| Abstract
| - The photochemistry of three pregna-1,4-diene-3,20-diones bearing a hydroxy or alkoxy group atC17 (4−6) has been examined. Irradiation at 254 or 366 nm, where absorption by the cross-conjugatedketone moiety in ring A is predominant or exclusive, causes the ‘lumiketone' rearrangement ofthis chromophore in low to medium quantum yield (Φr 0.05 to 0.31). On the contrary, irradiationat 310 nm, where the isolated ketone at C20 absorbs a large portion of light causes Norrish-Ifragmentation of that chromophore with a higher Φr (0.11−0.83). This leads to end-products arisingfrom the conversion of the C17 alkyl radical, in a way depending on the structure and the medium(reduction by hydrogen donating solvent, addition of oxygen when present). No intramolecular T−Tenergy transfer between the separated chromophores occurs. The ‘lumiketone' rearrangement occursindependently from the irradiation wavelength (Φr 0.06−0.18) with the strictly related androsta-1,4-dien-3-one 8 lacking the C20 ketone function.
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