18.104.22.168. Preparation of Sucrose Acetals (9 and 10a)
Two sucrose ketals, 4,6-0-isopropylidenesucrose (9) and 2,1':4,6-di-0-isopropylidenesucrose (10a) (Fig. 3), were prepared according to published procedures (13-16) with the minor modifications described below. A solution of sucrose (8.55 g, 25 mmol) in dry N,A-dimethylformamide (100 mL) containing molecular sieve pellets (Type 3A) was stirred with 2-methoxypropene (3.2 mL, 32.5 mmol, 9 and 12 mL, 125 mmol, 10a) in the presence of p-TSA. The reaction was allowed to proceed for 40 min at 70°C. The reaction mixture was then cooled, neutralized with sodium carbonate, and filtered and the solvent removed by rotary evaporation. The desired sucrose acetal was purified by column chromatography. The following yields were obtained: 3.2 g, 32% (9); 3.8 g, 36% (10a).
A crude mixture of isopropylidene sucrose was prepared as described for 10a. This typically contained some unreacted sucrose, 60-70% of 10a, 30-35% of 9, and a small quantity of a sucrose triacetal [2,1':4.6-di-0-iso-propylidene-6'-0-(1-methyl-1-methoxyethyl) sucrose] (16). This preparation was used for enzymatic esterification as obtained, after neutralization of the p-TSA catalyst with Amberlite IRA-400 OH anion-exchange resin, filtration, and removal of the solvent.
22.214.171.124. General Preparation of Sucrose Acetal 6'-O-Monoesters 10 b-f and Sucrose 6'-0-monoesters 11b-ff)
The approach is exemplified by the enzymatic esterification of 2,1':4,6-di-O-isopropylidene-sucrose (10a) with octadecanoic acid (stearic acid, C18:0) to form 6'-O-octadecanoyl-2,1':4,6-di-O-isopropylidenesucrose (10f) followed by cleavage of the acetal groups to yield 6'-O-octadecanoyl sucrose (11f, Fig. 3). Crude 2,1': 4,6-di-O-isopropylidenesucrose syrup (20.66 g) was incubated with stearic acid (13.80 g) at 75°C for 48 h with Novozyme™ 435 (1.70 g) with an initial addition of 21 mL of toluene to promote miscibility of the reagents. The solvent was allowed to evaporate during the first few hours of the reaction. After the completion of the reaction (typically 48 h), the resultant mixture was dissolved in ethanol and the enzyme was recovered by filtration. Unreacted fatty acid was then removed by passing the solution through a pad of basic alumina, and after evaporation of the ethanol in vacuo, the product was redissolved in ether (1:10 w/v). Unreacted sucrose acetals were then extracted with aqueous NaCl/K2CO3 (0.3 M each) and the organic solution was dried over MgSO4, filtered, and evaporated to dryness.
Hydrolysis of the isopropylidene groups to obtain the sucrose ester was carried out by dissolving the product (12.24 g) in a solution of acetonitrile:water:tetra-fluoroboric acid (500:5.5:0.5 v/v, 120 mL) and stirring at ambient temperature for 8 h. The solution was cooled in ice and the resultant precipitate (9.91 g, 11f) was filtered off. The reaction conditions for the acid-catalyzed hydrolysis of acetal groups were tailored for different esters (10b-e). Sucrose acetal monoesters 10b-d (10% w/v) were hydrolyzed in ethyl acetate:TFA:water 95:3:2 at 60°C for 8 h. TFA was then removed by azeotropic distillation with ether. Reaction conditions for 10e were identical to those described for 10f.
Final purification was achieved by column chromatography. The structure of the resultant monoester (>98% pure by GC) was determined to be 6'-O-stearoyl sucrose (11f) by 1H- and 13C-NMR spectroscopy and high resolution FAB-MS. This was in agreement with the structures assigned to the intermediate sucrose acetal ester (10f). Esters 11b-e were prepared and characterized in an analogous manner.
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