Photochemistry of nitroso compounds in solution. XV. Configuration and conformation of the enone oximes obtained from photoaddition of N-nitrosopiperidine to conjugated dienes

Abstract

In the presence of an acid, N-nitrosopiperidine photolytically adds to conjugated dienes to give 1:1 adducts in excellent yield. While the major products are the syn- and anti-isomers of piperidino α,β-unsaturated oxime derived from 1,4-addition, similar types of oximes derived from 1,2-addition are also obtained. The ratio of syn- and anti-oximes is determined by the relative configurational and the conformational stabilities of the two forms. The nuclear magnetic resonance (n.m.r.) chemical shifts and coupling constants provide useful information for the determination of the configuration and conformation of the oximes. A comparison of the n.m.r. data from α,β-unsaturated oximes indicates that the α-proton generally resonates at a lower field than the β-proton in contrast to the chemical shifts of α,β-unsaturated carbonyl compounds. The photoaddition of nitrosopiperidine to cycloheptatriene gave tropone oxime, probably through the elimination of piperidine from the 1:1 adduct.