PHOTOCYCLIZATION OF ORTHO-SUBSTITUTED CINNAMIC-ACIDS

Mono and di (i.e. 2, 6) o-chloro- and o-methoxycinnamic acids undergo photocyclization to give the corresponding coumarins. The reaction occ urs in aqueous and organic media, with a prototypical reaction giving evidence of being favored at pH > 6. Cyclization of the dimethoxy acid is relatively inefficient (Phi for the PSS = 0.0015), and a photostat ionary state of the cis/trans acids is formed early into the reaction, The photocyclization of the dichloro analog is more efficient (Phi ex ceeds 0.04) and therefore time dependent since product formation compe tes with trans/cis isomerization. Methyl o-chlorocinnamate also photoc yclizes (Phi for the PSS = 0.0022 in acetonitrile) but the o-methoxy e ster is virtually photoinert, It is proposed that the acid photocycliz es through intramolecular nucleophilic attack by the carboxylate group followed by heterolysis of the nucleofuge. Methyl o-chlorocinnamate a ppears to photocyclize through a [4 + 2] cycloaddition of the carbonyl group followed by homolysis of the Cl and Me moieties, possibly throu gh the intermediacy of a ketene as proposed by earlier workers.