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1. (WO2019016834) AN IMPROVED PROCESS FOR THE PREPARATION OF THIABENDAZOLE
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We claim:

1. An improved process for the preparation of thiabendazole of formula (I), which comprises the steps of:


(I)

(a) reacting 4-cyanothiazole of formula (II) with aniline of formula (III) in presence of hydrogen chloride gas and o-dichlorobenzene at atmospheric pressure, at a temperature between 135 to 150°C to obtain amidine hydrochloride of formula (IV);


(b) treating amidine hydrochloride of formula (IV) with 1.07 to 1.17 equivalents of sodium hypochlorite in presence of sodium carbonate to control an amount of 4-chloro thiabendazole and/or 5-chloro thiabendazole impurities, to obtain crude thiabendazole of formula (I); and

(c) purification of crude thiabendazole to pure thiabendazole of formula (I).

2. The process as claimed in claim 1 wherein, the lot wise addition of sodium hypochlorite is preferred in the said step (b).

3. The process as claimed in claim 1 wherein, an amount of 4-chloro thiabendazole (V) impurity is preferably not more than 0.80% w/w, more preferably not more than 0.50% w/w.

(V)

4. The process as claimed in claim 1 wherein, an amount of 5-chloro thiabendazole (VI) impurity is preferably not more than 0.80% w/w more preferably not more than 0.50% w/w.


(VI)

5. Thiabendazole as prepared by the process claimed in claim 1 wherein, an amount of 4-chloro thiabendazole (V) impurity is preferably not more than 0.80% w/w, more preferably not more than 0.50% w/w.

6. Thiabendazole as prepared by the process claimed in claim 1 wherein, an amount of 5-chloro thiabendazole (VI) impurity is preferably not more than 0.80% w/w, more preferably not more than 0.50% w/w.

7. Thiabendazole as prepared by the process claimed in claim 1 wherein, an amount of residual solvent methanol is not more than 3000 ppm and benzene is not more than 2 ppm.

8. Thiabendazole as prepared by the process claimed in claim 1 wherein, an amount of selenium is not more than 30 ppm, cobalt is not more than 5 ppm and molybdenum is not more than 300 ppm.

9. The process for purification of thiabendazole which comprises;

(i) converting crude thiabendazole to solid thiabendazole hydrochloride using concentrated hydrochloric acid in water;

(ii) charcoalizing thiabendazole hydrochloride;

(iii) filtering the solid thiabendazole hydrochloride;

(iv) dispersing thiabendazole hydrochloride in water;

(v) further treating with aqueous ammonia solution to obtain free thiabendazole; and

(vi) isolating pure thiabendazole by filtration.