A general approach to the synthesis of enantiopure 19-nor-Vitamin D 3 and its C-2 phosphate analogs prepared from cyclohexadienyl sulfone

Article abstract of DOI:10.1039/c2cc33054g

A synthesis of enantiopure 19-nor-Vitamin D₃ and its C-2 substituted cyclic phosphate analogs is achieved via in situ trapping of an α-sulfonyl anion with a CD-ring allyl chloride and 1,2-eliminative desulfonylation exploiting the basic properties of TBAF. The A-ring is prepared via anti-selective dithiane addition to vinyl sulfone and LiBH₄ mediated sequential bis reduction of an epoxy vinyl sulfone.

Full text of DOI:10.1039/c2cc33054g

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A general approach to the synthesis of enantiopure 19-nor-Vitamin D3
and its C-2 phosphate analogs prepared from cyclohexadienyl sulfonew
Vikas Sikervar, James C. Fleet and Philip L. Fuchs*
Received 27th April 2012, Accepted 28th May 2012
DOI: 10.1039/c2cc33054g
A synthesis of enantiopure 19-nor-Vitamin D3 and its C-2
substituted cyclic phosphate analogs is achieved via in situ
trapping of an a-sulfonyl anion with a CD-ring allyl chloride
and 1,2-eliminative desulfonylation exploiting the basic properties
of TBAF. The A-ring is prepared via anti-selective dithiane
addition to vinyl sulfone and LiBH4 mediated sequential bis
reduction of an epoxy vinyl sulfone.
1,25-Dihydroxyvitamin D3, [1a,25(OH)2D3] is an active meta-
bolite of Vitamin D3 and is involved in the regulation of
calcium homeostasis and bone metabolism. In addition, it
suppresses the growth of numerous human cancer cell lines
by inhibiting cell cycle progression and inducing cell death.1
Therapeutic applications have proven difficult because of
severe side effects such as hypercalcemia. Design of Vitamin
D3 analogs with effective antiproliferative activity combined
with reduced calcemic effects has been an extensive area of
research aimed at overcoming this hurdle.2 DeLuca et al.
reported in 1990 that the deletion of the C10,19 exocyclic
olefin renders the molecule less calcemic and significantly
increases stimulation of differentiation and growth inhibition
of tumor cells.3 This finding has spurred the synthesis of
19-nor-Vitamin D3 and its analogs, a number of which are
currently used as drugs.4 A number of analogs with C-2
substituted products are found to have higher cell differentiating
properties and VDR binding affinity.5 In addition, introduc-
tion of a phosphonate group in the upper side chain also
reduces the calcemic effects of these analogs.6 Phosphorus-
bearing A-rings and their effects on VDR binding affinity and
hypercalcemic properties are unknown. We now report the
synthesis and in vitro properties of four A-ring cyclic phosphate
analogs. The synthetic strategy as shown in Scheme 1 involves
the coupling of A-ring sulfone 4 with CD-ring allyl chloride 3.
The A-ring sulfone 4 is prepared by series of transformations
on cyclohexadienyl sulfone 5.
Scheme 1 Retrosynthetic analysis.
Scheme 2 Synthesis of 19-nor-1a(OH)2D3.
7
ultimate starting material, which is prepared on the kg scale.9
Application of the standard three-step synthesis provides
enantiopure epoxide 6,10 which suffers sequential double
conjugate reduction with LiBH4 to afford sulfone 7 in 2.8 : 1 dr
in 81% yield. O,C-bismetallation of 7 using 2.0 equiv of
n-BuLi gives the dianion, which is treated with allyl chloride
311 to deliver alkylated product 9. One-pot TBAF–mediated
desulfonylation with concomitant silyl ether deprotection
directly provides diene 10 in 70% overall yield from 7.
As shown in Scheme 2, a concise route to 19-nor-1a(OH)2D3
involves alkylation of an alkoxide-sulfonyl dianion derived
from 7 with allyl chloride 8.8 Achiral dienyl sulfone 5 is the
Department of Chemistry, Purdue University, West Lafayette, Indiana
47907, USA. E-mail: pfuchs@purdue.edu; Fax: +1765-494-0239;
Tel: +1765-494-5242
w Electronic supplementary information (ESI) available. CCDC
879401. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2cc33054g
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9077–9079 9077

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Scheme 5 NOE comparison of alcohols 22 and 23.
Scheme 3 Synthesis of sulfone 4.
Scheme 4 Synthesis of allyl sulfone 21.
To access the C3-hydroxymethyl A-ring needed for preparation
of the cyclic phosphate targets, dithiane is used as a hydroxy-
methyl synthon. Significantly, dienyl sulfone 12 (Scheme 3)
suffers conjugate addition12 of lithiated 1,3-dithiane giving
allyl sulfonyl anion 13, which was quenched with diphenyl-
disulfide providing the thermodynamic vinyl sulfide 14 as
a single regio- and stereoisomer.13,17 Dienyl sulfone 12 is
prepared via TBSOTf catalyzed opening of the epoxide 11
which in turn is prepared from dienyl sulfone 5 in 70% yield
and 99% ee using Jacobsen’s catalyst.14 HgO mediated depro-
tection of thioacetal 14 to the aldehyde15 and subsequent
reduction with NaBH4 gave alcohol 15. One-pot TBS protection,
1,4-elimination and mCPBA oxidation provided dienyl sulfone
16.16 Epoxidation using in situ generated TFDO proceeds with
6 : 1 dr. Double conjugate-reduction of epoxy vinyl sulfone 17
with LiBH4 gave the secondary alcohol, which provided
sulfone 4 in 56% yield with 4 90% purity upon silyl ether
protection (Scheme 3).
Scheme 6 Synthesis of cyclic phosphates.
with formic acid provided diastereomer 23 after formate
hydrolysis (Scheme 5).18
Sulfone 4 is metallated with 1.05 equiv of n-BuLi and the
sulfonyl anion thus created is quenched with 1 equiv of allyl
chloride 3 to generate the coupled product 24 (Scheme 6). The
intermediate 24 is treated with 20 equiv of TBAF to form the
dienes 1 and 2 in a 2 : 1 ratio in 78% yield. The mixture of
dienes 1 and 2 is subjected to reaction with 1.0 equiv of phenyl
dichlorophosphoridate to give the required targets 25, 26 27,
28 in 1 : 1.7 : 0.7 : 1.1 dr ratio and 60% yield. The four com-
pounds were separated on HPLC and stereochemistry assigned
based on NOESY, 31P and COSY spectra.w
Impact of the vitamin D analogs on the expression of
vitamin D-responsive genes in two human colon cancer cell
lines was examined: Caco-2 cells and SW480-ADH (Fig. 1)19.
These cells respond to 1,25 dihydroxyvitamin D3 treatment by
inducing the expression of various target genes. The most
responsive vitamin D regulated gene is for the enzyme CYP24
(a cytochrome P450 family member called 25 hydroxyvitamin D,
24 hydroxylase) – this is expressed in both cell lines. In SW480-
ADH cells, 1a,25(OH)2D3 treatment also induces expression
of E-cadherin, a tight junction protein that is a marker of cell
The stereochemistry of the dithiane addition to the dienyl
sufone 18 is verified by X-ray crystallography of allyl sulfone
21 (Scheme 4).
Stereochemistry of epoxide 17 is assigned by synthesis of
cross-conjugated dienyl alcohols 22 and 23 and comparing the
NOE difference between the two protons as shown in Scheme 4.
Addition of dimethylsulfonium methylide17 to epoxyvinyl
sulfone 17 gave alcohol 22, which on Mitsunobu inversion
c
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Notes and references
1 (a) D. Feldman, F. H. Glorieux and J. W. Pike, Vitamin D,
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(c) D. Trump, J. Muindi, M. Fakih, W. Yu and C. Johnson,
Anticancer Res., 2006, 26, 2551.
3 K. L. Perlman, R. R. Sicinski, H. K. Schnoes and H. F. DeLuca,
Tetrahedron Lett., 1990, 31, 1823.
4 L. A. Plum and H. F. DeLuca, Nat. Rev. Drug Discovery, 2010, 9, 941.
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7 For details of the earlier synthesis see: (a) K. L. Perlman and
H. F. DeLuca, Tetrahedron Lett., 1992, 33, 2937; (b) T. Hanazawa,
T. Wada, T. Masuda, S. Okamoto and F. Sato, Org. Lett., 2001,
3, 3975.
Fig. 1 Effect of phosphate analogs in Caco-2 and SW480-ADH cell
lines. The effect of phosphate modified A-ring analogs on expression
of two vitamin D responsive genes in the human colonic carcinoma cell
lines Caco-2 and SW-480ADH. (A) CYP24 mRNA in Caco-2, (B)
CYP24 mRNA in SW480-ADH, (C) E-cadherin mRNA in SW480-
ADH. Cells were treated for 6 h with varying doses of vitamin D
compounds or with concentration matched ethanol vehicle (EtOH).
RNA was isolated and analyzed by real time-PCR. Data are normal-
ized to the expression of the control gene RPLPO. Bars represent the
mean Æ SEM (n = 4). * significantly different from the ethanol control
group, # significantly different from all other groups, @ significantly
different from all but one group.
8 V. Sikervar, J. Fleet and P. L. Fuchs, J. Org. Chem., 2012, 77, 5132,
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9 J. David Meyers and P. L. Fuchs, J. Org. Chem., 2002, 67, 200–204.
10 (a) J. B. Evarts, Encyclopedia of Reagents for Organic Synthesis,
John Wiley & Sons, Ltd, DOI: 10.1002/047084289X.rn00434;
(b) M. Hentemann and P. L. Fuchs, Org. Lett., 1999, 1, 355.
11 H. Takikawa, N. Kamatani, K. Nakanishi, T. Tashiro, M. Sasaki,
H. Yoshida and Y. Mizushina, Biosci., Biotechnol., Biochem., 2008,
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12 (a) T. F. Braish, J. C. Saddler and P. L. Fuchs, J. Org. Chem., 1988,
53, 3647; (b) J. C. Saddler and P. L. Fuchs, J. Am. Chem. Soc.,
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14 G. R. Ebrahimian, X. M. Jourdin and P. L. Fuchs, Org. Lett.,
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differentiation. Phosphate analogs 25, 26, 27 and 28 were
found to be less active than 1a,25(OH)2D3 in Caco-2 cell lines.
While in SW480-ADH cell lines, phosphate analogs 25 and 26
showed less activity but 27 and 28 were found to have slightly
higher activity at 100 nM.
15 E. Vedejs and P. L. Fuchs, J. Org. Chem., 1971, 36, 366.
16 M. N. Noshi, A. El-Awa and P. L. Fuchs, J. Org. Chem., 2008,
73(8), 3274.
17 V. Sikervar and P. L. Fuchs, Chem. Commun., 2011, 47, 3472.
18 J. A. Dodge, J. S. Nissen and M. Presnell, Org. Synth., 1998, 9, 607.
19 (a) J.C. Fleet, M. DeSmet, R. Johnson and Y. Li, Biochemical,
2012, 61–76; (b) M. Cui, A. Klopot, Y. Jiang and J. C. Fleet, J. Cell
Physiol, 2009, 113; (c) P. L. Kovalenko, Z. Zhang, M. Cui,
S. K. Clinton and J. C. Fleet, BMC Genomics, 2009, 11, 26.
In summary, we have reported a highly convergent synthesis
of 19-nor-Vitamin D3 and its four cyclic phosphates from
dienyl sulfone 5.
We thank Arlene Rothwell and Karl Wood for providing
MS data. We thank John Harwood for providing some of the
NMR data. We thank Purdue Cancer center for providing the
Ross fellowship.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9077–9079 9079