Synergistic\nRh(II)- and Zn(II)-Catalyzed [3 + 3] Annulation\nof Diazoenals and α‑Hydroxy Ketones for the Direct Synthesis\nof 2<i>H</i>‑Pyrans, A Gateway Toward γ‑Pyrones

A new synergistic Rh(II)/Zn(II)-catalyzed [3 + 3] annulation\nhas\nbeen developed between diazoenals and α-hydroxy ketones enabling\nthe direct synthesis of 4-formyl-2<i>H</i>-pyrans. The reaction\ninvolves the formation of protic oxonium ylides from highly electrophilic\nRh-enalcarbenoids followed by Zn-templated regioselective intramolecular\naldol condensation. Subsequent investigations demonstrated that 4-formyl-2<i>H</i>-pyrans are unique precursors of γ-pyrones. The γ-pyrones\nwere obtained via Et<sub>3</sub>N-mediated oxidative deformylation.\nFurther, the triflic-acid-promoted double Schmidt reaction of 4-formyl-2<i>H</i>-pyrans provides synthetically important carboxamide-substituted\n4-cyano 2<i>H</i>-pyrans.