Total synthesis of ouabagenin and ouabain

Article abstract of DOI:10.1002/anie.200704959

(Chemical Equation Presented) The highly oxygenated steroid ouabagenin (1b) and its glycoside ouabain (1a) were prepared by a strategy based on a polyanionic cyclization. Starting building blocks A and B were combined to give the key intermediate C and transformed into 1b in 27 steps. Finally, ouabagenin (1b) was converted into ouabain (1a) in six steps (see scheme).

Full text of DOI:10.1002/anie.200704959

Communications
DOI: 10.1002/anie.200704959
Steroid Synthesis
Total Synthesis of Ouabagenin and Ouabain**
Hongxing Zhang, Maddi Sridhar Reddy, Serge Phoenix, and Pierre Deslongchamps*
Dedicated to Professor Zdenek Valenta on the occasion of his 80th birthday
Ouabain (1a), a cardioactive glycoside isolated from the bark
of the African ouabio tree (Acokanthera ouabio) by Arnaud
in 1888),[1] has received considerable attention since it was
discovered that an ouabain-like compound occurs naturally in
mammals and acts as an endogenous digitalis as proposed by
Szent–Gyorgyi.[2] After some debate, it was established that
the endogenous and plant derived ouabain are in fact
identical.[3] Ouabagenin (1b), the aglycone of ouabain, was
Scheme 1. Synthetic plan for ouabagenin and ouabain.
isolated for the first time in 1942 by Mannich and Siewert,[4]
(1b) and in turn ouabain (1a). Tetracycle D would be readily
available from tricycle C, which in turn could be produced
from the condensation of chiral building blocks A and B.
The initial steps of the synthesis, drawn from our previous
studies on steroid skeleton synthesis,[9–11] were successful
(Scheme 2). Thus, the union of Nazarov substrate 3[10b] with
freshly prepared cyclohexenone 2[11,12] in the presence of
Cs2CO3 at 08C followed by decarboxylation afforded tricycle
4 with an overall yield of 78%. Reduction of the resulting
aldehyde with Li(Et3CO)3AlH[13] and then protection of the
alcohol as its PMB ether[14] afforded aldol precursor 5 in 74%
who proposed a structure for the aglycone, and the structure
for ouabain, which was later proven to be correct.[5]
yield over two steps The aldol reaction to form the desired
tetracycle 6 in 83% yield occurred in the presence of
KHMDS. In order to reduce the ketone at C1 to give the
desired b stereochemistry, it was necessary to first deprotect
the alcohol at C11, by saponification of the acetate, which
assisted the subsequent surprisingly facile reduction of the C1
ketone with NaBH4 in EtOH at ꢀ788C to produce 7 with an
overall yield of 95%.
Oxidation of PMB ether 7 was carried out in a CH2Cl2
solution, which was rigorously dried with molecular sieves
prior to addition of DDQ, to afford orthoester 8 in 84%
yield.[15,16] From orthoester 8, formation of a silyl enol ether
using excess TBSOTf and oxidation to the enone with
DDQ[17] proceeded smoothly to afford the desired enone 9
with an overall yield of 90%. Reduction of the enone by using
sodium borohydride afforded the b-allylic alcohol, which
upon exposure to mCPBA underwent selective b-face epox-
idation to afford epoxide 10 with an overall yield of 81%. The
structure of 10 was ultimately confirmed by X-ray crystallog-
raphy.[18]
Progress towards the construction of these highly oxy-
genated steroids has been made recently,[6–8] but to date no
total synthesis has been reported. Our foray into this field was
based on a hypothesis that the polyanionic cyclization
(double-Michael addition followed by aldol condensation)
methodology developed by our research group[9] would allow
facile access to an appropriately functionalized tetracyclic
intermediate with the desired A/B cis, B/C trans, and C/D cis
ring junctions. Our initial studies[9,10] suggested a promising
synthetic route towards 14-b-OH steroidal intermediates, and
herein we report the completion of the first total synthesis of
ouabagenin (1b) and in turn ouabain (1a).
Our strategy was based on the initial rapid construction of
densely functionalized tetracycle D, (Scheme 1), which con-
tains, in principle, the functionalities required for ouabagenin
[*] Dr. H. Zhang, Dr. M. Sridhar Reddy, Dr. S. Phoenix,
Prof. Dr. P. Deslongchamps
Laboratoire de synth›se organique
Institut de Pharmacologie de Sherbrooke
UniversitØ de Sherbrooke
Sherbrooke, QuØbec, J1H 5N4 (Canada)
Fax: (+1)819-820-6868
Mesylation of the epoxy alcohol was followed by treat-
ment with excess LiBH4 in DME and subsequent hydrolysis
on silica gel, to afford 11. Thus, hydrogenolysis of the
mesylate took place as well as the desired reductive opening
of the epoxide. The resulting orthoester intermediate was
found to be unstable and was therefore hydrolyzed under
mild acidic conditions with silica gel producing mainly the
p-methoxybenzoate at the C1 position.[19] The overall yield for
these operations was 58%. Attempts to protect the C19 and
E-mail: pierre.deslongchamps@usherbrooke.ca
[**] We thank Dr. Vijay Kumar for his assistance in providing starting
materials. We also thank NSERCC (Ottawa) for financial support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Scheme 2. Synthesis of key fragment 14. Reagents and conditions: a) Cs2CO3, CH2Cl2, 08C, 85%; b) [Pd(PPh3)4], morpholine, THF, 92%;
c) Li(Et3CO)3AlH, THF, ꢀ788C, 89%; d) PMBOC(NH)CCl3, La(OTf)3 (0.1 equiv), Et2O, 08C, 83%; e) KHMDS, THF, 638C, 83%; f) K2CO3, THF,
MeOH, 98%; g) NaBH4, EtOH, ꢀ788C, 97%; h) DDQ, 4 Š M.S., CH2Cl2, 84%; i) TBSOTf, Et3N, CH2Cl2, 90%; j) DDQ, 2,6-di-tert-butyl-4-
methylpyridine, MeCN, 100%; k) NaBH4, EtOH, THF, ꢀ308C, 96%; l) mCPBA, NaHCO3, CH2Cl2, 85%; m) MsCl, py, CH2Cl2, 98%; n) LiBH4,
DME, RT, 24 h then reflux 8 h, then silica, 58%; o) Ac2O, py, DMAP, RT, 24 h, 90%; p) TBAF, THF, 08C!RT, 6 h, 88%; q) Hg(OAc)2, AcOH/
AcOOH (1:1), RT, 4 h, 93%; r) TBDPSCl, imidazole, CH2Cl2, 08C, 4 h, 75%; s) Ac2O, py, DMAP, CH2Cl2, 408C, 24 h, 65%. DDQ=2,3-dichloro-5,6-
dicyano-1,4-benzoquinone, DMAP=4-dimethylamino pyridine, DME=1,2-dimethoxyethane, KHMDS=potassium bis(trimethylsilyl)amide,
mCPBA=m-chloroperoxybenzoic acid, M.S.=molecular sieves, Ms=methanesulfonyl, PMB=p-methoxybenzyl, PMP=p-methoxyphenyl, py=pyr-
idine, TBAF=tetra-n-butylammonium fluoride, TBDPS=tert-butyldiphenylsilyl, TBS=tert-butyldimethylsilyl, Tf=trifluoromethanesulfonyl,
THF=tetrahydrofuran.
C11 hydroxy groups of 11 as acetates afforded only the C19
acetate. We considered this to be a suitable stage for
unmasking the C3 hydroxy group. Removal of the TBDPS
groupfollowed by Tamao oxidation [20] of 12 neatly furnished
13 in 93% yield. The primary hydroxy group of 13 was
reprotected as TBDPS ether and the secondary hydroxy
groups as acetates to give 14.
At this stage, with all the required stereochemistry
installed, we wanted to secure completely unambiguous
structure confirmation. For that, we opted for the degradation
of natural ouabain (1a)[21] to obtain the intermediate 14 and
compare it with the synthetic intermediate. Accordingly, the
acidic hydrolysis of natural ouabain[4,6e] and selective acety-
lation[22] of secondary alcohols gave 15 which upon hydrolysis
of the acetonide groupwith HCl in MeOH produced tetraol
16 in 52% yield over three steps (Scheme 3).
Transformation of the primary hydroxy group of 16 into
an acetate and the secondary hydroxy groupinto p-meth-
oxybenzoate gave 17 with an overall yield of 67%. Ozonolysis
of 17 followed by mild hydrolysis (KHCO3 solution) of the
resulting glyoxalate resulted in the somewhat less stable
hydroxy ketone 18. Reduction of 18 using NaBH4 in MeOH
followed by NaIO4-mediated oxidative cleavage of the
resulting 1,2-diol produced aldehyde 19. Again reduction of
17 and protection of the resulting primary hydroxy group as
the TBDPS ether cleanly furnished 14 in 37% overall yield.
This degradation product was identical to the synthetic
material and was used as a relay to complete the total
synthesis of ouabagenin (1b) and in turn ouabain (1a).
Thus, silyl ether cleavage of 14 with TBAF in THF and
then oxidation of the resulting primary hydroxy group again
gave aldehyde 19, which was transformed into 20 by rhodium-
catalyzed methylenation (Scheme 4).[23] After dihydroxyl-
ation of the olefin moiety in 20 with OsO4 and NMO, the
selective oxidation of the secondary hydroxy groupto afford
18 was achieved with NBS through the cyclic tin ether 21.[24]
Construction of the butenolide ring to obtain 17 was achieved
by exposing hydroxyketone 18 to triphenyl phosphorany-
lidene ketene[25] and subsequent hydrolysis of the ester groups
gave ouabagenin (1b). Since literature data needed to
confirm the structure is limited,[4,5,26] we decided to obtain a
pure sample of the natural product for an unambiguous
comparison. Ouabagenin acetonide 22,[6e,22] which was
obtained by degradation of ouabain (1a) was hydrolyzed by
using conc. HCl in MeOH to provide an authentic sample of
ouabagenin (1b). For further confirmation of the structure it
was then reprotected to give ouabagenin acetonide 22 by
using conc. HCl in acetone. The synthetic ouabagenin was
identical with the one obtained from degradation.[27]
Completion of the total synthesis of ouabain (1a) is
described in Scheme 5. Accordingly, we required suitably
functionalized coupling partners 25 and 26. Thus, perbenzo-
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Scheme 5. Preparation of glycoside partner 25 and completion of the
total synthesis of ouabain (1a). Reagents and conditions: a) BzCl, py,
DMAP, CH2Cl2, RT, 82%; b) AcBr, MeOH, CH2Cl2, RT; c) Ag2CO3,
acetone, H2O, RT, 58% over 2 steps; d) K2CO3, Cl3CCN, CH2Cl2, RT,
68%; e) Ac2O, py, DMF, DMAP, 508C, 78%; f) 0.5n Na2CO3, MeOH,
1 h, RT, 70%; g) TMSOTf, 4 Š M.S., CH2Cl2, RT, 90%; h) 2n HCl,
MeOH, RT, 2 h, 92%; i) 0.5n Na2CO3, MeOH, 2 h, RT, 88%. Ac=ace-
tyl, Bz=benzoyl.
Scheme 3. Degradation of natural ouabain. Reagents and conditions:
a) Conc. HCl, acetone, 08C, 10 days, 70%; b) Ac2O, py, DMF, 508C,
48 h, 83%; c) 1n HCl/MeOH (1:4), 36 h, 90%; d) Ac2O, py, CH2Cl2,
08C!RT, 6 h, 89%; e) p-anisoyl chloride, py, DMAP, CH2Cl2, 408C,
18 h, 76%; f) O3, CH2Cl2, ꢀ788C, 2 h, then Ph3P, RT, 15 h; g) KHCO3,
MeOH:H2O (1:1), RT, 3 h; h) NaBH4, MeOH, 08C, 30 min, 51% over
3 steps; i) NaIO4, EtOH/H2O (95:5), RT, 1 h, 82%; j) NaBH4, MeOH,
08C, 30 min, 88%; k) TBDPSCl, imidazole, CH2Cl2, 08C!RT, 4 h,
90%. DMF=N,N-dimethylformamide.
because our initial glycosylation trials of 22 were not
regioselective.
With both building blocks 25 and 26 in hand, our next task
was to tether the two parts of the molecule through an acetal
bridge. Thus coupling of 25 and 26 was carried out using
TMSOTf in CH2Cl2 at 08C to obtain 27 as the exclusive
isomer in 90% yield. Not unprecedented, the pleasing
outcome with glycosylation was a result of anchimeric
assistance.[28] For the global deprotection, we first treated 27
with mild acidic and then with mild basic conditions (the
reverse treatment did not work out to our satisfaction) to
furnish ouabain (1a) in 80% yield over the last two steps. The
synthetic compound was identical with an authentic sample of
the natural material.[21,27]
In conclusion, we have successfully completed the long-
awaited first total synthesis of ouabagenin (1b) and in turn
ouabain (1a) through a polyanionic cyclization strategy via a
key tetracyclic intermediate 14 in 19 steps. This in turn led to
the preparation of ouabagenin (1b) in eight steps. Finally,
ouabagenin (1b) was converted into ouabain (1a) in six steps.
Scheme 4. Completion of the synthesis of ouabagenin (1b). Reagents
and conditions: a) TBAF, THF, 08C!RT, 2 h, 90%; b) Dess–Martin
periodinane, CH2Cl2, 08C, 40 min, 75%; c) [(PPh3)3RhCl], PPh3, iPrOH,
TMSCHN2, THF, 16 h, 67%; d) OsO4, NMO, acetone/H2O (95:5), 6 h,
82%; e) nBu2SnO, benzene, reflux, 12 h; f) NBS, CHCl3, 10 min, 73%
over 2 steps; g) Ph3PCCO, TEA, benzene, RT, 12 h, 68%; h) 0.5n
Na2CO3, MeOH, 2 h, RT, 85%; i) Conc.HCl, MeOH, 508C, 4 h, 88%;
j) Conc.HCl, acetone, RT, 4 h, 80%. NBS=N-bromosuccinimide,
NMO=4-methylmorpholine N-oxide, TEA=triethylamine, TMS=tri-
methylsilyl.
Received: October 26, 2007
Published online: January 8, 2008
Keywords: cardioactive agents · cycloaddition · glycosylation ·
.
steroids · total synthesis
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