Stereocontrolled synthesis of trifluoromethylated (E)- or (Z)-ynenyl sulfones via sequential transformations

Abstract

Diethyl 1-(phenylsulfonyl)ethylphosphonate 1 was treated with n-butyllithium in tetrahydrofuran (THF) at −78 °C and the resulting carbanion 2 reacted with trifluoroacetic anhydride to give the trifluoroacylated phosphonate 3. Without isolation, 3 was attacked by lithium acetylides and elimination of phosphate anion afforded trifluoromethylated (Z)-ynenyl sulfones (Z-4) in 57–76% yields, while treatment of 3 with acetylenic Grignard reagents gave trifluoromethylated (E)-ynenyl sulfones (E-4) in 45–54% yields. The configuration of the products could be ascertained on the basis of the crystal structure. A possible mechanism for the explanation of stereochemical results is proposed.