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.