Non communicating photoreaction paths in some pregna-1,4-diene-3,20-diones

The photochemistry of three pregna-1,4-diene-3,20-diones bearing a hydroxy or alkoxy group at C17 (4−6) has been examined. Irradiation at 254 or 366 nm, where absorption by the cross-conjugated ketone moiety in ring A is predominant or exclusive, causes the ‘lumiketone' rearrangement of this chromophore in low to medium quantum yield (Φr 0.05 to 0.31). On the contrary, irradiation at 310 nm, where the isolated ketone at C20 absorbs a large portion of light causes Norrish-I fragmentation of that chromophore with a higher Φr (0.11−0.83). This leads to end-products arising from 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−T energy transfer between the separated chromophores occurs. The ‘lumiketone' rearrangement occurs independently 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.