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-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-thiopyrans are novel precursors for unstudied 2-thiopyran-2-ones and 4-thiopyran-4-ones. The 4-thiopyran-4-ones were obtained via a novel O/EtN-mediated oxidative deformylation. This methodology was applied to the short synthesis of structurally complex pyrimidine-fused 2-thiopyran via cascade Schmidt, Ritter, and intramolecular cyclization reactions.
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