Making use of crystallization-induced asymmetric transformations in solid state organic photochemistry: application to the enantioselective Yang photocyclization of endo-bicyclo[2.1.1]hexyl aryl ketones

Achiral endo-bicyclo[2.1.1] hexyl aryl keto-acid 1a exists in solution as a rapidly equilibrating 1: 1 mixture of enantiomers. When a solution of this compound is treated with an optically pure amine, a crystallization-induced asymmetric transformation of the second kind takes place, and crystals of only one of the two possible diastereomeric salts are deposited. Irradiation of the crystals leads to a diastereoselective Yang photocyclization reaction of the carboxylate anion portion of the salt, and following diazomethane workup to form the corresponding methyl ester, high yields of novel cyclobutanols of structure 2 are formed in enantiomeric excesses as high as 90%.