Enantioselective synthesis of a key A-ring intermediate for the preparation of 1α,25-dihydroxyvitamin D3

Article abstract of DOI:10.1021/ol102586w

A novel approach to the key A-ring α, β-unsaturated aldehyde 1, an important intermediate for the preparation of 1α,25-dihydroxyvitamin D₃, has been developed. The strategy started from the inexpensive starting material (R)-carvone with an ene reaction serving as the key step toward the potential synthesis of vitamin D₃ analogues bearing the modification at the C-2 position.

Full text of DOI:10.1021/ol102586w

ORGANIC
LETTERS
2011
Vol. 13, No. 1
86-89
Enantioselective Synthesis of A Key
A-Ring Intermediate for the Preparation
of 1r,25-Dihydroxyvitamin D3
Yan Chen* and Tong Ju
Tianjin Key Laboratory for Modern Drug DeliVery & High-Efficiency, School of
Pharmaceutical Science and Technology, Tianjin UniVersity, Tianjin 300072, China
cylcy2005@yahoo.com.cn
Received October 27, 2010
ABSTRACT
A novel approach to the key A-ring r, ꢀ-unsaturated aldehyde 1, an important intermediate for the preparation of 1r,25-dihydroxyvitamin D3,
has been developed. The strategy started from the inexpensive starting material (R)-carvone with an ene reaction serving as the key step
toward the potential synthesis of vitamin D3 analogues bearing the modification at the C-2 position.
1R,25-Dihydroxyvitamin D3 (calcitriol), the active form of
vitamin D3, is a multifunctional steroidal hormone that
regulates cell differentiation, cell proliferation, and the
immune system, in addition to its classical function in
calcium and phosphate metabolism.1 However, therapeutic
utility of calcitriol in the treatment of cancer and psoriasis
is limited by its potent calcemic effects.2 Thus, the research
for noncalcemic therapeutic agents and for convenient
methods of synthesizing modified calcitriol has been greatly
stimulated by medical needs. It is not surprising that more
than 3000 vitamin D3 analogues have been synthesized over
the past few decades.3 However, most of these synthetic
studies involved side-chain modification and the decoration
of the A-ring is not intensively reported.4 Therefore, the
construction of building blocks that could lead to vitamin
D3 analogues bearing the modification on the A-ring is of
important significance.
The R,ꢀ-unsaturated aldehyde 1 is an essential building
block for the synthesis of 1R,25-dihydroxyvitamin D3 in
Julia’s olefination approach5 (Figure 1). And in the enyne
approach,6 1 is a key intermediate which could furnish
the A-ring 3 by Corey-Fuchs homologation.7 Although the
majority of the syntheses of 1 utilized (S)-carvone as the
starting material,8 application of (R)-carvone remains unex-
ploited despite the obvious cost advantage over its enanti-
(3) (a) Zhu, G. D.; Okamura, W. H. Chem. ReV. 1995, 95, 1877–1952.
(b) Dai, H.; Posner, G. H. Synthesis 1994, 1383–1398. (c) Mathieu, C.;
Adorini, L. Trends Mol. Med. 2002, 8, 174–179. (d) Nagpal, S.; Na, S.;
Rathnachalam, R. Endocr. ReV. 2005, 26, 662–687. (e) Laverny, G.; Penna,
G.; Uskokvic, M.; Marczak, S.; Maehr, H.; Jankowski, P.; Ceailles, C.;
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3380. (c) Saito, N.; Suhara, Y.; Kurihara, M.; Fijishima, T.; Honzawa, S.;
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Plum, L. A.; Deluca, H. F. J. Med. Chem. 2007, 50, 6154–6164.
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1 1980, 1400–1404.
(1) (a) Feldman, D., Pike, J. W., Glorieux, F. H., Eds. Vitamin D, 2nd
ed.; Elsevier Academic Press: New York, 2005. (b) DeLuca, H. F. Nutr.
ReV. 2008, 66, s73–s87. (c) Adorini, L.; Penna, G. Nat. Clin. Pract.
Rheumatol. 2008, 4, 404–412. (d) White, P.; Cooke, N. Trends Endocrinol.
Metab. 2000, 11, 320–327. (e) Hammes, S. R. Proc. Natl. Acad. Sci. U.S.A.
2003, 100, 21680–21700. (f) Holick, M. F., Eds. Vitamin D; Humana Press:
Totowa, NJ, 1999.
(2) (a) Bouillon, R.; Okamura, W. H.; Norman, A. W. Endocr. ReV.
1995, 16, 200–257. (b) Carlberg, C.; Mourin´o, A. Expert Opin. Ther. Patents
2003, 13, 761–772. (c) DeLuca, H. F. J. Bone Miner. Metab. 1990, 8, 1–9.
(d) Bexkman, M. J.; DeLuca, H. F. Prog. Med. Chem. 1998, 35, 1–56. (e)
Stein, M. S.; Wark, J. D. Expert Opin. InVestig. Drugs 2003, 12, 825–840.
10.1021/ol102586w  2011 American Chemical Society
Published on Web 12/06/2010

Scheme 1
.
Enantioselective Synthesis of R,ꢀ-Unsaturated
Aldehyde 1 from (R)-Carvone
Figure 1. R,ꢀ-Unsaturated aldehyde 1 as a key intermediate to
synthesize calcitriol.
omer.9 We wish to report herein an efficient enantioselective
synthesis of the R,ꢀ-unsaturated aldehyde 1 from (R)-
carvone.
Compared with Okamura’s8b and Mourin´o’s8c approach,
our strategy is conceptually different (Figure 2): In the
95% yield.10 Regioselective syn-epoxidation of the allylic
double bond of 6 was achieved with m-CPBA in DCM at
-30 °C producing the desired epoxide 7 in 92% yield.
Mitsunobu reaction of this epoxy alcohol occurred with
complete inversion of the configuration at C-1 of 7 affording
nitrobenzoate 8.11 Ozonolysis12 of 8 at -78 °C in the
presence of methanol and in situ acylation of the resulting
hydroperoxide intermediate gave the acetate product 9
predominantly.13 After saponification of the ester, the
intermediate diol was protected with a MOM group which
is among the most used protecting groups in the area of
vitamin D3 synthesis.14,15 It was found that the undesired
epoxy ring-opened compound 13 was obtained as the major
product in this reaction. Fortunately, the conversion of 13
to the desired product 10 took place smoothly under basic
conditions. Thus, MOM ether 10 was achieved in a three-
step sequence in high yield from 9. The epoxide functionality
Figure 2. Comparison of the synthetic strategies to construct 1.
previous methods, the alkoxyl group at the C-1 position in
1 was introduced at the C-3 position of (S)-carvone by
epoxidation and subsequent ring-opening. In comparison, we
envisaged the installation of this group through a diastereo-
selective reduction of the ketone in (R)-carvone, and the
formyl group at the C-5 position in compound 1 was
assembled from (R)-carvone through an ene reaction with
actived formaldehyde followed by oxidation.
Our pathway synthesis of 1 is outlined in Scheme 1.
Diastereoselective reduction of (R)-carvone with LAH in
DCM at -78 °C afforded the sole isomer cis-carveol 6 in
(10) Bourque, L. E.; Cleary, P. A.; Woerpel, K. A. J. Am. Chem. Soc.
2007, 129, 12602–12603.
(11) Barbier, P.; Mohr, P.; Muler, M.; Masciadri, R. J. Org. Chem. 1998,
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67, 1580–1587.
(6) (a) Harrison, R. G.; Lythgoe, B.; Wright, P. W. Tetrahedron Lett.
1973, 14, 3649–3652. (b) Okamura, W. H. Acc. Chem. Res. 1983, 16, 81–
88. (c) Mascareiias, J. L.; Sarandeses, L. A.; Castedo, L.; Mourin´o, A.
Tetrahedron 1991, 47, 3485–3498. (d) Oves, D.; Ferrero, M.; Fernandez,
S.; Gotor, V. J. Org. Chem. 2003, 68, 1154–1157. (e) Rodriguez, R.;
Ollivier, C.; Stantelli, M. Tetrahedron Lett. 2004, 45, 2289–2292.
(7) Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 13, 3769–3772.
(8) (a) Baggiolini, E. G.; Iacobelli, J. A.; Hennessy, B. M.; Batcho, A. D.
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28, 2099–2102.
(13) The absolute configuration of ester groups at C-1 and C-3 of 9
was confirmed by X-ray crystal analysis (see Supporting Information).
(14) TBS was chosen as the protective group first, but it was found that
the TBS ether 15 had no reaction when treated with the active form of
formaldehyde 14. Considering if the steric hindrance of the TBS group
might be the initial problem, we turned our attention to the MOM group.
(9) Hatakeyama, S.; Numata, H.; Osanai, K.; Takano, S. J. Org, Chem.
1989, 54, 3515–3517.
Org. Lett., Vol. 13, No. 1, 2011
87

in compound 10 was removed by Zn powder and NaI to
reinstall the CdC bond to give product 11 in good yield.16
With the olefin 11 in hand, we studied the ene reaction
following the protocol of Yamamoto17 and were delighted
to isolate the single isomer 1218 in moderate yield from the
reaction of 11 with the active form of formaldehyde 14 which
was prepared by use of AlMe3, 2,6-diphenylphenol, and
trioxane. Without purification, primary alcohol 12 was further
subjected to oxidation and isomerization, and the final R,ꢀ-
unsaturated aldehyde 1 was successfully achieved in 92%
yield and 24% overall yield from (R)-carvone.
pylamide (LDA) and methyl iodide furnished a 3:2 diaster-
eomeric mixture of 6-methylcarvone, which upon low
temperature crystallization provided the major trans-6-
methylcarvone 20 in stereochemically pure form (Scheme
2).21 In the following reduction and epoxidation, it was found
Scheme 2. Modification of C-2 Position with Methyl Group
A significant feature of this new strategy was that the C-2
in product 1 maps onto the enolizable position of C-6 of
(R)-carvone and is thus amenable to functionalization with
a variety of electrophiles. Furthermore, it was found that
some functionalization of the C-2 position on the A-ring
increased the binding affinity for vitamin D receptor (VDR)
with potent agonistic activity. According to literature,19
nearly all of the analogues of 17 were prepared by Trost’s20
strategy, in which the A-ring part, 1,7-enyne 18, was obtained
from chiral monosaccharides (Figure 3). Therefore, our
that the methyl group had no effect on the yield and the
diastereoselectivity of these reactions, and the sole isomer
22 (the absolutely configurations were confirmed by X-ray
crystal analysis, Figure 4) was obtained in 90% yield over
Figure 3. Significant feature of this strategy.
strategy opens new possibilities for the preparation of vitamin
D3 analogues of therapeutic potential, particularly with
modifications at the C-2 position.
Figure 4. X-ray structure of 22.
Our preliminary investigation of this potential was directed
to preparation of the C-2 methylated analogue of calcitriol.
Kinetic methylation of (R)-carvone using lithium diisopro-
two steps. Subjecting 22, in which the key stereocenters have
been established, to our synthetic route in Scheme 1 and the
(15) Examples of the application of the MOM group in vitamin D3
synthesis: (a) Riveiros, R.; Rumbo, A.; Sarandeses, L. A.; Mourin´o, A. J.
Org. Chem. 2007, 72, 5477–5485. (b) Reino, C. G.; Vitale, C.; Maestro,
M.; Mourin´o, A. Org. Lett. 2005, 7, 5885–5887. (c) Moriaty, R. M.; Kim,
J.; Brumer, H. Tetrahedron Lett. 1995, 36, 51–54. (d) Nemoto, H.; Wu,
X.; Kurobe, H.; Ihara, M.; Fukumoto, K.; Kametani, T. Tetrahedron Lett.
1983, 24, 4257–4260. (e) Castedo, L.; Granja, J.; Maestro, M. A.; Mourin´o,
A. Tetrahedron Lett. 1987, 28, 4589–4590.
(19) (a) Suhara, Y.; Nihei, K.; Kurihara, M.; Kittaka, A.; Yamaguchi,
K.; Fujishima, T.; Konno, K.; Miyata, N.; Takayama, H. J. Org. Chem.
2001, 66, 8760–8771. (b) Takahashi, T.; Doi, T.; Hijikuro, I. J. Am. Chem.
Soc. 2001, 123, 3716–3722. (c) Saito, N.; Saito, H.; Anzai, M.; Yoshida,
A.; Fujishima, T.; Takenouchi, K.; Miura, D.; Ishizuka, S.; Takayama, H.;
Kittaka, A. Org. Lett. 2003, 5, 4859–4862. (d) Saito, N.; Matsunaga, T.;
Saito, H.; Anzai, M.; Takenouchi, K.; Miura, D.; Namekawa, J.; Ishizuka,
S.; Kittaka, A. J. Med. Chem. 2006, 49, 7063–7075. (e) Posner, G. H.;
Woodard, B. T.; Grawford, K. R.; Peleg, S.; Brown, A. J.; Dolan, P.;
Kensler, T. W. Bioorg. Med. Chem. 2002, 10, 2353–2365. (f) Posner, G. H.;
Cho, C. G.; Anjeh, T. E. N.; Johnson, N.; Horst, R. L.; Kobayashi, T.;
Okano, T.; Tsugawa, N. J. Org. Chem. 1995, 60, 4617–4628. (g) Posner,
G. H.; Johnson, N. J. Org. Chem. 1994, 59, 7855–7861.
(16) Li, Y.; Feng, J.; Wang, W.; Chen, J.; Cao, X. J. Org. Chem. 2007,
72, 2344–2350.
(17) Maruoka, K.; Conception, A. B.; Murase, N.; Oishi, M.; Hirayama,
N.; Yamamoto, H. J. Am. Chem. Soc. 1993, 115, 3943–3949.
(18) The configuration of the new chiral center was not determined.
88
Org. Lett., Vol. 13, No. 1, 2011

following literature work should lead to the C-2 methylated
analogue 17 (R ) R-Me).22
Further synthetic applications of this method are being
investigated in our laboratories.
In summary, we have developed a novel efficient enan-
tiospecific synthesis of the key A-ring synthon for the
preparation of 1R,25-dihydroxyvitamin D3 starting from
inexpensive (R)-carvone, in 11 steps and 24% overall yield,
with the ene reaction as the key step. The method has the
potential to be applied to the synthesis of vitamin D3
analogues bearing the modification at the C-2 position.
Acknowledgment. We acknowledge the National Cultiva-
tion Foundation (B) for New Faculty of Tianjin University
(TJU-YFF-08B68) for financial support. We also thank Dr.
Emile J. Velthuisen, GlaxoSmithKline, R.T.P., North Caro-
lina, for revising our English text.
Supporting Information Available: Detailed experimen-
tal procedures and spectral data for all new compounds (PDF)
and X-ray structural data of 9 and 22 (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(20) Trost, B. M.; Dumas, J.; Villa, M. J. Am. Chem. Soc. 1992, 114,
9836–9845.
(21) Srikrishna, A.; Kumar, P. R. Tetrahrdron Lett. 2002, 43, 1109–
1111.
(22) The approach from 22 to analogue 17 (R ) R-Me) is continued in
our lab. Herein, we wish to report this potential application of our new
strategy.
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