Self-Sensitized DiastereodifferentiatingZ-EPhotoisomerization of 3-, 4-, and 5-Benzoyloxycyclooctenes: Intra- versus Intermolecular Photosensitization

Abstract

Upon irradiation at 254 nm, (Z)-3-, 4-, and 5-benzoyloxycyclooctenes (1Z-3Z) efficiently isomerized through intraand/or intermolecular sensitization to the corresponding diastereomeric E-isomers (1E-3E) with low to moderate diastereomeric excesses (de's) of up to 43%. The major diastereomer produced from 1Z was switched from (1R*,3R*)-1E (26% de) to (1R*,3S*)-1E (-11% de) by simply increasing the substrate concentration from 1 mM to 50 mM. This unusual switching of product chirality is attributed to the opposite diastereoselectivities for intra-and intermolecular photosensitization. In contrast, 2Z and 3Z did not show such switching behavior, but consistently gave a single diastereomer of up to 9% and 43% de, respectively, over a wide range of substrate concentrations (0.04-50 mM), for which totally different mechanisms are responsible. Thus, the intra-and intermolecular sensitizations of 2Z exhibit the same diastereoselectivity, while only the intermolecular sensitization of 3Z can occur as a result of the hindered intramolecular approach of the benzoate substituent to the cyclooctene moiety.

Effective synthesis of bicyclodienes via palladium-catalyzed asymmetric allylic alkylation and ruthenium-catalyzed cycloisomerization

“…(Eluent for all: PE/EtOAc: 10/1). The isolated products correspond to the published NMR-data [66][67][68][69].…”

“…cyclopent-2-enyl benzoate: compound 3a: colorless liquid. Yield: 70%: It was synthesized according to the published procedure [66][67][68][69]: the spectroscopy data correspond to the data in ref. [66][67][68][69].…”