Chiroptical Studies of Labile or Difficult-to-Resolve Molecules Generated by Chiral Laser Photochemistry. 2. Products and Steric Course of the Phototransformation of the Racemate

Article abstract of DOI:10.1021/ja00345a026

This paper deals with a complex photoresolution of a racemate with circularly polarized light (CPL) which involves the phototransformation of the racemate into a chiral, labile compound.We show here how to obtain both the dichroism for the pure enantiomers and their absolute configuration, besides defining the stereochemical course for the phototransformation.This is exemplified by the irradiation of racemic 1-methoxybicyclo<3.2.0>hepta-3,6-dien-2-one (1) with ultraviolet, left CPL which led to a mixture containing both the photoresolved (-)(1S,5R)-1, with 0.3percent optical purity and Δε_max(365) = -2.4, and the labile product of the phototransformation of the racemate, (-)(1S,5S)-7-methoxybicyclo<3.2.0>hepta-3,6-dien-2-one ((-)(1S,5S)-2), with 1.6percent optical purity and Δε_max(350) = -2.0.Acidification of this mixture led to chemically and optically stable methyl (-)(1R)-4-oxo-2-cyclopentene-1-acetate ((-)(1R)-7), with 1.6percent optical purity, Δε_max(320) = -0.39, by selective hydrolysis of 2.The absolute configuration for 2 was assigned by correlation with the product of catalytic reduction of (-)(1R)-7, i.e., methyl (+)-3-oxocyclopentane-1-acetate, whose absolute configuration, (1R), is independently known.The above results show that the 1 -> 2 phototransformation is attained by complete inversion of configuration at the bridgeheads.The Kroning-Kramers theorem helped to elucidate the chiroptical properties for compounds 1, 2, and 7.