Oxidative Rearrangement of 2-Substituted Oxazolines. A Novel Entry to 5,6-Dihydro-2H-1,4-oxazin-2-ones and Morpholin-2-ones

A novel synthesis of 5,6-dihydro-2 H -1,4-oxazin-2-ones by SeO 2 -promoted oxidative rearrangement of 2-alkyl- and 2-(arylmethyl)oxazolines is described. Yields are good to excellent (up to 94%) with the highest yields obtained for 2-arylmethyl- and 2-neopentyl-substituted oxazolines. This reaction provides convenient access to novel 5-aryl-substituted dihydrooxazinones in high yield. The latter compounds are important “chiral glycine” synthons for asymmetric synthesis of α-amino acids. Since oxazolines are readily derived from carboxylic acids or their equivalents, this oxidative rearrangement constitutes an entry to synthesis of α-amino acids from carboxylic acids. A mechanism is proposed to account for the rearrangement involving a “nitrilium to acylium” 1,2-migration.