Synthesis of carbon‐ 14 labelled antimicrobial agents, I, synthesis of 1,4‐dihydro‐1‐methoxy‐6,7‐methylenedioxy‐4‐oxo‐quinoline‐3‐carboxylic‐3‐ 14 C acid (AB‐206‐3‐ 14 C)

1,4‐Dihydro‐1‐methoxy‐6,7‐methylenedioxy‐4‐oxoquinoline‐3‐carboxylic acid (I) (AB‐206), a new synthetic antimicrobial agent, was labelled with carbon‐14 at C‐3 position of the quinolinone ring for metabolic studies. The synthesis was achieved according to the reaction schemes shown in Fig. 1 and 4; which involved a newly devised key‐reaction, the condensation of diethyl malonate‐2‐ 14 C with ethyl orthoformate and 3,4‐methylene‐dioxyaniline in the presence of anhydrous zinc chloride as a catalyst to ethyl α‐carbethoxy‐β‐(3,4‐methylenedioxyanilino) acrylate‐2‐ 14 C (III). The overall radiochemical yield of AB‐206‐3‐ 14 C (I) was 9.6% from methanol‐ 14 C.