Medium-sized heterocycles are ubiquitous in biologically active natural products. Although transition metal catalyzed synthesis of 4- to 7-membered heterocyclic compounds is well-established, formation of medium sized cyclic molecules which are larger than 7-membered rings still has been challenging realm of organic synthesis. During our ongoing study on cooperative dual catalysis, we envisioned that the 9-membered 1,4-oxazonine heterocyclic compounds could be constructed through the synergistic Rh(II)/Pd(0) dual catalytic [6+3] dipolar cycloaddition reaction between the catalytically generated reactive intermediates, i.e., 1,3-dipole equivalent alpha-imino Rh-carbenoid formed from 1,2,3-triazoles 1 and 1,6-dipole equivalent zwitterionic alkoxy pi-allyl Pd(II)-complex formed from 4-vinyl-1,3-dioxan-2-one 2 (Scheme 1).
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