A-Ring-Modified 2-Hydroxyethylidene Previtamin D3Analogues: Synthesis and Biological Evaluation

Article abstract of DOI:10.1002/ejoc.201601263

To investigate the biological profile of the previtamin form of vitamin D, we synthesized new analogues of 19-nor-1α,25-dihydroxyprevitamin D₃bearing a 2-hydroxyethylidene moiety at the A-ring. The target compounds were prepared by convergent synthesis using a Sonogashira coupling between an A-ring enyne synthon and a CD-ring/side-chain vinyl triflate. We have demonstrated the versatility of shikimic acid as a starting material for the synthesis of the A-ring precursors. The binding affinity to vitamin D receptor (VDR) and human vitamin D binding protein (hDBP), and the MCF-7 cell antiproliferative activity were evaluated.

Full text of DOI:10.1002/ejoc.201601263

DOI: 10.1002/ejoc.201601263
Full Paper
Vitamin D Analogues
A-Ring-Modified 2-Hydroxyethylidene Previtamin D3 Analogues:
Synthesis and Biological Evaluation
Alba Hernández-Martín,[a][‡] Susana Fernández*[a] Annemieke Verstuyf,[b] Lieve Verlinden,[b]
and Miguel Ferrero*[a]
Dedicated to Professor Vicente Gotor on the occasion of his 70th birthday
Abstract: To investigate the biological profile of the previtamin have demonstrated the versatility of shikimic acid as a starting
form of vitamin D, we synthesized new analogues of 19-nor- material for the synthesis of the A-ring precursors. The binding
1α,25-dihydroxyprevitamin D3 bearing a 2-hydroxyethylidene affinity to vitamin D receptor (VDR) and human vitamin D bind-
moiety at the A-ring. The target compounds were prepared by ing protein (hDBP), and the MCF-7 cell antiproliferative activity
convergent synthesis using a Sonogashira coupling between an were evaluated.
A-ring enyne synthon and a CD-ring/side-chain vinyl triflate. We
Introduction
Over the last three decades, more than 3000 analogues of vita-
min D3 (1; Figure 1) have been synthesized because of the
therapeutic applications of 1α,25-dihydroxyvitamin D3 (2), the
hormonally active form of vitamin D3. In addition to its classical
role in the regulation of calcium homeostasis and bone
metabolism,[1] this hormone and its derivatives are associated
with the inhibition of cell growth, the stimulation of cell differ-
entiation, and regulation of the immune system, among other
activities.[2] Several laboratories have worked on the develop-
ment of clinically useful vitamin D3 drugs for a diverse set of
disease indications. As a result, more than nine analogues are
currently on the market in the United States, Japan, or Europe
Figure 1. Structures of vitamin D3 (1), 1α,25-dihydroxyvitamin D3 (2), and 19-
nor analogues (3).
for clinical uses, with many more in development.[3]
The crystal structures of vitamin D3 derivatives with different
biological activities in complex with the vitamin D receptor
(VDR) do not show significant differences, suggesting the
adaptability of the vitamin D3 ligand along with the potential
flexibility of the protein.[4] Several modifications in the vita-
min D3 skeleton have been reported.[5] 19-nor-Vitamin D3 ana-
logues (3), in which the exomethylene group at C-19 was re-
moved, have received much attention as compounds that are
characterized by the dissociation of cell differentiation and cal-
cemic activities.[6] In this context, the introduction of substitu-
ents at the C-2 position of the A-ring causes changes in the
biological profile compared with the parent 19-nor-vita-
min D3.[7] Among these substituents, the (2E)-hydroxyethyl-
idene group has been described as giving rise to higher VDR
affinity and higher ligand-dependent transcriptional activity
compared with the natural 1α,25-dihydroxyvitamin D3.[8] It was
found that the introduction of a (2E)-hydroxyethylidene substit-
uent into 19-nor-vitamin D derivatives with 22,24-diene, 22-oxa,
or 16-ene-22-thia-26,27-dimethyl modifications resulted in
higher potency in the transcriptional assay.[9] Furthermore, ana-
logues with the (20S) configuration showed a much higher abil-
ity to induce osteoclast formation and inhibition of the expres-
sion of CD86, a maturation marker of dendritic cells, than their
corresponding (20R)-configured counterparts.[10]
[a] Departamento de Química Orgánica e Inorgánica, Instituto Universitario
de Biotecnología de Asturias, Universidad de Oviedo,
33006 Oviedo, Asturias, Spain
E-mail: mferrero@uniovi.es
fernandezgsusana@uniovi.es
[b] Laboratorium voor Experimentele Geneeskunde en Endocrinologie,
Katholieke Universiteit Leuven,
Gasthuisberg, 3000 Leuven, Belgium
[‡] Current address: Bayer Pharma AG, Drug Discovery – Chemical
Development,
Friedrich-Ebert-Str. 217–333, 42117 Wuppertal, Germany
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejoc.201601263.
The hormone 1α,25-dihydroxyvitamin D3 exists in thermal
equilibrium (5–10 %) with 1α,25-dihydroxyprevitamin D3.[11]
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Most of the analogues that have been developed focus on the
vitamin D form due to the spontaneous isomerization of 1α,25-
(OH)2-pre-D3 (4) to the more stable 1α,25-(OH)2-D3 (2) by a
[1,7]-sigmatropic hydrogen shift (Scheme 1). However, scarce
examples of previtamin D analogues are described in the litera-
ture. These include the four A-ring diastereoisomers of 19-nor-
1α,25-(OH)2-pre-D3,[12] a 9,19-methano-bridged analogue of
1α,25-(OH)2-D3,[13] and a series of 2-substituted derivatives of
14-epi-previtamin D3,[14] in which the 14-epi-1α,25-(OH)2-pre-D3
form was major and dominant over 14-epi-1α,25-(OH)2-D3. In
our laboratory, we have synthesized conformationally locked
previtamin D3 derivatives with a 2-hydroxy, 2ꢀ-(3′-hydroxy-
propoxy), or 2ꢀ,3ꢀ-epoxy group.[15] To the best of our knowl-
edge, the only reported previtamin D analogue with activities
equivalent to 1α,25-(OH)2-D3 is characterized by the presence
of a trans-fused decalin CD-ring system.[16] This analogue, de-
veloped by Bouillon and co-workers, interacts as efficiently as
the natural hormone with the VDR and uses the same contact
points within the receptor as 1α,25-(OH)2-D3. Challenges remain
in the design of new analogues and the evaluation of their
biological actions in order to investigate structure/activity rela-
tionships.
Figure 2. Target hydroxyalkylidene-19-nor-previtamin D3 analogues.
of the 3- and 5-hydroxy groups and reduction of the ester to
an aldehyde, as described previously,[15a] generated compound
9. Transformation of this compound into enyne 10 was accom-
plished in 56 % yield by reaction with trimethylsilyldiazometh-
ane (TMSCHN2). For the introduction of the hydroxyethylidene
group, the free OH group at C-4 was oxidized with Dess–Martin
periodinane (DMP) to give ketone 11 as a single product ac-
cording to TLC. This compound proved to be unstable to stan-
Scheme 1. Equilibrium of 1α,25-dihydroxyvitamin D3 (2) and 1α,25-dihy-
droxyprevitamin D3 (4).
Based on these considerations, and on our ongoing interest
in the preparation and biological evaluation of vitamin D3 deriv-
atives,[17] in this paper we report the synthesis and biological
activities of a series of 19-nor-previtamin D3 analogues (5, 6,
and 7; Figure 2) that have the hydroxyethylidene group in dif-
ferent positions of the A-ring. The synthetic strategy involves
the coupling of a dienyne precursor of the A-ring with an enol
triflate of the CD-ring/side-chain fragment.
Results and Discussion
Chemistry
Scheme 2. Reagents and conditions: (a) DMP, CH2Cl2, room temp., overnight,
78 %; (b) (EtO)2P(O)CH2CN, nBuLi, THF, –40 °C, 1 h, 82 %; (c) DIBAL-H, toluene,
–78 °C, 1 h, 65 %; (d) NaBH4, EtOH, 0 °C, 45 min, 78 %; (e) TBDMSCl, imidazole,
CH2Cl2, 0 °C to room temp., 1 h, 97 %.
The A-ring precursors were synthesized starting from commer-
cially available shikimic acid (8; Scheme 2). Selective protection
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dard purification by column chromatography and was isolated
in 78 % yield (neutral silica gel).
Cyanomethylation of 11 with the ylide generated from di-
ethyl (cyanomethyl)phosphonate and n-butyllithium gave de-
rivative 12 as an approximately 60:40 mixture of (E)/(Z) isomers,
in 82 % yield. Both isomers were reduced with diisobutylalu-
minium hydride (DIBAL-H) followed by sodium borohydride to
give 2-(2-hydroxyethylidene) derivative 14. Protection of the
free hydroxy group of 14 with tert-butyldimethylsilyl chloride
(TBDMSCl) gave the desired A-ring synthons 15a [(E) isomer]
and 15b [(Z) isomer], which could be separated by semiprepara-
tive HPLC in excellent yield. The configuration of the ethylidene
unit was unambiguously determined by 2D NOESY experiments
(Figure 3). In 15a, a correlation cross-peak was observed be-
tween allylic proton 3-H and vinyl proton 1′-H. In addition, an
NOE was detected between 5-H and 2′-H. From these results,
the stereochemistry of the C-2 substituent of 15a was deter-
mined to be (E). On the other hand, derivative 15b showed a
cross-peak between 3-H and 2′-H, which confirmed a (Z) config-
uration. A weak NOE correlation between 5-H and 1′-H corrobo-
rated the assigned stereochemistry for 15b.
Scheme 3. Reagents and conditions: (a) MnO2, CH2Cl2, room temp., 16 h,
86 %; (b) (EtO)2P(O)CH2CN, nBuLi, THF, –40 °C, 2.5 h, 89 %; (c) DIBAL-H, tolu-
ene, –78 °C, 1 h, 85 %; (d) NaBH4, EtOH, 0 °C, 45 min, 65 %.
DIBAL-H, and the resulting aldehyde 19 was transformed into
alcohol 20 by treatment with NaBH4.
Figure 3. NOE correlations for 15a and 15b.
In the scale-up preparation of compound 10, migration of
the O-silyl group was occasionally observed, giving a mixture
of 3,5- and 4,5-disilyl ethers. This migration occurs typically un-
der basic conditions and proceeds intramolecularly via a penta-
coordinate silicon intermediate.[18] Depending on the batch of
nBuLi used for the transformation of 9 to 10, different ratios of
regioisomers from 99:1 to 1:1 (10/16) were observed. We postu-
late that this result is due to the action of LiOH, which is present
in the nBuLi solution to a greater or lesser extent. Both deriva-
tives were isolated by semipreparative HPLC for characteriza-
tion.
Figure 4. NOE correlations for 18 and 28.
We also explored an alternative pathway to the C-3-function-
alized A-ring precursor from shikimic acid using our reported
protocol[19] for the one-pot selective protection of the trans-
1,2-diol (Scheme 4). The resulting alcohol 21 was protected as
the tert-butyldimethylsilyl ether, and ester 22 was transformed
into aldehyde 24 by reduction with DIBAL-H and subsequent
oxidation of the resulting alcohol 23 with Dess–Martin periodi-
nane. Treatment of 24 with trimethylsilyldiazomethane pro-
vided 25 in 65 % yield. Removal of the silyl protecting group
For a practical separation of the unwanted isomer 16, a se- followed by oxidation of the hydroxy group gave the corre-
lective oxidation of the allylic alcohol at C-3 was carried out sponding keto compound 27. The latter underwent cyano-
with manganese dioxide to provide ketone 17. This could then methylation with diethyl (cyanomethyl)phosphonate to give 2-
be separated from alcohol 10 by standard column chromatog- hydroxyethylidene derivative 28 exclusively. The structure of 28
raphy. With compound 17 in hand, we explored the synthesis was proved by NOESY experiments (Figure 4). The cross-peaks
of the A-ring precursor having a 2-hydroxyethylidene group at- observed between 4-H/5-H and the OMe groups of the bis-
tached to the allylic position (Scheme 3). Thus, Wittig–Horner (acetal) allowed the assignment of the OMe groups. The NOE
reaction of compound 17 with diethyl (cyanomethyl)phosphon- detected between 1′-H and the OMe that correlated with 4-H
ate produced 18 selectively as the only isomer.
supported the (E) stereochemistry. Transformation of 28 into 31
was carried out using the same sequence of reactions shown
in Scheme 2 for 15.
The stereochemistry of the 2-hydroxyethylidene moiety of
18 was determined by analysis of the 2D NOESY spectrum. An
NOE was observed between 1′-H and 4-H, and a correlation
The synthesis of the previtamin D3 analogues was carried
cross-peak was detected between 1′-H and the tert-butyl moi- out by Sonogashira reaction as outlined in Scheme 5. Coupling
ety of the TBDMS group (Figure 4). Thus, this isomer must have of A-ring synthons 15a, 15b, 20, and 31 with the CD-ring/side-
the (E) configuration. Next, derivative 18 was treated with chain triflate 32[13,20] in the presence of a catalyst of bis(triphen-
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Scheme 4. Reagents and conditions: (a) TBDMSCl, imidazole, CH2Cl2, 0 °C to room temp., 16 h, 94 %; (b) DIBAL-H, Et2O, –78 °C, 4 h, 85 %; (c) DMP, CH2Cl2,
room temp., 2 h, 94 %; (d) TMSCHN2, nBuLi, THF, –78 °C to room temp., 1 h, 65 %; (e) TBAF, THF, 0 °C to room temp., 4 h, 95 %; (f) from 26, DMP, CH2Cl2,
room temp., 2 h, 91 %; (g) (EtO)2P(O)CH2CN, nBuLi, THF, –40 °C, 4 h, 80 %; (h) DIBAL-H, toluene, –78 °C, 1 h, 88 %; (i) NaBH4, EtOH, 0 °C, 45 min, 75 %;
(j) TBDMSCl, imidazole, CH2Cl2, 0 °C to room temp., 1 h, 80 %.
ylphosphine)palladium(II) acetate and copper(I) iodide yielded of the bis(acetal) protecting group in 40, including trifluoro-
dienynes 33, 34, 35, and 36, respectively. Next, catalytic acetic acid, pyridinium p-toluenesulfonate, trimethylsilyl
hydrogenation with the Lindlar catalyst generated derivatives bromide, and iron(III) chloride. However, the previtamin deriva-
37, 38, 39, and 40. Deprotection of the silyloxy groups with tive was found to be unstable under these conditions, and it
TBAF (tetrabutylammonium fluoride) gave the desired previta- was not possible to access 7 by this route.
mins 5, 6, and 7. Several reagents were tested for the removal
Biological Evaluation
The biological activity of the newly synthesized analogues was
tested in vitro, and the results are summarized in Table 1 and
Figures 5, 6, and 7.
We evaluated the affinity of compounds 5 and 7 for pig
mucosa cytosol VDR and for human vitamin D binding protein
(hDBP). Modification by the (2E)-hydroxyethylidene group at 19-
nor-1α,25-(OH)2-pre-D3 eliminated the affinity for VDR with re-
spect to (2E)-hydroxyethylidene-19-nor-1α,25-(OH)2-D3, which
was about twice as active as the natural hormone.[8] These re-
sults indicate that the 6-s-cis conformer 5 does not have the
ability that the 6-s-trans isomer has to access the VDR binding
pocket.[21] Compound 7, which has an (E)-hydroxyethylidene
group instead of the 1α-hydroxy group, also failed to bind to
the VDR (Figure 5). A similar pattern of relative activities of the
two derivatives was found in their ability to bind the hDBP
protein. Derivatives 5 and 7 showed significantly lower affinity
compared to 1α,25-(OH)2-D3 (Figure 6).
Next, we tested the potency of compounds 5 and 7 to inhibit
the proliferation of human breast adenocarcinoma MCF-7 cells.
Scheme 5. Reagents and conditions: (a) (PPh3)2Pd(OAc)2, CuI, Et2NH, DMF, 2 h,
The (2E)-hydroxyethylidene previtamin D3 analogue 5 was 25
70 % for 33, 90 % for 34, 65 % for 35, 60 % for 36; (b) H2, Lindlar catalyst,
times less potent than 1α,25-(OH)2-D3 in its inhibition of the
quinoline, hexane, 4 h, 90 % for 38, 60 % for 37, 90 % for 38, 70 % for 39,
proliferation of MCF-7 cells. Derivative 7, with the modification
at C-1, showed no effect on MCF-7 cell proliferation (Figure 7).
75 % for 40; (c) TBAF, THF, 0 °C to room temp., 4–18 h, 90 % for 5, 60 % for
6, 85 % for 7.
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Table 1. Biological activities of A-ring-modified 2-hydroxyethylidene previtamin D3 analogues.[a]
Compound
VDR [%]
hDBP [%]
MCF-7 [%]
1α,25-(OH)2-D3
(2E)-hydroxyethylidene-19-nor-1α,25-(OH)2-D3
100
200[b]
0
100
–
3
100
–
4
5
7
0
2
0
[a] Values are expressed as percentages of activity EC50 concentration relative to 1α,25-(OH)2-D3 (100 %). [b] Ref.[8]
Figure 5. Affinity of 1α,25-(OH)2-D3 and 2-hydroxyethylidene previtamin D3
analogues of 1α,25-(OH)2-pre-D3 for pig vitamin D receptor (VDR). 1α,25-
(OH)2-D3 (dots); 5 (triangles); 7 (squares).
Figure 7. In-vitro antiproliferative effects of 1α,25-(OH)2-D3 and 2-hydroxyeth-
ylidene previtamin D3 analogues of 1α,25-(OH)2-pre-D3 on MCF-7 breast can-
cer cells. Vehicle (diamonds); 1α,25-(OH)2-D3 (dots); 5 (triangles); 7 (squares).
and using the vinyl triflate of the CD-ring/side-chain fragment.
The hydroxyethylidene group was introduced by a Wittig–
Horner reaction. NOESY experiments were used to assign the
stereochemistry of the compounds. Their biological activity pro-
files were assessed in terms of their affinity for VDR and hDBP,
and their effects on MCF-7 cell proliferation. The C-2-substituted
6-s-cis analogue 5 showed no affinity for VDR, but demon-
strated very low affinity to hDBP, and showed some inhibitory
effect on MCF-7 cell proliferation. The analogue with a hydroxy-
ethylidene group at the 1-position, i.e., compound 7, was
unable to bind VDR, and was devoid of in-vitro antiproliferative
activity against MCF-7 cells.
Experimental Section
General Remarks: All reagents were bought from Aldrich at the
highest commercially available quality and used without further pu-
rification. All non-aqueous reactions were carried out under anhy-
drous conditions in dry, freshly distilled solvents. Reactions were
monitored by TLC carried out on 0.25 mm Merck silica gel plates
(60F-254). TLC plates were visualized using UV light and/or acidic
aqueous permanganate. Flash chromatography was carried out us-
ing silica gel 60 (230–400 mesh). Melting points were recorded with
samples in open capillary tubes. IR spectra were recorded as thin
films on NaCl plates with an infrared FT spectrophotometer (Agilent
or Perkin–Elmer). 1H, 13C, and DEPT NMR spectra were obtained
using Bruker instruments at 300.13, 400.13, or 600.13 MHz for 1H,
and 75.5, 90.61, or 150.90 MHz for 13C. The same spectrometers
were used for the acquisition of 1H,1H homonuclear (COSY and
Figure 6. Affinity of 1α,25-(OH)2-D3 and 2-hydroxyethylidene previtamin D3
analogues of 1α,25-(OH)2-pre-D3 for human vitamin D binding protein
(hDBP). 1α,25-(OH)2-D3 (dots); 5 (triangles); 7 (squares).
Conclusions
We have described the synthesis of new locked previtamin D
derivatives with a hydroxyethylidene moiety at the A-ring.
These target compounds were synthesized in order to investi-
gate important structure/activity relationships. All analogues
were successfully obtained by convergent synthesis, starting
from shikimic acid for the preparation of the A-ring precursors, NOESY) and 1H,13C heteronuclear (HSQC and HMBC) correlation
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spectra. Mass spectra (MS) and high-resolution mass spectra (HRMS)
were recorded with Hewlett–Packard 1100 and Bruker MicrOTOF-Q
mass spectrometers, respectively, under electrospray ionization (ESI)
conditions. Compounds 9,[15a] 10,[15a] 21,[19] 22,[22] and 23[22] have
been described previously. Full analytical data for new compounds
are given in the Supporting Information.
0.10 (s, 6 H, SiMe2), 0.12 (s, 3 H, SiMe), 0.90 (s, 9 H, SiCMe3), 0.91 (s,
9 H, SiCMe3), 2.60 (ddt, J = 17.5, 6.5, 1.2 Hz, 1 H, 6-H), 2.82 (ddt, J =
17.4, 5.7, 1.4 Hz, 1 H, 6-H), 2.94 (s, 1 H, 8-H), 4.55 (dd, J = 6.5, 5.7 Hz,
1 H, 5-H), 4.78 (apparent d, J = 3.8 Hz, 1 H, 3-H), 6.08 (m, 1 H, 2-H)
ppm. MS (ESI+): m/z (%) = 419 (100) [M + K]+, 403 (60) [M + Na]+.
(E)- and (Z)-(3R,5R)-3,5-Bis[(tert-butyldimethylsilyl)oxy]-4-
(E)-2-(2-Hydroxyethylidene)-19-nor-1α,25-dihydroxypre-
(cyanomethylidene)-1-ethynyl-1-cyclohexene (12): nBuLi (1.6
M
vitamin D3 (5): TBAF (1
M in THF; 117 μL, 0.117 mmol) was added
in hexanes; 1.3 mL, 2.129 mmol) was added to a stirred solution of
diethyl (cyanomethyl)phosphonate (419 mg, 2.367 mmol) in anhy-
drous THF (10 mL) at –40 °C. The mixture was stirred for 15–20 min,
after which time a solution of 11 (450 mg, 1.183 mmol) in anhy-
drous THF (10 mL) was added dropwise. The resulting deep red
solution was stirred at –40 °C for a further 1 h, after which the
mixture was quenched with saturated aqueous NH4Cl, and ex-
tracted with EtOAc. The organic layer was dried with Na2SO4, fil-
tered, and concentrated under vacuum. The residue was purified
by flash chromatography on silica gel (1–3 % Et2O/hexanes) to give
12 (392 mg, 82 %) as a white solid, as a mixture of (E)/(Z) diastereo-
isomers in a 60:40 ratio. 1H NMR [300.13 MHz, CDCl3; (E) isomer]:
δ = 0.09–0.17 (several s, 12 H, 4 SiMe), 0.89–0.94 (several s, 18 H, 2
SiCMe3), 2.42 (d, 1 H, J = 17.9 Hz, 6-H), 2.60 (m, 1 H, 6-H), 2.89 (s, 1
H, 8-H), 5.06 (dd, 1 H, J = 3.3, 2.3 Hz, 5-H), 5.20 (sa, 1 H, 3-H), 5.46
(d, 1 H, J = 2.0 Hz, 1′-H) and 5.94 (s, 1 H, 2-H) ppm; [(Z) isomer]: δ =
0.09–0.17 (several s, 12 H, 4 SiMe), 0.89–0.94 (several s, 18 H, 2
SiCMe3), 2.21 (ddd, J = 15.6, 9.5, 1.6 Hz, 1 H, 6-Hax), 2.66 (dd, J =
16.6, 6.2 Hz, 1 H, 6-Heq), 2.94 (s, 1 H, 8-H), 4.77 (m, 1 H, 5-H), 5.12
(d, J = 5.0 Hz, 1 H, 3-H), 5.58 (d, J = 1.8 Hz, 1 H, 1′-H), 6.08 (dd, J =
4.7, 2.2 Hz, 1 H, 2-H) ppm. MS (ESI+): m/z (%) = 426 (100) [M + Na]+.
dropwise to a stirred solution of 37 (11.5 mg, 0.013 mmol) in anhy-
drous THF (270 μL) at 0 °C and in darkness. After 5 min, the ice
bath was removed, and the reaction mixture was stirred at room
temperature overnight. Next, water was added, and the mixture
was extracted with EtOAc. The resulting organic layer was dried
with Na2SO4, and the solvents were evaporated to dryness. The
residue was purified by column chromatography on silica gel
(100 % EtOAc) to give 5 (5.4 mg, 90 %) as a white solid. 1H NMR
(400.13 MHz, [D6]acetone): δ = 0.75 (s, 3 H, 18-Me), 0.99 (d, J =
6.4 Hz, 3 H, 21-Me), 1.15 (s, 6 H, 26-Me, 27-Me), 0.84–2.65 (several
m), 3.11 (br. s, 1 H, OH), 3.65 (br. s, 1 H, OH), 3.89 (br. s, 1 H, OH),
4.02 (br. s, 1 H, OH), 4.22 (m, 2 H, 2′-H), 4.87 (s, 2 H, 1-H, 3-H), 5.37
(s, 1 H, 9-H) and 5.72–5.87 (several m, 4 H, 1′-H, 10-H, 7-H, and 6-H)
ppm. HRMS (ESI+): calcd. for C28H44NaO4 467.3132; found 467.3126.
(Z)-2-(2-Hydroxyethylidene)-19-nor-1α,25-dihydroxypre-
vitamin D3 (6): A procedure analogous to that described for the
synthesis of 5, starting from 38 (17 mg, 0.020 mmol), gave 6
(5.3 mg, 60 %) as a white solid (eluent gradient for column chroma-
tography: 90 % EtOAc/hexanes → 2 % MeOH/EtOAc). 1H NMR
(400.13 MHz, [D6]acetone): δ = 0.79 (s, 3 H, 18-Me), 0.99 (d, J =
6.5 Hz, 3 H, 21-Me), 1.15 (s, 6 H, 26-Me, 27-Me), 0.84–2.66 (several
m), 2.63 (dd, J = 16.1, 5.4 Hz, 1 H, 4-H), 3.10 (s, 1 H, OH), 3.64 (t, J =
5.5 Hz, 1 H, OH), 3.93 (d, J = 5.4 Hz, 1 H, OH), 4.02 (d, J = 5.4 Hz, 1
H, OH), 4.22 (apparent t, J = 5.3 Hz, 2 H, 2′-H), 4.42 (m, 1 H, 3-H),
4.97 (t, J = 4.2 Hz, 1 H, 1-H), 5.38 (s, 1 H, 9-H), 5.73–5.94 (several m, 4
H, 1′-H, 10-H, 7-H, and 6-H) ppm. HRMS (ESI+): calcd. for C28H44NaO4
467.3132; found 467.3142.
(E)- and (Z)-(3R,5R)-3,5-Bis[(tert-butyldimethylsilyl)oxy]-1-
ethynyl-4-(formylmethylidene)-1-cyclohexene (13): DIBAL-H (1
M
in toluene; 370 μL, 0.370 mmol) was added dropwise to a stirred
solution of 11 (50 mg, 0.123 mmol) in anhydrous toluene (1.2 mL)
at –78 °C. The mixture was stirred at this temperature for 1 h, and
then it was diluted with hexane. The solution was loaded directly
onto the top of a silica gel column and eluted with 2–3 % Et2O/
hexanes to give 13 (33 mg, 65 %) as a white solid [(E)/(Z), 60:40].
1H NMR [300.13 MHz, CDCl3; (E) isomer]: δ = 0.06–0.13 (several s,
12 H, 4 SiMe), 0.88–0.94 (several s, 18 H, 2 SiCMe3), 2.47 (dd, 1 H,
J = 17.6, 3.6 Hz, 6-H), 2.63 (m, 1 H, 6-H), 2.89 (s, 1 H, 8-H), 5.21 (br.
s, 1 H, 3-H), 5.51 (apparent t, 1 H, J = 3.8 Hz, 5-H), 6.02 (br. s, 1 H,
2-H), 6.12 (m, 1 H, 1′-H) and 10.17 (d, 1 H, J = 7.3 Hz, 2′-H) ppm; [(Z)
isomer]: δ = 0.06–0.13 (several s, 12 H, 4 SiMe), 0.88–0.94 (several s,
18 H, 2 SiCMe3), 2.27 (ddd, J = 16.8, 9.5, 2.4 Hz, 1 H, 6-Hax), 2.68 (m,
1 H, 6-Heq), 2.94 (s, 1 H, 8-H), 4.81 (ddd, J = 9.2, 6.4, 1.5 Hz, 1 H, 5-
H), 5.65 (d, J = 5.1 Hz, 1 H, 3-H), 6.12 (m, 1 H, 2-H), 6.25 (dd, J = 7.8,
1.5 Hz, 1 H, 1′-H), 10.11 (d, J = 7.8 Hz, 1 H, 2′-H) ppm. MS (ESI+):
m/z (%) = 429 (100) [M + Na]+.
(E)-1-(2-Hydroxyethylidene)-19-nor-2α,25-dihydroxypre-
vitamin D3 (7): A procedure analogous to that described for the
synthesis of 5, starting from 39 (6 mg, 0.008 mmol), gave 7 (3 mg,
85 %) as a white solid (eluent for column chromatography: 100 %
EtOAc). The reaction was monitored by TLC (20 % Et2O/hexanes, 3–
4 h). 1H NMR (600.13 MHz, [D6]acetone): δ = 0.79 (s, 3 H, 18-Me),
0.82–2.31 (several m), 0.99 (d, J = 6.6 Hz, 3 H, 21-Me), 1.15 (s, 6 H,
26-Me, 27-Me), 2.36 (dd, J = 17.5, 9.1 Hz, 1 H, 4-Hax), 2.65 (dd, J =
17.1, 5.1 Hz, 1 H, 4-Heq), 3.09 (s, 1 H, OH), 3.49 (s, 1 H, OH), 3.60
(apparent t, J = 5.5 Hz, 1 H, 5-H), 3.92 (br. s, 2 H, OH), 4.08 (br. s, 1
H, 4-H), 4.23 (m, 1 H, 2′-H), 4.30 (m, 1 H, 2′-H), 5.42 (s, 1 H, 9-H),
5.79 (d, J = 11.8 Hz, 1 H, 7-H), 5.85 (t, J = 6.8 Hz, 1 H, 1′-H), 5.94 (d,
J = 12.2 Hz, 1 H, 6-H), 6.42 (s, 1 H, 10-H) ppm. HRMS (ESI+): calcd.
for C28H44NaO4 467.3132; found 467.3157.
(E)- and (Z)-(3R,5R)-3,5-Bis[(tert-butyldimethylsilyl)oxy]-1-
ethynyl-4-(2-hydroxyethylidene)-1-cyclohexene (14): NaBH4
(11 mg, 0.290 mmol) was added portionwise to a stirred solution
of aldehyde 13 (117.8 mg, 0.290 mmol) in EtOH (3.6 mL) at 0 °C.
(3R,5R)-3,5-Bis[(tert-butyldimethylsilyl)oxy]-1-ethynyl-4-oxo-1-
cyclohexene (11): Dess–Martin periodinane (308 mg, 0.726 mmol)
was added to a stirred solution of 10 (185 mg, 0.484 mmol) in The mixture was stirred at this temperature for 45 min, then some
anhydrous CH2Cl2 (3.2 mL). The resulting suspension was stirred at
room temperature overnight. The solution was then diluted with
Et2O (6 mL) and a mixture of saturated aqueous Na2S2O3 and satu-
rated aqueous NaHCO3 (1:1 v/v; 6 mL). This generated a white sus-
pension, which turned clear after 10 min. The aqueous layer was
extracted with Et2O, the resulting organic layer was dried (Na2SO4),
and the solvents were evaporated to dryness. The residue was puri-
fied by flash chromatography on silica gel 60 Å (32–63 μm) pH 7
drops of cold water were added, and the mixture was extracted
with EtOAc. The resulting organic layer was dried (Na2SO4), filtered,
and concentrated under vacuum. The residue was purified by flash
chromatography on silica gel (10 % EtOAc/hexanes) to give 14
(92 mg, 78 %) as a colourless oil [(E)/(Z), 60:40]. 1H NMR
[300.13 MHz, CDCl3; (E) isomer]: δ = 0.07–0.12 (several s, 12 H, 4
SiMe), 0.87–0.94 (several s, 18 H, 2 SiCMe3), 2.33 (dd, 1 H, J = 17.3,
4.1 Hz, 6-H), 2.47 (m, 1 H, 6-H), 2.84 (s, 1 H, 8-H), 4.17 (dd, 1 H, J =
12.9, 6.7 Hz, 2′-H), 4.32 (dd, 1 H, J = 13.0, 6.8 Hz, 2′-H), 4.91 (apparent
(1–2 % Et2O/hexanes) to give ketone 11 (144 mg, 78 %) as a viscous,
1
colourless oil. H NMR (300.13 MHz, CDCl3): δ = 0.08 (s, 3 H, SiMe), t, 1 H, J = 4.1 Hz, 5-H), 4.97 (br. s, 1 H, 3-H), 5.73 (apparent t, 1 H,
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J = 6.7 Hz, 1′-H) and 6.05 (br. s, 1 H, 2-H) ppm; (Z isomer): δ = 0.07–
0.12 (several s, 12 H, 4 SiMe), 0.87–0.94 (several s, 18 H, 2 SiCMe3),
2.16 (m, 1 H, 6-H), 2.55 (dd, J = 16.7, 5.9 Hz, 1 H, 6-H), 2.89 (s, 1 H,
8-H), 4.26 (d, J = 6.9 Hz, 2 H, 2′-H), 4.60 (m, 1 H, 5-H), 5.01 (d, J =
4.9 Hz, 1 H, 3-H), 5.85 (t, J = 7.1 Hz, 1 H, 1′-H), 6.05 (br. s, 1 H, 2-H)
ppm. MS (ESI+): m/z (%) = 431 (100) [M + Na]+.
7.9 Hz, 1 H, 3-H), 6.04 (s, 1 H, 2-H) ppm. MS (ESI+): m/z (%) = 405
(100) [M + Na]+.
(4S,5R)-4,5-Bis[(tert-butyldimethylsilyl)oxy]-1-ethynyl-3-oxo-1-
cyclohexene (17): MnO2 (234 mg, 2.695 mmol) was added to a
stirred solution of 16 (103 mg, 0.269 mmol) in anhydrous CH2Cl2
(2.7 mL). The resulting suspension was stirred at room temperature
overnight, and then it was filtered through Celite. The filtrate was
concentrated under vacuum, and the resulting residue was purified
by flash chromatography (1.5 % EtOAc/hexanes) to give 17 (88 mg,
(E)- and (Z)-(3R,5R)-3,5-Bis[(tert-butyldimethylsilyl)oxy]-4-{2-
[(tert-butyldimethylsilyl)oxy]ethylidene}-1-ethynyl-1-cyclohex-
ene (15a and 15b): Imidazole (27 mg, 0.391 mmol) and TBDMSCl
(54 mg, 0.361 mmol) were added to a stirred solution of 14 (123 mg,
0.301 mmol) in anhydrous CH2Cl2 (1.5 mL) at 0 °C. After the addition
was complete, the ice bath was removed, and the reaction mixture
was stirred for 1 h. After this time, water was added, and the mix-
ture was extracted with CH2Cl2. The resulting organic layer was
washed with brine, dried with Na2SO4, filtered, and concentrated
under vacuum. The resulting residue was purified by flash chroma-
tography on silica gel (1 % Et2O/hexanes) to give 15 (152 mg, 97 %)
as a mixture of diastereoisomers. This mixture was further purified
by HPLC (Spherisorb W, 5 μm, 250 × 20 mm, 0.05 % iPrOH/hexanes,
4 mL min–1) to give 15a (64.6 mg) and 15b (56.3 mg), both as
colourless oils. Data for 15a [(E) Isomer]: 1H NMR (400.13 MHz,
CDCl3): δ = 0.05 (s, 3 H, SiMe), 0.06 (s, 3 H, SiMe), 0.07 (s, 3 H, SiMe),
0.08 (s, 3 H, SiMe), 0.10 (s, 3 H, SiMe), 0.11 (s, 3 H, SiMe), 0.88 (s, 9
H, SiCMe3), 0.90 (s, 9 H, SiCMe3), 0.94 (s, 9 H, SiCMe3), 2.29 (dt, J =
17.3, 1.6 Hz, 1 H, 6-H), 2.44 (ddd, J = 17.3, 6.3, 2.7 Hz, 1 H, 6-H), 2.83
(s, 1 H, 8-H), 4.23 (ddd, J = 13.1, 5.3, 1.4 Hz, 1 H, 2′-H), 4.37 (ddd,
J = 13.1, 7.2, 1.0 Hz, 1 H, 2′-H), 4.90 (apparent t, J = 3.5 Hz, 1 H, 5-
H), 5.03 (br. s, 1 H, 3-H), 5.60 (ddd, J = 7.1, 5.4, 1.7 Hz, 1 H, 1′-H),
6.05 (br. s, 1 H, 2-H) ppm. MS (ESI+): m/z (%) = 545 (100) [M + Na]+.
HRMS (ESI+): calcd. for C28H54NaO3Si3 545.3273; found 545.3292.
1
86 %) as a white solid. H NMR (300.13 MHz, CDCl3): δ = 0.079 (s, 3
H, SiMe), 0.082 (s, 6 H, 2 SiMe), 0.10 (s, 3 H, SiMe), 0.87 (s, 9 H,
SiCMe3), 0.91 (s, 9 H, SiCMe3), 2.44 (ddd, J = 18.0, 5.8, 1.7 Hz, 1 H,
6-H), 2.81 (ddd, J = 18.0, 4.0, 1.5 Hz, 1 H, 6-H), 3.51 (s, 1 H, 8-H),
3.90 (d, J = 7.2 Hz, 1 H, 4-H), 4.05 (ddd, J = 7.1, 5.9, 4.1 Hz, 1 H, 5-
H), 6.22 (s, 1 H, 2-H) ppm. MS (ESI+): m/z (%) = 381 (100) [M + H]+,
403 (75) [M + Na]+.
(E)-(4R,5R)-4,5-Bis[(tert-butyldimethylsilyl)oxy]-3-(cyanomethyl-
idene)-1-ethynyl-1-cyclohexene (18): A procedure analogous to
that described for the synthesis of 12, starting from 17 (85 mg,
0.224 mmol), gave, after 2.5 h, 18 (80 mg, 89 %) as a yellow solid
and a single diastereoisomer (eluent for column chromatography:
1
1.5 % EtOAc/hexanes). H NMR (300.13 MHz, CDCl3): δ = 0.06 (s, 3
H, SiMe), 0.07 (s, 3 H, SiMe), 0.08 (s, 3 H, SiMe), 0.12 (s, 3 H, SiMe),
0.87 (s, 9 H, SiCMe3), 0.90 (s, 9 H, SiCMe3), 2.31 (dd, J = 18.0, 5.4 Hz,
1 H, 6-H), 2.69 (dd, J = 18.1, 3.0 Hz, 1 H, 6-H), 3.26 (s, 1 H, 8-H), 3.85
(m, 1 H, 5-H), 4.03 (dd, J = 6.7, 0.9 Hz, 1 H, 4-H), 5.29 (s, 1 H, 1′-H),
6.91 (s, 1 H, 2-H) ppm. 13C NMR (75.5 MHz, CDCl3): δ = –4.7 (SiMe),
–4.5 (SiMe), –4.1 (2 SiMe), 18.1 (SiC), 18.2 (SiC), 26.0 (2 SiCMe3), 37.0
(C-6), 70.2 (C-5), 73.4 (C-4), 82.7 (C-8), 83.8 (C-7), 95.6 (C-1′), 116.5
(CN), 126.5 (C-1), 129.7 (C-2), 157.1 (C-3) ppm.
1
Data for 15b [(Z) Isomer]: H NMR (400.13 MHz, CDCl3): δ = 0.062
(E)-(4R,5R)-4,5-Bis[(tert-butyldimethylsilyl)oxy]-1-ethynyl-3-
(formylmethylidene)-1-cyclohexene (19): A procedure analogous
to that described for the synthesis of 13, starting from 18 (93 mg,
0.231 mmol), gave 19 (80 mg, 85 %) as a yellow oil (eluent for
column chromatography: 5–10 % EtOAc/hexanes). 1H NMR
(300.13 MHz, CDCl3): δ = 0.04 (s, 3 H, SiMe), 0.06 (s, 3 H, SiMe), 0.07
(s, 3 H, SiMe), 0.11 (s, 3 H, SiMe), 0.84 (s, 9 H, SiCMe3), 0.88 (s, 9 H,
SiCMe3), 2.29 (dd, J = 18.0, 3.8 Hz, 1 H, 6-H), 2.75 (apparent dt, J =
18.1, 3.0 Hz, 1 H, 6-H), 3.25 (s, 1 H, 8-H), 3.93 (apparent dt, J = 5.5,
3.8 Hz, 1 H, 5-H), 4.02 (d, J = 5.5 Hz, 1 H, 4-H), 5.87 (d, J = 8.0 Hz, 1
H, 1′-H), 7.35 (s, 1 H, 2-H), 10.18 (d, J = 8.0 Hz, 1 H, 2′-H) ppm. MS
(ESI+): m/z (%) = 407 (45) [M + H]+. HRMS (ESI+): calcd. for
C22H38NaO3Si2 429.2252; found 429.2256.
(s, 3 H, SiMe), 0.066 (s, 3 H, SiMe), 0.069 (s, 3 H, SiMe), 0.081 (s, 3 H,
SiMe), 0.084 (s, 3 H, SiMe), 0.10 (s, 3 H, SiMe), 0.87 (s, 9 H, SiCMe3),
0.90 (s, 9 H, SiCMe3), 0.93 (s, 9 H, SiCMe3), 2.15 (ddd, J = 16.5, 9.7,
2.3 Hz, 1 H, 6-Hax), 2.53 (dd, J = 16.7, 5.9 Hz, 1 H, 6-Heq), 2.88 (s, 1
H, 8-H), 4.29 (dd, J = 6.6, 1.4 Hz, 2 H, 2′-H), 4.58 (ddd, J = 9.3, 5.9,
1.3 Hz, 1 H, 5-H), 4.95 (d, J = 5.0 Hz, 1 H, 3-H), 5.74 (dt, J = 6.5,
1.6 Hz, 1 H, 1′-H), 6.06 (dd, J = 4.8, 2.3 Hz, 1 H, 2-H) ppm. MS (ESI+):
m/z (%) = 545 (100) [M + Na]+. HRMS (ESI+): calcd. for C28H54NaO3Si3
545.3273; found 545.3263.
(3R,4R,5R)-4,5-Bis[(tert-butyldimethylsilyl)oxy]-1-ethynyl-3-
hydroxy-1-cyclohexene (16): nBuLi (1.6
M
in hexanes; 1.53 mL,
2.455 mmol) was added to a stirred solution of TMSCHN2 (2.0
M
in
hexanes; 1.12 mL, 2.250 mmol) in anhydrous THF (8 mL) at –78 °C.
After 15 min at this temperature, a solution of 9 (790 mg,
2.045 mmol) in anhydrous THF (6.2 mL) was added, and the mixture
was stirred at –78 °C for a further 5 min. Then, the reaction mixture
was stirred further and allowed to reach room temperature over
16 h. The reaction was then stopped by the addition of water
(2 mL). The THF was then evaporated, and the resulting aqueous
phase was extracted with Et2O/H2O. The combined organic frac-
tions were washed with brine, dried (Na2SO4), and concentrated.
The crude residue was purified by flash chromatography on silica
gel (5 % EtOAc/hexanes) to give a 1:1 mixture of isomers 10/16
(442 mg, 56 %). The two compounds were separated by semi-
preparative HPLC (Kromasil 250 × 20 mm, 5 % EtOAc/hexanes,
7 mL min–1) to give 10 (217 mg, 28 %) and 16 (211 mg, 27 %). Data
(E)-(4R,5R)-4,5-Bis[(tert-butyldimethylsilyl)oxy]-1-ethynyl-3-(2-
hydroxyethylidene)-1-cyclohexene (20): A procedure analogous
to that described for the synthesis of 14, starting from 19 (65 mg,
0.160 mmol), gave 20 (43 mg, 65 %) as a viscous, yellow oil (eluent
for column chromatography: 10 % EtOAc/hexanes). 1H NMR
(300.13 MHz, CDCl3): δ = 0.04 (s, 3 H, SiMe), 0.05 (s, 3 H, SiMe), 0.06
(s, 3 H, SiMe), 0.10 (s, 3 H, SiMe), 0.85 (s, 9 H, SiCMe3), 0.87 (s, 9 H,
SiCMe3), 2.16 (dd, J = 17.7, 3.2 Hz, 1 H, 6-H), 2.67 (dt, J = 17.7, 2.8 Hz,
1 H, 6-H), 3.03 (s, 1 H, 8-H), 3.86 (apparent dt, J = 5.3, 3.6 Hz, 1 H,
5-H), 3.92 (d, J = 5.3 Hz, 1 H, 4-H), 4.33 (d, J = 6.9 Hz, 2 H, 2′-H),
5.65 (t, J = 6.9 Hz, 1 H, 1′-H), 6.70 (s, 1 H, 2-H) ppm. MS (ESI+): m/z
(%) = 447 (100) [M + K]+. HRMS (ESI+): calcd. for C22H40NaO3Si2
431.2408; found 431.2360.
1
for 16: White solid. H NMR (300.13 MHz, CDCl3): δ = 0.07 (s, 3 H,
(3R,4S,5R)-3-[(tert-Butyldimethylsilyl)oxy]-4,5-(2,3-dimethoxy-
butane-2,3-diyl-2,3-dioxy)-1-formyl-1-cyclohexene (24): A pro-
cedure analogous to that described for the synthesis of 11, starting
from 23 (170 mg, 0.438 mmol), gave, after 2 h, 24 (159 mg, 94 %)
as a white solid (eluent for column chromatography: 10 % EtOAc/
SiMe), 0.08 (s, 3 H, SiMe), 0.12 (s, 3 H, SiMe), 0.13 (s, 3 H, SiMe), 0.88
(s, 9 H, SiCMe3), 0.91 (s, 9 H, SiCMe3), 2.03 (d, J = 17.4 Hz, 1 H, 6-
Hax), 2.27 (d, J = 9.7 Hz, 1 H, OH), 2.50 (d, J = 17.6 Hz, 1 H, 6-Heq),
2.85 (s, 1 H, 8-H), 3.78 (m, 1 H, 4-H), 3.99 (m, 1 H, 5-H), 4.28 (d, J =
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1
hexanes). H NMR (300.13 MHz, CDCl3): δ = 0.11 (s, 3 H, SiMe), 0.13
yellow solid, as a single diastereoisomer (eluent for column chroma-
tography: 10 % EtOAc/hexanes). H NMR (400.13 MHz, CDCl3): δ =
1
(s, 3 H, SiMe), 0.89 (s, 9 H, SiCMe3), 1.27 (s, 3 H, Me), 1.29 (s, 3 H,
Me), 2.07 (ddd, J = 17.0, 10.3, 1.9 Hz, 1 H, 6-Hax), 2.73 (dd, J = 17.5,
6.0 Hz, 1 H, 6-Heq), 3.21 (s, 3 H, OMe), 3.24 (s, 3 H, OMe), 3.52 (dd,
J = 10.8, 3.8 Hz, 1 H, 4-H), 4.08 (apparent td, J = 10.6, 6.0 Hz, 1 H,
5-H), 4.44 (apparent t, J = 4.4 Hz, 1 H, 3-H), 6.57 (dd, J = 4.8, 2.2 Hz,
1 H, 2-H), 9.51 (s, 1 H, 7-H) ppm. MS (ESI+): m/z (%) = 355 (100) [M
– OCH3]+, 409 (65) [M + Na]+.
1.33 (s, 3 H, Me), 1.38 (s, 3 H, Me), 2.53 (ddd, J = 17.8, 10.3, 1.9 Hz,
1 H, 6-Hax), 2.61 (dd, J = 17.6, 6.0 Hz, 1 H, 6-Heq), 3.26 (s, 3 H, OMe),
3.27 (s, 3 H, OMe), 3.31 (s, 1 H, 8-H), 3.85 (apparent td, J = 10.5,
6.0 Hz, 1 H, 5-H), 4.30 (dd, J = 10.8, 2.2 Hz, 1 H, 4-H), 5.57 (s, 1 H,
15-H), 6.91 (d, J = 2.4 Hz, 1 H, 2-H) ppm. MS (ESI+): m/z (%) = 312
(100) [M + Na]+.
(3R,4S,5R)-3-[(tert-Butyldimethylsilyl)oxy]-1-ethynyl-4,5-(2,3-di-
methoxybutane-2,3-diyl-2,3-dioxy)-1-cyclohexene (25): nBuLi
(E)-(4R,5R)-4,5-(2,3-Dimethoxybutane-2,3-diyl-2,3-dioxy)-1-
ethynyl-3-(formylmethylidene)-1-cyclohexene (29): A procedure
analogous to that described for the synthesis of 13, starting from
28 (169 mg, 0.586 mmol), gave 29 (151 mg, 88 %) as a yellow solid
(eluent for column chromatography: 20 % EtOAc/hexanes). 1H NMR
(300.13 MHz, CDCl3): δ = 1.33 (s, 3 H, Me), 1.38 (s, 3 H, Me), 2.52
(ddd, J = 17.8, 10.5, 2.5 Hz, 1 H, 6-Hax), 2.63 (dd, J = 17.6, 6.0 Hz, 1
H, 6-Heq), 3.26 (s, 3 H, OMe), 3.27 (s, 3 H, OMe), 3.29 (s, 1 H, 8-H),
3.92 (apparent td, J = 10.6, 6.0 Hz, 1 H, 5-H), 4.33 (dd, J = 10.7,
2.1 Hz, 1 H, 4-H), 6.28 (d, J = 7.6 Hz, 1 H, 15-H), 7.39 (d, J = 2.1 Hz,
1 H, 2-H), 10.14 (d, J = 7.6 Hz, 1 H, CHO) ppm. MS (ESI+): m/z (%) =
315 (100) [M + Na]+.
(1.6
M
in hexanes; 472 μL, 0.755 mmol) was added to a stirred solu-
tion of TMSCHN2 (2.0
M
in hexanes; 346 μL, 0.692 mmol) in anhy-
drous THF (2.5 mL) at –78 °C. After 15 min at this temperature, a
solution of 24 (243 mg, 0.629 mmol) in anhydrous THF (2.5 mL) was
added, and the mixture was stirred at –78 °C for a further 5 min.
Then, the cooling bath was removed, and the reaction mixture was
allowed to reach room temperature over 1 h. After this time, the
reaction mixture was poured into H2O/Et2O, and the aqueous layer
was extracted with Et2O. The combined organic fractions were
washed with brine, dried (Na2SO4), and concentrated. The residue
was purified by flash chromatography on silica gel (5 % EtOAc/hex-
anes) to give 25 (156 mg, 65 %) as a white solid. 1H NMR
(300.13 MHz, CDCl3): δ = 0.08 (s, 3 H, SiMe), 0.11 (s, 3 H, SiMe), 0.89
(s, 9 H, SiCMe3), 1.28 (s, 3 H, Me), 1.29 (s, 3 H, Me), 2.26 (ddd, J =
17.0, 10.3, 2.4 Hz, 1 H, 6-Hax), 2.49 (dd, J = 17.1, 6.1 Hz, 1 H, 6-Heq),
2.87 (s, 1 H, 8-H), 3.23 (s, 3 H, OMe), 3.25 (s, 3 H, OMe), 3.48 (dd, J =
10.8, 3.8 Hz, 1 H, 4-H), 4.14 (apparent td, J = 10.5, 6.2 Hz, 1 H, 5-H),
4.22 (apparent t, J = 4.6 Hz, 1 H, 3-H), 6.07 (dd, J = 5.1, 2.1 Hz, 1 H,
2-H) ppm. MS (ESI+): m/z (%) = 351 (100) [M – OCH3]+, 405 (80) [M
+ Na]+.
(E)-(4R,5R)-4,5-(2,3-Dimethoxybutane-2,3-diyl-2,3-dioxy)-1-
ethynyl-3-(2-hydroxyethylidene)-1-cyclohexene (30): A proce-
dure analogous to that described for the synthesis of 14, starting
from 29 (85 mg, 0.291 mmol), gave 30 (64 mg, 75 %) as a yellowish
semisolid of low stability (eluent for column chromatography: 30 %
EtOAc/hexanes). 1H NMR (400.13 MHz, CDCl3): δ = 1.33 (s, 3 H, Me),
1.39 (s, 3 H, Me), 2.49 (m, 2 H, 6-H), 3.08 (s, 1 H, 8-H), 3.26 (s, 3 H,
OMe), 3.29 (s, 3 H, OMe), 3.81 (ddd, J = 10.6, 9.5, 7.0 Hz, 1 H, 5-H),
4.20 (d, J = 10.6 Hz, 1 H, 4-H), 4.30 (dd, J = 13.5, 6.0 Hz, 1 H, 16-H),
4.38 (dd, J = 12.9, 7.6 Hz, 1 H, 16-H), 6.01 (apparent t, J = 6.5 Hz, 1
H, 15-H), 6.74 (s, 1 H, 2-H) ppm.
(3R,4S,5R)-1-Ethynyl-3-hydroxy-4,5-(2,3-dimethoxybutane-2,3-
diyl-2,3-dioxy)-1-cyclohexene (26): TBAF (1
M in THF; 785 μL,
(E)-(4R,5R)-3-{2-[(tert-Butyldimethylsilyl)oxy]ethylidene}-4,5-
(2,3-dimethoxybutane-2,3-diyl-2,3-dioxy)-1-ethynyl-1-cyclo-
hexene (31): A procedure analogous to that described for the syn-
thesis of 15, starting from 30 (113 mg, 0.384 mmol), gave 31
(126 mg, 80 %) as a viscous, colourless oil (eluent for column chro-
matography: 5 % EtOAc/hexanes). 1H NMR (300.13 MHz, CDCl3): δ =
0.077 (s, 3 H, SiMe), 0.080 (s, 3 H, SiMe), 0.91 (s, 9 H, SiCMe3), 1.32
(s, 3 H, Me), 1.38 (s, 3 H, Me), 2.46 (apparent d, J = 7.9 Hz, 2 H, 6-
H), 3.04 (s, 1 H, 8-H), 3.25 (s, 3 H, OMe), 3.28 (s, 3 H, OMe), 3.79 (m,
1 H, 5-H), 4.17 (dd, J = 10.6, 2.1 Hz, 1 H, 4-H), 4.27 (ddd, J = 13.8,
5.2, 2.2 Hz, 1 H, 16-H), 4.45 (ddd, J = 13.8, 7.7, 1.6 Hz, 1 H, 16-H),
5.91 (apparent td, J = 6.7, 1.2 Hz, 1 H, 15-H), 6.73 (s, 1 H, 2-H) ppm.
HRMS (ESI+): calcd. for C22H36NaO5Si 431.2224; found 431.2233.
0.785 mmol) was added dropwise to a stirred solution of silyl ether
25 (120 mg, 0.314 mmol) in anhydrous THF (6 mL) at 0 °C and in
darkness. After 10 min at this temperature, the ice bath was re-
moved, and the reaction mixture was stirred at room temperature
until TLC indicated complete consumption of the starting material
(10 % EtOAc/hexanes, 2–3 h). The reaction was stopped by the addi-
tion of water, and then the mixture was extracted with EtOAc. The
resulting organic layer was washed with brine, dried with Na2SO4,
and concentrated. The residue was purified by flash chromatogra-
phy on silica gel (25 % EtOAc/hexanes) to give 26 (80 mg, 95 %) as
a white semisolid. 1H NMR (300.13 MHz, CDCl3): δ = 1.29 (s, 3 H,
Me), 1.33 (s, 3 H, Me), 2.30 (ddd, J = 17.0, 10.4, 2.6 Hz, 1 H, 6-Hax),
2.49 (dd, J = 17.1, 5.9 Hz, 1 H, 6-Heq), 2.77 (br. s, 1 H, OH), 2.91 (s, 1
H, 8-H), 3.25 (s, 3 H, OMe), 3.26 (s, 3 H, OMe), 3.60 (dd, J = 10.8,
4.2 Hz, 1 H, 4-H), 4.12 (apparent td, J = 10.6, 6.0 Hz, 1 H, 5-H), 4.28
(apparent t, J = 4.7 Hz, 1 H, 3-H), 6.18 (dd, J = 5.4, 2.5 Hz, 1 H, 2-H)
ppm. MS (ESI+): m/z (%) = 291 (100) [M + Na]+.
General Procedure for the Synthesis of 33–36: CuI (1.2 mg,
0.006 mmol), (Ph3P)2Pd(OAc)2 (1.4 mg, 0.002 mmol), and deoxygen-
ated anhydrous Et2NH (490 μL) were added to a stirred solution of
vinyl triflate 32 (27 mg, 0.056 mmol) and the appropriate A-ring
precursor 15a, 15b, 20, or 31 (0.071 mmol) in deoxygenated anhy-
drous DMF (490 μL). The mixture was stirred at room temperature
for 2 h, then water was added, and the mixture was extracted with
Et2O. The resulting organic layer was dried with Na2SO4, filtered,
and concentrated under vacuum. The residue was purified by flash
chromatography on silica gel (eluents: 1 % Et2O/hexanes for 33–34;
10 % Et2O/hexanes for 35; and 2 % EtOAc/hexanes for 36).
(4S,5R)-1-Ethynyl-4,5-(2,3-dimethoxybutane-2,3-diyl-2,3-di-
oxy)-3-oxo-1-cyclohexene (27): A procedure analogous to that de-
scribed for the synthesis of 11, starting from 26 (75 mg,
0.280 mmol), gave, after 2 h, 27 (68 mg, 91 %) as a white solid
(eluent for column chromatography: 20 % EtOAc/hexanes). 1H NMR
(300.13 MHz, CDCl3): δ = 1.32 (s, 3 H, Me), 1.41 (s, 3 H, Me), 2.72 (m,
2 H, 6-H), 3.25 (s, 3 H, OMe), 3.30 (s, 3 H, OMe), 3.59 (s, 1 H, 8-H),
4.11 (m, 1 H, 5-H), 4.27 (d, J = 11.5 Hz, 1 H, 4-H), 6.29 (d, J = 1.3 Hz,
1 H, 2-H) ppm. MS (ESI+): m/z (%) = 289 (100) [M + Na]+.
(E)-(1R,3R)-1,3-Bis[(tert-butyldimethylsilyl)oxy]-2-{2-[(tert-
butyldimethylsilyl)oxy]ethylidene}-6,7-dehydro-25-(trimethyl-
silyloxy)-19-nor-previtamin D3 (33): Colourless oil (70 %). 1H NMR
(300.13 MHz, CDCl3): δ = 0.05 (s, 3 H, SiMe), 0.06 (s, 3 H, SiMe), 0.07
(s, 3 H, SiMe), 0.08 (s, 3 H, SiMe), 0.10 (s, 3 H, SiMe), 0.107 (s, 3 H,
(E)-(4R,5R)-3-(Cyanomethylidene)-4,5-(2,3-dimethoxybutane-
2,3-diyl-2,3-dioxy)-1-ethynyl-1-cyclohexene (28): A procedure
analogous to that described for the synthesis of 12, starting from
27 (60 mg, 0.225 mmol), gave, after 4 h, 28 (52 mg, 80 %) as a SiMe), 0.113 (s, 9 H, TMS), 0.70 (s, 3 H, 18-Me), 0.88 (s, 9 H, SiCMe3),
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0.90 (s, 9 H, SiCMe3), 0.94 (s, 9 H, SiCMe3), 1.21 (s, 6 H, 26-Me, 27-
Me), 0.88–2.47 (several m), 4.23 (ddd, J = 13.1, 5.2, 1.3 Hz, 1 H, 2′-
H), 4.39 (ddd, J = 13.1, 7.2, 0.9 Hz, 1 H, 2′-H), 4.88 (t, J = 3.6 Hz, 1
H, 3-H), 5.01 (s, 1 H, 1-H), 5.58 (m, 1 H, 1′-H), 5.89 (s, 1 H, 10-H), 5.96
(d, J = 3.0 Hz, 1 H, 9-H) ppm. MS (ESI+): m/z (%) = 725 (100) [M –
(s, 9 H, SiCMe3), 1.21 (s, 6 H, 26-Me, 27-Me), 0.87–2.42 (several m),
2.47 (dd, J = 17.2, 3.9 Hz, 1 H, 4-H), 4.26 (dd, J = 12.2, 5.2 Hz, 1 H,
2′-H), 4.45 (dd, J = 12.6, 7.2 Hz, 1 H, 2′-H), 4.86 (t, J = 3.9 Hz, 1 H,
3-H), 4.96 (s, 1 H, 1-H), 5.42 (s, 1 H, 9-H), 5.54 (t, J = 5.3 Hz, 1 H, 1′-
H), 5.62 (s, 1 H, 10-H), 5.71 (d, J = 11.9 Hz, 1 H, 7-H), 5.83 (d, J =
OTBDMS]+. HRMS (ESI+): calcd. for C49H92NaO4Si4 879.5965; found 12.6 Hz, 1 H, 6-H) ppm. MS (APCI+): m/z (%) = 727 (100) [M –
879.5942.
OTBDMS]+. HRMS (ESI+): calcd. for C49H94NaO4Si4 881.6121; found
881.6107.
(Z)-(1R,3R)-1,3-Bis[(tert-butyldimethylsilyl)oxy]-2-{2-[(tert-
butyldimethylsilyl)oxy]ethylidene}-6,7-dehydro-25-(trimethyl-
silyloxy)-19-nor-previtamin D3 (34): Colourless oil (90 %). 1H NMR
(300.13 MHz, CDCl3): δ = 0.06 (s, 9 H, 3 SiMe), 0.08 (s, 6 H, 2 SiMe),
0.09 (s, 3 H, SiMe), 0.11 (s, 9 H, TMS), 0.70 (s, 3 H, 18-Me), 0.86 (s, 9
H, SiCMe3), 0.90 (s, 9 H, SiCMe3), 0.93 (s, 9 H, SiCMe3), 0.95 (d, J =
6.6 Hz, 3 H, 21-Me), 1.21 (s, 6 H, 26-Me, 27-Me), 1.22–2.26 (several
m), 2.51 (dd, J = 16.7, 6.0 Hz, 1 H, 4-H), 4.29 (dd, J = 6.6, 1.4 Hz, 2
H, 2′-H), 4.59 (dd, J = 8.7, 6.9 Hz, 1 H, 3-H), 4.94 (d, J = 5.3 Hz, 1 H,
1-H), 5.72 (dd, J = 6.6, 4.9 Hz, 1 H, 1′-H), 5.89 (dd, J = 5.0, 2.3 Hz, 1
H, 10-H), 5.99 (d, J = 3.0 Hz, 1 H, 9-H) ppm. MS (ESI+): m/z (%) = 725
(100) [M – OTBDMS]+. HRMS (ESI+): calcd. for C49H92NaO4Si4
879.5965; found 879.5920.
(Z)-(1R,3R)-1,3-Bis[(tert-butyldimethylsilyl)oxy]-2-{2-[(tert-
butyldimethylsilyl)oxy]ethylidene}-25-(trimethylsilyloxy)-19-
nor-previtamin D3 (38): Colourless oil (90 %). 1H NMR (400.13 MHz,
CDCl3): δ = 0.06 (s, 6 H, 2 SiMe), 0.07 (s, 6 H, 2 SiMe), 0.08 (s, 3 H,
SiMe), 0.09 (s, 3 H, SiMe), 0.12 (s, 3 H, TMS), 0.74 (s, 3 H, 18-Me),
0.87 (s, 9 H, SiCMe3), 0.90 (s, 9 H, SiCMe3), 0.93 (s, 9 H, SiCMe3), 1.21
(s, 6 H, 26-Me, 27-Me), 0.87–2.29 (several m), 2.53 (dd, J = 16.3,
5.6 Hz, 1 H, 4-H), 4.31 (s, 2 H, 2′-H), 4.53 (dd, J = 8.2, 6.1 Hz, 1 H, 3-
H), 4.94 (d, J = 4.7 Hz, 1 H, 1-H), 5.41 (s, 1 H, 9-H), 5.61 (d, J = 3.4 Hz,
1 H, 10-H), 5.71 (m, 2 H, 1′-H, 7-H), 5.83 (d, J = 12.2 Hz, 1 H, 6-H)
ppm. MS (APCI+): m/z (%) = 727 (100) [M – OTBDMS]+. HRMS (ESI+):
calcd. for C49H94NaO4Si4 881.6121; found 881.6099.
(E)-(2R,3R)-2,3-Bis[(tert-butyldimethylsilyl)oxy]-1-(2-hydroxy-
ethylidene)-25-(trimethylsilyloxy)-19-nor-previtamin D3 (39):
Yellow oil (70 %). 1H NMR (300.13 MHz, CDCl3): δ = 0.03 (s, 3 H,
SiMe), 0.05 (s, 6 H, 2 SiMe), 0.08 (s, 3 H, SiMe), 0.11 (s, 9 H, TMS),
0.73 (s, 3 H, 18-Me), 0.85 (s, 9 H, SiCMe3), 0.90 (s, 9 H, SiCMe3), 0.81–
2.78 (several m), 0.97 (d, J = 6.5 Hz, 3 H, 21-Me), 1.21 (s, 6 H, 26-Me,
27-Me), 3.83 (m, 1 H, 3-H), 3.93 (d, J = 5.7 Hz, 1 H, 2-H), 4.31 (m, 2
H, 2′-H), 5.44 (s, 1 H, 9-H), 5.62 (t, J = 7.0 Hz, 1 H, 1′-H), 5.78 (d, J =
12.0 Hz, 1 H, 7-H), 5.88 (d, J = 12.2 Hz, 1 H, 6-H), 6.34 (s, 1 H, 10-H)
ppm.
(E)-(2R,3R)-2,3-Bis[(tert-butyldimethylsilyl)oxy]-6,7-dehydro-1-
(2-hydroxyethylidene)-25-(trimethylsilyloxy)-19-nor-pre-
vitamin D3 (35): Viscous, yellow oil (65 %). 1H NMR (400.13 MHz,
CDCl3): δ = 0.04 (s, 3 H, SiMe), 0.05 (s, 3 H, SiMe), 0.06 (s, 3 H, SiMe),
0.10 (s, 3 H, SiMe), 0.11 (s, 9 H, TMS), 0.72 (s, 3 H, 18-Me), 0.85 (s, 9
H, SiCMe3), 0.88 (s, 9 H, SiCMe3), 0.96 (d, 3JHH = 6.5 Hz, 3 H, 21-Me),
0.98–2.30 (several m), 1.21 (s, 6 H, 26-Me, 27-Me), 2.16 (dd, J = 17.3,
3.6 Hz, 1 H, 4-H), 2.67 (d, J = 16.8 Hz, 1 H, 4-H), 3.85 (m, 1 H, 3-H),
3.91 (d, J = 5.4 Hz, 1 H, 2-H), 4.32 (m, 2 H, 2′-H), 5.61 (t, J = 7.0 Hz,
1 H, 1′-H), 6.01 (d, J = 3.0 Hz, 1 H, 9-H) and 6.55 (s, 1 H, 10-H) ppm.
MS (APCI): m/z (%) = 725 (100) [M – OH]+.
(E)-(2R,3R)-1-{2-[(tert-Butyldimethylsilyl)oxy]ethylidene}-2,3-
(2,3-dimethoxybutane-2,3-diyl-2,3-dioxy)-25-(trimethylsilyl-
oxy)-19-nor-previtamin D3 (40): Yellow oil (75 %). 1H NMR
(600.13 MHz, [D6]acetone): δ = 0.091 (s, 3 H, SiMe), 0.093 (s, 3 H,
SiMe), 0.10 (s, 9 H, TMS), 0.79 (s, 3 H, 18-Me), 0.91 (s, 9 H, SiCMe3),
1.00 (d, J = 6.6 Hz, 3 H, 21-Me), 1.215, 1.217 (2 s, 6 H, 26-Me and
27-Me), 1.25 (s, 3 H, Me), 1.31 (s, 3 H, Me), 0.86–2.42 (several m),
2.55 (dd, J = 16.8, 5.4 Hz, 1 H, 4-H), 3.18 (s, 3 H, OMe), 3.24 (s, 3 H,
OMe), 3.62 (apparent td, J = 10.6, 5.4 Hz, 1 H, 3-H), 4.09 (dd, J =
10.4, 1.6 Hz, 1 H, 2-H), 4.37 (ddd, J = 13.5, 6.1, 1.9 Hz, 1 H, 2′-H),
4.47 (ddd, J = 13.4, 7.0, 1.7 Hz, 1 H, 2′-H), 5.46 (br. s, 1 H, 9-H), 5.77
(apparent t, J = 5.6 Hz, 1 H, 2′-H), 5.84 (d, J = 11.9 Hz, 1 H, 7-H),
5.95 (d, J = 12.2 Hz, 1 H, 6-H), 6.45 (s, 1 H, 2-H) ppm.
(E)-(2R,3R)-1-{2-[(tert-Butyldimethylsilyl)oxy]ethylidene}-2,3-
(2,3-dimethoxybutane-2,3-diyl-2,3-dioxy)-6,7-dehydro-25-tri-
methylsilyloxy-19-nor-previtamin D3 (36): Viscous, colourless oil
1
(60 %). H NMR (600.13 MHz, [D6]acetone): δ = 0.097 (s, 3 H, SiMe),
0.101 (s, 12 H, SiMe, TMS), 0.73 (s, 3 H, 18-Me), 0.92 (s, 9 H, SiCMe3),
1.00 (d, J = 6.6 Hz, 3 H, 21-Me), 1.220, 1.222 (2 s, 6 H, 26-Me and
27-Me), 1.26 (s, 3 H, Me), 1.32 (s, 3 H, Me), 0.91–2.38 (several m),
2.41 (dd, J = 16.9, 5.8 Hz, 1 H, 4-H), 3.21 (s, 3 H, OMe), 3.23 (s, 3 H,
OMe), 3.70 (apparent td, J = 10.5, 5.9 Hz, 1 H, 3-H), 4.08 (d, J =
10.7 Hz, 1 H, 2-H), 4.38 (ddd, J = 13.8, 6.1, 1.6 Hz, 1 H, 2′-H), 4.47
(ddd, J = 13.8, 6.9, 1.7 Hz, 1 H, 2′-H), 5.82 (apparent t, J = 6.4 Hz, 1
H, 2′-H), 5.97 (apparent q, J = 3.6 Hz, 1 H, 9-H), 6.64 (d, J = 1.8 Hz,
1 H, 2-H) ppm.
In Vitro Biological Evaluation. Affinity for VDR: The affinity of
1α,25-(OH)2-D3 and its analogues to the vitamin D receptor was
evaluated by their ability to compete with [3H]1α,25-(OH)2-D3 for
binding to high-speed supernatant from intestinal mucosa homo-
genates obtained from normal pigs, as described previously.[23] The
relative affinity of the analogues was calculated from their concen-
tration needed to displace 50 % of the [3H]1α,25-(OH)2-D3 from its
receptor compared with the activity of 1α,25-(OH)2-D3 (assigned a
100 % value). Affinity for hDBP: Binding of vitamin D metabolites
and analogues to hDBP was carried out at 4 °C, as described previ-
ously.[24] [3H]1α,25-(OH)2-D3 and 1α,25-(OH)2-D3 or its analogues
General Procedure for the Synthesis of 37–40: A round-bot-
tomed flask containing Lindlar catalyst (30 mg), was exposed to a
positive pressure of hydrogen gas (balloon). Then, a solution of the
appropriate dienyne 33–36 (0.024 mmol) in hexanes (11 mL) and
quinolone (0.5 % v/v in hexanes; 30 μL, 0.001 mmol) was added.
The reaction mixture was stirred vigorously for 4 h, after which it
was filtered through Celite and washed with a mixture of hexanes/
Et2O. The filtrate was concentrated, and the crude residue was puri-
fied by flash chromatography on silica gel (eluents: 1 % Et2O/hex-
anes for 37–38; 15 % Et2O/hexanes for 39; and 4 % Et2O/hexanes
for 40).
were incubated with hDBP (0.18 μ
M) in a final volume of 1 mL
(0.01 Tris-HCl buffer and 0.154
M
M NaCl, pH 7.4) at 4 °C for 3 h.
(E)-(1R,3R)-1,3-Bis[(tert-butyldimethylsilyl)oxy]-2-{2-[(tert-
butyldimethylsilyl)oxy]ethylidene}-25-(trimethylsilyloxy)-19-
nor-previtamin D3 (37): Colourless oil (60 %). 1H NMR (300.13 MHz,
CDCl3): δ = 0.04 (s, 3 H, SiMe), 0.06 (s, 6 H, 2 SiMe), 0.07 (s, 3 H,
Phase separation was then obtained by the addition of cold dex-
tran-coated charcoal (0.5 mL). Cell-Proliferation Assays: As a meas-
ure of cell proliferation, [3H]thymidine incorporation of breast can-
cer MCF-7 cells (ATCC) was determined after a 72 h incubation pe-
SiMe), 0.10 (s, 3 H, SiMe), 0.105 (s, 3 H, SiMe), 0.113 (s, 3 H, TMS), riod with various concentrations of 1α,25-(OH)2-D3, analogues or
0.71 (s, 3 H, 18-Me), 0.88 (s, 9 H, SiCMe3), 0.90 (s, 9 H, SiCMe3), 0.95 vehicle, as described previously.[23]
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Acknowledgments
Financial support by the Spanish Ministerio de Ciencia e Innova-
ción (MICINN) (projects CTQ2011-24237 and CTQ2014-55015-P)
and the Principado de Asturias (project FC-15-GRUPIN14-002) is
gratefully acknowledged. A. H.-M. thanks the Spanish Ministerio
de Educación for a predoctoral FPU fellowship.
Keywords: Vitamins · Total synthesis · Chiral pool · Biological
activity · Structure-activity relationships
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Received: October 7, 2016
Published Online: ■
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Full Paper
Vitamin D Analogues
A. Hernández-Martín, S. Fernández,*
A. Verstuyf, L. Verlinden,
M. Ferrero* ........................................ 1–11
A-Ring-Modified
2-Hydroxyethyl-
idene Previtamin D3 Analogues:
Synthesis and Biological Evaluation
The synthesis of previtamin D3 ana- of the CD-ring/side-chain fragment.
logues with
a 2-hydroxyethylidene The binding affinity to vitamin D re-
group in the A-ring is described. The ceptor (VDR) and human vitamin D
strategy involves coupling of a di- binding protein (hDBP), and the MCF-
enyne precursor of the A-ring derived 7 cell antiproliferative activity is dis-
from shikimic acid with an enol triflate cussed.
DOI: 10.1002/ejoc.201601263
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim