Title of Invention	PROCESS FOR PREPARING LETROZOLE FREE FROM ITS REGIOSOMER AND OTHER IMPURITIES
Abstract	The proposed invention relates to the preparation of Letrozole free from its regioisomer (7) and other impurities using optionally solvent extraction and crystallisation.

Full Text

FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & The Patent Rules, 2003 PROVISIONAL SPECIFICATION (See section 10 and rule 13) TITLE OF THE INVENTION "PROCESS FOR PREPARING LETROZOLE FREE FROM ITS REGIOISOMER AND OTHER IMPURITIES" We, CADILA HEALTHCARE LIMITED, a company incorporated under the Companies Act, 1956, of Zydus Tower, Satellite Cross Road, Ahmedabad-380015, Gujarat, India. The following specification describes the invention: 20 OCT 2006 PROCESS FOR PREPARING LETROZOLE FREE FROM ITS REGIOISOMER AND OTHER IMPURITIES FIELD OF INVENTION Aromatase is an enzyme, which effects aromatisation of ring A in the metabolic formation of various steroid hormones, Various cancers for example breast cancer, are dependent upon circulating steroid hormones which have an aromatic ring A. Such cancers can be treated by removing the source of ring A aromatised steroid hormones, for example by the combination of oophorectomy and adrenalectomy. An alternative way of obtaining the same effect is by administering a chemical compound, which inhibits the aromatisation of the steroid ring A. Letrozole is a non-steroidal antineoplastic, claimed to inhibit the aromatase (oestrogen synthase) activity. It is useful in the treatment of advanced breast cancer in postmenopausal women. The growth of some cancers of the breast are stimulated or maintained by estrogens. Treatment of breast cancer thought to be hormonally responsive (i.e., estrogen and/or progesterone receptor positive or receptor unknown) has included a variety of efforts to decrease estrogen levels (ovariectomy, adrenalectomy, hypophysectomy) or inhibit estrogen effects (antiestrogens and progestational agents). These interventions lead to decreased tumor mass or delayed progression of tumor growth in some women. In postmenopausal women, estrogens are mainly derived from the action of the aromatase enzyme, which converts adrenal androgens (primarily androstenedione and testosterone) to estrone and estradiol. The suppression of estrogen biosynthesis in peripheral tissues and in the cancer tissue itself can therefore be achieved by specifically inhibiting the aromatase enzyme. Letrozole is a nonsteroidal competitive inhibitor of the aromatase enzyme system; it inhibits the conversion of androgens to estrogens. In adult tumor bearing females, Letrozole is as effective as ovariectomy in reducing uterine weight, elevating serum LH, and causing the regression of estrogen-dependent tumors. In contrast to ovariectomy, treatment with Letrozole does not lead to an increase in serum FSH. Letrozole selectively inhibits gonadal steroidogenesis but has no significant effect on adrenal mineralocorticoid or glucocorticoid synthesis. Letrozole inhibits the aromatase enzyme by competitively binding to the heme of the cytochrome P450 subunit of the enzyme, resulting in a reduction of estrogen biosynthesis in all tissues. Treatment of women with Letrozole significantly lowers serum estrone, estradiol 2 and estrone sulfate and has not been shown to significantly affect adrenal corticosteroid synthesis, aldosterone synthesis, or synthesis of thyroid hormones. BACKGROUND OF INVENTION Synthesis of Letrozole is reported in US 4,978,672 and EP 236,940. In above patents the synthesis of Letrozole starts with. 4-bromomethylbenzonitrile (1), which undergoes condensation with 1,2,4-triazole (2) to form 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3) as an intermediate. The compound of structural formula (3) is purified by column chromatography to remove 4-[(l,3,4-triazol-l-yl)methyl]benzonitrile (4) and followed by reaction with 4-fluorobenzonitrile (5) to afford Letrozole (6). SCHEME-1 Br HN-N W NC (D (6) LETROZOLE N-N V (7) LETROZOLE REGIOISOMER In the above process, the undesired intermediate 4-[(l,3,4-triazol-l-yl)methyl]benzonitrile (4) is formed during the course of the preparation 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3) in 10%w/w to 30%w/w. The undesired impurity 4-[(l,3,4-triazol-l-yl)methyl]benzonitrile (4) present with 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), further on reaction with 4-fluorobenzonitrile (5) leads to the formation of another impurity 4-[l-(4-cyanophenyl)-l-(l,3,4-triazol-l-yl)methyl]benzonitrile (7) in approximately same ratio. To control the formation of impurity of structural formula (7), it is required to make intermediate of structural formula (3) in its pure form. The separation of desired compound from isomeric impurities is of great importance. In basic patents US 4,978,672 and EP 236,940; chromatographic technique is used to separate 3 regioisomer (4), which has its own limitations on commercial scale. Purification by chromatography is an expensive and time consuming operation on plant scale. It also consumes lots of solvent and is hazardous for environment. To overcome the above problems, purification of intermediate (3) is reported in PCT application WO 2005/047269 via the hydrochloride salt formation of the mixture of product (3) along with regioisomer (4). Selective crystallisation of regioisomer as hydrochloride using approximately 8.5 volume diisopropyl ether, filtering the resultant and then isolation of the intermediate (3) as pure product from the filtrate in approximately 60%) overall yield. Finally washing the product with hexane or petroleum ether. In above PCT application, highly flammable solvents like diisopropyl ether, hexane and petroleum ether are used in process, which require high level of precautions and are never safe to handle on plant scale. SCHEME-2 -*- NC ^"^ fi T ii) 25% Ammonia (D NH2 Br N-N (8) ^ Br i)NaN02/HCI Another process reported in PCT application WO 2004/076409, discloses the different route to prepare the pure intermediate (3). The said patent discloses a reaction of 4-bromomethylbenzonitrile (1) with 4-amino-l,2,4-triazole (8) to give quaternary ammonium salt (9), which undergoes diazotisation reaction to give 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3) in approximately 59% molar yield. The process is complicated and involves lengthy steps and tedious operations. H w // However, if the starting material 4-bromomethylbenzonitrile (1) contains its raw material i.e. 4-toluonitrile as impurity, it also reacts with 4-fluorobenzonitrile and leads to the formation of 4,4',4"-methylidenetrisbenzonitrile (10) as another impurity in final product. 4 OBJECTS OF INVENTION It is an object of the present invention to overcome the use of highly flammable solvents and tedious operations, a safe and smooth process is required for making pure 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile(3). In order to obtain the pure Letrozole (6), we have planned to get intermediate (3) in its pure form and free from regioisomer (4). For the said purpose, we have planned to go for solvent extraction method and selectively extract or crystallise the desired intermediate 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), from a mixture with regioisomer 4-[(l,3,4-triazol-1 -yl)methyl]benzonitrile (4). SCHEME-3XT* HN-N l*J N-Ni > N (1) NcA^ V (3) V Separation Using Selective Extraction or Selective Crystallisation F-0~CN \ f^rV "^ « (5) ■irV (6) NCAJ (3) LETROZOLE Therefore, it is an important object of the present invention to provide Letrozole (6) with its regioisomer 4-[l-(4-cyanophenyl)-l-(l,3,4-triazol-l-yl)methyl]benzonitrile (7), preferably, less than 0.3%w/w, more preferably, less than 0.1%w/w and most preferably below the quantitation limit. It is another object of the present invention to provide an improved process for the preparation of Letrozole with its regioisomer 4-[l-(4-cyanophenyl)-l-(l,3,4-triazol-l-yl)methyl]benzonitrile (7), preferably less than 0.3%w/w, more preferably less than 0.1% w/w and most preferably, below the quantitation limit. It is another object of the proposed invention to provide Letrozole with impurity 4,4',4"-methylidenetrisbenzonitrile (10), preferably less than 0.2%w/w, more preferably less than 0.1%w/w and most preferably below quantitation limit. 5 It is another object of the present invention to provide an improved process for the preparation of Letrozole with impurity 4,4',4"-methylidenetrisbenzonitrile (10), preferably less than 0.2%w/w, more preferably less than 0.1%w/w and most preferably below the quantitation limit. It is another object of the present invention to provide Letrozole with intermediate impurity 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), preferably, less than 0.1%w/w and more preferably below quantitation limit. It is another object of the present invention to provide an improved process for the preparation of Letrozole with intermediate impurity 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), preferably less than 0.1%w/w, and more preferably below the quantitation limit. It is another object of the present invention to provide Letrozole with quaternary ammonium salt, preferably less than 0.1%w/w and more preferably quantitation limit. It is another object of the present invention to provide an improved process for the preparation of Letrozole, with quaternary ammonium salt, preferably less than 0.1%w/w and more preferably below the quantitation limit. It is another object of the present invention to provide Letrozole with any known or unknown impurities, preferably less than 0.1%w/w and more preferably below quantitation limit. It is another object of the present invention to provide an improved process for the preparation of Letrozole, with any known or unknown impurities, preferably less than 0. l%w/w and more preferably below the quantitation limit. In view to minimize the formation of quaternary ammonium salt, It is another object of the present invention to optimize the reaction conditions by employing the amount of 1,2,4-triazole (2), ranging from more than 1.5 mole equivalent to less than 4.4 mole equivalent with respect to 4-bromomethylbenzonitrile (1). During the preparation of 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), approximately 10-30%w/w w/w of its regioisomer 4-[(l,3,4-triazol-l-yl)methyl]benzonitrile (4) is also formed on reaction of 4-bromomethylbenzonitrile (1) with 1,2,4-triazole. Therefore, it is another object of the present invention to go forward for the reaction of crude mixture with 4-fluorobenzonitrile to get a mixture of Letrozole and its regioisomer 4-[l-(4-cyanophenyl)-l-(l,3,4-triazol-l-yl)methyl]benzonitrile (7), and separation of pure Letrozole either by using selective extraction in suitable solvent or by selective crystallisation methods. 6 It is another object of the present invention to provide 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3) free from its regioisomer 4-[(l,3,4-triazol-l-yi)methyl]benzonitrile (4) and other related impurities. It is another object of the present invention to provide an improve process for the preparation of 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3) free from its regioisomer 4-[(l,3,4-triazol-l-yl)methyl]benzonitrile (4) and other related impurities. It is another object of the present invention to provide 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3) with its regioisomer 4-[(l,3,4-triazol-l-yl)methyl]benzonitrile (4) which will be preferably less than 30%w/w, more preferably less than 0.3%w/w and most preferably below quantitation limit. It is another object of the present invention to provide an improved process for the preparation of 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), with its regioisomer 4-[(l,3,4-triazol-l-yl)methyl]benzonitrile (4), preferably less than 30%w/w, more preferably less than 0.3%w/w and most preferably below quantitation limit. Br SCHEME-4 NC (D HN-N V*J NC ■^r ON (3) NC N-N ■^r N (4) (5) N-^ Separation Using \^ „N Selective Extraction or Selective Crystallisation (6) LETROZOLE (6) LETROZOLE (7) LETROZOLE REGIOISOMER It is another object of the present invention to provide 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), with quaternary ammonium salt, preferably less than 0.1%w/w and more preferably below quantitation limit. 7 It is another object of the present invention to provide an improved process for the preparation of 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), with quaternary ammonium salt preferably less than 0.1%w/w and more preferably below the quantitation limit. It is another object of the present invention to provide 4-[(l,2,4-triazol-l-y1)methyl]benzonitrile (3), with any known or unknown impurity preferably less than 0.1%w/w and more preferably below quantitation limit. It is another object of the present invention to to provide an improved process for the preparation of 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), with any known or unknown impurities, preferably less than 0. l%w/w and more preferably below the quantitation limit. It is another object of the present invention to provide an improved process for the separation of 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3) in its pure form, from the mixture with undesired regioisomer 4-[(l,3,4-triazol-l-yl)methyl]-benzonitrile (4), using either by selective extraction in suitable solvent or by selective crystallisation. It is another object of the present invention to provide an improved process for the preparation of 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3) with regioisomer 4-[(l,3,4-triazol-l-yl)methyl]benzonitrile (4) and other related impurities preferably less than 0.3 %w/w and more preferably below quantitation limit. DISCRIPTION OF INVENTION The present invention relates to the preparation of Letrozole with its regioisomer 4-[l-(4-cyanophenyl)-l-(l,3,4-triazol-l-yl)methyl]benzonitrile (7), preferably less than 0.3 % w/w, more preferably less than 0.1% w/w and most preferably below quantitation limit and other impurities as discussed earlier preferably less than 0.1%w/w and most preferably below quantitation limits, which are arising due to impure intermediate 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3) contaminated with regioisomer 4-[(l,3,4-triazol-l-yl)methyl]benzonitrile (4) and quaternary ammonium salt. The present invention also relates to an improved process of Letrozole with its regioisomer 4-[l-(4-cyanophenyl)-l-(l,3,4-triazol-l-yl)methyl]benzonitrile (7), preferably less than 0.3 % w/w, more preferably less than 0.1% w/w and most preferably below quantitation limit and other impurities as discussed earlier below 0.1%w/w or below quantitation limits, which are arising due to impure intermediate 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3) contaminated with regioisomer 4-[(l,3,4-triazol-l-yl)methyl]benzonitrile (4) and quaternary ammonium salt. 8 The present invention also relates to the preparation of Letrozole with impurity 4,4',4"-methylidenetrisbenzonitrile (10), preferably less than 0.2%w/w, more preferably less than 0. l%w/w and most preferably below quantitation limit. The present invention also relates to an improved process for preparation of Letrozole with impurity 4,4',4"-methylidenetrisbenzonitrile (10), preferably less than 0.2%w/w, more preferably less than 0. l%w/w and most preferably below the quantitation limit. The present invention also relates to the preparation of Letrozole with intermediate impurity 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), preferably less than 0.1%w/w and more preferably below quantitation limit. The present invention also relates to an improved process for preparation of Letrozole with intermediate impurity 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), preferably less than 0.1%w/w, and more preferably below the quantitation limit. The present invention also relates to the preparation of Letrozole with quaternary ammonium salt formed during the course of reaction of 4-bromomethylbenzonitrile (1) with 1,2,4-triazole (2), preferably less than 0.1% w/w and more preferably below quantitation limit. The present invention also relates to an improved process of Letrozole with quaternary ammonium salt formed during the course of reaction of 4-bromomethylbenzonitrile (1) with 1,2,4-triazole (2), less than 0.1%w/w or below quantitation limits. Further, the present invention also relates to the preparation of 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3) with its regioisomer 4-[(l,3,4-triazol-l-yl)methyl]benzonitrile (4), which is preferably less than 0.3 % w/w, more preferably less than 0.1% w/w and most preferably below quantitation limit and other impurities as discussed earlier less than 0. l%w/w or below quantitation limits. The present invention also relates to an improved process for the of 4-[(l,2,4-triazol-l-y1)methyl]benzonitrile (3) with its regioisomer 4-[(l,3,4-triazol-l-yl)methyl]benzonitrile (4), preferably less than 0.3 % w/w, more preferably less than 0.1% w/w and most preferably below quantitation limit and other impurities as discussed earlier less than 0.1%w/w or below quantitation limits. The processes in accordance with the present invention are depicted in Scheme - 3 and Scheme - 4. During the formation of 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), its regioisomer 4-[(l,3,4-triazol-l-yl)methyl]benzonitrile (4), is also formed up to 10%w/w to 9 30%w/w, which will be removed either by the selective extraction using suitable solvent system or by the fractional crystallisation. Therefore, another objective of the proposed invention is to go forward with crude mixture of 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), and its regioisomer 4-[(l,3,4-triazol-l-yl)-methyl]benzonitrile (4 to obtain a mixture of Letrozole and its regioisomer 4-[l-(4-cyanophenyl)-l-(l,3,4-triazol-l-yl)methyl]benzonitrile (7), and isolation of pure Letrozole using either selective extraction by the suitable solvent system or by fractional crystallisation. The present invention also relates to the preparation of 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), with quaternary ammonium salt preferably less than 0.1%w/w and more preferably below quantitation limit. The present invention also relates to an improved process for the preparation of 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), with quaternary ammonium salt preferably less than 0.1% w/w and more preferably, below the quantitation limit. It is in view to minimize the formation of any quaternary ammonium salt, so the present invention also relates to optimize the reaction condition by employing the amount of 1,2,4-triazole (2), ranging from more than 1.5 mole equivalent to less than 4.4 mole equivalent with respect to 4-bromomethylbenzonitrile (1). The present invention also relates to the preparation of 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), with any known or unknown impurity preferably less than 0. l%w/w and more preferably below quantitation limit. The present invention also relates to an improved process for the preparation of 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3), with any known or unknown impurities, preferably less than 0. l%w/w and more preferably below the quantitation limit. The present invention also relates to an improved process for the separation of 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3) in its pure form, from the mixture with undesired regioisomer 4-[(l,3,4-triazol-l-yl)methyl]-benzonitrile (4), using either by selective extraction in suitable solvent or by selective crystallisation. The present invention also relates to an improved process for the preparation of 4-[(l,2,4-triazol-l-yl)methyl]benzonitrile (3) with regioisomer 4-[(l,3,4-triazol-l-yl)methyl]benzonitrile (4) less than 0.3%w/w and other related impurities preferably less than 0.1 %w/w and more preferably below quantitation limit. Thus the present invention provides a process for the preparation of Letrozole comprising; 10 a) reacting 4-bromomethylbenzonitrile of structural formula (1) with 1,2,4-triazole of structural formula (2) b) isolating crude intermediate of structural formula (3) containing up to approximately 0% to 30%w/w of regioisomer of structural formula (4) c) purifying crude intermediate (3) from its undesired regioisomer (4) either by using selective extraction in suitable solvent / mixture of solvents or by selective crystallisation d) reacting pure intermediate of structural formula (3) with 4-fluorobenzonitrile (5) and e) isolating Letrozole from the reaction mixture. Preferably, the amount of 1,2,4-triazole (2) is in the range of from more than 1.5 mole equivalent to less than 4.4 mole equivalent with respect to 4-bromomethylbenzonitrile of structural formula (1) In another embodiment, the present invention provides a process for the preparation of Letrozole comprising; a) reacting crude intermediate of structural formula (3) containing up to approximately 0% to 30%w/w of regioisomer of structural formula (4), with 4-fluorobenzonitrile (5); b) isolating crude Letrozole with its regioisomer up to approximately 30%w/w; and c) isolating pure Letrozole either by selective extraction method by suitable solvent system or by selective crystallisation method Dated this the 15th day of October 2005 11 ABSTRACT The proposed invention relates to the preparation of Letrozole free from its regioisomer (7) and other impurities using optionally solvent extraction and crystallisation. 12

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