DESC:FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
PROVISIONAL SPECIFICATION
(See section 10 and rule 13)

A PROCESS FOR THE PREPARATION OF NEBIVOLOL INTERMEDIATES

Vasudha Pharma Chem Limited
An Indian company having its registered office at
78/A, Vengalrao Nagar,
Hyderabad – 500038,
Telangana
India
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:
A PROCESS FOR THE PREPARATION OF MEBEVERINE INTERMEDIATES
FIELD OF THE INVENTION
The present invention relates to a novel process for the synthesis of Nebivolol intermediates by using enzymatic resolution.
BACKGROUND OF THE INVENTION
The drug compound having the adopted name “Nebivolol” is a highly selective ß1 blocker and has been found to be useful for the management of hypertension. Chemical names for nebivolol are: [2R*[R*[R*S*)]-a,a'-(iminobismethylene)bis[ 6-fluoro-2-chromanmethanol]; and (1RS,1'RS)-1,1'-[(2RS,2'SR)-bis(6-fluoro-3,4-dihydro-2H-1-benzopyran-2-yl)]-2,2'-iminodiethanol.

Nebivolol, in the form of its hydrochloride salt, is the active ingredient in products sold as BYSTOLIC®, and the product prescribing information discloses nebivolol to be a mixture of equal amounts of two enantiomers, having respectively the SRRR- and the RSSS-configurations. The SRRR configuration is called d-nebivolol and the RSSS configuration is called l-nebivolol.
It is known in the art that the synthesis of a,a'-[imino-bis(methylene)]bis [chroman-2-methanol] molecular structures is challenging for the skilled person because of the four asymmetric carbon atoms producing a mixture of 16 stereoisomers (in case of asymmetrical substitutions) or a mixture of 10 stereoisomers (in case of symmetrical substitutions). As apparent from the presence of symmetry in the nebivolol structure, a total of 10 stereoisomers may be generated.
Methods for preparation of nebivolol and its intermediate compounds are disclosed in European Patents 0145067 and 0334429. European Patent 0334429 B1 discloses a process for the preparation of specifically the RSSS isomer of nebivolol. The process for preparing RSSS nebivolol involves the use of hazardous reagents like thionyl chloride, sodium hydride, and diisobutyl aluminium hydride (DIBAL), expensive optically active reagents like (+)-1,2,3,4,4a,9,10,10a-octahydro-1,4a-dimethyl-7-(1-methylethyl-1-phenathrene methanamine [(+)-dehydroabietylamine], and uses cumbersome column chromatography and low temperatures. The processes also involve a large number of steps, thereby increasing the energy, manpower and time required to complete a production cycle, rendering the processes commercially disadvantageous.
European Patent 0744946 describes a process for obtaining nebivolol hydrochloride from a mixture containing the desired (RSSS+SRRR) nebivolol base, contaminated with the undesired (RSRR+SRSS) diastereomers, using ethanol as both a reactant and the recrystallization solvent. A major disadvantage of this process is a very low yield (6.6%) of the desired isomers (having the SRRR- and the RSSS-configuration) of nebivolol hydrochloride. Moreover, ethanol is a solvent that can be used only in controlled quantities due to regulatory requirements, and therefore its usefulness on an industrial scale is limited.
Patent EP 0145067 (Janssen Pharmaceutica NV) describes a method of preparing NBV which comprises synthesizing diastereoisomeric mixtures of 6-fluoro-3,4-dihydro-2-oxiranyl-2H-1-benzopyran in accordance with the synthetic scheme below

6-fluoro chroman carboxylic acid ethyl ester, derived from the esterification of the corresponding acid, is reduced with sodium dihydro bis-(2-methoxyethoxy)- aluminate to primary alcohol; the product is reacted with oxalyl chloride and then triethylamine at -60°C to give corresponding racemic aldehyde, which is then converted into 6-fluoro-3,4-dihydro-2-oxiranyl-2H-1-benzopyran (epoxide intermediate) as a mixture of (R5S), (S5R), (R5R) and (S5S) stereoisomers. Example 17 describes the chromatographic separation of the epoxide intermediate into two racemic mixtures (R5S)-, (S5R)-epoxides (Mixture A) and (S5S)-, (R5R)- epoxides (Mixture B), respectively, which represent the key intermediates of the process for preparing NBV.
Till date, it seems that the development of the art has not been able to propose a valid alternative to the expensive chromatographic separation process. It is clear that the prior art is addressed towards alternative methods for preparing the epoxide derivatives or open analogues thereof substantially aimed at preventing said chromatographic separation of the mixtures useful for preparing the active ingredient.
Despite the efforts of the research aimed at finding alternative processes, it would be topical and desirable to study methods for preparing the epoxide intermediate, which allow overcoming the drawbacks presented by the processes described in the art, in particular, with the aim of by-passing the chromatographic step.
We have now, found a simple and efficient synthesis of 6-fluoro-3,4-dihydro-2-oxiranyl-2H-1-benzopyran in the form of individual enantiomers useful in the preparation of nebivolol, via enzymatic resolution of the mixture containing the four possible stereoisomeric forms.
SUMMARY OF THE INVENTION
In the first embodiment the present application provides a process for the separation of 6-fluoro-3,4-dihydro-2-oxiranyl-2H-1-benzopyran, epoxide intermediate of compound of formula I

to give a compound of formula

as a diastereoisomerically pure compound of RS/SR configuration; and a compound of formula

as a diastereoisomerically pure compound of RR/SS configuration; characterized in that said separation is carried out by enzymatic resolution.
In the second embodiment the present application provides a process for the preparation of Nebivolol hydrochloride by separating 6-fluoro-3,4-dihydro-2-oxiranyl-2H-1-benzopyran, epoxide intermediate of compound of formula I

to give a compound of formula

as a diastereoisomerically pure compound of RS/SR configuration; and a compound of formula

as a diastereoisomerically pure compound of RR/SS configuration; characterized in that said separation is carried out by enzymatic resolution and then converting the epoxide intermediate to Nebivolol.

DETAILED DESCRIPTION OF THE INVENTION
The compound of formula I, which is the starting material, racemic epoxide can be prepared according to known techniques, in particular, according to the processes described in patent EP 145067 and in patent applications WO 2008/040528, EP 1803715 and EP 1803716.
The separation of the compound of formula I to give the diastereoisomerically pure compound of formula (I:RS/SR) and the diastereoisomerically pure compound of formula (I:RR/SS) is carried out by enzymatic resolution.
In the present application, the expression "to give a compound of formula (I:RS/SR) as diastereoisomerically pure compound of RS/SR configuration" means a compound obtained as a substantially pure mixture of the optical isomers of RS and SR configuration, i.e. of the enantiomers (R)-6-fluoro-3,4-dihydro-2((S)-oxiran-2-yl)-2H-chromen and (S)-6-fluoro-3,4-dihydro((R)-oxiran-2-yl)-2H-chromen.
In the present application, the expression "to give a compound of formula (I:RR/SS) as diastereoisomerically pure compound of RR/SS configuration" means a compound obtained as a substantially pure mixture of the optical isomers of RR and SS configuration, i.e. of the enantiomers (R)-6-fluoro-3,4-dihydro((R)-oxiran-2-yl)-2H-chromen and (S)-6-fluoro-3,4-dihydro((S)-oxiran-2-yl)-2H-chromen.
For the purposes of the present application the relative amount of four stereoisomers in the mixture of compounds of formula I is not relevant.
Thus, it is evident to the skilled person that the process of the application can be applied to partially resolved compounds of formula I wherein one or more stereoisomers are missing or, otherwise, present in a different percentage.
The known mixtures of stereoisomers obtained through the separation process object of the present application (Mixture A and Mixture B) are converted into the end product, Nebivolol, according to known techniques.
The enzymatic resolution of the mixture of the two enantiomers of the 6-fluoro-3,4-dihydro-2-oxiranyl-2H-1-benzopyran is performed with the enzyme catalyst is selected from a lipase, esterase or protease. The enzyme may be selected from the group comprising of Candida Antarctica Lipase B (CALB), Novozyme-435, Codexis CDX-036 and Novozyme Promea.
6-fluoro-3,4-dihydro-2-oxiranyl-2H-1-benzopyran may be resolved into its two enantiomers with high stereoselectivity through enantioselective resolution catalysed by CALB.
This strain provides a good example of activity, however, given the rather diffused nature of activity in a wide variety of related strains, the scope of the invention is not meant to be limited only thereto. The microorganisms and their enzymatic activity may be used for resolution of the racemic compound, 6-fluoro-3,4-dihydro-2-oxiranyl-2H-1-benzopyran in a stereoselective manner, considerably enriched in enantiomer (R5S). e.g. 93-100% of enantiomeric excess with a 45-50% of conversion, and leave the enriched residual in the enantiomer (S5R).
It is therefore produced the (R)-6-fluoro-3,4-dihydro((R)-oxiran-2-yl)-2H-chromen with an enantiomeric excess of >70%, preferably of >80% and even more preferably of >90%, while the (S)-6-fluoro-3,4-dihydro((S)-oxiran-2-yl)-2H-chromen remains with an enantiomeric excess of >70%, preferably of >80% and even more preferably of >90%.
The resolution may be conducted on any mixture of enantiomers, but generally the racemate is used.
The resolution is preferably conducted at a pH 8-11, preferably 8.5-10.0.
The resolution temperature may be comprised between 10 and room temperature, but preferably between 20 and 25°C.
The reaction carried out in an aqueous or aqueous / organic medium leads resolution of epoxide mixture to one of the two enantiomers in a selective manner. The resolution proceeds quickly and with a high stereoselectivity.
Recovery of both compounds is possible by processes known to a person skilled in the art and preferably through a series of acid-base extractions.
Both compounds are then used for the Nebivolol synthesis. Through processes known to a person skilled in the art or the process mentioned herein.
The epoxide, (R)-6-fluoro-3,4-dihydro-2((S)-oxiran-2-yl)-2H-chromen is reacted with 0.5-2 equivalents of benzylamine to provide N-benzyl nebivolol which is then debenzylated to provide Nebivolol.

DEFINITIONS
The following definitions are used in connection with the present application unless the context indicates otherwise.
The terms "about," "general, "generally," and the like are to be construed as modifying a term or value such that it is not an absolute. Such terms will be defined by the circumstances and the terms that they modify as those terms are understood by those of skill in the art. This includes, at very least, the degree of expected experimental error, technique error and instrument error for a given technique used to measure a value.
All percentages and ratios used herein are by weight of the total composition and all measurements made are at about 25 °C and about atmospheric pressure, unless otherwise designated. All temperatures are in degrees Celsius unless specified otherwise. As used herein, the terms "comprising" and "comprises" mean the elements recited, or their equivalents in structure or function, plus any other element or elements which are not recited. The terms "having" and "including" are also to be construed as open ended. All ranges recited herein include the endpoints, including those that recite a range between two values. Whether so indicated or not, all values recited herein are approximate as defined by the circumstances, including the degree of expected experimental error, technique error, and instrument error for a given technique used to measure a value.
The term "optional" or "optionally" is taken to mean that the event or circumstance described in the specification may or may not occur, and that the description includes instances where the event occurs and instances where it does not.
Room temperature as used herein refers to ‘the temperatures of the thing close to or same as that of the space, e.g., the room or fume hood, in which the thing is located’. Typically, room temperature can be from about 20 °C to about 30 °C, or about 22 °C to about 27 °C, or about 25 °C.
The reaction time should be sufficient to complete the reaction which depends on scale and mixing procedures, as is commonly known to one skilled in the art. Typically, the reaction time can vary from about few minutes to several hours.
The reactions of the processes described herein can be carried out in air or under an inert atmosphere. Typically, reactions containing reagents or products that are substantially reactive with air can be carried out using air-sensitive synthetic techniques that are well known to the person skilled in art.
The isolation may be effected by methods such as, removal of solvent, crash cooling, flash evaporation, rotational drying, spray drying, thin-film drying, agitated nutsche filter drying, freeze drying, or any other suitable fast evaporation technique.
Suitable temperatures for isolation may be less than about 120 °C, less than about 80 °C, less than about 60 °C, less than about 40 °C, less than about 30 °C, less than about 20 °C, less than about 10 °C, less than about 0 °C, less than about -10 °C, less than about -40 °C or any other suitable temperatures.
Certain specific aspects and embodiments of the present application will be explained in more detail with reference to the following examples, which are provided for purposes of illustration only and should not be construed as limiting the scope of the present application in any manner.
EXAMPLES
Example 1: Resolution of epoxide mixture
To a round bottom flask charge acetonitrile (125 ml), epoxide mixture (25 gr), benzylamine (8.27 gr) and lipozyme CALB (5 gr) at room temperature and stirred for 24 hours. Cool the reaction mass to 2-3 oC and stirred for 2 hours and washed with acetonitrile (12.5 ml). The material was dried under vacuum at 65 oC to provide the titled compound.

Dated this on 26th day of Jun, 2023

Signature: ___________________
Name: Dr. Phani Kumar Balsu
Vasudha Pharma Chem Limited ,CLAIMS:We Claim:
1) A process for the separation of 6-fluoro-3,4-dihydro-2-oxiranyl-2H-1-benzopyran compound of formula I

to give a compound of formula

as a diastereoisomerically pure compound of RS/SR configuration; and a compound of formula

as a diastereoisomerically pure compound of RR/SS configuration; characterized in that said separation is carried out by enzymatic resolution.

2) A process for the preparation of Nebivolol comprising, separating 6-fluoro-3,4-dihydro-2-oxiranyl-2H-1-benzopyran, epoxide intermediate of compound of formula I

to give a compound of formula

as a diastereoisomerically pure compound of RS/SR configuration; and a compound of formula

as a diastereoisomerically pure compound of RR/SS configuration; characterized in that said separation is carried out by enzymatic resolution and then converting the epoxide intermediate to Nebivolol.

Dated this on 26th day of June, 2023

Signature: ___________________
Name: Dr. Phani Kumar Balsu
Vasudha Pharma Chem Limited