Ring-closing photoreactions

The photoreaction of the cyanouracil (262) with cyclo-octene, hex-l-ene, and acetoxyethylene in acetonitrile yields the adducts (266a—c) and the rearranged adducts (267a—c). ° The reaction is proposed to occur by the addition of the olefin to the excited state of the uracil yielding the biradical (268). This biradical can either ring close to yield the cycloadduct (266) or else cyclize to form... 

The photochemical reactivity of alkenes is also of great interest [1,2]. Studies in this area have led to an expansion of the synthetic utility of these substances. Typical photochemical reactions include cis-trans isomeriza-tions, inter- and intramolecular cycloadditions, photooxidations, and electrocyclic ring opening and closing of conjugated dienes and polyenes. Many of these photoreactions have thermal counterparts. In contrast,... 

The structural formula of l,2-dicyano-l,2-bis-(2,4,5-trimethyl-3thienyl)ethene, and 6-nitro-l, 3, 3 -trimethylspiro[2H-l-benzopyrane-2,2 -indoline], referred to here as DE and SP, respectively, and their photochemical isomers are shown in Figure 3.14. The DE and SP chromophores have two photochemical isomers, a stable isomer and a thermally unstable isomer, namely the open-ring and close-ring forms for DE and the spiropyran and photomerocyanine for SP. The stable and thermally unstable isomers are henceforth referred to here as the A and B isomers, respectively- Light irradiation produces photoreaction in both the A—>B and A< B directions, and the thermal reaction proceeds in the A<—B direction. In contrast to the colored photomerocyanine form, which usually fades after several minutes at room temperature, the colored close-ring form of DE is stable for more than three months at Both the A and B isomers of DE and SP can... 

Fig. 19.19 (a) Stereoview of the special pair in the photoreaction center. Rings I of the chlorophyll molecules are stacked upon each other, and the magnesium atom of each chlorophyll is coordinated by an acetate group from the other molecule, (b) Close-up of the nearer chlorophyll molecule from part (a). The unattached acetate group is from the other chlorophyll molecule. (Modified from Deisenhofer, J. Epp. O. Miki, K. Huber, R. Michel. H. J. Mol. Biol. 1984, 180, 385-398. Reproduced with permission.]... 

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