Photofragmentations of 3-and 4-Membered Rings

2 Photofragmentations of 3- and 4-Membered Rings - Flash photolysis of e/jf/o-7-chlorodibenzo[fl,c]bicyclo[4.1.0]heptane (87) generates phenanthrene and chlorocarbene (CHC1). The carbene can be trapped with pyridine, to give an ylide with a strong absorption at 374 nm and a lifetime of many tens of microseconds. Lifetimes of the carbene in several solvents were estimated from the yields of this ylide at various concentrations of pyridine. 

Efficient deprotection of thioketals to the corresponding ketones can be induced by photosensitized single electron transfer to n-acceptors such as 9,10-dicyanoanthracene. Similar reactions are observed for oxathiolanes, but the method is unsatisfactory for protected aldehydes. 

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