Abstract
Optically active 1- or 3-acyl-sn-glycerols were synthesized from 2,3- or 1,2-isopropylidene-sn-glycerols, respectively. The 2,3- or 1,2-isopropylidene-sn-glycerols were condensed with appropriate long saturated or unsaturated fatty acids and the resulting acyl isopropylidene compounds were treated with dimethylboronbromide at - 50 degrees C to give the title compounds. The ketal cleavage of acyl isopropylidene-sn-glycerols by dimethylboronbromide to produce the long 1- or 3-acyl-sn-glycerols was effective and gave good yields (70-90%). The reaction conditions were mild and there was no acyl migration, as shown by optical rotation of the monoacyl-sn-glycerols. The synthesis of 2,3-isopropylidene-sn-glycerol was improved to give an overall yield of 40% from L-arabinose. L-Arabinose was first converted to its 1,1'-diethylmercapto derivative and then condensed with 2-methoxypropene to yield 1,1'-diethyl-mercapto-4,5-isopropylidene-L-arabinose. Oxidation of this compound with sodium periodate followed by reduction with sodium borohydride under alkaline conditions yielded 2,3-isopropylidene-sn-glycerol [alpha]22D = -14.90 degrees, neat (Lit. 8 [alpha]22D = -14.5 degrees, neat; 14 [alpha]25D = -10.8 degrees; methanol C, 16.9). The optical purity of isopropylidene-sn-glycerols was determined as benzoyl derivatives on a high performance liquid chromatographic column packed with a chiral stationary phase.
Highlights
The fatty acids lauric, palmitic, stearic, behenic, lignoceric, oleic, and linoleic acids and L( + )-arabinose were purchased from Sigma Chemical Company
The optical rotations were taken on an automatic polarimeter, Autopol-I1 (Rudolph Research, Flanders, NJ)
The difficulty encountered during the synthesis of monoacyl-sn-glycerols is the removal of the protected isopropylidene group which becomes increasingly difficult with increasing acyl chain length and impractical with stearic acid or longer chains
Summary
The fatty acids lauric, palmitic, stearic, behenic, lignoceric, oleic, and linoleic acids and L( + )-arabinose were purchased from Sigma Chemical Company The following conditions, column, and solvent system were used for the optical purity determination of the enantiomeric isopropylidene-sn-glycerol. As the T L C showed the presence of some starting material, an additional 2.9 g (0.04 mol) of 2-methoxypropene was added dropwise and stirred for 1 hr By this time, the reaction was complete and was stopped by the addition of 5 g of sodium carbonate and stirred for 10 min and filtered. Sodium periodate (64.2 g; 0.3 mol) was dissolved in distilled water (600 ml) and was chilled in an ice-salt bath to 0°C To this solution, 29.6 g of 1,l'-diethylmercapto-4, 5-isopropylidene-L-arabinose(0.1 mol) was added in portions over about 20 min while stirring. DCC (226 mg; 1.1 mmol) dissolved in carbon tetrachloride (10 ml) was added to the reaction mixture over about 10-15 min at room temperature.
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