Field of invention
This invention relates to a novel process for the synthesis of temozolomide (3-methyl-8-
aminocarbomyl-imidazo[5,l-d]-l,2,3,5-tetrazin-4(3H)-one), an antitumor compound starting fromfrom 8-carboxy-3-methyl-[3H]-imidazo[5,l-d]-l,2,3,5-tetrazin-4-one by three step synthesis via acid chloride, converting to its N-tert-butyl carboxamide followed by simple dealkylation with strong acid.
Back ground of the invention
Temozolomide is an oral alkylating agent and shown to be active against a broad spectrum of tumors in vivo, particularly used for the treatment of refractory anaplastic astrocytoma, a type of cancerous brain tumor.
Temozolomide has also demonstrated activity against recurrent glioma and is as effective as dacarbazine in patients with advanced metastatic melanoma. A derivative of imidazotetrazine, temozolomide is the prodrug of MTIC {3-methyl-(triazen-l-yl) imidazole-4-carboxamide). Temozolomide is chemically known as 3-methyl-8-aminocarbomyl-imidazo[5,l-d]-l,2,3,5-tetrazin-4(3H)-one, depicted by the structure as shown below.
Temozolomide is an imidazotetrazine derivative of the alkylating agent dacarbazine, undergoes rapid chemical conversion in the systemic circulation at physiological pH to the active compound, monomethyl triazeno imidazole carboxamide (MTIC). Both temozolomide and dacarbazine are prodrugs of MTIC. The cytotoxicity of MTIC is thought to be due primarily to methylation of DNA at the O6 position of guanine. Temozolomide exhibits schedule-dependent antineoplastic activity by interfering with DNA replication.

A recent randomized trial confirms that a concomitant temozolomide chemotherapy with radiation significantly improves progression free survival and overall survival in glioblastoma multiforme patients.
Temozolamide, 3-methyl-8-aminocarbomyl-imidazo[5,l-d]-l,2,3,5-tetrazin-4(3H)-one (1) is reported by Lunt et. al. in U.S.Patent No. 5,260,291. The total synthesis of temozolomide has been reported in J. Med. Chem. 1984, 27, 196-201 is shown in Scheme 1.

In the above synthesis, 5-amino-lH-imidazolo-4-carboxamide (2) is converted into 5-diazo-lH-imidazolo-4-carboxamide (3); which is further reacted with methylisocyanate in dichloromethane (DCM) to yield temozolamide (1). However, methylisocyanate is highly toxic reagent and is avoided transporting on industrial scale, hence alternative methods were looked into. But, none of these methods are suitable for large scale synthesis as most of the starting material 5-amino-lH-imidazole-4-carboxamide (2) remains un-reacted thus the yields obtained are poor.
US Patent 6,844,434 describes a process for preparation of temozolamide, wherein, the reaction of compound (4) with methylhydrazine in DMF at 0° to obtain compound (5) which is then treated with BU4NI in a mixture of CH3CN / THF at 60° for 1hr followed by the addition of H5IO6 gives temozolamide (1) as shown in Scheme 2.


The above method can be extended to make N-alkyl compounds using other alkyl substituted hydrazines. However, this approach is also more tedious in its operations and suffers in low overall yields.
From the above discussion it is very clear that there is a persistent need to develop a more suitable synthetic route for industrial scale up. Therefore, the present inventors have now developed a novel synthetic route for preparation of temozolomide to meet the industrial requirements as referred to herein above.
Object of the invention:
The object of the present invention is to provide a cost-effective, high yielding consistent process for the synthesis of temozolomide, which is economical, eco-friendly and suitable for industrial scale up.
Summary of the invention
Accordingly, the present invention provides a novel process for preparation of temozolamide from the corresponding acid, i.e. 8-carboxy-3-methyl-[3H]-imidazo[5,l-d]-l,2,3,5-tetrazin-4-one.

In one aspect, the invention provides a process for conversion of temozolamide acid to the temozolomide acid chloride by treating the temozolomide acid with thionyl chloride in dimethylformamide.
In another aspect, the temozolomide acid chloride is reacted with excess of ter.butyl amine in presence of suitable solvent to obtain N-tert. Butyl-3-methyl-4-oxoimidazo(5,l-d)-l,2,3,5-tetrazine-8-carboxylate in excellent yields.
In a further aspect of the invention, N-tert. Butyl-3-methyl-4-oxoimidazo (5,l-d)-l,2,3,5-tetrazine-8-carboxylate is treated with strong acid to obtain temozolomide.
Yet in another aspect, the invention provides a process for preparation of N-tert. Butyl-3-methyl-4-oxoimidazo (5,l-d)-l,2,3,5-tetrazine-8-carboxylate, wherein temozolomide acid is reacted with tert.butyl amine in presence of peptide coupling agent.
Detailed description of the invention
While the invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated, the examiners and embodiments are not to be construed to restrict the scope of the invention. The present invention describes-a more practical, economical and efficient synthesis for the preparation of highly pure temozolomide in good yields.
According to the present invention, there is provided a process for synthesis of temozolomide, from 8-carboxy-3-methyl-[3H]-imidazo[5,l-d]-l,2,3,5-tetrazin-4-one, temozolamide acid.
Temozolomide acid was described by Wang et.al. in US2006/0047117 and can also be made from the ethyl ester (7) described by Wang et. al.(J.Org. Chem. 1997, 62 7288-7294) by simple alkaline hydrolysis or from 8-cyano derivative (8) by following known art disclosed in organic synthesis.


Accordingly, in one embodiment of the present invention, temozolamide acid (6) is converted to the corresponding acid chloride (9) by refluxing with chlorinating agent in presence of suitable organic solvent. The acid chloride (9) thus obtained was heated with excess of tert-butylamine in dichloromethane (DCM) or dimethylformamide (DMF) to yield N-tert-butyl-3-methyl-4-oxoimidazo (5,l-d)-l,2,3,5-tetrazine-8-carboxylate (10).
The temozolamide acid chloride (9) can be prepared by using suitable chlorinating agents selected from oxalyl chloride, thionyl chloride and phosphorous pentachloride and the like known in the art. The suitable solvent used for chlorination reaction is selected from the group consisting of chlorinated hydrocarbons, dimethylformamide, N-methylpyrrolidone, N,N-dialkylaniline or their combination thereof.
The chlorination reaction is better carried out using suitable chlorinating agent, thionyl chloride preferably in dichloromethane (DCM) in presence of catalytic amount of dimethylformamide (DMF) at temperature ranging from 25 to 50°C for 2 to 4 hrs- After completion of reaction, excess thionyl chloride is distilled, and the reaction mass is stripped with hydrocarbon solvent such as toluene to afford temozolomide acid chloride as a solid.


The acid chloride (9) thus obtained either as a solid or in solution, is treated with excess of alkyl amine; preferably tert-butylamine in dichlromethane to give N-tert-butylcarboxamide (10) in excellent yield. The amine used in excess also act as a base for the condensation reaction. This conversion is conveniently carried out at a temperature of -5 to 10°C.
In another embodiment, the invention provides an alternate process for preparation of N-tert-butylcarboxamide (10) wherein, temozolamide acid (6) is reacted with tert-butylamine, in presence of suitable peptide coupling agent selected from DCC, TBTU, HBTU, HATU, PyBOP and the like.
The above reaction is carried out preferably using (4-dimethylaminopyridine) DMAP as base. The reaction is carried out in suitable solvents such as tetrahydrofuran, dimethylacetatmide, N-methyl pyrrolidone and dimethylformamide or mixtures thereof, preferably in dimethylformamide. The reaction is carried out preferably in dimethylformamide at temperature between 0 to 25°C
According to another embodiment, the invention is directed to the preparation of temozolamide (1) from tert-butyl derivative of temozolomide (10) by dealkylating the compound (10) in suitable acid. Representative examples of acid used for dealkylation reaction is selected from trifluoroacetic acid or sulphuric acid or trifluoromethanesulfonic acid or their combination thereof.

Accordingly, tert-butyl derivative of temozolomide (10) is reacted in trifluoroacetic acid and refluxing for 10 to 24 hrs followed by pouring the solution over cold isopropyl alcohol or suitable medium including cold water. The solid thus obtained is more than 90% in yield with purity exceeding 99.8%.
Alternately, this reaction can also be carried out by dissolving ter-butyl derivative of temozolomide in concentrated sulphuric acid, stirring the reaction mass for few hrs and pouring the solution over cold isopropyl alcohol to give temozolamide in 90% yield with purity greater than 99%.
The conversion of tert- butyl derivative of temozolomide to temozolomide can also be carried out using suitable dealkylating agents like boron trichloride, boron tribromide in chlorinated or hydrocarbon solvent.
The following examples, which include preferred embodiments, will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purpose of illustrative discussion of preferred embodiments of the invention.
Examples:
Preparation of temozolamide chloride (9)
19.5 g of temozolamide carboxylic acid (6), was taken in a 150 ml of DCM and to it
thionyl chloride (23.8 g) was added along with catalytic quantity of dimethylformamide.

The contents are refluxed for 3 hrs. Excess thionyl chloride was removed, toluene (50 ml) was added and removed completely to give temozolamide chloride 15.5 g as light yellowish solid m.p. 71 - 74°C. The compound was immediately converted to the N-tert-butylcarboxamide (10).
Example 2:
Preparation of N-tert-butyl-3-methyl-4-oxoimidazo-(5,l-d)-l,2,3,5-tetrazin-8-
carboxylate (10).

The acid chloride (9 ; 21.3 g) was taken in DCM and to this tert-butylamine (21.9 g) was added at 0° and then stirred at room temperature for 12 hrs. After removal of the solvent, cold water was added and filtered to yield 20.5 g of colourless solid m.p. 131.5 to 133°C.
NMR 1.5 ppm (Me3C-N) 4.05 (Me-N-)7.30 (NH-CZO) 8.36 (CH-imidazole ring)
Example 3
Preparation of compound 10 from the Temozolamide acid(6)
The acid (6, 5.0g) was taken in THF(25ml) and to this TBTU(10.4g) was added at 0°(ice-
salt) followed by addition of tert-butylamine and DMAP(6.1g) and stirred the contents for
30 minutes at 0° and then for 12 hrs at room temperature. The contents were filtered, then
cold water was added and the precipitate was collected. It was then washed with hexane
to give 4.5g(70.3%) as pale yellow powder.
The NMR spectrum was identical with the product obtained from the acid chloride (9).
Example 4
Preparation of Temozolamide
The N-tert-butyl-3-methyl-4-oxoimidazo-(2,l-d)-l,2,3,5-tetrazin-8-carboxylate (20 g)
was taken in excess of trifluoroacetic acid (100ml) and refluxed for 20 hrs. The excess
organic acid was partly removed under reduced pressure and the residual mass was
poured over isopropanol to obtain a white solid, characterized as temozolamide (HPLC
purity > 99%).

We claim,
1. A process for preparation of Temozoloamide of formula (1) comprising the
steps of:
(a) chlorinating temozoloamide acid of formula (6) with a chlorinating agent
in suitable solvent to yield an acid chloride of formula (9);

(b) reacting the acid chloride of formula (9) with excess of alkyl amine to give
amide, a compound of formula (10); or alternatively reacting
temozolamide acid (6) with an amine in presence of suitable peptide
coupling agent to yield compound of formula (10) and

(c) treating the amide (10) with an acid, to yield temozolomide (1).
2. The process as claimed in claim 1, wherein the chlorinating agent is selected
from thionyl chloride or oxalyl chloride or PCI5.
3. The process as claimed in claim 1, wherein said suitable solvent used step a) is
selected from the group consisting of chlorinated hydrocarbons,
dimethylformamide, N-methylpyrrolidone, . N,N-dialkylaniline or their
combination thereof.
4. The process as claimed in claim 1 and 3, wherein said chlorination reaction is
carried out in chlorinated hydrocarbons preferably in dichloromethane in
presence of catalytic amount of dimethylformamide at temperature ranging from
25 to 50°C.

5. The process as claimed in claim 1, where in the said alkyl amine is ter. Butyl
amine.
6. The process in claimed in claim 5, wherein said alkyl amine is also acted as base
for said reaction.
7. The process as claimed in claims 1, 5 and 6, wherein, step b) is carried out at a
temperature of-5 to 30°C.
8. The process as claimed in claim 1, wherein the acid used step c) is selected from
trifluoroacetic acid or sulphuric acid or trifluoro methanesulfonic acid or their
combination thereof.
9. The process as claimed in claim 1, wherein the peptide coupling agent used in
step b) is selected from the group consisting of TBTU, HBTU, HATU and
PyBOP.
10. The process as claimed in claim 1 and 9, wherein said reaction is carried out in
presence of suitable organic solvent and dimethylaminopyridine as base.
11. The process as claimed in claims 1,9 and 10, wherein said solvent is selected
from the group consisting of tetrahydrofuran, dimethylacetatmide, N-methyl
pyrrolidone and dimethylformamide.
12. A process for preparing Temozoloamide as substantially described herein with
reference to the foregoing examples 1 to 3.
Dated this 31st day of October 2006