The present invention relates to a method of epimerizing a C-24 epimer of a vitamin D analog.
The present invention relates to analogs of vitamin D and derivatives thereof that have multiple asymmetric centers and to methods for epimerizing such vitamin D analogs aad derivatives.
RELATED APPLICATIONS
The present application claims the benefit of the filing date of United States Provisional Patent Application 60/388,520, filed June 13, 2002.
BACKGROUND OF THE INVENTION
In the preparation of Vitamin D analogs, in particluiar those haA'ing a structure analogous to I, a defined stereochemistry for (he hydroxyl group at C-24 is necessary for full expression of the biological activity. As a specific example, the 24S epimer is desired for, for example, MC 903, (Structure I calcipotriene)

(Figure Remove)

The standard process to make these vitamin D analogs is illustrated in Scheme I. The intermediate II (C-22 aldehyde vitamin D analogue) is reacted with a WIttig reagent to afford the unsaturated ketone IH which is then reduced by, for example, sodium boiohydride, to give IV as a mixture of C-24 epimers. See, e.g., Calverley Tetrahedron 4609-4619,1987). The desired epimer, in this case the 24 S epimer is then separated by standard methods (for example, cinematography) and the undesired epimer, in this case
the 24 R epimer, is discarded. Thus, waste is high and overall yield and process productivity are correspondingly low.
Scheme I

(Figure Remove)
We have discovered a method whereby the undesired epimer, here 24R, pure or contaminated with residual undesired 24S epinier, can be converted into a mixture enriched in the desired 24S epimer. This enriched mixture can be recycled to the separation step and thus the overall process productivity can be increased, waste reduced, and production costs lowered.
SUMMARY OF THE INVENTION
hi one aspect, the present invention relates to a method of epimerizing a C-24 epimer of a vitamin D analogue having a hydroxyl group substituent on an asymmetric allylic carbon atom at the 24 position comprising the steps of esterifying the hydroxyl group on the asymmetric allylic carbon atom at the 24 position with an esterifying agent and contacting the ester with an epimerization-active solid.
In another aspect, the present invention relates to a method of epimerizing a C-24 epimer of a vitamin I) analogue having a hydroxyl group substituent on an asymmetric allylic carbon atom at the 24 position comprising the steps 6f acetylating the hydroxyl group on the asymmetric allylic carbon atom at the 24 position with an acetylating agent and contacting the acetate ester so formed with an epimerization-active solid.
In another aspect, the present invention relates to a method of epimerizing a C-24 epimer of a vitamin D analogue having a hydroxyl group substituent on an asymmetric ailylic carbon atom at the 24 position comprising the steps of acetylating the hydroxyl group on the asymmetric allylic carbon atom at the 24 position with an acetylating agent and contacting the acetate ester so formed with an epimerization-active solid selected from the group consisting of silica gel, aluminum oxide, florisil, and zeolite.
In another aspect, the present invention relates to a method of epimerizing a C-24 epimer of a vitamin D analogue having a hydroxyl group substituent on an asymmetric allylic carbon atom at the 24 position comprising the steps of acetylating the hydroxyl group on the asymmetric allylic carbon atom at the 24 position with an acetylating agent and contacting the acetate ester so formed with an epimerization-active solid selected from group consisting of silica gel, aluminum oxide, florisil, and zeolite, wherein the contacting is on a column packed with the epimerization-active solid.In yet another aspect, the present invention relates to a method of epimerizing a C-24 epimer of a vitamin D analogue having a hydroxyl group substituent on an asymmetric allyiic carbon atom at the 24 position comprising the steps of acetylating the hydroxyl group on the asymmetric allylic carbon atom at the 24 position with an acetylating agent and contacting the acetate ester so formed in a solvent selected from dichloromethane, hexane, heptane, toluene, ethyl acetate, and mixtures of these, with an epimerization-active solid selected from group consisting of silica gel, aluminum oxide, florisil, and zeolite.
In still another aspect, the present invention relates to a method of epimerizing a C-24 epimer of a vitamin D analogue having a hydroxyl group substituent on an asymmetric allylic carbon atom at the 24 position (a C-24 OH epimer) comprising the steps of acetylating the hydroxyl group on the asymmetric allylic carbon atom at the 24 position
with an acetylating agent selected from acetic acid, acetyl chloride, and acetic anhydride, and «contacting» the acetate ester so formed with an epirnerization-active solid.
In yet another aspect, the present invention relates to a method of epimerizing an epimeric C-24 ester of a vitamin D analogue having structure V or VIII.

OH
(Figure Remove)
wherein RI and Ra are independently selected from hydrogen or a hydrogen protecting group and Ra is a C\ - Ce linear or branched alkyl or cycloalkyl group, comprising the step of contacting the C~24 epimeric ester with an epimerization-active solid.
In still another aspect, the present invention relates to a method of epimerizing a C-24 epimeric ester of a vitamin D analogue having structure V or VHI.

(Figure Remove)

wherein RI and R2 are independently selected from hydrogen or a hydrogen protecting group and Rj is a cycloalkyl group, comprising the step of contacting the C-24 epimeric ester with an epimerization-active solid.

DETAILED DESCRIPTION OF THE INVENTION
As used herein, ambient temperature refers to a temperature between about 18 °C and about 35 °C, preferably about 25 °C.
We herein disclose a novel process whereby the alcohols of general structures IV OT VHare esterified with, for example, acetic anhydride or acetylchloride to the corresponding C-24 esters. The process is illustrated in Scheme H with the 24 R compounds V and VIII. However, the process is equally applicable when the epimeric analog has the S configuration at C-24, or with mixtures epimeric anologs including both R and S configurations at C-24.
Esteriiication can be carried out by conventional means, or it can be carried out by enzyme-assisted esteriiication using, for example, vinyl acetate as disclosed in copending United States Patent Application Serial No. 10/339,226, filed January 9, 2003.
The esters are epimerized by contact with an epimerization-active solid, at ambient or elevated temperature, to afford, after hydrolysis, mixtures of the corresponding C-24 hydroxyl epimers that have a relatively liigher concentration of the epimer originally absent or, in the case of mixtures, originally present in lesser amount.
We have found that, under certain conditions, an autohydrolysis can take place during the epimerization on the active solid. However, the empimerization can be carried-out without hydrolysis. The hydrolysis step can be carried out as a separate step using methods well known in the art. See Scheme III
Scheme II

(Figure Remove)

(Figure Remove)

Scheme III
(Figure Remove)

The present invention relates to epimers of vitamin D analogs. Epimers are cliastereomers - that is configurational isomers that are not enantiomers - in compounds having more that one asymmetric carbon atom, wherein the configuration at only of the asymmetric centers differs between the two members of the epimeric pair. The process of converting one member of the epimeric pair to the other, by changing the stereochemistry al only one asymmetric center, can be referred to as epimerization.
The relative excess of one epimer over the other in a mixture of two epimers can be expressed in terms of diasteromeric excess, d.e., defined as [E] - £2], where E/ is the mole fraction of epimer i. Percent diastereomeric excess, % d.e., can be defined as 100 • [Ei - Ei]. Methods of determining d.e. and % d.e. are well known to one skilled in the stereochemistry of organic compounds.
The present invention provides a method for epimerizing a C-24 ester, especially an acetate, of a vitamin D analogue. Vitamin D analogs useful in the practice of the present invention are structurally related to cholecalciferol and, to the extent possible, the numbering system of the parent, cholesterol, is used. Vitamin D analogs useful in the practice of the present invention have an asymmetric allylic carbon atom that bears a free or derivatized hydroxyl group at C-24. Vitamin D analogs that are epimers, analogs, or structural or positional isomers, with respect to the exocyclic double bond on the A ring of a calciptotriene analogue, as exemplified by structures I, V, and VIE are particular objects of the present invention.

In the structures, RI and R2 can be the same or different and are independently selected from hydrogen or hydroxyl protecting group. Preferred protecting groups are silyl protecting groups, for example trimethyl silyl, triethyl silyl, and, especially, t-butyl dimethyl silyl
By the method of the present invention, a first C--24 epimer (either 24 S or 24 R) of a vitamin D analogue, alone or including some amount of a second epimer of a vitamin D analogue that has a configuration opposite to that of the first epimer at C-24, is converted to a mixture that is relatively enriched in the second epinler, relative to the stating mixture. That is, the diastereomeric excess of the starting epimer or mixture of epimers is decreased from its initial value.
The method of the present invention includes the step of contacting a C-24 ester of an epimer, or mixture of epimers, of a vitamin D analogue having an asymmetric allylic carbon atom at the 24 position with an epimerization-active solid. The ester can be obtained from any source. For example, the ester can be obtained by esterifying, using any esterification technique known in the art (including enzyme promoted esterification), a single epimer or a mixture of epimers of a vitamin D analogue having an OH group on the allylic carbon atom at position 24 for which it is desired to decrease the diastereometric excess. The ester is contacted in a suitable solvent. Suitable solvents include dichloromethane, hexane, heptane, toluene, ethyl acetate, and mixtures of these. When a solvent is used, hexane is a preferred solvent. The contacting can be at ambient temperature, or it can be at an elevated temperature, for example 35° C or 40°C.
Epimerization-active solids are oxides of Group II, HI, or IV elements. Examples of epimerization-active solids include silica gel, aluminum oxide, florisil, and zeolite, to mention just a few. The yield of the epimerization can be improved and the generation of
i
byproducts reduced if a small amount of amine or inorganic base (e.g. K/zCOs) are combined with the epimerization-active solid.
Preferably, the epimerization-active solid has a broad particle size distribution; for example from about 0.06 to about 0.20 mm or from about 0.04 to about 0.20 mm.
hi the contacting step, epimerization of the C-24 ester occurs and the original percent diastereometric excess, which can be up to 100%, is decreased. That is, the relative amount of the epimer originally absent or present in a relatively smaller amount is

increased. The contacting can be batchwise or it can be continuous. Batchwise contacting can be carried out in a suitable reactor provided with agitation, for example a flask or stirred tank reactor, or it can be carried out by loading the ester on a column, e.g. a chromatography column, packed with epimerization-active solid and eluted with a suitable solvent or mixture of solvents that can be any of the solvents discussed above.
In preferred embodirnents,the solution of epimerized ester is separated from the epimerization-active solid. The epimerization-active solid is then washed at least once with a solvent. The solvent can be the same as the solvent in which the ester was contacted, or it can be different. Washing can be batchwise or it can be continuous (e.g. in a column). The washings are combined with the separated solution.
If desired, the mixed epimeric esters can be isolated, for example by removing any solvent(s) used, and then used for any purpose. If a mixture of epimeric alcohols is desired, the contacting step can be followed by a hydrolysis step in which the mixture of epimeric esters, now having a relatively larger amount of the epimer originally absent or present in a lesser amount, is hydrolyzed. If the epimeric C-24 alcohols are desired, the mixture of epimeric esters can be separated and the hydrolysis carried out on the desired epimer by any conventional method known in the art. For example, hydrolysis can be carried out in a suitable hydrolysis solvent using potassium carbonate. A mixture of methanol and tetrahydrofuran is an example of a suitable hydrolysis solvent.
Following hydrolysis, the epimeric C-24 alcohols can be isolated by, for example, removing the hydrolysis solvent using conventional techniques, for example with the aid of a rotary evaporator.
If desired, the hydrolysis step can be carried out in the presence of the epimerization-active solid without first isolating the ester. That is, epimerization and hydrolysis of the C-24 epimeric esters can be carried out in "one pot". Whether hydrolysis occurs in the presence of the epimerization-active solid is principally a function of the contacting time. The skilled artisan will recognize that this time may vary depending on, for example, the temperature of contacting, and the amount of water in the epimerization active solid. When the contacting is with silica gel and carried out at ambient temperature, a contacting time of about one hour is preferred. Longer contacting times can be tolerated without hydrolysis if alumina is the epimerization-active solid.
The esters and free alcohol are easily separated by chromatographic techniques and the hydrolysis can be monitored by chromatography, for example thin layer chromatigraphy (TLC).
Whether hydrolysis is carried out in situ (i.e. in "one-pot") or in a separate reaction step, the C-24 epimeric vitamin D analogs having a hydroxyl group on the carbon atom at the 24 position can be isolated and purified by any conventional-means.
The present invention in several of its embodiments is illustrated in the following non-limiting examples.
Example I Oae-ppt batch process on silica gel in jiexajae
To a round bottom 250 ml flask containing a solution of acetate IX (Ra~ cyclopropyl, R2= R3= ter^butyldimethylsilyl (TBDMS), d.e. >97%,7.28 g) in hexane (180 ml), was added silica gel type 60 (0.043-0.060 mm, 71.93 g). The mixture was stirred slowly for 4 hr and then filtered. The silica gel was washed with ethyl acetate (300 ml), followed by a 10% solution of methanol in ethyl acetate. The filtrate and washings were combined and evaporated to d^ness at 38°C/20 nibar to afford a crude product (6.55 g) containing 79.3% of a mixture of the epimeric alcohols IV (Rj~ cyclopropyl, R2= RI= TBDMS) with d.e. - 9%. The overall yield of TV was 81%.
Results for the batch epimerization of the acetate IX (R3= cyclopropyl, Rj Ra= tert-butydimethylsilyl), obtained as above but with shorter reaction time promoted by different types of silica gel in hexane, at various temperatures and silica gel/substrate ratios, are presented in Table 1.
Table 1. Results of the one-pot batch process promoted by silica gel in hexane

(Table Remove)
(Table Remove)
Legend:
SG/Hex - Silica Gel/hexane ratio i
Purity - total content of alcohols IV (a and b, R3= cyclopi'opyl, R2= Rj= TBDMS)
in isolated product
HE - diastereomeric excess
Example 2 2-step batch process on silica gel following hydrolysis with
To a round bottom 50 ml flask containing a solution of acetate IX (Ra= cyclopropyl, R2= R:>= tert-butyldimethylsilyl, TBDMS, d.e. >97%1.02g) in hexane (20 ml), was added silica gel grade 40 (0.063-0.200 mm, 4.61 g). After stirring for about an hour, the contents of the flask were filtered and the residual silica gel was washed with ethyl acetate (50 ml). The filtrate and washing were combined and evaporated to dryness at 38aC/20 mbar lo afford a crude product (0.935 g) containing 50.1% of a mixture of acetates X (R3= cyclopropyl, R2=R3= tert-butyldimethylsilyl, TBDMS) with d.e. 7.4%. These were dissolved in Methanol/THF (2.5ml: 2.5 ml), anhydrous potassium carbonate (0.61 g) was added and the mixture was stirred overnight. The solvent was removed under reduced pressure and the residue was extracted with hexane (3 X 30 ml). The extracts were combined and evaporated to afford 0.873 g of a crude product, containing 76.2% of a mixture of alcohols IV (Ra= cyclopropyl, R2= RS= tert-butyldimethylsilyl, TBDMS) with d.e. 7%. in 72% yield.
Example 3 2-step batch process on Alumimum oxide
To a round bottom 100 ml flask containing a solution of acetate EX (Rs= cyclopropyl, R2= R3= TBDMS, d.e. >97% 2.2g) in hexane (44 ml), was added basic aluminum oxide (90, 0.063-0.200 23.1 g). After stirring over 6 hours the content was filtered and the residual alumina was then washed with ethyl acetate (80 ml). The filtrate and washing were combined and evaporated to dryness at 38°C/20 mbar to afford a crude
product (1.97 g) containing 59% of a mixture of acetates X (R3= cyclopropyl, R2= Ra= tert-butyldimethylsilyl, TBDMS). The residue was dissolved in Methanol/dichlorometliane (3.5ml: 2.5 ml), anhydrous potassium carbonate (1.3 g) was added and the mixture was stirred overnight. The solvent was removed under reduced pressure and the residue was extracted with hexane (2 X 40 ml). The extracts were combined and evaporated to afford 1.68 g of a crude product, containing 93% of a mixture of alcohols IV with d.e. 69%. Yield 98%.
Example 4
The general procedure outlined above for the two-step batchwise process was employed in each of the experimental runs discussed in the table below.
Results for the two-step batch epimerization of acetates DC (Ry= cyclopropyl, Rj= R3= TBDMS) promoted by different types of silica gels in various solvents, under various silica gel: substrate ratios at 20°C, are presented in Table 2. Results of the two-step batch epimerization of acetate IX R3= cyclopropyl, R2= Ra= te/i-butyldimethylsilyl, TBDMS) promoted by different types of active solids are presented in Table 3.
(Table Remove)
Legend: SG/Sub - silica gel/substrate ratio. SG/S - silica gel/solvent ratio. SH - solvent for hydrolysis. YH - extent of hydrolysis of acetate X (R3= cyclopropyl, R[= R2= TBDMS) during epimerization. Purity - total content of alcohols IV (R3= cyclopropyl, R'j= R2= TBDMS) in final product. DE -diastereomeric excess; Yield - overall yield of alcohols IV and VTI (R3= cyclopropyl, RI= R2= TBDMS) after epimerization-hydrolysis; Solvents: DCM - dichloromethane, TBME - tert-butyl methyl ether
Table 3. Results of the 2-step batch process promoted by different active solids in various solvents at 20°C

(Table Remove)
Solid/Sub - active solid/substrate ratio
SoI/S- active solid/solvent ratio
CH - conditions for hydrolysis: C - K2C03 in methanol/DCM, D - autohydrolysis on the active solid
YS — extent of hydrolysis of acetate X (R3=: cyclopropyl, RI= R2= TBDMS) during epimerization
Purity - total content of alcohols IV and VII (R3= cyclopropyl, Rj= R2= TBDMS) in final product
DE -diastereomeric excess
Yield - overall yield of alcohols TV and VII (R3= cyclopropyl, Rj= R2= TBDMS) after epirnerization -hydrolysis
DCM dichloromethane
Example 5 Epimerizatioa iu a column packed with sjlica gel followed by hydrolysis with Id
A solution of acetate IX (R3= cyclopropyl, RI= R2= TBDiVIS, 1.032 g, d.e. >97%) in dichloromethane (DCM) (2 ml) was placed on the top of a column (ID 18 mm) packed with 30g of silica gel grade 60 (0.040-0.063 mm). The column was eluted with DCM (50 ml), followed by a mixture of DCM:EA:water (40:10:0.05 ml). The eluates were combined and evaporated to afford 1.058 g of a crude product. This was dissolved in tetrahydroiuran (5 ml) and anhydrous potassium carbonate (0.660 g) was added, followed by methanol (10 ml), and the mixture was stirred overnight at 20-22°C. The solvent was removed under reduced pressure and the residue was extracted with hexane (3 x 30 ml). The extracts were combined, washed with water (-10 ml) and evaporated to afford 1.057 g of a crude product containing a mixture of alcohols IV (Ry= cyclopropyl, R,= R3= TBDMS) d.e. 78%. Yield -100%.
Example 6
The general column method discussed above was repeated with different silica gels as the epimerization-active solid. The results are given below.
Results of the epimerization of esters (IX)(R3= cyclopropyl, RI= R.2— TBDMS) at 20°C in a column packed with various silica gels of different granule size, using various solvents for the elution, are presented in Table 4. Note that the ester in No. 65 is ofpropionate.
Table 4. Results of the epimerization of esters IX (Ra= Cyclopropyl, RI= R2= TBDMS) on a silica gel column, followed by hydrolysisrwith K2CO3 In MeOH/THF
(Table Remove)
Example 7 Epimerizatioa of acetate VI (Ry= cydopropyl^R^ R2= TBDMS)
To a round bottom 25 ml flask containing mixture of VI and IV a (0.37g 60:40, R3= cyclopropyl, R,=; R2= TBDMS) dissolved in hexane (5 ml), was added silica gel 60 (0.040-0.063 mm, 2.31 g). The mixture was stirred slowly for 1.5 h then filtered. The
silica gel was washed with ethyl acetate (50 ml). The filtrate and washings were combined and evaporated to dryness at 38°C/20 mbar n j afford a crude product (0.3g), containing a mixture of diastereomeric alcohols IV enriched with alcohol IV a (R3= cyclopropyl, R,= R2= TBDMS), d.e.18.4%
Example 8 Epimerization of acetate VI (Bj= cvclopropvl, Ri=Rf= H)
To a round bottom 50 ml flask containing a solution of the acetate VI (0.44g^ R3= cyclopropyl, Ri^Ra^ H, d.e.>98%) in a mixture of DCM (5 ml) and hexane (5 ml), was added silica gel grade 60 (0.040-0.063 mm, 4.86 g). The mixture was stirred slowly for 24 h. HPLC analysis showed that the reaction mixture contained a mixture of alcohols VII (R3= cyclopropyl, Ri=R2= H) d.e.= 20%.
Example 9 Epimerization of alcohol VIII (Rr=cydopropyl, R]=R2= TBDMS) in situ via enzyme-promoted acylation with vinyl acetate
To a round bottom 50 ml flask containing a solution of alcohol VHI (Ra™ cyclopropyl, Ri=R2= TBDMS, 0.502 g, 0.69 mmol, d.e. >98%) in hexane (10 ml), were added lipase from Pseudomonas sp. (0.15 g,) and vinyl acetate (0.29 ml, 3.12 mmol). The mixture was stirred at room temperature for 3 h. The mixture was diluted with hexane (20 ml) then anhydrous potassium carbonate (1.69 g, 16.7 mmol), followed by silica gel 60 (0.043-0.060 mm, 4.96 g) were added. The mixture was stirred for 2 days and filtered. The cake was washed with ethyl acetate (2 x 25 ml) and evaporated imder reduced pressure to afford 0.486 g of crude product containing 76% of a racemic mixture of the alcohols IV (R,= cyclopropyl, R,=R2= TBDMS), d.e. 4.5%. Yield 83%.
Example 10 Epimerization on silica gel column of alcohol VIII (R^ cyclopropyl, R^Rf= TBDMS) in situ via the Mitsiinobu reaction
To a round bottom 10 ml flask containing a solution of alcohol VIII (Ry= cyclopropyl, R!=R2= TBDMS, 0.502 g, 0.7 mmol, d.e>98%), triphenylphosphine (0.378 g, 1.4 mmol, and/7-nitrobenzoic acid (0.239 g, 1.4 mmol) hi anhydrous tetrahydrofuran (6 ml), was added a solution of diisopropyl azodicarboxylate (0.30 g, 1.4 mmol) in anhydrous tetrahydrofuran (4 ml). The mixture was heated to 60°G and stirred for 4.5 h, then extracted with hot hexane (50°C, 3x10 ml). The extracts were combined and evaporated under reduced pressure to afford 1.066 g of a crude product containing the
nitrobenzoyl ester at C-24 with d.e. 98%. This was placed on top of a column (ID18 mm) packed with 20 cc of silica gel 60 (0.040-0.063 mm). The column was eluted with gradient of hexane-EA to afford 0.263 g, containing -70% of racemic mixture of the alcohols IV (R3= cyclopropyl, R,=R2= TBDMS), d.e. 5%. Yield 37%.
Example 11 Epimerization of alcohol VIII CR^= cyclopropvl, RHR3= TBPMS) using silica gel treated with triethyl amine
To a round bottom 1000 ml flask containing a solution of acetate IX (R3= cyclopropyl, R2= R3=TBDMS, d.e. >97% 20 g) in hexane (400 ml) and triethylamine (2ml, 14.4mmol), was added silica gel grade 60 (0.043-0.060 mm, 200 g). The mixture was stirred slowly for 7 h and then filtered. The silica gel was washed with ethyl acetate (2X200 ml). The filtrate and wasliings were combined and evaporated to dryness at 38°C/20 mbar to afford a crude product (20.6 g) containing 80.3% of a mixture of the epimeric alcohols IV (R3= cyclopropyl, R2= RI= TBDMS) with d.e. = 0.4%. The overall yield of IV was 85%.
Example 12 Preparation Calcipotriepe (I)
A solution of alcohol IVa = VIH (R]=R2=/e7-/-butyldimethylsilyl, R3=cycloprpyl) (15.3g, 24mmol), 9-acerylanthracene (1.6g, 7.2mmol), and triethylamme 350jaL in toluene (1200ml) contained in a photochemical reactor and cooled to 5-8 °C. Irradiated with light from a high pressure ultra-violet lamp to completion (~ 45 min.) and reaction mixture was transferred to evaporation together with 2x100ml toluene from rinsing the reactor. Drying tinder high vacuum gave crude alcohol VII a 15.3g.
The alcohol Vila was dissolved in THF (450ml) and tetrabutylammonium fluoride (45ml, 45mmol) was added. The mixture was heated at 40 °C for 2 hours under atmosphere of nitrogen and evaporated to dryness. The residue was dissolved in ethyl acetate (1200ml) and washed with 2% sodium bicarbonate solution (2X150ml) following with 250rnl brine. After drying with Na2SO4 the solution was evaporated to dryness. The residue was chromatographed on silica gel (1200g) eluted with mixture of ethyl acetate in hexane. Collection of the appropriate fractions (checked by TLC) and svaporation gave 7g of a foam.
Three grams of the product were crystallized from methyl formate to give 2.6g Dalcipotriene structure I.
Example 13 - Enzymatic Esterification
To a stirred solution of C-24 epimeric alcohol mixture of structure IV, Ri=R2= tert butyldimetllylsilyl and Rs-cyclopropyl, (20 gr, 3.1.2 mmol) and vinyl acetate (5.8ml, 62.4 mmol) in hexane (60ml) was added 0.56gr Alcaligenes sp. Lipase. The mixture was stirred for 3 hours at 25 + 3 °C after which time the HPLC analysis showed essentially complete conversion of epimer C-24 (R) to the acetate. The remaining nonesterified C-24 (S) alcohol was >99% diastereomeric excess (by HPLC). The solution was filtered and concentrated to dryness. The residue was chromatographed on pre-treated silica gel with 5% ethyl acetate in hexane then with ethyl acetate to give C-24 acetate compound IX (llgr) aid C-24 alcohol having the opposite configciration at C-24 (7.4gr).
The purity profile of the product was as follows.

(Table Remove)
Example 14 Enzymatic Esterification
To a vial containing 100 mg (0.16 mmol) of mixed C-24 epimers of C-24 alcohol of structure TV, withRi=R2= tert-butyldirnethylsilyl and R3=cyclopropyl, (65:35 isomer ratio), 0.044 ml (0.47 mmol) vinyl acetate and 1.5 ml diisopropyl ether, 10 mg of Alcaligenes sp. lipase was added. The mixture was stirred at room temperature for 2 hours after which time the HPLC analysis showed the presence of 65% compound IX and 35% of the alocihol having opposite configuration at C-24.

We Claim:
1. A method of decreasing the diastereometric excess of a starting mixture of epimers of a vitamin D analog, which analog is represented by one or the other structures IV or VII

(Figure Remove)

wherein Ri and R2 can be the same or different and are selected from hydrogen or silyl hydroxyl protecting group and Ra is a Ci-Ce alkyl or cycloalkyl group, comprising the steps of:
a) acetylating the hydroxyl group at the 24 position with
an acetylating agent to form the corresponding acetate ester,
b) providing a solution of the acetylated vitamin D analog
in a solvent selected from hexane, heptane, toluene, benzene,
dihloromethane, ethyl acetate, and mixtures of these,
c) contacting the solution with an epimerization-active
solid whereby the ester is epimerized to obtain a solution of epimers of
esterified vitamin D analog having a diastereometric excess that is less
than that of the starting mixture of epimers, and
d) hydrolyzing the epimerized esters to obtain a mixture
of epimers of vitamin D analog having a diastereometric excess that is
less than that of the starting mixture.
2. The method as claimed in claim 1 wherein Rais cyclopropyl.
3. The method as claimed in claim 1 wherein Ri and R2 are t-butyl dimethylsilyl.

4. The method as claimed in claim 1 wherein the acetylating agent is selected from acetyl chloride and acetic anhydride.
5. The method as claimed in claim 1 wherein the acetylating is by enzyme-asisted acetyaltion with vinyl acetate.
6. The method as claimed in claim 1 wherein the epimerization-active solid is selected from the group consisting of silica gel, aluminum oxide, florisil, and zeolite.
7. The method as claimed in claim 6 wherein the epimerization-active solid is combined with an amine or an inorganic base.
8. The method as claimed in claim 7 wherein the inorganic base is potassium carbonate.
9. The process for making calcipotriene comprising the steps of:
a) acetylating an alcohol selected from XI and XII, wherein Ri and Ra are independently H or a silyl hydroxyl protecting group,

(Figure Remove)

b) contacting a solution of the acetate in a solvent selected
from hexane, heptane, toluene, benzene, dichloromethane, ethyl acetate,
and mixtures of these with an epimerization-active solid,
c) separating the solution of epimerized ester from the
epimerization-active solid,
d) washing, at least once, the separated epimerization-
active solid with a solvent that can be the same or different from the
solvent in which the ester to be epimerized was contacted,

e) combining the washings with the separated solution of
epimerized ester, and
f) removing solvent to obtain the epimerized ester.
10. A method of decreasing the diastereometric excess of a starting
mixture of epimers of a vitamin D analog substantially as herein
described with reference to foregoing examples.
11. The process for making calcipotriene substantially as herein
described with reference to foregoing examples.