Accessing dihydro-1,2-oxazine via Cloke−Wilson-Type annulation of cyclopropyl carbonyls: application toward the diastereoselective synthesis of pyrrolo[1,2‑b][1,2]oxazine

Abstract

A convenient additive-free synthesis of dihydro-4H-1,2-oxazines via a Cloke−Wilson-type ring expansion of the arylsubstituted cyclopropane carbaldehydes with the hydroxylamine salt is introduced. Comparatively less active cyclopropyl ketones also follow a similar protocol if supplemented by catalytic p-toluene sulfonic acid monohydrate. The transformation is performed in an open-to-air flask as it shows negligible sensitivity toward air/moisture. Dihydro-4H-1,2-oxazines when subjected to cycloaddition with the cyclopropane diester afford a trouble-free formulation of the valued hexahydro-2H-pyrrolo[1,2-b][1,2]oxazine derivatives. A cascade one-pot variant of this two-step strategy offers a comparable overall yield of the final product.