Abstract
In the 1.3-dipolar cycloaddition of the cyclic nitrone 1c and the acyclic nitrones 8a-h 12a b with dimethyl maleate in refluxing chloroform not only the expected cycloadducts with cis-configuration of the two ester groups were formed but also such with trans-configuration. This phenomenon is not limited to chloroform, but was also observed in polar solvents as cyclohexane and n-hexane. However there is no clue for either a non-concerted reaction course or a subsequent conversion of the cis-products to trans-product in general. Rather conversion of dimethyl maleate to dimethyl fumarate seems to be responsible for the formation of the trans-substituted cycloadducts. This conversion can be induced by small quantities of N-alkylhydroxylamine formed from slight decomposition of nitrones under the reaction conditions or by small quantities of nitrone derivatives possessing a NOH moiety such as N-hydroxy enamine tautomers or nitrone dimers.
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