Competitive [4 + 2] and [3 + 2] cycloadditions of nitrosoalkenes to the imino bond of bicyclic 1,2-oxazines

Abstract

The bicyclic 1,2-oxazines derived from the cycloaddition of α-nitrosostyrene to either 2,5-dimethyl-furan (11) or cyclopentadiene (15) react further with nitrosostyrene or other nitrosoalkenes. Only 3 + 2 addition is observed with (11) to give the tricyclic nitrones (12) and (13), shown by X-ray analysis of (12) to be cis-anti-cis fused, with a central boat conformation. The adduct (15) gives both nitrones (16)–(18) and the 4 + 2 cycloadducts of the 1,2,5 oxadiazines (21)–(23). The latter mode of addition is favoured by nitrosoalkenes with electron withdrawing α-substituents. The oxadiazine (23), derived from 3-nitrosobut-3-en-2-one is cis-syn-cis fused with a central chair conformation in the crystal state, but exists in solution in equilibrium with a small amount of the boat conformation.

The requirements for addition to the imino bond are demanding, many other model systems related to (11) or (15) failing to react.