The photochemical reactions of some azo derivatives of substituted pyrazoli
n-5-one were examined. Irradiation of 4-arylazopyrazolin-5-one 1-3 gave the
corresponding hydrate derivatives 4-6 via reduction of the ate group and r
earrangement of the pyrazolinone ring. The photo a-cleavage of (-CO-N-Ph) i
n the pyrazolinone ring is suggested leading to the formation of a diradica
l. component followed by cyclization to give the final rearrangement produc
ts.