FORM2
THE PATENT ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
1. TITLE OF THE INVENTION
"METHOD FOR PRODUCTION OF 2,3-DIHYDROXY-l,2-BENZlSOTHIAZOL-3-ONE-l,l-DIOXIDE"
2. APPLICANT
1. NAME : OMKAR SPECIALITY CHEMICALS LTD.
2. NATIONALITY: INDIAN
3. ADDRESS : B-34, M.I.D.C. BADLAPUR (E), DIST-THANE,
MAHARASHTRA, PIN CODE : 421503, INDIA
The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
Present invention relates to an improved method for preparing 2, 3-dihydroxy-l,2-benzisothiazol-3-one-l,l-dioxide from alkyl anthranilate. More specifically it involves diazotization of alkyl anthranilate, treating obtained diazotized compound with thionyl chloride, monovalent or divalent copper salt and water to get alkyl benzoate-o-sulfochloride and alkyl benzoate-o-sulfinic acid. These products are first oxidized and then subjected to amidation to obtain 2, 3-dihydroxy-l,2-benzisothiazol-3-one-l,l-dioxide. This invention also provides a process for producing pure sodium salt of 2, 3-dihydroxy-l, 2-benzisothiazol-3-one-1,1 -dioxide.
BACKGROUND OF THE INVENTION
Over the last century, 2,3-dihydroxy-l,2-benzisothiazol-3-one-l,l-dioxide and its salts have been used in a variety of beverages, foods, cosmetics and pharmaceuticals. Its primary function is to provide sweetness without adding calories, and it is used in the foods and beverages such as soft drinks, fruit juices, in other beverages bases or mixes; table-top sweeteners in tablet, powder or liquid form; processed fruits; chewing-gum and confections; gelatin desserts, jams and toppings; and sauces and dressings.
Worldwide consumption of 2, 3-dihydroxy-l,2-benzisothiazol-3-one-1,1 -dioxide and its salts in 1995 was approximately 28,000 tons and number of companies around the world manufacture 2,3-dihydroxy-l,2-benzisothiazol-3-one-l,l-dioxide and its salt forms. Most commonly used method for synthesis of 2, 3-dihydroxy-l, 2-benzisothiazol-3-one-l, 1-dioxide is the basic synthetic route described by Remsen and Fahlberg in which toluene is treated with chlorosulfonic acid to produce ortho- and para-toluenesulfonyl chloride. Subsequent treatment with ammonia forms the corresponding toluene sulfonamides. Ortho-toluene - sulfonamide is separated from mixture of para and ortho-toluenesulfonamide. which is further then oxidized and heated to obtain 2,3-dihydroxy-l,2-benzisothiazol-3-one-1,1-dioxide, drawback of above process is occurrence of impurity i.e. ortho-toluenesulfonamide in 2,3-dihydroxy-l,2-benzisothiazol-3-one-1,1 -dioxide.

US patent 4464537 disclosed a process for 2.3-dihydroxy-1.2-benzisothiazol-3-one-l,l-dioxide synthesis by diazotization of methyl anthranilate, for this reaction maintaining desired temperature involves high energy consumption at commercial plant level. Second drawback of above method is use of hazardous fumes of sulphur dioxide for sulfonation of 2-carbomethoxy-benzenediazonium chloride, maximum yield by this process is about 93.1%.
German patent no. 4564 disclosed a synthesis of 2, 3-dihydroxy-l, 2-benzisothiazol-3-one-1, 1-dioxide which also involves diazotizing an anthranilic acid ester and reacting the diazonium salt with SO?. This process is however evidently unsatisfactory, both economically and technically, because, in order to avoid handling substantial volumes of water, the diazotization must be carried out with nitrous fumes and the further conversion must be carried out with liquid SO2.
Prior arts methods mentioned herein above either generate impurities which are difficult to remove from the products and/or have used sulfur dioxide as a reactant either in gaseous form or as liquid. However, it has been noticed that sulfur dioxide when used in gaseous form, handling due to corrosive nature is difficult, and charging measured amount in reaction becomes difficult. Secondly the gas is easily liquefied, dead volume remaining in between the valves and piping are to be drained with utmost care and safety measures are to be followed strictly. Withdrawal and charging rates of sulfur dioxide from cylinders to reactors should be in controlled manner, if faster rates are applied it will lead to pipe sweating and frost formation, in extreme cases ice formation thus lowering the rate of input of measured amount of gas required for the process. Sulfur dioxide cylinders require special storage.
Since 2, 3-dihydroxy-l, 2-benzisothiazol-3-one-l, 1-dioxide is a mass produced product it has been always important to develop a simplified process and also to increase the efficiency of the process. Inventors of the present invention have developed an improved process having in-situ generation of sulfur dioxide through the reaction of thionyl chloride and acidic water from diazotized mass which simplifies the procedure as it replaces use of gaseous cylinders either to be used as gas or liquid. In the present invention thionyl

chloride when reacts with water from the acidic diazotized mass produces sulfur dioxide and hydrogen chloride out of which sulfur dioxide is consumed towards the preparation of 2, 3-dihydroxy-l, 2-benzisothiazoI-3-one-l, 1-dioxide.
The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
OBJECT OF THE INVENTION
• The main objective of the present invention is to provide an improved method for the preparation of 2, 3-dihydroxy-l,2-benzisothiazol-3-one-l,l- dioxide.
• Another object of the present invention is to provide a process for synthesis of 2. 3-dihydroxy-1,2-benzisothiazo!-3-one-l,1-dioxide and salts thereof which involves in-situ formation of sulphur dioxide gas for sulfonation, hence avoid handling of hazardous and toxic fumes of sulphur dioxide.
• Yet another object of the present invention is to provide method for preparing 2,3-dihydroxy-l,2-benzisothiazol-3-one-l. 1-dioxide with ease and convenience.
• Further object of the present invention is to pure sodium salt of 2,3-dihydroxy-l,2-benzisothiazol-3-one-1,1 -dioxide.
SUMMARY OF THE INVENTION
Present invention provides a process for preparation of 2, 3-dihydroxy-l, 2-benzisothiazol-3-one-1,1 -dioxide comprising:
a) diazotizing alkyl anthranilate,
b) mixing obtained diazotized product of step a) with thionyl chloride taken in water immiscible solvent, monovalent or divalent copper salt and water,
c) discarding the aqueous layer and oxidizing the remaining reaction mixture,

d) amidating the resultant mixture of step c) to form 2, 3-dihydroxy-l, 2-benzisothiazol-3-one-l,l-dioxide.
It further provides a process for producing pure sodium salt of 2, 3-dihydroxy-1, 2-benzisothiazoI-3-one-l,l-dioxide comprising treating 2, 3-dihydroxy-l, 2-benzisothiazol-3-one-1,1-dioxide with sodium hydroxide and purifying obtained sodium salt by giving carbon and sodium dithionite treatment.
DESCRIPTION OF THE INVENTION
While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.
To facilitate the understanding of this invention, a number of terms are defined below. Terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as "a", "an" and "the" are not intended to refer to only a singular entity, but include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the invention, but their usage does not delimit the invention, except as outlined in the claims.
Present invention relates to improved method for preparing 2,3-dihydroxy-l,2-benzisothiazol-3-one-l,l-dioxide. This method utilizes in-situ formed sulphur dioxide, making it easy, simple and convinent to carry at commercial scale. It also provides the process for preparation of its pure sodium salt.
Accordingly, in one aspect the invention provides, a process for preparation of 2, 3-d'ihydroxy-1, 2-benzisothiazol-3-one-1,1 -dioxide comprising:
a) diazotizing alkyl anthranilate,

b) mixing obtained diazotized product of step a) with thionyl chloride taken in water
immiscible solvent, monovalent or divalent copper salt and water,
c) discarding the aqueous layer and oxidizing the remaining reaction mixture,
d) amidating the resultant mixture of step c) to form 2. 3-dihydroxy-l, 2-benzisothiazoi-3-one-1.1-dioxide.
The reaction scheme is represented below as scheme 1:

The reaction step a) is the diazotiasation reaction carried out by reacting alkyl anthranilate with an alkali metal salt of nitrous acid in presence of aqueous hydrochloric acid. Anthranilates. advantageously are the lower alkyl e.g. methyl anthranilate. ethyl anthranilate or isobuty! anthranilate.

This reaction is preferably carried out at temperature below 10°C and more preferably at or below 0°C. The temperature range is achieved by addition of ice as cubes or slab. Other sophisticated techniques for chilling are also available. The alkali metal salts of nitrous acid preferably alkali metal nitrite used in amount required to complete diazotization reaction. The non limiting examples of such alkali metal nitrite include sodium nitrite, potassium nitrite etc.
The reaction step b) is mixing step where diazotized product of step a) is mixed with thionyl chloride taken in water immiscible solvent, monovalent or divalent copper salt and water. If the product is directly employed from step a) then it is already in the water medium and no additional water may be required but, if it is dried before using in step b) then, sufficient water shall be added to it. Also, if the copper salt is added in its aqueous solution form then water quantity may be sufficient eliminating the requirement of additional water in mixing step b). Preferable water immiscible solvent is one or more selected from group consisting of ethylene dichloride, methylene dichloride or chloroform. The non limiting examples of monovalent or divalent copper salts are cuprous chloride, cupric chloride etc. These salts are suitably employed either alone or in the form of mixture thereof. The temperature for mixing is preferably below 10°C and after completing addition it is increased gradually to 50°C. The mixture is stirred for 60 minutes to 120 minutes and more preferably for 90 minutes while maintaining temperature between 30°C to 70°C. After this, the mixture is allowed to settle and aqueous layer is discarded.
The remaining reaction mass is oxidized using suitable oxidizing agents. The non limiting examples of such oxidizing agents are hydrogen peroxide, chlorine etc.
After oxidation, the resultant mixture is subjected to amidation to form 2, 3-dihydroxy-l, 2-benzisothiazol-3-one-l,l-dioxide. Preferably, amidation is carried out with aqueous ammonia solution and at pH above 10. The product is obtained by separating the aqueous phase and acidifying it to precipitated product. Suitable mineral acids or any other acid donating H+ ions or any mixture thereof may be employed for acidification of aqueous phase below pH 1. Hydrochloric acid is amongst one of them.

According to second aspect of the invention, a process for producing pure sodium salt of 2, 3-dihydroxy-l, 2-benzisothiazol-3-one-1,1-dioxide comprising treating 2, 3-dihydroxy-l, 2-benzisothiazol-3-one-1,1 -dioxide with sodium hydroxide and purifying obtained sodium salt by giving carbon and sodium dithionite treatment. The salt product obtained through this purification is devoid of color impurities also.
The purification process is as shown in below scheme 2.

The details of the invention, its objects and advantages are explained hereunder in greater detail in relation to the non-limiting exemplary illustrations.
EXAMPLES:
Ingredients of composition in weight percentage range or in other unit in following conditions:
EXAMPLE 1: Preparation of 2,3-dihydroxy-l,2-benzisothiazol-3-one-l,l-dioxide
In a reaction vessel one, (0.00032 mmoles) of concentrated hydrochloric acid and 9 liter water with 7.5 kg ice were added under stirring. Then (0.00009 mmoles) of methyl anthranilate was added slowly to above solution. The reaction mass was chilled to 0°C and added with 15 liter aqueous solution of sodium nitrite (containing (0.0001 mmoles) of sodium nitrite in water). The diazotization completion was confirmed using starch-iodide paper.
In second reaction vessel prepared the solution of cupric chloride by adding (0.00002 mmoles) of cupric chloride in 20 liter water. In third reaction vessel prepared the mixture of (0.00019 mmoles) of thionyl chloride by mixing in 48.75 kg ethylene dichloride.

In reaction vessel one, under vigorous stirring poured the cupric chloride solution (0.016
mole) and thionyl chloride mixture (0.0002 mmoles) at controlled rate below temperature
10°C which was after completion of addition, heated to 50°C and maintained for 90
minutes. Then the reaction mass was allowed to settle and the aqueous layer was discarded
after extracting with fresh ethylene dichloride. The extracts were added to ethylene
chloride layer and 0.0001 mmoles of chlorine gas was passed slowly through it by
maintaining temperature between 20°C to 30°C. After chlorine treatment, 0.0011 mmoles
of 28% aqueous ammonia solution was added and stirred for 30 minutes at pH more than
10.
After maintaining mass at 35°C to 40°C for 60 minutes the phases were allowed to separate
and aqueous layer was acidified with concentrated hydrochloric acid to pH below 1. 2,3-
dihydroxy-1,2-benzisothiazol-3-one-l,l-dioxide was precipitated out in the form of white
crystal slurry.
Yield: 90%.
Melting point: 225°C to 230°C.
EXAMPLE 2: Preparation of pure Sodium salt of 2,3-dihydroxy-l,2-benzisothiazol-3-one-l,l-dioxide:
Product of example 1 was dissolved in aqueous sodium hydroxide solution (0.00006
mmoles NaOH in 12.5 liter of water). pH was adjusted to 7.0 to 7.5. Added carbon and
(0.0005 mmoles) sodium dithionite to absorb the colour impurities. Filtered and
concentrated to specific gravity 1.29-1.31 at 90°C. Cooled and centrifuged at 10°C to
20°C.
The isolated sodium 2,3-dihydroxy-l,2-benzisothiazol-3-one-l,1 -dioxide was air dried at
35°C to 40°C.
Melting point: 227°C to 229°C
Purity by HPLC: 99%
Yield: 95%.
All of the compositions and/or methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the

compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the invention. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit scope and concept of the invention as defined by the appended claims.

WE CLAIM:
1. A process for preparation of 2, 3-dihydroxy-l, 2-benzisothiazol-3-one-1,1 -dioxide
comprising:
a) diazotizing alky! anthranilate,
b) mixing obtained diazotized product of step a) with thionyl chloride taken in water immiscible solvent, monovalent or divalent copper salt and water,
c) discarding the aqueous layer and oxidizing the remaining reaction mixture,
d) amidating the resultant mixture of step c) to form 2, 3-dihydroxy-l. 2-benzisothiazol-3-one-l, I-dioxide.

2. The process of claim 1, wherein diazotization of alkyl anthranilate in step a) is carried out by reacting alkyl anthranilate with an alkali metal salt of nitrous acid in presence of aqueous hydrochloric acid.
3. The process of claim 1, wherein alkyl anthranilate in step a) is selected from methyl anthranilate, ethyl anthranilate and isobutyl anthranilate.
4. The process of claim 3, wherein alkyl anthranilate is methyl anthranilate
5. The process of claim 1, wherein the mixing in step b) is done below the temperature 10°C and after complete mixing temperature is slowly increased to maintain within the range of 30°C to 70°C for 60 to 120 minutes.
6. The process of claim I, wherein water immiscible solvent in step b) is one or more selected from group consisting of ethylene dichloride, methylene dichloride or chloroform.
7. The process of claim 6, wherein the water immiscible solvent in step b) is ethylene dichloride.

8. The process of claim I, wherein monovalent or divalent copper salt in step b) is selected from group consisting of cuprous chloride and cupric chloride and any mixture thereof.
9. The process of claim 1, wherein the oxidization of remaining reaction mixture in step c) is carried out with hydrogen peroxide or with chlorine.
10. The process of claim 9, wherein the oxidization of remaining reaction mixture in step c) is carried out with chlorine gas.
11. The process of claim 1, wherein the amidation in step d) is carried out with aqueous ammonia solution and at pH above 10 to form 2, 3-dihydroxy-l, 2-benzisothiazol-3-one-1,1-dioxide.
12. The process of claim 11, further comprises isolating formed 2, 3-dihydroxy-l, 2-benzisothiazol-3-one-l,l-dioxide from aqueous phase by acidifying the aqueous phase to precipitate the product.
13. A process for producing pure sodium salt of 2, 3-dihydroxy-l, 2-benzisothiazol-3-one-1,1-dioxide comprising treating 2, 3-dihydroxy-l, 2-benzisothiazol-3-one-1,1-dioxide with sodium hydroxide and purifying obtained sodium salt by giving carbon and sodium dithionite treatment.