1.3- Butadiene photocycloaddition reactions

Many examples of photocycloaddition reactions of olefins and polyenes yielding cyclobutane derivatives have been reported. Irradiation of mixtures of butadiene and 1,1-dichloroethylene in the presence of a... 

An interesting extension of the forementioned generalizations has recently been reported.43 When a dilute solution of 2,3-dimethyl-l,3-butadiene 11a is irradiated in the presence of a large excess of benzo-phenone, a product 16 arising from the photocycloaddition reaction is observed. This anomalous result has been explained as involving the 77,77- triplet of the diene, formed by triplet-triplet transfer from the n,n triplet of benzophenone, which then reacts with ground-state benzophenone. The critical steps are shown in structures 14-16. 

The utility of dioxopyrrolines in [2 + 2]-photocycloaddition reactions was comprehensively demonstrated by Sano et al. [66]. Substrate 55, for instance, underwent a clean reaction with 2-trimethylsilyloxy-butadiene to provide bicyclic HTproduct 56 as a single diastereoisomer (Scheme 6.22). Inline with previous observations [67], the vinyl group was positioned exo relative to the five-membered ring, and the silyloxy... 

A theoretical study of the Diels-Alder reactions between 1,3-butadiene and, respectively, cyclopentadiene and 2H-phosphole, has revealed a remarkable similarity between the two reactions. Further studies of photocycloaddition reactions of phosphole moieties have also been reported. Transition metal complexes of phospholide anions continue to attract attention, and in particular the chemistry of phosphaferrocene systems remains a major interest . The past year has also seen significant activity in the chemistry of di- and tri-phospholes, related polyphospholide anions, and also heterodiphosphole systems. Routes have been developed to the diphosphonio-l,2-diphospholes 356,... 

With conjugated dienes, photocycloaddition of carbonyl compounds occurs at one of the double bonds to give vinyloxetanes. An interesting example is the reaction of acetone with 2-methyl-l,3-butadiene, which gave the two oxetanes (60) and (61) in a ratio of 3 1 and a total yield of about 20% (72JA8761). Other alkenes which have been used for photosynthesis of oxetanes include enol ethers, ketene acetals, enamines, allenes and diketene, with the reaction of the last compound with benzaldehyde illustrated in equation (105) (75CPB365). 

The inverse electron demand Diels-Alder reaction of 1-oxa-l,3-butadiene 1 (a tautomer of triformylmethane) to dihydrofuran 2. obtained by photocycloaddition of benzaldehyde to furan, gives a mixture of two diastereomeric adducts with moderate yields and a d.r. (exolendo) 87 135. 

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