Stereochemical aspects of novel 5,6-dihydro-4H-1,3-oxazines

Novel polysubstituted 5,6-dihydro-4H-1,3-oxazines are synthesized by 1,4-cy cloaddition of unactivated olefins with N-acyliminiums from benzotriazole p recursors. The configuration and conformation of the products are deduced f rom NMR investigation. The regio- and exo-endo stereochemistry of the cyclo addition are discussed. Conformations of compounds of six types of substitu tion (6-mono; 6,6-, 5,6- and 4,6-di; 4,6,6- and 4,5,6-tri) are rationalized on the basis of axial-1,3-repulsion and the anomeric effect.