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 occurs 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 photostationary state of the cis/trans acids is formed early into the reaction, The photocyclization of the dichloro analog is more efficient (Phi exceeds 0.04) and therefore time dependent since product formation competes with trans/cis isomerization. Methyl o-chlorocinnamate also photocyclizes (Phi for the PSS = 0.0022 in acetonitrile) but the o-methoxy ester is virtually photoinert, It is proposed that the acid photocyclizes through intramolecular nucleophilic attack by the carboxylate group followed by heterolysis of the nucleofuge. Methyl o-chlorocinnamate appears to photocyclize through a [4 + 2] cycloaddition of the carbonyl group followed by homolysis of the Cl and Me moieties, possibly through the intermediacy of a ketene as proposed by earlier workers.