A process for the preparation of a compound of formula I:wherein,R1 represents hydrogen, cyano, nitro, halogen, or acylR2 represents aryl, alkenyl, alkynyl, halogen substituted alkyl, halogen substituted alkenyl, or halogen substituted alkynylR3 represents hydrogen or NR6R7 wherein R6 and R7 each independently represent hydrogen, alkyl, alkenylalkyl, alkynylalkyl, formyl, optionally halogen substituted alkanoyl, optionally halogen substituted alkoxycarbonyl, or alkoxymethyleneamino, halogen, or R6 and R7 together with, the N atom attached form a heterocycle andR4 represents hydrogen, alkyl, aryl, or heteroarylthe process including oxidizing a compound of formula II:
