B agents, can, in principle, be used for the simultaneous delivery of antitumor drugs such as DOX [13].

Zhang and coworkers studied the behavior of nanoparticles (NPs) formed by self-assembly of amphiphilic poly[N-(2-hydroxypropyl)methacrylamide] (pHPMA) copolymers bearing cholesterol side groups (39) as potential drug carriers for solid tumor treatment (Figure 3) [14].

The behavior of such NPs in human serum albumin (HSA) protein environment was evaluated using mixed solutions of NPs from polymer conjugates with or without the anticancer drug doxorubicin bounded to them, 39 and 40, respectively. The authors found that in the absence of DOX, a small amount of HSA molecules bind to the cholesterol groups of the NPs by diffusing through the loose pHPMA shell or get caught in meshes formed by the pHPMA chains. On the other hand, the presence of DOX strongly hinders these interactions, and for that reason the delivery of DOX by these NPs in the human body is not affected by the presence of HSA [14].

Recently, Singh and coworkers reported the biofunctionalization of the surface of β-cyclodextrin nanosponge 41 (β-CD-NSP) with cholesterol, expecting to improve its cellular binding ability. The β-CD-NSP was functionalized by grafting cholesterol hydrogen succinate (CHS) through a coupling reaction, affording β-CD-NSP-CHS 42 (Scheme 9) [15].

The cytotoxicity assays showed that β-CD-NSP 41 was nontoxic and that the surface biofunctionalized with CHS 42 improved both the therapeutic and drug delivery efficacy of DOX. The experimental results also demonstrated that CHS grafting may enhance DOX adsorption due to the hydrophobic charge on the surface. Therefore, the surface-engineered CD-NSP could be used as a carrier for low water-soluble small drug molecules to improve solubility and bioavailability in site-specific drug delivery systems [15].

In attempting to develop an intelligent drug delivery for cancer chemotherapy, Li et al. synthesized dual redox/pH-sensitive amphiphilic copolymer 44 and cholesterol-modified poly(β-amino esters)-grafted disulfide poly (ethylene glycol) methyl ether [PAE(-SS-mPEG)-g-Chol]. The precursor PAE-SS-mPEG 43 was successfully synthesized via Michael-type step polymerization using disulfide linkage-containing PEG segment. Finally, cholesterol was incorporated into the hydroxy-pendant group trough an esterification reaction, affording the copolymer PAE(-SS-mPEG)-g-Chol 44 (Scheme 10) [16].

The authors verified the interesting physicochemical properties of copolymer 44, namely redox and pH sensitivity. Doxorubicin-loaded hybrid polymer-lipid NPs (DOX-HDPLNPs) were prepared, and drug-loading capacity, delivery efficacy, and redox- and pH-triggered drug release behavior in vitro were studied. The results showed that DOX-HDPLNPs enhanced loading capacity and improved cellular uptake ability, as well as serum stability. The anticancer potential in tumor-bearing mice was addressed, indicating that the DOX-HDPLNPs prepared with redox- and pH-sensitive copolymer with disulfides and PEGylated lipid could efficiently enhance therapeutic efficacy with low cytotoxicity and side effects. Both in vitro and in vivo experiments indicated that DOX-HDPLNPs enhanced therapeutic efficacy with high cellular uptake and negligible cytotoxicity compared to the free drug DOX. Therefore, HDPLNPs can be considered to be smart delivery systems for hydrophobic anticancer drug delivery [16].

Tran et al. developed a copolymer in 2014, constituted of polynorbonene-cholesterol/ poly(ethylene glycol) [P(NBCh9-b-NBPEG)] 45, that undergoes self-assembly to form a long circulating nanostructure capable of encapsulating the anticancer drug DOX with high drug loading (Figure 4) [17].

The authors found that the doxorubicin-loaded nanoparticles (DOX-NPs) were effectively internalized by human cervical cancer cells (HeLa) and that they showed dose-dependent cytotoxicity. Moreover, the DOX-NPs showed good in vivo circulation time and preferential accumulation in tumor tissue with reduced accumulation in the heart and other vital organs, and significantly inhibited tumor growth in tumor-bearing severe combined immunodeficient (SCID) mice. Based on these results, DOX-NPs can become useful carriers in improving tumor delivery of hydrophobic anticancer drugs [17].

A new series of amphiphilic diblock terpolymer poly(6-O-methacryloyl-d-galactopyranose)- -b-poly(methacrylic acid-co-6-cholesteryloxyhexyl methacrylate) bearing attached galactose and cholesterol grafts [PMAgala-b-P(MAA-co-MAChol)s] 49 were prepared via Reversible Addition Fragmentation chain Transfer (RAFT) copolymerization followed by deprotection of galactose in the presence of trifluoroacetic acid (TFA) (Scheme 11) [18].

The new terpolymers (49) were studied for in vitro DOX release, and the results revealed high stability of the DOX-loaded terpolymer micelles under neutral conditions and significantly fast responsive DOX release. In addition, the results of fluorescence microscopy revealed that the DOX encapsulated in the synthesized diblock terpolymer PMAgala₁₈-b-P(MAA₂₆-co-MAChol₉)/DOX micelles could be uptaken and delivered into cell nuclei in an efficient way, and their intracellular trafficking pathway could be altered compared to the free DOX control. The new terpolymers (49) could therefore be strongly considered for future smart nanoplatforms toward efficient antitumor drug delivery [18].

In 2014, a reduction-responsive polymersome based on the amphiphilic block copolymer PEG-SS-PAChol 52 was developed. The synthesis of 52 was achieved using PEG-SS-Br 50, a versatile atom transfer radical polymerization (ATRP) macroinitiator, and a cholesterol-containing acrylate 51, using CuBr as a catalyst and N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) as a ligand (Scheme 12) [19].

The polymersome 52 was studied to come up with robust nanocarriers able to release their content inside the cells upon contact with the intracellular reducing environment. The physical crosslinking by a smectic phase of 52 in the hydrophobic sublayer, as well as the introduction of a disulfide bridge that links the hydrophilic PEG and hydrophobic blocks present in 52, were key features that gave stability, robustness, and reduction sensitivity to the polymersome. The results showed sensitivity of the block copolymer 52 to reduction, and the fluorescence dequenching of calcein both in glutathione (GSH) solution and in vitro with the mouse macrophage cells pretreated with GSH-OEt demonstrated the breakdown of polymersome under reduction conditions. To achieve significant calcein release, high concentrations of GSH and long incubation times were necessary. These reduction-responsive polymersomes (52) could be used as drug carriers with very long circulation profiles and slow release kinetics [19].

Recently, two new sterol-anchored polyethylene glycols, 55 and 58, were reported as potential alternatives to conventional phosphatidylethanolamine-PEGs. Their synthesis relied on the esterification reaction of cholesterol derivatives 53 and 56 with PEGs 54 and 57, as depicted in Scheme 13 [20].

The authors studied the biophysical properties of liposomes containing these two sterol-anchored PEGs, 55 and 58, which exhibited an array of canonical PEGgylated-liposome behaviors including retention of encapsulated small molecules, low serum protein adsorption, and reduced cellular uptake, yet they did not exhibit long circulation [20].

Polymeric micelles are known for their variety of therapeutic applications. In this field, two amphiphilic polymers were successfully synthesized using hyaluronic acid (HA), cholesterol, and octadecanoic acid as hydrophobic groups. Only the synthesis of cholesterol containing polymer HA-SA-CYS-Chol 60 is depicted in Scheme 14, since the other hydrophobic groups do not fit in the scope of this paper. Nevertheless, the authors concluded that different properties of hydrophobic groups of the amphiphilic carrier are closely implicated in the stability and drug-loading capacity of the amphiphilic carrier and micelles. HA-SA-CYS-Chol 60 presented a lower critical micellar concentration, producing docetaxel (DTX)-loaded micelles of a smaller particle size, higher encapsulation efficiency, and drug loading, when compared to the other hydrophobic tails [21]. Furthermore, in vivo animal studies revealed very good tumor-targeting properties and efficient antitumor effects at very low concentrations, with low systemic toxicity of HA–SA–CYS–Chol 60 micelles [21].

A new liposomal formulation for drug delivery purposes was recently developed, based on the N-terminal cholesterol conjugation with a mitochondria-penetrating peptide (MPP) sequence, consisting of four amino acids [phenylalanine-arginine-phenylalanine-lysine (FRFK)]. More specifically, the synthesis of cholesterol-phenylalanine-arginine-phenylalanine-lysine (Chol-FRFK) 64 was achieved by coupling cholesteryl chloroformate 7 with amino acid-bound resins (62), followed by resin cleavage using TFA and the removal of protecting groups (Scheme 15) [22]. The authors developed the liposomes using dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) and Chol-FRFK 64 for delivery of the hydrophobic drug antimycin A specifically targeted toward mitochondria and lung cancer A549 cells. The results indicated that this formulation can effectively deliver the encapsulated drug to the mitochondria because of the small size and moderately cationic charge of the liposomes, enabling cellular uptake with low toxicity. The liposomes were found to be stable for long periods at room temperature, and they acted synergistically with antimycin A, leading to the complete disruption of inner membrane potential [22].

In 2016, six new cholesterol-derived cationic lipids, 68–73, were synthesized via ether or ester linkages with different head groups (Scheme 16), which were used to create cationic liposomes for nonviral gene delivery vectors [23]. The authors studied the relationship between the structure of the synthesized lipids and the transfection efficiency and optimized gene transfection conditions of the liposomes. They found that the chemical structure of head groups and the linkage between cholesterol and head groups play important roles in gene delivery efficiency. Furthermore, lipids 69 and 73 exhibited higher transfection efficiency and lower toxicity than those of the tested commercial liposomes DC-Chol and lipofectamine 2000, even in the presence of serum [23].

In 2015, Vulugundam and coworkers reported the design and synthesis of new redox-active monomeric 76 and 77, and dimeric (gemini) 79 and 80, cationic lipids based on ferrocenylated cholesterol derivatives for the development of gene delivery systems (Scheme 17). The cationic cholesterols 76 and 77, as well as 79 and 80, were incorporated into co-liposomes and shown to be transfection-efficient. The authors found that redox activities of co-liposomes and their lipoplexes could be regulated using the alkyl ferrocene moiety. The vesicles possessing ferrocene in the reduced state induced an efficient gene transfection capability using pEGFP-C3 plasmid DNA in three cell lines, even better than the commercial lipofectamine 2000 (Lipo 2000). This evidence suggests that these redox-driven systems could be used in gene delivery applications where transfection needs to be performed spatially or temporally [24].

A series of macrocycle polyamine (cyclen and 1,4,7-triazacyclononane (TACN))-based cationic lipids 85 and 88, bearing cholesterol as a hydrophobic tail, were synthesized through ring-opening reactions (Scheme 18). These cationic lipids, 85 and 88, were used in combination with 1,2-dioleoyl-sn-glycero- -3-phosphoethanolamine (DOPE) to prepare lipoplexes, which efficiently condense DNA into nanoparticles with a proper size and zeta potential [25].

Lipid 85, containing cyclen as a headgroup, demonstrated lower toxicity and better transfection efficiency (TE) in vitro, when compared to the commercial reference lipofectamine 2000 in both 7402 and A549 cancer cells. Furthermore, the authors rationalized the good serum tolerance of 85 due to the presence of a hydroxy group in its structure. These promising results indicated that cationic-lipid 85 should be considered for nonviral gene vectors in in vivo applications [25].

Aiming to extend the existent library of polycationic amphiphiles, Puchkov et al. designed and synthesized a new molecule, 92, based on triethylenetetramine and cholesterol (a spermine analogue containing the same number of amino groups but differing in the number of methylene units). The synthesis of the polycationic amphiphile 92 was based on the selective transformation of primary amines into secondary ones via nitrobenzenesulfonamides, and the molecule of cholesterol was incorporated through alkylation of bis(sulfonamide) 89 with bromo derivative of cholesterol 90 (Scheme 19) [26]. The authors used the triethylenetetramine-based amphiphile 92 to prepare cationic liposomes and concluded that the transfection properties of delivery nucleic acids in eukaryotic cells were inferior to those with amphiphiles based on spermine. Despite the polyamines (triethylenetetramine and spermine) having the same number of amino groups, their distribution was significantly different, which may have resulted in the difference in their transfection activity [26].

A newly designed arginine-conjugated cholesterol derivative, 94, was recently reported for the preparation of cationic liposomes and their interaction with paclitaxel (PTX), a widely used anticancer drug. The synthesis of cholesterol-arginine ester (CAE) conjugate 94 was carried out through N-amidation of l-arginine ethyl ester 93 with cholesteryl chloroformate 7 (Scheme 20) [27].

The authors conducted molecular dynamic simulations as well as in vitro studies with the PTX-loaded liposomes. The results showed that these cationic liposomes enhanced loading efficiency and stability over the conventional liposomes, which can be rationalized based on the hydrogen bonding between CAE and PTX and the deeper penetration of PTX in the bilayer. Moreover, these novel liposomes demonstrated improved cytotoxicity in three different cell lines (MDA MB 231, H5V, and HDMEC) and enhanced endothelial cell migration inhibition compared to conventional liposomes. The absence of genotoxicity makes cholesterol-arginine ester 94 an interesting biocompatible cationic ligand in drug delivery applications [27].

The design and synthesis of thermosensitive polymers of N-(2-hydroxypropyl)methacrylamide mono/dilactate of different molecular weights and composition bearing a cholesterol anchor 98 (Chol-pHPMAlac) was reported in 2014 (Scheme 21). These new cholesterol-based polymers 98 were incorporated onto liposome formulations loaded with DOX. The authors concluded that the release of DOX from such liposome formulations was effective at low temperatures and could be adjusted according to the grafting density of Chol-pHPMAlac 98. Chol-pHPMAlac 98 with a cloud point of 19.0 °C and a M_n of 10.0 kDa showed interesting releasing features because it was stable at body temperature, releasing its content only under hyperthermia conditions. These releasing features make these liposomes interesting for local drug delivery using hyperthermia [28].

Recently, Asayama and coworkers reported a byproduct-free PEGylation method for the modification of insulin. The strategy involves the reaction of cholesterol chloroformate 7 with aminopropyl mPEG in the presence of triethylamine to afford the conjugate Chol-U-Pr-mPEG 99 (Scheme 22), complexation with insulin in aqueous solution, and subsequent freeze-drying [29].

The Chol-U-Pr-mPEG/insulin complex not only preserved the insulin conformation, but also was shown to be effective in its protection from hydrolysis by protease and in the suppression of blood glucose levels in mice [29].

3. Anticancer, Antimicrobial, and Antioxidant Compounds

Many new cholesterol derivatives bearing a wide range of bioactive scaffolds have been developed in the search for new anticancer, antimicrobial, or antioxidant agents with improved efficacy. In this context, Rodríguez et al. described an efficient synthesis of (6E)-hydroximinosteroid homodimers (105) linking two steroidal monomers at position 3 of the steroid scaffold via ruthenium-catalyzed cross-metathesis reaction (Scheme 23). The synthesis of the precursor monomers was carried out through a five-step reaction sequence starting from cholesterol 28 (Scheme 23) [30].

The cytotoxic activity of (6E)-hydroximinosteroid homodimers (105) was evaluated in vitro using human lung carcinoma A549, colon adenocarcinoma HCT-116, human Caucasian glioblastoma multiform T98G, and human pancreatic adenocarcinoma PSN1 cells. Only homodimer 105 (n = 2) showed selective cytotoxicity against HCT-116 cells: However, it presented no activity against the remaining cell lines. Nevertheless, the monomer counterparts 106 and 107 showed better cytotoxic activity against all cell lines when compared to homodimer 105 [30].

Richmond et al. reported the synthesis of four new (6E)-hydroximinosteroids (109), starting from the corresponding ketones (108) derived from cholesterol. The authors evaluated the cytotoxicity of all the prepared compounds (109) and compared the results to those of five polyhydroxylated sulfated analogs (110) (Scheme 24) [31].

Upon evaluation of the cytotoxic activity of the steroidal oxime 109 against two prostate carcinoma cell lines (PC-3 and LNCaP), the authors concluded that oxime 109 (R¹ = R⁴ = OH, R² = R³ = H) was the most active compound for PC-3, while for LNCaP the trisulfated analog 110 (R⁵ = H, R⁶ = OSO₃Na) was the most active one [31].

A new greener methodology involving steroidal epoxides as intermediates for the synthesis of steroidal β-aminoalcohols was recently reported. The synthesis of β-aminoalcohol 112 involved two steps: i) Epoxidation of cholesterol 28 conducted by m-chloroperoxybenzoic acid (m-CPBA); and ii) solvent-free aminolysis of epoxide 111 mediated by sulfated zirconia (Scheme 25) [32].

The antiproliferative activity of the cholesterol-based β-aminoalcohol 112 was evaluated using MCF-7 cells, and the results showed better cytotoxic effects than cholesterol 28 itself, either by crystal violet staining (CVS) or 3-(4,5-dimethylthiazo-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays. Furthermore, cell images obtained by Harris’ hematoxylin and eosin staining protocol evidenced formation of apoptotic bodies because of the presence of cholesterol β-aminoalcohol 112 in a dose-dependent fashion [32].

The synthesis of new steroidal 5α,8α-endoperoxides starting from different steroids, including cholesterol, was reported, involving a four-step synthetic protocol. It involved the introduction of a diene in the cholesterol 28 structure through allylic bromination followed by elimination, and finally a photoinduced formation of the cholesterol-based 5α,8α-endoperoxide 115 (Scheme 26) [33].

The authors evaluated the in vitro antiproliferative activities of the 5α,8α-endoperoxides against human cancer cell lines derived from various human cancer types, such as human hepatocellular cancer cell lines (HepG2, SK-Hep1) and human breast cancer cell lines (MDA-MB-231, MCF-7). It was found that some compounds exhibited potent anticancer activities through inducing cancer cell apoptosis against the four tested cancer cell lines, particularly the cholesterol-based 5α,8α-endoperoxide 115, which was the most promising derivative, presenting IC₅₀ values ranging from 8.07 to 12.25 μM [33].

A six-step synthetic route based on cholesterol 28 as a starting material was designed to prepare two new steroidal thiadiazole derivatives, 121 (R = H, Me), with an A-homo lactam and a B-norsteroidal skeleton (Scheme 27) [34]. The antiproliferative activity of compounds 118–121 against various cancer cell lines was evaluated, and the results showed that compounds 120 (R = Ph) and 121 (R = Me) displayed excellent selective inhibition to the A-549 (human lung carcinoma) cell line, with IC₅₀ values of 7.8 and 8.0 µM, respectively [34].

In 2017, Martínez-Pascual et al. reported a new three-step method for the synthesis of 6a-aza-b-homo steroidal lactam 124 using cholesterol 28 as a starting material. This new methodology involved the formation of a hydroximino intermediate 123 obtained in a two-step sequence from the starting cholesterol 28 (Scheme 28) [35].

The new compound 124 was evaluated as an antiproliferative agent against six human solid tumor cell lines, displaying only moderate activity against the screened cell lines [35].

D’yakonov et al. synthesized two new hybrid compounds based on cholesterol and 1,14-tetradeca-(5Z,9Z)-dienedicarboxylic acid, 127 and 129, which were synthetic analogues of natural (5Z,9Z)-dienoic acids. The synthetic methodology relied on the preparation of cholesterol-based oximes 126 and 128 and their further esterification using 1,14-tetradeca-(5Z,9Z)-dienedicarboxylic acid (Scheme 29) [36]. The authors evaluated the in vitro cytotoxic activities of the synthesized compounds 126–129 against Jurkat (leukemia), K562 (myelogenous leukemia), U937 (lung), HeLa (cervical), and Hek293 (kidney) human cell lines. The results showed that the hybrid molecules 127 and 129 efficiently induced apoptosis of the studied cell lines and were substantially more cytotoxic than their cholesterol oxime precursors 126 and 128 [36].

Cholesterol 28 was used as a template for the synthesis of a series of 2-methoxybenzoate analogs, bearing function groups such as carbonyl 131, hydroxyl 132, and thiosemicarbazones 133, which were evaluated as potential new anticancer agents. The synthetic route involved the reaction of cholesterol 28 with 2-methoxybenzoyl chloride and the subsequent functionalization of the 7 position of the steroid core with several functional groups (Scheme 30) [37].

All of the synthesized cholesterol derivatives were evaluated for their in vitro antiproliferative activities against CNE-2 (nasopharyngeal), BEL-7402 (liver), HepG2 (liver), and Skov3 (ovarian) human cancer cells, as well as HEK-293T human kidney epithelial cells. The results demonstrated that the presence of the 7-hydroxy group (compound 132) doubled the antiproliferative activity over the nonhydroxylated compound 130. Furthermore, none of the evaluated compounds showed inhibitory activity on HEK-293T normal cells, making them good candidates for cancer treatment [37].

The synthesis of a bis(cyclam)capped cholesterol lipid (139) was recently reported by Peters and coworkers, who also evaluated its bioactivity using primary chronic lymphocytic leukemia (CLL) cells. The synthesis of the bis(cyclam)capped cholesterol lipid relied on a four-step methodology, as depicted in Scheme 31 [38]. It was found that the bis(cyclam)capped cholesterol lipid 139 was water-soluble and self-assembled into micellar and nonmicellar aggregates in water. The authors also found that the bis(cyclam)capped cholesterol lipid 139 was as effective as the commercial drug AMD3100 in reducing chemotaxis along CXCL12 gradients, showing that 139 may be effective in disrupting the migration of CLL cells into protective niches such as the bone marrow and lymphoid organs [38].

In 2015, a paper describing the synthesis, as well as the antimicrobial and cytotoxic activities, of ten pharmacophoric motifs through CuAAC of chloroquinoline and glucose azide substrates with propargyl compounds such as chalcones, theophylline, and cholesterol was published. Within the scope of this review, only the synthesis of cholesterol-based derivatives 141 and 143 is presented (Scheme 32). Interestingly, the results from the antimicrobial evaluation showed that among the ten synthesized conjugates, triazole 143 exhibited the highest antibacterial activity against E. coli and S. aureus, and moderate antifungal activity against A. flavus and C. albicans. Furthermore, the sugar-cholesterol conjugate 143 displayed the best in vitro cytotoxic activity against the prostate cancer PC3 cell line [39].

Two cholesterol derivatives (3β-azidocholest-5-ene (144) and (3β)-3-(prop-2-yn-1-yloxy)- -cholest-5-ene (20)) were used as starting materials for the preparation of three-motif pharmacophoric conjugates including cholesterol, 1,2,3-triazole, and either a chalcone, a lipophilic residue, or a carbohydrate tag [40]. The first set of cholesterol conjugates was prepared through the reaction of 3β-azidocholest-5-ene 144 with propargylated chalcones or lactose derivatives under CuAAC conditions, affording chalcone conjugates 145 and 146 and lactose conjugates 147 and 148 (Scheme 33) [40].

A second set of cholesterol conjugates was prepared once again through CuAAC of (3β)-3-(prop-2-yn-1-yloxy)cholest-5-ene (20) with azido alkanols (149) and 3β-azidocholest-5-ene (144), affording cholesterol-triazole alkanols (150) and a triazole-linked cholesterol dimer (152), respectively (Scheme 34) [40]. Furthermore, compound 150 was converted in the respective bromo alkane 151 through a substitution reaction in the presence of CBr₄ (Scheme 34) [40]. A carbohydrate-tagged set of cholesterol compounds was prepared by the CuAAC reaction of (3β)-3-(prop-2-yn-1-yloxy)- -cholest-5-ene (20) with the appropriate glycosyl azides 153 and 155, affording compounds 154 and 156, respectively, upon cleavage of the acetyl protecting groups (Scheme 34) [40].

Another carbohydrate-tagged compound, 159, was synthesized through the reaction of cholesterol 28 with an appropriate glycosyl donor 157 in a three-step protocol, as depicted in Scheme 35 [40]. The authors screened all the cholesterol conjugates for their in vitro antimicrobial and anticancer activities. Among all compounds, the chalcone-triazole-cholesterol derivative 145 (R = NMe₂) was the one with the most promising antimicrobial activity, being as active as the controls against E. coli, S. aureus and C. albicans. Concerning the cytotoxic potential of the cholesterol conjugates, the cholesterol-triazole-lactoside congener 147 displayed the best in vitro cytotoxic effect against the prostate cancer PC3 cell line, with similar cytotoxicity to that of DOX, used as a control [40].

A new methodology for the synthesis of steroidal pyrazolines (162) through the reaction of cholest-5-en-7-ones (160) with 2,4-dinitrophenylhydrazine (161) was reported by Shamsuzzaman and coworkers in 2016 (Scheme 36) [41]. The reaction proceeded by a well-known 1,4-/1,2-addition/dehydration mechanism to an α,β-unsaturated carbonyl compound. The new steroid-based pyrazolines (162) were evaluated for their in vitro antibacterial activity against three different strains (E. coli, Corynebacterium xerosis, and S. epidermidis), in which compound 162 (R = H) was the most active against C. xerosis and S. epidermidis, with minimum inhibitory concentrations similar to the positive control gentamicin. Compound 162 (R = H) also demonstrated moderate activity against fungal strains Mucor azygosporus, Claviceps purpurea, and A. niger, being the most effective compound tested. The in vitro anticancer activity against five human cancer cell lines (SW480 (colon), HeLa (cervical), A549 (lung), HepG2 (hepatic), HL-60 (leukemia)) of pyrazolines (162) was also screened, with the chlorinated compound 162 (R = Cl) the most active [41].

The same research group reported a green simple synthesis of steroidal 2H-pyran-2-ones (163), starting from 3-substituted cholest-5-en-7-ones (160) and ethyl acetoacetate in the presence of chitosan as an ecofriendly heterogeneous catalyst (Scheme 37) [42]. The synthesized steroidal 2H-pyran-2-ones (163) were tested in vitro against two cancer cell lines (HeLa (cervical) and Jurkat (leukemia)) and one normal cell line (PBMC: Peripheral blood mononuclear cell). All the tested compounds (163) exhibited moderate-to-good activity against the two human cancer cell lines and were less toxic against the noncancer cell line. Furthermore, the antioxidant potential of these new compounds (163) was also evaluated, exhibiting lower 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) radical scavenging activity than the positive control, ascorbic acid [42].

A series of new steroidal pyrimidine derivatives (167) was prepared through the multicomponent reaction of cholestan-6-ones (164) with urea (166) and benzaldehyde (165) in the presence of trimethylsilyl chloride (TMSCl) as catalyst (Scheme 38) [43]. The antitumor activity of these steroidal pyrimidine-functionalized scaffolds (167) was screened against three human cancer cell lines, MDA-MB231 (breast), HeLa (cervical), and HepG2 (hepatic), and one noncancer normal cell line, PBMC, by MTT assay. All tested compounds showed cytotoxicities against the three cancer cell lines. Particularly, compound 167 (R = H) exhibited the highest cytotoxicity against the three cancer cell lines. However, all cases were lower than DOX, used as a positive control [43]. The authors also addressed the antioxidant activity of the pyrimidine compounds (167), concluding that these new compounds presented reduced DPPH radical, hydroxyl radical, nitric oxide radical, and H₂O₂ scavenging potential than l-ascorbic acid, used as a control. Moreover, the IC₅₀ values pointed out that the scavenging activity of the tested compounds were in the order of nitric oxide radical < hydrogen peroxide < DPPH radical < hydroxyl radical [43].

The cases in which the attachment of a heterocycle in the steroid backbone changes the biological properties of the steroid molecule are not so rare, and often are an interesting platform for the development of new pharmacophores. In this context, Saikia et al. reported the synthesis of steroidal heterocyclic compounds (170) through the solvent-free microwave-assisted epoxide ring opening with nitrogen nucleophiles [44]. The first series of N-heterocycles was synthesized by the reaction of nitrogen nucleophiles with the epoxide 169, which was prepared in a three-step synthetic route starting from cholesterol acetate 125 (Scheme 39) [44].

The synthesis of another set of N-heterocycles, 173, was accomplished using a mixture of epoxides (171 (α) and 172 (β) (4:1)) as starting materials, which were obtained through the direct epoxidation of cholesterol acetate 125 (Scheme 40) [44]. It is worth noticing that compound 173 was obtained as a diastereomeric mixture, which upon recrystallization in ethanol provided pure alcohol 173.

The authors also considered the dehydration of the obtained cholesterol-based N-heterocycles (170 and 173), which was successfully accomplished using a catalytic amount of sulfuric acid in acetic acid, affording compounds 174 and 175, respectively (Scheme 41) [44].

Finally, the in vitro antibacterial activity of all compounds was evaluated, and the N-heterocycles 170 (Het = 4-nitroimidazole, piperidine, morpholine, thiomorpholine, tetrahydroisoquinoline) and dehydrated N-heterocycles 174 (Het = 4-nitroimidazole, morpholine) demonstrated moderate effects against the tested microorganisms (E. coli, P. syringae, B. subtilis, P. vulgaris and S. aureus). Specifically, compound 170 (Het = piperidine, morpholine, and thiomorpholine) inhibited all the tested strains, and the 170 (Het = tetrahydroisoquinoline) derivative showed inhibition against three gram-negative bacterial strains, E. coli, P. syringae, and P. vulgaris. The authors also concluded that the removal of the hydroxyl group decreased the antimicrobial activity of the tested compounds [44].

Recently, Morake and coworkers synthesized a series of artemisinin-cholesterol conjugates, 177, 179, 180, 182, 184, 186, and 188, expecting that the putative cholesterol transporters may enhance the activity of the parent drug (artemisinin) against malaria and tuberculosis [45]. The conjugates were designed to have different O- or N-linkers, such as ether, ester, and carbamate, varying the length of each linker as well. The first set of conjugates, 177, 179–180, was synthesized from cholesterol 28 or cholesteryl chloroformate 7 with dihydroartemisinin 178 or artesunate 176 (Scheme 42) [45].

A second set of conjugates, 182, 184, and 186, was synthesized starting from a specific artemisinin derivative, 181 or 183, appropriately substituted with a piperazine group at C-10, through reaction with the appropriate cholesterol derivative, 7 or 185 (Scheme 43) [45].

Furthermore, the authors designed a final set of compounds bearing a carbamate linker, 188. The synthesis of this set of compounds was carried out through an amidation reaction of cholesteryl chloroformate 7 with the appropriate amine derivative, 187, bearing different lengths of alkyl chains (Scheme 44) [45].

The antimalarial activity of the novel artemisinin-cholesterol conjugates 177, 179, 182, 184, 186, and 188 were evaluated against Plasmodium falciparum (Pf) NF54, K1, and W2 strains, in which the conjugates of 186 (N-linked artemisinin-cholesterol conjugates) were the most active derivatives. However, the potency of these compounds was lower than the precursors artemether and artesunate. The authors rationalized these results based on the low solubility in the culture medium given by cholesterol moiety, which may have affected the efficacies of the artemisinin-cholesterol conjugates. On the other hand, concerning the activities against Mycobacterium tuberculosis (Mtb) H37Rv, the conjugates displayed enhanced efficacy over the parent drug artemisinin [45].

The synthesis of three new cholesterol conjugates, 190, 193, and 194, via CuAAC reaction was recently reported [46]. These conjugates were prepared either to have a ferrocene-chalcone moiety 190 or sugar moieties 193 and 194 as well, both linked by a triazole group (Scheme 45) [46].

The antimicrobial activities of these cholesterol conjugates were evaluated in vitro against E. coli, S. aureus, A. flavus, and C. albicans. Surprisingly, the authors found that the cholesterol conjugate bearing ferrocene-chalcone moiety 190 was completely inactive against all the tested bacteria. On the other hand, sugar conjugates 193 and 194 showed moderate inhibitory activity against E. coli, A. flavus, and C. albicans, being even less potent than control compounds ampicillin and amphotericin B [46].

Employing a one-pot multicomponent reaction procedure using (thio)semicarbazide hydrochloride 196 and ethyl 2-chloroacetoacetate 195 allowed the preparation of a series of steroidal oxazole and thiazole derivatives (197) (Scheme 46) [47].

The antimicrobial activity of the new steroidal compound 197 was evaluated against two gram-negative (E. coli and P. aeruginosa) and two gram-positive bacterial strains (S. aureus and L. monocytogenes). Additionally, the bioactivity against pathogenic fungi (C. albicans and C. neoformans) was also addressed. The authors found that most of the compounds exhibited good antibacterial and antifungal activity against the tested strains. In addition, the compounds also showed interesting antibiofilm activity against S. aureus biofilm. Molecular docking studies showed effective binding of the steroidal compound 197 with amino acid residues of DNA gyrase and glucosamine-6-phosphate synthase through hydrogen bonding interactions [47].

Given the increasing importance of steryl ferulates [3-O-(trans-4-feruloyl)sterols] in pharmaceutical applications, Begum and coworkers reported the microwave-assisted synthesis of steryl ferulates from several steroids [48]. The synthesis of cholesterol-based steryl ferulate 199 is exemplified in Scheme 47, in which microwave (MW) irradiation played a crucial role in the esterification step with trans-4-O-acetylferulic acid 198 [48].

The authors evaluated the antioxidant capacity (DPPH radical scavenging, total antioxidant capacity, and reducing power) of all synthesized steryl ferulates in comparison to equimolar mixtures of steryl ferulates and γ-oryzanol (a natural mixture of steryl ferulates, abundant in cereal bran layers). The results showed that the mixture of steryl ferulates and γ-oryzanol was a better radical scavenger than most individual ferulates, including the cholesterol-based one, 199 [48].

4. Cholesterol-Based Liquid Crystals

A liquid crystal is basically a state of matter that has properties between those of conventional liquids and those of solid crystals. The classification of liquid crystals was proposed in the 19th century and is based on molecular arrangement. Since then, liquid crystals have been divided into smectic (from the Greek word “smegma”, meaning soap) and nematic (from the Greek word “nema”, meaning thread) crystals. In smectic liquid crystals, molecules are arranged so that their major axes are parallel, and their centers of mass lie in one plane. There are many different smectic phases characterized by different types and degrees of positional and orientational order. The most common ones are the smectic A phase, in which the molecules are oriented along the layer normal, and the smectic C phase, in which the molecules are tilted away from it. Nematic phases are the simplest liquid crystalline phases formed, since they only have long-range orientational order (of, e.g., molecules, columns) and no degree of long-range translational order [49]. There is also a chiral variant of nematic or smectic phases, when the molecules of the liquid crystalline substance are chiral, with these phases denoted N* or Sm(A/B)* (an asterisk denotes a chiral phase), respectively. These phases are often called the cholesteric phases, because they were first observed for cholesterol derivatives [49].

In 2014, Hiremath reported the synthesis of two new series of cholesterol-biphen-4-yl 4-(n-alkoxy)benzoate conjugates (203), linked through either odd-parity or even-parity spacers (Scheme 48) [50]. The compounds in 203 are optically active, and both series of conjugates show a frustrated liquid crystalline state, with a thermodynamically stable twist grain boundary phase with a chiral smectic C structure (TGBC*) over an exceedingly wide thermal range [50].

The author explained such behavior based on the combined effect of extended geometry (conformation), strong chirality, and the enantiomeric excess of the molecules. Furthermore, the conjugates of 203 with an odd-parity spacer show an additional phase, the blue one. The clearing transition temperatures and the associated enthalpies alternate where the odd members exhibit lower values compared to those of even members. These results clearly demonstrate that the geometry (rod-like and bent conformation) and the thermal behavior of the conjugates of 203 are greatly influenced by the spacer parity [50].

A series of similar conjugates of 206, containing cholesterol, triazole, and biphenylene units, were synthesized via CuAAC chemistry (Scheme 49). Different flexible spacers were introduced in the system to evaluate the effect on the mesophase formation as well as the influence of the presence of a triazole linker [51].

The authors concluded that short (n = 5 and 6) and medium (n = 7, 8, and 9) alkyl spacers exhibit enantiotropic SmA* and monotropic SmC* phases, whereas the conjugate possessing the longest spacer (n = 10) favors the formation of enantiotropic SmA and N* phases. A close correlation between the transition temperatures and the increase in the length of the methylene spacer was also observed, and a higher clearing point was observed for the even spacers. Further comparison studies with (S)-2MBbip-n-Chol 207 (Scheme 49) demonstrated that the triazole ring plays a crucial role in the mesophase formation, wherein apart from the molecular dipole, the subtle electrostatic interaction and van der Waals forces enhance the SmC* phase [51].

A study involving the design, synthesis, and mesomorphic properties of the first examples of cholesterol-based calixarene liquid crystals was reported in 2015 by Guo and coworkers [52]. Novel cholesterol-1,3-bis-substituted calix[4]arene 209 and cholesterol-tetra-substituted calix[4]arene 210 derivatives were synthesized by reacting cholesterol-chlorinated derivatives (208) with calix[4]arene, as depicted in Scheme 50.

The liquid crystalline behaviors of cholesterol-calix[4]arene compounds 209 and 210 were studied, and both showed excellent mesomorphic properties of the columnar molecular arrangement of the calix[4]arene bowlic column, with cholesterol units as ancillary lateral columns. Furthermore, the authors demonstrated that compounds with longer spacers and more cholesterol units, such as 210, are better for good mesomorphic properties [52].

Following this study, similar calix[4]arene-cholesterol derivatives with Schiff-base bridges (213) were synthesized (Scheme 51), and the influence of complexation behaviors on their mesomorphic properties was investigated [53]. Like the previous cholesterol-calix[4]arene compounds (210), these Schiff-base bridged compounds (213) presented mesomorphic properties with a molecular arrangement of the calixarene bowlic column and Schiff-base cholesterol units as ancillary lateral columns as well. However, upon complexation with AgClO₄, no mesophase was observed, suggesting that the mesomorphic properties of compound 213 could be tuned by ion-complexation behavior [53].

Recently, novel columnar liquid crystals (LCs) based on symmetric hairpin-shaped cholesterol tetramers with Schiff-base spacers were prepared, and their mesomorphic behaviors were investigated by different techniques. The new molecules were synthesized through the reaction between a cholesterol dimer, 214, and phenylenediamines or bis-hydrazides working as spacers containing hydrogen bonds, affording compounds 215 and 216 (Scheme 52) [54].

The results indicated good hexagonal columnar liquid crystalline behaviors, with three molecules arranged as a disc of the columnar hexagonal state. In addition, the symmetric cholesterol tetramers with rigid cores or hydrogen-bonding cores strongly favored the formation of a columnar mesophase [54].

The preparation of a series of tetramers (218), based on azobenzene decorated with cholesterol units, was also recently reported. These oligomeric compounds bearing different alkyl spacers were synthesized by reacting azobenzene tetracarboxylic acid (217) with cholesteryl derivatives (200) (Scheme 53) [55].

Among the synthesized compounds, it was found that oligomers with n = 1, 5, and 8 exhibited an enantiotropic N* phase, while the other oligomers showed a monotropic N* phase, upon cooling from an isotropic state. Interestingly, oligomers with n = 1 and 8 formed spherulites in their crystalline state, dispersed for hundreds of micrometers in the case of the oligomer with n = 1. Moreover, both oligomers (n = 1 and 8) had photoisomerization in dilute solutions and Langmuir monolayers, in opposition to the liquid crystalline state, in which no photoisomerization was observed [55].

Cholesterol-based nonconventional liquid crystals have been studied by Gupta and coworkers. They reported the synthesis of novel functional discotic oligomeric materials based on 3,4,9,10-tetrasubstituted perylene, one of which bore the cholesterol units of 220 (Scheme 54) [56].

The cholesterol derivative 220 was found to be a nonconventional LC at room temperature: However, a monotropic nematic (N*) phase on cooling was achieved. The authors also demonstrated that the combination of rod and disc-like moieties sufficiently perturbed the molecular shape to yield calamitic mesophases. Additionally, this hybrid material showed interesting fluorescence emission properties, making it suitable for a range of optoelectronic applications [56].

Recently, the synthesis of perylene derivatives with two (223) or four cholesterol units (225) at bay-position or both in bay-position and imide position, respectively, was reported (Scheme 55). The authors addressed the influence of the number as well as the position of the cholesterol units on the mesomorphic and photophysical properties of these new liquid crystals [57]. The authors concluded that more cholesterol units significantly lowered the mesophase temperature, created wider scopes of phase transfer temperatures, and increased the fluorescence. Furthermore, it was found that a longer spacer between perylene and cholesterol units was ideal for mesomorphic properties as well as to enhance the fluorescence of the compounds [57].

A year later, Chen et al. reported the synthesis of three different perylene-based liquid crystals bearing different bay-rigid spacers (228). These new liquid crystals were synthesized starting from a perylene derivative (227) with six alkyl chains on the imides positions by coupling two phenyl (biphenyl or naphthyl)-bridging cholesterol units (226) at bay positions (Scheme 56) [58]. Investigations addressing the mesomorphic properties of these perylene-based compounds (228) demonstrated that all derivatives ordered hexagonal columnar liquid crystalline behaviors, despite the functionalization of the bay positions with aromatic spacers. Derivatives with larger and rigid aromatic spacers presented higher phase transition temperatures as well as smaller scopes of mesophase temperatures. The authors also concluded that rigid and larger aromatic groups showed stronger emission and higher fluorescence quantum yield. These results suggested that by adjusting the structures of spacers on the bay position, both mesomorphic and photophysical properties are likely to be tuned depending on the purpose of the liquid crystal [58].

Aiming to explore the potentially interesting mesomorphic properties of liquid crystals, Champagne and coworkers reported the synthesis of a synthetic liquid crystal dimer (233) and two of its monomer analogues (231) based on cholesterol mesogens [59]. The synthesis relied on the CuAAC reaction of a cholesteryl azide (229) with α,ω-di-O-propargyl-TEG (232) and O-monopropargylated-TEG (230) linkers, as depicted in Scheme 57. Several experimental studies were carried out, showing that both monomers (231) as well as the dimer (233) formed a smectic A liquid crystalline phase with comparable layer spacing. The authors explained this feature by the formation of a bilayer structure in the case of the monomers (231) and a monolayer structure for the dimer (233). Concerning the thermal stability of the self-assembled phases, the clearing temperature increased around 10 °C from 231 (R = Ac) to 231 (R = H). Molecular modeling studies rationalized the features of the liquid crystalline phases based on the different chemical functional groups present in each class of materials, allowing different kinds of intermolecular interactions, such as dipole-dipole interaction, hydrogen-bonding, as well as London dispersion forces, which greatly affected the self-assembly behavior of the three cholesterol derivatives [59].

The synthesis of four new aliphatic polycarbonate copolymers (mPEG₄₃-b-P(MCC-C_n)₅₁ (236) (n = 1–4) containing cholesteryl groups as side chain mesogenic units was achieved through the coupling reaction between mPEG₄₃-b-PMCC₅₁ (235) with a side carboxyl group and chiral cholesteryl derivatives (234) with different numbers of methylene groups, bearing a terminal hydroxyl group (Scheme 58) [60].

The authors studied the liquid crystal behavior of both chiral cholesteryl compounds (234) and the block copolymers based on cholesterol (236). The results demonstrated that the chiral compounds (234) exhibited an enantiotropic mesophase of an SmA phase and cholesteric phase except for 234 (n = 1), which only showed an SmA phase. The block copolymers showed an enantiotropic mesophase of an SmA phase except for mPEG₄₃-b-P(MCC-C₁)₅₁ (236) (n = 1), with the mesophase temperature range of the copolymers (236) being greater than those of the corresponding chiral compounds (234). It was also concluded that a longer spacer tended to stabilize the mesophase more than a shorter one and showed a wide mesophase range. These new polycarbonate copolymers with longer spacers based on cholesterol exhibited mesophase states below body temperature, which makes them good candidates for drug delivery applications [60].

The synthesis of the cholesterol-triazine-BODIPY trimers 239 and 240 with one or two cholesterol units involved the reaction of cyanuric chloride-substituted BODIPY derivative 238 with an esterified cholesterol derivative (237), using different reaction conditions (Scheme 59) [61].

The cholesterol-triazine-BODIPY trimers 239 and 240 exhibited distinct mesomorphic properties, dependent on the number of cholesterol units. The one-cholesterol unit derivative 239 showed nematic liquid crystal behavior, while the two-cholesterol unit 240 was a hexagonal columnar liquid crystal. The photophysical properties of both compounds were also addressed, and the authors concluded that both derivatives presented good fluorescence intensities with higher quantum yields and larger Stokes shifts when compared to their precursors. The authors claimed that this study reported the first examples of cholesterol-BODIPY liquid crystals, in which the introduction of a cholesterol unit was favorable for both liquid crystalline behavior and improved fluorescence [61].

The synthesis of two series of λ-shaped dicholesteryl-based conjugates, 242 and 245, containing a Schiff base core linking two cholesteryl ester units was reported. The first series of compounds was prepared based on a Williamson etherification between the Schiff-base (241) and cholesteryl bromo-alkanoates (200) to afford XSB-n-Chol (n = 4–10) derivatives (242) (Scheme 60) [62]. The synthesis of SB-10-Chol (244) was slightly different and involved the alkylation of 2,4-dihydroxybenzadehyde (243) by cholesteryl bromo-decanoate (200) followed by condensation with 4-aminophenol to afford OHSB-10-Chol (245) (Scheme 60) [62]. The study of the liquid crystal properties of the conjugates 242 and 245 indicated that the compounds had enantiotropic chiral nematic behavior, with an exception for short conjugates, which formed an additional SmA phase along with the narrow intermediary TGB phase. All compounds showed mesogenic properties, as they could form oily streaks, fan-shaped filaments, and Grandjean textures in the liquid crystalline state. The authors also found that long spacer compounds vitrified to form stable cholesteric glassy states instead of crystallization. Furthermore, the mesomorphic temperature range increased alongside the length of the spacer (from n = 4 to n = 10), showing an odd-even alternation on the clearing and transition temperatures [62].

In 2015, Frizon and coworkers described the synthesis of and preliminary studies on the thermal and photophysical properties of selenium liquid crystals containing cholesterols 247, 249, 251, and 253. The synthesis of three new series of selenide 247/251 and diselenide compounds 249/253 was accomplished via esterification of cholesterol 28 with the appropriate selenide 246/250 or diselenide acid 248/252 (Scheme 61) [63].

All synthesized compounds presented good thermal stability. Six of them showed liquid crystal properties, in which selenide 251 and alkyl diselenides 249 (n = 2) and 249 (n = 3) exhibited an SmC* mesophase, whereas aryl diselenide 253, with higher structural rigidity, showed a chiral enantiotropic smectic A (SmA*) mesophase. Furthermore, all these new selenide-cholesterol compounds showed higher glutathione peroxidase-like activity than the standard ebselen, with selenide 249 (n = 2) the most active [63].

A series of glycosteroids (256) constituted by cholesterol and distinct glycosidic moieties were synthesized by coupling propargyl 1-S-propargyl d-glucose, d-galactose, or l-rhamnose (255) to cholesterol scaffold 254 through a CuAAC reaction (Scheme 62) [64]. This study aimed to analyze if the sugar structure as well as the heteroatom linked to the anomeric position had an impact on the liquid-crystalline properties of the glycosteroids (256). The mesomorphic temperature range found for the glycosteroids (256) was higher than that generally reported in the literature, but similar to that reported for other glycosteroids. All the studied glycosteroids (256) showed great phase stability compared to those already studied, and interestingly, glycosteroids (256) (sugar = d-glucose; X = S) showed no decomposition even at 200 °C. These results offer new possibilities in the development of new high-temperature captors or detectors [64].

5. Cholesterol-Based Gelators

Low molecular weight organic gelators (LMOGs) are small organic molecules that self-assemble in water or organic solvents, forming a 3D network that entraps the liquid phase, resulting in gel formation. In recent years, these classes of compounds have attracted much attention because of their range of applications, for example as alternative biomaterials for drug delivery or tissue engineering [65,66]. New generations of steroidal low molecular mass gelators (LMGs) are usually designed through the assembly of various building units such as a steroid derivative (S), a linker unit (L), and often an aromatic platform (A) around which the steroid units can be positioned through linkers. The good gelation ability of the steroidal LMGs led to the development of a series of steroid-based gelators commonly classified as ALS, arranged in A(LS)₂, A(LS)₃, LS, or LS₂ molecular types [65].

In 2014, an interesting study was reported involving the design of an uncommon class of cholesteryl-based triangular A(LS)₃-type low molecular mass gelators and the exploration of their gelation and anion-sensing applications. The design strategy was based on placing three cholesteryl derivatives using linker units around melamine or benzene-1,3,5-tricarbonyl chloride as aromatic platform precursors. The synthesis of compounds 257 and 259 involved the reaction of cholesteryl chloroformate 7 with different amines in one- or two-step procedures (Scheme 63) [67].

This study also involved the evaluation of gelation and self-assembly properties of this new class of compounds by comparing them to the existing cholesteryl-based LMGs. The results indicated that the gelation and self-assembly properties of compounds 257 and 259 could be controlled by modification of the structural features of the A(LS)₃-type molecule. Increasing the length of the linker units, the fibrous xerogel networks assembled into more porous fiber networks. Moreover, the authors found that the compounds 257 and 259 could be used as selective sensors for F⁻, and their selectivity could be enhanced by increasing the chain length of their linker units [67].

Two new cholesterol-based compounds (261) were also reported as fluoride-responsive organogels. Their design was based on the coupling of compounds in 260, bearing azo units as the chromophore and a pyrazole group as the anion acceptor, with the cholesteryl chloroformate 7 (Scheme 64) [68].

The authors observed that structural modifications on the benzyl core of compound 261 (R = H or NO₂), hydrogen bonding, hydrophobic interactions, as well as π-π stacking interactions, had considerable influence on the gel-sol transition properties. Moreover, they also found that the gel was selectively fluoride-responsive among the tested anions, expressing gel-sol transition and red-purple color changes easily detected by the naked eye [68].

Following the purposes of the selective detection of F⁻, a new coumarin-based supramolecular gelator (267) was designed [69]. The reported compound 267 follows a simple architecture that bears a coumarin-appended 1,2,3-triazole coupled with cholesterol, synthesized in a six-step route as depicted in Scheme 65. The coumarin moiety acts as a fluorescence signaling unit, the 1,2,3-triazole as a linker and as an anion binding site, and cholesterol as a hydrophobic surface.

The authors concluded that cooperative hydrogen bonding between phenolic OH and a 1,2,3-triazole ring as well as hydrophobic-hydrophobic interactions of the cholesteryl groups in compound 267 played a crucial role in the formation of an organogel. Furthermore, it was demonstrated that compound 267 organogel was sensitive for F⁻ and HP₂O₇³⁻ detection by means of gel phase transformation as well as fluorimetrically, showing considerable changes in emission properties [69].

A novel cholesterol-based organogelator containing D-A (donor-acceptor) pairs (salicylaldehyde and naphthalimide units) (272) was synthesized [70]. The synthetic strategy relied on the introduction of the electron-rich salicylaldehyde group into a naphthalimide-based organogelator through a Schiff-base reaction (Scheme 66). This cholesterol-based organogelator (272) was found to form stable and chiral gels with different optical properties and morphologies in several organic solvents. An interesting feature of compound 272 was the changing of the color and emission color of the organogel in benzene, which varied from yellow-green to red during the thermoreversible sol-gel transformation, demonstrating for the first time solvent-controlled multiple color emission achieved in a monocomponent gel system. This feature makes the organogel 272 quite suitable for applications in optical switches, sensors, and smart materials [70].

To develop new supramolecular gelators, Panja and coworkers synthesized pyrrole and furan-based pyridine/pyridinium bisamides containing cholesteryl units in their architecture [71]. The synthesis of cholesterol-based bisamides (274) was achieved through the coupling reaction of cholesteryl chloroacetate derivate 262 with the pyridine ring nitrogens in bisamide 273 (Scheme 67).

The gelation properties of both bisamide 273 and bisamides with a cholesteryl unit attached (274) were evaluated. In aqueous DMSO, compound 274 (X = O) exhibited nongelation properties, while compound 274 (X = NH) produced a light yellow colored gel. This suggests that the heteroatom of the aromatic linker played a crucial role in gelation. The organogel formed by compound 274 (X = NH) revealed itself to be a good anion sensor, since the gel state was selectively ruptured into solution in the presence of F⁻ and AcO⁻ anions. Interestingly, the gel rupture induced by F⁻ was recovered upon the addition of Fe³⁺. This feature is very useful in the visual distinction of F⁻ from AcO⁻ anions [71].

A different kind of fluorescent organogelator based on cholesterol containing benzothiadiazole fluorophores 276 and 278 was designed and synthesized by Sun and coworkers (Scheme 68). The authors aimed to understand the role of hydrogen bonding and π–π interactions and to study the changes of fluorescent properties in the process of gelation of cholesterol-based π-conjugated organogels [72].

The authors studied three methods of gel preparation (heating-cooling process, ultrasonic treatment, and mixed solvents, at room temperature) and found that π–π and H-bonding interactions should be the key contributors in forming gels of 276, while in gel formations of 278, only π–π interactions seemed to matter. The obtained results suggest that these two multiple-stimuli responsive luminescent gels, 276 and 278, can be used as smart soft materials sensitive to temperature, solvent, ultrasound, and Hg²⁺ [72].

Recently, Panja and Ghosh reported three related works involving cholesterol conjugates bearing three different moieties (dithioacetal 280, diaminomalononitrile 281, and diazine 282 functional groups) for sensing a series of cations such as Hg²⁺, Cu²⁺, Ag²⁺, and Fe²⁺ [73,74,75]. The three cholesterol conjugates were synthesized using the same three-step methodology, except for the final step, which involved the reaction of the intermediate benzaldehyde 279 with 1-dodecanethiol, diaminomalononitrile, and hydrazine to afford cholesterol conjugates 280, 281, and 282, respectively (Scheme 69). The cholesterol-dithioacetal conjugate 280 was used for the detection of Hg²⁺ and incorporated two distinct components: i) A cholesterol motif to assist the self-assembly of the molecules through hydrophobic interaction; and ii) a thiol part that was used as the reaction-based recognition unit of the molecule [73].

The authors studied the sensing mechanism for Hg²⁺ of the cholesterol-dithioacetal conjugate, realizing that the specific Hg²⁺-induced deprotection of the thioacetal functionality of 280 resulted in sol-to-gel transition in DMF/H₂O (1:1, v/v) through the formation of precursor aldehyde 279. The authors also claimed that this was the first chemodosimeter that functions as a selective “naked-eye” Hg²⁺-detector by showing in situ sol-to-gel conversion [73].

The cholesterol-diaminomalononitrile conjugate 281 was found to form supramolecular gels in dimethylformamide (DMF)/H₂O and 1,2-dichlorobenzene, as confirmed by rheological studies. In addition, the authors verified that the gel formed in DMF/H₂O was more stable and robust than the one obtained from 1,2-dichlorobenzene, due to strong intermolecular forces among the gelators in DMF/H₂O. Furthermore, it was also established that cholesterol-diaminomalononitrile 281 gel was selective for visual recognition of Hg²⁺ and Cu²⁺ ions, and for sensing hydrazine based on the dosimetric interaction of the malononitrile motif with hydrazine [74].

Concerning the cholesterol-diazine conjugate 282, the authors demonstrated that it could form nice gels with Ag⁺ and Fe³⁺ ions in a CHCl₃/CH₃OH mixture solvent, using the diazine moiety as a metal ion binding site. The gelator 282 was able to distinguish Ag⁺ and Fe³⁺ with the aid of tetrabutylammonium chloride, tetrabutylammonium bromide or fluoride, and ammonium thiocyanate. Furthermore, the authors proved that there was no interference of Fe²⁺ ions in the detection of Fe³⁺ ions, as in the case of most chemosensors and gelators [75].

The effect of different spacer lengths containing two, three, five, six, ten, or twelve carbon atoms on cholesterol-based azobenzene organogels 285 and 286 was investigated [76]. For this purpose, a series of seven azobenzene-cholesterol compounds was synthesized through esterification reactions of cholesterol derivatives of 283 (bearing different spacer lengths) with 4′-carboxy-4-methoxyazobenzene 284 carried out in the presence of N,N′-dicyclohexylcarbodiimide (DCC) and dimethylaminopyridine (DMAP) in dichloromethane, as depicted in Scheme 70.

Typical reversible trans-cis and cis-trans isomerization of the azobenzene units was observed upon UV-Vis irradiation, giving the compounds 285 and 286 recoverable photoresponsive properties. Differential scanning calorimetry studies revealed that the spacer length plays a crucial role in the gelation phenomenon. Interestingly, among the tested compounds, only 285 (n = 6) could form a gel, and in specific solvents such as ethanol, isopropanol, and butan-1-ol. Furthermore, the authors concluded that the solvents, intermolecular H-bonding, and van der Waals interactions affected the aggregation mode and morphology of the gels [76].

In 2016, a study was reported on liquid crystal (LC) and gelation-based self-assembly, as well as the photoresponsive behavior of a new unsymmetrical azobenzene-cholesterol based dimesogen, 288 [77]. This molecule assembles a CN group at one end and a cholesterol carbonate, fixed through an oxyethylene spacer, to the opposite end of the azobenzene unit (Scheme 71).

Compound 288 presented the capacity of acting as a chiral mesogenic dye dopant to induce a high helical-twisting chiral phase in the common nematic phase of 5CB. In addition, the gels of 288 formed in organic solvents exhibited multiple stimuli-responsive behaviors upon exposure to environmental stimuli such as temperature, light, and shear forces. The photoresponsive character was also proven in solution, in LC and gel states. These properties give to compound 288 potential applications in displays, as chiral mesogenic dye dopants, photochemical molecular switches, and new versatile LMGs [77].

A new series of liquid crystal gelators (290) with photoresponsive and aggregation-induced emission (AIE) properties was synthesized by connecting cholesterol derivatives 200 and tetraphenylethylene (an important AIEgen) to a central azobenzene moiety through esterification reaction (Scheme 72) [78]. The authors included variations in the alkyl chain spacer (n = 0, 1, 3, 5) to adjust the distance between cholesterol and azobenzene, while a fixed alkyl chain was placed between azobenzene and tetraphenylethylene (Scheme 72). The liquid crystal properties of compounds in 290 were assessed, and the results showed that all compounds exhibited, in pure state, smectic A LC phases, enantiotropic for 290 (n = 0) and (n = 3), but monotropic for 290 (n = 1) and (n = 5). The gelation properties of compound 290 demonstrated that 290 (n = 3) and (n = 5) form stable gels in appropriate solvents or solvent mixtures, while 290 (n = 0) and (n = 1) cannot form gels in a range of solvents. An interesting feature of both 290 (n = 3) and (n = 5) LMOGs is that they have significantly enhanced emissions induced by molecular self-assembly into fibril or ribbon-like nanostructures [78].

Three new cholesteryl-based A(LS)₂- and A(LS)₃-type LMGs, 292, 294, and 296, without hydrogen bond linkers, were reported in the literature, synthesized through esterification reactions of acid chlorides 291, 293, and 295 with cholesterol 28 in the presence of DMAP (Scheme 73) [79]. The study of the gelation properties in various organic solvents indicated that the number and position of the substituents in the cholesteryl moieties attached to a benzene ring had a great influence on the gelation as well as in the aggregation behaviors of the A(LS)₂- and A(LS)₃-type LMOGs. Among these three gelators, 294 and 296 showed efficient gelation abilities even without hydrogen bond linkers, in contrast with the meta-substituted 292, which did not gelate in any tested solvent [79].

Recently, the synthesis of a new pillar[6]arene-functionalized cholesterol derivative (298), acting as an LMG, was reported in the literature [80]. In this new compound, the host–guest pillar[6]arene 300 was linked to a cholesterol unit by the long alkyl chain, as well as amide groups (Scheme 74). This new pillar[6]arene-cholesterol 298 was found to form an organogel in cyclohexane/hexan-1-ol (10:1, v/v), which was reversibly responsive to temperature, share stress, and partially host–guest interaction introduced by ferrocenyl iminium derivative 299. In the case of the addition of ferrocenyl iminium derivative 299, the organogel could be tuned into a solution and tuned back into the organogel upon addition of per-butylated pillar[6]arene 300. This interesting feature could be explained on the basis of host–guest interactions of individual 300 with cationic guest 299 that bound with pillar[6]arene-cholesterol gelator 298 [80].

In 2015, the development of a new kind of self-healing, degradable, and biocompatible polypeptide hydrogel based on self-assembly between cholesterol-modified triblock poly(l-glutamic acid)-block-PEG-block-poly(l-glutamic acid) [(PLGA-b-PEG-b-PLGA)-g-Chol] 302 and β-cyclodextrin (β-CD)-modified poly(l-glutamic acid) (PLGA-g-β-CD) 303 (Figure 5) was reported in the literature [81].

The authors observed that the hydrogel formation was based on the host and guest linkage between β-cyclodextrin (β-CD) and cholesterol, and that their viscoelastic behavior depended on polymer concentration as well as the β-CD/Chol molar ratio. Those hydrogels showed very interesting self-healing capabilities, good cytocompatibility, excellent flexibility, and quick colorant diffusion. With all these features, it is anticipated that these self-healable hydrogels may have important applications in tissue engineering [81].

6. Bioimaging Applications

Imaging techniques, particularly fluorescence imaging techniques, have become powerful tools for noninvasive visualization of biological processes in real time with high spatial resolution. Methods to “see into the body” or “see into cells” are essential for the diagnosis and treatment of a disease, as well as for research into the basic processes of life. Therefore, bioimaging techniques to visualize physiological or pathophysiological changes in the body and cells have become increasingly important in biomedical sciences [82].

The synthesis of a series of BODIPY-based fluorogenic dyes was reported, involving the CuAAC reaction of a nonfluorescent BODIPY-azide, 304, with a series of nonfluorescent alkyne molecules, including O-propargylated cholesterol 20 (Scheme 75) [83]. The most interesting molecule was the cholesterol-linked dye 305, which presented red-shifted absorption and emission wavelengths and displayed its preferential accumulation at the intracellular membranes over the plasma membrane of HeLa cells. This result offers potential applications of cholesterol-BODIPY conjugate 305 in the bioimaging of cholesterol trafficking in living cells and organisms [83].

Byrd and coworkers reported the synthesis of a crosslinker containing two independent cholesterol units, with or without a photoaffinity label, guided by computational methods based on a model for the transfer of a cholesterol molecule between two proteins, NPC1 and NPC2 [84]. The synthesis of crosslinker 314 (without a photoaffinity label) involved several steps, especially because of the demanding six-step synthetic route of one of the portions that constitutes the crosslinker 314 (Scheme 76) [84].

Another cholesterol-based crosslinker (322) with a photoaffinity label was also synthesized (Scheme 77) [84]. The synthesis of such a compound involved two stages: i) The preparation of an appropriate carboxylic acid cholesterol moiety (318) (Scheme 78) [84]; and ii) the linkage between compounds 318 and 312 (previously synthesized) (Scheme 76) [84].

The authors claimed that with the appropriate connection of the two cholesterol molecules 314 and 322, both proteins (NPC1 and NPC2) are simultaneously occupied in a manner that stabilizes the protein–protein interaction, allowing detailed structural analysis of the resulting complex. Furthermore, the introduction of a photoaffinity label in one of the cholesterol moieties, 322, should allow the covalent attachment of one of the units into its respective protein-binding pocket. The compounds synthesized in this work may be interesting tools for studying the transfer of cholesterol between cholesterol-binding proteins [84].

Two cholesterol-based fluorescent lipids, 326 and 329, were synthesized using nitrobenzoxadiazole (NBD) or rhodamine B, respectively, linked by an ether alkyl chain (Scheme 79). Compounds 326 and 329 were incorporated into liposome formulations, aiming to create and validate their use as fluorescent probes for lipoplex tracking, without interfering with green fluorescent protein (GFP) [85]. The authors concluded that both compounds 326 and 329 did not interfere with the expression of GFP plasmid, obtaining live cell images without any interference. Furthermore, microscopic observations clearly showed that these fluorescent lipids had minimal self-quenching and photobleaching effects. The results indicated that the synthesized compounds 326 and 329 may be considered for the development of fluorescent probes to trace the intracellular trafficking of cholesterol-derived cationic liposomes [85].

Reibel et al. prepared radiolabeled-¹⁸F polymer compounds based on linear PEG 332 and novel linear-hyperbranched amphiphilic polyglycerol (hbPG) 334, using cholesterol 28 as a lipid anchor via CuAAC chemistry of propargylated compounds 330 and 333 with radiolabeled-¹⁸F azide 331 (Scheme 80) [86].

The authors also carried out direct labeling of cholesterol 28 with ¹⁸F (Scheme 80) and performed in vivo positron emission tomography (PET) studies as well as ex vivo biodistribution studies in mice with both polymers (Ch-PEG₂₇-CH₂-triazole-TEG-¹⁸F 332 and Ch-PEG₃₀-hbPG₂₄-CH₂-triazole-TEG-¹⁸F 334) and ¹⁸F-cholesteryl fluoride 336. These three new derivatives were incorporated into liposome formulations. The results showed that both polymers 332 and 334 were quickly excreted by renal function, whereas ¹⁸F-cholesteryl fluoride 336 showed some retention in the lung, liver, and spleen. Liposome formulations with the new polymers showed different physical properties from those of the conventional liposomes with ¹⁸F-cholesteryl fluoride 336, as well as fast uptake by the liver, spleen, and lung. Furthermore, the novel hbPG-polymer liposomes of 334 showed similar behavior to the PEG-shielded vesicles, enhancing multifunctionality without the loss of pharmacokinetic properties. This approach opens new possibilities in the field of polymer tracking in vivo and liposome tracing in mice via PET [86].

In 2015, Palakollu and Kanvah designed and synthesized cholesterol-conjugated chromophores of α-cyanostilbene/diene 338 and 340 exhibiting intramolecular charge transfer (ICT) and aggregation-induced enhanced emission (AIEE). Compounds 338 and 340 were easily prepared from the reaction of cholesterol chloroformate 7 with either a stilbene 337 or diene derivative 339 (Scheme 81) [87].

The authors carefully studied the absorption and emission properties of both cholesterol conjugates 338 and 340 and their parent chromophores 337 and 339. An ICT behavior was observed for diene compounds 339 and 340, whereas for stilbene compounds 337 and 338 a remarkable AIEE behavior was detected. The lack of AIEE characteristics in dienes may be explained by the competing nonradiative losses due to double bond flexibility. Nevertheless, the most interesting conclusion of the optical properties study was that the random aggregates formed by stilbene 337 in aqueous media became highly ordered upon cholesterol conjugation 338. Furthermore, the interaction with sodium cholate stimulated the formation of self-assembled structures in nanoscale dimensions, making these conjugates the starting point for the development of several bioimaging probes [87].

In 2016, Wercholuk and coworkers synthesized a fluorescent-labeled cholesterol molecule (342) by treating cholesteryl chloroformate 7 with 4-amino-1,8-naphthalimides (341) (Scheme 82) [88]. The authors expected that such conjugates might serve one of two roles, depending on whether the toxicity of the fluorophore was retained in the conjugates: As reporters for following in vivo uptake or catabolism of cholesterol, or as “Trojan horse” antibiotics. The results pointed out that the new compounds (342) emitted blue light in nonpolar solvents, and its lipid portion incorporated into liposomal membrane bilayers quickly, leaving the fluorophore exposed to the external aqueous environment. Compounds in 342 were incubated with Mycobacterium smegmatis mc2 155, which displayed stable integration of the fluorescent-labeled cholesterols into bacterial membranes in vivo. Although fluorophores are toxic to prokaryotic cells, the new cholesterol conjugates (342) are not, and therefore they could be considered for the evaluation of cholesterol uptake in prokaryotic organisms [88].

In the same year, Bernhard et al. reported an interesting paper in which they studied two strategies for the bioconjugation of bombesin (BBN), a well-known peptide, the receptor of which is overexpressed at the surface of tumor cells and which has been conjugated in several probes [89]. They used subphthalocyanines (SubPcs), which are interesting probes for optical imaging. One of these strategies involved the entrapping of SubPc into a liposome and subsequently grafting BBN to the SubPc-containing liposome to afford a biovectorized liposome. The synthesis of cholesterol derivatives 346 and 347 used in their work was achieved by the reaction of dimethylaminopropyne 344 or 3-azidodimethylpropylamine 345 with cholesterol bromo ester 343 to afford cholesteryl-ammonium species 346 (alkynyl) and 347 (azide), respectively (Scheme 83) [89].

Once the cholesteryl-ammonium species 346 and 347 were prepared, the pre-bioconjugation strategy started from grafting the biomolecule to one liposome’s component (i.e., cholesterol additive) prior to the preparation of the liposome, to afford BBN-cholesterol conjugates 348 and 349. The conjugation of BBN-azide with cholesteryl-alkyne 346 (i.e., pre-functionalization by copper-catalyzed click chemistry) was carried out in the presence of copper sulfate and sodium ascorbate as the reducing agents (Scheme 84) [89]. Alternatively, BBN-bicyclononyne and cholesteryl-azide 347 were reacted without the Cu catalyst to afford conjugate 349 (Scheme 84) [89]. This strategy was employed using liposomes containing graftable cholesterol derivatives, revealed itself as a more suitable approach in addressing the stability of SubPcs, and was achieved by copper-free click-chemistry on the outer face of the liposome. This study demonstrated that both azido- and alkynyl-liposomes are good entry points for a bioconjugation or biovectorization approach (on the outer face of the liposome), which offers a second chance for fluorophores with no reactive functional group available on their backbone, a way of imitating bioconjugation with a biomolecule (i.e., an indirect approach offered to achieve future site-specific targeting of tumors) [89].

A series of new hybrid compounds (Ch-DAINs), 355, 356, and 360, bearing a green fluorescent protein-chromophore analogue, 4-(diarylmethylene)imidazolinone (DAIN), and a cholesten or cholestane, was recently reported as a candidate for viscosity-dependent and cholesterol-responsive fluorescent molecules [90]. The synthesis of Ch(en)-DAINs 355 and 356 was carried out through a condensation reaction of methyl imidates 352 or 358 (obtained from cholestenone through Beckmann rearrangement followed by methylation) with N-(diarylmethylene) glycinates 353 or 354 (Scheme 85) [90]. Likewise, Ch(an)-DAINs 359 and 360 were obtained following the same synthetic strategy with an additional double-bond hydrogenation step (Scheme 85) [90].

Among the tested compounds, cholesten DAINs 355 and 356 increased their fluorescence intensity in viscous solvents such as triglycerides. Besides, compound 355 showed good cholesterol-responsive emission, which increased linearly with the amount of cholesterol in the lipid bilayer. The responsiveness displayed by cholesten DAIN 355 to cholesterol was improved relatively to the known viscosity probes, 9-(2,2-dicyanovinyl)julolidine (DCVJ) and Laurdan [90].

7. Synthetic Applications

The regio- and stereoselective formation of O-glycosidic bonds between carbohydrates and steroids is still a demanding process, despite the considerable progress in carbohydrate chemistry in the last years. The direct electrochemical glycosylation of steroids is an alternative: However, it has several drawbacks. In attempting to solve the problem, Tomkiel et al. screened several derivatives of cholesterol as sterol donors in electrochemical reactions with sugar alcohols [91]. The authors tested sixteen cholesterol substrates in the presence of 1,2:3,4-di-O-isopropylidene-α-d-galactopyranose (362), concluding that cholesteryl diphenylphosphate 361 was the best compound for the purpose, affording 3β-O-(1′,2′:3′,4′-di-O-isopropylidene-α-d-galactopyranos-6′-yl)-cholest-5-ene (363) in 54% yield (Scheme 86) [91].

Following this work, the same authors reported in 2015 the use of 3α,5α-cyclocholestan-6β-yl alkyl and aryl ethers (364) as a cholesteryl donor in the electrochemical synthesis of glycoconjugates (363) (Scheme 87) [92]. The reaction worked well for all the tested compounds, but the best yields were achieved for ethyl, benzyl, phenyl, and tert-butyldimethylsilyl (TBDMS) ethers (51%, 50%, 58%, and 52%, respectively). Unfortunately, an isomerization side reaction was observed for the less reactive cholesteryl esters, affording the compounds in 365 (Scheme 87) [92].

To develop step-economy syntheses of cholesteryl glycosides, Davis and coworkers reported a methodology for the synthesis of α-d-cholesteryl glycosides 369 and 372, using a one-pot per-O-trimethylsilyl glycosyl iodide glycosylation (Scheme 88) [93]. The methodology relied first on the generation of glucosyl or galactosyl iodide through the reaction of per-O-TMS glucoside 366 or 370 with iodotrimethylsilane (TMSI), which was directly cannulated into a solution of cholesterol, tetrabutylammonium iodide (TBAI), and N,N-diisopropylethylamine (DIPEA), and the mixture was stirred for 2 days at room temperature. After that, the product was treated with methanol and Dowex-50WX8-200 acidic resin to remove the silyl protecting groups, affording compounds 367 and 371 (Scheme 88) [93]. These glycosides were subsequently esterified using regioselective enzymatic acylation of the 6-hydroxy group with tetradecanoyl vinyl ester 368 (Scheme 88) [93].

This methodology involving the glycosylation of cholesterol followed by enzymatic regioselective acylation allowed expansion of the acylated α-cholesteryl glycoside inventory to include galactose analogues. The glycosylation of per-O-silylated glucose provided better α-selectivity (39:1) than past syntheses (8:1 α-selectivity) and higher glycosylation yields due to the armed nature of per-O-silyl donors [93].

Mao and coworkers developed a novel glycosyl coupling reaction, involving a photoinduced direct activation mechanism of thioglycosides (373) and subsequent O-glycosylation in the absence of photosensitizer [94]. In their studies, the authors used several sugars, amino acids, and cholesterol 28 (75%) as substrates (Scheme 89). The authors showed that the activation of thioglycosides upon UV irradiation followed by the oxidation of Cu(OTf)₂ led to the in situ formation of species that could undergo glycosylation to afford glycosides without the need for a photosensitizer. The proposed mechanism involved i) homolytic cleavage of a C-S bond to generate a glycosyl radical and ii) oxidation to an oxacarbenium ion promoted by Cu(OTf)₂, and sequential O-glycosylation [94].

In 2015, Davis and coworkers reported the synthesis of cholesteryl-α-d-lactoside 378 via generation and trapping of stable β-lactosyl iodide 376. The iodide derivative 376 was prepared quantitatively under non-in situ anomerization and metal-free conditions by reacting commercially available β-per-O-acetylated lactose 375 with trimethylsilyl iodide [95]. The introduction of cholesterol occurred under microwave conditions to afford the corresponding glycoconjugate 377 in 59% yield (Scheme 90). Cholesterol glycoconjugate 377 was further deacetylated using sodium methoxide to afford cholesteryl α-d-lactoside 378 in 88% yield (Scheme 90). This glycosylation method can be employed on sterically demanding nucleophiles such as cholesterol and has potential applications in accessing structurally diverse cholesteryl glycoside analogs [95].

A new efficient method for the synthesis of cholesteryl glucosides starting from sucrose 379 was recently developed [96]. This method lays down a five-step synthetic route that involves the initial protection of disaccharide 379 hydroxy groups, and upon acidic hydrolysis at its anomeric center, the pyranosyl moiety 381 is converted into trichloroacetimidate derivative 383 (Scheme 91).

The final two steps rely on the formation of the glycosidic bond to cholesterol 28 followed by the removal of the protecting groups, affording the desired cholesteryl glucoside 384 (Scheme 91). The authors claimed that the major advantage of this strategy was the use of the readily available and cheap sucrose 379 as starting material. In addition, the methodology proved to be fast, cost-saving, and high-yielding, representing a competitive preparation method for these natural compounds [96].

In 2014, Algay and coworkers extensively explored the versatility of nitrile oxide alkyne cycloaddition (NOAC) chemistry for the formation of cholesterol conjugates anchored by way of a polar, aromatic, metabolically stable isoxazole nucleus [97]. The first series of compounds produced in this paper involved i) the microwave-assisted formation of propargyl ethers (386) in 62%–70% yield (Scheme 92a), and ii) the reaction of cholesterol propargyl ethers (386) with phenyl nitrile oxide (generated in situ from benzaldehyde oxime upon exposure to an ethanolic solution of chloramine-T) (Scheme 92b). This last reaction was carried out at room temperature or under microwave heating depending on the length of the spacing between the bulky lipid and the reacting alkyne, affording isoxazoles (387) in fair to excellent yields (35%–91%) [97]. The authors extended a bit further this reaction to prepare biologically relevant cholesterol fluorescent probes such as steroid–coumarin (391) (75%) and steroid–azobenzene conjugates (389) (56%) (Scheme 92). It is known that long-chain hydrophilic linkers are very attractive for bioconjugation and therefore, in this paper, the authors also synthesized three new ether-linked isoxazole-cholesterol conjugates (396) in 29%–58% yield (Scheme 92) [97].

Another series of isoxazole-cholesterol conjugates (401) was also prepared, starting from cholesterol chloroformate 7 and bearing an amidocarbamate linker following the four-step synthetic route depicted in Scheme 93 [97].

The nontrivial synthesis of aryl ethers of natural alcohols drove the authors to test the NOAC chemistry in the assembly of aryl ether cholesterol conjugates [97]. Therefore, isoxazole-linked aryl cholesterol ether 404 was prepared from the aldehyde-functionalized aryl ether 402 through subsequent oximation and cycloaddition reactions, as depicted in Scheme 94.

Finally, the authors used the potential of NOAC chemistry to prepare a steroidal glycoconjugate, 407, and the selective tethering of one or two cholesterol units, 409 and 410, respectively, to a thymidine skeleton was demonstrated by trapping of the same dipole by 5′-protected mono- or bis-propargylated thymidines (Scheme 95) [97].

In 2016, Alarcón-Manjarrez and coworkers reported the synthesis of two dimeric steroidal terephthalates, 415 and 416, from epimeric 4,5-seco-cholest-3-yn-5-ols 413 and 414, using a five-step synthetic route with cholesterol 28 as a starting material [98]. The synthetic route first involved the Oppenauer oxidation of cholesterol 28, followed by epoxidation, to afford a mixture of epoxides (411) (α:β = 1:4) (Scheme 96). Then, an Eschenmoser-Tanabe fragmentation followed by carbonyl group reduction provided the epimeric alkynols 413 and 414 in a 1:2 ratio (Scheme 96). Finally, the treatment of each one of the epimeric alkynols 413 and 414 with terephthaloyl chloride led to the symmetrical axial and equatorial dimers 415 and 416, respectively (Scheme 96) [98].

The authors proceeded to crystallographic analysis of the compounds and concluded that the facial hydrophobicity of the steroidal skeletons had crucial influence on the crystal packing in which the dimeric molecules were forced to accommodate these fragments only with a few hydrogen-bonding interactions. This feature originated a cisoid conformation for 415 and a linear conformation for 416 [98].

Shibuya et al. reported in 2016 the synthesis of (24S)-hydroxycholesterol (24S-OHChol) esters, which are involved in neuronal cell death, through catalysis with acyl-CoA:cholesterol acyltransferase-1 (ACAT-1) [99]. The authors studied the esterification of (24S)-OHChol 417 with cis-oleoyl chloride under basic conditions and obtained mono-oleates 418 and 419 and bis-oleate 420 in 39%, 9%, and 20% yields, respectively (Scheme 97). The protection of (24S)-OH with a trifluoroacetyl group was also attempted, affording mono-trifluoroacetates 421 and 422 in 33% and 14% yields, respectively, and the bis-trifluoroacetate 423 in 21% yield (Scheme 97) [99]. The authors took advantage of the mono-trifluoroacetate 421 to prepare the stearoyl and palmitoyl esters 427 and 428 in 68% and 75% yields, respectively, as depicted in Scheme 98 [99]. Finally, the authors also reported the use of esters of unsaturated long-chain fatty acids, such as linoleic (LA), arachidonic (AA), and docosahexaenoic (DHA), to react with cholesterol derivative 422 in order to prepare linoleate 430, arachidonoate 431, and docosahexaenoate 432 esters, in 52%, 74%, and 66% yields, respectively, in a two-step synthetic route, as depicted in Scheme 99 [99].

Recently, Sarkar et al. reported interesting work dealing with the preparation of diverse ring-A or ring-B oxo-functionalized steroids in a green fashion involving solvent-free solid supports [100]. The authors used cholesterol derivatives such as 4β-hydroxycholesterol 433, which was functionalized into three different keto-steroids, 434, 435, and 436, in 55%, 10%, and 10% yields, respectively, employing p-toluenesulfonic acid and SiO₂ (silica 60–120 mesh) as solid support (Scheme 100) [100]. Interestingly, if the reaction was attempted in the solution phase at room temperature using either dichloromethane or ethanol as solvents, cholest-4-en-3-one 434 was obtained exclusively in 64% and 60% from dichloromethane and ethanol, respectively. The procedure on solid silica was applied to the other cholesterol derivative, namely 4β,7α-dihydroxycholesterol 437, which was converted into four distinct keto-steroids: (i) cholest-5-en-7-one (438, 8%), (ii) cholesta-3,5-dien-7-one (439, 13%), (iii) cholesta-4,6-dien-3-one (440, 17%), and (iv) 5a-cholestane-4,7-dione (441, 7%) (Scheme 100) [100]. It is worth noticing that if the reaction of 4β,7α-dihydroxycholesterol 437 was carried out in dichloromethane as solvent, cholesta-4,6-dien-3-one (440, 54%) was found to be the only product formed. This was found to be a facile procedure for the synthesis of dienone 440 from cholesterol via triol 437 [100].

Cholesterol derivatives can also be used as starting materials for the synthesis of fused nitrogen heterocycles. This was the case for 4-cholesten-3-one 350, which was involved in the preparation of A-ring dehydropiperazine 443 (90% yield) through a microwave-assisted annulation reaction with ethylenediamine 442 in the presence of basic alumina (Scheme 101) [101].

The proposed mechanism should encompass the initial oxidation of the allylic protons of the conjugated ketone via enolate intermediate to afford a diketo intermediate. Then, the condensation with ethylenediamine followed by a Michael addition and autoxidation reactions afforded the dehydropiperazine derivatives [101].

Recently, Ansari and coworkers reported an efficient and green synthetic method for the preparation of steroidal pyridines [102]. Such methodology relied on the utilization of MgO NPs as a heterogeneous, mild, and reusable catalyst, in a multicomponent one-pot protocol, taking advantage of the usefulness of the microwave irradiation as an alternative heating source. The series of substituted fused pyridines (444) were obtained in 80%–89% yield from the reaction of steroidal ketones (164) with malononitrile/methylcyanoacetate, benzaldehyde, and ammonium acetate in ethanol using MgO NPs as a catalyst (Scheme 102) [102].

One of the key mechanistic steps in this kind of multicomponent reaction is the standard Knoevenagel condensation of benzaldehyde and malononitrile/methyl cyanoacetate. The effect of MgO NPs can be rationalized on this basis since they are known as a highly effective heterogeneous base catalyst for Michael addition and Knoevenagel condensation reactions with Mg²⁺ (Lewis acid) and O²⁻ (Lewis base) sites along with various cationic and anionic vacancies in the lattice [102].

The A-ring of cholesterol was also functionalized by fusing pyrimidines at the steroidal 2,3-position. These new steroidal compounds were synthesized through a microwave-assisted three-component reaction of 2-hydroxymethylene-3-ketosteroid (445), benzaldehydes (446), and ammonium acetate, affording cholesterol-fused pyrimidines (447) in good yields (78%–88%) (Scheme 103) [103].

The authors’ mechanism was based on: (i) microwave-assisted reaction of ammonia (released from decomposition of ammonium acetate) with 2-hydroxymethylene-3-ketosteroid to afford a β-aminoketoimine intermediate; (ii) their condensation reaction with benzaldehydes to afford a diamine intermediate; and (iii) cyclization and subsequent auto-oxidation to give the cholesterol-fused pyrimidines [103].

A two-step method for the preparation of steroid-fused 4,6-diaryl substituted pyridines has been reported [104]. The synthetic protocol relied on the Michael addition of 5α-cholestan-3-one 448 with chalcones (generated in situ by the base-catalyzed reaction of acetophenones (449) and benzaldehydes (446)), affording 3,5-diaryl-1,5-dicarbonyl 5α-cholestan-3-one derivatives (450) (88%–94%) (Scheme 104). Then, the intermediates (450) were used as substrates in a microwave-assisted solid phase reaction with urea in the presence of BF₃·OEt₂ to give 4,6-diaryl substituted pyridines (451) in good yields (81%–93%) (Scheme 104) [104].

The authors proposed a mechanism for the formation of a pyridine ring, which may start with the release of ammonia by urea under microwave heating, which forms an imine by reaction with one carbonyl group. Next, the BF₃·OEt₂-promoted nucleophilic attack of the imine NH-group on the activated carbonyl functionality facilitated an aza-cyclization reaction, affording a 1,4-dihydropyrinine intermediate upon which aromatization gave the desired 4,6-diarylpyridines [104].

In 2015, Schulze and coworkers developed a new method for the synthesis of model asphaltene compounds. The reported methodology was based on a multicomponent cyclocondensation reaction of 2-aminoanthracene 452 with aromatic aldehydes and 5-α-cholestan-3-one 448 (Scheme 105) [105].

The authors found that the actual catalyst for this reaction was indeed hydriodic acid, which is formed in situ from the reaction of iodine with water. Carrying the reaction under anhydrous conditions, it was proven that iodine itself did not promote the reaction, as generally assumed. Using this methodology, the authors prepared a library of optically active steroidal naphthoquinolines (453) in acceptable yields (40%–53%) [105].

8. Miscellaneous

The design of (supra)molecular switches and machines has a key feature that relates to the control of mechanical motions at the molecular level. In this field, rotaxanes have attracted much attention because they offer the possibility of restricting the freedom of motion to some well-defined pathways, such as the translational motion of a rotaxane’s ring along its axis in a shuttling manner. The synthesis of a novel nonsymmetrical bistable pH-sensitive rotaxane with a cholesterol stopper at one end and a tetraphenylmethane group at the other end (457), has been reported [106]. The synthesis of both terminal ends was challenging, and therefore we only describe here the final step, which consisted of joining both axes of the nonsymmetrical rotaxane, the alkyne 454, and the azide 455 through CuAAC chemistry, affording compound 456 (Scheme 106). The formation of a pH-sensitive bistable rotaxane 457 was achieved by methylation of the triazole ring using methyl iodide (Scheme 106). The authors verified that the crown ether part changed its preferred position on the axis because of the protonation state of a secondary amine. More specifically, the crown ether was located around the secondary ammonium ion as the best binding site in the protonated form. On the other hand, NMR analysis showed that upon deprotonation of the ammonium ion, the triazolium ion became the better binding site, which caused the ring to shuttle along the axis toward this position (Scheme 106) [106].

Venkataraman and coworkers reported the two-step synthesis of cholesterol-functionalized aliphatic N-substituted 8-membered cyclic carbonate monomer 459 (Scheme 107) [107]. Cholesterol-based monomer 459 was employed in organocatalytic ring-opening polymerization to produce PEGylated amphiphilic diblock copolymers (using a commercially available macroinitiator), polyethylene glycol monomethyl ether (mPEG-OH) 460 (Scheme 107). The authors evaluated the behavior of these copolymers in aqueous media, concluding that they self-assembled to form unique nanostructures, including disk-like micelles. The experimental results also suggested that the prepared copolymers can be used as inexpensive steric stabilizers for liposomes, making them suitable for several biomedical applications [107].

Recently, a cholesterol-modified poly(l-cysteine) copolymer, 466, that can undergo unusual micelle-to-vesicle transformation of polypeptides triggered by oxidation, was synthesized following a three-step protocol starting from cholesteryl 3-bromopropylcarbamate 462 (Scheme 108) [108]. The thioether groups in the side chains of 466 were further oxidized to the corresponding sulfone derivative 467 (Scheme 108). The authors demonstrated that oxidation of the thioether groups in the side chains could change the packing characteristics of cholesterol groups and the peptide backbone, resulting in the transformation of a β-sheet to an α-helix conformation, combined with an interesting morphological transition from micelle-like structures to vesicles. Moreover, changing the secondary structure as well as the morphology endowed the polymer assemblies with excellent specificity for controlled payload release and improved cell interaction in response to ROS. These interesting formulations had excellent anticancer properties both in vitro and in vivo [108].

Gramine [N-(1H-indol-3-ylmethyl)-N,N-dimethylamine] is a well-known indole derivative and is often used as synthon for the preparation of a large variety of substituted indoles with important biological activities. In this context, Kozanecka and coworkers reported the use of gramine (470) to synthesize cholesterol (471) and cholestanol dimers (472) consisting of two molecules of sterols connected by an N(CH₃)₂ group (Scheme 109) [109].

These new steroid dimers (471 and 472) were shown to interact in vitro with the human erythrocyte membrane, changing the discoid erythrocyte shape, which resulted in induced stomatocytosis or echinocytosis. The authors also demonstrated that these new dimers were capable of interfering with membrane phospholipid asymmetry and loosening the molecular packing of phospholipids in the bilayer at sublytic concentrations. Moreover, the dimers 471 and 472 possessed a higher capacity for changing the erythrocyte membrane structure and its permeability than steroids alone did [109].

A new multifunctional pyridine derivative was synthesized and studied as an efficient initiator for the polymerization of diethylvinylphosphonate (DEVP). The authors used a new pyridine compound (473) in the thiol-ene click reaction (a well-established coupling method) to link together poly-DEVP and thiocholesterol 95 (Scheme 110) [110].

Compound 474 exhibited good thermal response and low cytotoxicity against human embryonic renal cell lines (HEK-293) and immortalized human microvascular endothelial cells (HMEC-1). It was concluded that the introduction of the thiocholesterol anchor unit was advantageous regarding toxicity when compared to polymers without functionalization. The thiocholesterol conjugate 474 is interesting for many applications, since it is water-soluble, thermo-responsive, and biocompatible [110].

To take advantage of the important biological properties of cholesterol and glutathione for the cells, a cholesterol-glutathione (Chol-GSH) bioconjugate (478) was designed and used as a model amphiphilic biomolecule to make a co-assembly with lysozyme using a dialysis-assisted approach [111]. The synthetic route toward the Chol-GSH bioconjugate 478 involved a five-step reaction sequence, including esterification, 1,3-dipolar cycloaddition, and thiol‒disulfide exchange reactions (Scheme 111).

The authors applied a dialysis-assisted method of Ch-GSH and lysozyme to prepare bioactive self-assembled structures, which showed that hydrophobic cholesterol located in the walls, and hydrophilic GSH and lysozyme on the inner and outer surfaces. This result was explained based on the electrostatic interaction between GSH and lysozyme, which provided a driving force for the self-assembly, maintaining the bioactivity of lysozyme in the self-assembly process [111].

9. Conclusions

In this review, the role of cholesterol-based compounds in different research areas such as drug delivery, biological activities, liquid crystals, gelators, bioimaging, and purely synthetic applications was highlighted. In the drug delivery field, several examples of cholesterol derivatives were highlighted due to their applications in preclinical and clinical liposomal drug formulations to decrease membrane fluidity and provide favorable drug retention properties. Furthermore, in the last few years, some series of new cholesterol derivatives have also been developed for pharmacological applications as anticancer, antimicrobial, or antioxidant agents. In the bioimaging field, cholesterol has been used as a lipid anchor attached to fluorophores to study cellular membrane trafficking, imaging of cholesterol density, and liposome tracing, among many other bioimaging applications. The fact that cholesterol conjugates have much scientific interest in the field of materials science due to their liquid crystal phase behavior, as well as the ability to promote self-organization and hydrophobic interactions in aqueous media (gelation properties), was also demonstrated in this review. In this review, a general perspective was given of the main applications of cholesterol derivatives in several research fields, but also a concise perspective of the advances in their synthetic chemistry. Therefore, we described the synthetic pathway for different cholesterol derivatives alongside the corresponding application of the new compounds to furnish a general view from the synthetic and biological aspects of the most recently reported cholesterol-based compounds.