Background of the Invention 'Paclitaxel'having the formula (2), widely known as'Taxol'in chemical literature is a naturally occurring compound from the bark of Pacific as well as Himalayan Yew tree and exhibits a broad spectrum of anti-tumor activity. It has been approved in various countries for use in the treatment of ovarian and breast cancers and clinical trials are under progress in many countries for its use in several other types of cancers as well. During the time of human clinical trials of Paclitaxel of the formula (2), a number of SAR (Structure Activity Relationship) studies on taxol were carried out all over the world and and highlighted the crucial structural features which are responsible for its anti-cancer activity. Consequently, several structural modifications were attempted on taxol at specific positions in baccatin III part as well as in side chain acid part which presumably do not diminish the anti-cancer activity of taxol molecule.

Docetaxel of the formula (1) is one such synthetic derivative of paclitaxel of the formula (2) which showed most promising anti-tumor activity in clinical trials. It was developed as an anti-cancer drug by Rhone-Poulenc Rorer for the treatment of cancer. Docetaxel has been approved in several countries for the treatment of breast cancer recently. Docetaxel has the molecular formula (1), which is similar to paclitaxel of the formula (2) except that a free hydroxyl group at C-10 position of baccatin part and a t-butoxycarbonyl (t-boc) group at C-3'nitrogen of the side chain acid part. side chain acid baccatin part side chain acid baccatin part HgC-HsC partpart H3C H3C H0100OH H Ac 00 OH 0. 0 00 H 3C CH 3_ 'O, v 3, _ _ '\ N 3= y, w zu _ = O H OH= H O CN3 H OH H-'_ H O CH3 OH O O o O O i ( 9) 2) Since the total synthesis of docetaxel or paclitaxel is not a commercially viable process due to the lengthy synthetic steps as well as the high cost involved, the option of a semi-synthetic approach to paclitaxel or docetaxel has been persued in several laboratories starting from a precursor molecule 10-deacetylbaccatin III (10-DAB III) of the formula (3) (3) Compared to paclitaxel having the formula (2), 10-deacetylbaccatin III (10- DAB III) of the formula (3) is a relatively abundant precursor molecule obtained from several species of the Yew tree (genus Taxus) grown in various parts of the world.

Considering the relative chemical reactivity differences of the four hydroxyl groups (C-1, C-7, C-10 & C-13) in 10-DAB III of the formula (3), it is necessary to protect the C-7, & C-10 hydroxyl groups before undertaking the esterificåtion of C-13 hydroxyl group in the compound of the formula (3) to produce docetaxel of the formula (1) or paclitaxel of the formula (2). Although a few synthetic approaches were established for the efficient esterification of highly hindered C-13 hydroxyl group in the compound of the formula (3), very few 7,10-protected 10-deacetylbaccatin III precursors were known in the literature for such esterification processes. Since the C-7 hydroxyl group in. compound of the formula (3) is highly vulnerable to epimerisation under basic reaction conditions, selection of a suitable and easily removable protecting group for C-7 OH is a crucial parameter in a semi-synthetic approach for the synthesis of docetaxel or paclitaxel. Secondly, the esterification of the side chain acid with sterically hindered C-13 hydroxyl group in suitably protected 10-deacetylbaccatin III is another parameter which requires considerable attention. Finally, easy accessibility of a suitably protected chiral side chain acid of the formula (4), where Rl & R2 independently represent hydrogen, lower alkyl, phenyl, substituted phenyl groups which corresponds to the side chain acid part of the docetaxel of the formula (1) and a cost effective method of the side chain acid preparation also accounts for a salient feature of a good synthesis of both docetaxel as well as paclitaxel.

In continuation of our research activities in taxane chemistry, we initiated our efforts to develop a short and cost effective approach for the synthesis of docetaxel of the formula (1). Consequently, we recognized the need for a new protecting group for C-7 and C-10 hydroxyl groups in 10-deacetylbaccatin III of the formula (3), which can be easily deprotected under mild reaction conditions. Accordingly, we developed new and an improved semi-synthetic process for the synthesis of docetaxel of the formula (1) using an imidazolylcarbonyl group as C-7 and C-10 hydroxy protecting group.

Therefore the present invention involves a novel and an efficient semi-synthetic approach comprising a short synthetic sequence and milder reaction conditions to produce docetaxel of the formula (1).

Detailed Description of the Invention The objective of the present invention is to provide an improved process for the preparation of'Docetaxel'of the formula (1), by a semi-synthetic route which is short and cost-effective starting from a suitably protected 10-DAB III of the formula (3) and a suitably protected side chain acid of the formula (4).

Accordingly, the starting compound 10-deacetylbaccatin III of the formula (3) has been isolated from the leaves of the plant'Taxus baccata'or'Taxus wallichiana' available in western ghats of India following the literature procedure.

Accordingly, the present invention provides an improved process for the preparation of docetaxel of the formula (1) which comprises, i) reacting 10-deacetylbaccatin III of the formula (3) with a compound of the formula (5) where the'Het'represents heterocyclic group in the presence of solvent to produce the novel 7,10-diprotected 10-deacetylbaccatin III of the formula (6) (6) where the'Het'group has the meaning given above, ii) esterifying the C-13 hydroxyl group in the compound of the formula (6) defined with a side chain acid having the formula (4) where Rl & R2 independently represent hydrogen, lower alkyl, phenyl, substituted phenyl groups, in the presence of a base, and a solvent to furnish the novel C-13 coupled product of the formula (7) (7) where Rl, R2 and"Het"have the meaning given above. iii) deprotecting the protecting groups at C-7, C-10 & oxazolidine group in compound of the formula (7) under acidic reaction conditions to obtain an intermediate amino alcohol of the formula (8) and (8) iv) converting the compound of the formula (8) to docetaxel of the formula (1) using a base and ditert. butyl dicarbonate.

The compound of the formula (5) employed in the reaction step (i) is a heterocyclic compound, preferably an aromatic heterocyclic compound, more preferably the heterocyclic group is an imidazole.

The transformation of 10-deacetylbaccatin III of the formula (3) to 7, 10- diprotected 10-deacetylbaccatin III of the formula (6) may be achieved in a sizgle step by reacting. the compound of the formula (3) with a commercially available heterocyclic compound such as carbonyldiimidazole (CDI) at a temperature in the range of 20-60 °C preferably at room temperature in 85-90% yields.

The solvents employed in the reaction may be chosen from ethereal solvents such as 1,4-dioxane, dimethoxyethane, tetrahydrofuran or aromatic solvents such as benzene, toluene or chlorinated solvents such as dichloroethane, dichloromethane, chloroform but preferably chlorinated solvents.

The appropriate compounds of the formula (6) obtained by the reaction may be purified by crystallization methods or column chromatography using silicagel preferably of 6-120 mesh.

The 7,10-diprotected 10-deacetylbaccatin III of the formula (6) was reacted with a side chain acid of the formula (4) in the presence of a base to produce the C-13 adduct of the formula (7). In the compound of the formula (4), Rl and R2 may preferably represent methyl group.

The base used in this reaction may be carbodiimide bases such as dicyclohexyl carbodiimide or carbonate bases such as dipyridylcarbonate along with 4-N, N- dimethylaminopyridine as a promoter of the reaction.

The solvents used in the reaction may be selected from chlorinated solvents such as dichloromethane, chloroform, 1,2-dichloroethane, carbon tetrachloride or aromatic solvents such as benzene, toluene, hexane, 1,2-dichlorobenzene, xylene or ethereal solvents such as diethyl ether, tetrahydrofiiran, 1,2-dimethoxyethane or amides such as dimethylformamide, dimethylacetamide.

The reaction may be carried out at a temperature in the range of 20 to 80 °C but preferably between 20-40 °C.

The appropriate compounds of the formula (7) obtained by the reaction may be isolated by known methods and if required purified by crystallization or column chromatography using silicagel preferably of 100-120 mesh. We have observed that the crystallization of the compound of the formula (7) provides a high purity compound. For such crystallizations, solvents such as chloroform, acetone, diethyl ether, diisopropyl ether, pet-ether, hexane or cyclohexane were found to be preferable.

The deprotection of the protecting groups at C-7, C-10 carbons and the oxazolidine group present in the compound of the formula (7) may be carried out by the treatment with an acid to obtain the amino alcohol of the formula (8).

The acid used in the reaction may. be selected from organic acids such as formic acid, acetic acid, trifluoroacetic acid either neat or in aqueous form.

The solvent used in the reaction may be selected from water, tetrahydrofuran, diethyl ether, 1,4-dioxane, acetonitrile, methanol, and ethanol.

The temperature employed for the reaction may be in the range of-10 to 30 °C but preferably in the range of-5 to 5 °C.

The amino alcohol of the formula (8) may be derivatised with tert. butoxycarbonyl (Boc) group using ditert. butyl dicarbonate in the presence of a base.

The base used in the reaction may be selected from sodium bicarbonate, sodium carbonate, and potassium carbonate.

The solvent employed in the reaction may be selected from tetrahydrofuran, diethyl ether, 1,4-dioxane, 1,2-dimethoxyethane and ethyl acetate.

The chiral protected side chain acid of the formula (4) used in the present invention may be prepared in two steps from a known vinyl alcohol of the formula (9) (A. E. Greene et. al., J. Org. Chem., 56,6939 (1991) following the method described in our US patent number 5,763,477. The method of its preparation involves: (i) the reaction of vinyl alcohol of the formula (9) with a reagent of the formula (10) where 1t1 & R2 independently represents hydrogen, lower alkyl, phenyl, substituted phenyl groups under acidic reaction conditions to obtain an oxazolidine derivative of the formula (11), (ii) oxidation of the double bond in the compound of the formula (11) to produce the corresponding oxazolidine acid of the formula (4) in which 1t1 & R2 represent the groups as mentioned above.

The terms representing R'and R in the above process have the following definitions throughout the present invention.

The term'lower alkyl"denotes a univalent, branched or straight chain hydrocarbon containing 1 to 8 carbon atom. Representative of the alkyl groups may be methyl, ethyl, propyl, isopropyl, butyl, sec. butyl, tert. butyl, pentyl, iso pentyl, tert.

Pentyl, hexyl, isohexyl, octyl.

The term'lower alkoxy'denotes lower alkyl groups as defined above attached via oxygen linkage to the rest of the molecule. Representative of those groups are methoxy, ethoxy, isopropoxy, tert. butoxy, hexyloxy, heptoxy and octoxy.

The term'halogen'represents chlorine, bromine or fluorine. The term'Ph' represents phenyl.

The term'substituted'phenyl group used in the present invention refers to those substituents which can be selected from the groups such as hydroxyl, lower alkyl, halogen, lower alkoxy, amino, lower alkylamino. Examples of such groups are 4-hydroxyphenyl, 4-methoxyphenyl, 4-fluorophenyl, 4-tert. butyl phenyl, 3-N, N- dimethylaminophenyl.

Thus the process described in the present invention has significant advantages over the existing processes in terms of less number of steps, milder reaction conditions, inexpensive reagents, simple crystallization methods to obtain high purity compounds at each step and therefore, amenability for large scale preparation also the final compound docetaxel so obtained by following this process is of high purity as observed by the HPLC (High performance liquid chromatography) method.

Accordingly the present invention provides an efficient route for the preparation of docetaxel on a commercial scale.

The process of the present invention is described in detail in the example given below which are provided by way of illustration only and therefore should not be construed to limit the scope of the present invention.

Example 1: Step (i): Preparation of novel 7,10- di (imidazolyl) carbonyloxy-10-deacetylbaccatin III of the formula (6) where'Het'represents an imidazole group: 18g of 10-deacetylbaccatin III of the formula (3) was treated with 26.5g of carbonyldiimidazole (5 equiv.) dissolved in 1. 7L of dry chloroform and stirred at 25°C till the starting material was consumed as seen by TLC. The reaction mixture was then diluted with chloroform and washed with water and brine solution and dried over anh. sodium sulfate. Concentration of the solvent afforded 27g of crude solid which was washed with 250 ml of chloroform. The organic layer was separated and the resulting solid was dried under vacuum over lh to get 23 g of the title compound 95% HPLC purity. MP.: 193-195° C ; IR (vma,) : 3296,1766,1728,1395,1284,1241,1176, 998, 762 cm 1 ; IH NMR (200 MHz, DMSO-d6) ; 88. 07 (s, 1H), 8.05 (s, l H), 8. 01 (d, J=8Hz, 2H), 7.75-7.52 (m, 3H), 7.45 (s, 1H), 7.29 (s, 1H), 7.09 (s, 1H), 7.05 (s, lH), 6.39 (s, 1H), 5.61 (t, J=2.4 Hz, 1H),), 5.51 (d, J=7 Hz, 1H), 5.06 (d, J=8 Hz, 1H),), 4.75 (s, D20 exchangeable, 1H), 4.71 (br t, 1H), 4.13 (br t, 2H), 3.90 (d, J=8Hz, 1H), 2.85-2.62 (m, lH), 2.26 (s, 3H), 2 : 03 (s, 3H), 1.81 (s, 3H), 1.06 (s, 3H), 1.02 (s, 3H); 13C NMR (DMSO-d6) : 5 202. 10,170.31,165.27.149.19,147.39,147.17,137.36,136.81, 133.43,130.70,129.95,9.61,28.78,117.63,117.22,82.70,79.39,78 .79,76.87,75.88, 74.02,66.09,55.67,46.39,42.34,39.10,32.64,26.42,22.21,20.85, 15.12,10.55.

Step (fui) : Preparation of 13- (2'-phenyl-4', 4'-dimethyl-N-Boc-oxazolidine-l-carbonyloxy) 7,10-di (imidazolyl) carbonyloxy-10-deacetylbaccatin III of the formula (7): 5g of 7, 10-Di (imidazolyl) carbonylyloxy-10-deacetylbaccatin III of the formula (6) obtained in step (i) was treated with 5. 5g of oxazolidine acid of the formula (4) where Rl and R2 are methyl, in the presence of 3.54g (2.5equiv.) of dicyclohexylcarbodiimide (DCC) and 0.8g (1 equiv.) of 4-dimethylaminopyridine (DMAP) suspended in 1L of toluene and stirred the contents at 25 °C until the starting baccatin derivative was consumed. At the end, reaction mixture was filtered and the filtrate was diluted with ethylacetate. The organic layer was washed with water, 5% sodium bicarbonate solution, brine and dried over anh. sodium sulfate. Concentration of the solvent under reduced pressure afforded 9.7 g of crude solid. Crystallization of the solid material in diethyl ether-pet. ether solvent mixture produced 6.9 g of the title compound having 97% HPLC purity. MP.: 181-183°C ; IR: 3440,1766,1728,1395,1249,1172,1095,764,105 Cm-1 ;'H NMR (CDC13) ; 6 8.05 (d, J=7.8 Hz, 1H), 8.03 (s, lH), 7.93 (s, 1H), 7.72-7.46 (m, 3H), 7.41-7.23 (m, 5H), 7.30 (s, 1H), 7.18 (s, 1H), 7.04 (s, 2H), 6.46 (s, lH), 6.29 (t, J=8Hz, 1H), 5.80-5.60 (m, 2H), 5.10 (brs, 1H), 4.95 (d, J=8Hz, 1H), 4.48 (d, J=6Hz, 1H), 4.33 (d, J=8Hz, 1H), 4.16 (d, J=8Hz, 1H), 3.96 (d, J=6.6Hz, 1H), 2.90-2.70 (m, 1H), 2.30 (d, J=8Hz, 1H), 2.07 (s, 3H), 1.97 (s, 3H), 1.90 (s, 3H), 1.89-1.80 (m, 2H), 1.82 (s, 3H), 1.77 (s, 3H), 1.30 (s, 3H), 1.20 (s, 3H), 1.23-1.05 (m, 9H) ; 13 C MR (CDC13) : 8 200.26,170.25, 169.53,166.38,151.31,147.55,147.10,143.68,37.30,136.71,133.7 0, 131.22,130.52,129.84 (3C), 128.88,128.54 (3C), 127.64,126.24 (2C), 117.10, 116.90,96.73,83.30,80.72,80.04,78.37.77.84,75.90,75. 64,74.07,70.91,63.99, 55.85,46.37,42.88,35.45,33.63,32.90,27.77 (3C), 26.34,25.39,24.67,21.26,14.67, 10.70.

Step (iii) & (iv): Preparation of Docetaxel of the formula (1) : 2g of 13- (2'-phenyl-4', 4'-dimethyl-N-Boc-oxazolidine-1-carbonyloxy)-7, 10- di (imidazolyl) carbonyloxy-10-deacetylbaccatin III of the formula (7) obtained in step (ii) was treated with 80ml of 50% aqueous trifluoroacetic acid at-10°C and stirred the contents below 0 °C until most of the starting material was consumed. At the end, reaction mixture was poured into ice water and neutralized with 5 % sodium bicarbonate solution. The aqueous layer was extracted with ethyl acetate (1L) and the organic layer was washed with brine solution and dried over anh. sodium sulfate.

Concentration of the solvent provided 1.7g of material containing the amino alcohol of the formula (8) which was used in the next reaction directly.

To the above amino alcohol dissolved in 50ml of dry tetrahydrofuran was added di. tert. butryl dicarbonate (0. 75g, 1.5 equiv.) in 20ml of tetrahydrofuran followed by solid sodium bicarbonate (330mg, 1.6 equiv.). The reaction mixture was stirred at 20°C until the starting amino alcohol was disappeared as observed by TLC. At the end, the reaction mixture was diluted with 500 ml of ethyl acetate and the combined organic layer was washed with water and brine solution. Concentration of the organic layer afforded crude Docetaxel (1.6g) which was purified over flash column chromatography using 200-400 mesh silica gel and ethyl acetate-chloroform as eluent to obtain 400mg of 97% pure docetaxel.

Example 2: Preparation of novel 7,10-di (2'-methylimidazolyl) carbonyloxy-10- deacetylbaccatin III of the formula (6) where'Het'represents 2'- methylimidazole group : 3g of 10-deacetylbaccatin III of the formula (3) treated with 4.3g of 1, 1-carbonylbis (2- methyldiimidazole) (5 equiv.) dissolved in 300ml of dry chloroform and stirred at 25 °C till the starting material was consumed as seen by TLC. The reaction mixture was then diluted with chloroform and washed with water and brine solution and dried over anh. sodium sulfate. Concentration of the solvent afforded 4.2g of crude solid, which was washed with 80ml of chloroform. The organic layer was separated and the resulting solid was dried under vacuum over 1 hour to get 3.2 g of the title compound in 93 % HPLC purity.

Example 3: Preparation of novel 7,10-di (piperidinyl) carbonyloxy-10-deacetylbaccatin III of the formula (6) where'Het9 represents a piperidin group: 1.8g of 10-deacetylbaccatin III of the formula (3) was treated with 2.65g of 1, 1,- carbonyldipiperidine (5 equiv.) dissolved in 170ml of dry dichloromethane and stirred at 25 °C till the starting material was consumed as seen by TLC. The reaction mixture was then diluted with dichloromethane and washed with water brine solution and dried over anh. sodium sulfate. Concentration of the solvent afforded 2.7g of crude solid, which was washed with 30ml of chloroform. The organic layer was separated and the resulting solid was dried under vacuum over lh to get 1.2 g of the title compound in 96% HPLC purity.

Advantages of the invention: (1) The process involves a simple three step semi-synthetic approach using abundantly available naturally occurring compound 10-deacetylbaccatin III as the starting material.

(2) A new one step synthetic process was developed for the protection of both C-7 and C-10 hydroxy groups present in 10-deacetylbaccatin III without encountering epimerisation of C-7 substituent. The protecting group used in this process is also a novel protecting group as far as taxane chemistry is concerned.

(3) The purification methods used for each of the process steps were simple enough for getting consistent results in terms of yields and purity of the products.

(4) The process steps involves simple organic acids and bases available at low cost and milder reaction conditions which gave the intermediate products in high yields.

(5) Finally, the novelty of the present invention includes (i) single step protection of both the C-7 & C-10 hydroxyl groups in 10-deacetylbaccatin III in high yields (ii) single step deprotection of three protecting groups at C-7, C-10 & oxazolidine moiety in step (iii) of the process.