Synergistic Rh(II)- and Zn(II)-Catalyzed [3 + 3] Annulation of Diazoenals and α-Hydroxy Ketones for the Direct Synthesis of 2-Pyrans, A Gateway Toward γ-Pyrones.

A new synergistic Rh(II)/Zn(II)-catalyzed [3 + 3] annulation has been developed between diazoenals and α-hydroxy ketones enabling the direct synthesis of 4-formyl-2-pyrans. The reaction involves the formation of protic oxonium ylides from highly electrophilic Rh-enalcarbenoids followed by Zn-templated regioselective intramolecular aldol condensation. Subsequent investigations demonstrated that 4-formyl-2-pyrans are unique precursors of γ-pyrones. The γ-pyrones were obtained via EtN-mediated oxidative deformylation. Further, the triflic-acid-promoted double Schmidt reaction of 4-formyl-2-pyrans provides synthetically important carboxamide-substituted 4-cyano 2-pyrans.