Photocyclization of Ortho-Substituted Cinnamic Acids

Article abstract of DOI:10.1021/jo00112a018

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 (Φ 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 (Φ exceeds 0.04) and therefore time dependent since product formation competes with trans/cis isomerization.Methyl o-chlorocinnamate also photocyclizes (Φ 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.