NEW PATENT, WO 2016178162, TENOFOVIR, CBZ INVESTMENTS LTD.

NEW PATENT

WO2016178162,  SYNTHESIS OF INTERMEDIATES USED IN THE MANUFACTURE OF ANTI-HIV AGENTS

CBZ INVESTMENTS LTD. [--/GB]; 122, Feering Hill, Feering, Colchester, Essex CO5 9PY (GB)

PRADHAN, Braja Sundar; (IN).
AMARNATH, Kommireddy; (IN).
VEMPALA, Naresh; (IN).
RAHMAN, Md. Ataur; (IN)

PRADHAN, Braja Sundar

Process for preparing 9-[2-(hydroxyl)propyl]adenine and 9-[2-phosphonomethoxypropyl]adenine, useful as intermediates in the synthesis of anti-HIV agents such as tenofovir disoproxil fumarate (TDF) and tenofovir alafenamide fumarate (TAF).

Represents the first patenting to be seen from CBZ that focuses on tenofovir. Gilead Sciences, under license from the Rega Institute of Medical Research, had developed and launched TDF.

Gilead was also developing TAF, an isopropylalanilyl phenyl ester prodrug of TDF. In November 2016, TAF was reported to be in pre-registration phase.

The present invention relates to a process of preparing intermediates of Formula (I). The process comprises of reacting compound of Formula (III) with compound of Formula (V) in the presence of a solvent selected from an alcohol, ether or water to form compound of Formula (I) wherein, R1 is selected from –NH2, Cl, Br, NHCOR", wherein R" is alkyl, aryl, Schiff's base of formula N=CHR', wherein R' is alkyl or aryl; R2 is selected from H, alkyl; R3 and R4, each independently is H; R5 and R6, each independently is H, alkyl; R7 is H, alkyl; and R8 is H, alkyl.

The US Patent US 5935946A discloses a method for preparation of 9-[2-(R)-(hydroxyl)propyl]adenine. The method involved coupling (R)-1 ,2-propylene carbonate with adenine under basic condition in N, N-dimethylformamide (DMF) at 130°C for 18-30 h to furnish the said product. The referred intermediate compound, (R)-1 , 2-propylene carbonate in turn was prepared in two steps from (S)-Glycidol. In the first step, (S)-Glycidol was subjected to reductive hydrogenation conditions to prepare (R)- 1 ,2-propanediol. In the following step, the diol reacted with diethyl carbonate under suitable reaction conditions to furnish the said carbonate.

A process reported in Technical Reports, Volume 9, Number 14, page 2/7, Joint American Chemical Society, 59th Northwest and 18th Rocky Mountain Regional Meeting, Logan, Utah, June 6-9, 2004 by Robert C. Ronald and John M. Whitaker discloses an entirely different process for the synthesis of the said intermediate compound, (R)-1 , 2-propylene carbonate. The synthesis used ethyl (S)- lactate as the starting material. It was transformed to an intermediate compound, (S)-2-tosyloxy-1 -propanol, which, in the subsequent step, reacted with CO2 in the presence of a phase transfer reagent, 18-crown-6, to provide the required (R)-1 ,2-propylene carbonate.

The process reported in the US Patent US 5935946A suffers from various drawbacks. The first drawback lies in the coupling between (R)-1 ,2-propylene carbonate and adenine. The use of such a high boiling solvent such as Ν, Ν-dimethylformamide as the reaction medium and conducting the reaction at as high a temperature as 130°C for a long duration of time do throw operational and work up challenges in commercial production. Secondly, the preparation of the important intermediate compound, (R)-1 ,2-propylene carbonate, throws a bigger challenge; its preparation when taken into account the preparation of (S)-Glycidol is cumbersome, involving many steps including a step of reductive hydrogenation using a fairly expensive heavy metal catalyst. The second process for preparing (R)-1 ,2-propylene carbonate as reported in Technical Report does not offer any significant advantage even though it uses an inexpensive starting material; the use of the phase transfer reagent, 18-crown-6, renders the process commercially unattractive.

Scheme 1

The reaction scheme for preparing compound of Formula II is represented below:

Scheme 2

Formula II

L-(+)-Lactic axid VIII IX

a. i. Methanol ii. Thionyl cMoride c. i. NaBH4 ii. Methanol / DicMoromethane

d. K2C03

Scheme: 3

Example: 1 Preparation of Methyl L-(+)-lactate (VIII)

Thionyl chloride (96.72 ml) was added drop by drop to a solution of L-(+) lactic acid (100 g) in methanol (500 ml) at -10°C over a period of 1 h. After the completion of addition, the reaction mixture was warmed to room temperature and stirred at this temperature until the reaction was complete as indicated by TLC. The reaction mixture was concentrated at 90°C to furnish an oil which was dissolved in dichloromethane (250 ml). The organic layer was washed with saturated sodium bicarbonate solution (2 X 150 ml), dried over sodium sulfate and concentrated at 50°C to give VIII; yield: 79 g.

Example: 2 Preparation of Methyl 2-(S)-methylsul†onyloxypropionate (IX)

Triethylamine (107 ml) was added to a cold, stirred solution of methyl (S)-lactate (40 g) in dichloromethane (400 ml) -10°C. Methanesulfonyl chloride (38.8 ml) was added to the reaction mixture drop by drop over a period of 1 h at this temperature, and the reaction mixture was stirred at this temperature for about 2 h when the reaction was complete, as indicated by TLC. The reaction mixture was poured on cold water (400 ml) and stirred for 10 minutes. The separated organic layer was washed with 5 % dilute hydrochloric acid (1 X 80 ml), saturated sodium bicarbonate solution (1 X 100 ml), brine (1 X 100 ml), dried over sodium sulfate and concentrated at 50°C to furnish the desired product; 48.2 g.

Example: 3 Preparation of 2-(R)-(+)-propylene oxide (V)

Method: A

Methanol (6 ml) was added to a solution of the mesylated methyl (S)-lactate (3 g) in dichloromethane (9 ml). The reaction mixture was cooled to -10°C and sodium borohydride (748 mg) was added to the mixture in divided portions over a period of 0.5 h at this temperature. After 4 h at this temperature, when the reaction was complete as indicated by TLC, the reaction mixture was diluted with water (10 ml) and dichloromethane (20 ml) and warmed to room temperature. After 10 minutes at room temperature, layers were separated. The organic layer was washed with

10 % aqueous citric acid solution (1 X 10 ml), brine (1 X 10 ml), dried over sodium sulfate and concentrated under reduced pressure at 40°C to give mesylated alcohol; yield: 1 .1 g. The crude mesylated alcohol was treated with potassium carbonate, and distilled to furnish (R)-(+)-propylene oxide.

Example: 4 Preparation of 2-(R)-hydroxypropanol (XII)

Sodium borohydride (1 .81 g) was added to a stirred, cold solution of methyl (D)-lactate (5 g) in methanol (25 ml) at -5°C in divided portions over a period of 0.5 h. The reaction mixture was stirred at this temperature for 3 h when the reaction was complete as indicated by TLC. The reaction mixture was neutralized with concentrated hydrochloric acid until the pH of the mixture attained the range 6- 7, and then concentrated under reduced pressure at 40°C. The residue thus obtained was dissolved in ethyl acetate (30 ml) and filtered. The filtrate was washed with water (1 X 20 ml), brine (1 X 5 ml), dried over sodium sulfate and concentrated under reduced pressure at 40°C to give the product XII; yield: 1 .97 g.

Example: 5 Preparation of 2-(R)- (+)-propylene oxide (V)

Method: B

Pyridine (1 .58 ml) was added to a solution of the diol (XII) (0.5 g) in dichloromethane (6 ml) at room temperature. 4-Tolunesulfonyl chloride (1 . 5g) was added at -78°C and the reaction mixture was allowed to warm to room temperature. The reaction mixture was stirred at this temperature

until the reaction was complete as indicated by TLC. The reaction mixture was poured into cold water to separate the layers. The organic layer was washed with 10% dilute hydrochloric acid, water, brine, dried over sodium sulfate and concentrated at 40°C to furnish a residue which was purified to furnish the said compound XIII; yield: 0.7 g. The alcohol was was treated with potassium carbonate, and distilled to furnish (R)-(+)-propylene oxide.

Example: 6 Preparation of 9-[2-(R)-(Hydroxy)propyl]adenine (I)

Method-A

Sodium hydroxide (3 g) was added to a stirred suspension of adenine (10 g) in water (50 ml) at room temperature, and the reaction mixture was heated to 100-1 10°C. After 5 hrs at this temperature, the reaction mixture was allowed to cool down to 25°C. Ammonium chloride (3.96 g) and 2-(R)-(+) propylene oxide (6.44 g) were added in succession to the reaction mixture at this temperature. The reaction mixture was maintained at this temperature until the reaction was complete as indicated by TLC. The reaction mixture was concentrated under reduced pressure at 55°C to furnish the desired product, contaminated with unidentified polar impurities. The crude material was purified by column chromatography using a solvent system comprising of methanol and dichloromethane (1 : 9) as the eluant to furnish 9-[2-(R)- (hydroxyl)propyl]adenine as a white solid; yield: 5.5 g.

Example: 7 Preparation of 9-[2-(R)-(Hydroxy)propyl]adenlne (I)

Method-B

Potassium tert-butoxide (4.15 g) was added to a stirred suspension of adenine (10 g) in methanol (150 ml) at 0°C. The reaction mixture was allowed to warm to room temperature, and stirred at this temperature for 30 minutes. R-(+)-Propylene oxide (6.44 g) was added and the reaction mixture was stirred at this temperature until the reaction was complete, as indicated by TLC. The reaction mixture was neutralized with methanesulfonic acid and filtered. The filtrate was concentrated under reduced pressure to furnish the desired product, contaminated with unidentified polar impurities. The crude material was purified by column chromatography using a solvent system comprising of methanol and dichloromethane (1 :9) as the eluant to furnish 9-[2-(R)-(hydroxyl)propyl]adenine as a white solid; yield: 6.2 g.

Example: 8 Preparation of 9-[2-(R)-(Hydroxyl)propyl]adenine (I) Method-C

Potassium carbonate (204 mg) was added to a stirred suspension of adenine (5 g) in a solvent mixture comprising of methanol (13 ml) and N,N-dimethylformamide (2.5 ml) at 0°C. The reaction mixture was allowed to warm to room temperature. (R)-(+)-Propylene oxide (3.78 ml) was added to the reaction mixture in three divided batches over a period of 20 h, and the reaction mixture was stirred at this temperature until the reaction was complete, as indicated by TLC. The reaction mixture was neutralized with methanesulfonic acid and concentrated under reduced pressure. Toluene (15 ml) was added to the residue thus obtained. The reaction mixture was cooled to 0°C and stirred at this temperature for 2 h to maximize the precipitation of the desired compound. The precipitated material was filtered and dried to furnish 9-[2-(R)-(hydroxyl)propyl]adenine, crude yield: 6.8 g. The product was contaminated with unidentified polar impurities. The crude material was carried over to the following step without any purification.

Example: 9 Preparation of 9-[2-(R)-(hydroxyl)propyl]adenine (I)

Method-D

Magnesium di-tert-butoxide (630 mg) was added to a stirred suspension of adenine (5 g) in a solvent mixture comprising of methanol (15 ml) and N,N-dimethylformamide (15 ml) at 0°C. The reaction mixture was warmed to room temperature. (R)-(+)-Propylene oxide (3.78 ml) was added to the reaction mixture in three divided batches over a period of 20 h and the mixture was stirred at room temperature until the reaction was complete, as indicated by TLC. The reaction mixture was then neutralized with methanesulfonic acid and concentrated under reduced pressure. Toluene (15ml) was added to the residue thus obtained. The reaction mixture was cooled to 0°C and stirred at this temperature for 2 h to maximize the precipitation of the desired compound. The precipitated material was filtered and dried to furnish 9-[2-(R)-(hydroxyl)propyl]adenine, crude yield: 6.4 g. The product was contaminated with unidentified polar impurities. The crude material was carried over to the following step without any purification.

Example: 10 Preparation of 9-[2-(R)-(Hydroxyl)propyl]adenine (I) Method: E

Magnesium di-tert-butoxide (100 mg) was added to a stirred suspension of adenine (2 g) in methanol (12 ml) at 0°C. After 6 h at this temperature, sodium hydroxide (24 mg) and (R)-(+)Propylene oxide (1 .25 g) were added in succession to the reaction mixture. The mixture was allowed to warm to room temperature, stirred at this temperature until the reaction was complete as indicated by TLC and concentrated under reduced pressure. Toluene (6 ml) was added to the residue thus obtained. The reaction mixture was cooled to 0°C and stirred at this temperature for 2 h to maximize the precipitation of the desired compound. The precipitated material was filtered and dried to furnish 9-[2-(R)-(hydroxyl)propyl]adenine, crude yield: 3.2 g. The product was contaminated with unidentified polar impurities. The crude material was carried over to the following step without any purification.

Example: 11 Preparation of 9-[2-(R)-(phosphonomethoxy)propyl] adenine (II).

Method-A

Magnesium di-tert-butoxide (51 mg) was added to a stirred suspension of adenine (2 g) in methanol (12 ml) at 10°C. After 4 h at this temperature, sodium hydroxide (12 mg) and (R)-(+)-propylene oxide (1 .25 g) were added in succession to the reaction mixture. The reaction mixture was allowed to warm to room temperature and stirred at this temperature until the reaction was complete, as indicated by TLC. Methanol was evaporated under vacuum. N, N-Dimethylformamide (16 ml) was added to the crude reaction mixture thus obtained and the mixture was heated to 60-70°C. Magnesium di-tert-butoxide (7 g) was added to the reaction mixture in four divided batches over a period of 15 minutes at this temperature. The mixture was heated to 80-90°C, and stirred at this temperature for 30 minutes. Diethyl p-toluenesulfonyloxymethylphosphonate (13.2 g) was added drop by drop to the reaction mixture over a period of 4 h, and the mixture was stirred at this temperature until the reaction was complete, as indicated by TLC. N, N-Dimethylformamide was distilled out under vacuum at 90-100°C. Aqueous hydrobromic acid (48% w/w, 30 ml) was then added to the residue and the reaction mixture was heated to gentle reflux. After approximately 20 h at this condition, the reaction mixture was allowed to cool down to room temperature and filtered. The filtered solid was washed with dichloromethane (10 ml). The washing was concentrated to furnish a residue. The residue was combined with the filtrate and the combined filtrate was washed with dichloromethane (2 X 10 ml). To the aqueous layer, an aqueous solution of sodium hydroxide (50 %) was added until the pH attained 2.1 -3. After several hours at room temperature, the aqueous layer was cooled to 0- 5°C and stirred at this temperature for further hours to maximize precipitation of the desired product from the solution. The precipitated solid was filtered, washed with cold water (1 X 5 ml), acetone (2X 5 ml) and dried to obtain 9-[2-(R)-(phosphonomethoxy)propyl]adenine; yield: 1 .42 g.

Example: 12 Preparation of 9-[2-(R)-(phosphonomethoxy)propyl] adenine (II)

Method-B

Magnesium di-tert-butoxide (1 .76 g) was added in four divided batches to a stirred suspension of 9-[2-(R)-(hydroxyl)propyl)adenine] (1 g) in N, N-dimethylformamide (3 ml) at 60-70°C over a period of 1 h. The reaction mixture was heated to 90°C and diethyl chloromethylphosphonate (1 .93 g) was added drop by drop to the reaction mixture over a period of 4 h. The reaction mixture was maintained at this temperature until the reaction was complete, as indicated by TLC. Ν, Ν-Dimethylformamide was distilled out under vacuum at 90-100°C. To the residue thus obtained, aqueous hydrobromic acid (48% w/w, 10 ml) was added and the reaction mixture was heated to gentle reflux. After approximately 20 h at this condition, the reaction mixture was allowed to cool down to room temperature and filtered. The filtered solid was washed with dichloromethane (10 ml). The washing was concentrated to furnish a residue. The residue was combined with the filtrate and the combined filtrate was washed with dichloromethane (2 X 10 ml). To the aqueous layer, an aqueous solution of sodium hydroxide (50%) was added until the pH attained 2.1 -3. After several hours at room temperature, the aqueous layer was cooled to 0-5 °C and stirred at this temperature for further hours to maximize precipitation of the desired product from the solution. The precipitated solid was filtered, washed with cold water (1 X 5 ml), acetone (2 X 5 ml) and dried to obtain 9-[2-(R)-(phosphonomethoxy)propyl]adenine; yield: 0.7 g.

Example: 13 Prepration of diethyl (chloromethyl)phosphonate (VI)

Pyridine (108 ml) was added to a stirred solution of (hydroxymethyl)phosphonic acid (50 g) in benzene (300 ml) at room temperature. The reaction mixture was heated to 60°C. Thionyl chloride (100 ml) was added to the mixture over a period of 30 min at this temperature. After 1 h at this temperature, the reaction mixture was allowed to cool down to room temperature, stirred for 3 h at this temperature and cooled to 10°C. The precipitated solid was filtered and washed with benzene (100 ml). The washing was combined with the filtrate and the combined filtrate was concentrated at 100°C to furnish an oil. The oil was fraction distilled under reduced pressure to furnish a

colourless liquid; yield: 22 g. The liquid was cooled 0°C. Ethanol (40 ml) was added to the liquid at this temperature. The reaction mixture was allowed to warm to room temperature, stirred for 3 h at this temperature and diluted with dichloromethane (100ml). The combined organic layer was washed with water (2 χ 50 ml), brine (50 ml), dried over sodium sulfate and concentrated under reduced pressure to furnish diethyl (chloromethyl)phosphonate as a colorless liquid; yield: 18 g.

Ph.D. CSIR Indian Institute of Chemical Technology, Hyderabad, India
Advisor: Dr. J. S. Yadav
M.Sc. Jamia Millia Islamia, New Delhi, India

mdataur_rahman@fas.harvard.edu

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