Stereoselective hydrocoupling of optically active 3-trans-cinnamoyloxazolidinones by electroreduction

Reductive hydrocoupling of chiral 3-trans-cinnamoyloxazolidinones was studied by an electro chemical method. The electroreduction was performed conveniently at a constant current using an undivided cell. The stereoselectivity of the hydrodimers was strongly affected by the electrolyte employed. Electroreduction of (S)-4-isobutyl-3-trans-cinnamoyloxazolidinone 1a in 0.3 M Et4NOTs/AN gave a mixture of two diastereomers of all-trans cyclized hydrodimer 2a, and the selectivity was R,S,R/S,R,S = 85:15. On the other hand, the reduction of 1a in 1.7 M LiClO4/THF afforded a diastereomeric mixture of hydrodimers in a selectivity of R,R/S,S/R,S = 5:52:43. The stereoselectivities were explained by considering stable conformations of intermediate anion radicals, that is, syn-Z type for naked anion radicals and anti-Z type for lithiated anion radicals. Semiempirical calculations also supported this hypothesis. Electroreductions of (S)-4-isobutyl-3-cis-cinnamoyloxazolidinone and (S)-4-isobutyl-3-phenylpropioloyloxazolidinone gave 2a in the same stereoselectivity as electroreduction of 1a did. The electroreductive hydrocoupling was not inhibited by para and meta substitution on the aryl group of 3-trans-cinnamoyloxazolidinones. An ortho substitution, however, hindered the hydrocoupling and lowered the stereoselectivity of the hydrodimers. Electroreduction of 3-trans-cinnamoyloxazolidinethione and thiazolidinethione gave trans-3,4-diphenylcyclopentanone as a product, and the stereoselectivities were similar to that obtained from the corresponding oxazolidinone.