Highly enantioselective synthesis of (–)-chokol A

Abstract

Stereoselective introduction of a methyl group into the (3S)-2-methylcyclopentanone 3, followed by Claisen rearrangement of the vinyl ether 10 derived from the allyl alcohol 7 provided separable methyl esters 6a and 6b. Reduction of 6a and subsequent homologation of the side-chain afforded the pivaloyl ester 16 after protection of the primary alcohol 15 with pivaloyl chloride. Cleavage of the double bond of compound 16, followed by sodium borohydride reduction of the resultant aldehyde 17, produced a mixture of the diols 18 and 19 leading to (–)-chokol A 5. Its enantiomeric excess was determined based on examination of the ¹H NMR spectrum of its MTPA ester 25.