Dioxinone chromophore

Winkler and Shao85 have examined the first bond formation in the photoaddition of labeled dioxinones 143 and 146. Irradiation of 143 afforded an epimeric mixture of the more strained photoproducts 145, while irradiation of 146 gave 148 as a single product. These results are consistent with diradicals 144 and 147 as the intermediates in the photoaddition of 143 and 146, respectively, pointing out that the first bond formation in both cases takes place exclusively at the /1-carbon of the dioxinone chromophore (Scheme 31). 

The role of the dioxinone chromophore on the selectivity in the first bond formation, was examined by comparison to the enone analogs 149 and 151, lacking the /1-oxygen found in the dioxinone chromophore. Irradiation of 149 and 151 afforded only the fraws-fused photoproducts 150 and 152 respectively, which indicates that in contrast to dioxinones, the first bond formation in the enone photocycloadditions can take place from either the a- or /1-carbon of the enone via six-membered ring of the corresponding diradical intermediate (Scheme 32). 

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