Hydroboration of terpenes—VIII: Cyclic hydroboration of d-(+)-limonene. A stereoselective synthesis of d-(−)-(1R, 2R, 4R)-limonene-2,9-diol and d-(−)-(1R, 2R, 4R)-carvomenthol

Abstract

d-(+)-Limonene can be converted into the corresponding bicyclic organoborinate intermediate, B-methoxy-4,8-dimethyl-2-borabicyclo[3.3.1]nonane, by cyclic hydroboration with borane in THF, followed by methanolysis, and distillation of the product. Alternatively, cyclic hydroboration of d-(+)-limonene with thexylborane provides the related bicyclic organoborane intermediate, B-thexyl-4,8-dimethyl-2-borabicyclo[3.3.1]nonane. Oxidation of the respective intermediates produces d-(−)-(1R, 2R, 4R)-limonene-2,9-diol. Protonation of the bicyclic thexyl intermediate, followed by oxidation, provides d-(−)-(1R, 2R, 4R)-carvomenthol. These results suggest that the cyclic hydroboration of dienes can provide a valuable means for controlling the exact site of hydroboration, leading to a stereoselective synthesis of alcohols.