The cycloaddition of α-nitrosostyrenes to olefins. Investigations of the scope and mechanism of the reaction

Abstract

Cycloaddition of α-nitrosostyrene (1a) to several substituted styrenes is shown to give 3-phenyl-5,6-dihydro-4H-1,2-oxazines (2) in moderate yield. Reactions with 4-nitro-α-nitrosostyrene (1b) give better yields of the corresponding adducts (3). The preferred conformations of the adducts are deduced from their n.m.r. spectra. The observed regio- and stereo-selectivities of the additions are interpreted in terms of a one-step addition in which the olefins act as the donor components.

The first example of a formal 1,3-dipolar addition of a vinyl nitroso compound is provided by the formation of the nitrone (5) as a minor product of the reaction of α-nitrosostyrene with 2-methoxypropene. The relative yield of this nitrone is not increased by the change to a more polar solvent, nor by the use of α-bromoacetophenone anti-oxime in place of the usual syn isomer as the precursor of α-nitrosostyrene. It is suggested that this, and other minor products (4) isolated from the cycloadditions, may be formed because of steric inhibition of the usual Diels–Alder-type process.