Process for preparation of amisulpride

1) A process for the preparation of amisulpride (I) which involves following steps:

• 1) methylation of 4-amino-salicylic-acid (VI) to 4-amino-2-methoxy methyl benzoate (VII), optionally in the presence of phase transfer catalyst,
• 2) conversion of 4-amino-2-methoxy methyl benzoate (VII) to 4-amino-2-methoxy-5-thiocyano methyl benzoate (VIII),
• 3) ethylation of 4-amino-2-methoxy-5-thiocyano methyl benzoate (VIII) to form 4-amino-2-methoxy-5-ethyl thio methyl benzoate (X),
• the intermediate compound (X) is converted to amisulpiride (I) by two routes (a) or (b):
• Route (a):
• 4a) hydrolysis of 4-amino-2-methoxy-5-ethyl thio methyl benzoate (X) to 4-amino-2-methoxy-5-ethyl thio benzoic acid (IX),
• 4b) oxidation of 4-amino-2-methoxy-5-ethyl thio benzoic acid (IX) with a suitable oxidizing agent in presence of sodium tungstate or ammonium molybdate to give 2-methoxy-4-amino-5-ethyl-sulfonyl benzoic acid (IV),
• Route (b):
• 5a) oxidation of 4-amino-2-methoxy-5-ethyl thio methyl benzoate (X) to 2-methoxy-4-amino-5-ethyl-sulfonyl methyl benzoate (XI) with a suitable oxidizing agent,
• 5b) hydrolysis of 2-methoxy-4-amino-5-ethyl-sulfonyl methyl benzoate (XI) to give 2-methoxy-4-amino-5-ethyl-sulfonyl benzoic acid,(IV and;
• 6) coupling of 2-methoxy-4-amino-5-ethyl-sulfonyl benzoic acid (IV) with 1-ethyl-2-amino methyl pyrrolidine (V)