Design and synthesis of a 1α,25-dihydroxyvitamin D3 dimer as a potential chemical inducer of vitamin D receptor dimerization

Article abstract of DOI:10.1021/ol990878z

(matrix presented) A dimer comprising two 1α,25-dihydroxyvitamin D₃ units linked by an alkyl side chain at C-11 was synthesized with a view to the simultaneous binding of two vitamin D receptor (VDR) molecules and the consequent induction of VD

Full text of DOI:10.1021/ol990878z

ORGANIC
LETTERS
1999
Vol. 1, No. 7
1005-1007
Design and Synthesis of a
1r,25-Dihydroxyvitamin D Dimer as a
3
Potential Chemical Inducer of Vitamin D
Receptor Dimerization
,†
Jose´ Pe´rez Sestelo,† Antonio Mourin˜o,‡ and Luis A. Sarandeses*
Departamento de Qu´ımica Fundamental e Industrial, UniVersidade da Corun˜a,
E-15071 A Corun˜a, Spain, and Departamento de Qu´ımica Orga´nica y Unidad
Asociada al C.S.I.C., UniVersidade de Santiago de Compostela,
E-15706 Santiago de Compostela, Spain
qfsarand@udc.es
Received July 28, 1999
ABSTRACT
A dimer comprising two 1r,25-dihydroxyvitamin D units linked by an alkyl side chain at C-11 was synthesized with a view to the simultaneous
3
binding of two vitamin D receptor (VDR) molecules and the consequent induction of VDR dimerization. The short, convergent synthesis uses
a stereoselective cuprate addition to introduce the linking side chain and a key ruthenium olefin metathesis as the dimerization step.
1R,25-Dihydroxyvitamin D3 (calcitriol, 1c) (Figure 1), the
hormonally active form of vitamin D3 (1a),1 besides regulat-
ing the metabolism of calcium and phosphorus, promotes
cell differentiation, inhibits the proliferation of tumor cells,
and induces some biological functions related to the immune
system.2 Calcitriol acts in the cell nucleus by binding to the
vitamin D receptor (VDR), a member of the nuclear receptor
superfamily that can interact with DNA as a homodimer
† Universidade da Corun˜a.
‡ Universidade de Santiago de Compostela.
(1) Vitamin D3 is transformed to 1R,25-(OH)2-D3 (1c) through 25-(OH)-
D3 (1b) by two specific enzymatic hydroxylations.
Figure 1. Vitamin D3, 25-(OH)-D3, and 1R,25-(OH)2-D3.
(2) For a general review of vitamin D chemistry and biology, see: (a)
Vitamin D: Chemistry, Biology and Clinical Applications of the Steroid
Hormone; Norman, A. W., Bouillon, R., Thomasset, M., Eds; Vitamin D
Workshop, Inc.: Riverside, CA, 1997. (b) Zhu, G.-D.; Okamura, W. H.
Chem. ReV. 1995, 95, 1877.
(VDR-VDR) or a heterodimer with the retinoid X receptor
(VDR-RXR).3 The role of dimeric VDR structures in
activation of gene transcription is not clear.
10.1021/ol990878z CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/10/1999

Recently, it has been found possible to influence many
cell processes involving proteinic receptors by means of
dimeric molecules with two protein-binding regions, which
can act by facilitating the dimerization of the ligand receptor
and have become known as chemical inducers of dimeriza-
tion (CIDs).4 This effect is based on the importance that
proximity and orientation have in the protein function during
the information transfer process. The few CIDs that have
been synthesized to date include the dimers of the immuno-
suppressive agents FK506 (FK1012) and cyclosporin A
[(CyA)2].5 In this Letter we describe the synthesis of the first
calcitriol dimer, which we designed to bind two molecules
of VDR and hence, hopefully, facilitate the formation of
VDR-VDR dimers with a view to the exploration and
possible exploitation of their role in transcription control.
The design of the calcitriol dimer took into account the
fact that the triene system and the hydroxyl groups at
positions 1R and 25 are crucial for the biological activity of
calcitriol6 and that the incorporation of new functional groups
alters its biological activity dramatically.3,7 These consider-
ations, and the absence of other functional groups in the
vitamin D structure, ruled out direct dimerization, and we
therefore decided to construct a molecule consisting of two
calcitriol moieties linked by an alkyl chain attached to each
calcitriol at ring C, which is probably the least active part
of the vitamin D structure (Scheme 1). Specifically, we aimed
ketone 4 and phosphine oxide 5 using the Wittig-Horner
approach9 and the introduction of the olefinic side chain by
cuprate chemistry.10
The synthesis of 3 starts with known ester 6.11 Addition
of MeLi (3 equiv), followed by oxidation with PDC and
protection with TESOTf at -78 °C, afforded ketone 7 (73%,
three steps) (Scheme 2). Treatment of 7 under Saegusa
Scheme 2. Synthesis of Ketone 4
reaction conditions,12 followed by reaction of the resulting
enone 8 with the cuprate derived from 4-bromobutene, gave
1
4 in 90% yield as the only diastereoisomer detected by H
NMR spectroscopy.
Formation of the vitamin D triene system was achieved
in 87% yield by the reaction between ketone 4 and the ylide
derived from phosphonium oxide 5 containing ring A,13 to
give the protected vitamin D analogue 3 (Scheme 3). Olefin
Scheme 1. Retrosynthetic Analysis
Scheme 3. Synthesis of Vitamin 3 and Dimerization
to obtain dimer 2 by dimerization of the alkenylic vitamin
D analogue 3 using an olefin metathesis reaction.8 The
retrosynthetic analysis includes the formation of 3 from
metathesis of 3 with Grubbs’s catalyst14 (10 mol %) in
CH2Cl2 at room temperature afforded, after 4 h, an insepa-
rable mixture of 3 and its dimer with loss of ethylene.
(3) Freedman, L. P.; Lemon, B. D. In Vitamin D; Feldman, D., Glorieux,
F. H., Pike, J. W., Eds.; Academic Press: San Diego, 1997; Chapter 9, pp
127-148.
1006
Org. Lett., Vol. 1, No. 7, 1999

Treatment of the mixture with TBAF in THF allowed the
isolation of dimer 9 as a 5:1 mixture of alkene isomers (35%)
and deprotected vitamin 3 (52%). Remarkably, the olefin
metathesis does not affect the triene system. Unfortunately,
attempts to obtain calcitriol dimer 2 by chemoselective
hydrogenation of the nonconjugated double bond led to the
fully saturated dimer.
To circumvent this problem we decided to carry out the
olefin metathesis reaction and hydrogenation before con-
struction of the triene system. Treatment of olefinic ketone
4 with catalytic amounts of (Cy3P)2Cl2RudCHPh14 (10 mol
%) provided dimeric ketone 10 as a mixture of alkene
isomers (6:1) in 46% yield together with recovered starting
ketone (50%) (Scheme 4). Catalytic hydrogenation of the
olefinic mixture 10 for 12 h at atmospheric pressure using a
rhodium catalyst gave dimeric ketone 11 in quantitative yield.
Treatment of 11 with the ylide derived from phosphonium
oxide 5 (2.2 equiv) led to stereoselective formation of
protected calcitriol dimer 2a in excellent yield (85%).
Removal of the six silyl groups with TBAF in THF (rt, 24
h) afforded the desired calcitriol dimer 2 in 74% yield.
In conclusion, we have prepared the first dimer containing
two calcitriol moieties, in each of which the VDR-binding
region is unobstructed. The synthesis is short and convergent
and should allow the preparation of an entire family of
dimeric vitamin D structures. The length of the linker could
be modulated to optimize binding with the receptor and the
biological response associated with the binding of two VDR
molecules. Biological evaluation of dimer 2 in terms of its
VDR binding, dimerization induction capacities, and influ-
ence on signal transduction mechanisms are currently
underway.
Acknowledgment. We thank the DGES (Spain, PM97-
0166), Xunta de Galicia (XUGA 10305A98), and the
University of A Corun˜a for financial support. J.P.S. thanks
the Spanish Ministry of Education and Culture for a research
grant. We also thank Mrs. Paulina Freire for reproducing
some experiments and Mr. Carlos Gregorio for preparation
of the A ring phosphine oxide.
Scheme 4. Synthesis of Calcitriol Dimer 2
Supporting Information Available: Complete charac-
terization data (1H NMR and 13C NMR and mass spectral
data) for all new compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
OL990878Z
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