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 and 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 having a structure analogous to I, a defined stereochemistry for the 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 IT (C-22 aldehyde vitamin D analogue) is reacted with a Wittig reagent to afford the unsarurated ketone HI which is then reduced by, for example, sodium boroiydrid^togivelVasamixtureofC^epimei-s. See, e.g., CalverleyTetrahedron 4609-4619,1987). The desired epimer, in this case the 24 S epimer is then separated by standard methods (for example, chromatography) 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.
ScherneJ
(Figure Remove)

o

NaBH,

We have discovered a method whereby the undesired epimer, here 24R, pure or contaminated with residual undesired 24S epimer, 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
In 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 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-acti ve 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 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 epimerizati on-active solid selected irom 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 tile 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.
Jta 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 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 in a solvent selected from dichloromethane, hexarie, heptane, toluene, ethyl acetate, and mixtures of these, with an epimerization-active solid selected Scorn 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 aiydroxyl 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 epimerization-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 VIH.
(Figure Remove)

V

OH

OH

wherein Rj andRz are independently selected from hydrogen or a hydrogen protecting group and RS is a d - Cg 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)

V

OH

OH

wherein RI and R2 are independently selected from hydrogen or a hydrogen protecting group and R3 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 or VII are esterified with, for example, acetic anhydride or acetylchloride to the corresponding C-24 esters. The process is illustrated in Scheme n with the 24 R compounds V and VHL 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.
Esterification can be carried out by conventional means, or it can be carried out by enzyme-assisted esteriJEication 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 higher 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 EL
Scheme
(Figure Remove)

Scheme IE
(Figure Remove)
The present invention relates to epimers of vitamin D analogs. Epimers are diastereomers - 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 at only one asymmetric center, can be referred to as epimerizat'on.
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 [Ei - Ea], where E/ is the mole fraction of epimer i. Percent diastereomeric excess, % d.e., can be defined as 100 • [E] - £2]. 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 VUI are particular objects of the present invention.
In the structures, R] 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 K) 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 epiraer, relative to the strating mixtee. 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 analogize 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, El, 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^COa) are
i
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 nun.
In 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 columnrpacked with epitnerization-active solid and eluted with a suitable solvent or mixture of solvents that can be any of the solvents discussed above.
Jn preferred embodiments,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 solvents) 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 metiianol 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 1 One-pot batch process on silica gel in liexane
To a round bottom 250 ml flask containing a solution of acetate IX (Ra= cyclopropyl, R2= R3= tert-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
I
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 dryness at 38°C/20 mbar to afford a crude product (6.55 g) containing 79.3% of a mixture of the epimeric alcohols IV (Rs= 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, R2 RS= tert-butydimethylsilyl), obtained as above but with shorter reaction time promoted by different types of silica gel ha 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

No Silica Gel (SG), Type, Particle Size Range^ mm) SG/substrate, g/g SG/Hex, g/ml Temp,
°C ' - Time, hr. Purity,
% DE,
% Yield, %
39 40, 0.063-0.200 10.4 0.5 20 6 71 6 64
40 40, 0.063-0.200 4.7 0.27 20 23 57 4 51
41 60, 0.040-0.063 9.8 0.53 20 6 78.2 10 79
42 60, 0.040-0.063 4.3 0.24 20 23 59.2 3 55
43 40, 0.063-0.200 4.9 0.27 40 3.5 53.3 2 47
44 60, 0.040-0.063 4.3 0.23 40 3.5 54 3 53
45 40, 0.063-0.200 17 0.88 20 2.5 79.2 6 76
46 60, 0.040-0.063 13.2 0.58 20 3.0 81.3 9 80
47 40, 0.063-0.200 16.4 0.71 0-5 25 78 5 82
48 60, 0.040-0.063 13.7 0.58 0-5 26 73 9 . 70
49 60,0.040-0.063 14.5 0.57 20 3.5 81.5 9 79
50 60, 0.040-0.063 9.9 0.40 20 4 79 9 81
53 60, 0.040-0.063 10.1 0.59 20 3.5 73.6 4.6 67
57 60, extra pure, 0.063-0.200 9.9 0.53 20 25 52.3 10 49
58 100, 0.063-0.200 9.9 0.43 20 24 66.3 16 56
Legend:
SG/Hex - Silica Gel/hexane ratio j
Purity- total content of alcohols IV (a and b, R3= cycloptopyl, R2= R3= TBDMS)
in isolated product
DE - 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 (R3= cyclopropyl, R2= R3= 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 3 8°C/20 mbar to afford a crude product (0.93 5 g) containing 50.1 % of a mixture of acetates X (R3= cyclopropyl, R2=R3= tert-butyldimetliylsilyl, 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 (R3= cyclopropyl, R2= R3= tert-butyldimethylsilyl, TBDMS) with d.e. 7%. in 72% yield.
Example 3 2-step batch process on Aluminium oxide
To a round bottom 100 ml flask containing a solution of acetate IX (Ry= 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, Rz= Ra= tert-butyldrmethylsilyl, TBDMS). The residue was dissolved in Methanol/dichloromethane (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 IX (Rs= cyclopropyl, Ra= RS= 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= R3= ferf-buryldimethylsilyl, TBDMS) promoted by different types of active solids are presented in Table 3.

Table 2. Results of the 2-step batch process promoted by silica gel in various solvents (at20°C)
No ffica Gel (SG),Type, article Size Range (mm) G/Sub,
tfr olvent Co-solvent,
% v/v) G/S, g/ml 'line, SH lYH, (Purity, % 1 % DE, field,
(L 0.063-0.200 DCM' .37 5 MeOH/THF ND |61 -5 55
0, 0.063-0.200 .5 DCM .49 MeOH/THF ND 71 -2 ' 58
11 60, 0.063-0.200 2.6 DCM - .15 .75 MeOH/THF 17 89 50 9
12 40, 0.063-0.200 4.1 DCM - .18 .92 MeOH/THF 28 82 7 1
13 40, 0.2-0.5 3,2 DCM. - .18 .83 MeOH/THF 7 89 |44 6
14 60, 0.04-0.063 .5 DCM - .08 MeOH/THF 4 89.6 156 02
15 40, 0.063-0.200 2.1 DCM - 0.11 MeOH/THF 8 84.8 J25 96
16 60, 0.04-0.063 0.7 DCM; - 0.04 MeOH/THF 11 90.5 |67 92
17 40, 0.063-0.200 1.05 DCM - 0.05 MeOH/THF 14 87.6 43 105
18 40, 0.063-0.200 1.02 DCM. - 0.06 MeOH/THF 17 86.3 22 83
19 40, 0.063-0.200 0.79 DCM . 0.04 MeOH/THF 18 88.6 |40 95
21 40, 0.063-0.200 6.02 DCM - 0.31 MeOH/THF 30 80.2 \2 72
26 40, 0.063-0.200 4.6 DCM MeOH (1) 0.20 1 MeOH/DCM 10 73.6 50 72-
27 40, 0.063-0.200 4.5 DCM TEA (1) 0.23 1 MeOH/DCM 13 92 69 85
28 40, 0.063-0.200 4.5 rlexarie - 0.23 1 MeOH/DCM 50 76.2 7 72
29 40, 0,063-0.200 4.6 3exane - 0.22 2 MeOH/DCM 68 74 [5 70
30 60, 0.063-0.200 3.5 Hexane - 0.19 1 MeOH/DCM 44 83.6 1 16 80
31 60, 0.063-0.200 3.3 Hexane - 0.18 2 MeOH/DCM 56 79.2 J8 70
34 40, 0.063-0.200 9.7 Hexane - 0.57 1 MeOH/DCM 63 82.8 (6 76
35 40, 0.063-0.200 9.3 Hexane . - 0.56 2 MeOH/DCM 64 79.3 8.2 .61
36 60, 0.040-0.063 9.0 Hexane - 0.53 1 MeOH/DCM 63 - 82.8 U 76
3V 60, 0.040-0.063 8.7 Hexane • . « 0.50 2 MeOH/DCM 71 79.5 |9 7?.
38 40, 0.063-0.200 3.7 Hexane Water, 0.5 0.22 1 MeOH/DCM 50 77,7 (6 69
54 60, 0.040-0.063 13.4 Toluene - 0.48 26 Toluene 51 51.6 1-10 51.6
55 40, 0.063-0.200 14.7 Toluene! - 0.50 26 Toluene 51 59.4 -9 51
56 60, 0.040-0.063 10.2 TBME . - 0.49 72 MeOH/DCM 51 1 83.9 |94 37
Legend: SG/Sub - silica gel/substrate ratio. SG/S - silfca gel/solvent ratio. SH - solvent for hydrolysis. YH - extent of hydrolysis of acetate X (R3= cyclopropyl, 5.,= R2= TBDMS) during epimerization. Purity - total content oif alcohols IV (R3= cyclopropyl, R;== R2= TBDMS) in final product. DE -diastereomeric excess; Yield - overall yield of alcohols IV and VH (R3= cyclopropyl, RI= R2= TBDMS) after epimerization-liydrolysis; 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

No Active solid, Particle Size Range (mm) Solid/Sub-ratio,
R/R Solvent Soys,
g/ml Time, h CH YH,
% Purity, % DE,
% Yield, % Notes
32 Celite 521 4.1 DCM . 0.20 1 C 0 94.9 96 100
33 Molecular sieves, 4A 3.9 DCM 0.20 1 c 0 94.1 92 73
51 Aluminum oxide basic 90, 0.063-0.200 mm 10.5 Hexane 0.52 6 D 23 91 70 90
52 Aluminum oxide neutral 90, 0.063- 0.200 mm 10.1 Hexane 0.51 6 D 38 93 69 98
59 Florisil, 60-100 mesh 9.7 Hexane 0.32 24 D ND ND ND ND Non-selective decomposition
60 Florisil, activated at 675°C, 60-100 mesh 9.7 Hexane 0.32 24 C 4 81.5 .62 79
Legend:
Solid/Sub - active solid/substrate ratio
Sol/S- active solid/solvent ratio
CH - conditions for hydrolysis: C - K2C03 in methanol/DCM, D - autohydrolysis on the active solid
YH - extent of hydrolysis of acetate X (R3= cyclopropyl, Rt= R2= TBDMS) during epimerization
Purity - total content of alcohols IV and VII '(R3= cyclopropyl, Rf* R2= TBDMS) in final product
DE -diastereomeric excess
Yield - overall yield of alcohols TV and VII (R3= cyclopropyl, Ri=Ra= TBDMS) after epimerization -hydrolysis
DCM dichloromethane

Example 5 Epimerization in a column packed with
silica gel followed by hydrolysis with. KaCQa
A solution of acetate IX (R3= cyclopropyl, RI= R2= TBDMS, 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 tetrahydrofuran (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 (3x30 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 (R3= cyclopropyl, R,= R2= 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, R,= R2= 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 (Rs= cyclopropyl, Rj= R2= TBDMS)-on a-silica gel column, followed fry hydrolysls-withr K2CO3 Iff MeOH/THF
No Ester TypeSG, Granule size, mm SG/Sub, tlfK Eluent Purity,
% DE,
% Yield,
%
3 Ac 40, 0.2-0.5 7.5 DCM,EA 93 81 Quant
5 Ac 40, 0.063-0.200 7.3 DCM.EA 91 69 Quant
65 PrCO 60, 0.063-0.200 7.5 Hexane, EA 78 10 61
66 Ac 60, 0.063-0.200 15 Hexane, EA+2.5%AcOH 91 93 87
67 Ac 60, 0.063-0.200 15 DCM 86 49 85
Example 7 Epimerization of acetate VT (R,= cyclopropy], R,= R,= TBDMS)
To a round bottom 25 ml flask containing mixture of VI and IV a (0.37g 60:40, R3= cyclopropyl, Rj= 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 dryriess at 38°C/20 mbar to afford a crude product (0.3 g), containing a mixture of diastereomeric alcohols IV enriched with alcohol IV a (Rs= cyclopropyl, RI= R2= TBDMS), d.e. 18.4%
Example 8 Epimerization of acetate VI (Rg= cvclopronyl. Ri=Rr= H)
To a round bottom 50 ml flask containing a solution of the acetate VI (0.44gj 1 R3= cyclopropyl, .Ri=R2= 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, Rr=R2=H) d.e.= 20%.
Example 9 Epimerization of alcohol VHI (R*= cyclopropyl, Ri=Ri= TBDMS) in situ via enzyme-promoted acylation with vinyl acetate
To a round bottom 50 ml flask containing a solution of alcohol VUI (R3= 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 under reduced pressure to afford 0.486 g of crude product containing 76% of a racemic mixture of the alcohols IV (R3= cyclopropyl, R,=R2= TBDMS), d.e. 4.5%. Yield 83%.
Example 10 Epimerization on silica gel column of alcohol VUI (Rf= cyclopropyl, Ri=Rr= TBDMS) in situ via the Mitsunobu reaction
To a round bottom 10 ml flask containing a solution of alcohol VIII (R3= cyclopropyl, Ri=R2= TBDMS, 0.502 g, 0.7 mmol, d.e>98%), triphenylphosphine (0.378 g, 1.4 mmol, and^-nitrobenzoic acid (0.239 g, 1.4 mmol) in 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, Ri=R2= TBDMS), d.e. 5%. Yield 37%.
Example 11 Epimerization of alcohol VIII flfa= cyclopropyl, RHfo- TBDMS) using silica gel treated with triethvl 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 washings 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= R]= TBDMS) with d.e. = 0.4%. The overall yield of IV was 85%.
Example 12 Preparation Calcipotriene (I)
A solution of alcohol IVa = VIH (R]=R2=ter^utyldimethylsilyl, R3=cycloprpyl) (15.3g, 24mmol), 9-aceryl anthracene (1.6g, 7.2mmol), and triethylamine 350pL 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 iiniier high vacuum gave crude alcohol VBT 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 250ml brine. After drying with Na2SO4 the solution was evaporated to dryness. The-residue was chromatographed on silica gel (1200g) ehited with mixture of ethyl acetate in hexane. Collection of the appropriate fractions (checked by TLC) and evaporation gave 7g of a foam.
Three grams of the product were crystallized from methyl formate to give 2.6g Calcipotriene structure I.

Example 13 - Enzymatic Esterification
To a stirred solution of C-24 epimeric alcohol mixture of structure IV, Ri=Ra= fert butyldimethylsilyl and R3=cyclopropyl, (20 gr, 3.1.2 mmol) and vinyl acetate (5.8ml, 62.4 mmol) in hexane (60ml) was added Q.56gcAlcaligenes sp. Lipase. The mixture was stirred for 3 hours at 25 ±3 °C after which time the HPLC analysis showed essentially complete conversion of epinier C-24 (R) to the acetate. The remaining nonesterified C-24 (£) 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) and C-24 alcohol having the opposite configuration at C-24 (7.4gr).
The purity profile of the product was as follows.

Compound \Purityprofile Instrument and method
X Alcohol vnr Merck-Hitachi
rv 93.1 0.2 - Model: L-6200A intelligent pump. Mobile phase : 0.5% amyl alcohol in hexane
Alcohol 0.5 89.0 1.6

Example 14 Enzymatic Esterification
To a vial containing 100 mg (0.16 mmol) of mixed C-24 epimers of C-24 alcohol of structure IV, with Ri=R2=: fer^butyldimethylsilyl and R3=cyclopropyl, (65:35 isomer ratio), 0.044 ml (0.47 mmol) vinyl acetate and 1.5 ml diisopropyl ether, 10 mg ofAlcaligenes 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 for making mixed epimers of a vitamin D analog having a hydroxyl substituent on an asymmetric allylic carbon atom at the C-24 position, starting from an individual C-24 OH epimer or a mixture of C-24 OH epimers having an initial diastereometric excess, the steps of:
a} esterifying the hydroxyl group on the asymetric allylic carbon atom at the 24 position of the epimer or epimers with an esterifying agent,
b) contacting a solution of the ester in a solvent with an
epimerization-active solid, whereby the ester is epimerized, and
c) hydrolyzing the epimerized ester to obtain a mixture of C-24
epimers having a hydroxyl substituent on the asymmetric allylic carbon
at position 24, wherein the diasteromeric excess of the mixture is less
than the initial diastereomeric excess.
2. The method as claimed in claim 1 wherein the epimerization-active solid is combined with an amine or an inorganic base.
3. The method as claimed in claim 2 wherein the inorganic base is potassium carbonate.
4. The method as claimed in claim 1 wherein the contacting is at a temperature of about 35° C.
5. The method as claimed in claim 1 wherein the ester is in solution in a solvent selected from hexane, heptane, toluene, benzene, dihloromethane, ethyl acetate, and mixtures of these.
6. The method as claimed in claim 1 comprising, after the contacting step, (b), the steps of:
1) separating the solution of epimerized ester from the epimerized-active solid,

2) 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,
3) combining the washings with the separated solution of
epimerized ester, and
4) removing solvent to obtain the epimerized ester.
7. A method for making mixed epimers of a vitamin D analog substantially as herein described with reference to foregoing examples.
Dated this 19th day of October, 2006.
[DEEPAK KUMAR]
OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANTS]