title: On the regioselectivity of the intramolecular [2 + 2]-photocycloaddition of alk-3-enyl tetronates.
abstract: The simple diastereoselectivity and the regioselectivity of the [2 + 2]-photocycloaddition of alk-3-enyl tetronates were studied, depending on the nature of the substituent R in the α-position. Fourteen tetronates were synthesized and their intramolecular photocycloaddition reactions performed. If R was an alkoxycarbonyl group and if the olefin, which underwent the intramolecular addition, was sterically congested, crossed photoproducts prevailed and were formed in yields between 35 and 65%. In all other cases, the straight photoproducts were obtained as the main reaction products in yields ranging from 39 to 84%. The structures of the photochemical products were thoroughly investigated by one- and two-dimensional NMR experiments. In all cases, the diastereoselectivitiy was excellent and only a single diastereoisomer was formed. A mechanistic model is proposed, which explains the regioselectivity of the [2 + 2]-photocycloaddition reaction.
authors: Lehrstuhl für Organische Chemie I, Technische Universität München, Lichtenbergstr. 4, 85747 Garching, Germany.
authors: Roland
authors: R
authors: Weixler
authors: Jörg P
authors: JP
authors: Hehn
authors: Thorsten
authors: T
authors: Bach