IL24642A - Method for preparation of 6-thioxo purines from 6-methylthio purines - Google Patents

Description

Method for Preparation of 6-thioxo purines from 6-methylthio This invention relates to purine derivatives and more particularly to a novel method of preparing 3-methyl-6-thioxo-3,6-dihydro purines, and to certain novel purines derivatives so prepared.

These compounds are useful as growth inhibitors against bacterias and plant cells and as antitumor agents.

The conversion of alkylthio-3H-purines into the corresponding thioxo compounds is frequently an essential step in the synthesis of N-alkylated-6-thioxo-3,6-dihydro purines, and it is an object of the present invention to provide a method of demethylating the thio ether group of 3-methyl-6-methylthio-3H-purines and converting the latter to the corresponding 3-methyl-6-thioxo-7H-3,6-dihydro purines under very mild conditions.

According to the invention, a method of preparing a 3-methyl-6-thioxo-7H-3,6-dihydro purine comprises reacting the corresponding 3-methyl-6-(methyl-thio)-3H-purine with hydrogen sulphide in the presence of concentrated aqueous ammonia. The reaction can be carried out at room temperature, that is to say without the use of any external heating.

The method of the present invention can be applied to the preparation of the new compounds 3,8-dimethyl-6-thioxo-7H-3,6-dihydro purine: and 3-methyl-8-phenyl-6-thioxo-7H-3,6-dihydro-purine: CH3 from the corresponding 3,8-dimethyl-6-methylthio-3H-purine : and 3-methyl-8-phenyl-6-methylthio-3H-purine: which are themselves new compounds.

The novel starting materials may be conveniently prepared by reacting the corresponding 6-methylthio-3H-purine with methyl iodide in dirnethyljformamide in accordance with the method of Bergmann and Kleiner (Israel Journal of Chemistry, 1963, 1 , 477).

The method of the present invention can also (6-thiotheophylline) : from ^-dimethyl-^-methylthio-f-oxo-3H-t72-dihydro-purine: CH3 which is itself a new compound and which may be prepared by reacting 6-thioxo-theophylline with methyl iodide in aqueous caustic soda solution.

It is believed that the ease with which the demethylation is effected in the method of the present invention depends on the known fact that a 3-methyl group confers marked activity to the 6-position of purines. That this activity is not due to the fixation of a double bond in the 1 ,2-position but rather to the presence of a quinonoid imidazole-ring is shown by the fact that the corresponding ethers of 6-thioxanthine, such as the compound (VI) above, are readily converted into the corresponding thioxo-purine (V), and is confined by the fact that 6-methylthio purine itself does not react under the conditions of the present invention.

It is believed that the demethylation reaction of the present invention proceeds by splitting of the bond between the sulphur atom and the purine ring, the hydrosulphide ion performing a nucleophilic attack on carbon atom 6, thus releasing the 6-methylthio anion.

This invention is illustrated by the following examples Example 1. (a) Preparation of 3,8-dimethyl-6-methylthio-3H-purine (111) A suspension of S-methyl-e-thioxo^H-vl^-dihydro purine (22g), as described by Bergmann and Tamari (J. Chem. Soc. 1961 , p. 4468), in dry dimethylformamide (250 ml.) and freshly distilled methyl iodide (30 ml.) was refluxed for 2 hours. A further portion of methyl iodide ( 15 ml.) was then added and refluxing continued for a further 2 hours. The solvent was then removed in vacuo, the residue dissolved in water and an excess of concentrated ammonia was added to the acidic solution. Upon cooling, a white precipitate formed, which crystallised from acetonitrile in colourless needles (m.p. 195°C). Yield 12g. (47%).

Analysis: Calculated for C8 H i0 N 4S: C, 49.5 ; H, 5.2; N, 28.9 ; S, 16.5% Found: C, 49.4; H, 5.6 ; N, 28.8 ; S, 16.3% Preparation of 3,8-dimethyl-6-thioxo-7H-3,6-dihydro-purine (1) 3 ,8-D imethyl-6-me thy 1 thio-3 H-purine, prepared as above, (2 g.) was suspended in 25% aqueous ammonia (50 ml.) and hydrogen sulphide was bubbled through the suspension at room temperature for 20 minutes.

The reaction set in within a few minutes, as indicated by the rise in temperature and by dissolution of the suspended material. The final solution was brought to dryness in vacuo and the residue recrystallised from glacial acetic acid as sharp prisms, decomposing at 300°C. Yield 60%.

Analysis: Calculated for C 7H8 N4 S; C, 46.7; H, 4.4; N, 31.1 ; S, 17.8%.

Found: C, 46.7 ; H, 4.6; N, 30.8; S, 17.9%.

Example 2. (a) Preparation of 8-phenyl-6-thioxo-7 ej^H-Z^-dihydro purine A mixture of 8-phenylhypoxanthine (22g.), as described by Bergmann and Tamari (loc. ci ), phosphorus pentasulphide ( l OOg.) and dry 0-picoline (500ml.) was stirred and refluxed for 4 hours. The solvent was removed in vacuo and the residue treated with water (200ml.) at 70°C for 1 hour. The insoluble portion was triturated with 2N sodium hydroxide solution and the mixture passed through celite which retained the fine suspension of sulphur.

The solution was then decolourised with charcoal and acidified with acetic acid. The brown product was further purified by dissolving it in 5% sodium carbonate solution and treating the hot solution with charcoal. The sodium salt which crystallised on cooling was again dissolved in hot water and a hot, saturated solution of ammonium chloride was added. Light yellow needles were obtained, decomposing at above 300°C. Yield 19g. (80%).

Analysis: Calculated for CU H8 N4 S: C,57.9: H,3.5 ; N, 24.6%.

Found: C,57.2: H,3.6; N,24.2%. (b) Preparation of 3-methyl-8-phenyl-6-methylthio-3H-purine (IV) A solution of S-phenyl-e-thioxo-V^et-i^H-^^-dihydro-purine (39g.), prepared as described above, in dimethylformamide (600ml.) and freshly distilled methyl iodide (75ml.) was refluxed for 2 hours. A further portion of methyl iodide (40ml.) was then added and refluxing continued for a further 2 hours. The mixture, when cooled, precipitated a yellow product. The latter was dissolved in water and the pH adjusted to 10 by addition of 2N NaOH solution. The substance crystallised from isopropanol in while needles, m.p. 196-198°C. Yield 20g. (48%).

Analysis: Calculated for C13H 12 N4 S: C,60.9; H,4.7; N,12.9; S,12.5% Found: C.61 . 1 ; H,4.7; N.22. 1 ; S, 12.6%. (c) Preparation of 3-methyl-8-phenyl-6-thioxo-7H-3,6-dihydro-purine (II) 3-methyl-8-phenyl-6-methylthio-3H-purine (2g.) prepared as above, was dissolved in dimethylformamide ( 10ml.) and 0. 1 volumes of concentrated ammonia were added. Hydrogen sulphide was then bubbled through at room temperature for 20 minutes. The solution was brought to dryness in vacuo and the residue recrystallised from dilute acetic acid as yellow macrocrystalline aggregates, decomposing at 280-285°C. Yield 80% Analysis: Calculated for C12 H i0N4 S: C,59.5 ; H,4.1 ; N.23. 1 ; S, 13.2%.

Found: C, 59.5; H,4. 1 ; N.23.3 ; S, 13.5%.

Example 3.

Preparation of l S-dimethyl-e-metliylthio-^-oxo-SH-l^-dihydro-purine (VI) 6-Thiotheophylline (2.5g.), prepared as described by Woolridge and Slack (J. chem. Soc. 1962, p i 863), was dissolved in N NaOH solution ( 15 ml.) and the solution stirred with methyl iodide (2.5 ml.) at room temperature for 4 hours. After neutralization with acetic acid, the mixture was brought to dryness in vacuo.

The residue was extracted with isopropanol and the crude product recrystallised first from acetonitrile and then from isopropanol to form colourless needles of m.p. 189-191°C. Yield 0.6g. (22%).

Analysis: Calculated for C8 H I0N4 OS: C,45.7; H,4.8; N,26.7; S, 15.2%.

Found: C,45.8; H,5.0; N,26.2; S, 15.0%.

The new compound could be demethylated to 6-thiotheophylline by the process of the present invention as described in Example 1 with a yield of 80%.

The purity of the new compounds prepared in Example 1 to 3 was checked by measuring their molar extinction and by paper chromatography. The descending method was applied to Whatman No. 1 paper, using the following solvents: A - Ethanol: dimethylformamide: 25% ammonia . = 60:20:20 v/v B - Isopropanol: dimethylformamide: 25% ammonia = 65:25: 10: v/v C - n-Butanol: acetic acid: water = 60: 15:25 v/v Spots were located by their fluorescence under a Mineral light ultraviolet lamp, emitting light of about 255μ.

The physical constants so obtained are given in Tables I and II below.

TABLE I Physical constants of 3-methyl-6-methylthio-3H-purines m maaxx(mM) i ln solvent COMPOUND og„c emax RF *+ in Flue atPH8.0 A B C rmc 3,8-Dimethyl-6-methylthio-3 H 239 4.05 0.68 0.73 0.81 Green-yellow purine (III) 312 4.24 3-Methyl-6-methyl- 228 3.92 thio-8-phenyl-3H- 250 4.01 0.82 0.76 Blue purine (IV) 337 4.28 351 4.22 ls3,-Dimethyl-6-methylthio-J-oxo- 257 3.84 0.69 0.83 Blue-violet 3H-*S-dihydro-purine (VI) 334 4.11 All RF values refer to theophylline (Rp = 0.68 in all solvents) as standard.

TABLE II Physical constants of 3-methyl-6-thioxo-7H-3,6-dihydro purines (τημ) 'Vnax Rp + in solvent COMPOUND log emax Fluorescence at pH 8.0 A B C 3,8-Dimethyl-6-thioxo-7H- 3,6-dihydro purine 248 3.94 0.68 0.56 0.56 Yellow-green (I) 336 4.33 3-Methyl-8-phenyl-6-thioxo- 7H-3,6-dihydro-purine 261 4.31 0.75 0.68 Yellow (HI) 361 4.57 + All RF values refer to theophylline (RF = 0.68 in all solvents) as standard.

Claims (4)

1. WE CLAIM: A method of preparing a 3-methyl-6-thioxo-7H-3,6- dihydro-purine which comprises reacting the corresponding 3-methyl-6-methylthio-3H-purine with hydrogen sulphide in the presence of concentrated aqueous ammonia.

2. A method according to Claim 1, wherein the reaction is carried out without external heating.

3. A method according to claims 1 and 2 for the preparation of 3-methyl-6-thioxo-7H-3,6-dihydro-purine of the general formula: where R is a methyl group or a phenyl group which comprises reacting a compound of the formula: where R is as defined above, with hydrogen sulphide in presence of concentrated aqueous ammonia.

4. A method according to Claim 3, wherein the 3-methyl- 6-methylthio-¾H 3;6 dihydre purine is prepared by reacting the corresponding 6-methylthio-3H-purine with methyl iodide in dimethylformamide. A method according to claims 1 and 2 for preparing 6-thiotheophylline of the formula: k which comprises reacting a compound of the formula: with hydrogen sulphide in the presence of concentrated aqueous ammonia. A method of preparing 3-methyl-6-thioxo-7H-3,6-dihydro- purines substantially as hereinbefore described with reference to Example 1. New compounds of the general formula: where R is methyl or phenyl 3 , 8-D imethy 1-6- thioxo-7H-3 , 6-dihy dro-purine 3-Methyl-8-phenyl-6-thioxo-7H-3,6-dihydro-purine 10. 3,5-Dimethyl-6-methylthio-4-oxo-3H-4,5-dihydro-purine