Five-Membered Cyclic Carbonates: Versatility for Applications in Organic Synthesis, Pharmaceutical, and Materials Sciences

This review presents the recent advances involving several applications of five-membered cyclic carbonates and derivatives. With more than 150 references, it covers the period from 2012 to 2020, with special emphasis on the use of five-membered cyclic carbonates as building blocks for organic synthesis and material elaboration. We demonstrate the application of cyclic carbonates in several important chemical transformations, such as decarboxylation, hydrogenation, and transesterification reactions, among others. The presence of cyclic carbonates in molecules with high biological potential is also displayed, together with the importance of these compounds in the preparation of materials such as urethanes, polyurethanes, and flame retardants.


Introduction
Organic carbonates are important synthetic intermediates in a number of reactions [1,2]. They are characterized by presenting a carbonyl group flanked by two oxygen atoms and may be linear or cyclic. Five-membered cyclic carbonates can be obtained through the combination of CO2 with diols and epoxides [3][4][5][6] or by conversion of glycerol into glycerol carbonate and derivatives ( Figure 1) [7]. important starting materials, which have been widely exploited for different applications, such as their potent antiwear action when added to fuels [8]. For example, glycerol carbonate (GC) 1a was used as a monomer in the synthesis of hyperbranched aliphatic polyethers [9] and polyurethanes [10]. The versatility of GC-derived five-membered cyclic carbonates as synthons in organic synthesis was explored by Rollin and co-workers [11][12][13][14]. However, their extensive reactivity was reviewed only recently [15][16][17][18][19][20][21]. Therefore, being interested in the chemical versatility of the carbonate moiety, we provide here an updated review of the recent advances involving different applications of five-membered cyclic carbonates and derivatives in organic synthesis.
For an optimized discussion, this review presents papers published between 2012 and 2020, organized in three sections, according to the potential application of the synthesized compounds in chemistry, biology, and material science studies.

Decarboxylation of Cyclic Carbonates
The decarboxylation reaction of glycerol carbonate (GC) 1a with concomitant generation of CO2 is a typical method used to obtain the 1,2-anhydroglycerol derivative denominated glycidol, which has a variety of industrial uses, such as in surface coatings, as a gelation agent in solid propellants, as a stabilizer for natural oils and vinyl polymers, and as a demulsifier (emulsion breaker). Glycidol is also used as an intermediate in the synthesis of esters and amines and it can be of high-value in the production of epoxy resins, polyurethanes, and in the controlled etherification of glycerol to form polyglycerols [22,23].
At the industrial level, glycidol can be produced through oxidation of allyl alcohol using hydrogen peroxide in the presence of a titanium silicate catalyst [24]. However, all conventional processes used to prepare glycidol generate undesirable byproducts; therefore, producing glycidol from glycerol derivatives stands as an interesting alternative to minimize the generation of waste.
Kim, Lee, and co-workers [25] performed the decarboxylation of GC 1a to glycidol 2a using various ionic liquids (ILs) as catalysts (0.5 mol%) at 175 °C under reduced pressure (2.67 kPa) for 30 min, releasing CO2 as the only co-product (Scheme 1). After studying the catalytic efficiency of anions (with a range of sodium salts) and cations (with a range of metallic chlorides), the authors discovered that both the yield and selectivity in producing glycidol 2a could be improved by using ILs. The best results were attained using [bmim] nitrate and iodide (59-73% yields In a continuous process, a solvent was used to minimize the contact between the IL and glycidol while simultaneously removing 2a as soon as it formed. The use of high-boiling solvents, such as glycol dimethyl ether, dibenzyl ether, and dibutyl phthalate, increased the yield (83-98%) and selectivity in both batch and continuous reactions. The mechanism of the decarboxylation of 1a was theoretically investigated under [EMIm]NO3 catalysis and computational calculations showed that the decarboxylation of 1a to 2a to proceeded in two steps: (1) nitrate-assisted ring-opening of 1a, followed by formation of a 3-membered ring; (2) proton-transfer from HNO3 to an oxygen atom of the carbonate group, along with the simultaneous loss of CO2. It is worth mentioning that Li and co-workers [26] recently developed a similar protocol for synthesizing epoxybutane from 1,2-butanediol by using the ionic liquid 1-butyl-3-methylimidazolium bromide in batch or continuous processes.
With a view to getting away from conventional heating, the conversion of 1a to 2a was studied through alternative methodologies: solvent-free thermal activation at 150 °C over 23 h (about 35% conversion yield), ultrasound activation at ca. 60 °C (44-48% conversion yield), and microwave activation in liquid phase at 150 °C over 30 min (48-71% conversion yield). In all cases, the authors used a ZSM-5 zeolite or a zinc oxide-supported nanoscaled cobalt oxide (RT-10CoZn, 5 wt%) as catalysts (Scheme 2) [27]. In thermal activation, the ZSM-5 zeolite showed 100% selectivity for 2a, while it did not exceed 60% with RT-10CoZn. In ultrasonic activation, RT-10CoZn showed 100% selectivity for 2a, whereas in microwave activation, RT-10CoZn reaches 71% conversion yield to 2a with total selectivity, indicating that this catalyst delivers the best performance in the shortest reaction time (30 min vs. 6-7 h under ultrasonic activation and 23 h under thermal activation). The hypothetical mechanism is based on the polymerization of 1a to poly-(glyceryl-1,2-carbonate), which then forms 2a through CO2 release. The reuse of both heterogeneous catalysts was not mentioned. Lee and co-workers [28] studied the use of a binary zinc-lanthanum catalytic system (ZnO/La2O3; Zn:La ratio = 1:9) in the decarboxylative process. When 10 wt% of the mixed oxides was employed in the reaction, 2a was obtained with 76% yield after 2 h reaction under reduced pressure (0.05 MPa) in a nonspecific solvent below 180 °C (Scheme 2). The evaluation of the catalytic activity showed that an increase in the La molar ratio correlates with a yield increase of 2a, due to equilibrium between acidic and basic sites on the surface of the catalyst. However, when the reaction was performed in the presence of La2O3 alone, 2a was obtained with 47% yield only.
Mahajani and co-workers [29] produced 2a by using the same ZnO/La2O3 system, albeit with a molar ratio for Zn/La of 4:1. In this work, the performance of the catalyst was studied up to four re-uses (about 64-97% conversion) and a decrease of activity was noted after each cycle, because of the adsorption of carbonaceous side products on the catalytic basic sites. This problem was minimized by calcination of the catalyst to restore its activity. As the major objective of the authors was to synthesize 1a by dimethyl carbonate transesterification with glycerol, no additional studies were carried out to increase the catalyst's activity in preparing 2a.
Hyunjoo Lee and co-workers [30] described a one-pot consecutive glycidol synthesis method from glycerol, whereby carbonylation of glycerol 3a with urea first takes place under high pressure using a homogeneous zinc catalyst to produce GC 1a. Subsequently, decarboxylation is used to obtain the desired 2a product (Scheme 3). This second reaction proceeds in acetonitrile in the presence of zinc acetate to give glycidol 2a with 67% yield after 1.5 h. According to the authors, the high catalytic activity of Zn(OAc)2 can be attributed to the formation of the intermediate species [Zn(NH3)x•(OAc)2]. Scheme 3. One-pot consecutive glycidol synthesis method from glycerol.
An original decarboxylation process involving GC 1 was described by Falivene and co-workers [31] through an organocatalytic coupling reaction between the bromolactide 4 and carbonates 1, in the presence of the heterocyclic carbene 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), resulting in the formation of chiral keto-diesters 5 (Scheme 4). The reaction was found to proceed at 100 °C in times ranging from 13 h to 15 h, delivering the desired products in high yields (72-93%). More recently, the use of palladium catalysis for the decarboxylation of GC 1a was reported [32,33]. Kleij and co-workers [32] described the synthesis of ethers via a Pd-catalyzed reaction of carbonates 6 with phenols 7 in THF in the presence of cesium carbonate (Cs2CO3) and phosphoramidite or phosphine ligands (Scheme 5). The authors observed that the choice of the ligand influences the regioselectivity of the reaction. Thus, by changing the ligands it was possible to direct the reaction to prepare either branched-8 or linear allylic aryl ethers 8. Reacting the cyclic carbonates at 0 °C for 36 h led to aryl ethers 8, while running the same reaction at room temperature for 12 h mainly provided 9. The products were obtained in moderate to excellent yields (49-98%) and with considerable enantio-induction. In a closely related work, Guo and co-workers [33] developed an efficient protocol to construct densely functionalized quaternary carbons with the concomitant formation of a new Csp 3 -Csp 3 bond through decarboxylation of carbonate 6. The reaction was carried out in DMF at room temperature for 12 h, using palladium catalysis with the L2 ligand and resulted in the formation of branched aldehydes 10 (32-91%) and substituted acroleins 11 (26-84%).

Hydrogenation Reaction
Catalytic hydrogenation of organic carbonates is of synthetic interest as an environmentally benign approach to fundamental synthetic building blocks, such as alcohols and diols [34,35]. In 2012, Han and co-workers [36] developed a process to obtain ethylene glycol and methanol via catalytic hydrogenation of cyclic carbonates by using (PNP)Ru II pincer complexes as catalysts (Scheme 6). In this work, ethylene glycol was prepared through insertion of CO2 into epoxides (omega process), as an option for the recycling of CO2. This catalytic process allowed them to obtain a variety of ethylene glycols (EGs) 12 with 96-99% yields, with co-production of methanol (up to 99% yield) from different cyclic carbonates 1. The reactions proceeded over 4-10 h in THF at 140 °C under 50 atm of H2 and in the presence of catalytic amounts of the Ru II complex I (0.02-0.10 mol%) and t BuOK (1.0 equivalent relative to the catalyst) in a 125 mL autoclave. It is noteworthy that when using GC 1a protected with a methyl or a benzyl group, the corresponding EGs 12 were obtained with 99% and 97% yields, respectively, after 4 h. Additionally, it was observed that sterically hindered cyclic carbonates 1 required longer reaction times or higher amounts of the catalyst. The protocol was successfully applied to a six-membered ring carbonate, giving the respective diol after 2 h of reaction. Moreover, the protocol was used in the depolymerization of poly(propylene carbonate) (PPC), resulting in propane-1,2-diol and methanol after 24 h, both with 99% conversion. Scheme 6. Catalytic hydrogenation of cyclic carbonates by using (PNP)Ru II pincer complexes as catalysts.
Similarly, the Ru-lutidine-bridged N-heterocyclic carbene complex (Ru complex II) was used as a catalyst in the hydrogenation of ethylene carbonate 1c (R = H) (Scheme 6) [37]. In this work, glycol and methanol were obtained with 92% and 42% yields, with 99% conversion of the carbonate under 5 MPa of H2 over 12 h at 130 °C. A N-butyl-substituted CNC-pincer ligand and RuHCl(CO)(PPh3)3 were applied to prepare the Ru-NHC complex, formed in situ in the presence of t BuOK (10 mol%) as a base. From the optimization studies, the authors concluded that the high catalytic activity might be ascribed to the metal-ligand cooperation through aromatization-dearomatization of the lutidine framework. Furthermore, air and moisture-stable Ru-based complexes were applied for indirect conversion of CO2.
In the same line, the pincer pyridine-bridged N-heterocyclic carbene-ruthenium complex (Ru complex III) exhibited high catalytic activity in the hydrogenation of cyclic carbonates 13 (Scheme 7) [38]. When carried out in THF in the presence of K3PO4 and H2 (50 atm) at 140 °C, with a catalyst loading of 0.5 mol%, the reaction provided nearly quantitative yields of the corresponding diol 12 and methanol. Even when using a catalytic amount of a weak base and a low catalyst loading, a range of substrates with varying steric congestion underwent the transformation. The production of diols and methanol was reported by using copper-silica nanocomposite catalysts with a uniform Cu dispersion, prepared using a precipitationgel method (Cu-SiO2-PG) (Scheme 8) [39]. This system promoted hydrogenation of cyclic carbonates 1 (87-97% conversion), providing methanol and diols 12 at conversion rates of 29-98% in both batch and fixed-bed continuous flow reactors, under relatively mild conditions. All the cyclic carbonates involved gave the respective diols in high yield, with the exception of GC 1a, which provided glycerol with 29% yield only. A catalytic process using a basic zwitterionic salt as an alternative to inorganic bases was employed to synthesize glycols through hydrolysis of cyclic carbonates [40]. Six examples of glycols 12 were prepared with 64-98% yields by using 1-6 wt% zwitterionic 1,3-dimethylimidazolium-2-carboxylate (DMImC), cyclic carbonate 1/H2O = 1:1.5 (molar ratio), at 140 °C and under 0.4 MPa for 30 min to 6 h (Scheme 8). A reaction time of 24 h was required to convert 4-phenyl-1,3-dioxolan-2-one 1b into the corresponding vicinal diol 12a with 90% yield. The catalyst DMImC was recovered and re-used six times at 140 °C, 0.4 MPa for 30 min, keeping good selectivity and high yield.
Based on the experimental results, NMR spectroscopy, and theoretical calculations, a possible catalytic cycle was suggested (Scheme 9). Generated by formation of the conjugated acid DMImCH, the hydroxide ion promotes opening of the cyclic carbonate 1b, forming the intermediate I, which is converted to II by intramolecular hydrogen transfer. Then, II is converted into a possible imidazolium alkylcarbonate salt III or IV by reaction with DMImCH. Subsequent elimination of DMImC and production of II occurs. The reaction of II with OHaffords V with CO2 and H2O release to the reaction medium. Finally, a proton from DMImCH is captured by V, producing the desired glycol 12a while regenerating the catalyst for a new cycle. The indirect hydrogenation of CO2 via the ethylene carbonate intermediate 1c to produce ethylene glycol 12b and methanol was proposed by using mesoporous silica (KIT-6, MCM-41, and SBA-15) as the support for copper-based catalysts (Scheme 10, condition a) [41]. This attractive approach showed 100% conversion of carbonate 1c, yielding 94.7% of diol 12b and 62.3% of CH3OH 7a in only 4 h reaction time over Cu/SBA-15 under the optimal conditions. This copper-based catalyst exhibited superior catalytic activity with turnover numbers of 22.0 and 11.4 (mol mol −1 ) towards diol and methanol, respectively. It is noteworthy that the higher catalytic performance was due to the synergic effect between the Cu 0 and Cu + species that co-exist on the surface of the catalysts and also to a larger metallic surface area on SBA-15 when compared with KIT-6 and MCM-41. Additionally, the thermodynamic calculation showed the ethylene glycol hydrogenation to be exothermic (ΔrH θ m = −71.59 kJ mol −1 ) and thermodynamically favorable (ΔrG θ m = −25.62 kJ mol −1 ). In another study, the same authors reported the selective hydrogenation of ethylene carbonate 1c with 100% of conversion over Cu/SiO2 catalysts, prepared using the ammonia evaporation method, with silica sol as the silica source (Scheme 10, condition b) [42]. The 10%Cu/SiO2-AE catalyst with moderate copper loading exhibited better catalytic activity to co-produce methanol 7a with 70.8% yield and EG 12c at 98.0% yield. After catalyst characterization, the authors disclosed the synergic effects of Cu 0 and Cu + species in this reaction; while Cu 0 species promoted H2 dissociation, Cu + species were adsorbed on the carbonyl group of EC. Moreover, in catalyst reusability experiments, the 10%Cu/SiO2-AE catalyst showed agglomeration of Cu and Cu2O particles, which were responsible for the gradually decreasing activity after consecutive recycling runs.
The most frequently used methods to produce diols are described above, which involve catalytic hydrogenation or hydrolysis of cyclic carbonates using organic solvents. Fu and co-workers recently published a new procedure for the conversion of cyclic carbonates 1 to diols 12 in overheated water, without an external catalyst or additive (Scheme 11) [44]. The reaction of ethylene carbonate proceeded smoothly to give the desired glycol in high yield (99%) along with CO2 as a byproduct. The reaction was studied at different temperatures (100 °C to 250 °C) with water filling of 25% for 2 h. In general, a higher temperature was better in promoting the reaction and the methodology was extended to methyl-and ethyl-substituted ethylene carbonates. With these substrates, 10 h was needed to provide the corresponding diols with 59% and 62% yield, respectively. Scheme 11. Catalytic hydrogenation or hydrolysis of cyclic carbonates using organic solvents.
The same diols can be obtained from cyclic carbonates via a deprotection reaction. In this sense, the new heterogeneous zinc/imidazole catalyst was evaluated in various reactions such as transesterification and transcarbonation [45]. A small amount of the suitable zinc/imidazole catalyst immobilized on polystyrene resin proved effective to prepare 3-phenoxypropane-1,2-diol 12 from 4-(phenoxymethyl)-1,3-dioxolan-2-one 1d after 6 h in refluxing methanol (Scheme 11).
Werner and co-workers reported on the first use of an earth-abundant metal complex as a catalyst for the transfer hydrogenation of carbonate 14, opening an indirect route for the reduction of CO2 to methanol (Scheme 14) [48]. An advantage of this method, according to the authors, is the use of isopropanol as a hydrogen source, avoiding the use of flammable hydrogen under high pressures. Using a metal-based catalyst, it was possible to perform the transfer hydrogenation of GC 1a selectively, using the PNP pincer-type iron complex C17H38BrFeNOP2 15. This reaction occurred at 140 °C over 6 to 24 h, obtaining the respective diols 12 and MeOH as products with 51-97% and 39-92% yield, respectively.

Transesterification of Cyclic Carbonates
The transesterification of cyclic carbonates, especially ethylene carbonate 1c, to give non cyclic carbonates has attracted much attention, due to their role as building blocks for the production of chemical intermediates in the polymer industry [49].
Hahm and co-workers [50]  In a closely related study, Saptal and Bhanage [51] developed the synthesis of a series of bifunctional, DABCO-based, hydroxyl-functionalized ILs, which were applied in the conversion of carbon dioxide into valuable chemicals, such as cyclic carbonates, the transesterification of which was also considered. Here, transesterification of ethylene carbonate 1c with methanol to prepare dimethyl carbonate 16a proceeded at a moderate temperature in the presence of different ionic liquids. The [DABCO-PDO][I], functionalized with propane diol functional groups, gave the highest conversion rate and yield for dimethyl carbonate 16a, with 93% yield after 4 h at 80 °C (Scheme 15). One major advantage of this protocol is that no transition metal, solvent, or additive is used.

Substitution Reaction
Cyclic carbonates can be used to prepare new organochalcogen derivatives. The interest in compounds containing sulfur, selenium, and tellurium atoms has been continuously increasing, due to their potential applications as synthons for the synthesis of heterocycles [52], in cross-coupling reactions [53,54], and in asymmetric synthesis [55,56]. Those compounds have applications in materials science [57][58][59], and as key intermediates in the synthesis of natural products [60][61][62].
Perin and co-workers [63] recently reported the synthesis of 4-arylsulfanylmethyl-1,3-dioxolan-2-ones under microwave irradiation (MW) using an environmentally benign KF/Al2O3/PEG-400 system [64]. By applying KF/Al2O3 as an efficient heterogeneous base, 3-O-tosyl glycerol 1,2-carbonate 1h (TGC) reacted with alkyl, aryl, or heteroaryl thiols 17 to provide good yields of the corresponding sulfanylated carbonates 1 after short times of MW irradiation at 60 °C. This green methodology was extended to thiophenols bearing either electron-donating (CH3) or electron-withdrawing groups (Cl and F) in the para-and ortho-positions, to provide with good yields the respective 4-arylthio-1,2-glycerol carbonates. However, when the strong electron-donating amino group was present in the ortho-position (2-aminobenzenethiol), only 65% yield of the desired product was obtained, due to the disfavoring proton abstraction from the thiol (Scheme 16). The conversion was extended, albeit in lower yields, to 2-mercaptobenzothiazolyl and dodecyl derivatives. The possibility of developing a selective method for the synthesis of 1,3-bis-arylthiopropan-2-ols 18 by incorporation of two thiol units was explored (Scheme 17). By reacting a second equivalent of thiol 17 under 15 h conventional heating at 90 °C, the corresponding disubstituted products 18 were obtained in good to excellent yields. When conducted at 90 °C for 3 h in a microwave reactor (irradiation power of 200 W), the same compounds 18 were isolated in slightly lower yields [63]. The same year, Perin's group reported the selective synthesis under stoichiometric control of 4-chalcogenomethyl-1,3-dioxolan-2-ones and 1,3-bis(organylchalcogenyl)propan-2-ols [46]. To obtain the GC-derived chalcogenoethers 1, diaryl diselenides 19 or disulfides 20 (0.6 equivalent) were first reduced using NaBH4/PEG-400 under inert atmosphere, before the addition of TGC 1h (Scheme 18). A library of eleven compounds 1 was prepared with 55-92% yield after 2 h reaction at 50 °C. The protocol was extended to aliphatic diselenides, and when dibutyl diselenide was used, the corresponding selenide was isolated with 74% yield. In general, diaryl disulfides lead to better yields when compared to the seleno-analogues. During the optimization process, it was observed that increasing the amount of diaryl diselenides 19 or disulfides 20 to 1.2 equivalent led to ring-opening of the carbonate unit of 1h over 1 h, resulting in symmetrical and unsymmetrical 1,3-bis(organochalcogenyl)propan-2-ols 18 in moderate to excellent yields (Scheme 19). The reaction works well when either electron-donating or electron-withdrawing groups are present in the aromatic unit R. It is worth noting that when using dimesityl diselenide in the synthesis of symmetrical compounds 18, 3 h reaction was needed to reach 94% yield of the expected product. Moreover when using 1,2-bis(2-chlorophenyl)disulfide in the synthesis of unsymmetrical compounds 18', the respective product was isolated with 40% yield after 1 h reaction.

Scheme 19. Synthesis of symmetrical and unsymmetrical 3-bis(organochalcogenyl)propan-2-ols.
The stereoselective construction of cyclic carbonate systems is another attractive line of studies that has been explored. In this sense, racemic vinyl-substituted cyclic carbonates 6 are important substrates for copper-mediated SN2 reactions to access useful hydroxyl functionalities 9 and 21 in a single step ( Table 1, entries 1-3) [65], or for combining a palladium catalytic system and a boron reagent to obtain chiral alcohols 12 and ethers 8 ( Table 1, entries 4-6) [66].
In 2016, the catalytic and stereoselective formation of (Z)-1,4-but-2-enediols 9 was reported through decarboxylative hydration of VECs 6 in the presence of water as a nucleophile [86]. The reaction was performed in DMF/H2O at room temperature, using a catalytic Pd precursor, Pd/bis(sulfoxide) and a bidentate phosphine ligand. The expected unsaturated 1,4-diols 9 were obtained with 50-98% yields with >99:1 Z-stereoselectivity (Scheme 22). While different groups such as aryl, naphthyl, and heteroaryl were efficiently incorporated into diols 9, it is important to highlight that some specific substitutions, e.g., o-haloaryl or 3-pyridyl, required forced conditions with higher temperatures and larger amounts of Pd-catalyst (5 mol%) and phosphine ligand (10 mol%) in order to reach preparative yields. In order to illustrate the good reactivity of the (Z)-1,4-but-2-enediols 8, some chemical modifications were made, such as m-CPBA oxidation to the epoxide 2b, high-yield conversion into the cyclic orthoester 34, and copper-catalyzed chemoselective oxidocyclization to the isomeric lactones 35a and 35b (Scheme 23).  [87]. Performed in dichloroethane or toluene over 24-48 h at temperatures ranging from r.t. to 80 °C, the reaction allowed exploration of an ample range of substrates, with yields exceeding 70% in most cases (see Figure 3 for selected examples). In this extensive study, carbonate 1e was converted into compound 39a (94% yield; 97% ee) by asymmetric O-allylation of 4-bromophenol 7b (Scheme 24). Rueping and co-workers [88] described the alkaline earth metal-catalyzed reduction of cyclic and linear organic carbonates using a readily available magnesium catalyst (Scheme 25). These mild reaction conditions provided an indirect route for the conversion of CO2 into valuable alcohols. By using a low catalyst loading and avoiding ligands, a broad scope of compounds 40 was prepared via the selective hydroboration with HBPin 41, all with excellent yields. Specifically, cyclic carbonates 1 bearing substituents such as phenyl, methyl, hexyl, t Bu, as well as glycerol derivatives, were efficiently reduced with excellent yields, ranging from 88% to 95% within 3 h. The authors also developed olefination of arylpyridines and indoles with 30-91% yield using unactivated aliphatic olefins under the same conditions, albeit using pivalic acid as a promoter. A selection of the prepared compounds is shown in Figure 4. Under manganese catalysis, VEC 1e was employed to functionalize aryl compounds bearing electron donor groups to obtain the hydroxybutenylated derivatives 47a-c [90]. The manganese complex [MnBr(CO)5] was used in the presence of sodium acetate over 5 h reaction time, allowing the synthesis of twelve allylic alcohols with moderate to excellent yields (54-97%) (Scheme 27). Another example of ruthenium catalysis was described by Guillaume, Carpentier, and co-workers [92] to obtain α-and α,ω-telechelic cyclocarbonate polycyclooctenes 50 and 51, macromolecules having two reactive end groups that are able to react selectively with other molecules to create new bonds [93]. The high-yielding reaction between cyclooctene 52 and VEC 1e or acryloyl derivative 1k was promoted by a Grubbs 2nd generation ruthenium catalyst in THF at 40 °C (Scheme 29). However, the authors did not define the polymerization degrees. The polymers characterized by 1 H NMR are important as a starting backbone for the preparation of new materials for applications in different fronts, such as surfactants or NIPUs. Silylating agents are broadly used to insert different functional groups on the surfaces of organic or inorganic materials. In this line, the synthesis of functionalized symmetrical and unsymmetrical disilazanes was reported with excellent yields (81-95%) by using the Karstedt catalyst Pt2[(CH2=CHSi(CH3)2)2O]3 [94]. Kucinski and co-workers reported the reaction between olefins and the 1,1,3,3-tetramethyldisilazane 53 in toluene at 60-110 °C over 3-24 h (Scheme 30). Among the olefins involved, when VEC 1e was reacted with 53, the bifunctional unsymmetrical disilazane 54 was obtained with 85% yield after only 5 h, thus showing the relevant tolerance of the cyclic carbonate unit. To perform the C-H allylation of indoles by insertion of VEC 1e, Yu and co-workers [95] described the use of mechanochemistry, cobalt(III) catalyst [Cp*Co(MeCN)3][SbF6]2, silver acetate, and silica gel (Scheme 31). This reaction allowed the functionalization of fifteen indole derivatives with 1e under solvent-free conditions, delivering the desired products 55 in modest to high yields (28-98%). It is worth highlighting that the reactions were run for only 2 h using a ball mill (800 rpm), which characterizes this method as an environmentally friendly protocol. In 2012, Bensemhoun and Condon [96] reported the first creation of carbon-carbon bonds with glycerol 1,2-carbonate (Scheme 32). They proposed conjugate addition reactions of activated glycerol 1,2-carbonate derivatives 1 onto electron-deficient olefins 57 based on an electrochemical process using nickel complexes as catalysts in conjunction with a consumable anode. Halogen (I, Br) or pseudohalide (N3, OTs) derivatives 1 and certain electron-deficient olefins 57 such as methyl vinyl ketone, butyl acrylate, and acrylonitrile were used as substrates and the reactions were conducted in an undivided cell fitted with a nickel grid as the cathode and an iron rod as the anode. The first step of the reaction was a short pre-electrolysis process at room temperature; after 15 min, the starting materials and NiBr2·3H2O were added to the system for the electrolysis at 76 °C under a constant 0.2 A current, until full consumption of the GC. The regioselective conjugate addition was effective and provided the new carbonates in good yields. This sacrificial anode-based electroreductive process is compatible with the presence of the carbonate function. A comparison of product yields using DMF or propylene carbonate (PC) as a green solvent gave good results in both cases. In 2020, Glorius and co-workers [97] described the synthesis of α,β-disubstituted γ-butyrolactones 58 through cooperative iridium catalysis and N-heterocyclic carbene organocatalysis (Scheme 33). Considering the importance and prevalence of γ-butyrolactones in biologically active natural products, in this study the authors prepared several compounds 58 with moderate to excellent yields, with control of the relative and absolute configuration of the two formed stereocenters, using an enantioand diastereodivergent [3 + 2] annulation reaction. These lactones were prepared from VEC 1e and trans-cinnamaldehydes 59 using a solution of [Ir(COD)Cl]2 (2 mol%), Et3N, L4 (8 mol%), and 4C (10 mol%) in toluene (0.1 M) under Ar for 48-96 h. This combination of catalysts was responsible for the effectiveness and selectivity, with the Ir-π-allyl intermediates seeming well suited for nucleophilic addition of the NHC-enolate, giving the desired compounds. An extended study involving two chiral catalysts allowed selective access to all four possible stereoisomers of the γ-butyrolactone products. The usefulness of this strategy was illustrated in the synthesis of the naturally occurring lignan (−)-hinokinin.

Other Reactions
Due to their favorable nutritional properties, 1,3-diglycerides, i.e., 1,3-sn-diacylglycerols 3a, are popular in the food industry. Interestingly, these diglycerides may be synthetized by an efficient and benign two-step, one-pot process, without any solvent (Scheme 34) [98]. A practical method used fatty acid anhydrides 60 and GC 1a under DABCO catalysis to produce the industrially relevant diglycerides 3a with good yields and high purity. Long-chain anhydrides 60 of lauric, myristic, palmitic, and stearic acids 61 led in all cases to the esterified carbonates, which were isolated in similar yields after 7 h reaction. Two plausible mechanisms were suggested for their transformation into 3a. First, the liberated fatty acid 61 reacts with DABCO to form an ammonium salt. The carboxylate anion can subsequently attack either the alkylene carbon (route 1) or the carbonyl group of the esterified carbonates (route 2), providing the desired 3a. The formation of the product was monitored by observing the liberation of CO2 and the disappearance of the carbonate IR band of the esterified carbonates. Poly(ionic liquid)s (PILs) represent an emerging interdisciplinary topic due to their potential applications in chemistry, physics, materials science, and electrochemistry [99,100]. In this sense, the application of nonporous PIL beads with swelling properties as catalysts for the reaction of ethylene carbonate with aniline was proposed by Zhang and co-workers (Scheme 35) [101]. After preparation of cross-linked poly(ionic liquid)s by direct radical copolymerization of N-vinyl imidazolium ionic liquids with sodium acrylate, the authors used them in the conversion of various carbonates. Due to its swelling ability, P[VBIM][AA] presents catalytic activities similar to the homogeneous ionic liquid monomers. However, cyclic carbonates with a low dielectric constant, such as propylene-and 1,2-butylene carbonate, showed different swelling abilities, thus decreasing the catalytic activity and providing the corresponding oxazolidones 62 in lower yields (50% and 2%) than with ethylene carbonate 1 (99%). The proposed method allowed seven aryloxypropanediols to be obtained, three of which were pharmaceutically important products, namely guaiphenesin, mephenesin, and chlorphenesin ( Figure 5). To check the recyclability of excess glycerol and of the catalyst after the first run, mephenesin was extracted with toluene, then 1 equivalent of o-cresol, 1.4 equivalent of glycerol, and 1.4 equivalent of diethyl carbonate were added to the recovered residue. The resulting mixture was then stirred at 105-110 °C over 28 h. Although mephenesin could be obtained with good yields (76-80%) after three consecutive runs, the study of the re-use of the glycerol and base was not further developed. A tetrabutylammonium-fluoride-mediated hydroxyalkylation process for phenols 7 with carbonate 1c was developed to access different aryl β-hydroxyethyl ethers 64 [103]. In this process, the corresponding products were obtained with 88-98% yields by using TBAF.3H2O (1 mol%) in DMF as the solvent at 170 °C, under N2 atmosphere over 13 min to 21 h of reaction (Scheme 37). Different substituted phenols were employed and good yields were obtained. The authors also extended this protocol to gram-scale and flow synthesis of multiflorol 64e, with 90% isolated yield. Olsén and co-workers [104] reported the synthesis of densely functionalized carbamates 65 and 65' through ring-opening of cyclic carbonates 14. The highlight of this study was the fact that unprotected α-amino acids, such as glycine 66a, were used in aqueous medium to provide a variety of new alkenes, carboxylic acids, thiols, and alcohols linked to a central carbamate motif (Scheme 38). The reactions were performed over 2 h at room temperature in the presence of triethylamine (2 equivalent). Optimization studies showed the necessity of 4 equivalent of the amino acid 66 to achieve opening of the cyclic carbonate ring. On reaction of the cyclic carbonates 14 with glycine 66a, the corresponding carbamates 65 and 65' were obtained with 75-91% yields, while different amino acids 66b, containing one stereocenter, led to carbamates 65''a-d with 94-97% conversion and 20-70% yields (NMR evaluation).

Alkene Polymerization
The synthesis of different monomers with at least two reactive groups for further polymerization is an attractive approach, especially from the point of view of the polymer industry. While one of the reactive groups can be used for copolymerization, the other one can be set aside for further transformations, namely for crosslinking [106]. In this sense, the presence of an unsaturated site on the monomer facilitates subsequent polymerization. On the other hand, the presence of a cyclic carbonate moiety as a second reactive site results in polymers with low reactivity, which allows better control of crosslinking compared with isocyanate or epoxide functions, for example. It is worth noting that the reactions on the double bond of the monomer, which can keep the cyclic carbonate unit intact, are critical for later functionalization of these monomers [19].
In this sense, Lapinte and co-workers [107] described the efficient synthesis of glycerol-based co-oligomers functionalized with alcohol, cyclocarbonate, or acetal pendant groups. The process involves free radical chain transfer polymerization using 2-mercaptoethanol (ME) as the chain transfer agent and glycerol-derived acrylate monomers. Initially, GC 1a was reacted with acryloyl chloride 69 in the presence of Et3N to provide glycerol carbonate acrylate (GCA) 1k with 80% yield (Scheme 41). The free radical telomerization of GCA to oligoGCAs was performed in the presence of AIBN and ME at 80 °C. During the telomerization process, the decomposition of AIBN causes the formation of a thiyl radical from ME, which further reacts with a monomer unit to form radical species, resulting in the product carrying a hydroxyl head group. A similar procedure was conducted using solketal instead of GC. Those materials did not exhibit branching or gelation and were totally soluble in the tested solvents. Additionally, the carbonate ring or acetal protective groups on oligomers can be selectively removed under basic or acidic conditions. Seeking to obtain amphiphilic polymers, Robin and co-workers [108] have prepared thiol monomers 71 by the esterification of oleic and lauric acids with 2-mercaptoethanol. These intermediates were employed in AIBN-induced free-radical polymerization with glycerin carbonate acrylate (GCA) 1k in acetonitrile solution at 80 °C. In this telomerization process, two oleate polymers 72 were obtained with 65% (for n = 27) and 73% (for n = 65) yield, together with one laurate polymer with 62% yield (for n = 24), as shown in Scheme 42. The fatty acid fragment is responsible for the hydrophobic character of the polymer; in order to prepare amphiphilic derivatives 73 from telomers 72, the base-catalyzed hydrolysis of the cyclic carbonate unit was used to unmask hydroxyl groups. In aqueous media, the hydrolyzed polymers 73 were able to form aggregates with different numbers of micellar units, with critical micellization concentrations in the range of 10 to 60 mg·L −1 . These values increase with the length of the hydrophilic moiety and decrease with the growth of the hydrophobic moiety length and the presence of double bonds, which leads to control of the diameter of the micelles. Patti and co-workers [110] promoted the reaction of the double bond of polystyrene polymers bearing pendant cyclic carbonate groups BMD 80a and BBD 80b via radical photopolymerization to develop new alternative materials for application in nanocomposites of sodium smectite, such as the one used in saline leachates, keeping the cyclic carbonate unit intact in the polymerization step. The synthesis of BMD 80a was performed via the reaction between GC 1a and 4-vinylbenzyl bromide 81a in the presence of sodium hydride in THF (Scheme 44). The analog BBD 80b was similarly synthesized by reacting 4-(4-hydroxybutyl)-1,3-dioxolan-2-one with 81a. The monomers 80a and 80b were photopolymerized over 24 h under nitrogen atmosphere using AIBN and a 12-cm-high UV lamp. The corresponding polymers were obtained with 93% (PBMD 82a) and 74% yields (PBBD 82b). On treatment of 82a with a saline solution (3 M NaCl), a nanocomposite with disordered intercalation was formed. Alternatively, 82a and 82b were intercalated in the presence of the same saline solution, providing partially disordered intercalates nanocomposites. Plasseraud and co-workers [111] prepared monoglycerides 83 by reacting CG 1a or glycidol 2a with carboxylic acids 61. These intermediates 83 were converted into the corresponding monoglyceride bis-acrylates 84, which underwent polymerization under UV irradiation in the presence of 2-hydroxy-2-methyl-1-phenyl-propan-1-one (Darocur 1173 ® ) at room temperature (Scheme 45). When GC 1a was employed (step 1), the desired product was isolated with 14% yield only (even after 24 h of reaction) due to side-reactions. In contrast, when glycidol 2a was used in excess (carboxylic acid 61:2a = 1:2), the intermediate 83 was obtained in yields above 80% after only 50 min.

Ring-Opening Polymerization
The class of five-and six-membered cyclic alkylene carbonates is large and varied, offering a wide range of monomeric systems for ring-opening polymerization (ROP), leading to new polymers [112][113][114]. Gnaneshwar and co-workers [115] reported the synthesis of a α-(cyclic carbonate), ω-Hydroxyl/Itaconic acid asymmetric telechelic poly(ε-caprolactone), by ROP followed by modification of the cyclic carbonates with 2-phenylethylamine (Scheme 46). Firstly, compound 86 was prepared with 96% yield by reacting GC 1a as a functional bio-based initiator with ε-caprolactone 27 in the presence of tin 2-ethylhexanoate (110 °C, argon flow). The authors reported that the ROP technique allows control of the molecular weights, as well as a linear correlation of the molecular weight vs. monomer conversion. The reactivity of compound 86 was demonstrated through the modification with 2-phenylethylamine to give hydroxylurethane 87 using a non-isocyanate route. In 2016, Möller and co-workers developed an anionic ring-opening copolymerization technique for cyclic carbonate 1c and tert-butyl glycidyl ether 2c, followed by the removal of the protective group (Scheme 47) [116]. The hydroxy-functionalized PEG derivatives were submitted to sequential transcarbonylation, transalkylation, and fragmentation, aiming to replace carbonate groups with ether linkages. Poly(ethylene oxide-co-tert-butyl glycidyl ether)hydroxyl telechelic copolymer 88a was synthetized via the reaction between 1c and 2c under mono-mode microwave irradiation, using CsOH as an initiator. After 15 h at 180 °C, 1c was totally consumed, and the product 88a, with molecular weight values of 750-1700 g.mol −1 , was obtained. Further, poly(ethylene oxide-co-glycidyl ether) 88b was obtained after the removal of the tert-butyl protective group, generating hydrophilic reactive hydroxyl sites in the copolymer. In 2017, Marcos-Fernández and co-workers [117] described the degradation of poly(ethylene terephthalate) by ethylene carbonate (EC) in the presence of potassium hydroxide, a low-cost catalyst. Degradation of PET at 130 °C produced short-chain oligomers with approximately 33% of carbonate units inserted in their structure, resulting from the ring-opening of EC with partial decarboxylation. Furthermore, the reaction of bis(2-hydroxyethyl) terephthalate (BHET) with EC in the absence of KOH produced difunctional oligomers with hydroxyl terminal groups, bearing aromatic rings esterified with polyether short chains. Generally, such oligomers could be applied as macroglycols for the preparation of thermoplastic polyurethanes.

Raw Material Derivatives
A raw material is a basic material that is used for the generation of a new material with greater application potential. Currently, glycerol represents one of the most versatile and available raw materials due to the large production of biodiesel, from which large tonnages of glycerol co-product are obtained [118]. Another highly abundant raw material is xylan, a major biopolymer associated with plant hemicelluloses [119]. Taking into account their abundance, it is interesting to explore ecologically sound applications for these raw materials.
In 2016, two methodologies describing the functionalization of xylan through reaction with cyclic carbonate 1e were reported. Saake and co-workers [120] first described the functionalization of xylan 92 to deliver modified polysaccharides 93 (Scheme 49A) through a DBU-catalyzed reaction in DMSO at 140 °C. Two different xylans were evaluated, with a special focus on the effects of the reaction time, temperature, and amount of xylan in the reaction mixture. Samples were analyzed mainly using NMR spectroscopy regarding their degree of substitution (DS) and molar substitution (MS). The products were further characterized by FTIR spectroscopy, mass spectrometry, and SEC, and also regarding their water solubility.
A few months later, the same group [121] reported that differences in temperature and time were determining factors in the synthesis of xylan derivatives. Considering the decarboxylation process that occurs in the reaction medium, the authors observed a significant difference between reactions performed at 140 °C and at 160 °C. When the reaction proceeds at 140 °C, derivatives with low substitution levels are preferably obtained, with longer reaction times being required to obtain highly substituted xylanes 92. Moreover, as DMSO undergoes degradation at high temperatures, a reaction proceeding at 140 °C is preferred. The remaining hydroxyl group on the hydroxyvinylethyl appendage of xylan 93 then reacts with another vinyl carbonate molecule, leading to product 94 (Scheme 49B).

Scheme 49. Synthesis of xylan derivatives by route A or B.
In 2016, Oliveira and co-workers [122] used two renewable materials for the synthesis of the glyceryl glycoside 95. The Ferrier allylic rearrangement involving GC 1a and tri-O-acetyl-D-glucal 96 provided a mixture of diastereomers 95 with 31% yield, together with degradation products (Scheme 50). Considering their possible effects on the reaction, both the reaction time and the quantity of Lewis acid were reduced, resulting in a modest increase in the yield to 40%. Finally, an optimized set of mild conditions was established-by stirring a mixture of glucal, GC 1a (1.2 equivalent), BF3.Et2O (0.1 equivalent). and 4Å molecular sieves in CH2Cl2 at −40 to −20 °C for 10 min provided glycoside 95 with 84% yield (diastereoisomeric mixture) after flash column chromatography. This protocol was used to prepare the starting material for a CuAAC reaction to induce the formation of glucoglycerol-1,2,3-triazole derivatives. Scheme 50. Synthesis of the glyceryl glycoside 95.

Heterocyclic Compounds
Heterocyclic systems represent the most general structural unit of many natural compounds; they have valuable biologically properties and a wide range of applications in the pharmaceutical industry [123,124]. Oxa-and aza-heterocycles atoms are the most representative, which have been used to access new compounds in chemical industries [125]. In the following sections, we mention studies from recent years that have shown efficient synthetic methodologies for the functionalization of heterocycles via the insertion of different substituents in the chemical structure.
The oxazolidin-2-ones 62 were prepared through the reaction between cyclic carbonates 1 and aryl amines 63 in the presence of 1-butyl-3-methylimidazolium acetate ([bmim]OAc) as an eco-efficient catalyst (Scheme 51) [126]. NMR spectroscopy and DFT calculations demonstrated that both the anion and the cation of the ionic liquid catalyst cooperatively activate the starting materials via formation of hydrogen bonds. Thus, several compounds 62 were obtained with good to excellent yields after 9 h at 130 °C. Ethylene-, propylene-, and 4-phenoxymethylethylene carbonates 1 were reacted with a range of arylamines 63. The carbonylation reaction using different cyclic carbonates showed that steric hindrance negatively influenced the reactivity in the following order: ethylene carbonate > propylene carbonate > 4-phenoxymethylethylene carbonate. In the same work, a [bmim]OAc-catalyzed synthesis approach for aryl bis-oxazolidin-2-ones 97 from cyclic carbonates 1 and aryl-or heteroaryldiamines 98 was devised. Ethylene-and propylene carbonate smoothly reacted with aromatic diamines to form the corresponding bis-oxazolidin-2-ones 97 in good yields (Scheme 52). In addition, the imidazolium-based ionic liquids also exhibited high catalytic activity to prepare 3-aryl-[1,3]-oxazinan-2-ones from trimethylene carbonate and [1,3]-dithiolan-2-ylidene-arylamines from ethylene trithiocarbonate [126]. Due to the versatility of glycerol carbonates, several investigations have been conducted, aiming to develop protocols for the introduction of the GC unit in heterocyclic systems. In 2012, Sackus and co-workers [127] reported the functionalization of 1-phenyl-1H-pyrazol-3-ol 99 with TGC 1h. The synthesis of phenylpyrazole derivatives has attracted the attention of synthetic organic chemists because of their broad spectrum of biological activities. In this protocol, 1h reacts with compound 99 in DMF at room temperature in the presence of K2CO3 to provide chemoselectively for 4-{[(1-phenyl-1H-pyrazol-3-yl)oxy]methyl}-1,3-dioxolan-2-one 100 with 71% yield (Scheme 53).
The same group [14] reported the use of 1h as a bis-electrophile for the N-glyceryl functionalization of diverse aza-aromatic systems 101-106 (Scheme 54). The ambident benzimidazole 101 was converted into the carbonate 107 with 76% yield after 5 h, whereas under the same conditions, benzotriazole 102 provided a mixture of 1-and 2-substituted regioisomers (85% global yield). For less acidic aza-heterocyclic systems, a decrease in the yield was observed, e.g., 1H-indole 108 led to the N-alkylated compound 109 with only 34% yield. Comparatively, methyl 1H-indole-3-carboxylate 104, with the N-H bond under the influence of the electron-withdrawing group, was more reactive than unsubstituted indoles, providing the expected product with 48% yield. Carbazole 105 reacted in a solvent-dependent way to provides mixtures with 60% yield for N-alkylated product 110 and the isomeric glycidyl carbamate. When reacting 1,2,3,4-tetrahydrocyclopenta[b]indole as the substrate in DMF, the corresponding glycidyl carbamate was obtained exclusively with 51% yield. Various chemistshave evaluated the performance of benzimidazole-derived carbonates as building blocks for useful transformations. The synthetic potential of the prepared compounds was evaluated in reductive or hydrolytic conditions, involving selective carbonate ring-cleavage using diverse nucleophiles ( Figure 6) [14]. A new series of N-phenyl-4,5-dibromopyrrolamides and N-phenylindolamides was synthetized and a selected set of compounds was evaluated against the enzyme DNA gyrase from Escherichia coli and Staphylococcus aureus [128]. Zidar and co-workers used ethylene carbonate to prepare N-aryloxazolidin-2-ones according to Scheme 55. Initially, 3-(4-nitrophenyl)oxazolidin-2-one 62a was obtained with 81% yield by reacting 4-nitroaniline 63c with ethylene carbonate 1c at 100 °C in the presence of DBU. Subsequent reduction of the nitro group of 62a provided the corresponding arylamine, which was further coupled with 4,5-dibromo-pyrrole-2-carboxylic acid or indole-2-carboxylic acid to give the target arylcarboxamides 115 and 116, respectively. However, these compounds showed weaker activities against both E. coli and S. aureus In 2016, Kim and co-workers [129] described a rhodium(III)-catalyzed, heteroatom-directed C-H allylation process with allylic phosphonates and allylic carbonates. Specifically, the mild and site-selective C-H allylation of 2-arylbenzo[d]thiazole 117 with vinyl carbonate 1e provided the allylic alcohol 118 (90% yield) with an E/Z ratio of 7.5:1 (Scheme 56). This protocol was suitable for a wide range of substrates and may be applicable for accessing new bioactive heterocyclic compounds. A one-pot synthesis method for new N-3-substituted 2-thiohydantoins 119 was reported starting from GC (Scheme 57) [130]. In this study, a tandem Staudinger reaction was devised to convert primary azides into isothiocyanates, which were further reacted with α-amino esters. Namely, glycerol carbonate azide 1n led to isothiocyanate 1o, which was reacted without isolation with α-amino ester 120 to provide the N-3-glycerylated compound 119 with 68% overall yield. In 2017, Zhao and co-workers [131] described the construction of nine-membered heterocycles via a [5+4] cycloaddition reaction between N-tosyl azadienes 31 and substituted vinylethylene carbonates 6 under Pd catalysis, involving highly diastereoselective functionalization (Scheme 58). The benzofuran-fused heterocycles 30 were isolated with 85-92% yields after 15 h of reaction. This strategy was applied with high efficiency to various carbonates and azadienes bearing para-, meta-, or orthosubstituents at the aryl ring with both electron-withdrawing and electron-donating characters. When the unsubstituted vinyl carbonate 1e was reacted with various N-tosyl azadienes 31, the kinetically preferred five-membered ring structure was formed through [3+2] cyclization, and the spirocyclic products 121 were isolated in good yields and with high diastereoselectivity. The same group [132] reported the synthesis of [5,5]-and [6,5]-spiro-heterocycles 123 and 124 through the reaction of aurones 122 with vinylethylene carbonates 6 under Lewis-acid-assisted Pd catalysis (Scheme 59). Using a simple approach and using commercially available catalysts, the scope of the [3+2] cycloaddition was extended to several enones 122 and carbonates 6 bearing electron-donating or electron-withdrawing substituents at either para-or ortho-positions on the aryl group, as well as to naphthyland furyl derivatives. Under the optimized conditions, substituted [5,5] products 123 were obtained in high yields, except when alkyl vinylethylene carbonates 6 were used. Additionally, the protocol was applied in [4+2] cycloadditions, targeting the formation of [6,5] spirocyclic structures 124. A range of substrates bearing different functionalities was examined, as well as other thia-or aza-heterocyclic-based enones. Generally, the expected products were obtained in very good yields and uniformly excellent diastereoselectivity (>20:1) was observed for all prepared compounds. Guo and co-workers developed an efficient Pd-catalyzed formal [5+3] cycloaddition process to prepare N,O-containing eight-membered heterocycles 126 (Scheme 60) [133]. In this work, zwitterionic allylpalladium intermediates were generated in situ from vinylethylene carbonates 6, which acted as 1,3-dipolarophiles in the cycloaddition. Under this Pd-catalyzed protocol, the reaction between N-iminoquinazolinium ylides 125 and carbonates 6 provided tricyclic compounds 126 in high yields with excellent regioselectivities. The optimal conditions were applied to alkyl-or aryl-substituted vinylethylene carbonates and different azomethine imines, including N-quinazolinium and N-isoquinolinium ylides, providing the expected [5+3] cycloadducts. The reaction mechanism involves the formation of the zwitterionic allylpalladium intermediate Ackermann and co-workers developed an air-and water-tolerant manganese(I) catalyst to perform the C-H allylation of heteroarenes 127 and arenes 128 with vinyl carbonates 14 through "in-water" decarboxylative C-H/C-O cleavages (Scheme 61) [134]. In a typical procedure, aromatic compounds were reacted at 100 °C for 16 h with an excess of carbonate 14 in the presence of MnBr(CO)5 (10 mol%) and NaOAc (20 mol%), using trifluoroethanol (TFE) or water as the solvent. C-H Allylations of indoles 127, for instance, proceeded with good yields and high levels of chemo-, site-, and regioselectivity. In 2017, Wang and co-workers [135] explored the domino C−H/N−H allylation of arylimidates under cobalt(III) catalysis (Scheme 62). In this work, the combined C−H and N−H allylation was reported for the one-step synthesis of isoquinolines. The reaction between different vinyl carbonates 14 and substituted arylimidates 131 delivered the desired decorated vinyl isoquinolines 132 in good yields. The protocol was extended to arylimidates 131 bearing electron-donating or electron-withdrawing groups, such as fluorine, chlorine, ester, ketone, and amide groups. The method was also suitable for carbonates 14 bearing aryl groups. In the same year, Ji and co-workers described the synthesis of a new heterogeneous catalyst, the ionic liquid 1-butyl-3-methylimidazolium acetate ([bmim][OAc]) immobilized on MIL-101-NH2, denoted as IL(OAc − )-MIL-101-NH2. This catalyst was used in the reaction between phenylamine 63 and propylene carbonate 1b to generate the oxazolidinones of interest 133 (Scheme 64) [137]. The method was extended to several anilines, allowing the synthesis of ten new oxazolidinones 133 with yields that varied from 68 to 94% after 9 h of reaction.

Bioactive Urethanes and Derivatives
Carbamates or urethanes have attracted much attention due to their biological relevance as mimic fragments of pharmaceutical compounds, such as efavirenz [138] and retigabine (Figure 7) [139]. In other respects, urethanes are monomers that are largely used in the synthesis of polyurethanes, one of the most used raw materials in global industry [10]. Polyurethanes are also versatile thermoplastic elastomers employed in the preparation of biocompatible components, including artificial hearts, feeding tubes, catheters, surgical drains, and others [140]. In 2012, Endo and co-workers [141] reported the synthesis of a branched cationic polyurethane 134 via the polycondensation of 4-chloromethyl-1,3-dioxolan-2-one 1o with diethylenetriamine (DETA) 63d in ionic liquid as the solvent (Scheme 65). The reaction of 1o and DETA 63d was achieved using a 2:1 molar ratio for the monomers. When carried out in molten imidazolium salts, the polymerization yields were significantly higher than those obtained using conventional solvents. The authors reported that due to its ammonium group, the polymer can serve as a catalyst and a capsule for DNA, whereby it can be used as a gene delivery carrier. The complexation with DNA was confirmed by dynamic light scattering (DLS) experiments. By reacting primary amines 63e, alkyl halides, and diverse carbonates 1 bearing cationic, hydrophobic, or amphiphilic groups, Moller and co-workers [142] prepared poly(ethyleneimine)s (PEI) and amphiphilic monodisperse compounds (Scheme 66). These compounds were assayed for their ability to inhibit the proliferation of microorganisms (E. coli, S. aureus, and B. subtilis) and for their hemolytic activity, which was influenced by the microstructure and length of the PEI alkyl chain. The results led to the conclusion that the microstructure of the amphiphilic monodisperse compounds has a significant influence on their antibacterial properties and the hemolytic activity.
Compounds with alkyl chains directly linked to cationic groups have high antibacterial efficiency for long (C-14 to C-18) alkyl chains. Scheme 66. Preparation of poly(ethyleneimine)s (PEI) and amphiphilic monodisperse compounds.

Other Compounds with Biological Activity
Several reactions have been explored for the synthesis of glycerol carbonate derivatives with interesting biological activities. In this sense, Sebastiano and co-workers [143] investigated the condensation of ferrocenemethanol 139 with glycerol derivatives and related amines in the absence of solvent and catalyst. The reactions were performed under neat homogenous conditions and eleven novel compounds were described. Among the tested alcohols, GC 1a was reacted with 139 in the presence of catalytic CO2 or without any catalyst, leading to the corresponding ferrocenylmethyl ether 140 with 92% and 97% yields, respectively (Scheme 67). Ferrocenylmethyl ether 140 three other ferrocenyl analogues were tested in vitro as fungicides and were shown to inhibit fungi growth against Botrytis cinerea and Penicillium spp. Recently, Zhang and co-workers [144] described the copper-catalyzed enantioselective synthesis of highly substituted β-amino alcohols 141 starting from ethynylated carbonates 6 and arylamines 63. The reaction was performed at −20 °C in toluene and in the presence of triethylamine and (S,S)-L as a copper ligand (Scheme 68). Using this procedure, fourteen novel β-amino-β-ethynyl alcohols 141 were obtained with 65-99% yields. When reacting ethynylated carbonates bearing naphthyl or heterocyclic groups under the optimal conditions, the efficacy of the method was maintained, giving the desired products with 93% and 98% yields, respectively. In contrast, the only aliphatic amine tested, t butylamine, yielded the respective β-amino alcohol with only 75% yield. The reaction enantioselectivity (determined by HPLC) presented er ratios varying from 77:23 to 94:6. In order to illustrate the potential usefulness of the β-amino-β-ethynyl alcohols 141, the authors tested several reactions, such as hydrogenation or Sonogashira coupling. They also performed a click reaction between 141a and antiretroviral Zidovudine 142 to efficiently prepare a triazolyl derivative of 142 (85% yield; 92:8 er) (Scheme 69). Zidovudine, a FDA-approved drug used for preventing and treating AIDS, is included in the World Health Organization's List of Essential Medicines [145].

Urethanes and Polyurethanes
As already mentioned in Section 2.3, polyurethanes (PUs) are a valuable group of materials that have applications in a range of industrial segments, in competition with other plastics, ceramics, metals, and rubbers, both in domestic and industrial fields [146]. Due to their multiple applications and high global consumption, the production of polyurethanes (24 Mtons by 2020) represents around 6% of all the plastics produced every year. Many different kinds of polyurethanes, such as flexible and rigid foams, elastomers, adhesives, and coatings, are broadly manufactured and applied, namely as rollers, shoe soles, seals, and engineering components for electrical encapsulation and in the mining industry [147,148]. They also have important applicability as components of medical implants in surgery [146].
The traditional route for the synthesis of PUs involves the reaction of isocyanates with alcohols (Scheme 70) [147]. Despite the versatility of isocyanates as building blocks, these compounds are carcinogenic, mutagenic, and reprotoxic and are classified as "very harmful" chemicals by the European Union. Additionally, the synthesis of isocyanates themselves involves a reaction between the lethal phosgene gas and an amine salt [148]. For these reasons, the development of new routes to access PUs involving less toxic reagents became synonymous with the synthesis of non-isocyanate polyurethanes. The main alternative methods developed so far are (i) the cyclic carbonate pathway (polyaddition), (ii) transurethanization (polycondensation), and (iii) ring-opening polymerization and rearrangements [146,147,149,150]. In particular, the synthesis of PUs via the polyaddition of amines on cyclic carbonates has been explored in recent years as an environmentally friendly route (Scheme 70). The studies published between 2012 and 2020 on the isocyanate-free construction of the urethane nucleus (the carbamate function) can be grouped according to the structure of the cyclic carbonate reacting with the amine: glycerol carbonate 1a, polycyclic carbonate (poly-1), ethylene carbonate 1c, and ethylene carbonate derivatives (deriv-1) (Scheme 71).

Flame Retardants
Reducing the occurrence and severity of fire is a major issue, which has fostered the development of new products able to deter or stop flame propagation. Flame retardants are chemicals added into a combustible material in order to increase the ignition temperature, reduce the rate of burning, limit the spread of flames, and minimize, if not eliminate, the generation of smoke. The most important active species in flame-retardant families are halogens (chlorine and bromine), phosphorus, and water [189,190].
In 2014, Morgan and co-workers [191] evaluated the heat release of polyurethanes modified with boron-and phosphorus-containing flame retardants using pyrolysis combustion flow calorimetry (PCFC). The observed results demonstrated that both the flame retardant's structure and its environment in the polymer greatly affect the flammability behavior. Among the tested potential flame retardants, the phosphorylated glycerol carbonate 145 (Scheme 72) did not inhibit melt flow in the polyurethane during decomposition, thus reflecting the low flame retardant potential of 145 when incorporated in a PU. In a subsequent study, the same group [192] reported the synthesis and flammability testing of a new class of phosphorus-containing flame-retardant molecules incorporated into a PU. A new mixed phosphate ester 148 was prepared in two steps, by first converting GC 1a into its dichlorophosphate 147, which was further reacted with glycidol 2a (2 equivalent) to give 148, containing two epoxide units and a glycerol carbonate moiety (Scheme 73). The fire-retardant performance of 148 as an additive was evaluated using PCFC, showing middling levels of flame-retardant potency. In 2016, Sonnenschein and co-workers reported the alkoxylation of bisphenols S and A by propylene carbonate to prepare new hydroxylated polymerizable building blocks 149-159 (Scheme 74) [193]. The thermo-oxidative evaluation of these useful low molecular weight molecules showed significantly better thermo-oxidative stability for sulfone polyols compared to other bisphenol-A derivatives, as measured by gas evolution and pyrolysis GC-mass spectrometry techniques. Moreover, bisphenol-S polyols possess higher viscosities than their non-sulfone counterparts of similar molecular weight-a property which is presumably linked to the increased interchain hydrogen bonding potential provided by the sulfone moiety.

Energy and Electronics
For further development of lithium-ion batteries, it is essential to prepare less flammable and easily processable materials. In this sense, Rieger and co-workers [194] studied the structure and ionic conductivity of liquid crystals containing propylene carbonate units (Schemes 75 and 76). The interest in this class of compounds is due to their thermal stability, high dielectric constant, and good lithium ion conductivity at room temperature [195]. The authors prepared type 160 liquid crystalline molecules bearing a cyclic carbonate endpoint (Scheme 75). The ionic conductivity of the tetrafluorinated compound 160a was compared with that of the non-fluorinated analogue 161a (Scheme 76), while 160b was compared to the tetra(ethylene oxide) (TEO) analogue 162a (Scheme 75), which bears the same fluorinated mesogenic core. The fluorinated precursor 163 was obtained in 2 steps from 2,3,5,6-tetrafluorophenol, while the non-fluorinated precursor 164a was prepared from 4-hydroxybenzoic acid. Liquid crystals 160a and 161a were obtained with satisfactory yields via the esterification with GC 1a under mild reaction conditions. The TEO analogue 162 was synthesized with 65% yield via a sequential BH3-THF reduction of 163c, followed by the Cs2CO3-catalyzed etherification of the benzylic alcohol 166 using the TEO tosylate 167a. Scheme 75. Synthesis of liquid crystalline molecules bearing a cyclic carbonate endpoint 160.
After mixing the synthesized compounds with lithium bis(trifluoro-methanesulfonyl)imide (LiTFSI), the authors observed that Li + self-assembly was driven by the interaction with the carbonate units via coordination with the free electron pairs of oxygen. Furthermore, the cyclic carbonate moiety present in 160 and 161 enhanced the lithium salt dissociation as compared to the TEO derivative 162. Measurements by small-and wide-angle X-ray scattering (SAXS/WAXS) demonstrated that the compounds align spontaneously in the smectic phase when cooled from the isotropic melt. The formation of a smectic layer improves the ionic conductivity by enhancing the ion mobility parallel to the layer plane. Another interesting study involving GC 1a was reported by Kim and co-workers in 2016 [196]. In this work, 1a was reacted with furfuryl amine 63g to provide the urethanes 168a-b (Scheme 77). After a few more reaction steps, a surface of polydimethylsiloxane-based polyurethane cross-linked by Diels-Alder adducts 169 was prepared. In the sequence, silver nanowire (AgNW) was deposited on this surface via vapor-assisted solution process, resulting in a highly flexible, transparent, and healable electrode. In 2020, Cui and co-workers [197] synthesized several fluorescent macrocycles 171 through the iridium-catalyzed intermolecular decarboxylative coupling of vinylated carbonate 1e with readily available isatoic anhydrides 170 (Scheme 78). The best reaction conditions were obtained with [Ir(cod)(OMe)]2 (2.0 mol%) as the catalyst, 2,2'-bipyridine as the chelating N,N-donor ligand, and a mixture DCM/acetone (3:1) as the solvent. The sequential allyl-amination-macrolactonization upon extrusion of CO2 provided the expected benzo-fused, 14-membered aza-macrolides 171 with moderate yields after 12 h of reaction. The scope was evaluated using isatoic anhydrides 170 bearing electron-donating (Me and CH3) and electron-withdrawing groups (halides and CF3). Azamacrolides 171 showed favorable fluorescence emission in the 397-430 nm range. An online Raman analysis technique for monitoring of the production of biofuels was developed to study the transesterification reaction of canola oil with dimethyl carbonate [198]. The reaction was monitored by measuring the height of the C=C stretching mode of the unsaturated fatty acids present in the oil. The transesterification of the triglycerides was catalyzed using a triazabicyclodecene-modified, Mg-Al layered double hydroxide (TBD-LDH). The methyl fatty esters 172 were obtained together with esterified glycerol carbonates, which further reacted with dimethyl carbonate to form additional 172 and the bis-carbonate 1p (Scheme 80).

Non-Ionic Surfactants
Surface-active agents (surfactants) are a class of compounds that present outstanding physical properties as solubilizers, with a wide range of application in the most varied industry sectors, for example being used in the soap industry, in cosmetics, and in the food area [199]. Among the range of surfactants, non-ionic structures are used in diverse areas, such as in cleaning products, textile processing, pulp and paper processing, emulsion polymerization, and agrochemicals. An important characteristic of these materials is the absence of a hydrophilic group with filler bound to their hydrophobic chain [200,201].
In this regard, Corma, Iborra, and co-workers [202] prepared glycerol-carbonate-derived fatty esters 1q using a hybrid Nafion-silica composite (Nafion SAC-13) as an acid catalyst in the absence of solvent [203]. In this work, the desired compounds were prepared through direct esterification of GC 1a with carboxylic acids of different chain lengths; the protocol led to the formation of several side-products, such as monoglycerides 12 and 173, as well as the triglyceride 174. Two different possible reaction routes were proposed by the authors: Several parameters were evaluated in this reaction, such as the solvent, heterogeneous catalysts, zeolites, and hybrid organic-inorganic acids. The best results for the esterification of GC 1a were obtained by using 3.5 wt% of Nafion-silica (with respect to the total weight of the reactants) at 100 °C. The highest activity was exhibited by the Nafion SAC-13 nanocomposite, which proved to be an excellent esterification catalyst, while other ion exchange resins showed lower activity and selectivity. It has to be pointed out that when increasing the chain length in 61 from hexanoic to lauric and palmitic acids, the conversion rate decreased considerably from 98% to 71% and 31%, respectively.

Miscellaneous
In addition to the plethora of chemical transformations presented in the previous sections in which glycerol carbonate is involved, the versatility of this glycerol derivative has allowed it to be employed in singular polymerization reactions, leading to diversified products with significant potential for application in various materials. In this line, the synthesis of new fluorinated terpolymers bearing cyclocarbonate pendant groups to ensure the crosslinking of the polymer chains was reported [205]. The new cyclocarbonate-and triazole-functionalized fluorinated terpolymer 176 was obtained in three steps from chlorotrifluoroethylene 76, as shown in Scheme 83. The glycerol carbonate vinyl ether monomer 1l was obtained with 45% yield via a transetherification reaction between GC 1a and ethoxyethylene 177 under palladium catalysis. The key co-polymerization step involving 1l, 76, and 2-chloroethyl vinyl ether 178 was catalyzed by n butylperoxypivalate and proceeded in a HFP solution at 74 °C, leading to the terpolymer 179. After two more steps, the triazolyl-modified terpolymer 176 was obtained as a membrane, which was removable from the glassy substrate via immersion in 1M aqueous hydrochloric acid. After the final conditioning process, it was verified that the developed semi-interpenetrating polymer network (semi-IPN) blended membranes presented lower water uptake and better mechanical properties compared to non-crosslinked membranes. In the same year, Sacripante and co-workers [206] described the preparation of polyester resins derived from rosin acids for application as a xerographic toner. Rosin acids obtained from pulp byproduct (tall oil), gum, or wood rosins are more sustainable monomers than those from food biomass. Reacting renewable dehydroabietic acid 180 with GC 1a (3.3 equivalent) in the presence of catalytic tetraethylammonium iodide at 150 °C provided a mixture of dehydroabietates 11e, 181, and 182. After adding more 180, the reaction was continued for 8 h at the same temperature to deliver a mixture of the monoesters 12e and 181 (61.9% yield) and 182 (12.5%), together with the isomeric diesters 183 (12.4%) and 184 (5.3%). This mixture of 182, 183, 184, and 12e rosin adducts was reacted with various organic diacids to provide novel polyester resins with a glass transition of 35-53.9 °C, which is the suitable temperature range for toner applications (Scheme 84). A promising approach in green and sustainable chemistry is the direct use of CO2 by combining the coupling reaction and free-radical polymerization to efficiently produce polyacrylates bearing cyclic carbonate groups. In 2016, Zhang, Du. and co-workers [207] described the use of a new nano-lamellar double-metal cyanide complex, Zn-Co(III) DMCC/CTAB, a catalyst that allows the preparation of highly transparent polyacrylates with specific UV absorption. The reaction of CO2 with glycidyl methacrylate 185 followed by free radical polymerization (FRP) of the resulting alkenyl carbonate ester DOMA 1s provided new polyacrylates, known as PDOMAs (Scheme 85). AIBN (1.5 mol% related to 1s) was used as the radical initiator in the FRP, which was conducted in DMF at 80 °C. The resulting polymer, poly(2-oxo-1,3-dioxolane-4-yl)methyl methacrylate (PDOMA) 186, which is soluble in strong polar solvents, has tunable number-average molecular weights (12.0-132.0 kg·mol −1 ), high glass transition temperatures (121.0-140.4 °C), and a high thermal decomposition temperature (5 wt%, 257 °C). PDOMAs 186 presented excellent visible light transmittance (93%) and nearly 100% UV absorbance in the wavelength range of 200-313 nm. In 2016, Carbonnier and co-workers [208] designed a synthetic route involving GC methacrylate 1s and glycol bis-methacrylate 189 for an FRP preparation of monolithic columns. Further surface modification with allylamine and thiols provided functionalized polymeric monoliths through a cyclic carbonate opening and thiol-ene click addition (Scheme 86). Subsequent possible immobilization of metallic nanoparticles allowed these materials to be successfully used as supports for chromatographic applications or as microreactors for flow catalysis.

Conclusions
The versatility of five-membered cyclic carbonates in organic synthesis and materials science has been recognized for years, as demonstrated in a number of papers. In this review, the studies reported from 2012 to the present were covered as an update of previously published contributions [5,[15][16][17][18][19][20][21].
The cyclic carbonate nucleus has attracted considerable attention among researchers in different areas since it can be implemented as a substrate to access several classes of compounds with broad functional diversity. Our review covers miscellaneous uses of this structural unit as a starting material for the preparation of biologically active compounds, attractive molecules for the materials industry, and versatile building blocks in organic synthesis. In particular, the preparation of urethanes and polyurethanes should be highlighted as one of the most important applications of five-membered cyclic carbonates, since this represents an isocyanate-free and environmentally friendly route to these valuable polymers.
The possibility of using glycerol and CO2 (as demonstrated more recently) as readily available, cheap, renewable starting materials to prepare cyclic carbonates reinforces the role of such versatile motifs in the chemical industry. It is expected that investigating the reactivity and the physicochemical properties of cyclic carbonates will continue to attract the attention of synthetic and organic materials chemists for years to come. There are still some challenges to be overcome in the comprehensive development about cyclic carbonates, namely increases in the range and selectivity of the reactions involved in polymer synthesis and the greenness of most of the protocols engaged in the functionalization of this building block.
We hope the present review can awaken in the reader an interest in the development of new reactions and applications for the well-established range of cyclic carbonates, especially those derived from glycerol. This might well become a viable way to add value to glycerol issued from regrowing resources, with nobler applications for chemicals, pharmaceuticals, agrochemicals, and materials.