- Noncommunicating photoreaction paths in some pregna-1,4-diene-3,20-diones
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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.
- Ricci, Andrea,Fasani, Elisa,Mella, Mariella,Albini, Angelo
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- General patterns in the photochemistry of pregna-1,4-dien-3,20-diones
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The photochemistry of six pregna-1,4-dien-3,20-diones has been compared and found to involve both the cyclohexadienone moiety in ring A and the isolated ketone at C-20. The two reactions take place proportionally to the fraction of light absorbed by each chromophore. The cross-conjugated ketone absorbs predominantly or exclusively at both 254 and 366 nm and undergoes the lumi rearrangement to bicyclo[3.1.0]hex-3-en-2-one. The quantum yield of the reaction diminished somewhat with increasing λexc, e.g., for prednisolone Φ254 nm = 0.42, Φ366 nm = 0.3. A much stronger lowering is caused by halogen substitution in position 9 (by a factor of 3 for F, > 50 for Cl), apparently due to a shortened triplet lifetime caused by heavy atom effect. At 310 nm, both chromophores absorb to a comparable degree and both may react. The reaction at C20 ketone involves either quite efficient α-cleavage (C17-C20) for compounds bearing an acetal or hydroxyl function at C17 or less effective (by a factor of ca. 10) hydrogen abstraction from the 18-methyl group in the other cases (finally resulting in Norrish II fragmentation or Yang cyclization). The results allow generalizing how the substitution pattern surrounding each chromophore affects the photoreactivity at that site and the competition between the two modes, allowing predicting the photochemistry of this family of antiinflammatory drugs.
- Ricci, Andrea,Fasani, Elisa,Mella, Mariella,Albini, Angelo
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p. 4361 - 4366
(2007/10/03)
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