On the reaction of anthranilic acid with thionyl chloride : Iminoketene intermediate formation

It has been reported that sulfinamide anhydride is formed when anthranilic acid is treated with thionyl chloride under anhydrous conditions. Garin et al. reported that no sulfinamide anhydride was formed in the reaction of anthranilic acid with thionyl chloride. They reported that 2-(sulfinylamino) benzoyl chloride was formed, in accordance with an earlier observation, under the same reaction conditions. Our interest in the addition reactions of 2-(sulfinylamino) benzoyl chloride and the physiological activities of its products prompted us to re-examine the reaction between anthranilic acid and thionyl chloride. The present paper not only confirms the formation of 2-(sulfinylamino) benzoyl chloride in the reaction of anthranilic acid 1 with thionyl chloride but also suggests the formation of iminoketene 7 under the reaction conditions. 2-(Sulfinylamino) benzoyl chloride 2 on treatment with azomethines 3a-d in dry benzene solution afforded dihydroquinazolin-4 (1H)-ones 4a-d in good yield along with a small amount of dibenzo [b,f] [1,5] diazocine-6,12-(5H,11H)-dione 5 (Scheme I, Table I). Dibenzo [b,f] [1,5] diazocine-6,12-(5H,11H)-dione 5 was also isolated, on keeping 2 for 24 hr. The formation of 4a-d and 5 was envisaged through the formation of an iminoketene intermediate. The course of the reaction was monitored by GCMS and IR spectrometers. The reaction of anthranilic acid and thionyl chloride was carried out under dry nitrogen and the reaction product was distilled under reduced pressure to remove excess thionyl chloride and HCl. An immediate GCMS analysis under dry conditions of the reaction mixture indicated the presence of 2-(sulfinylamino) benzoyl chloride (m/z = 201). No peak was observed at m/z = 183, indicating the absence of sulfinamide anhydride as proposed by Kametani et al.. Besides the absence of peak at m/z = 119 also indicated the absence of formation of iminoketene under the dry conditions. However when the reaction mixture was exposed to atmosphere for 35 minutes, peak at m/z 119 (45%) and also at m/z 64 (100%), 36 (25%) and 38 (8.2%) were developed plausibly due to the formation of iminoketene, sulphur dioxide, HCl and HCl respectively.