Stereoselectivity in the Reactions between the Anion of 4-Phenyl-5,6-dihydro-2H-thiopyran 1-Oxide and Electrophiles

Abstract

Treatment of the anion(17), prepared from the six- membered ring allylic sulfoxide 4-phenyl-5,6-dihydro-2H-thiopyran 1-oxide (8) with primary alkyl halides at -78°, affords trans-2-alkyl-4-phenyl-5,6-dihydro-2H-thiopyran 1-oxides exclusively. The stereospecificity of these reactions thus corresponds to that obtained from the related saturated cyclic sulfoxides . However, when the anion (17) is treated with 2-iodopropane (-78° → 20°) a mixture of 2-cis and 2-trans products results. Independent experiments indicate that the lack of stereoselectivity is due to anion exchange reactions at the higher temperature required to effect alkylation.

2-cis and 2-trans Adducts also are obtained in reactions of the anion (17) with benzaldehydes (-78° → 0°), and from X-ray crystallographic analyses it has been established that the cis adducts predominate, with little selectivity in attack on the two enantiotopic faces of the aldehyde.