Low-temperature photosensitized oxidation of a guanosine derivative and formation of an imidazole ring-opened product.

An organic-soluble guanosine derivative, 2',3',5'-O-(tert-butyldimethylsilyl)guanosine (1), was prepared and its photosensitized oxidation was carried out in several solvents at various temperatures. Singlet oxygen is the reactive oxidizing agent responsible for this reaction. Neither an endoperoxide nor a dioxetane intermediate was detected by low-temperature NMR even at -78 degrees C. A product (A) with an oxidized imidazole ring was the only major product detected at room temperature; this compound could be isolated by low-temperature column chromatography and was characterized by (1)H and (13)C and mass spectroscopy. CO(2) was the other major product. A small amount of the corresponding 8-oxo-7,8-dihydroguanosine derivative B was detected during the initial stage of the photooxidation and was shown to be intermediate in the formation of two products of extensive degradation, C and D. Reaction of 1 with the singlet oxygen analogues 4-methyl-1,2,4-triazoline-3,5-dione (MTAD) and 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) gave products consistent with a proposed mechanism involving the rearrangement of an initially formed endoperoxide to give A and B from reaction of 1 with singlet oxygen.

Substituent Effects on Rates and Stereoselectivities of Conrotatory Electrocyclic Reactions of Cyclobutenes. A Theoretical Study.

Substituent effects on the geometries and conrotatory electrocyclic ring openings of cyclobutenes were studied. This work extends the original investigations to many more substituents and provides a comprehensive theory of substituent effects on geometries and reaction rates. The effects of substitution at the 1 position are minimal; donor substituents raise the activation energy slightly, and powerful acceptor substituents slightly lower the activation energy. Substituents on C(3) cause small distortions of the cyclobutene geometry, in the same direction as the favored stereochemistry of reaction. Donors prefer outward rotation, while strong acceptors prefer inward rotation. The activation energy changes and cyclobutene geometrical perturbations were found to correlate with Taft sigma(R)(0) parameters. Amino, hydroxy, fluoro, chloro, methyl, cyano, formyl, and vinyl substituents were studied in the 1 position. Boryl, dimethylboryl, nitroso, formyl, nitro, carboxyl (neutral, protonated, and deprotonated), cyano, trifluoromethyl, sulfoxyl, sulfonyl, sulfinic acid, imino, N-protonated imino, ammonio, ethynyl, methyl, mercapto, chloro, fluoro, hydroxyl, amino, lithium oxy, vinyl, and acetyl were calculated as substituents in the 3 position. Comparisons with experimental results are given when available, and predictions are made in other cases.