Bicyclic alkene, ring-rearrangement

Very highly constrained bicyclic alkenes rearrange via carbene intermediates to ring-contracted products. Thus, bicyclo[3.3.0]oct-2-ene gave 6-methylenebicyclo[3.2.0]heptane (38), and bicy-clo[2.2.1]hept-2-ene gave 5-methylenebicyclo[2.1.1]hexane (39).113... 

Corey and Kania later prepared the bicyclic marine diterpenoid (+)-dolabellatrie-none via the Ireland-Claisen rearrangement (Scheme 4.132) [125]. Ring contraction of the 15-membered ring lactone occurred with both high diastereo- and enantioselectivity via the Z-(0)-B-ketene acetal. The alkene and carboxyhc acid group were manipulated in several steps to form the isopropylidene cyclopenta-... 

Coyle has summarized the photochemistry of carboxylic acid derivatives. For arene carboxylic acid esters it has been shown that [2-1-2]-cycloaddition competes with hydrogen abstraction by the excited ester from an allylic position of the alkene. The addition of methyl benzoate 17 to 2-methyl-2-butene gave a 1 1 mixture of the Paterno-Bilchi adduct 18 and the coupling product 19. Less electron-rich alkenes (e.g., cyclopentene) did add preferentially toward the benzene ring of 17 in an ortho- and metacycloaddition manner. Furans could also be added photochemically to methyl benzoate and other aren-ecarboxylic acid esters. The resulting bicyclic oxetanes could be transformed into a series of synthetically valuable products. [2-1-2]-Cycloadducts and/or their cleavage or rearrangement products have also been described for photoreactions of alkenes with diethyl oxalate,benzoic acid, " and carbamates. ... 

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