Enantiomerically Pure Octahydronaphthalenone and Octahydroindenone: Elaboration of the Substrate Overcame the Specificity of Yeast-Mediated Reduction

Substrate specificity on the reduction of bicyclic diketones with a yeast strain, Torulaspora delbrueckii IFO10921, was investigated. Although this yeast efficiently reduces the isolated carbonyl group involved in the (S)-enantiomer of Wieland-Miescher ketone with high enantioselectivity (E = 126), the introduction of a substituent on the octahydronaphthalene skeleton as well as the structural change into an octahydroindene skeleton retarded the enzymatic reduction and the enantioselectivity fell to E = 5-16. Further structural variation into a bicyclo[3.3.0] skeleton led to an exclusive 1,4-conjugate reduction of the α,β-unsaturated carbonyl group, and the above results suggested the participation of plural oxidoreductive enzymes in the whole cell. In turn, among the 2,2-disubstituted cycloalkanediones there were found good substrates to give the corresponding hydroxy ketone equivalents by yeast-mediated reduction. The products were isolated as cyclic hemiacetals, such as (1S,6S)-3-ethyl-3-hydroxy-6-methyl-2-oxabicyclo[4.4.0]decan-7-one and (1S,6S)-3-hydroxy-3,6-dimethyl-2-oxabicyclo[4.3.0]nonan-7-one. In addition, the reduction worked well with use of an air-dried, long-term preservable cell preparation. The subsequent chemical transformation warranted the stereochemistry and the stereochemical purity of the products, which are related to octahydronaphthalenone and octahydroindenone systems that, in turn, are of considerable value as starting materials for terpenoid synthesis.