Total synthesis of coralloidolides A, B, C, and E

Article abstract of DOI:10.1002/anie.200906126

"Chemical Equation Presented" Mapping the matrix: Several coralloidolides, members of a Mediterranean branch of the furanocembranoid family of diterpenes, have been synthesized. The total syntheses include biomimetic transformations that often occur with high chemoselectivity, thus obviating the need for protecting-group manipulations. The fascinating reactivity of 2,5-diene-1,4dione moieties was explored in detail.

Full text of DOI:10.1002/anie.200906126

Angewandte
Chemie
DOI: 10.1002/anie.200906126
Biomimetic Synthesis
Total Synthesis of Coralloidolides A, B, C, and E**
Thomas J. Kimbrough, Paul A. Roethle, Peter Mayer, and Dirk Trauner*
Dedicated to Klaus Rꢀmer on the occasion of his 70th birthday
The furanocembranoids are a steadily growing family of
natural products that beautifully demonstrate how nature
generates complexity and biological activity through oxida-
[
1]
tive diversification of low-oxidation-state precursors. The
biosynthesis of these diterpenoids involves the formation of a
1
4-membered cembrane ring from geranylgeranyl pyrophos-
phate, followed by oxidative transformations that initially
install furan and the butenolide moieties. These heterocycles
often engage in further oxidative processes, which underlie
the remarkable skeletal and biological diversity of the
furanocembranoids.
[
2]
[3]
In previous publications, we and others have identified
rubifolide (1) (Scheme 1) as a possible biosynthetic precursor
of bipinnatin J (2) and numerous other complex diterpenoids,
[4]
such as intricarene and bielschowskysin. We have now
expanded this proposed set of biosynthetically related
molecules to include the coralloidolides, a family of diterpe-
noids isolated from the alcynoacean coral Alcyonium cor-
[
5]
alloides by Pietra et al. As such, they were the first
furanocembranoids to be found in a Mediterranean organism,
in contrast to most other members of the family, which are of
Caribbean origin.
It is intriguing to speculate that the coralloidolides are
naturally derived from rubifolide (1). Rubifolide (1) has been
found in other tropical corals, such as Gersemia rubiformis, as
well as in a nudibranch, Tochuina tetraquetra, but it has not
[
6]
been isolated from A. coralloides. In the biosynthesis
1
1,12
presumably epoxidation of the electrophilic D
double
bond yields coralloidolide A (3). Oxidative cleavage of the
furan ring of 3 then affords coralloidolide E (4), which
features a prominent 2,5-diene-1,4-dione moiety (Scheme 1).
This functional group lends itself to several alternative
reaction pathways, resulting in the formation of other
members of the coralloidolide family. In the first of these,
hydration of the dienedione functionality and transannular
opening of the epoxide in 4 would give the tetracyclic
coralloidolide B (5). It is conceivable that this intricate bis-
Scheme 1. Rubifolide and its proposed biosynthetic relations with
bipinnatin J and the coralloidolides.
acetal could rearrange to yield coralloidolide D (6). Alter-
natively, selective tautomerization of 4 and double-bond
isomerization could afford dienone enol 7, which could
undergo transannular aldol addition to the C6 carbonyl to
afford coralloidolide F (8). Finally, a second mode of tauto-
merization and aldol addition (via 9), followed by shifting of
the double bond to the thermodynamically more stable
[*] Dr. P. Mayer, Prof. D. Trauner
Department of Chemistry, University of Munich
Butenandtstrasse 5–13 (F4.086), 81377 Munich (Germany)
Fax: (+49)892-180-77972
E-mail: dirk.trauner@cup.uni-muenchen.de
T. J. Kimbrough, Dr. P. A. Roethle
Department of Chemistry, University of California Berkeley
Berkeley, CA 94720 (USA)
[7]
position, would afford coralloidolide C (10).
We have recently described a short synthesis of racemic
[
2a]
bipinnatin J (2) and its near-quantitative transformation to
[
**] This work was supported by a Novartis Young Investigator Award.
We thank Dr. Michele D’Ambrosio (Universitꢀ di Trento) for
providing us with authentic samples of coralloidolides.
[
2b]
rubifolide (1) (Scheme 2). Our efficient synthetic approach
puts us in a position to test the proposed biosynthetic
relationships in the laboratory in depth and identify condi-
tions for the selective interconversion of the coralloidolides.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906126.
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Communications
We were able to obtain an X-ray crystal structure of 4,
which provided useful insights into the mechanism and
diastereoselectivity of this transformation. As can be seen in
Scheme 3a, the molecule adopts a conformation, wherein the
Scheme 2. Total synthesis of coralloidolides A, B, C, and E.
Given the multitude of electrophilic positions in 2,5-diene-
,4-diones and the range of possible enol forms they can form,
these investigations proved to be a challenging exercise in
chemoselective synthesis.
Scheme 3. X-ray crystal structure of 4 (a) and diastereoselectivity of
the transannular cyclizations (b).
1
We now report the synthesis of coralloidolides A, B, C,
and E by means of selective oxidations and transannular
cyclizations. Our sequence starts with a chemoselective
nucleophilic epoxidation of rubifolide (1), which provided
coralloidolide A (3) as a single diastereomer (Scheme 2).
Oxidative cleavage of the furan ring in 3 with mCPBA
proceeded with a similar degree of chemoselectivity and
afforded coralloidolide E (4). This key compound was then
subjected to numerous reaction conditions to explore its
transformations into other members of the family. In most
cases, this led to the consumption of starting material and
formation of a large number of intractable products. After
extensive experimentation, however, we found that treatment
of 4 with scandium triflate in its hydrated form in dioxane led
C4–C8 segment of the dienedione moiety of 4 is fully planar.
The remaining C3 carbonyl group resides in an almost
perpendicular orientation. (The dihedral angle C5-C4-C3-O
is ꢀ1068.) Although this places the oxygen of the C6 carbonyl
group in close proximity (within 3.2 ꢀ) to the electrophilic
C11 of the epoxide, the oxocarbenium ion 11 resulting from
direct nucleophilic attack would afford an exceedingly
strained isomer 12 upon hydration. Therefore, it appears
more likely that the transannular epoxide opening proceeds
in a stepwise fashion via the hydrate 13, as originally
[
5a]
suggested by Pietra et al.
It is plausible that scandium
triflate plays a twofold role in this process, catalyzing both the
initial hydration of the dienedione and the subsequent
intramolecular nucleophilic attack of the resulting diol 13 to
afford coralloidolide B (5).
Our attempts to affect transannular aldol additions en
route to coralloidolides F (8) and C (10) proved equally
challenging. After considerable experimentation, we found
that treatment of 4 with a large excess of 1,8-diazabicyclo-
[
8]
to its clean conversion into coralloidolide B (5). Interest-
ingly, this reaction proceeds well in only dioxane as solvent;
attempts to carry out the reaction in acetone, DMF, or a
mixture of dioxane and water reduced the reaction rates and
gave significantly diminished yields.
2
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[
5.4.0]undec-7-ene (DBU) gave coralloidolide C (10) in
acetate (!17) or water, followed by Paal–Knorr-type furan
modest yield. Since several intermediates were observed in
the course of this reaction by thin-layer chromatography, and
since all of them ultimately converged to coralloidolide C
formation. The exo-methylene isomer 16 has been previously
observed upon dissolution of coralloidolide E (4) in
[D ]pyridine and appears to be readily formed under a
5
[5c]
(
10), it appears that 10 is the thermodynamic minimum of the
variety of acidic and basic conditions.
Given its facile
series of natural products. Treatment with other bases,
including LiHMDS, LDA, excess triethylamine, and pyridine,
failed to give any aldol addition products.
Again, the conformation of 4 in the crystal can be used to
explain the diastereoselectivity of the successful cyclization,
provided the C3 carbonyl cannot reorient itself during the
reaction owing to the constraints of the macrocyclic ring
formation, it is likely that 16 exists in organisms that produce
coralloidolides and thus qualifies as a genuine natural
product. It is also interesting to speculate whether oxidation
of the C18 methyl group in furanocembranoids can proceed
through the mechanism depicted in Scheme 4 or requires
enzymatic hydroxylation. It should be noted, however, that
hydroxymethylene derivatives of type 19 are rarely, if ever,
observed among furanocembranoids.
(
Scheme 3b). Intramolecular nucleophilic attack of its eno-
late 14 occurs on the face of the C3 carbonyl that is oriented
toward the macrocycle and affords intermediate 15, proto-
nation of which and subsequent double-bond isomerization
then yields coralloidolide C (10).
Other attempts to convert 4 into 8 or 10 through
transannular aldol additions and double-bond isomerizations
have failed but have yielded additional interesting results
The selective oxidation of the C18 methyl group could
also be achieved by other chemical methods. For instance,
treatment of bipinnatin J with excess DDQ gave aldehyde 20
in moderate yield. This provides another example of a highly
selective oxidation of the fascinating furanocembranoid
framework.
In summary, we have reported the first total synthesis of
the furanocembranoids coralloidolides A, B, C, and E.
Racemic coralloidolides B and C were synthesized without
recourse to protecting-group chemistry, each in 13 steps,
starting from the simple materials shown in Scheme 2. Several
highly selective transformations have been discovered, which
will undoubtedly find utility in synthetic approaches toward
other members of the furanocembranoid class of natural
products. Our synthetic work provides insight into the
biogenetic relationships within this family and adds to the
matrix of chemical and biosynthetic relations among furano-
cembranoids.
(
1
Scheme 4). For example, treatment of 4 with acetic acid gave
8-acetoxycoralloidolide A (18) as the only identifiable
product. Similarly, when 4 was subjected to a mixture of
aqueous sulfuric acid and acetone only the corresponding 18-
hydroxycoralloidolide A (19) was isolated. Presumably, these
transformations proceed through the intermediacy of double-
bond isomer 16, which undergoes conjugate addition of either
Received: October 30, 2009
Published online: March 9, 2010
Keywords: biomimetic synthesis · chemoselectivity ·
.
furanocembranoids · Lewis acids · total synthesis
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[
[
[
7] A similar proposal for the formation of coralloidolide C has been
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Scheme 4. Selective functionalizations at C18 of 4.
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