Yang photoenolization

There are four distinct processes initiated by y-hydrogen abstraction in excited carbonyl compounds Norrish type II photoelimination, Yang photo-cyclization (cyclobutanol formation), Yang photoenolization (o-xylylenol formation), and (3-cleavage of radicals from carbons adjacent to the radical sites of the 1,4-biradicals. Some of these require unique structures and generate distinct products. 

The cyclization reaction of a biradical intermediate to form cyclobutanol (304 see also Yang cyclization Section 6.3.5) and the Diels Alder cycloaddition of the photoenols with dienophiles are common subsequent processes (Scheme 6.134).981,982 Both processes are often stereospecific, typically involving a single, long-lived ( )-photoenol. 

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