Ethynylogization of a coarctate fragmentation

Coarctate reactions form a separate class of elementary closed-shell proces ses in addition to polar and pericyclic reactions. Hence, they also follow a different homology principle. Whereas vinylogous polar and pericyclic rea ctions differ in the length of the reacting system by a double bond, coarct ate reactions can be homologized (ethynylogized) by extending a known syste m by a triple bond, The prediction, which is based on theoretical considera tions, is confirmed experimentally by the fragmentation of cyclopropylethyn yl nitrene to cyano acetylene and ethylene, a reaction that is "ethynyloguo us" to the known fragmentation of cyclopropyl nitrene Co ethylene and HCN.