1. An improved process for the preparation of Metolazone (1)
W
comprising:
a) oxidizing N-(5-chloro-2-methyl-4-suIfamoyIphenyl) acetamide (5)
l
in presence of a suitable oxidizing agent and base to form 2-acetamido-4-chloro-5-sulfamoylbenzoic acid (4);
b) condensing intermediate (4) with O-toluidine (3)
in presence of a suitable carboxyl activating agent to form 7-chloro-2-methyl-4-oxo-3-o-tolyl-3,4-dihydroquinazoline-6-sulfonamide (2);

c) reducing intermediate (2) using a suitable reducing agent to obtain 7-chloro-2-methyI-4-oxo-3-o-tolyI-l,2,3,4-tetrahydroquinazoIine-6-sulfonamide (Metolazone) (1); and
d) purifying Metolazone (1) from suitable solvents.

2. The process according to claim 1, wherein the oxidizing agent is selected from a group comprising of potassium permanganate, potassium dichromate, potassium nitrate.
3. The process according to claim 1, wherein the base used in step a) may be selected from a group comprising triethyl amine, diisopropylethylamine, diethyl amine, isopropyl amine, morpholine, N-methyl morpholine, pyridine, ammonia, potassium tertiary butoxide, potassium ethoxide, sodium methoxide, lithium tertiary butoxide, , lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, cesium hydroxide , sodium carbonate, potassium carbonate, magnesium carbonate, sodium bicarbonate, potassium bicarbonate or mixtures thereof.
4. The process according to claim 1, wherein the carboxyl activating agents used in step b) is selected from a group comprising of phosphorus trichloride, phosphorous pentachloride, thionyl chloride, t-butyloxycarbonyl hydrazide or mixtures thereof.
5. The process according to claim 1, wherein the reducing agents used in step c) is selected from a group comprising of sodium borohydride, lithium aluminium hydride, diisobutyl aluminium hydride (DIBAL), lithium triethyl borohydride or mixtures thereof.

6. A process for the purification of Metolazone (1) comprising of:
1. providing a solution of Metolazone (1) in a suitable aprotic solvent;
2. filtering the reaction mixture through micron filter;
3. removing the solvent under vacuum;
4. adding suitable protic solvent to the solid;
5. heating the reaction mixture; and
6. isolating Metolazone (1).

8. The process according to claim 6, wherein the solvent is selected from a group comprising of acetone, acetonitrile, ethyl acetate, dichloromethane, methyl tertiary butyl ether, water, methanol, ethanol, n-propanol, n-butanol t-butanol, isopropyl alcohol or mixtures thereof.
9. The process according to claim 1, wherein the intermediate (5) is prepared comprising:

a) Acetylation of 5-chloro-2-methylalinine (8) to form N-(5-chloro-2-methyl phenyl) acetamide (7);
b) sulphonation of intermediate (7) to form 4-acetamido-2-chloro-5-methylbenzene-1-sulfonyl chloride (6) in presence of a suitable sulphonating agent; and
c) amination of intermediate (6) in presence of aqueous ammonium hydroxide to form N-(5-chloro-2-methyl -4-sulfamoylphenyl) acetamide (5).
10. The process according to claim 9, wherein the suitable sulfonating agent used
in the present invention is selected from a group comprising of chlorosulfuric
acid, sulphuric acid, sulphonic acid and oleum.