Synthesis and biological evaluation of novel, peripherally selective chromanyl imidazolethione-based inhibitors of dopamine beta-hydroxylase
J Med Chem 2006, 49(3): 1191

PATENT

in the processes .

(J?) -5- (2-Aminoethyl) -1- (6, 8-difluorochroman-3-yl) -1, 3-dihydroimidazole-2 -thione hydrochloride (the compound of formula 1, below) is a potent, non-toxic and peripherally selective inhibitor of ϋβΗ, which can be used for treatment of certain cardiovascular disorders. Compound 1 is disclosed in WO2004/033447 , along with processes for its preparation.

1

The process disclosed in WO2004/033447 involves the reaction of ( R) – 6 , 8 -difluorochroman-3 -ylamine hydrochloride (the structure of ( R) -6, 8-difluorochroman-3 -ylamine is shown below as compound QA) , [4 – ( tert-butyldimethylsilanyloxy) -3 -oxobutyl] carbamic acid tert-butyl ester and potassium thiocyanate .

QA

(R) -6 , 8-difluorochroman- 3 -ylamine (compound QA) is a key intermediate in the synthesis of compound 1. The stereochemistry at the carbon atom to which the amine is attached gives rise to the stereochemistry of compound 1, so it is advantageous that compound QA is present in as pure enantiomeric form as possible. In other words, the (R) -enantiomer of compound QA should be in predominance, with little or no (S) enantiomer present. Thus, the process for preparing compound QA will advantageously produce compound QA with as high enantiomeric excess (ee) as possible.

Advantageous processes for preparing, for example, the compound of formula QA have now been found. In one aspect, the processes involve a biotransformation step. In another aspect, the processes involve chemical transformation. The processes may also be employed in the preparation of similar precursors useful in the production of other peripherally-selective inhibitors of dopamine -β -hydroxylase .

WO2008/136695 discloses a compound of formula YA, its (R) or (S) enantiomer, a mixture of its (R) and (S) enantiomers, or pharmaceutically acceptable salts thereof.

YA

The (R) -enantiomer of the compound of formula YA has been found to be a potent dopamines-hydroxylase inhibitor having high potency and significantly reduced brain access.

As disclosed in WO2008/136695 , the compound of formula YA may be prepared by reacting the compound of formula 1 with benzaldehyde under reductive alkylation conditions. In particular, (R) -5- (2 -aminoethyl ) -1- (6 , 8-difluorochroman-3 -yl) – 1 , 3 -dihydroimidazole-2 -thione and benzaldehyde may be reacted in the presence of a solvent or mixture of solvents, and a reducing agent such as sodium cyanoborohydride or sodium triacetoxyborohydride .

The compound of formula W may be prepared using a process as disclosed herein from the nitro chromene compound M.

The compound of formula WA may also be prepared using a process comprising bromination of 2 , 4 -difluorophenol to give bromophenol, alkylation of bromophenol with 4 -chloro-3 -oxo butanoate to give ketone followed by cyclization and decarboxylation to produce compound WA.

WA

According to an aspect of the present invention, there is provided the following 2 -part synthetic route from the starting material 2 , 4 -difluorophenol to (R) -5- (2 -aminoethyl ) -1- (6 , 8-difluorochroman-3 -yl) -1 , 3 -dihydroimidazole-2 – thione

hydrochloride :

Part (1)

Preferred reagents and conditions:

a) HMTA, CF₃COOH, 115°C, 18 hours

b) CH₂CHCN, DABCO, DMF, water, 70°C, 16 hours

c) H₂S0₄, AcOH, 100°C, 1 hour

d) NaClO, NaOH, MeOH, 25°C, 24 hours

e) (R) -C3 -TunePhosRu (acac) 2 S/C 3000, 30 bar H₂, MeOH, 80°C, 20 hours

f) Water, 2-propanol, reflux to 20°C

g) 40% KOH, MeOH, reflux, 24 hours

h) L-tartaric acid, ethanol, water, RT, 1 hour

Part (2)

Preferred reagents and conditions

a’) methyl vinyl ketone, t-BuONa, EtOAc, EtOH, 40-50°C, 2-3 hours

Br₂, MeOH, 20-25°C, 5 hours

water, reflux, 1 hour

KOH, AcOH, reflux, 1 hour

HCl, water, 2-propanol, 75 °C, 4 hours

KSCN, AcOH, 100°C, 2-4 hours

NaHC0₃, water, EtOH

NaBH₄, 2-propanol, THF, water, 20-25°C, 16 hours

HCl, 2-propanol, water, reflux, 1-2 hours

The ( R ) -5- (2-Aminoethyl) -1- (6, 8-difluorochroman-3 -yl) -1,3-dihydroimidazole-2 – thione hydrochloride

EXAMPLES

Example 1

Nitro chromene synthesis

To 3 , 5-difluoro-2-hydroxybenzaldehyde (lOg, 63mmol, leq) , di-n-butylamine (4.1g, 32mmol, 0.5eq) , phtalic anhydride (18.7g, 126mmol, 2eq) in toluene (500mL) was added nitroethanol (5.75g, 63mmol, leq) . The round bottomed flask fitted with a dean stark apparatus was refluxed for 18h. The mixture was cooled and nitroethanol (5.75g, 63mmol, leq) was added. The resulting reaction mixture was then reflux for 12h. After cooling, the solution was evaporated down to approximately 150mL and purified over silica gel (eluent ethyl acetate : hexane 1:1) this gave several fractions that contained only the product by TLC, these was evaporated under reduced pressure to yield 1.8g which was 100% pure by HPLC aera. Several more fractions were collected containing a mixture of product and starting material. These were combined and washed with 2% NaOH solution (2x50mL) to remove starting material. The organic layer was washed with water (50mL) , dried over sodium sulfate and evaporated under reduced pressure to give 2.49g of brown solid ( 100% pure by HPLC aera) . More fractions were collected. These were combined, washed with 2% NaOH solution (3xl00mL) , water (lOOmL) and dried over sodium sulfate. This was then filtered and evaporated down in vacuum to yield 6.14g of a brown solid which was 91.3% pure by HPLC aera. 6 , 8 -difluoro-3 -nitro-2H-chromene (9.90g, 73.4%) was obtained as a brown solid.

Example 2

Nitro chromene synthesis with column purification

To a solution of isobenzofuran-1 , 3 -dione (4,68 g, 31,6 mmol) , 3 , 5-difluoro-2 -hydroxybenzaldehyde (2,5 g, 15,81 mmol) in Toluene (25 ml) was added 2 -nitroethanol (2,88 g, 31,6 mmol). The resulting mixture was heated to reflux overnight (Dean stark) .

The reaction conversion was checked by TLC (eluent PE/EtOAc 9:1) . A yellow spot was observed and corresponds to the expected product .

Reaction was cooled to room temperature and a plug of silica gel was performed. A pale brown solid (3.9g) was obtained. “””H-NMR showed presence of product and starting material. The solid was dissolved in diethylether and the organic layer was washed with aqueous sodium carbonate, dried over Na₂S0₄, filtered and concentrated under reduced pressure. A pale brown solid (1.7g,) was obtained. The ¹H-NMR was indicated no starting material but still polymer from nitroethanol and residue of phtalic anhydride. A second silica plug (eluent: PE/EtOAc 95:5) was done. A pale yellow solid (1.5g) was obtained. ¹H-NMR of solid showed only product and polymer. The solid was recrystallized from methanol/water . A pale yellow solid (1.05g, 31.2%) was obtained.

Example 3

Nitro chromene synthesis without column purification

To a solution of isobenzofuran- 1 , 3 -dione (18,74 g, 127 mmol) , 3 , 5-difluoro-2 -hydroxybenzaldehyde (10 g, 63,3 mmol) in Toluene (100 ml) was added 2 -nitroethanol (6,86 ml, 95 mmol) . The resulting mixture was heated to reflux for 24h (Dean stark) .

The reaction conversion was checked by HPLC and by ¹H-NMR. Only 50% conversion was obtained.

The reaction mixture was cooled to room temperature and diluted with DCM (lOOmL) and 1M NaOH solution (200mL) .

The biphasic system was stirred for 30 minutes and then separated (very difficult to see phase separation) . The aqueous layer was washed with DCM (50mL) and the combined organic layers were washed twice with water (2x50ml) , dried over sodium sulfate. The filtered organic layer was concentrated under reduced pressure. To the residue was added methanol (50mL) . The methanol was then removed by distillation under reduced pressure. A brown solution precipitated when most of the methanol was removed. More methanol was added and more solid crushed out then few drops of water was added to increase the product precipitation. The brown slurry was stirred for 30 minutes and filtered. The brown solid was washed with methanol/water (1:9, 5mL) and dried in a vacuum oven at 40°C for 12h.6, 8-difluoro-3 -nitro-2H-chroraene (4,9 g, 22,99 mmol,) was obtained as brown solid in 36.3% yield.

HPLC showed a purity of 98% and ¹H-NMR confirmed the structure and purity around 95%

Example 4

Reduction of nitro chromene to nitro-alkane (racemic mixture)

To a suspension of 6 , 8 -difluoro-3 -nitro-2H-chromene (213mg, 0,999 mmol) and silica (0,8 g, 0,999 mmol) in a mixture of CHC1₃ (10 ml) and IPA (3,4 ml) at 0°C was added portion wise sodium borohydride (95 mg, 2,498 mmol). The resulting mixture was stirred at 0°C for 45 minutes. Reaction conversion was checked by HPLC. 1 mL of acetic acid was added at 0°C and the resulting mixture was stirred for 30 minutes at room temperature. The slurry was filtered and the silica was washed with DCM. The filtrate was diluted with ethyl acetate and water and the biphasic system was separated. The aqueous layer was back extracted with ethyl acetate. The combined organic layers were washed with brine, dried over MgS0₄, filtered and concentrated under reduced pressure.

6 , 8-difluoro-3 -nitrochroman (196mg, 0,911 mmol, 91 % yield) was obtained as a pale yellow oil.

Example 5

Preparation of 6 , 8 -difluorochroman-3 -one from nitro chromene

A solution of 6, 8-difluoro-3 -nitro-2H-chromene (lOOmg, 0,469 mmol) in acetic acid (0.5 ml) is added slowly to a stirred slurry of iron (262 mg, 4,69 mmol) in acetic acid (1 ml) at 60.deg. C. The reaction mixture is stirred at 60. °C for 2 hour then allowed to cool to room temperature and stirred overnight. The reaction mixture is poured onto ice-water (30 ml) and filtered through Celite. The solid was wash with dichloromethane (DCM) (50 ml) . The organic portion is separated and washed with water (2 x 30 ml) and brine (30 ml) , dried over MgS0₄, filtered and concentrated in vacuo to give a brown oil. 6,8-difluorochroman-3 -one (75 mg, 0,407 mmol, 87 % yield) was obtained as a brown oil.

Example 6

Preparation of 6 , 8-difluorochroman-3 -one from methyl 6,8-difluoro-2H-chromen-3 -yl-carbamate

Methanol (1000m ml) was added to a slurry of methyl fluoro-2H-chromen-3 -yl -carbamate (250 g, 1.037 mol) hydrogen chloride 6N (2000 ml, 12 mol) at room temperature. The resulting mixture was reflux and stirred for 2 hours. Reaction monitored by HPLC.

Reaction was not complete but was stopped in order to avoid degradation of the product. The yellow solution was cooled to room temperature. A slurry (two type of solid) was observed and diluted with diethyl ether (300mL) . The resulting slurry was stirred at 5°C for 1 hour then filtered. The yellow solid was washed with water. The resulting wet yellow solid was suspended in diethylether (400mL) and petroleum ether (PE) (400mL) was added. Slight yellow solid was stirred at room temperature overnight, filtered and washed with PE (300mL) , dried in a vacuum oven at 30 °C for 4h. The wet sample was checked by NMR. No starting material was detected. A pale yellow solid (72.5g, solid 1) was obtained. The mother liquors were concentrated to dryness. A yellow solid was obtained, suspended in diethyl ether and PE. The slurry was then stirred for 4 hours, filtered, washed with PE . A dark yellow solid (4.5g, solid 2) was obtained. Solid 1 (2g) was diluted in DCM and washed with water (pH =6). The organic layer was then dried over Na₂S0₄, filtered, concentrated to dryness. A crystalline pale yellow solid (1.9g, solid 3) was obtained. NMR showed the same purity for solid 3 as for solid 1. The remaining part of solid 1 was then diluted in DCM. The resulting organic layer was washed with water, dried over Na₂S0₄, filtered and then concentrated to dryness. Slight yellow crystalline solid (68.5g, solid 4) was obtained. NMR confirmed high quality material.

Loss on Drying (LOD) : 1.03% .

Example 7

Biotransformation: Transaminases

Codexis transaminases ATA-025, ATA-251 and ATA-P2-A07 recognized 6 , 8 -difluorochroman-3 -one as the substrate and produced the corresponding 6 , 8 -difluorochroman-3 -amine .

PATENT

WO 2004033447

WO 2008094056

WO 2008143540

WO 2009064210