(R)-2-Methyl-CBS-oxazaborolidine → (S)-2-(4-t-butyldimethylsilyloxy-1,3-dihydro-6-ethyl-7-methyl-3-oxoisobenzofuran-5-yl)-1-cyclopentenyl-1-hydroxyethane

Participants

Reaction Conditions

Reaction Steps

Cyclopentyl (4-t-butyldimethylsilyloxy-1,3-dihydro-6-ethyl-7-methyl-3-oxoisobenzofuran-5-yl methyl)methanone (4.2 g) was treated with a 1M toluene solution of (R)-tetrahydro-1-methyl-3,3-diphenyl-1H,3H-pyrrolo[1,2-c][1,3,2]oxazaborole (3 ml). The toluene was evaporated in vacuo and methylene chloride added (2 ml). The reaction mixture was cooled to -30° C. and borane/dimethyl sulfide (0.33 ml) was added three times at 45 minute intervals. The reaction mixture was stirred 16 hours at -30° C. and quenched by addition of 1M hydrogen chloride/ether (3 ml). Toluene was added (10 ml) and the solution filtered, diluted with ether (200 ml), washed with aqueous hydrochloric acid, aqueous sodium bicarbonate and brine. Drying and evaporation gave an oil which was purified by silica gel chromatography to give (S)-2-(4-t-butyldimethylsilyloxy-1,3-dihydro-6-ethyl-7-methyl-3-oxoisobenzofuran-5-yl)-1-cyclopentenyl-1-hydroxyethane. Further purification by crystallization from t-butylmethyl ether gave material with an enantiomeric excess of 97.8% (Chiracel OD-H, 85:15 hexane:i-propanol, 0.8 ml/minute, minor 8.3 minutes, major 9.2 minutes).

Provenance & Evidence