Synthesis and Biological Evaluation of Four New Ricinoleic Acid-Derived 1-O-alkylglycerols

A series of novel substituted 1-O-alkylglycerols (AKGs) containing methoxy (8), gem-difluoro (9), azide (10) and hydroxy (11) group at 12 position in the alkyl chain were synthesized from commercially available ricinoleic acid (12). The structures of these new synthesized AKGs were established by NMR experiments as well as from the HRMS and elementary analysis data. The antimicrobial activities of the studied AKGs 8–11 were evaluated, respectively, and all compounds exhibited antimicrobial activity to different extents alone and also when combined with some commonly used antibiotics (gentamicin, tetracycline, ciprofloxacin and ampicillin). AKG 11 was viewed as a lead compound for this series as it exhibited significantly higher antimicrobial activity than compounds 8–10.


Introduction
Natural 1-O-alkylglycerols (AKGs) 1 are bioactive ether lipids present in body cells and fluids. They are precursors of ether phospholipids, which participate in structures and functions of membranes in certain cells such as white blood cells or macrophages. AKGs are also found in bone marrow lipids and in milk [1]. Marine sources of AKGs such as the liver oil of certain shark species or rat fish (elasmobranch fishes) contain high levels of these compounds as a mixture of few species varying by length and unsaturation or saturation of the alkyl chain.
Experimental studies were performed during the last century, aiming to demonstrate whether AKGs from SLO had biological properties and beneficial effects. Indeed, several studies did observe interesting effects such as hematopoiesis stimulation [3], lowering radiotherapy-induced injuries [4], reducing tumor growth [5] and improving vaccination efficiency [6,7]. However, in most cases, conclusions were mainly impaired by the poor definition of the mixtures used in terms of purity as well as chemical composition. It was then established that the alkyl chain is bound to the glycerol backbone at the sn-1 position, thus leading to an S configuration at the asymmetric carbon [8] (Figure 1). To assess the biological activity of each individual AKG, Legrand et al. [9][10][11] reported the antitumor activity (against lung cancer in mice) of each of the six prominent components 2-7 of the natural mixture. These derivatives were obtained in pure form by total synthesis and it was observed that the biological activity was heavily dependent upon the unsaturation of the alkyl chain. When this chain was saturated, the corresponding 1-O-alkylglycerols 2-5 exhibited little or no activity. However, when it was monounsaturated 6-7, a good antitumor activity was observed, thus indicating that the antitumor activity of the natural SLO mixture was heavily related to its unsaturated components ( Figure 2). Currently, resistance to the existing antibiotics and increasing numbers of diseases result in identifying new drug candidates with new forms of activity. Thus, synthesized natural or non-natural AKGs derivatives of natural fatty acids could be one new source of drug delivery systems of antibiotics. Additionally, defined synthetic routes for these targets will facilitate further investigation of biological activities, as natural AKGs were found present in various human cells, but only in trace quantities. Among known natural fatty acids, ricinoleic acid (RA) is one of the major fatty acids occurring in castor oil (almost 90%) [12]. Such a high concentration of this unusual unsaturated fatty acid may be responsible for castor oil's remarkable healing abilities. It is known to be effective in preventing the growth of numerous species of viruses, bacteria, yeasts and molds [13,14]. Due to the many beneficial effects of this fatty acid component, the use of castor oil can be applied topically to treat a wide variety of health complaints [15], and it also has pharmacological effects on the human gastrointestinal tract [16]. An RA-based glycine derivative was reported to exhibit excellent antimicrobial and anti-biofilm activities against the tested Gram-positive bacterial strains and specifically against various Candida strains [17], and it is used for the preparation of several bioactive molecules [18][19][20].
The presence of the hydroxyl group in RA provides a functional group location for performing a variety of chemical reactions including esterification, halogenation, dehydration, alkoxylation and nucleophilic substitution. In this direction, non-natural AKG 8-11 derivatives from ricinoleic acid have been synthesized in a stereo-controlled manner. Taking into account RA and natural alkylglycerols' beneficial effects, herein, we report the synthesis of new non-natural RA-derived methoxy, gem-difluorino, azide, and hydroxy-substituted 1-O-alkylglycerols 8-11 and their respective antimicrobial activities (Figure 3). The hydroxyl functionality of RA makes the castor oil a natural polyol providing oxidative stability to the oil and a relatively long shelf life, compared to other oils, by preventing peroxide formation. As a result, this unique functionality allows the castor oil to be used in industrial applications such as paints, coatings, inks and lubricants [30]. With that in mind, the AKG 11 was designed as an analog of 8, without a methoxy group at the 12 position in the alkyl chain, to study the influence of the hydroxyl group in the AKG's alkyl chain, and the structure-activity relationship (SAR) by comparing their biological activity, respectively. The preparation of the AKG 11 with the hydroxyl group required a protection-deprotection sequence to yield the penultimate intermediate alcohol 21.
Attempt to protect the hydroxy group of 13 using 2-(bromomethyl)naphthalene in the presence of sodium hydroxide and of tetra-n-butylammonium bromide in DMSO for 18 h at rt failed. Thereafter, a trace of 20 was observed when chlorotriisopropylsilane in the presence of DIPEA in dichloromethane was used to protect 13 as silyl ether [31]. When DIPEA was replaced by imidazole as a base and in DMF, 20 was obtained in 61% yield. Afterward, 20 was reduced to the penultimate intermediate alcohol 21 in 75% yield using Red-Al in diethyl ether (Scheme 2). Compound 21, upon mesylation conditions using mesyl chloride in DCM, and Et 3 N provided 22 in 76% yield, which was then alkylated under anhydrous conditions with 2,3-isopropylidene-sn-glycerol 18 in DMF in the presence of sodium hydride to provide acetonide 23 in 68% yield. Sodium hydride was used as a base instead of KOH to eliminate the by-products formed when reacting 22 with 18. Following this, silyl ether group was removed using TBAF in THF at rt for 20 h to provide 24 in 92% yield. Easy acetonide cleavage on 24 under acidic conditions (0.05 equiv of p-toluenesulfonic acid monohydrate in MeOH/H 2 O (9:1)) gave 11 in 84% yield (Scheme 2). We envisioned that the AKG 9 with a gem-difluorinated group in the alkyl chain could exhibit more biological activity than other AKGs studied, as compounds containing a difluoromethylene group were reported to exhibit excellent biological activities [32]. Moreover, introduction of fluorine atoms in molecules heavily modifies their physical, chemical and physiological properties. These fluorinated compounds have found many applications in pharmaceutical and agrochemical fields [33]. Thus, a gem-difluorinated 26 key intermediate for the synthesis of 9 was obtained by a classic two-step sequence. Oxidation of 13 by PCC in DCM provided the ketone 25 in 68% yield, which was then subjected to fluorination at rt using (diethylamino)sulfur trifluoride (DAST) in DCM, and the fluorinated product 26 was obtained in 54% yield. Following this, 26 was reduced to alcohol 27 in 79% yield using Red-Al in Et 2 O, which upon treatment with mesyl chloride in DCM in the presence of Et 3 N furnished 28 in 68% yield. Alkylation of 28 in DMSO with 2,3-isopropylidene-sn-glycerol (18) in the presence of 50% aqueous sodium hydroxide and tetra-n-butylammonium bromide gave the expected product 29 in 63% yield, along with 6% of a by-product 30 (Scheme 3). Compound 29 was obtained as a pure green oil after column chromatography on florisil gel and showed spectroscopic properties ( 1 H and 13 C NMR as well as correlation spectra) in full agreement with its structure, and confirmation was made by HRMS (ESI, m/z) that showed 441.3149 for [M + Na] + . Sodium hydroxide solution (50%) in H 2 O was used instead of others bases (NaH, KOH) to decrease the formation of the by-product 30. Acetonide cleavage on compound 29 under acidic conditions using catalytic amount of p-toluenesulfonic acid monohydrate in MeOH/H 2 O (9:1) provided 9 in 92% yield (Scheme 3). At last, the AKG 10 was designed as another analog with an azide group at the same position in the alkyl chain to evaluate the beneficial effects of the azide group on the biological activity, and to estimate the dissimilarity between the studied AKGs. Subsequently, 10 was obtained in a classical seven step sequence. Starting under mesylation conditions of methyl ricinoleate 13 using mesyl chloride and Et 3 N in DCM, 31 was obtained in 65% yield, which under substitution of the intermediate mesylate group by S N 2 substitution reaction using sodium azide in DMSO provided 32 in 80% yield. Dibal in Et 2 O was used to reduce the ester functionality of 32 into alcohol 34, but surprisingly an aldehyde 33 was formed in 68% yield instead of the expected alcohol 34. Dibal was chosen as a reducing agent for its non-interaction with the azide group. Moreover, use of another reducing system such as Zn-AlCl 3 was reported to reduce the azide 32 into the amino derivative [17,19]. Thereafter, aldehyde 33 was then reduced into alcohol 34 in 74% yield using NaBH 4 in EtOH, which upon reaction with mesyl chloride in DCM in the presence of Et 3 N provided 35 in 59% yield (Scheme 4). Following this, alkylation of 35 in DMSO with 2,3-isopropylidene-sn-glycerol (18) in the presence of 50% aqueous NaOH and tetra-n-butylammonium bromide provided a~1:1 mixture of two compounds: the expected product 36 along with a closer polar by-product 37, arising from a subsequent elimination of the azide group as indicated in Scheme 4. An attempt to separate the mixture of compounds (36,37) failed when subjected to maleic anhydride in cyclohexane for 72 h at 45 • C with vigorous stirring. Under these conditions, only 37 was supposed to react with maleic anhydride, leading to a polar product other than 36, which could ease separation via column chromatography. Compound 36 was then obtained without a trace of by-product 37 by column chromatography on basic alumina gel, and visualization of these two compounds (36 and 37) was easily followed on TLC plates as they stained differently with an acidic ethanolic solution of p-anisaldehyde. Easy acetonide cleavage on 36 under acidic conditions (p-toluenesulfonic acid monohydrate in MeOH/H 2 O (9:1)) provided 10 in 97% yield (Scheme 4). Supplementary Materials of all synthetic compounds for this sequence is attached in a file as Figures S4.1-S4.14.
The synthesized compounds 8-11 were evaluated for their respective antimicrobial activities.
AKGs 8-11 were also tested in combination with some commonly used antibiotics ( Table 2). The results showed that synergistic effects could be obtained in some cases, especially when the AKG 8 was combined with gentamicin, and compound 11 was combined with ciprofloxacin and ampicillin. More than four-fold increase of the activity of these antibiotics was recorded on the three selected bacteria, suggesting that the study should be emphasized on such combinations. The overall activity could be considered as important, mainly when viewed that most of the organisms used were antibiotic resistant. This study therefore provides supportive data for the potential use of the studied 1-O-alkylglycerols, in particular AKG 11, as well as in combination with some antibiotics for the treatment of microbial infections. However, this is to be confirmed by further toxicological studies.

General Experimental Procedures
Moisture sensitive reactions were performed under nitrogen. Anhydrous tetrahydrofuran (THF) and diethyl ether (Et 2 O) were obtained by percolation through a column of a drying resin. Anhydrous N,N-dimethylformamide (DMF) over molecular sieves was used as commercially supplied (Acros). Room temperature (rt) means a temperature generally in the range of 18-20 • C. Column chromatography was performed over silica gel Kielselgel 60 (40-60 µm) or basic alumina (Brockmann activity II; basic; pH 10 ± 0.5). Routine monitoring of reactions was carried out using Merck silica gel 60 F 254 TLC plates (TLC: thin layer chromatography) purchased from Fluka and visualized by UV light (254 nm) inspection followed by staining with an acidic ethanolic solution of p-anisaldehyde, or with a solution of phosphomolybdic acid (5 g in 100 mL 95% ethanol). Infrared spectra were recorded with a Thermo Nicolet Avatar 250 FTIR and were reported using the frequency of absorption (cm −1 ). 1 H NMR spectra (400.13 MHz) and 13 C NMR spectra (100.61 MHz) were recorded on an Avance 400 Bruker spectrometer using TMS as an internal standard. Multiplicity was tabulated using standard abbreviations: s for singlet, d for doublet, dd for doublet of doublets, t for triplet, q for quadruplet, dtt for doublet of triplets of triplets and m for multiplet (br means broad). Quite obvious first-order 1 H NMR multiplets were analyzed. As a helpful guidance for this analysis, two articles of Hoye et al. appeared in 1994 and 2002 [34,35]. NMR spectra were processed with zero filling (512 k or 1024 k points). Sometimes resolution enhancement in 1 H NMR using Traficante facilitated the assignments. Specific rotations were measured on a Perkin Elmer 341 polarimeter, with a cell of 1 dm long and a Na-or Hg-source (Na at 589 nm; Hg at 578 nm, 546 nm, 436 nm and 365 nm), and concentrations were expressed in g/100 mL. High resolution mass spectra (HRMS) were recorded using a MicrO-Tof-Q II spectrometer under electrospray using methanol as solvent. Microanalyses were performed with a CHNS analyzer.
The compound 2,3-isopropylidene-sn-glycerol (18) (≥95% pure) was purchased from Alfa Aesar (France, article # B23037); and ricinoleic acid (12) (15) In a flask containing a solution of 13 (625 mg, 2.0 mmol), tetra-n-butylammonium bromide (709.2 mg, 2.2 mmol, 1.2 equiv) in DMSO (2.0 mL) with stirring, was added finely crushed (with a mortar and pestle) sodium hydroxide (250 mg, 6 mmol, 3 equiv) and methyl iodide (0.63 mL, 10 mmol, 5 equiv). The reaction flask was flushed under nitrogen, tightly stoppered and protected from light by wrapping with an aluminum foil. After stirring overnight for 18 h, TLC monitoring showed that the reaction was mostly done. An aqueous solution of 10% citric acid (10 mL) was added, and extraction was done with petroleum ether/EtOAc (80:20). Organic layers were dried over Na 2 SO 4 , and solvent was removed under reduced pressure. Then, the residue was purified by column chromatography on basic alumina (5 g, 0%-0.2% acetone in petroleum ether) to afford 15 as a colorless oil (471 mg, 72%).  4 and (CH 2 ) 4 CH 2 CH 2 CO 2 Me), 0.88 (t, 3H, J = 6.9 Hz, CH 3 ). 13 (16) Red-Al (0.52 mL,~3 M in toluene, 1.56 mmol, 1.2 equiv) was added dropwise to a cooled solution of 15 (422 mg, 1.29 mmol) in anhydrous Et 2 O (4 mL) at 0 • C with stirring and under nitrogen. After the addition, the stirring was continued overnight for 18 h at 0 • C (use of a Dewar with ice-cooling). TLC monitoring confirmed disappearance of the starting material. A solution of citric acid (400 mg) in distilled water (5 mL) was added to the reaction mixture, which was allowed to stir again for 30 min. Extraction was done with petroleum ether/EtOAc (80:20). Organic layers were dried over Na 2 SO 4 , and solvent was evaporated under reduced pressure. The crude product was then purified by column chromatography on basic alumina (5 g, 0%-3% acetone in petroleum ether) to afford 16 as a colorless oil (318 mg, 82%  (17) To a stirred solution of 16 (6.64 g, 22.2 mmol), Et 3 N (4.7 mL, 33.4 mmol, 1.5 equiv) in DCM (67 mL) under nitrogen at −30 • C, mesyl chloride (2.2 mL, 28 mmol, 1.25 equiv) in DCM (9 mL) was added dropwise. The addition of mesyl chloride was completed by rinsing with DCM (3 × 0.3 mL). The corresponding mixture was then stirred for 15 h at −30 • C. TLC monitoring (elution with DCM, mesylate showed far bigger R f than starting alcohol with this eluent) showed completion of the reaction, and distilled water (75 mL) was added to quench the reaction. Extraction was done with DCM. Organic layers were washed with brine and dried over Na 2 SO 4 . Solvent was removed under reduced pressure to provide the crude product as a light yellow oil. The crude product was then purified by column chromatography on silica gel (10 g, 0%-2% acetone in petroleum ether) to provide 17 as a colorless oil  5 and (CH 2 ) 5 CH 2 CH 2 OMs), 0.88 (t, 3H, J = 6.9 Hz, CH 3 ). 13      To a solution of acetonide 19 (4.65 g, 11.2 mmol) in methanol (45.1 mL), was added p-toluenesulfonic acid monohydrate (107.3 mg, 0.55 mmol, 0.05 equiv) and distilled water (4.5 mL). The flask was then purged with nitrogen, stoppered and dipped in a preheated bath at 60 • C. Stirring was maintained for 5 h at 60 • C. TLC monitoring showed completion of the reaction, and sodium bicarbonate (52.1 mg, 0.62 mmol, 0.055 equiv) was added. Stirring was continued for 1 h at 60 • C. Methanol was then removed under reduced pressure, and the crude product was purified by column chromatography on silica gel (20 g, 0%-5% acetone in petroleum ether and then petroleum ether + 5% acetone + 12% methanol) to afford 8 as a colorless oil (4.16 g, 99%). R f = 0.03 (petroleum ether/acetone 90:10). 1 (20) To a vigorously stirred solution of 13 (625 mg, 2.0 mmol) and imidazole (334 mg, 4.9 mmol, 2.45 equiv) in DMF (1.6 mL), which was cooled under nitrogen at 0 • C, was added dropwise triisopropylsilyl chloride (0.53 mL, 97% pure, 2.4 mmol, 1.2 equiv). The corresponding mixture was allowed to stir for 48 h at rt. TLC monitoring showed completion of the reaction. Petroleum ether/EtOAc (80:20) was added. After washing with brine and drying over MgSO 4 , solvent was removed under reduced pressure, and the residue was purified by column chromatography on silica gel (5 g, 0%-0.2% acetone in petroleum ether) to provide 20 as a colorless oil (574.7 mg, 61%).  3 , due to the shielding effect on neighboring CH, CH and CH 3 of isopropyl groups being very close, so coupling was not seen and these signals were superimposed, HSQC showed CH at 1.061 ppm and CH 3 at 1.059 ppm), 0.88 (t, 3H, J = 6.9 Hz, CH 3 ). 13  3.2.9. (R,Z)-12-((Triisopropylsilyl)oxy)octadec-9-en-1-ol (21) In a flame-dried two-necked flask, a solution of 20 (574.7 mg, 1.22 mmol) in anhydrous Et 2 O (3.7 mL) was cooled at 0 • C under nitrogen. A solution of Red-Al (65% in toluene,~3 M, 0.5 mL, 1.5 mmol, 1.23 equiv) was added dropwise under stirring. Stirring was continued for 5 h at 0 • C. TLC monitoring confirmed completion of the reaction. Citric acid (400 mg) and distilled water (5 mL) were added to the mixture and stirring was continued for 30 min. Extraction was then done with petroleum ether/EtOAc (80: 20), and organic layers were dried over Na 2 SO 4 . Solvent was evaporated under reduced pressure, and the crude product was purified by column chromatography on silica gel (2.5 g, 0%-0.5% acetone in petroleum ether) to afford 21 as a colorless oil (509.6 mg, 94%).

(R,Z)-12-((Triisopropylsilyl)oxy)octadec-9-en-1-yl methanesulfonate (22)
To a solution of 21 (509.6 mg, 1.15 mmol) and Et 3 N (0.245 mL, 1.75 mmol, 1.5 equiv) in DCM (4.6 mL) with stirring and under nitrogen cooled at −50 • C, was added dropwise mesyl chloride (0.112 mL, 1.45 mmol, 1.25 equiv) in DCM (0.6 mL). Transfer of mesyl chloride was completed by rinsing twice with a few drops of DCM. The reaction mixture was then allowed to warm up slowly in 2 h up to −5 • C. TLC monitoring (elution with DCM) showed completion of the reaction, and distilled water (5.8 mL) was added to quench the reaction. Extraction was done with DCM. Organic layers were washed with brine and dried over Na 2 SO 4 . DCM was removed under reduced pressure, and the crude product was purified by column chromatography on silica gel (4 g, 0%-1% acetone in petroleum ether) to afford 22 as a colorless oil (453 mg, 76%). R f = 0.41 (petroleum ether/acetone 80:20).  (23) A 60% dispersion of sodium hydride in mineral oil (29.5 mg, 0.7 mmol, 2.5 equiv) was washed three times with petroleum ether under nitrogen and with stirring. Anhydrous DMF (0.2 mL) was then added and the mixture was cooled at 0 • C. Following this, a solution of 2,3 isopropylidene-sn-glycerol 18 (50.2 mg, ≥95% pure, 0.36 mmol, 1.25 equiv) in DMF (0.2 mL) was added dropwise to the mixture, and the flask containing 18 was rinsed with DMF (2 × 0.1 mL). The corresponding mixture was allowed to stir for 10 min at 0 • C, and a solution of 22 (153 mg, 0.29 mmol) in DMF (0.2 mL) was added to the resulting white suspension. Transfer of 22 was completed by rinsing with DMF (2 × 0.1 mL). The mixture was then vigorously stirred overnight for 16 h at rt. TLC monitoring showed completion of the reaction, and 10% aqueous ammonium acetate was added as a buffer. Extraction was done with petroleum ether. Organic layers were dried over Na 2 SO 4 . Solvent was removed under reduced pressure, and the crude product was purified by column chromatography on silica gel (1.5 g, 0%-0.5% acetone in petroleum ether) to provide 23 as a colorless oil (110.8 mg, 68%). R f = 0.71 (petroleum ether/acetone 95:5).    (24) To a stirred solution at of 23 (278 mg, 0.5 mmol) in anhydrous THF (1.5 mL) under nitrogen, which was cooled at -20 • C, was added a 1 M solution of TBAF in THF (0.67 mL, 0.67 mmol, 1.3 equiv). The reaction mixture was then left under stirring for 20 h at rt. TLC monitoring showed completion of the reaction, and distilled water (1.5 mL) was added to the mixture. Extraction was done with EtOAc and organic layers were washed with brine and dried over Na 2 SO 4 . Solvent was removed under reduced pressure, and the crude product was purified by column chromatography on silica gel (2.5 g, 0%-4% acetone in petroleum ether) to provide 24 as a colorless oil (184.2 mg, 92%). To a solution of 24 (1.26 g, 3.15 mmol) in methanol (12.6 mL), was added p-toluenesulfonic acid monohydrate (30 mg, 0.16 mmol, 0.05 eq) and distilled water (1.26 mL). The flask was then purged with nitrogen, stoppered and dipped in a preheated bath at 60 • C. Stirring was maintained for 4 h at 60 • C. TLC monitoring showed completion of the reaction, and sodium bicarbonate (14.5 mg, 0.173 mmol, 0.055 equiv) was added and stirring was continued for 1 h at 60 • C. Methanol was removed under reduced pressure, and the crude product was purified by column chromatography on silica gel eluting with 0%-5% acetone in petroleum ether to remove non polar impurities and then with petroleum ether + 5% acetone + 12% methanol to give 11 as a colorless oil (945.6 mg, 84%).    (25) Pyridium chlorochromate (18.06 g, 83.8 mmol, 2.6 equiv) was suspended in DCM (111.7 mL) with stirring for 5 min. Following this, a solution of methyl ricinoleate 13 (10 g, 32 mmol) in DCM (15 mL) was added rapidly to the mixture and the flask containing 13 was rinsed with DCM (3 x 2 mL). Stirring was pursued for 1 h at rt under nitrogen. TLC monitoring showed completion of the reaction, and petroleum ether/EtOAc (90:10) (111.7 mL) was added. The resulting mixture was filtered over a short plug of silica gel with rinsing of silica gel by petroleum ether/EtOAc (90:10). Evaporation of the filtrate under reduced pressure followed by the purification of the crude by column chromatography on silica gel (55 g, 0%-1% acetone in petroleum ether) afforded 25 as a colorless oil (6.73 g, 68%). R f = 0.36 (petroleum ether/acetone 90:10). To a solution of 25 (6 g 19.3 mmol) in DCM (22.5 mL) with stirring and under nitrogen at rt, was added dropwise DAST (6.22 mL, 47 mmol, 2.4 equiv). The corresponding mixture was stirred for 21 days at rt. TLC monitoring showed that the reaction was mostly done and saturated aqueous sodium bicarbonate (32 mL) plus water (20 mL) were added to quench unreacted DAST. After partitioning and extraction of the aqueous layer with DCM, combined organic layers were washed with brine and dried over Na 2 SO 4 . DCM was removed under reduced pressure, and the crude product was purified by column chromatography on silica gel (20 g). First, elution with 0%-0.5% acetone in petroleum ether) afforded 26 as a colorless oil (3.48 g, 54%). R f = 0.39 (petroleum ether/acetone 95:5). Then unreacted 25 (2.00 g, 33%) was eluted with 1% acetone in petroleum ether.  (27) To a stirred solution of 26 (199.5 mg, 0.6 mmol) in anhydrous Et 2 O (2 mL) under nitrogen, which was cooled at 0 • C, was added dropwise a solution of Red-Al (65% in toluene,~3 M, 0.3 mL, 0.9 mmol, 1.5 equiv); then stirring was continued for 5 h at 0 • C. TLC monitoring confirmed completion of the starting material. A solution of citric acid (400 mg) in water (5 mL) was added, and stirring was still continued for 30 min. Extraction was done with petroleum ether/EtOAc (80: 20), and organic layers were dried over Na 2 SO 4 . Solvent was evaporated under reduced pressure, and the crude product was purified by column chromatography on silica gel (4 g, 0%-2% acetone in petroleum ether) to afford 27 as a colorless oil (141.3 mg, 77%). R f = 0.21 (petroleum ether/acetone 85:15). This reduction was subsequently performed on a larger scale (15 ×), and the crude alcohol, which was thus obtained, was used as such for the next step.  (28) To a stirred solution of crude 27 (made from 3.09 g of 26, 9.3 mmol) and Et 3 N (1.95 mL, 14.0 mmol, 1.5 equiv) in DCM (28 mL) under nitrogen, which was cooled at −35 • C, was added dropwise mesyl chloride (0.9 mL, 11.6 mmol, 1.25 equiv) in DCM (3.7 mL). Transfer of mesyl chloride was completed by rinsing with DCM (3 × 0.2 mL). The reaction mixture was then allowed to warm up slowly to −5 • C (in 4-5 h) and TLC monitoring (elution with DCM) showed completion of the reaction. Distilled water (96 mL) was added to quench the reaction, and extraction of the aqueous layer was done with DCM. Combined organic layers were washed with brine and dried over Na 2 SO 4 . DCM was removed under reduced pressure, and the crude product was purified by column chromatography on silica gel (10 g, 0%-1% acetone in petroleum ether) to afford 28 as a colorless oil (2.44 g, 69% from 26). To a stirred mixture of 28 (115 mg, 0.3 mmol), n-Bu 4 NBr (24.2 mg, 0.075 mmol, 0.25 equiv), DMSO (0.6 mL) and 50% aqueous NaOH (63 µL, 1.2 mmol of NaOH, 4 equiv), was added (R)-solketal 18 (49 mg, ≥95% pure, 0.35 mmol, 1.17 equiv). The corresponding mixture was stirred for 5 h at 60 • C. TLC monitoring showed completion of the reaction, and distilled water was added. Extraction was done with petroleum ether/EtOAc (80:20). Organic layers were washed again with brine and dried over Na 2 SO 4 . Solvent was removed under reduced pressure, and the crude product was purified by column chromatography on silica gel (5 g, 0%-0.5% acetone in petroleum ether) to provide 29 as a colorless oil (79.3 mg, 63%). R f = 0.44 (petroleum ether/acetone 95:5). 13   To a solution of acetonide 29 (971.9 mg, 2.32 mmol) in methanol (9.3 mL) and distilled water (0.93 mL) was added p-toluenesulfonic acid monohydrate (22.1 mg, 0.116 mmol, 0.05 equiv). The flask was then purged with nitrogen, stoppered and dipped in a preheated bath at 60 • C. Stirring was maintained for 5 h at 60 • C. TLC monitoring showed completion of the reaction, and sodium bicarbonate (10.9 mg, 0.13 mmol, 0.056 equiv) was added to the mixture and the stirring was continued for 1 h at 60 • C. Methanol was removed under reduced pressure, and then the crude product was purified by column chromatography on silica gel (6 g, 0%-5% acetone in petroleum ether and then petroleum ether + 5% acetone + 12% methanol) to provide 9 as a green oil (811.2 mg, 92%). R f = 0.04 (petroleum ether/acetone 90:10).
[α] To a stirred solution of 13 (10.0 g, 32.0 mmol) and Et 3 N (9.15 mL, 65.5 mmol, 2.05 equiv) in DCM (80 mL), which was cooled under nitrogen cooled at −40 • C, mesyl chloride (5.0 mL, 64.0 mmol, 2.0 equiv) in DCM (10 mL) was added dropwise. Transfer of mesyl chloride was completed by rinsing with DCM (2 × 1 mL). The reaction mixture was allowed to warm up slowly to −10 • C and then further stirred for 4 h at −10 • C. TLC monitoring showed completion of the reaction, and distilled water (120 mL) was added. Extraction of the aqueous layer was done with DCM. Combined organic layers were washed with brine and dried over Na 2 SO 4 . DCM was removed under reduced pressure, and the crude product was purified by column chromatography on silica gel (36 g, 0%-1% acetone in petroleum ether) to afford 31 as a colorless oil (8.   To a solution of 31 (9.1 g, 23.3 mmol) in DMSO (34.9 mL), was added NaN 3 (2.6 g, 40.0 mmol, 1.7 equiv). The flask was then purged with nitrogen, stoppered and dipped in a preheated bath at 80 • C. Stirring was maintained for 16 h at the same temperature. TLC monitoring showed completion of the reaction, and it was brought back to rt, then quenched with an aqueous solution of NH 4 Cl. Extraction was done with petroleum ether/EtOAc (80:20, 185 mL), and organic layers were washed with brine and dried over Na 2 SO 4 . Solvent was removed under reduced pressure to obtain the crude product as a light-yellow oil. The crude product was then purified by column chromatography on silica gel (30 g, 0%-0.5% acetone in petroleum ether) to provide 32 as an oil (6.26 g, 80%). To a vigorously stirred solution of 32 (5.063 g, 15 mmol) in Et 2 O (60 mL), which was cooled at −80 • C under nitrogen, was added dropwise a solution of Dibal (25% in hexane, 28 mL, 34.5 mmol, 2.3 equiv). The resulting mixture was allowed to stir for 2 h at −80 • C. TLC monitoring showed completion of the reaction. A saturated aqueous solution of potassium sodium tartrate (25 mL) was added, and stirring was continued for 10 min. Extraction was then done with petroleum ether/EtOAc (80:20). Organic layers were washed with brine and dried over Na 2 SO 4 . Solvent was evaporated under reduced pressure, and the crude product was used as such for the next step. For analytical purposes, the crude reaction product, which was initially obtained from 10 times less product, was purified by column chromatography on silica gel (5.2 g, 0%-1% acetone in petroleum ether) to afford 33 as an ochre oil (313. To a vigorous stirred solution of 33 (crude made from 5.063 g, 15 mmol of 32) in ethanol (15 mL), which was cooled at 0 • C under nitrogen, was added NaBH 4 (435 mg, 11.25 mmol, 0.75 equiv). The resulting mixture was allowed to stir for 1 h at 0 • C. TLC monitoring showed completion of the reaction. About 10-20 drops of acetone were added, and ethanol was removed under reduced pressure. After addition of water, extraction with petroleum ether/EtOAc (80:20), organic layers were washed with brine and dried over Na 2 SO 4 . Solvent was removed under reduced pressure, and the crude product was purified by two successive column chromatographies on silica gel (15 g, 0%-2% acetone in petroleum ether) to afford 34 as an ochre oil (3.44 g, 74% from 32). R f = 0.36 (petroleum ether/acetone 85:15). 1  To a stirred solution of 34 (3.93 g, 12.7 mmol) and Et 3 N (2.7 mL, 19.1 mmol, 1.5 equiv) in DCM (38 mL), which was cooled at -50 • C under nitrogen, was added dropwise mesyl chloride (1.25 mL, 15.9 mmol, 1.25 equiv) in DCM (5.0 mL). Transfer of mesyl chloride was completed by rinsing with DCM (3 × 0.2 mL). The mixture was then allowed to stir for 4 h at −50 • C and then allowed to warm up slowly to −5 • C. TLC monitoring (elution with DCM) showed completion of the reaction, and distilled water (50 mL) was added to quench the reaction. Extraction was done with DCM, and organic layers were washed with brine and dried over Na 2 SO 4 . DCM was removed under reduced pressure, and the crude product was purified by two successive column chromatographies on silica gel (15 g and then 40 g, 0%-3% acetone in petroleum ether) to provide 35 as an ochre oil (2.89 g, 59%). R f = 0.61 (petroleum ether/acetone 80:20). 1  To a stirred mixture at rt of 35 (2.48 g, 6.4 mmol), n-Bu 4 NBr (515.8 mg, 1.6 mmol, 0.25 equiv), DMSO (12.8 mL) and 50% aqueous NaOH (0.67 mL, 2 equiv) was added (R)-solketal 18 (1002.8 mg, ≥95% pure, 7.21 mmol, 1.13 equiv). The resulting mixture was then stirred overnight for 15 h at 35 • C. TLC monitoring showed completion of the reaction, and distilled water (40 mL) was added. Extraction was done with petroleum ether/EtOAc (80: 20), and organic layers were washed again with brine and dried over Na 2 SO 4 . Solvent was removed under reduced pressure, and the crude product was purified by two successive column chromatographies on silica gel (11 g and then 30 g, 0%-0.5% acetone in petroleum ether) to afford 36 as an ochre oil (1.41 g, 52%). R f = 0.56 (petroleum ether/acetone 95:5). To a solution of 36 (610.7 mg, 1.44 mmol) in methanol (5.8 mL), was added p-toluenesulfonic acid monohydrate (13.7 mg, 0.072 mmol, 0.05 equiv) and distilled water (0.58 mL). The flask was then purged with nitrogen, stoppered and dipped in a preheated bath at 60 • C. Stirring was maintained for 4 h at 60 • C. TLC monitoring showed completion of the reaction, and sodium bicarbonate (6.7 mg, 0.08 mmol, 0.055 equiv) was added to the mixture, and stirring was continued for 1 h at 60 • C. Methanol was removed under reduced pressure, and the crude product was purified by column chromatography on silica gel (5 g, 0%-5% acetone in petroleum ether and then petroleum ether + 5% acetone + 12% methanol) to provide 10 as an ochre oil (539.1 mg, 97%). R f = 0.03 (petroleum ether/acetone 80:20). IR (KBr) ν 3396, 2928, 2855, 2100, 1464, 1254, 1125, 1037 cm −1 . microbial growth [37][38][39]. The solution obtained was then added to MHB and serially diluted two fold (in a 96-well microplate). Then, 100 µl of inoculum 1.5 × 10 6 CFU/mL was prepared in MHB. The plates were covered with a sterile plate sealer, then agitated to mix the contents of the wells using a plate shaker and incubated at 30 • C for 48 h (M. audouinii) or at 37 • C for 24 h (other organisms). The assay was repeated three times. Gentamicin, chloramphenicol (bacteria) and nystatin (fungi) were used as positive controls. Wells containing MHB, 100 µl of inoculum and DMSO to a final concentration of 1% served as negative controls. The MIC of samples was then detected following an addition (40 µL) of 0.2 mg/mL p-iodonitrotetrazolium chloride and incubated at 37 • C for 30 min. Viable bacteria reduced the yellow dye to a pink color. MIC was defined as the lowest sample concentration that prevented this change and exhibited complete inhibition of bacterial growth.
Bacterial enumeration was performed on E. coli LMP701 as described by Stenger et al. [40]. Cells were treated with samples at their MIC and 4× MIC values as previously determined using XTT assay, and incubated at 37 • C. Viable cells were then determined at 0, 30, 60, 120, 240 and 480 min by performing 10-fold serial dilutions of this suspension in 0.9% saline. Gentamicin was used as reference drug whilst 0.9% saline and DMSO to a final concentration of 1% was used as control. All dilutions were placed on nutrient agar plates that were then incubated at 37 • C for 18 h. Bacterial colonies were then enumerated, and the total CFU/mL at each time was deduced.

Conclusions
A series of novel 1-O-alkylglycerol compounds 8-11 were synthesized from cheap ricinoleic acid (12). The structures of these compounds were characterized by NMR experiments as well as from the HRMS and elementary analysis data. AKGs 8-11 were evaluated for their respective antimicrobial activities. All compounds exhibited antimicrobial activity to different extents alone. Additionally, some beneficial synergistic effects were observed when AKG 8 was combined with gentamicin, and when AKG 11 was combined with ciprofloxacin and ampicillin. AKG 11 was viewed as a lead compound for this series, as it exhibited significant antimicrobial activity alone and when combined with some antibiotics compared with 8-10. It is now evident that non-natural synthesized 1-O-alkylglycerols can be further explored as a new source of drugs, and can be used in diverse preparations of pharmaceutical importance.