Ring contraction 2 - with cyclopropene

One of the major areas of study in the photochemistry of heterocycles is the photorearrangement of five-membered heteroaromatic systems. Various mechanisms have been proposed to account for these transformations.123 A ring contraction-ring expansion pathway, via an intermediate cyclopropene, appears to be involved in the rearrangement of derivatives of furan. The isolation of an intermediate (140) of this type together with an allene (141) has been effected on irradiation of 2,4-di-tert-butylfuran (142), as shown in Scheme 9.124 Similar transformations have been reported in perfluorotri-... 

Related carbenes, i.e. 3//-benzocycloheptatrien-3-ylidene and 5/7-dibenzo[a,c]cycloheptatrien-5-ylidene generated from the corresponding tosylhydrazones, thermally or photolytically, isomerize to benzo-12 or dibenzobicyclo[4.1.0]hepta-2,4,6-trienes, respectively. In contrast to cycloheptatrienylidene, 5/7-dibenzo[a,c]cycloheptatrien-5-ylidene underwent ring contraction to form a cyclopropene derivative 13 before reaction with alkenes. 

There is interest in cycloaddition of diazoalkanes to cycloalkenes for various reasons. Small rings (cyclopropene and cyclobutene) react easily and often in good yield as a result of their angle-strained double bond. Their products with diazomethane, 2,3-diazabicyclo[3.1.0]hex-2-ene (6.56) and 2,3-diazabicyclo[3.2.0]hept-2-ene (6.57) and their substituted derivatives are interesting for synthetic purposes, e. g., by azo-extrusion leading to ring contraction. 

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