Total synthesis of vinigrol

Article abstract of DOI:10.1002/anie.201304624

Carbocyclic cage fight: The substrate-controlled total synthesis of vinigrol features a strategic oxidative dearomatization/Diels-Alder cycloaddition reaction and a subsequent palladium-catalyzed cyclization cascade to construct the carbocyclic core. The C4, C9, and C12 stereocenters were installed using either reduction or oxidation reactions, and the diterpenoid core was unraveled by a ring fragmentation reaction. Copyright

Full text of DOI:10.1002/anie.201304624

Angewandte
Chemie
Communications
Natural Products Synthesis
Q. Yang, J. T. Njardarson,* C. Draghici,
F. Li
&&&& — &&&&
Total Synthesis of Vinigrol
Carbocyclic cage fight: The substrate
controlled total synthesis of vinigrol fea-
tures a strategic oxidative dearomatiza-
tion/Diels–Alder cycloaddition reaction
and a subsequent palladium-catalyzed
cyclization cascade to construct the
carbocyclic core. The C4, C9, and C12
stereocenters were installed using either
reduction or oxidation reactions, and the
diterpenoid core was unraveled by a ring
fragmentation reaction.
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
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DOI: 10.1002/anie.201304624
Natural Products Synthesis
Total Synthesis of Vinigrol**
Qingliang Yang, Jon T. Njardarson,* Cristian Draghici, and Fang Li
Diterpenoids are an important and structurally diverse class
of natural products. In 1987, Hashimoto and co-workers
reported a new diterpenoid with a unique bridged bicyclic
core densely decorated with eight stereocenters.[1] The
authors named this new natural product vinigrol. As evident
from the three structural perspectives shown in Figure 1,
Figure 1. Vinigrol structural perspectives.
Figure 2. Vinigrol retrosynthesis.
vinigrolꢀs architecture is quite striking. Most notable is the
axial four-carbon tether which bridges the two six-membered
rings of the decalin in such a way that a very rigid and compact
framework results. Vinigrol has been evaluated in numerous
biological assays and shown to impact platelet aggregation,[2]
and act as a tumor necrosis factor (TNF) antagonist,[3] as well
as displaying other interesting properties.[4] Vinigrolꢀs unpre-
cedented structure and intriguing biological profile have
prompted numerous attempts at its synthesis.[5] In 2009,
Baran[6] and co-workers reported the first total synthesis of
vinigrol and a few years later Barriault[7] published a formal
synthesis of vinigrol.
direct installation of the C8 and C9 methyl stereocenters, and
would be assembled in two steps by dearomatization of the
pyrogallol ether 5. The components for the oxidative dearo-
matization precursor (5) would be synthesized from the
known compounds 7[9] and 8.[10]
Synthesis of the oxidative dearomatization precursor is
presented in Scheme 1. After iodination of 8, the terminal
olefin was selectively dihydroxylated (9) and oxidatively
cleaved to afford the aldehyde 10. Horner–Wadsworth–
Emmons olefination with the phosphonate 11[11] yielded the
Although details of our strategy have evolved over time,[8]
we have remained faithful to several design features. The
following reactions have been and still are critical to our
vinigrol synthesis plan: 1) one-pot oxidative dearomatization/
Diels–Alder reaction, 2) tandem 6-exo-trig cyclizations to
form the carbocyclic core, 3) fragmentation to unravel the
bridged bicyclic architecture, and 4) use of the rigid carbocy-
clic core to install stereocenters in a substrate-controlled
manner. Our retrosynthetic analysis for vinigrol is outlined in
Figure 2. Initial plans called for a late-stage carbanion-
mediated fragmentation of the rigid cagelike structure 2,
wherein the functional groups for this key reaction would
originate from the ketal 3. The cage architecture (4) would
[*] Prof. J. T. Njardarson, Dr. F. Li
Department of Chemistry and Biochemistry, University of Arizona
1306 E. University Blvd., Tucson, AZ 85721 (USA)
E-mail: njardars@email.arizona.edu
Q. Yang, Dr. C. Draghici
Department of Chemistry and Chemical Biology, Cornell University
Baker Laboratory, Ithaca, NY 14853-1301 (USA)
[**] We thank the NIH-NIGMS (RO1 GM086584) for generous financial
support of this program.
Scheme 1. Synthesis of oxidative dearomatization precursor. DIAD =
diisopropylazodicarboxylate, DIBAL=diisobutylaluminum hydride,
NMO=N-methylmorpholine-N-oxide, NIS=N-iodosuccinimide,
THF=tetrahydrofuran.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304624.
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Chemie
enoate 12, which was then reduced to the corresponding
allylic alcohol. Attachement of the aromatic moiety (7) was
accomplished using a Mitsunobu reaction. To set the stage for
the key oxidative dearomatization reaction, the carboxylate
needed to be converted into a free phenol and the adjacent
hydroxy group protected in such a way that it was electroni-
cally deactivated to guide the oxidative dearomatization
reaction towards the more hindered site and suppress
unwanted opening of the intermediate acetal. These goals
were accomplished by first treating 14 with DIBAL and then
protecting the free phenol as b,b,b-trifluoroethyl ether (15).
Dakin oxidation then converted 15 into the phenol 16.[12]
After optimization of the oxidative dearomatization
reaction, we were rewarded with high yields of intramolecular
Diels–Alder cycloadduct 17 (Scheme 2). A Heck cyclization
Scheme 3. Fragmentation—synthesis of the vinigrol core.
DMP=Dess–Martin periodinane, m-CPBA=meta-chloroperbenzoic
acid, Ms=methanesulfonyl.
iridium catalyst first coordinates to the furan oxygen atom
prior to hydrogen transfer.
We next turned our attention to opening up the tetracyclic
cage by fragmentation (Scheme 3). After a few unsuccessful
attempts at carbanion fragmentation, we chose to pursue
a Wharton fragmentation (27!28) strategy. Towards that
end, the acetal 23 was opened and the resulting alcohol
oxidized to an aldehyde (24), which we found could be
oxidized selectively using a Baeyer–Villiger reaction and then
exhaustively reduced (25). Although 25 is potentially com-
petent for fragmentation, the C7 epimer would be better
aligned. Inversion[15] and derivatization of the secondary
alcohol as a mesylate (27) was straight forward. We were
delighted to find that the desired Wharton fragmentation[16]
reaction proceeded in high yield to produce the vinigrol core
(28).
To complete the total synthesis of vinigrol we needed to
address three final challenges: 1) convert the C12 ketone into
an isopropyl group, 2) add the C4 hydroxy group, and
3) deprotect the C8a trifluoroethyl ether. We started our
endgame journey by first tackling the C12 isopropyl group
installation (Scheme 4).
Scheme 2. Synthesis of the carbocyclic core of vinigrol.
cascade afforded the carbocyclic core of vinigrol (18) in only
two steps from 16. During the Heck cyclization 11% of the
product isomerized to a trisubstituted olefin isomer of 18. This
was inconsequential for the next step as both isomers were
expected to afford 19 in the subsequent step. We next turned
our attention to the installation of the congested C9 and
C8 stereocenters. Hydrogenation of the carbocyclic cage
predictably afforded the C9 methyl stereocenter (19). The
C8 methyl stereocenter proved far more challenging to install.
All attempts at converting the C8 ketone of 19 into a meth-
ylene group (22) using either direct olefination or addition/
dehydration strategies failed. This problem was finally solved
by first converting 19 into 20, and derivatizing the resulting
alcohol[13] as a xanthate (21) which was then subjected to
Chugaev elimination (22). Unfortunately, when the exo olefin
of 22 was subjected to classical heterogeneous hydrogenation
conditions only the undesired C8 epimer was obtained.
Remarkably, this problem could be solved by using iridium-
catalyzed directed hydrogenation (23).[14] Presumably, the
After hydrogenation of 28, we added a vinyl cerium
reagent[17] to 29. The complete selectivity in this addition step
is the result of steric control. Deoxygenation or dehydration
of the tertiary hydroxyl group in 30 proved tricky as unwanted
Grob fragmentation of the trifluoromethyl ether proved quite
facile. This problem was finally alleviated by employing
Burgessꢀ reagent for the dehydration (31).[18] Conjugate
additions and reductions of the enone afforded primarily
the undesired C12 epimer of the methyl ketone. By perform-
ing a hydrogenation in the presence of potassium hydroxide
the desired thermodynamically more stable C12 epimer (32)
could be accessed selectively.[19] After a Wittig olefination
(33) and reduction the installation of the C12 isopropyl group
was completed (34). We next turned out attention to the
C4 hydroxy group. Removal of the methyl ether was accom-
plished using selenium dioxide (35),[20] and set the stage for
substrate-controlled installation of the C4 hydoxy group by
a directed epoxidation (36). Conversion of the primary
alcohol into an iodide and subsequent reductive zinc-medi-
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gic applications and deprotection of a trifluoroethyl ether. We
are currently using the lessons learned from this synthetic
journey to streamline the synthesis.
Received: May 29, 2013
Published online: && &&, &&&&
Keywords: dearomatization · hydrogenation · natural products ·
.
terpenoids · total synthesis
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Diels–Alder reaction, which coupled with a Heck cyclization
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steps from a simple precursor. Efforts are underway to render
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