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Asymmetric induction in the solid state photoisomerization of some three-membered ring compounds Chong, Ching Wah (Kenneth)


The ionic chiral auxiliary approach to asymmetric induction was investigated in two types of photoisomerizations of cyclopropyl compounds in the solid state. Three different tri-substituted cyclopropyl carboxylic acids were prepared. Upon irradiation, the achiral starting materials undergo cis-trans isomerization to form chiral products. Salts were formed between the achiral acids and optically pure amines. The resulting chiral samples were irradiated in the solid state, and the photoproducts analyzed for enantiomeric purity. Enantiomeric excesses ranging from poor to excellent were obtained. The enantioselectivity observed is controlled by both topochemical and chiral conformational effects. The same method of asymmetric induction was applied to study the solid state photochemistry of two achiral cyclopropyl ketoacids, which underwent the Norrish type II cleavage reactions to give chiral olefins. Enantioselectivities ranging from moderate to excellent were observed when optically active salts of these acids were irradiated in the crystalline state. The study presents the first successful asymmetric induction in a Norrish type II cleavage reaction. The starting materials are thermally reactive and undergo the enolene rearrangement to give the same chiral olefins. The salts prepared from one of the ketoacids remained solid at the reactive temperature, and the product showed low to moderate ee.

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