Design and synthesis of novel 20-epi analogues of calcitriol with restricted side chain conformation

Article abstract of DOI:10.1055/s-2005-872224

The design and efficient preparation of two 20-epi analogues of calcitriol with restricted side chain conformation is described. The formation of the tetrahydropyran ring was achieved via zinc chloride mediated etherification of alcohols. Docking experime

Full text of DOI:10.1055/s-2005-872224

LETTER
2163
Design and Synthesis of Novel 20-epi Analogues of Calcitriol with Restricted
Side Chain Conformation
Synthesisof
Nov
a
el20-
e
pi
A
r
nalogues
l
of Ca
l
o
citriol s Fernández,a Generosa Gómez,a Carmen Lago,a Edelmiro Momán,b Yagamare Fall*a
a
Departamento de Química Orgánica, Facultad de Química, Universidad de Vigo, 36200 Vigo, Spain
b
Centre for Synthesis and Chemical Biology, Department of Pharmaceutical and Medicinal Chemistry,
Royal College of Surgeons in Ireland, 123 St. Stephen’s Green, Dublin 2, Ireland
Fax +34(986)812262; E-mail: yagamare@uvigo.es
Received 31 May 2005
thesis of analogues of 1 with more selective (or even dif-
Abstract: The design and efficient preparation of two 20-epi ana-
logues of calcitriol with restricted side chain conformation is de-
scribed. The formation of the tetrahydropyran ring was achieved via
zinc chloride mediated etherification of alcohols. Docking experi-
ments show that these novel analogues could be interesting synthet-
ic targets.
ferent) biological effects (Figure 1).2
Most of the analogues of 1 synthesized so far show mod-
ifications at the side chain but only a limited number of
these analogues are in use as drugs.2f The structural mod-
ifications capable of substantially enhancing the antipro-
liferative potency of 1 include: incorporation of an oxygen
atom at C-22, methyl homologation at C-26 and C-27, ho-
mologation at C-24, epimerization at C-20 and incorpora-
tion of rigid, conformationally restricting structural units.3
The natural 1a,25-(OH)2-D3 and several synthetic ana-
logues have been recently co-crystallized within the ac-
tive site of the ligand binding domain (LBD) of the human
vitamin D receptor (VDR).4 This structural information is
crucial for the rational design of more potent and selective
analogues of the hormone. Pre-orientation of the side
chain in a productive conformation is expected to facili-
tate receptor–ligand binding by lowering the entropic bar-
rier and, hence, a number of semi-rigid analogues of
calcitriol with restricted flexibility of the side chain have
been reported. Yamada and co-workers showed that ho-
mologation at C-22 provided analogues with restricted
side chain conformation and in some cases enhanced bio-
logical properties.5 For example, analogue 4 is 100 times
more efficient than 1,25-dihydroxyvitamin D3 (1) in cell
differentiation, although its affinity for the vitamin D re-
ceptor (VDR) is one-seventh of that of 1, whereas ana-
logue 3 shows the highest VDR binding activity so far
known.
Key words: synthesis, vitamin D, analogues, nitriles, diols, drugs
1a,25-Dihydroxyvitamin D3 [1, 1a,25-(OH)2-D3, calcitri-
ol], the hormonally active form of vitamin D31 (2, chole-
calciferol), exerts control over important physiological
processes, including calcium and phosphorus metabolism,
cellular differentiation and immune reactions.2 However,
the clinical utility of this hormone for treatment of cancers
and skin disorders is limited by its hypercalcaemic effects.
There is accordingly much interest in the design and syn-
R1
OH
H
H
1 R1, R2 = OH
2 R1, R2 = H
HO
OH
HO
R2
3
As part of our research program on the synthesis of vita-
min D analogues with restricted side chain conformation,
we previously reported the synthesis of analogues of 1
modified at C-22.6 We now report our preliminary results
on the docking and synthesis of 20-epi analogues of cal-
citriol with cyclic side chain.
X
Y
OH
H
H
a X = CH2, Y = O
b X = O, Y = CH2
Inspection of the available crystal structures suggests that
bulky side chains can be effectively hosted within the ac-
tive site of the receptor. Therefore, we decided to study
the effect of carbocyclic side chains on the binding and se-
lectivity of the vitamin D analogues. Our docking calcula-
tions indicate that analogues with hexacyclic side chains
should have an enhanced affinity for the receptor as such
side chains contribute to improve the receptor–ligand
complementarity. For instance, our calculations suggest
that compounds 5a and 5b should exhibit a similar bind-
ing energy to that of the natural ligand. The decrease in the
HO
OH
HO
OH
5
4
Figure 1 Structure of calcitriol and some of its analogues with
restricted side chain conformation.
SYNLETT 2005, No. 14, pp 2163–2166
Advanced online publication: 20.07.2005
DOI: 10.1055/s-2005-872224; Art ID: G18705ST
© Georg Thieme Verlag Stuttgart · New York
0
2
.0
9
.2
0
0
5

2164
C. Fernández et al.
LETTER
binding energy due to the less efficient H-bonds between
C25–O and the histidines H305 and H397 is, according to
our in silico studies, compensated by more efficient van
der Waals interactions of the new side chains with the li-
pophilic residues of the active site. In our experiments, the
automated docking program Autodock17 reproduced with
a high degree of accuracy the reported competent confor-
mation of 1a,25-(OH)2-D3 in complex with the VDR-
LBD. The docking energy computed by Autodock for the
natural substrate, –15.5 kcal/mol, is similar to the docking
energies computed for 5b and 5a: –15.5 and –15.4 kcal/
mol, respectively (Figure 2).
OH
OH
i
ii
H
H
HO
7
TBSO
6
CN
CN
OTES
iv
iii
Br
H
H
TBSO
8
TBSO
OTES
10
OH
9
OH
OTES
OTES
v
+
H
H
TBSO
11
12
TBSO
R1
R2
X
Y
OH
vi
H
H
TBSO
TBSO
14
13
Scheme 1 Reagents and conditions: (i) see ref. 7, 8; (ii) (a) p-TsCl,
pyridine, r.t. (72%); (b) NaCN, DMSO, 90 °C (94%); (iii) LDA, THF,
–78 °C, 9 (93%); (iv) DIBAL-H, CH2Cl2, –10 °C, HCl, NaBH4,
MeOH (11: 17%; 12: 44%); (v) n-Bu4NF, THF, r.t. (13a: 97%; 13b:
94%); (vi) ZnCl2, ClCH2CH2Cl, r.t. (14a: 76%; 14b: 93%).
Figure 2 Most stable conformations of 5b (left) and 5a (right)
superimposed to the active conformation of calcitriol.
The introduction of a hydroxyl group in selected positions
of the side chains of these compounds is predicted to im-
prove the docking energies by approximately 2.5 kcal/mol
(data not shown). However, the preparation of such com-
pounds would require important synthetic efforts. In order
to test the accuracy of our computational predictions, we
decided to synthesize and test compounds 5a and 5b,
readily available by using a modification of a synthetic
methodology developed by us for the preparation of other
vitamin D analogues.6 If the preliminary biological tests,
currently underway, confirm our binding model, we plan
to accomplish the synthesis of more complex structures
based upon the scaffold presented here. The synthesis of
the cyclic side chain moieties is outlined in Scheme 1.
matography as colorless oils (17 and 44%, respectively).
At this stage the stereochemistry of 11 and 12 at C22 was
unknown. Both diastereoisomeric alcohols were selec-
tively desilylated to give diols 1310 in excellent yields. Di-
ols 13 were subjected to a very efficient zinc chloride
mediated etherification13 to give 14,10 which contained the
desired cyclic side chain. The silyl protecting group of 14
was removed on refluxing THF affording alcohols 15.10
We were delighted to see that 15a on recrystallization
from ethyl acetate–hexane afforded crystals, which were
subjected to X-ray crystallographic analysis, establishing
its structure to be that shown in Figure 3.
Alcohol 7 was readily obtained from the Inhoffen–Lyth-
goe diol 6 using known methodology.7,8 Tosylation of al-
cohol 7 and displacement of the tosylate with sodium
cyanide afforded nitrile 8,9,10 which was deprotonated
with two equivalents of LDA in THF at –78 °C and react-
ed with bromide 911 to afford nitriles 1010 as a mixture of
unseparable diastereoisomers. Reaction of the nitriles 10
with DIBAL-H in dichloromethane at –10 °C and subse-
quent acid work up afforded the corresponding aldehydes
which were taken up in methanol and reacted with excess
of sodium borohydride12 to afford a mixture of alcohols
1110 and 12,10 which were cleanly separated by flash chro-
Figure 3 X ray structure of 15a.
Synlett 2005, No. 14, 2163–2166 © Thieme Stuttgart · New York

LETTER
Synthesis of Novel 20-epi Analogues of Calcitriol
2165
Oxidation of alcohols 15 with pyridinium dichromate af-
forded ketone 16,10 so setting the stage for the Wittig–
Horner reaction with phosphine oxide 1714 leading to the
desired analogues 5a15 and 5b16 [5a (54%); 5b (52%); two
steps (Scheme 2)].
Acknowledgment
This work was supported by a grant from the Xunta de Galicia
(PGIDIT04BTF301031PR). We also thank the NMR service of the
CACTI, University of Vigo, for NMR studies.
References
X
Y
X
Y
(1) Bouillon, R.; Okamura, W. H.; Norman, A. W. Endocr. Rev.
1995, 16, 200.
(2) For general reviews of vitamin D chemistry and biology,
see: (a) Vitamin D: Chemistry, Biology and Clinical
Applications of the Steroid Hormone; Norman, A. W.;
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Inc.: Riverside CA, 1997. (b) Feldman, D.; Glorieux, F. H.;
Pike, J. W. Vitamin D; Academic Press: San Diego, 1997.
(c) Pardo, R.; Santelli, M. Bull. Soc. Chim. Fr. 1985, 98.
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(3) (a) Figadère, B.; Norman, A. W.; Henry, H. L.; Koeffler, H.
P.; Zhou, J.-Y.; Okamura, W. H. J. Med. Chem. 1991, 34,
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R. L.; Holick, M. F. J. Nutr. Biochem. 1993, 4, 49.
(c) Mathiasen, I. S.; Colston, K. W.; Binderup, L. J. Steroid
Biochem. Mol. Biol. 1993, 46, 365. (d) Martínez-Pérez, J.
A.; Sarandeses, L.; Granja, J.; Palenzuela, J. A.; Mouriño, A.
Tetrahedron Lett. 1998, 39, 4725. (e) Fernandez-Gacio, A.;
Vitale, C.; Mourino, A. J. Org. Chem. 2000, 65, 6978.
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G. H. Bioorg. Med. Chem. 2001, 9, 1691. (g) Varela, C.;
Nilsson, K.; Torneiro, M.; Mouriño, A. Helv. Chim. Acta
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Regulation, Structure-Function Analysis and Clinical
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(4) Rochel, N.; Wurtz, J. M.; Mitschler, A.; Klaholz, B.; Moras,
D. Mol. Cell 2000, 5, 173.
i
H
H
TBSO
15
14
OH
X
Y
iii
ii
Ph2PO
H
O
16
TBSO
OTBS
X
17
Y
H
iv
5a, b
TBSO
OTBS
18
a X = CH2, Y = O
b X = O, Y = CH2
(5) (a) Yamamoto, K.; Takahashi, J.; Hamano, K.; Yamada, S.;
Yamaguchi, K.; DeLuca, H. F. J. Org. Chem. 1993, 58,
2530. (b) Yamamoto, K.; Yan Sun, W.; Ohta, M.; Hamada,
K.; DeLuca, H. F.; Yamada, S. J. Med. Chem. 1996, 39,
2727. (c) Masuno, H.; Yamamoto, K.; Wang, X.; Choi, M.;
Ooizumi, H.; Shinki, T.; Yamada, S. J. Med. Chem. 2002,
45, 1825.
Scheme 2 Reagents and conditions: (i) n-Bu4NF, THF, reflux (15a:
77%; 15b: 77%); (ii) PDC, CH2Cl2, r.t. (16a: 94%; 16b: 99%); (iii)
17; n-BuLi, THF, –78 °C [18a (70%); 18b (69%)]; (iv) n-Bu4NF,
THF, r.t. [5a (77%); 5b (76%)].
(6) (a) Fall, Y.; Fernández, C.; González, V.; Vitale, C.;
Mouriño, A. Abstracts of the 11th Workshop on Vitamin D,
Nashville, Tennessee, May 27–June 1, 2000. (b) Fall, Y.;
Fernández, C.; Vitale, C.; Mouriño, A. Tetrahedron Lett.
2000, 41, 7323. (c) Fall, Y.; Fernández, C.; González, V.;
Mouriño, A. Synlett 2001, 1567. (d) Fall, Y.; Fernández, C.;
González, V.; Mouriño, A. Tetrahedron Lett. 2001, 42,
7815.
Methods
The 1.8 Å resolution crystal structure of an engineered ligand bind-
ing domain of human VDR4 (Protein Data Bank code 1DB1) was
used for the docking experiments. The ligands and the protein were
prepared for docking with MOE (Molecular Operating Environ-
ment, Chemical Computing Group Inc., Montreal) and Autodock
Tools. Autogrid17 was used to generate the grid maps for an approx-
imately 23 × 23 × 23 Å3 cell box centered on the ligand. Docking
experiments were carried out using the Lamarckian genetic algo-
rithm implemented in the automated docking program Autodock17,
version 3.0. Default parameters were employed except for: number
of GA runs, 256; maximum number of energy evaluations,
2.5 × 106; and maximum number of generations, 2.7 × 105.
(7) Fernández, B.; Martínez Pérez, J. A.; Granja, J. R.; Castedo,
L.; Mouriño, A. J. Org. Chem. 1992, 57, 3173.
(8) Fujishima, T.; Konno, K.; Nakagawa, K.; Kurobe, M.;
Okano, T.; Takayama, H. Bioorg. Med. Chem. 2000, 8, 123.
(9) (a) Leyes, G. A.; Okamura, W. H. J. Am. Chem. Soc. 1982,
104, 6099. (b) Sardina, F. J.; Mouriño, A.; Castedo, L. J.
Org. Chem. 1986, 51, 1264. (c) Fall, Y.; Torneiro, M.;
Castedo, L.; Mouriño, A. Tetrahedron 1997, 53, 4703.
(10) All new compounds exhibited satisfactory 1H NMR and 13
C
NMR, analytical, and/or high-resolution mass spectral data.
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C. Fernández et al.
LETTER
(11) Andrews, D. R.; Barton, D. H. R.; Hesse, R. H.; Pechet, M.
M. J. Org. Chem. 1986, 51, 4819.
(12) Wovkulich, P. M.; Barcelos, F.; Batcho, A. D.; Sereno, J. F.;
Baggiolini, E. G.; Hennessy, B. M.; Uskokovic, M. R.
Tetrahedron 1984, 40, 2283.
(13) Sunggak, K.; Ki Nam, C.; Sungbong, Y. J. Org. Chem. 1987,
52, 3917.
(14) Mouriño, A.; Torneiro, M.; Vitale, C.; Fernandez, S.; Pérez-
Sestelo, J.; Anné, S.; Gregorio, C. Tetrahedron Lett. 1997,
38, 4713.
31.35 (CH3), 29.12 (CH2), 26.96 (CH2), 24.86 (CH2), 23.45
(CH2), 22.20 (CH2), 21.57 (CH3), 14.30 (CH3), 12.10 (CH3).
HRMS: m/z calcd for C28H45O3: 429.3369; found: 429.3390.
(16) Compound 5b: 1H NMR (300 MHz, CDCl3): d = 6.36 (1 H,
d, J = 11.3 Hz, =CH-6 or =CH-7), 6.01 (1 H, d, J = 11.3 Hz,
=CH-6 or =CH-7), 5.31 (1 H, s, CH2-19), 4.99 (1 H, s, CH2-
19), 4.42 (1 H, dd, J = 8.0, 4.3 Hz, CHOH), 4.23 (1 H, m,
CHOH), 3.53 (1 H, t, J = 11.3, CH2O), 3.43 (1 H, dd,
J = 11.1, 3.0 Hz, CH2O), 2.82 (1 H, dd, J = 10.5, 3.7 Hz),
2.58 (1 H, dd, J = 9.9, 3.3 Hz), 2.30 (1 H, dd, J = 13.5, 6.4
Hz), 1.19 (3 H, s, CH3-26 or CH3-27), 1.17 (3 H, s, CH3-26
or CH3-27), 0.77 (3 H, d, J = 6.8 Hz, CH3-21), 0.54 (3 H, s,
CH3-18). 13C NMR (CDCl3): d = 147.67 (C=), 142.93 (C=),
132.99 (C=), 124.96 (CH=), 117.14 (CH=), 111.74 (CH2=),
71.35 (C-25), 70.82 (CH), 66.87 (CH), 65.85 (CH2), 56.38
(CH), 53.08 (CH), 46.02 (C-13), 45.27 (CH2), 42.89 (CH2),
39.80 (CH2), 38.70 (CH), 36.55 (CH2), 36.27 (CH), 31.34
(CH3), 29.69 (CH2), 27.25 (CH2), 23.52 (CH2), 22.12 (CH2),
21.51 (CH3), 19.73 (CH2), 14.39 (CH3), 11.89 (CH3).
HRMS: m/z calcd for C28H45O3: 429.3369; found: 429.3377.
(17) Morris, G. M.; Goodsell, D. S.; Halliday, R. S.; Huey, R.;
Hart, W. E.; Belew, R. K.; Olson, A. J. J. Comput. Chem.
1998, 19, 1639.
(15) Compound 5a: 1H NMR (300 MHz, CDCl3): d = 6.37 (1 H,
d, J = 11.3 Hz, =CH-6 or =CH-7), 6.01 (1 H, d, J = 11.30 Hz,
=CH-6 or =CH-7), 5.31 (1 H, s, CH2-19), 4.99 (1 H, s, CH2-
19), 4.42 (1 H, dd, J = 7.69, 4.27 Hz, CHOH), 4.22 (1 H, m,
CHOH), 3.66 (1 H, d, J = 11.52 Hz, CH2O), 3.47 (1 H, t,
J = 11.38 Hz, CH2O), 2.82 (1 H, m), 2.59 (1 H, m), 2.31 (2
H, m), 1.18 (3 H, s, CH3-26 or CH3-27), 1.16 (3 H, s, CH3-
26 or CH3-27), 0.81 (3 H, d, J = 6.80 Hz, CH3-21), 0.52 (3
H, s, CH3-18). 13C NMR (CDCl3): d = 147.67 (C=), 143.08
(C=), 132.91 (C=), 125.15 (CH=), 117.20 (CH=), 111.90
(CH2=), 71.01 (C-25), 70.87 (CH), 66.87 (CH), 62.76 (CH2),
56.35 (CH), 53.50 (CH), 46.16 (C-13), 45.30 (CH2), 42.89
(CH2), 39.85 (CH2), 39.13 (CH), 37.89 (CH), 37.16 (CH2),
Synlett 2005, No. 14, 2163–2166 © Thieme Stuttgart · New York