Synthetic strategies toward non-fused unsaturated 1,3-oxazines: an update

1,3-Oxazines are important six-membered heterocycles demonstrating diverse biological activities and photochromic properties. Most of the known 1,3-oxazines are benzo-fused; however, in recent decades, numerous reports on the synthesis of monocyclic unsaturated 1,3-oxazine derivatives have appeared in the literature. New approaches toward these compounds, including inter- and intramolecular ring expansions, as well as various (5 + 1)-, (4 + 2)-, and (3 + 3)-annulations, have been discovered, opening access to 1,3-oxazines with unique substitution patterns. These methods are based on metal-catalyzed transformations, multicomponent and photoinduced reactions, etc. In this review, we discuss synthetic approaches to non-fused unsaturated 1,3-oxazine derivatives, mainly reported after 2000, with special attention to novel reactions and their mechanistic aspects. Herein, we will spotlight advances in the synthesis of 2H-, 4H-, and 6H-1,3-oxazines, 1,3-oxazinones and 1,3-oxazinediones with carbonyl group(s) at different positions.