RELATED APPLICATIONS

The present application claims the benefit of priority to Indian complete patent application No 202121028288 filed on June 23, 2021 and entire complete specifications.

FIELD OF THE INVENTION

The present invention provides a process for preparation of alicyclic acid dianhydride.

BACKGROUND OF THE INVENTION

The present invention relates to a process of preparing an alicyclic acid dianhydride characterized by cyclization of 2, 3, 5 - tricarboxylic acid cyclopentane acetic acid (TCMP) without a catalyst.

TCMP is a useful compound in the field of electronic materials as a raw material for polyimide, a curing agent for epoxy resin, and the like in terms of heat resistance, light transmission, electrical characteristics (insulation, dielectric constant), and the like. JP2004182658 disclosed a method for producing 1, 2, 4-tricarboxy-3- carboxymethylcyclopentane (TCMP), characterized by nitric acid oxidation of hydroxydicyclopentadiene.

US3080418 disclosed a vanadium pentoxide and sodium nitrite catalyzed process for the preparation of (trans,cis,trans-l,2,4-tricarboxy-cis-3-carboxy) methyl cyclopentane. CN109503617 disclosed synthesis of 3-carboxymethyl-l,2,4-pentamethylene tricarboxylic acids-1, 4:2, 3-dianhydride involving ozone oxidation of dicyclopentadiene as the key step.

CN104193612 disclosed a preparation method of 2,3,5-tricarboxylic cyclopentyl acetic acid by a catalytic reaction between dicyclopentadiene, Osmium tetraoxide and Sodium periodate.

CN102533279 disclosed method for preparing polyamic acid including repeating units by copolymerization and imidization reaction that may be synthesized from an acid dianhydride and/or a diamine. W02021005888 disclosed synthesis of polyamic acid, liquid crystals alignment agent, liquid crystals that can be obtained by reacting a tetracarboxylic dianhydride and a diamine compound.

JP2005208465 disclosed a polyamic acid obtained by reacting with diacid anhydride having a specific structure with a diamine. The diamine used in the patent document is only of two types having one thioether group (-S-) is described in which the benzene rings are mainly bonded by an ether group (-0-).

JP2008274234 disclosed a resin composition having a structure with one or more polyamic acids, a cured product obtained by curing the resin composition, and an optical member made of the same. A conventional method for producing alicyclic acid dianhydride, a known method available in JP4035365B2 in which acetic anhydride and tetracarboxylic acid are subjected to toluene under reflux for a few hours. However this method causes intense heat generation, temperature control during the progress of the reaction becomes challenging.

Hence, there is need to provide an industrial production method by improving the yield of the target product as lesser by-products are produced, easy temperature control by improving the reaction conditions to mild conditions, reducing the generation of by product NO2 gas, and mixing impurities in the target product.

As a result of intensive research in order to solve the above-mentioned problems, the inventors of the present invention have found an economical production method without using a catalyst that is advantageous in terms of yield and purification under the mild reaction conditions that can be carried out in the process, safe operation, high purity of product, pollution free, suitable in large scale industry.

In order to achieve the above objects, the present inventors have conducted diligent research and prepared a protocol for synthesis of 3 - methyl - 1,2,4 - pentamethylene three carboxylic acid - 1,4:2, 3- dianhydride and pharmaceutically acceptable salts thereof.

OBJECTIVE OF THE INVENTION

An objective of the invention is to provide a method for synthesis of 3 - methyl - 1,2,4 - pentamethylene three carboxylic acid - 1,4:2, 3- dianhydride with optimized reaction conditions. Another objective of the invention is to provide an efficient method for synthesis of 3 - methyl - 1,2,4 - pentamethylene three carboxylic acid - 1,4: 2, 3- dianhydride with high yield.

Another objective of the invention is to provide an efficient method for synthesis of 3 - methyl - 1,2,4 - pentamethylene three carboxylic acid - 1,4:2, 3- dianhydride with higher purity.

Another objective of the invention is to provide an economical process for synthesis of 3 - methyl - 1,2,4 - pentamethylene three carboxylic acid - 1,4:2, 3- dianhydride.

Another objective of the invention is to provide time saving process for synthesis of 3 - methyl - 1,2,4 - pentamethylene three carboxylic acid - 1,4:2, 3- dianhydride.

Another objective of the invention is to provide a process with minimum steps for synthesis of 3 - methyl - 1,2,4 - pentamethylene three carboxylic acid - 1, 4:2,3- dianhydride.

SUMMARY OF THE INVENTION In one aspect the present invention relates to a process for preparation of alicyclic dianhydride.

According to another aspect, the present invention provides a process of preparation of 3 methyl -l,2,4-pentamethylene-3-carboxylic acid-1, 4:2, 3-dianhydride from alicyclic tetracarboxylic acid. According to another aspect, the present invention provides a process of preparing alicyclic dianhydride comprising of the following steps: (i) a. mixing 25% aq. sulphuric acid and dicyclopentadine by stirring continuously and further heating the reaction mixture to obtain hydroxydicyclopentadine;

(i) b. adding nitric acid dropwise to the mixture of solvent and hydroxydicyclopentadine (DCPD-OH) obtained in step (i)a while stirring, continuously stirring the mixture by maintaining the RPM below 120 to obtain crude alicyclic tetracarboxylic acid which is further recrystallized to obtain alicyclic tetracarboxylic acid;

(ii) adding an organic solvent to the alicyclic tetracarboxylic acid obtained in step (i)b and further subjecting the mixture to inert atmosphere; (iii) adding acetic anhydride to the reaction mixture of step (ii) by stirring and maintaining the inert atmosphere;

(iv) heating the reaction mixture obtained in step (iii) to obtain a clear solution, wherein the reaction mixture slowly starts precipitating into a solid, continuing heating in inert atmosphere; (v) cooling the reaction mixture obtained in step (iv) at room temperature, further stirring and then filtering the product, washing with organic solvent and drying under vacuum to obtain alicyclic dianhydride.

According to an embodiment, the present invention provides a process of preparing alicyclic anhydride wherein, the reaction mixture in step (i)a is stirred for over 13 to 16 hours; preferably 14 to 15 hours; More preferably 15 hours. According to an embodiment, the present invention provides a process of preparing alicyclic anhydride wherein, the reaction mixture in step (i)a is heated to 100-110 °C and maintained for 15 hrs.

According to another embodiment, the solvent used in step (i)b is selected from group consisting of aliphatic halogenated hydrocarbons; More particularly, the solvent is selected from group consisting of 1, 2-dichloroethane, nitromethane, nitroethane, acetic acid, combination thereof. More particularly solvent is 1 , 2-dichloroethane.

According to another embodiment, wherein in step (i)b nitric acid is added drop wise for over 5-6 hrs to the mixture of solvent and hydroxy dicyclopentadine while stirring. According to another embodiment, the reaction mixture in step (i)b is further continuously stirred for 10 to 12 hours.

According to an embodiment, the stirring in step (i)b is below 120 RPM.

According to an embodiment, the stirring in step (i)b is below 120 RPM; preferably RPM ranging between 90 to 130, both inclusive; more preferable RPM ranging between 100 to 120, both inclusive.

According to an embodiment, wherein the crude alicyclic tetracarboxylic acid obtained in step (i)b is purified by adding the crude product to ethyl acetate while stirring and heating to get clear solution; continuing stirring at room temperature to form white precipitate which is further filtered and washed with chilled Ethyl acetate and dried under vacuum to obtain the alicyclic tetracarboxylic acid.

According to a further embodiment, in step (ii) an organic solvent is added to alicyclic tetracarboxylic acid obtained in step (i)b and further the mixture is subjected to inert atmosphere.

According to another embodiment, acetic anhydride is added to the stirred reaction mixture obtained in step (ii) by maintaining the inert atmosphere.

According to another embodiment, the inert atmosphere is made of using inert gas is selected from the group consisting of N2, Ar, CO, CO2, ¾, CH4, and combinations thereof, more preferably nitrogen.

According to an embodiment, the organic solvent used in step (ii) is selected from substituted or unsubstituted aromatic hydrocarbons; particularly the organic solvents are selected from group consisting of benzene, toluene, tetrahydrofuran and combination thereof; more particularly organic solvent is toluene. According to another embodiment, the reaction mixture in step (iii) is stirred for 5 to 30 min; particularly 10 to 15 minutes.

According to a further embodiment, the reaction mixture obtained in step (iv) is heated at about 100 to 130°C, preferably at 120°C.

According to another embodiment, the reaction mixture in step (iv) is continuously heated at 120°C for about 2 to 3 hours. According to another embodiment, the reaction mixture in step (v) is cooled at 25 °C to 30°C and stirred for about 1 to 2 hours.

According to an embodiment, the molar ratio of nitric acid to hydroxydicyclopentadine is about 1:4 to 1: 6. According to an embodiment, the molar ratio of alicyclic tetracarboxylic acid to acetic anhydride is about 1:1.5 to 1:2 and more preferably 1: 1.7.

According to an embodiment, the organic solvent used in step (v) is selected from aliphatic hydrocarbons; particularly the solvents are selected from groups consisting of heptane, hexane and pentane; more preferably hexane. According to another embodiment, the present invention provides a process of preparation of 3-(Carboxymethyl)-l,2,4-cyclopentanetricarboxylic acid 1,4:2, 3 dianhydride comprising of the following steps:

(i) a. mixing 25% aq. sulphuric acid and dicyclopentadine by stirring continuously over 15 hours and further heating to obtain hydroxydicyclopentadine (a);

(i) b. adding nitric acid dropwise to hydroxydicyclopentadine (DCPD-OH) obtained in step (i)a in presence of a solvent and simultaneously stirring over 5 to 6 hours and stirring the mixture continuously for 10 to 12 hours characterized and RPM should be below 120 to obtain 2, 3, 5 - tricarboxylic acid cyclopentane acetic acid (b);

(ii) subjecting 2, 3, 5 - tricarboxylic acid cyclopentane acetic acid (b) of step (i)b and toluene to inert atmosphere;

(iii)stirring the reaction mixture obtained in step (ii) for 10 to 15 minutes and adding acetic anhydride by maintaining inert atmosphere; (iv) Heating the reaction mixture obtained in step (iii) at 100 to 120°C to obtain a clear solution which slowly starts precipitating continuously heating in inert atmosphere; and

(v) cooling the reaction mixture obtained in step (iv) at 25 to 30°C (room temperature) and further stirring, filtering, washing with hexane and dried in vacuum to obtain 3-(Carboxymethyl)-l,2,4-cyclopentanetricarboxylic acid

1,4:2, 3 dianhydride (c)

According to an embodiment, in step (i)a 25% aq. sulphuric acid and dicyclopentadine are mixed by continuous stirring. The reaction mixture is heated to 100-110°C and maintained for 15 hr. The reaction mass is allowed to stand for 30min at 50-55°C and layers are separated. The organic layer is washed with Hot- water followed by 10% sodium hydroxide solution, further the aqueous layer is again extract with ethyl acetate. Both the organic layer are combined and dried over sodium sulphate, distilled out at 60- 65°C to obtain hydroxydicyclopentadine. According to an embodiment, in step (i)b nitric acid is added drop wise to the mixture of hydroxydicyclopentadine and 1 ,2-dichloroethane which results into increase in the temperature of reaction mixture to 45-50°C. The mixture is stirred while adding the nitric acid over 5-6 h, the reaction temperature raised to a maximum of 60°C which is further gradually raised to 60-65°C, stirring is continued for 12 h and RPM should be below 120. Further reaction solution is collected and analyzed by LC-MS. The unreacted hydroxydicyclopentadine disappears and l,2,4-tricarboxy-3- corboxymethylcyclopentane (75-80% area) and 1,2,3,4-tetracarboxycyclopentane (10- 15% area) is obtained. The reaction solution is further distilled out completely under vacuum to obtain crude l,2,4-tricarboxy-3-corboxymethylcyclopentane. In another embodiment, in step (i)b crude l,2,4-tricarboxy-3- corboxymethylcyclopentane is heated with Ethyl acetate to get clear solution. This solution is further stirred for 10-20 min at room temperature to form white precipitate, the precipitate is filtered, washed with chilled ethyl acetate and dried under vacuum at 65-70°C to get highly pure 1, 2, 4-tricarboxy-3-corboxymethyl cyclopentane.

In another embodiment, the invention provides the process for preparation of alicyclic dianhydride without catalyst.

In another embodiment, the invention provides the process for preparation of alicyclic dianhydride with high purity and yield. In another embodiment, the invention provides the process for preparation of alicyclic dianhydride with purity more than 98%.

In another embodiment, the invention provides the process for preparation of alicyclic dianhydride with yield more than 80%.

In another aspect the present invention provide economical process for preparation of alicyclic dianhydride.

In another aspect the present invention provide the process for preparation of alicyclic dianhydride with minimum steps.

In another aspect the present invention provide process for preparation of alicyclic dianhydride which is economical yet suitable for large scale. DETAILED DESCRIPTION OF THE INVENTION

The present invention describes the process for the preparation of 3-methyl - 1, 2, 4 - pentamethylene 3-carboxylic acid -1,4:2, 3-dianhydride and pharmaceutically acceptable salts thereof. To clarify the above and other purposes, features, and advantages of this invention, specific embodiment of this invention is especially listed and described in detail with the examples as follows. The principal and mode of operation of this invention have been described and illustrated in its embodiment. At the outset, a person skilled in the art will appreciate that this invention may be practiced otherwise than is specifically described and illustrated. The invention should not be limited by the above described embodiment, method, and examples, but by all embodiments and methods within the scope and spirit of the invention. Also, in the following description of the invention, certain terminology may be used for the purpose of reference only, and is not intended to be limiting. Definitions and Use of Terms

As used in this specification and in the claims which follow, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "an ingredient" includes mixtures of ingredients.

Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value and/or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.

The present invention describes the process of preparation of 3 - methyl - 1, 2, 4 - pentamethylene three carboxylic acid - 1, 4:2, 3 - dianhydride and pharmaceutically acceptable salts thereof wherein in the first step, 2, 3, 5 - tricarboxylic acid cyclopentane acetic acid was synthesized using hydroxydicyclopentadine, nitric acid and 1,2-dichloroethane.

During the synthesis of 2, 3, 5 - tricarboxylic acid cyclopentane acetic acid, a part of the reaction solution was characterized by LC-MS wherein a major and minor product was formed, l,2,4-tricarboxy-3-corboxymethylcyclopentane was 75-80 area% and 1, 2,3,4- tetracarboxy cyclopentane was 10-15 area%. The reaction was stopped and the solvent was removed by distillation, obtained crude product to this Ethyl acetate was added and the mixture was further heated to get clear solution and stirred for 15 h at room temperature, white precipitate was formed, filter and washed with chilled Ethyl acetate, dried under vacuum at 65-75°C to get product with high Purity. Further the synthesis of alicyclic anhydride was performed following the method described below in Scheme I.

Scheme: I Step-I: Preparation of hydroxydicyclopentadine

Example 1: 500g of dicyclopentadiene and 1520 g of 25% sulphuric acid solution charge into 5L three-necked round bottom flask, reaction mass heat to 100-110 °C and maintain for 15 hr. The complete of reaction was monitored by TLC and after completion of reaction, reaction mass allow to stand for 30min at 50-55 °C and layers was separated. The organic layer was wash with 250ml of hot- water followed by 250 ml of 10% sodium hydroxide solution, after that aqueous layer was again extract with Ethyl acetate 250 ml, thereafter both organic layer was combine and dried over sodium sulphate, distilled out at 60-65°C to obtain final product wt: 600 g. TLC mobile phase: n-Hexane (Rf:0.1). The fraction boiling at 180-200°C (9 m bar) was yellow liquid 485 g Yield: 85% GC purity: 96-98%.

¹ H-NMR(CDC ppm):1.401-1.542(m,lH),1.65-1.736(m,2H),2.497-2.591(t,lH),

2.81 l-2.859(s, 1H), 3.002-3.160(t, 1H), 3.415-3.576(t,lH), 4.076-4.187(s,lH), 5.581- 5.603(d,lH), 5.759-5.774(d,lH)5.822-5.844(m,lH)5.917-5.938(m,lH).

Example 2: 200 g of Dicyclopentadiene and 608 g of 25% sulphuric acid solution charge into 3L three-necked round bottom flask and reaction mass heat to 100-110 °C and maintain for 20 h. The completion of reaction was monitored by TLC and after completion of reaction the reaction mass allowed to stand for 30min at 50-55 °C and layers were separate. The organic layer was washed with 100ml of Hot- water followed by 100 ml of 10%sodium hydroxide solution, after that aqueous layer again extract with Ethyl acetate 100 ml. The both organic layers was combined and dried over sodium sulphate, distilled out at 60-65°C to obtained final product 243 g. TLC mobile phase: n- Hexane (Rf:0.1) The fraction boiling at 180-200°C(9 m bar) was yellow liquid 186 g Yield: 82% GC purity: 92-94%.

Example 3: 100 g of Dicyclopentadiene and 304 g of 25% sulphuric acid solution charge into 1L three-necked round bottom flask and reaction mass heat to 100-110 °C and maintain for 5-6 hr. The completion of reaction was monitored by TLC and after completion of reaction the reaction mass was allowed to stand for 30min at 50-55 °C and layers were separate, organic layer was wash with 50 ml of Hot- water followed by 50 ml of 10%sodium hydroxide solution, after that aqueous layer again extract with Ethyl acetate 50 ml. The both organic layers were combined and organic layer was dried over sodium sulphate, distilled out at 60-65°C to obtain final product wt: 115 g TLC mobile phase: n-Hexane (RfO.l).The fraction boiling at 180-200°C(9 m bar) was yellow liquid 91 g Yield: 80% GC purity: 92-94%.

Step-II: Preparation of 2, 3, 5 - tricarboxylic acid cyclopentane acetic acid

Example 4: 2, 3, 5 - tricarboxylic acid cyclopentane acetic acid

200 g of hydroxydicyclopentadine and 300 mL of 1,2-dichloroethane were charged into a 3L three-necked round bottom flask. Subsequently, the temperature was raised to 45- 50°C. 800 ml of 73% nitric acid was added to the reaction mixture drop wise over 5-6 h while continuing stirring. The internal temperature was raised to a maximum of 60°C. Thereafter, the temperature was gradually raised to 60-65°C and stirring was continued for 12 hrs with RPM should be below 120. A part of the reaction solution was collected and analyzed by LC-MS. As a result, unreacted hydroxydicyclopentadine was disappeared and obtained l,2,4-tricarboxy-3-corboxymethylcyclopentane (75-80% area) and 1,2,3,4-tetracarboxycyclopentane (10-15% area). The reaction solution was distilled out completely under vacuum to obtained crude product 276 g. Ethyl acetate 200 ml was added to the crude product and was heated to get clear solution. The solution is further stirred for 15 hrs at room temperature to form a white precipitate. The precipitate was filtered and washed with chilled Ethyl acetate-30ml and dried under vacuum at 65-70°C to obtained the final product 175 g, yield: 87.5%, Purity 98-99%. ¹ H-NMR (DMSO-d ₆ , 5ppm):l.868-1.942(m,lH), 2.175-2.236(m,lH), 2.297-2.371

(M,2H), 2.371-2.639(m,lH), 2.702-2.760(m,lH), 3.015-3.076(m,lH),3.076-

3.187(m, 1 H), 12.359(brs,4H). MASS: 259.6(M-H), 100), 241.5. Example 5: 2, 3, 5 - tricarboxylic acid cyclopentane acetic acid

200 g of hydroxydicyclopentadine and 400 mL of Nitrobenzene were charged into a 3L three-necked round bottom flask. Subsequently, the temperature was raised to 45-50°C. 800 ml of 73% nitric acid was added to the reaction mixture drop wise over 5-6 h while continuing stirring. The internal temperature was raised to a maximum of 60°C. Thereafter, the temperature was gradually raised to 60-65°C and stirring was continued for 12 hrs with RPM should be below 120. A part of the reaction solution was collected and analyzed by LC-MS. As a result, unreacted hydroxydicyclopentadine was disappeared and obtained l,2,4-tricarboxy-3-corboxymethylcyclopentane (75-80% area) and 1,2,3,4-tetracarboxycyclopentane (10-15% area). The reaction solution was distilled out completely under vacuum to obtained crude product 251 g. Ethyl acetate 200 ml was added to the crude product and was heated to get clear solution. The solution is further stirred for 15 hrs at room temperature to form a white precipitate. The precipitate was filtered and washed with chilled Ethyl acetate-20ml and dried under vacuum at 65-70°C to obtained the final product 169 g, yield: 84.5%, Purity 98-99%. Example 6: 2, 3, 5 - tricarboxylic acid cyclopentane acetic acid 200 g of hydroxydicyclopentadine and 400 mL of acetic acid were charged into a 3L three-necked round bottom flask. Subsequently, the temperature was raised to 45-50°C. 800 ml of 73% nitric acid was added drop wise added to the to the reaction mixture over 5-6 h. The internal temperature was raised to a maximum of 60°C. Thereafter, the temperature was gradually raised to 60-65°C and stirring was continued for 12 h and RPM should be below 120. A part of the reaction solution was collected and analyzed by LC-MS. As a result, unreacted hydroxydicyclopentadine disappeared and 1,2,4- tricarboxy-3-corboxymethylcyclopentane (71-75% area) and 1, 2,3,4- tetracarboxycyclopentane (13-16% area). Further the reaction solution was distilled out completely under vacuum to obtained crude product (245 g). The ethyl acetate (100 ml) was added to the crude product and heated further to get clear solution. The solution is stirred for 15 hrs at room temperature to form white precipitate. The precipitate is further filtered and washed with chilled Ethyl acetate-20ml and dried under vacuum at 65-70°C to get final product 155 g, yield: 77.5%, Purity 98-99%. Step-III: Preparation of 3 - methyl - 1, 2, 4 - pentamethylene three carboxylic acid - 1, 4:2, 3 - dianhydride

Example 7: 3 - methyl - 1, 2, 4 - pentamethylene three carboxylic acid - 1, 4:2, 3 - dianhydride

The (1000 ml) Toluene was added to (200 g) 2, 3, 5 - tricarboxylic acid cyclopentane acetic acid in nitrogen atmosphere. The reaction mixture stirred for 10-15 min and added (300 ml) acetic anhydride (purity 98%) while maintaining the nitrogen atmosphere. The reaction mixture is further heated to 120°C to get clear solution and continued heating till get the white solid precipitate. The heating was continuing for 2-3 hrs while maintaining the temperature at 120°C. The reaction mixture was then cool to room temperature at 25-30°C and stirred for 1 h. The reaction mixture was filtered and washed with hexane and dried in vacuum to obtain the final product (155 g) as white solid, yield: 90% and purity was 99.67%. H NMR(DMSO-d ₆ , d ppm): 2.125-2.179(m,lH),2.418-2.499(m,lH),2.791- 2.857(m, lH),2.961-3.077(m,2H), 3.094-3.302(d,lH), 3.358-3.596(d,lH), 3.974- 3.998(d,lH). Example 8: 3 - methyl - 1, 2, 4 - pentamethylene three carboxylic acid - 1, 4:2, 3 - dianhydride

The (1000 ml) Benzene was added to (200 g) 2, 3, 5 - tricarboxylic acid cyclopentane acetic acid in nitrogen atmosphere. The reaction mixture stirred for 10-15 min and added 300 ml of acetic anhydride (purity 98%) while maintaining the nitrogen atmosphere. The reaction mixture is further heated to 80-85°C to get clear solution and continued heating till get the white solid precipitate. The heating was continuing for 2-3 hrs while maintaining the temperature at 80-85°C. The reaction mixture was then cool to room temperature at 25-30°C and stirred for 1 h. The reaction mixture was filtered and washed with hexane and dried in vacuum to obtain the final product (152 g), white solid, yield: 88% and purity was 98.51%. Example 9: 3 - methyl - 1, 2, 4 - pentamethylene three carboxylic acid - 1, 4:2, 3 - dianhydride

The (1000 ml) tetrahydrofuran was added to (200 g) 2, 3, 5 - tricarboxylic acid cyclopentane acetic acid in nitrogen atmosphere. The reaction mixture stirred for 10-15 min and added 300 ml of acetic anhydride (purity 98%) while maintaining the nitrogen atmosphere. The reaction mixture is further heated to 65-70°C to get clear solution and continued heating till get the white solid precipitate. The heating was continuing for 2-3 hrs while maintaining the temperature at 65-70°C. The reaction mixture was then cool to room temperature at 25-30°C and stirred for 1 h. The reaction mixture was filtered and washed with hexane and dried in vacuum to obtain the final product (147 g), white solid, yield: 85% and purity was 98.30%.

Example 10: 3 - methyl - 1, 2, 4 - pentamethylene three carboxylic acid - 1, 4:2, 3 - dianhydride

The 600 ml of trifluoroacetic anhydride (purity 98%) was added to the (200 g) 2, 3, 5 - tricarboxylic acid cyclopentane acetic acid in nitrogen atmosphere. The reaction mixture was stirred for 10-15 minutes while maintaining the nitrogen atmosphere. The reaction mixture is further heat to 35-40°C to get clear solution and continued heating till get the white solid precipitate. The heating was continuing for 15 hr at 35-40°C. The reaction mixture was then cool to room temperature at 25-30°C and stirred for 1 h. The reaction mixture was filtered and washed with hexane and dried in vacuum to obtain the final product (146 g), white solid and yield: 84% and purity was 99.5%.