Switching the Reactivity of Dihydrothiopyran-4-one with Aldehydes by Aqueous Organocatalysis: Baylis–Hillman, Aldol, or Aldol Condensation Reactions

An aqueous medium containing catalytic amounts of a tertiary amine was employed to direct the chemoselectivity of the reaction of aldehydes with 1a. With DBU, 2 was formed at room temperature as a rare exemplary of Baylis-Hillman reactions in heterocyclic enones. DABCO alternated the pathway toward an aldol reaction to form syn/anti mixtures of 3 with the syn isomers being the major products. With Et(3)N, aldol condensation dominated.