Stereoselective hydrocoupling of cinnamic acid esters by electroreduction: application to asymmetric synthesis of hydrodimers.

The electroreduction of Ar-substituted methyl cinnamates in acetonitrile gave all-trans cyclized hydrodimers stereoselectively (58 approximately 90% de). In all cases, small amounts (<10% yield) of meso hydrodimers were also formed. The electrolysis was performed conveniently using an undivided cell at a constant current. The transition states for the hydrocoupling were calculated with semiempirical methods. The all-trans cyclized hydrodimers were transformed to C(2)-symmetric dl-3,4-diaryladipic acids and trans-3,4-diarylcyclopentanones. The chiral auxiliary [(1R)-exo]-3-exo-(diphenylmethyl)borneol, prepared from (1R)-(+)-camphor, was highly effective for the stereoselective hydrocoupling of its cinnamates by electroreduction. From the resulting hydrodimers, (3R,4R)-3,4-diaryladipic acid esters and (3R,4R)-3,4-diarylhexane-1,6-diols were synthesized in 87-95% ee.