[3+3] Annulation of Diazoenals and α-Mercapto Ketones via Protic Sulfonium Ylides: Direct Synthesis of 2 H -Thiopyrans, Innovative Progenitors for Unstudied 2 H -Thiopyran-2-ones and 4 H -Thiopyran-4-ones.

Herein, we report a new Rh(II)/Sc(III)-catalyzed [3+3] annulation between diazoenals and α-mercapto ketones for the direct synthesis of 4-formyl-2 H -thiopyrans. The reaction proceeds via protic sulfonium ylides derived from highly electrophilic Rh-enalcarbenoids, followed by regioselective intramolecular aldol condensation. Further studies revealed that 4-formyl-2 H -thiopyrans are novel precursors for unstudied 2 H -thiopyran-2-ones and 4 H -thiopyran-4-ones. The 4 H -thiopyran-4-ones were obtained via a novel O 2 /Et 3 N-mediated oxidative deformylation. This methodology was applied to the short synthesis of structurally complex pyrimidine-fused 2 H -thiopyran via cascade Schmidt, Ritter, and intramolecular cyclization reactions.