Cyclization of alkoxyiminyl radicals onto olefins: Formation of 2-alkoxy-delta(1)-pyrrolines, 4, 5-dihydrooxazoles and 5, 6-dihydrol-4h-1, 3-oxazines

Alkoxyiminyl radicals, generated by photolysis of N-bromo imidates, undergo exo-1, 5 and exo-1, 6 cyclization onto olefins on the O-alkyl side chains giving good yields of 4, 5-dihydrooxazoles and 5, 6-dihydro-4H-1, 3-oxazines, respectively. exo-1, 5 Cyclization onto an olefin on the iminyl side chain gives 2-alkoxy-Δ¹-pyrrolines. 4, 5-Dihydrooxazole formation is more favourable than cyclization to 2-alkoxy-Δ¹-pyrrolines and both reactions are irreversible. These preferences are supported by MNDO molecular orbital calculations which predict that both processes are exothermic but cyclization onto the O- alkenyl side chain has a lower ΔH^‡.