Process for the preparation of calcipotriol

Field of the invention

The invention relates to the process for preparation of cholecalciferol derivatives having double bond in a side chain, particularly calcipotriol (calcipotriene) .

Calcipotriol, (lα, 3β, 5Z, IE, 22E, 24S)-24-cyclopropyl-9, 10- secochola-5,7,10(19),22-tetraene-l,3,24-triol, is the synthetic vitamine D3 analog exhibiting an antiproliferative activity, launched for the treatment of psoriasis.

Background of the invention

The preparation method of calcipotriol, developed by Calverley (WO 87/00934; Tetrahedron 43, 4609 (1987)), is based on a Wittig reaction of C-22 aldehyde derivative of cholecalciferol, having triene (5E,7E) configuration, with triphenylphosphorane cyclopropyl ketone ylide. The reduction of the side chain carbonyl group to C-24 alcohol, gives the mixture of C-24 diastereomers. The disadvantage of the process is the formation of triphenylphosphine oxide as a side product which is water insoluble and difficult to remove from the reaction mixture. The desired (24S)-alcohol needs to be separated by column chromatography. In that synthetic method, more than a half of the amount of obtained product is being lost. As the consequence, the undesired product, [5E), (24 S) isomer, is to be subject to photoisomerization to yield the product of a proper (52), (24 S) configuration.

In the International Patent publication WO 2005/095336, in the Wittig-Horner reaction the more reactive phosphonate derivative instead of triphenylphosphorane is used. Phosphate ester formed in the reaction, unlike triphenylphosphine oxide, is soluble in water and thus can be easily removed from the reaction mixture. Despite that improvement, the main disadvantage of Calverley's synthetic pathway

still remains and it concerns the necessity of C-24 diastereomers separation in the last step.

Another method of calcipotriol synthesis (Synlett, 1990, 157) consists in condensing C-22-seleneacetal with the fragment of the side chain, (S)-2-[(tert-butyl)dimethyl]silyloxy-2-cyclopropylacetyl aldehyde, resulting in the formation of the mixture of diastereoisomeric 23-hydroxy-22-methylselenides, which are treated with methane sulfonyl chloride and triethylamine. In two following steps hydroxyl and selenemethyl groups are removed, resulting in the formation of the mixture of (5E\,(22E/Z\ olefins as the protected triols. The mixture is chromatographically separated and the obtained product of (5E), [22E) configuration is subject to anthracene sensitized photoisomerization of (5£,7£^-triene to (5Z,7E)-triene, followed by the removal of silyl groups. On account of some drawbacks concerning work with seleneorganic componds (unpleasant odor, toxicity and low stability of methylselenol), lack of stereoselectivity at the step of selenium removal from (α-hydroxy)methylselenides, this method is not suitable in a larger scale production.

Another possible way of obtaining calcipotriol accompanied by its C-24 epimer is disclosed in Japanese patent application JP 08325226 A2. This method is based on coupling of two synthons: calcipotriol A ring - (4i?,6S)-4,6-di(t-butyl)dimethylsilyloxy-7-octen-l- yn and 7-bromo derivative, consisting of calcipotriol CD rings, under Heck reaction conditions, followed by the removal of protecting groups. The synthesis of both synthons used in a coupling-cyclization reaction proved to be difficult and multistep process.

In the Japanese patent application JP 06316558 A2 the preparation of (7Z) -calcipotriol isomer from appropriately substituted cholesta-5,7-dien during photochemical or termal rearrangement process is disclosed.

The prior art methods for the preparation of calcipotriol have some disadvantages, connected with easy isomerization of asymmetric center C-20 of starting C-22 aldehydes, lack of selectivity

in the step of C-24 keton reduction and use of preparative chromatography purification step. These factors adversely affect the implementation of the described syntheses in routine laboratory practice. A number of methods were developed to increase the total yield of the process, whereas undesired (24.R)-isomer is transformed into the mixture enriched with the desired (24S) epimer. In the International Patent publication WO 03/ 106412 the method of regaining of desired calcipotriol (24S) epimer is disclosed. The process is based on the racemization of calcipotriol C-24 p- nitrobenzoate under the Mitsunobu reaction conditions using diisopropyl azadicarboxylate.

In the International Patent publication WO 2006/024296 in turn, epimerization of C-24 alcohol in aqueous organic medium under acidic conditions is claimed. In this process no additional hydroxyl group transformation is necessary. Although the latter procedure is superior to the former one described in WO 03/ 106412 as the esterification as well as alcohol hydrolysis are avoided, the problems of diastereomers separation and discarding of half of the obtained racemic mixture still exist.

Some attempts were undertaken towards calcipotriol preparation in direct olefination method under Julia-Kocienski protocol, known in the art [P. R. Blakemore at al., Synlett, 26 (1998); P.R. Blakemore at al., J. Chem. Soc. , Perkin Trans. I, 955 (1999)], applying a phenyltetrazole sulfon. This method was applied in the synthesis of cholecalciferol derivatives [Kutner, A., Przem. Chem., 85(5), 322 (2006)]. The attempts to overcome the difficulties with low transformation rate of thiophenyltetrazole derivative into appropriate sulfonylphenyltetrazole have failed. In addition, the high cost of commercially available main reagent, l-phenyltetrazole-5-thiol, may be the limitation in the implementation of this method in the production scale.

In the International Patent publication WO 03/087048, method of calcipotriol preparation with the use of benzothiazole sulfon is disclosed. Use of benzothaizole sulfon was first reported for the direct aldehydes olefination by Sylvestre Julia [J. B. Baudin at al., Tetrahedron Lett. 32, 1175 (1991)]. In the Sylvestre Julia's procedure, condensation of deprotonated benzothiazole sulfon and aldehyde under basic conditions proceeds, followed by subsequent cyclization and rearrangement, accompanied by sulphur dioxide elimination. As a reaction result, olefin is formed as well as water soluble benzothiazolone salt [J.B. Baudin at al., Bull. Soc. Chim. Fr. 130, 336 (1993); Bull. Soc. Chim. Fr. 130, 856 (1993O].

Oxidation of benzothiazole sulfide to sulfon, unlike phenyltetrazole sulfide, proceeds in moderate yield (62%). Crystalline C-22 sulfonylbenzothiazole derivative is a convenient, advanced intermediate in the synthesis of calcipotriol. However, under the reaction conditions with the use of benzothiazole sulfon, (222) byproduct is formed in over 10% yield. The removal of this by-product up to the pharmaceutically accepted level is accomplished due to multi-step crystallization process. The elaborate purification procedure affects the total yield of the synthetic process.

In the art, Marc Julia's [M. Julia, J. M. Paris, Tetrahedron Lett. 1973, 4833] olefination method was one of most frequently used, when C=C double bond formation in a molecule was necessary. In this process aldehyde reacted with phenyl sulfonyl anion, generated in situ when treated with n-butyl lithium; obtained intermediate was subsequently functionalized and subject to reductive elimination with sodium amalgam to yield alkene.

Regardless of popularity of this method, it has not been applied in direct condensation of cholecalciferol C-22 phenylsulfonyl derivative, having methyl group at alfa position in respect to phenylsulfonyl substituent, and aliphatic aldehyde having at alfa position bulky substituent, such as terf-butyl-diphenylsilyl. In light of WO 03/087048, that type of hydroxyl protection facilitates

purification and isolation of the reaction product. The silyl protection also enables the starting aldehyde detection by UV spectroscopy in TLC and HPLC chromatography, and causes the increase of molecular density and decrease of volatility thereof. Now, it has unexpectedly been found that cholecalciferol C-22 phenylsulfonyl derivative of Formula 2 reacts in good yield with alfa- hydroxy aldehyde of Formula 3, the side chain precursor bearing bulky silyl substituent, in the condensation reaction under Julia protocol.

Description of the invention

1. The invention provides the process for preparation of calcipotriol,

comprising the steps of:

(a) reacting a C-22 phenylsulphonyl derivative of cholecalciferol of Formula 2,

wherein R ₁ and R2 are the same or different and represent hydroxyl protecting groups, with a silyl derivative of alfa-hydroxy aldehyde of Formula 3,

wherein R3 represents silyl group of formula Si(R^(Rs)(Ro), where R4-R6 are the same or different and represent Ci-β alkyl or phenyl groups, in the presence of a strong organic base in aprotic solvent, to yield the diastereomeric mixture of alfa-hydroxysulfones of Formula 4,

wherein the substituents R ₁ -Ra are as defined above, (b) performing a reductive desulfonation of alfa-hydroxysulfones of the Formula 4 obtained in step (a) with sodium amalgam, to yield the olefination product of Formula 5,

(c) removing the hydroxyl protecting groups under standard conditions,

(d) purification of the product obtained in step (c). As organometallic componds, lithium, sodium or potassium organic derivatives, preferably n-butyl lithium, are used.

As aprotic solvents, phosphoric acid alkyl amides or urea alkyl derivatives, preferable hexamethylphosphorous triamide (HMPT) or tetrahydrofurane (THF), are used. Hydroxyl protecting group of phenylsulfone of formula 2 is any group routinely used in vitamin D chemistry, such as for example acyl, alkylsilyl or alkoxyalkyl group. Acyl group includes alkanoyl and carboxyalkanoyl groups, having 1 - 6 carbon atoms, preferably acetyl

group. Usual alkoxyalkyl groups are methoxymethyi, ethoxyethyl, tetrahydrofuranyl and tetrahydropyranyl groups. Most common silyl groups are trialkylsilyl, dialkylarylsilyl, alkyldiarylsilyl, triarylsilyl group, such as for example trimethylsilyl, triethylsilyl, t-butyl- dimtheylsilyl, triphenylsilyl group.

In the preferred embodiment of the invention, the hydroxyl groups in both reagents are protected with the same silyl substituents, eg. t-butyldimethylsililoxy groups.

Deprotection of hydroxyl groups is carried out under basic conditions. Usually used silyl groups are removed when treated with tetrabutylammonium fluoride, in organic solvents such as THF or acetone, often in the presence of water.

5 or 10% sodium amalgam used in dehydroxy-desulfonation reaction is commercially available. Starting compound, C-22 phenylsulfone of Formula 2, is the key synthon used in the vitamin D chemistry. It is crystalline compound of high stability, that enables its easy purification to required pharmaceutical purity before is being reacted with an aldehyde. The process according to the invention enables the successful preparation of calcipotriol in a direct chemical synthesis, avoiding the diastereomeric mixture separation. The results of chromatography with chiral stationary phase proved, that the amount of undesired (22Z)-isomer reached only 2,2% in a crude reaction mixture. The contents of this impurity can be reduced to below 0,3% during one or two crystallizations.

Unexpectedly it has been discovered that the product obtained by the process of the invention and subsequently purified by standard procedure, such as the column chromatography and crystallization, is free of mercury. The traces of mercury were not detected, even using very sensitive analytical methods, such as atomic absorption method.

The invention is illustrated by the following examples, which should not be construed as any limitation of its scope.

Examples Example 1

(5Z,7£^-(lS,3i?,24S)-l,3-bis(t-butyldimethylsilyloxy)- (22#,23i?/22i?,23S/22S,23#/22S,23S)-22-sulfonylphenyl-23- hydroxy-24-cyclopropyl-24-£-butyldiphenylsilyloxy-9, 10-secochola- 5,7, 10(19)-trienes (4).

(5Z,7E)-( 1 S,3Rj- 1 ,3-bis(t-butyldimethylsilyloxy)-22- sulfonylphenyl-23,24-dinor-9,10-secochola-5,7, 10(19)-trien of

Formula 2 (384 mg, 0,55 mmol) was dissolved in THF (2 mL). The flask was placed in a cooling bath (-68 ⁰ C) on a magnetic stirrer under Ar. While stirring n-butyl lithium (1,6 M in THF, 350 μL, 0.56 mmol)) was added drop- wise with a syringe. Stirring was continued for 30 min. at -68 ⁰ C. Next, (2S)-2,2-cyclopropyl-t- butyldiphenylsilyloxyetanal of Formula 3 (200 mg, 0,59 mmol) was slowly added drop-wise. Cooling bath was removed after 30 min. and the reaction mixture was stirred for additional 2 h. at ambient temp. Brine (1 mL) was added, organic phase was separated and dried over Na2SO4 (100 mg). After filtration solvents were removed under reduced pressure. The residue was filtered through silica gel (2 g). The mixture of crude products (5Z,7E)-(lS,3i?,24S)-l,3-bis(t- butyldimethylsilyloxy)-(22i?,23/?/22i?,23S/22S,23i?/22S,23S) -22- sulfonylphenyl-23-hydroxy-24-cyclopropyl-24-t-butyldiphenyls ilyloxy- 9, 10-secochola-5,7, 10(19)-trienes of Formula 4 (500 mg, 88%) was obtained, as colorless powder.

Example 2

(5Z,7E,22£^-(l S,3i?,24S)-l,3-bis(t-butyldimethylsilyloxy)-24- cyclopropyl-24-t-butyldiphenylsilyloxy-9, 10-secochola-5,7, 10(19),22- tetraen (5).

The mixture of (5Z,7E)-(lS,3i?,24S)-l,3-bis(t- butyldimethylsilyloxy)-(22J?,232?/ 222?,23 S/ 22 S,23R/ 22 S,23 S)-22- sulfonylphenyl- 23 -hydroxy- 24 -cyclopropy 1- 24 - t-buty ldipheny lsilyloxy- 9, 10-secochola-5,7, 10(19)-trienes of Formula 4 (500 mg, 0.45 mmol) was dissolved in THF (1 mL), in a round-bottom flask of 10 mL capacity. The flask was placed on a magnetic stirrer and saturated methanolic Na2HPO4 solution (2 mL), followed by sodium amalgam Na/ Hg (1,2 g) were added. The reaction mixture was vigorously stirred at ambient temp, for 2 h. The solution was decanted and the solvents were removed under reduced pressure. The product was extracted with hexane (3 x 5 mL) from dry residue. Hexane was evaporated and the residue was filtered through silica gel (2 g). {5Z,7E,22E)-{ 1 S,3R,24S\- 1 ,3-bis(f-butyldimethyldimethylsilyloxy)-24- cyclopropyl-24-t-butyldiphenylsilyloxy-9, lO-secochola-5,7, 10(19),22- tetraen of Formula 5 was obtained (276 mg, 64%), as colorless powder.

Example 3 {5Z,7E,22 Ej-[I S,3fl,24S)-24-cyclopropyl-9, 10-secochola- 5,7, 10(19),22-tetraen-l,3,24-triol (l).

(5Z,7E,22E)-{1 S,3i?,24S)- 1 ,3-bis(t-butyldimethylsilyloxy)-24- cyclopropyl-24-t-butyldiphenylsilyloxy-9, 10-secochola-5,7,10(19),22- tetraen of Formula 5 (276 mg, 031 mmol) was dissolved in THF (2 mL) under Ar. The solution was warmed at heating bath (+60 ⁰ C) on magnetic stirrer. The tetrabutylammonium fluoride solution (1 M in THF, 1,0 mL, 1,0 mmol) was added drop-wise and stirring was continued for 1,5 h (+60 ⁰ C). After removal of a heating bath, the solution was cooled down to 2O ⁰ C and 1 mL of brine was added. Organic phase was separated, dried over Na2SO4(200 mg), filtered and condensed under the vaccum. The crude product was contaminated with 2,2% of (222)-isomer of calcipotriol (HPLC). The

residue was filtered through silica gel (2 g). When the solvents had been removed, the resulting solid was crystallized from ethyl acetate (500 μL). (5Z,7E,22EHlS,3i?,24S)-24-cyclopropyl-9,10-secochola- 5,7,10(19),22-tetraen-l,3,24-triol of Formula 1 was obtained (89 mg, 68%), as colorless crystals, of 98,5% purity (HPLC), UV λ _max (EtOH) 265,0, 212,0 nm, λmin 229,0 nm; IR v 3401, 2949, 2927, 2869, 1631, 1432, 1376, 1325, 1246, 1064, 981, 911, 797 cm- ⁱ ; ¹ H-NMR (δ, ppm): 0,57 (3H, s, 18-CH ₃ ), 1,04 (3H, d, J=8 Hz, 21-CH ₃ ), 3,42 (IH, m, 24- H), 4,23 (IH, m, 3-H), 4,43(1H, m, 1-H), 5,00 (IH, bs, 19Z-H), 5,35 (IH, bs, 19E-H), 5,50 (2H, m, 22-H i 23-H), 6,01 (IH, d=11.2 Hz, 7- H), 6,38 (IH, d=l 1.2 Hz, 6-H). 0,3% of (22Z)-isomer was detected.