Stereoselective aldol reaction of alpha-seleno carbonyl compounds: preparation of (Z)-alpha,beta-unsaturated carbonyl compounds

The aldol reaction of the titanium enolates of alpha -seleno esters in the presence of Ph3P or Ph3PO gave the products with high stereoselectivity fav oring the syn isomers. Reaction of alpha -seleno ketones with TiCl4 in the presence of 2 equiv. of Et3N, and subsequently with aldehydes, gave the ald ol products with high syn selectivity. The stereoselectivity in the aldol r eaction of 3-pentanone also increased by using an excess amount of Et3N. Th e aldol products thus obtained from the alpha -seleno carbonyl compounds co uld be stereospecifically converted to (Z)-alpha,beta -unsaturated carbonyl compounds by treatment with pyridine. (Z)-Alkylidenecyclopentanones were e xclusively formed by treatment of the syn-aldol products with Et3N in the d ark. (C) 2001 Elsevier Science Ltd. All rights reserved.