REGIOCHEMISTRY IN ELECTROPHILIC REACTIONS OF PROPANEDITHIO-SUBSTITUTED ALLYLIC ANIONS INFLUENCED BY THE GAMMA-SUBSTITUENTS

The dienyl anion 1f, generated from metallation of 2-(1,3-pentadienyl)-1,3-dithiane with n-BuLi in THF, reacted exclusively at the C-1 position (alpha-site) with alkylating agents and ketones, but it showed some tendency toward the C-3 position (gamma-site) in the reactions with benzyl bromide and aliphatic aldehydes. No reaction at the C-5 position (epsilon-site) was observed in any studied case. Examination of the related propanedithio-substituted allylic anions having various gamma-substituents revealed that the regiochemistry was remarkably influenced by the gamma-substituents and the attacking electrophiles. The electronic effect according to the principle of hard and soft acids and bases is proposed to account for the observed regiochemistry, while the steric factor was minor. Two trienes, obtained by alkylations of the dienyl anion If with 4-bromo-1-butene and 5-pentenyl methanesulfonate, were applicable to intramolecular Diels-Alder reactions to give bicyclic compounds.