Cyclization of Alkoxyiminyl Radicals Onto Olefins: Formation of 2-Alkoxy-Delta(1)-Pyrrolines, 4,5-Dihydrooxazoles and 5,6-Dihydrol-4h-1,3-Oxazines

Abstract

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^‡.