Furans from 3-acylcyclopropenes

The formation of furans from 3-acylcyclopropenes is catalyzed by copper(I) but not copper(II) salts, and can also be brought about photochemically. With an optically active cycloprop-2-enecarboxylate, racemization in a photochemical reaction occurred 2.5 times faster than furan formation, supporting the intermediacy of a singlet vinylcarbene in equilibrium with the cyclo-propene. ... 

While the photolysis of (164) can lead to reasonable yields of acylcyclopropenes, no cyclopropenes are detected from the corresponding reactions of (165) however, if the photolysis of (165, R = Me, R = H) is carried out in the presence of furan, a 2-oxa-bicyclohexane is isolated. This is apparently derived by addition of the carbene (166) to the 2,3-bond. The oxabicycle in turn rearranges to a single triene (167) 128). 

Valence isomer formation is a feature also ot the photochemistry of naphthalenes (3.33) and anthracenes for naphthalenes, as for benzenes, the extent of steric crowding helps to determine which tvpe of valence isomer predominates, since there is more severe interaction in the hicyclnhexadiene products than in the benzvalene products. Amongst five-membered heteroaromatic compounds there are many known ring photoisomerizations that involve conversion of a 2-substituted to a 3-substitutud system (e.g. 3.34). In some cases non-aromatic products can be isolated, such as hicyt lo[2 1. OJpentene analogues from thiophenes (3,35), or acylcyclopropenes from furans (3. Jb) related species may be... 

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See also in sourсe #XX -- [ ]

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