In the cycloaddition of heteroaromatic N-ylides to symmetrically substituted trans olefins, the anti ylide exclusively participates if the ylide is carbonyl-stabilized, while the syn ylide does if it has a substituent of non-carbonyl type. The cycloadducts isomerize into thermodynamically more stable isomers through a retro reaction.
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Tsuge et al. (1985) studied this question.
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