Photocyclization of 2,6-Dichlorocinnamic Acid Derivatives to 5-Chlorocoumarin

On UV irradation, 2,6-dichlorocinnamic acid and its esters undergo photocyclization with elimination of the elements of HC1 or RC1 (R = alkyl or aryl) to yield 5-chlorocoumarin (2); amide derivatives yield the corresponding imino analogues. Low-temperature irradiation monitored by infrared and optical spectrophotometry allowed the identification of o-quinomethyl ketene as one of the intermediates of this reaction and suggested a mechanism for the photocycloelimination.