6-s-cis locked analogues of the steroid hormone 1α,25-dihydroxyvitamin D3. Synthesis of novel A-ring stereoisomeric 1,25-dihydroxy-3-epi-19-nor-previtamin D3 derivatives

Article abstract of DOI:10.1021/jo000443l

Efficient syntheses of A-ring synthons 24 and 32 are described from hydroxy ester 16, which is easily available on a preparative scale from (-)-quinic acid. Key features of the syntheses were (a) the ability to selectively perform desilylations in the presence of p-nitrobenzoate esters and (b) the excellent yield and complete stereospecificity with which the configuration of alcohols 16, 18, and 26 could be inverted under Mitsunobu conditions. Thus, A-ring synthons 24 and 32 were both prepared in 35-38% yield (eight steps) from the common precursor 16. The coupling of A-ring synthons 24 and 32 with the appropriate CD-ring/side chain fragment 7 provides access to novel 6-s-cis locked analogues of steroid hormone 1α,25-dihydroxyvitamin D₃: 1α,25-dihydroxy-3-epi-19-nor-previtamin D₃ (37) and 1β,25-dihydroxy-3-epi-19-nor-previtamin D₃ (38), which are unable to undergo rearrangement to the respective vitamin D form by virtue of the absence of the C-19 methyl group. Compounds 37 and 38 can be used as tools for studying the genomic and nongenomic mechanisms of action of the previtamin form of the hormone 1α,25-dihydroxyvitamin D₃.

Full text of DOI:10.1021/jo000443l

J . Org. Chem. 2000, 65, 5647-5652
5647
6-s-cis Lock ed An a logu es of th e Ster oid Hor m on e
1r,25-Dih yd r oxyvita m in D3. Syn th esis of Novel A-Rin g
Ster eoisom er ic 1,25-Dih yd r oxy-3-epi-19-n or -p r evita m in D3
Der iva tives
Mo´nica D´ıaz, Miguel Ferrero, Susana Ferna´ndez, and Vicente Gotor*
Departamento de Quı´mica Orga´nica e Inorga´nica, Facultad de Qu´ımica, Universidad de Oviedo,
33071-Oviedo, Spain
VGS@sauron.quimica.uniovi.es
Received March 24, 2000
Efficient syntheses of A-ring synthons 24 and 32 are described from hydroxy ester 16, which is
easily available on a preparative scale from (-)-quinic acid. Key features of the syntheses were (a)
the ability to selectively perform desilylations in the presence of p-nitrobenzoate esters and (b) the
excellent yield and complete stereospecificity with which the configuration of alcohols 16, 18, and
26 could be inverted under Mitsunobu conditions. Thus, A-ring synthons 24 and 32 were both
prepared in 35-38% yield (eight steps) from the common precursor 16. The coupling of A-ring
synthons 24 and 32 with the appropriate CD-ring/side chain fragment 7 provides access to novel
6-s-cis locked analogues of steroid hormone 1R,25-dihydroxyvitamin D3: 1R,25-dihydroxy-3-epi-
19-nor-previtamin D3 (37) and 1â,25-dihydroxy-3-epi-19-nor-previtamin D3 (38), which are unable
to undergo rearrangement to the respective vitamin D form by virtue of the absence of the C-19
methyl group. Compounds 37 and 38 can be used as tools for studying the genomic and nongenomic
mechanisms of action of the previtamin form of the hormone 1R,25-dihydroxyvitamin D3.
Sch em e 1
In tr od u ction
Vitamin D3 is successively hydroxylated in the liver
(at C-25) and then in the kidney (at C-1R) to produce the
seco-B steroid 1R,25-dihydroxyvitamin D3 [1R,25-(OH)2-
D3] (1, Scheme 1), which is the hormonally active form
of vitamin D. This metabolite exhibits a much broader
spectrum of biological activities than originally thought,
beyond its classic functions as a steroid hormone in
regulating intestinal calcium absorption (ICA) and bone
calcium mobilization (BCM), via genomic mechanisms1
by interaction with a nuclear/cytosol vitamin D receptor
(n-VDR) to regulate gene transcription. 1R,25-(OH)2-D3
also affects the human immune system, inhibits cell
proliferation, and promotes cellular differentiation.2
There is clear evidence that this secosteroid can also
generate biological responses via nongenomic pathways3
by interaction with a membrane vitamin D receptor
(m-VDR), which generates rapid biological responses
believed to be independent of direct interaction with the
genome. These include the rapid stimulation of intestinal
(1) (a) Lowe, K. E.; Maiyar, A. C.; Norman, A. W. Crit. Rev. Eukar.
Gene Exp. 1992, 2, 65. (b) Pike, J . W. Annu. Rev. Nutr. 1991, 11, 189-
216. (c) Norman, A. W., Bouillon, R., Thomasset, M., Eds. Vitamin D:
Gene Regulation, Structure-Function Analysis and Clinical Applica-
tion; Walter de Gruyter: Berlin, 1991. (d) Norman, A. W. Vitamin D:
The calcium Homeostatic Steroid Hormone; Academic Press: New
York, 1979.
(2) (a) Norman, A. W., Bouillon, R., Thomasset, M., Eds. Vitamin
D, A Pluripotent Steroid Hormone: Structural Studies, Molecular
Endocrinology, and Clinical Applications; Walter de Gruyter: Berlin,
1994. (b) Uskokovic, M. R. Bioorg. Med. Chem. Lett. 1993, 3, 1783-
1896.
(3) (a) Norman, A. W.; Nemere, I.; Zhou, L. X.; Bishop, J . E.; Lowe,
K. E.; Maiyar, A. C.; Collins, E. D.; Taoka, T.; Sergeev, I.; Farach-
Carson, M. C. J . Steroid Biochem. Mol. Biol. 1992, 41, 231-240. (b)
Deboland, A. R.; Nemere, I.; Norman, A. W. Biochem. Biophys. Res.
Commun. 1990, 166, 217-222.
Ca2+ transport, a process known as transcaltachia.4 The
efficacy of 1R,25 analogues as chemotherapeutic agents
in a variety of human disease states is extremely promis-
ing,5 including both nongenomic and genomic pathways.
(4) (a) Nemere, I.; Norman A. W. Endocrinology 1986, 119, 1406-
1408. (b) Nemere, I.; Yoshimoto, Y.; Norman A. W. Endocrinology 1984,
115, 1476-1483.
(5) (a) Ettinger, R. A.; DeLuca, H. F. Adv. Drug. Res. 1996, 28, 269-
312. (b) Bouillon, R.; Okamura, W. H.; Norman, A. W. Endocrine Rev.
1995, 16, 200-257.
10.1021/jo000443l CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/10/2000

5648 J . Org. Chem., Vol. 65, No. 18, 2000
D´ıaz et al.
Ch a r t 1
Ch a r t 2
1R,25-(OH)2-D3 exists as two conformationally inter-
equilibrating forms (6-s-trans 1 and a minor form 6-s-cis
1′), which are present in slow chemical equilibrium (5-
10%) with 1R,25-dihydroxyprevitamin D3 [1R,25-(OH)2-
pre-D3] (2). Pure pre-1R,25 standing in solution rear-
ranges to 1R,25 with a half-life of 13.5 h at 37 °C, whereas
when the C-19 methyl group is deuterated,6 rearrange-
ment proceeds with a half-life of 81 h. However, whereas
1R,25 is significantly more active than pre-1R,25 (in a
pentadeuterated form)6 in assays reflecting genomic
responses, the latter was equally as active as 1R,25 in
assays for measuring nongenomic effects.7 Thus, 6-s-cis-
1R,25 (1′) was proposed as the active conformer in
eliciting nongenomic effects, with pre-1R,25 (2) simply
behaving as an excellent analogue of this conformer.8 1R,-
25-(OH)2-Lumisterol (3, Chart 1), a 6-s-cis analogue,9 has
been established to be equally active in eliciting the
nongenomic activity of transcaltachia as both pre-1R,25
and 1R,25. In contrast, in terms of both transcaltachia
and binding to m-VDR, 1R,25-(OH)2-7-dehydrocholesterol
(4) is less active than 1R,25-lumisterol. Thus, there is a
significant level of stereoselectivity in mediating these
nongenomic responses. Different analogues can induce
different shapes of protein, resulting in different confor-
mations of the protein-analogue complex, each leading
to different biological responses. It became of interest,
then, to prepare locked analogues of the 6-s-cis conformer
of 1R,25, incapable of isomerizing to its 6-s-trans confor-
mation, which is thermodynamically more stable.
triflate 7 with A-ring silyl-protected precursors of 8 and
9, respectively. Spectral data for these compounds are
collected in the Supporting Information. To develop
useful vitamin D analogues to understand the mecha-
nism of the action of the steroid hormone 1R,25-(OH)2-
D3, as a result of the importance of A-ring stereochem-
istry for binding the ligand to the hormone receptor and
as a part of our research program,10 we need to complete
the A-ring stereoisomeric series of 1,25-(OH)2-19-nor-pre-
D3. It is the purpose of this article to describe the
preparation of the appropriate A-ring precursors 10 and
11 to allow us access to novel 1,25-(OH)2-3-epi-19-nor-
pre-D3 analogues 37 and 38 (Scheme 4). As starting
material we use (-)-quinic acid, an inexpensive and
commercial available compound.
Resu lts a n d Discu ssion
A-Rin g Syn th eses. In a previous communication,10a
we described the synthesis of A-ring stereoisomeric
synthons 8 and 9 (Chart 2) through an efficient sequence
in high overall yield from (-)-quinic acid. Key features
of this approach were the selective deprotection of a
disilyl ether in an R,â-unsaturated ester derivative and
the excellent yield of the Mitsunobu reaction, which takes
place with total inversion of configuration. To prepare
the two other remaining stereoisomers of A-ring synthons
(3S,5S)-10 and (3R,5S)-11, stereoselective reduction of
hydroxy ketone (S)-12 with sodium triacetoxyborohy-
dride11 to prepare 11 and with sodium borohydride to
prepare 10 was chosen as a reasonable approach. Com-
Our efforts have been aimed at synthesizing the A-ring
diastereomers of 1R,25-(OH)2-19-nor-pre-D3, which are
unable to undergo rearrangement to the respective
vitamin D form by virtue of the absence of the C-19
methyl group. We have already reported10a the synthesis
of 1R,25-(OH)2-19-nor-pre-D3 (5) and 1â,25-(OH)2-19-nor-
pre-D3 (6) (Chart 2) through the coupling of CD-ring
(6) Curtin, M. L.; Okamura, W. H. J . Am. Chem. Soc. 1991, 113,
6958-6966.
(7) Norman, A. W.; Okamura, W. H.; Farach-Carson, M. C.; Alle-
waert, K.; Branisteanu, D.; Nemere, I.; Muralidharan, K. R.; Bouillon,
R. J . Biol. Chem. 1993, 268, 13811-13819.
(8) Okamura, W. H.; Midland, M. M.; Norman, A. W.; Hammond,
M. W.; Dormanen, M. C.; Nemere, I. Ann. N.Y. Acad. Sci. 1995, 761,
344-348.
(10) (a) D´ıaz, M.; Ferrero, M.; Ferna´ndez, S.; Gotor, V. Tetrahedron
Lett. 2000, 41, 775-779. (b) Ferrero, M.; Ferna´ndez, S.; Gotor, V. J .
Org. Chem. 1997, 62, 4358-4363. (c) Ferna´ndez, S.; D´ıaz, M.; Ferrero,
M.; Gotor, V. Tetrahedron Lett. 1997, 38, 5225-5228. (d) Ferna´ndez,
S.; Ferrero, M.; Gotor, V.; Okamura, W. H. J . Org. Chem. 1995, 60,
6057-6061.
(9) (a) Okamura, W. H.; Midland, M. M.; Hammond, M. W.; Rahman,
N. A.; Dormanen, M. C.; Nemere, I.; Norman, A. W. J . Steroid Biochem.
Mol. Biol. 1995, 53, 603-613. (b) Dormanen, M. C.; Bishop, J . E.;
Hammond, M. W.; Okamura, W. H.; Nemere, I.; Norman, A. W.
Biochem. Biophys. Res. Commun. 1994, 201, 394-401.

6-s-cis Locked Analogues of 1R,25-Dihydroxyvitamin D3
J . Org. Chem., Vol. 65, No. 18, 2000 5649
Sch em e 2
pound (S)-12 was expected to form by inversion of (R)-
12, which is readily obtained from 8 by selective Dess-
Martin oxidation of the allylic OH group. However a
problem arose when attempts to invert the configuration
of the OH group in (R)-12 were found to consistently lead
to spontaneous aromatization.
Another rational but unsuccessful strategy was the
synthesis of diol (3S,5S)-10 through 3,5-disilyl-protected
compound 14. (-)-Methyl 5-epi-shikimate 13 was
obtained10c on a large scale in five steps in 60% overall
yield from shikimic acid or alternatively from (-)-quinic
acid. Dess-Martin oxidation of the allylic alcohol in 10
provided the (S)-hydroxy ketone 12, which by simple
treatment with sodium triacetoxyborohydride11 gave
exclusively the A-ring stereoisomer (3R,5S)-11. Here,
however, a problem was encountered when we tried to
apply Barton’s procedure12 to remove the C-4 hydroxyl
group in 14. At the beginning, we followed a procedure
similar to that for (3S,5R)-8,13 in which the A-ring has
the natural anti configuration. In our case, as a result of
the steric hindrance of the 4-hydroxyl group in compound
14, it was not possible to introduce the phenoxythiono-
carbonate group with the corresponding chloride in
DMAP. It was necessary to use stronger conditions (bases
t
such as NaH or BuOK), but in these cases migrations of
Sch em e 3
the O-silyl group were observed. Alternative methods of
removing the hydroxyl group at C-4, such as its trans-
formation into a good leaving group (by tosylation or
mesylation) and subsequent removal with hydride, were
also unsatisfactory.
At this point, we decided to change our strategy, taking
into account the availability of compound 16 (Scheme 2)
through the selective deprotection of the allylic hydroxyl
group of the corresponding disilyl derivative, an inter-
mediate in the route to A-ring synthon 9. Full experi-
mental details for the conversion of (-)-quinic acid to key
starting material 16, as well as full spectral data for all
compounds involved in this route are shown in the
Supporting Information section. To synthesize A-ring
precursors (3S,5S)-10 and (3R,5S)-11, we have used the
efficient pathways shown in Schemes 2 and 3, respec-
tively. A-Ring protected synthon (3S,5S)-24 was prepared
via the allylic alcohol 16, which was synthesized as
previously described in eight steps with 31% overall yield
from (-)-quinic acid.10a To perform the subsequent trans-
formations, the OH group at C-5 had to be free and the
OH group at C-3 had to be protected. Because of the
impossibility of selectively deprotecting the C-5 hydroxyl
group of the fully protected precursor of ester 16, protec-
tion of the allylic alcohol is required. From various
possibilities, we chose to convert 16 to the p-nitrobenzoate
ester 17 on the basis that the same ester will be obtained
after Mitsunobu reaction14 over compound 18 and also
would be stable under acid-catalyzed desilylation condi-
tions. Thus, reaction of 16 with p-nitrobenzoyl chloride
(PNBCl) in pyridine gave an almost quantitative yield
of 17, and subsequent treatment with HCl in MeOH
afforded 18. This was fortunate, as attempted desilylation
with TBAF was unsuccessful as a result of the fact that
fluoride ion in THF was a sufficiently strong base to lead
to migration of the PNB group, giving a mixture of 3-
and 5-OPNB esters. To invert the C-5 hydroxyl group in
18, we applied Mitsunobu’s procedure with p-nitrobenzoic
acid in THF. The completely inverted di-PNB ester 19
was obtained with 85% yield. Methanolysis of the PNB
esters was carried out at 0 °C with 0.7 equiv of NaOMe.
Protection of the resulting diol 20 was necessary to afford
the desired transformation from methyl ester to alkyne
and to couple this A-ring fragment with CD-ring/side
chain synthon 7. When the OH group was left unpro-
tected, the yield of coupled product decreased signifi-
cantly. Transformation of the ester 21 into the aldehyde
23 was best carried via a two-step sequence: reduction
with DIBAL-H followed by oxidation with pyridinium
chlorochromate (PCC). Formation of the enyne 24 was
accomplished in a good yield by reaction with trimeth-
ylsilyldiazomethane. The overall yield of 24 from 16 via
this eight-step sequence was 38%.
(11) Thompson, S. H. J .; Mahon, M. F.; Molloy, K. C.; Hadley, M.
S.; Gallagher, T. J . Chem. Soc., Perkin Trans. 1 1995, 379-383.
(12) Barton, D. H. R.; Motherwell, W. B. Pure Appl. Chem. 1981,
53, 15-31.
(13) Sarandeses, L. A.; Mascaren˜as, J . L.; Castedo, L.; Mourin˜o, A.
Tetrahedron Lett. 1992, 33, 5445-5448.
(14) (a) Hughes, D. L. Org. Prep. Proc. Int. 1996, 28, 127-164. (b)
Mitsunobu, O. Synthesis 1981, 1-28.

5650 J . Org. Chem., Vol. 65, No. 18, 2000
D´ıaz et al.
Sch em e 4
at the 25-position (steroid numbering) and then protected
as a trimethylsilyl ether. Posterior kinetic enol formation
afforded triflate 7. A-Ring synthons 24 or 32 were coupled
to CD-triflate 7 in the presence of bis[triphenylphosphine]-
palladium(II) acetate-copper(I) iodide catalyst and di-
ethylamine in DMF. The resulting silyloxy dienynes 33
or 34 were deprotected with TBAF to afford the trihy-
droxydienynes 35 or 36, respectively. Careful catalytic
hydrogenation of triols 35 or 36 in MeOH, in the presence
of Lindlar catalyst and quinoline, generated 3-epi-previ-
tamins 37 or 38.
The structural assignment of the compounds described
1
in this paper is based on the analysis of their H and 13C
NMR spectra and DEPT experiments. The correct as-
1
signment was confirmed by H-13C heteronuclear cor-
relation experiments.
In summary, we have developed a novel and practical
route to A-ring precursors 24 and 32 from a single
intermediate (16) easily available on a preparative scale
from (-)-quinic acid. Key features of the synthesis were
the ability to selectively deprotect silyl ethers in the
presence of p-nitrobenzoate esters and the excellent yield
and stereoselectivity of the Mitsunobu reaction, which
occurs with complete inversion. Coupling of 24 and 32
to the appropriate CD-ring/side chain fragment 7 pro-
vided access to the novel 6-s-cis-3-epi locked analogues
37 and 38. These syntheses will allow the effect of
stereochemistry at C-1 and C-3 on the biological activity
of 1,25-(OH)2-19-nor-pre-D3 to be studied.
Exp er im en ta l Section 18
Gen er a l. Reagents were purchased from Aldrich or Fluka.
Solvents were distilled over an appropriate desiccant under
nitrogen. Syntheses of CD-triflate 716 and allylic alcohol 1610a
were previously reported.
We envisaged that the synthesis of 32 could be
achieved by simultaneous inversions both OH groups in
the corresponding diol of 16. However, although the
Mitsunobu process gave the desired inversion at both C-3
and C-5, the yield was very poor and large amounts of
starting material were recovered. As a result, we turned
our attention to carrying out the inversion stepwise.
Thus, 16 under standard Mitsunobu conditions with
p-nitrobenzoic acid resulted in esterification of the alcohol
with total inversion of configuration (Scheme 3). Cleavage
of the silyl group in 25 with HCl in MeOH yielded 26; as
in the case of 17, TBAF treatment led to PNB migrations
and thus was not useful. Inversion of the desilylated OH
group by reaction with p-nitrobenzoic acid under Mit-
sunobu conditions provided diester 27, in which both C-3
Meth yl (3S,5R)-5-[(ter t-Bu tyld im eth ylsilyl)oxy]-3-[(p-
n itr oph en ylcar bon yl)oxy]cycloh ex-1-en ecar boxylate (17).
p-Nitrobenzoyl chloride (1.15 g, 6.22 mmol) was added in
portions to a stirred solution of compound 16 (1.00 g, 3.11
mmol) in pyridine (10 mL) at 0 °C under nitrogen atmosphere.
The mixture was stirred for 1 h at room temperature, then
diluted with water, and extracted with methylene chloride. The
combined organic layers were evaporated under reduced
pressure to leave a residue which was purified by flash
chromatography (10% EtOAc/hexane) to obtain 1.32 g (98%)
of benzoate ester 17: 1H NMR (CDCl3, 400 MHz) δ 0.08 (s,
3H, MeSi), 0.09 (s, 3H, MeSi), 0.88 (s, 9H, Me3CSi), 1.92 (ddd,
2
3
2
1H, H4a, J HH 13.4, J HH 6.4, 2.7 Hz) 2.11 (ddd, 1H, H4e, J HH
13.4, J HH 8.0, 5.4 Hz), 2.30 (dddd, 1H, H6a′,
2J HH 18.2, J HH
3
3
5.6, 4J HH 1.9, 1.9 Hz), 2.64 (dddd, 1H, H6e′, 2J HH 18.2, 3J HH 4.5,
4J HH 2.0, 2.0 Hz), 3.76 (s, 3H, OMe), 4.26 (m, 1H, H5), 5.86 (m,
1
and C-5 have undergone inversion. H NMR spectra of
3
4
1H, H3), 6.94 (ddd, 1H, H2, J HH 1.9, J HH 1.9, 1.9 Hz), and
the product at each step showed the presence of only one
diastereoisomer and confirmed that both Mitsunobu
reactions proceeded with complete inversion. Methanoly-
sis and the previously described reaction sequence from
20 to 24 was followed to obtain enyne 32 in 35% overall
yield starting from 16.
Syn th eses of P r evita m in D An a logu es. To synthe-
size stable previtamin analogues 37 and 38 (Scheme 4),
we used a convergent route15 starting from the protected
A-ring synthon precursors 24 or 32 and the known CD-
triflate 7.16 The synthesis of the CD-ring/side chain
fragment was accomplished through ozonolysis of vitamin
D3 to obtain Grundmann’s ketone,17 which was oxidized
8.22 (m, 4H, ArH); HRMS calcd for C17H20NO7Si (M+ - Bu)
t
378.1009, found 378.0990.
Meth yl (3S,5R)-5-Hyd r oxy-3-[(p-n itr op h en ylca r bon yl)-
oxy]-cycloh ex-1-en eca r boxyla te (18). To a solution of 17
(1.00 g, 2.23 mmol) in methanol (5 mL) were added 2 drops of
concentrated HCl. After being stirred at room temperature for
2 h, the reaction mixture was evaporated; yield 630 mg
(16) (a) Maynard, D. F.; Trankle, W. G.; Norman, A. W.; Okamura,
W. H. J . Med. Chem. 1994, 37, 2387-2393. (b) Also see refs 6, 10a,
and 15b.
(17) Inhoffen, H. H.; Quinkert, G.; Schu¨zt, S.; Kampe, D.; Domagk,
G. F. Chem. Ber. 1957, 90, 664-673.
(18) For all products, full spectral data are given in the Supporting
Information and selected 1H and 13C NMR signals are also presented
in the Experimental Section. The purity of compounds was estimated
by a combination of GC and/or HPLC and NMR analysis. The level of
purity is indicated by the inclusion of copies of 1H and 13C NMR spectra
in the Supporting Information.
(15) (a) Mascaren˜as, J . L.; Sarandeses, L. A.; Castedo, L.; Mourin˜o,
A. Tetrahedron 1991, 47, 3485-3498. (b) Barrack, S. A.; Gibbs, R. A.;
Okamura, W. H. J . Org. Chem. 1988, 53, 1790-1796.

6-s-cis Locked Analogues of 1R,25-Dihydroxyvitamin D3
(88%): 1H NMR (CDCl3, 300 MHz) δ 2.00-2.20 (m, 2H, H4),
J . Org. Chem., Vol. 65, No. 18, 2000 5651
gummy residue was filtered through a short column of fluo-
rosil. The filtrate was concentrated to afford 990 mg of 23,
whose purity was sufficiently pure for direct use in the next
step. This aldehyde is unstable and could only be kept a couple
of days in the refrigerator under nitrogen. 23: 1H NMR (CDCl3,
300 MHz) δ 0.07 (s, 6H, 2MeSi), 0.11 (s, 3H, MeSi), 0.13 (s,
3H, MeSi), 0.89 (s, 9H, Me3CSi), 0.91 (s, 9H, Me3CSi), 1.67 (ddd,
2
3
2.32 (m, 1H, H6a′, J HH 12.0, J HH 4.2 Hz), 2.60 (br s, 1H, OH),
2
3
2.78 (m, 1H, H6e′, J HH 12.0, J HH 3.0 Hz), 3.75 (s, 3H, OMe),
4.32 (m, 1H, H5), 5.87 (m, 1H, H3), 6.95 (m, 1H, H2), and 8.21
(m, 4H, ArH); HRMS calcd for C15H15NO7 (M+) 321.0848, found
321.0849.
Meth yl (3S,5S)-3,5-Di[(p-n itr op h en ylca r bon yl)oxy]-cy-
cloh ex-1-en eca r boxyla te (19). To a stirred solution of 18
(710 mg, 2.21 mmol) in THF (30 mL) at 0 °C under nitrogen
were added PPh3 (638 mg, 2.43 mmol), 4-nitrobenzoic acid (428
mg, 2.43 mmol), and diisopropyl azodicarboxylate (0.47 mL,
2.43 mmol). The mixture was stirred for 2 h at room temper-
ature and then evaporated under reduced pressure to leave a
residue which was purified by flash chromatography (gradient
eluent 10-40% EtOAc/hexane); yield 884 mg (85%): 1H NMR
(CDCl3, 300 MHz) δ 2.27-2.37 (m, 1H, H4a), 2.51-2.59 (m,
1H, H4e), 2.64-2.74 (m, 1H, H6a′), 2.87-2.93 (m, 1H, H6e′), 3.79
(s, 3H, OMe), 5.46 (m, 1H, H5), 5.88 (m, 1H, H3), 7.03 (m, 1H,
H2), and 8.05-8.24 (m, 8H, ArH); HRMS calcd for C21H14N2O9
(M+ - MeOH) 438.0699, found 438.0702.
2
3
1H, H4a, J HH 12.0, J HH 12.0, 10.2 Hz), 1.98 (dddd, 1H, H6a′
,
3
4
2J HH 17.2, J HH 9.6, J HH 2.8, 2.8 Hz), 2.18 (m, 1H, H4e), 2.61
(dd, 1H, H6e′, 2J HH 17.4, 3J HH 5.2 Hz), 3.84 (dddd, 1H, H5, 3J HH
3
12.0, 9.2, 5.5, 3.5 Hz), 4.55 (dddd, 1H, H3, J HH 10.0, 5.7, 3.9,
1.9 Hz), 6.50 (m, 1H, H2), and 9.50 (s, 1H, H7).
(3S,5S)-3,5-Di[(ter t-bu tyld im eth ylsilyl)oxy]-1-eth yn yl-
n
cycloh ex-1-en e (24). BuLi (1.8 mL of 1.6 M in hexane, 2.88
mmol) was added to a solution of trimethylsilyldiazomethane
(1.32 mL of 2.0 M in hexane, 2.64 mmol) in THF (2 mL) at
-78 °C under nitrogen. To this solution was added compound
23 (900 mg, 2.4 mmol) in THF (2 mL). The reaction mixture
was stirred at -78 °C for 1 h and then was allowed to reach
room temperature overnight. The THF was evaporated and
the residue was poured into water/EtOAc. The aqueous layer
was extracted with EtOAc, washed with brine, dried (Na2SO4),
and evaporated in a vacuum. A subsequent flash chromatog-
raphy (gradient eluent 1-10% EtOAc/hexane) afforded 660 mg
(75% yield from 22) of compound 24: 1H NMR (CDCl3, 300
MHz) δ 0.07 (s, 3H, MeSi), 0.07 (s, 3H, MeSi), 0.08 (s, 3H,
Meth yl (3S,5S)-3,5-Dih yd r oxycycloh ex-1-en eca r boxy-
la te (20). A solution of MeONa in MeOH, prepared in situ by
addition of Na (46 mg, 2.00 mmol) to MeOH (2 mL), was added
dropwise to a solution of 19 (800 mg, 1.70 mmol) in MeOH (4
mL). The reaction was stirred at room temperature under
nitrogen, until TLC showed the disappearance of starting
material. The mixture was neutralized with solid NH4Cl and
the solvent was evaporated. The residue was purified by flash
chromatography (gradient eluent 60-80% EtOAc/hexane), and
the product was dried in vacuo; yield 204 mg (70%): 1H NMR
MeSi), 0.09 (s, 3H, MeSi), 0.89 (s, 9H, Me3CSi), 0.90 (s, 9H,
3
Me3CSi), 1.57 (ddd, 1H, H4a
,
2J HH 12.0, J HH 12.0, 10.2 Hz),
2.07-2.20 (m, 2H, H4e + H6a′), 2.33 (m, 1H, H6e′), 2.85 (s, 1H,
3
H8), 3.81 (dddd, 1H, H5, J HH 12.0, 9.4, 5.7, 3.5 Hz), 4.38 (m,
2
3
(MeOH-d4, 200 MHz) δ 1.67 (ddd, 1H4a, J HH 11.8, J HH 11.8,
1H, H3), and 5.97 (m, 1H, H2); HRMS calcd for C16H29O2Si2
9.8 Hz), 2.25 (dddd, 1H, H6a′, J HH 17.0, J HH 9.2, J HH 3.6, 2.7
(M+ - Bu) 309.1706, found 319.1707.
2
3
4
t
Hz), 2.45 (m, 1H, H4e), 2.85 (dddd, 1H, H6e′,2J HH 17.0, 3J HH 5.6,
Meth yl (3R,5R)-5-[(ter t-Bu tyld im eth ylsilyl)oxy]-3-[(p-
n itr oph en ylcar bon yl)oxy]cycloh ex-1-en ecar boxylate (25).
The same procedure as the one described for 19 yielded 25
(98%): 1H NMR (CDCl3, 300 MHz) δ 0.02 (s, 3H, MeSi), 0.03
3
4
J HH 3.2, 1.7 Hz), 3.93 (s, 3H, OMe), 4.03 (dddd, 1H, H5, J HH
9.8, 5.9, 3.3, 2.1 Hz), 4.58 (m, 1H, H3), and 7.06 (m, 1H, H2);
HRMS calcd for C8H12O4 (M+) 172.0736, found 172.0739.
Meth yl (3S,5S)-3,5-Di[(ter t-bu tyld im eth ylsilyl)oxy]-cy-
cloh ex-1-en eca r boxyla te (21). TBDMSCl (900 mg, 6.38
mmol) was added to a solution of 20 (580 mg, 3.37 mmol) and
imidazole (459 mg, 6.38 mmol) in CH2Cl2 (5 mL) at 0 °C under
nitrogen. The reaction mixture was stirred at room tempera-
ture for 2 h, diluted with CH2Cl2 and washed with water. The
aqueous layer was extracted with CH2Cl2 and the combined
organic layers were dried (Na2SO4) and evaporated in a
vacuum. The residue was purified by flash chromatography
(5% EtOAc/hexane); yield 1.31 g (97%): 1H NMR (CDCl3, 300
MHz) δ 0.06 (s, 6H, 2MeSi), 0.08 (s, 3H, MeSi), 0.09 (s, 3H,
(s, 3H, MeSi), 0.81 (s, 9H, Me3CSi), 1.78 (ddd, 1H, H4a
13.2, J HH 11.5, 11.5 Hz), 2.20 (dddd, 1H, H6a′, J HH 17.6, J HH
,
2J HH
3
2
3
4
5.7, 3.2, J HH 3.2 Hz), 2.33 (m, 1H, H4e), 2.64 (dd, 1H, H6e′
,
2J HH 17.6, J HH 4.3 Hz), 3.70 (s, 3H, OMe), 3.97 (m, 1H, H5),
3
5.74 (m, 1H, H3), 6.77 (m, 1H, H2), and 8.16 (m, 4H, ArH);
t
HRMS calcd for C17H20NO7Si (M+ - Bu) 378.1009, found
378.1002.
Meth yl (3R,5R)-5-Hyd r oxy-3-[(p-n itr op h en ylca r bon yl)-
oxy]-cycloh ex-1-en eca r boxyla te (26). The same procedure
as the one described for 18 yielded 26 (85%): 1H NMR (CDCl3,
2
3
300 MHz) δ 1.93 (ddd, H4a, J HH 12.4, J HH 10.8, 8.1 Hz), 2.42
2
3
MeSi), 0.88 (s, 9H, Me3CSi), 0.89 (s, 9H, Me3CSi), 1.58 (ddd,
(m, 2H, H4e+H6a′), 2.78 (dd, 1H, H6e′, J HH 17.5, J HH 5.2 Hz),
3.70 (s, 3H, OMe), 4.16 (m, 1H, H5), 5.82 (m, 1H, H3), 6.91 (m,
1H, H2), and 8.25 (m, 4H, ArH); HRMS calcd for C15H15NO7
(M+): 321.0848, found 321.0856.
3
1H, H4a
H
,
2J HH 12.0, J HH 10.4, 10.4 Hz), 2.10 (m, 2H, H4e
+
3
6a′), 2.60 (dd, 1H, H6e′
,
2J HH 17.2, J HH 5.6 Hz), 3.72 (s, 3H,
OMe), 3.80 (m, 1H, H5), 4.43 (m, 1H, H3), and 6.67 (m, 1H,
H2); HRMS calcd for C20H40O4Si2 (M+) 400.2465, found 400.2457.
(3S,5S)-3,5-Di[(ter t-bu tyld im eth ylsilyl)oxy]-1-h yd r oxy-
m eth ylcycloh ex-1-en e (22). DIBAL-H (7.4 mL of 1.0 M in
toluene, 7.48 mmol) was added dropwise under nitrogen to a
solution of 21 (1.00 g, 2.48 mmol) in anhydrous toluene (20
mL) at -78 °C, and the reaction was stirred for 2 h at the
same temperature. NH4Cl (aqueous) was added (∼2 mL) and
the mixture was warmed to room temperature, diluted with
Et2O, and filtered through a short column of silica gel, using
additional Et2O to elute the column. The filtrate was concen-
trated and the residue further purified by flash chromatog-
raphy (gradient eluent 5-10% EtOAc/hexane); yield 887 mg
(96%): 1H NMR (CDCl3, 300 MHz) δ 0.05 (s, 3H, MeSi), 0.06
(s, 3H, MeSi), 0.07 (s, 3H, MeSi), 0.08 (s, 3H, MeSi), 0.87 (s,
Meth yl (3R,5S)-3,5-Di[(p-n itr op h en ylca r bon yl)oxy]-cy-
cloh ex-1-en eca r boxyla te (27). The same procedure as the
one described for 19 yielded 27 (85%): 1H NMR (CDCl3, 400
2
3
MHz) δ 2.25 (ddd, 1H, H4a, J HH 13.6, J HH 6.4, 2.9 Hz), 2.42
(ddd, 1H, H4e, 2J HH 13.6, 3J HH 8.1, 5.5 Hz), 2.61 (dddd, 1H, H6a′
2J HH 18.5, 3J HH 5.9, 4J HH 1.8, 1.8 Hz), 2.97 (dddd, 1H, H6e′, 2J HH
,
3
4
18.5, J HH 4.9, J HH 2.0, 2.0 Hz), 3.78 (s, 3H, OMe), 5.62 (m,
3
4
1H, H5), 5.94 (m, 1H, H3), 7.05 (ddd, 1H, H2, J HH 3.7, J HH
1.9, 1.9 Hz), and 8.19 (m, 8H, ArH); HRMS calcd for C21H14N2O9
(M+ - MeOH) 438.0699, found 438.0703.
Meth yl (3R,5S)-3,5-Dih yd r oxycycloh ex-1-en eca r boxy-
la te (28). The same procedure as the one described for 20
yielded 28 (75%): 1H NMR (MeOH-d4, 300 MHz) δ 1.95 (ddd,
2
3
2
1H, H4, J HH 13.2, J HH 6.4, 2.8 Hz), 2.09 (ddd, 1H, H4, J HH
13.2, 3J HH 8.1, 5.3 Hz), 2.35 (dddd, 1H, H6, 2J HH 17.8, 3J HH 6.1,
4J HH 2.0, 2.0 Hz), 2.79 (dddd, 1H, H6, 2J HH 17.8, 3J HH 4.6, 4J HH
2.0, 2.0 Hz), 3.94 (s, 3H, OMe), 4.32 (m, 1H, H5), 4.66 (m, 1H,
9H, Me3CSi), 0.88 (s, 9H, Me3CSi), 1.55 (ddd, 1H, H4a,
2J HH
3
12.0, J HH 10.0, 10.0 Hz), 1.93-2.20 (m, 3H, H4e + 2H6), 2.35
(br s, OH), 3.80 (m, 1H, H5), 3.96 (m, 2H, H7), 4.36 (m, 1H,
H3), and 5.52 (m, 1H, H2); HRMS calcd for C15H31O3Si2 (M+
tBu) 315.1812, found 315.1810.
-
H3), and 7.06 (m, 1H, H2); HRMS calcd for C8H8O2 (M+
2H2O) 136.0524, found 136.0495.
-
(3S,5S)-3,5-Di[(ter t-bu tyld im eth ylsilyl)oxy]cycloh ex-1-
en eca r ba ld eh yd e (23). PCC (1.16 mg, 5.36 mmol) was added
to a solution of compound 22 (1.00 g, 2.68 mmol) in anhydrous
CH2Cl2 (5 mL). The reaction mixture was stirred at room
temperature for 2 h under nitrogen. Et2O was added and the
Meth yl (3R,5S)-3,5-Di[(ter t-bu tyld im eth ylsilyl)oxy]-cy-
cloh ex-1-en eca r boxyla te (29). The same procedure as the
one described for 21 yielded 29 (96%): 1H NMR (CDCl3, 300
MHz) δ 0.06 (s, 6H, 2MeSi), 0.07 (s, 3H, MeSi), 0.09 (s, 3H,
MeSi), 0.86 (s, 9H, Me3CSi), 0.90 (s, 9H, Me3CSi), 1.69 (ddd,

5652 J . Org. Chem., Vol. 65, No. 18, 2000
D´ıaz et al.
2
3
2
1H, H4a, J HH 12.9, J HH 6.4, 2.8 Hz), 1.81 (ddd, 1H, H4e, J HH
hexane) afforded 61 mg (70%) of triol 35: 1H NMR (CDCl3,
400 MHz) δ 0.67 (s, 3H, Me-18), 0.93 (d, 3H, Me-21, J HH 6.4
3
2
3
3
12.9, J HH 7.9, 4.9 Hz), 2.15 (dd, 1H, H6a′, J HH 17.8, J HH 5.6
Hz), 2.54 (dd, 1H, H6e′
OMe), 4.19 (dddd, 1H, H5, J HH 7.3, 5.5, 4.9, 2.6 Hz), 4.54 (m,
1H, H3), and 6.79 (m, 1H, H2); HRMS calcd for C20H40O4Si2
(M+) 400.2465, found 400.2456.
3
,
2J HH 17.8, J HH 4.5 Hz), 3.74 (s, 3H,
Hz), 1.19 (s, 6H, Me-26 + Me-27), 1.00-1.60 (m, 12H), 1.70-
2.80 (m, 12H), 4.20 (m, 1H, H3), 4.27 (m, 1H, H1), 5.96 (m, 1H,
H9 or H10), and 6.13 (m, 1H, H9 or H10); HRMS calcd for
3
C
26H40O3 (M+) 400.2977, found 400.2994.
(3R,5S)-3,5-Di[(ter t-bu tyld im eth ylsilyl)oxy]-1-h yd r oxy-
m eth ylcycloh ex-1-en e (30). The same procedure as the one
described for 22 yielded 30 (97%): 1H NMR (CDCl3, 300 MHz)
δ 0.05 (s, 3H, MeSi), 0.06 (s, 3H, MeSi), 0.07 (s, 6H, 2MeSi),
0.87 (s, 9H, Me3CSi), 0.88 (s, 9H, Me3CSi), 1.73 (m, 2H, H4),
1â,25-Dih yd r oxy-6,7-d e h yd r o-3-ep i-19-n or -p r e vit a -
m in D3 (36). The same procedure as the one described for 35
yielded 36 (75% from 32): 1H NMR (CDCl3, 400 MHz) δ 0.67
3
(s, 3H, Me-18), 0.94 (d, 3H, Me-21, J HH 6.7 Hz), 1.20 (s, 6H,
Me-26 + Me-27), 1.00-1.50 (m, 12H), 1.60-2.20 (m, 11H), 2.5
1.90 (dd, 1H, H6a′, 2J HH 17.0, 3J HH 7.1 Hz), 2.12 (br s, OH), 2.22
(dd, 1H, 3J HH 17.4, 4.8 Hz), 4.14 (m, 1H, H3), 4.44 (m, 1H, H1),
3
2J HH 17.0, J HH 4.8 Hz), 3.98 (m, 2H, H7), 4.16
3
(dd, 1H, H6e′
,
5.97 (dd, 1H, H9, J HH 6.5, 3.4 Hz), and 6.03 (m, 1H, H10);
(m, 1H, H5), 4.40 (ddd, 1H, H3, 3J HH 8.3, 4.6, 4.6 Hz), and 5.63
(m, 1H, H2); HRMS calcd for C19H38O2Si2 (M+ - H2O) 354.2410,
found 354.2411.
HRMS calcd for C26H40O3 (M+) 400.2977, found 400.2986.
1R,25-Dih yd r oxy-3-epi-19-n or -p r evita m in D3 (37). A
flask containing Lindlar catalyst (32 mg) was exposed to a
positive pressure of hydrogen gas (balloon). MeOH (1 mL) was
added and the mixture was cooled to 0 °C. To this suspension
was added quinoline (0.24 mL, 0.17 M in hexane) and 35 (20
mg, 0.05 mmol) in MeOH (2 mL). The reaction was stirred at
(3R,5S)-3,5-Di[(ter t-b u t yld im et h ylsilyl)oxy]cycloh ex-
1-en eca r ba ld eh yd e (31). The same procedure as the one
described for 23 yielded 31 (100%): 1H NMR (CDCl3, 300 MHz)
δ 0.04 (s, 3H, MeSi), 0.06 (s, 3H, MeSi), 0.11 (s, 3H, MeSi),
0.12 (s, 3H, MeSi), 0.85 (s, 9H, Me3CSi), 0.91 (s, 9H, Me3CSi),
1.70 (ddd, 1H, H4a, 2J HH 12.8, 3J HH 7.3, 2.3 Hz), 1.94 (ddd, 1H,
i
20-25 °C and was carefully monitored by TLC (20% PrOH/
hexane) to avoid over-reduction. After 3 h the mixture was
filtered on Celite and concentrated to afford a residual oil
which was purified by flash chromatography (gradient eluent
2
3
2
H
4e, J HH 12.5, J HH 7.1, 6.0 Hz), 2.13 (dd, 1H, H6a′, J HH 18.0,
3J HH 4.2 Hz), 2.41 (ddd, 1H, H6e′, J HH 17.9, J HH 4.3, J HH 2.1
Hz), 4.22 (m, 1H, H5), 4.70 (m, 1H, H3), 6.62 (m, 1H, H2), and
9.49 (s, 1H, H7).
2
3
4
i
10-15% PrOH/hexane) to afford 14 mg (70%) of previtamin
37: 1H NMR (CDCl3, 400 MHz) δ 0.71 (s, 3H, Me-18), 0.95 (d,
3
(3R,5S)-3,5-Di[(ter t-bu tyld im eth ylsilyl)oxy]-1-eth yn yl-
cycloh ex-1-en e (32). The same procedure as the one de-
scribed for 24 yielded 32 (70% from 30): 1H NMR (CDCl3, 300
MHz) δ 0.07 (s, 6H, 2MeSi), 0.08 (s, 6H, 2MeSi), 0.89 (s, 9H,
3H, Me-21, J HH 6.5 Hz), 1.21 (s, 6H, Me-26 + Me-27), 1.00-
2.50 (m, 24H), 4.18 (m, 1H, H3), 4.31 (m, 1H, H1), 5.40 (m,
1H), and 5.82 (m, 3H); HRMS calcd for C26H40O2 (M+ - H2O)
384.3028, found 384.3020.
Me3Si), 0.90 (s, 9H, Me3Si), 1.74 (m, 2H, H4), 2.06 (dd, 1H, H6a′
,
1â,25-Dih yd r oxy-3-epi-19-n or -p r evita m in D3 (38). The
same procedure as the one described for 37 yielded 38 (75%):
1H NMR (CDCl3, 400 MHz) δ 0.65 (s, 3H, Me-18), 0.90 (d, 3H,
3
2
3
2J HH 16.1, J HH 6.3 Hz), 2.39 (dd, 1H, H6e′, J HH 16.1, J HH 3.1
Hz), 2.82 (s, 1H, H8), 4.15 (m, 1H, H5), 4.43 (m, 1H, H3), and
6.07 (m, 1H, H2); HRMS calcd for C20H38O2Si2 (M+) 366.2410,
found 366.2411.
3
Me-21, J HH 6.6 Hz), 1.16 (s, 6H, Me-26 + Me-27), 0.90-2.50
(m, 24H), 4.06 (m, 1H, H3), 4.41 (m, 1H, H1), 5.35 (m, 1H),
and 5.79 (m, 3H); HRMS calcd for C26H42O3 (M+) 402.3134,
found 402.3125.
1R,25-Dih yd r oxy-6,7-d e h yd r o-3-ep i-19-n or -p r e vit a -
m in D3 (35). CuI (4 mg, 0.02 mmol), Pd(PPh3)2(OAc)2 (5 mg,
0.01 mmol) and Et2NH (1 mL) were added to a solution of 7
(105 mg, 0.22 mmol) and 24 (81 mg, 0.24 mmol) in DMF (1
mL). The reaction mixture was stirred at room temperature
for 1 h under nitrogen atmosphere and then poured into water
and extracted with diethyl ether. The combined ether fractions
were washed with brine, dried over Na2SO4, filtered and
concentrated to give 165 mg of crude 33, which was sufficiently
pure for the next step. Although it was possible to purify this
compound by flash chromatography, it decomposed in a few
hours.
Ack n ow led gm en t. Financial support from CICYT
(Spain; project BIO98-0770) and Fundacio´n Pr´ıncipe de
Asturias (“Dr. Severo Ochoa” Award to M.F.) is grate-
fully acknowledged. We also thank the Ministerio de
Educacio´n y Cultura (Spain) for postdoctoral fellowships
(S.F. and M.F.) and Solvay-Duphar (Wesp, Holland) for
the generous gift of vitamin D3, which was used for
preparation of Grundmann’s ketone.
TBAF (1.2 mL of 1.0 M in THF, 1.2 mmol) was added to a
solution of the crude 33 in THF (4 mL) at 0 °C and the reaction
was stirred at room temperature for 12 h. THF was evaporated
and the crude residue was poured into water/EtOAc. The
aqueous layer was extracted with EtOAc and the combined
organic layers were dried (Na2SO4) and evaporated in a
vacuum. Flash chromatography of the crude (85% EtOAc/
Su p p or tin g In for m a tion Ava ila ble: Complete 1H and
13C NMR spectral data in addition to mp, optical rotations,
IR, microanalysis, and MS (and/or HRMS) data. This material
is available free of charge via the Internet at http://pubs.acs.org.
J O000443L