Stereoselective reduction of the ketone group in α-allyl β-keto esters

Stereoselective reduction of the ketone carbonyl group in α-allyl-substituted β-keto esters with sodium tetrahydridoborate in the presence of 2 equiv of MnCl 2 quantitatively afforded the corresponding syn -isomeric alcohols. The reduction of the same substrates with L-selectride [LiBH( s -Bu) 3 ] in anhydrous THF was characterized by low chemoselectivity, and anti -isomeric alcohols were formed in about 50% yield. Under analogous conditions, α-allyl ketones smoothly reacted with L-selectride to give syn -isomeric alcohols in quantitative yield. The corresponding anti isomers were synthesized by the Mitsunobu reaction of the syn isomers with formic acid, followed by alkaline hydrolysis.