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The present invention describes an eco-friendly and cost effective process for the synthesis of teri-butyl (3R,5S)-6-oxo-3,5-dihydroxy-3,5-0-isopropylidene-hexanoate [I]
tert-b tyl (3R,5S)-6-oxo-3,5-dihydroxy-3,5-0-isopropylidene-hexanoate [I] [CAS No. 124752-23-4] is key intermediate for the preparation of statins such as Atorvastatin (Tetrahedron 63, 2007, 8124 -8134), Cerivastatin (Journal of Labeled Compounds and Radiopharmaceuticals, 49, 2006 311-319), Fluvastatin [WO2007125547; US 4739073], Pitavastatin [WO2007/132482; US2012/22102 Al, WO2010/77062 A2; WO2012/63254 Al ; EP 304063; Tetrahedron Letters, 1993, 34, 513 – 516; Bulletin of the Chemical Society of Japan, 1995, 68, 364 – 372] and Rosuvastatin [WO2007/125547 A2; WO2011/132172 Al ; EP 521471]. Statins are used for treatment of hypercholesterolemia, which reduces the LDL cholesterol levels by inhibiting activity of HMG-CoA reductase enzyme, which is involved in the synthesis of cholesterol in liver.
Compound [I] is generally obtained by various methods of oxidation of teri-butyl 2- ((4R,65)-6-(hydroxymethyl)-2,2-dimethyl-l,3-dioxan-4-yl)acetate [compound II] and are discussed in details hereinafter. In addition, various methods for synthesis of compound [II] are also elaborated below.
US patent Number 5278313 describes a process for synthesis of compound [II]
(Schemel). In the said process, (5)-methyl 4-chloro-3-hydroxybutanoate has been obtained in only 70% yield through whole cell enzymatic reduction of methyl 4-chloro-3- oxobutanoate, which has a necessity of special equipment such as fermenters as well as other microbial facilities such as sterile area, autoclaves, incubator for growing seed culture, etc.
(S)-mefhyl 4-chloro-3-hydroxybutanoate upon reaction with teri-butyl acetate in presence of LiHMDS or LDA at -78°C, yielded (S)-ieri-butyl 6-chloro-5-hydroxy-3- oxohexanoate, which was further transformed to corresponding diol through syn selective reduction in presence of methoxydiethyl borane/sodium borohydride at -78°C. The diol thus obtained was converted to compound [II] .
The overall yield for this process is low and required special equipment such as fermenters, etc and in addition to that, this process is not cost effective due to use of costly reagent such as methoxydiethyl borane.
Moreover, methoxydiethylborane is highly pyrophoric (Encyclopedia for organic synthesis, editor in chief L. Paquette; 2, 5304; Published by John and Wiley Sons;
Organic Process Research & Development 2006, 10, 1292-1295) and hence safety is a major concern.
EP 1282719 B l (PCT application WO 01/85975 Al ) discloses a process for synthesis of compound ( R, 5S)-tert-bv y\ 3,5,6-trihydroxyhexanoate from (S)-tert-b tyl-5,6-dihydroxy-3-oxohexanoate through a) asymmetric hydrogenation in presence of a chiral catalyst e.g. di-mu-chlorobis-[(p-cymene)chlororuthenium(II)] along with an auxiliary such as (IS, 2S)-(+)-N- (4-toluenesulfonyl)-l ,2-diphenylethylenediamine as ligand, which gave desired product only in 70% diastereomeric excess (de); b) Whole cell enzymatic reduction of (S)-tert- butyl 5,6-dihydroxy-3-oxohexanoate to obtain compound (3R, 5S)-tert-bv y\ 3,5,6-trihydroxyhexanoate in 99% de (80% yield).
It is needless to mention that it has necessity of fermenter and other microbiological equipment (Scheme 2).
Moreover, conversion of (2>R,5S)-tert-bv y\ 6-acetoxy-3,5-dihydroxyhexanoate to tert-bv yl 2-((4R,65)-6-(acetoxymethyl)-2,2-dimethyl-l ,3-dioxan-4-yl)acetate was accomplished in only 25% yield and also required the flash chromatography for isolation of desired product.
Thus, overall yield for this process is poor and process is not operation friendly especially at large scale hence cannot be considered feasible for commercial manufacturing.
EP1317440 Bl (PCT Application WO 02/06266 Al) has disclosed the process for synthesis of compound [II] from 6-chloro-2,4,6-trideoxy-D-erythro-hexose (Scheme 3) .
In the said patent application 6-chloro-2,4,6-trideoxy-D-erythro-hexose was converted to (4R, 65)-4-hydroxy-6-chloromethyl-tetrahydropyran-2one with excess of bromine in presence of potassium bicarbonate, which liberates environmentally undesired gas i.e. carbon dioxide.
Moreover, starting material i.e. 6-chloro-2,4,6-trideoxy-D-erythro-hexose is not commercially available and conversion efficiency of starting material at large scale towards (4R, 65)-4-hydroxy-6-chloromethyl-tetrahydropyran-2-one is only 67%.
US Patent No. 6689591 B2 has demonstrated the whole cell enzymatic reduction of teri-butyl 6-chloro-3,5-dioxohexanoate to compound [II] (Scheme 4).
In the said process, whole cell enzymatic reduction is not specific; yield for desired product is only 34% and other partially reduced products are also obtained.
Hence, further purification is required for obtaining the desired compound. Thus, this process is not suitable for commercial scale.
Tatsuya et al (Tetrahedron Letters; 34, 1993,513 – 516) has reported synthesis of compound [I] from derivative of L-tartatric acid (Scheme 5).
In the said process, tartaric acid di-i‘sopropyl ester is doubly protected by tert-butyldimethylsilyl group, which was reacted with dianion of teri-butyl acetoacetate to give β, δ-diketo ester compound.
β,δ-diketo ester was reacted with 2 equivalent of diisobutylaluminium hydride (which is a pyrophoric reagent) to afford -hydroxy,8-keto ester in only 60% yield.
This process is not industrially viable as overall yield is very low and also because of use of costly and pyrophoric reagents/chemicals.
US7205418 (PCT application WO03/053950A1) has described the process for synthesis of compound [II] from (S)-ieri-butyl-3,4-epoxybutanoate (Scheme 6).
The overall yield for this process is very low and moreover, it required the diastereomeric separation of teri-butyl 2-(6-(iodomethyl)-2-oxo-l,3-dioxan-4-yl)acetate by flash chromatography.
Since overall requirement of title compound is very high, any operation involving flash chromatography will tend to render the process commercially unviable.
Fengali et al (Tetrahedron: Asymmetry 17; 2006; 2907-2913) has reported the process for synthesis of compound [II] from racemic epichlorohydrin (Scheme 7).
In this process, racemic epichlorohydrin was converted to corresponding nitrile intermediate through reaction with sodium cyanide; nitrile intermediate thus obtained was further resolved through lipase catalyzed stereo-selective esterification to obtain (5)-4-(benzyloxy)-3-hydroxybutanenitrile and (R)-l-(benzyloxy)-3-cyanopropan-2-yl acetate;
separation of desired product i.e. (S)-4-(benzyloxy)-3-hydroxybutanenitrile having 98% de (40% yield) was done by column chromatography.
Needless to mention a commodity chemical like compound [I] cannot be manufactured by such a laboratory method, which involved number of steps.
Bode et al (Organic letters, 2002, 4, 619-621) has reported diastereomer- specific hydrolysis of 1,3-diol-acetonides (Scheme 8).
In this publication, duration of the reaction for diastereomer- specific hydrolysis of 1,3, diol-acetonides is reported to be 4 h, however, in our hand it was observed that hardly any reaction took place in 4 h, which made it non-reproducible.
In addition to that, separation of desired product is achieved by flash chromatography and it is needless to mention that any process which involved flash chromatography would render the process to be commercially unviable.
Hence, additional innovation needs to be put in for making the process industrially viable.
CN 101613341A has reported the process for synthesis of compound [II] (Scheme
In the same patent application tert-b tyl (S)-6-chloro-5-hydroxy-3-oxohexanoate was synthesized through Blaise condensation of (5)-4-chloro-3-hydorxy-butanenitrile with zinc enolate of tert butyl bromo acetate.
In the literature, synthesis of tert-bv yl (S)-6-chloro-5-hydroxy-3-oxohexanoate was reported through Blaise condensation of silyl protected (5)-4-chloro-3-(trimethylsilyl)oxy-butanenitrile with zinc enolate of tert butyl bromo acetate, in good yield (Synthesis 2004, 16, 2629-2632). Thus, protection of hydroxy group in (5)-4-chloro-3-hydorxy-butanenitrile is imperative.
In the said Chinese patent application, in claim 7, it was mentioned that solvent used for conversion of tert-bv yl (5)-6-chloro-5-hydroxy-3-oxohexanoate to ( R,5S)-tert-butyl 6-chloro-3,5-dihydroxyhexanoate is anyone or mixture of more than one from tetrahydrofuran, ether, methanol, ethanol, n-propanol, /so-propanol and ethylene glycol.
However, in enablement the only example using mixture of solvent was that of THF-methanol (Experimental section, Example 4: The preparation of (R,5)-6-chloro-3,5- dihydroxyhexanoate) and same outcome was expected in other individual or mixture of solvents.
Claim 8 of CN 101613341A mentioned that reduction was carried out by any one or mixture of more than one reducing agents such as sodium borohydride, potassium borohydride, lithium aluminum hydride, diethylmethoxy borane, triethyl borane and tributyl borane.
It implies that either any one of the reducing agents or a mixture of the same can be employed. From reaction mechanism it is very much clear that diethylmethoxy borane, triethyl borane and tributyl borane form the six membered complex between optically active hydroxyl and carbonyl group, which gets reduced by sodium borohydride, signifying that individually diethylmethoxy borane, triethyl borane and tributyl borane are not reducing agents
Moreover, in claims 12 and 13 (Experimental section, Example 4: The preparation of (R,S)-6-chloro-3,5-dihydroxyhexanoate), it is mentioned that reduction should be carried out in temperature range -80 °C to -60 °C, implying that reaction would not work beyond this temperature range i.e. it would work in the temperature window of -80 °C to -60 °C only.
Summarizing, the teachings of the application are not workable.
Wolberg et al (Angewandte Chemie International Edition, 2000, 4306) has reported that diastereomeric excess for syn selective reduction using mixture of diethyl methoxy borane/sodium borohydride of compound [VI] gave 93% de for compound [VIII], which required further re-crystallization to obtain compound [VIII] in 99% de and 70% yield.
Thus, all the reported methods for stereo-selective hydride reduction of compound [VI] were achieved through mixture of trialkyl borane or diethyl methoxy borane & sodium borohydride in THF, at -78°C. As mentioned earlier, trialkyl borane or diethyl methoxy borane are pyrophoric in nature; in addition to that anhydrous THF is costly and moreover, reaction required large dilution.
Hence, there is need for developing efficient, environment friendly, cost effective and green process for stereo-selective reduction compound [VI].
B) The process of Oxidation of compound [II] to compound [I] has been discussed in following literature processes.
4) Trichloroisocyanuric acid/ TEMPO (CN 101747313A)
5) Oxidation of compound [II] to compound [I] through IBX [CN101475558A].
It would be evident that most of the reported methods are not “green” and
environmentally benign; none of the reported methods use molecular oxygen as oxidizing agent in presence of metal catalyst/co-catalyst.
Example 18: Process for synthesis of tert-butyl 2-((4R,6S)-6-formyl-2,2-dimethyl-l,3-dioxan-4-yl)acetate [I]
A reactor was charged with 1.1 g of copper (I) chloride and 10 mL of acetonitrile. 2-2′ Bipyridyl (156 mg) and TEMPO (156 mg) were added to the reactor under oxygen environment at 25°C. A solution of (6-Hydroxymethyl-2,2-dimethyl-[l,3]dioxan-4-yl)-acetic acid tert-butyl ester 2.6 g in 26 mL DCM was added dropwise over a period of 10 min into it. The reaction mass was stirred at 40°C and progress of reaction was monitored on GLC, which shows that 90% conversion for desired product.
Example 19: Process for synthesis of tert-butyl 2-((4R,6S)-6-formyl-2,2-dimethyl-l,3-dioxan-4-yl)acetate [I]
A reactor was charged with 1.1 g of copper (I) chloride and 10 mL of dichlorome thane. 2-2′ Bipyridyl (156 mg) and TEMPO (156 mg) were added to the reactor under oxygen environment at 25°C. A solution of (6-Hydroxymethyl-2,2-dimethyl-[l,3]dioxan-4-yl)-acetic acid tert-butyl ester 2.6 g in 26 mL DCM was added dropwise over a period of 10 min into it. The reaction mass was stirred at 40°C and progress of reaction was monitored on GLC, which shows that 90% conversion for desired product.