Stereoselective aldol reaction of α-seleno carbonyl compounds: preparation of (Z)-α,β-unsaturated carbonyl compounds

The aldol reaction of the titanium enolates of α-seleno esters in the presence of Ph3P or Ph3PO gave the products with high stereoselectivity favoring the syn isomers. Reaction of α-seleno ketones with TiCl4 in the presence of 2 equiv. of Et3N, and subsequently with aldehydes, gave the aldol products with high syn selectivity. The stereoselectivity in the aldol reaction of 3-pentanone also increased by using an excess amount of Et3N. The aldol products thus obtained from the α-seleno carbonyl compounds could be stereospecifically converted to (Z)-α,β-unsaturated carbonyl compounds by treatment with pyridine. (Z)-Alkylidenecyclopentanones were exclusively formed by treatment of the syn-aldol products with Et3N in the dark.