A Divergent Synthesis of Functionalized Unsaturated δ-Lactones from α-Alkenoyl-α-carboxyl Ketene Dithioacetals

A divergent synthesis of functionalized unsaturated δ-lactones 2, 3, 4, and 5 has been developed starting
from the readily available α-alkenoyl-α-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 α-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).