Electrocyclic ring closure of the methylenephosphonium ion 1, which is in equilibrium with 2, leads to the base-induced formation of 3 with the novel four-membered ring. Compounds of type 2 are considerably more stable than the corresponding pure hydrocarbons and can thus be isolated and characterized by X-ray crystallography.
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Heim et al. (1993) studied this question.
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