Cyclopropanes Cycloreversions

A few studies have been carried out on the parent four- and five-membered cyclic sulfones—for thietane 1,1-dioxide (30) by Scala and Colon65 and for thiolane 1,1-dioxide (sulfolane) (31) by Honda and coworkers66 and, later, by Schuchmann and von Sonntag67. In the former compound, the major photochemical process, in the vacuum UV range, is the initial production of a trimethylene (C3H6) biradical and S02 (equation 9). In both the solid- (77 K) and gas-phase photolyses, formation of a triplet biradical appears to be favored. As well as the expected cyclopropane and propylene, ethylene is also obtained during these photolyses, presumably by a cycloreversion process (equation 10). 

It is noteworthy that irradiation of tetracyclo[6.2.0.0 . 0 ]decan-9-one resulted in cycloreversion and formation of ketene and vinylcyclopropane tricyclo[5.1.0.0 ]oct-5-ene instead of ring cleavage and decarbonylation. A C-C double bond a to a cyclopropane ring also resulted when spiro bicyclo[2.2.1]hept-5-ene-7,l -cyclopropane -2-one eni/o-epoxide underwent a deep-seated rearrangement upon irradiation through Corex to give 5-oxo-spiro[2.4]hept-6-ene-4-ylacetaldehyde in 50% yield. ... 

Similarly, numerous other oligocyclic ring systems containing cyclopropane units have been obtained according to this method.The reaction is mechanistically interpreted as occurring via edge attack of the silver ion followed by a cationic rearrangement. Softer metal catalysts, such as rhodium, nickel and palladium, mostly lead to alternative products (e.g. [2-F2] cycloreversion). ... 

Plentiful examples of [C2 + C J cycloreversions of cyclopropanes have been reviewed. Most examples of this transformation are initiated photochemically, "" thermally initiated Ci-ex-trusions have only been observed for carbenes stabilized with electron-donating or fluorine substituents as well as with the participation of transition metal catalysts. ... 

In the cycloreversion of cyclopropane under vacuum-UV irradiation (<180 nm) the formation of ethene and methylene were detected. °... 

Enzymes may also catalyze transformations of a highly selective and nniqne natnre snch as in transformations of small molecnles. Examples are the fragmentation of 1-amino-cyclopropane-1-carboxylate to the frnit ripening hormone ethylene, the cycloreversion of thymine dimers in DNA repair, the synthesis of isopenicUhn and the rednction of molecn-lai nitrogen (N2) to ammonia... 

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