Carbenes alkoxycarbene fragmentation

At this point, participation of the allyl side chain must 1 e recognized, because an n-butoxy derivative reacts in a different fashion. At least two routes, A and B, are open to pivotal intermediate IV (see Scheme 23.1). While the electron reorganization that leads to alkoxycarbene V (route A) and dihydro-furan (VII) via its addition to the terminal alkene would be justified by the high temperatures employed at which other carbenes have been thermally generated, the prior cyclization of zwitterion IX (route B) and its ensuing 1,2-hydrogen shift followed by C-C bond fragmentation in X—the natural precursor of products II and VII—would perhaps be more feasible. 

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