Single-Step Modular Synthesis of Unsaturated Morpholine N-Oxides and Their Cycloaddition Reactions.

A single-flask procedure for the generation of α-keto-N-alkenylnitrones through a Chan-Lam coupling and subsequent spontaneous 6π electrocyclization of these intermediates for the synthesis of 2H-1,4-oxazine N-oxides has been developed for a variety of α-ketooximes and alkenylboronic acids. This transformation provides a new approach to C-substituted unsaturated morpholine derivatives that are poised to undergo further functionalization for the preparation of a diverse array of novel heterocyclic structures. The scope of the new method for the synthesis of 2H-1,4-oxazine N-oxides is discussed, in addition to initial studies describing the cycloaddition reactivity of these new heterocyclic intermediates.