The stereochemical structures of the four adducts formed between oleic acid methyl ester (OAME) and 4,5\u27,8-trimethylpsoralen (tmPso) have been determined. Assignment of the tmPso \u27H NMR spectrum was accomplished by analogy to two coumarin model compounds and with the use of homonuclear decoupling and resonance enhancement. Assignment of the lH NMR spectra for the OAME-tmPso adducts was made by analogy to the spectra of OAME and tmPso and using 2D &resolved and COSY analyses. The configurations of the cyclobutyl rings in these adducts was determined by The stereochemical structures of the four adducts formed between oleic acid methyl ester (OAME) and 4,5\u27,8-trimethylpsoralen (tmPso) have been determined. Assignment of the tmPso \u27H NMR spectrum was accomplished by analogy to two coumarin model compounds and with the use of homonuclear decoupling and resonance enhancement. Assignment of the lH NMR spectra for the OAME-tmPso adducts was made by analogy to the spectra of OAME and tmPso and using 2D &resolved; and COSY analyses. The configurations of the cyclobutyl rings in these adducts was determined by MM2 energy minimization calculations, homonuclear \u27H NOE analysis, and comparison of products obtained with cis-OAME and trans-EAME (elaidic acid methyl ester). Only four of the eight possible disastereomeric adducts are detected. These adducts have the cis-cis- , cis-cis-HT, trans-cis- , and trans-cis-HT configurations. The lack of formation of the other isomers may be due to the geometric requirements of exciplex formation. The mechanism of the reaction was established to involve initial bond formation at the 4 position of tmPso, most likely to form a diradical intermediate. The rate of dissociation of the trans diradical is much faster than ring closure, in contrast to the cis diradical whose rate of ring closure is at least as fast as dissociation. The rate of cis-trans isomerization of the 9,lO-bond of the fatty ester portion of the diradical is faster than ring closure for the cis diradical and slower than ring closure for the trans diradical
