A divergent synthesis of functionalized unsaturated delta-lactones from alpha-alkenoyl-alpha-carboxyl ketene dithioacetals

A divergent synthesis of functionalized unsaturated delta-lactones 2, 3, 4, and 5 has been developed starting from the readily available alpha-alkenoyl-alpha-carboxyl ketene dithioacetals 1 in high to excellent yields under mild reaction conditions. Thus, 6-substituted 3-(1,3-dithiolan/dithian-2-ylidene)-3H-pyran-2(6H)-ones 2, obtained from a consecutive reduction with NaBH4 and acidic workup of 1 via a novel vinylogous Pummerer cyclization, can be further transformed into alpha-pyranones 3, 4, and 5 upon a sequential isomerization catalyzed by triethylamine (to give 3), followed by dethioacetalization (to give 4) or a formylation with Vilsmeier reagent (to give 5).