Photochemical -photocydoaddition

A combined photochemical and biocatalytic approach provided access to bicyclo [4.2.0]octanes via a novel strategy. The Cu-catalyzed [2 + 2] photocydoaddition of... 

Due to their reduced aromatic character, naphthalene derivatives more easily undergo [2 + 2] photocydoaddition to give access to cyclobutane derivatives [49]. In some cases, the formal [4 + 2] addition is competitive. In contrast to the ground-state Diels-Alder reaction, the [4 + 2] products from these photochemical reactions result from a multistep reaction. Upon irradiation, the 1-cyanonaphthalene derivative 45... 

Photochemical reactions provide a classical access to four-membered ring compounds that generate major interest in organic synthesis, notably as intermediates in multistep syntheses. The [2 + 2] photocycloaddition of a,(3-unsaturated carbonyl and carboxyl compounds with alkenes and [2 + 2] photocydoaddition of ketones with alkenes (the Paterno-Buchi reaction) are discussed in Chapters 6 and 7, respectively. Yet, aside from these transformations, a variety of further reactions provides a systematic access to four-membered rings that possess a wide structural variation. Four-membered ring compounds may also be created via less-systematic photochemical transformations, many of which can be carried out without additional chemical activation. As a consequence, such transformations are rendered not only very convenient but also extremely interesting within the context of green chemistry. ... 

Pyridones can react photochemically along several reaction channels [94]. Besides [4 + 4]-photodimerization and 4jt]-ring closure, [2 + 2]-photocydoaddition reactions are possible in an a,P- or in a y,8-mode relative to the carbonyl carbon atom. With regard to the former reaction pathway, the [2 + 2]-photocydoaddition of olefins to 4-alkoxypyridones appears to be synthetically most useful (vide infra). 

Next, the homochiral crystals were utilized for subsequent diastereoselective reactions. Diastereoselective 2+2 photocydoaddition reaction of amide 20 with methacrylonitrUe was examined. The reaction with methacrylonitrile proceeded effectively, stereo-, and regiospecifically. The amide 20 before crystallization was irradiated in the presence of methacrylonitrile with a high-pressure mercury lamp at 20°C until most of the starting amide was consumed (2 hrs). The photochemical reaction occurred effectively, and 2 + 2 cycloadducts were obtained in 100% chemical yields both diastereomers were endo isomers, minor (lS,2aR,8bR)-23, major (lR,2aS,8bS)-23, and the de value was -25%. Since epamerization was not observed at 20°C, it seems that the de value of the photoproducts should be attributed to the ratio of the diastereomers of the amide 20 before crystallization (-20% de). 

Asymmetric photocydoaddition of naphthamide with a diene using the provisional molecular chirality in a chiral crystal. Photochem. and Photobiol. Sci. Vol. 10, pp. 1387-1389... 

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