Concise total syntheses of variecolortides A and B through an unusual hetero-Diels-Alder reaction

Article abstract of DOI:10.1002/anie.201006154

A fusion of certain anthraquinones and diketopiperazines is an apt description of the variecolortides, a family of unusual fungal alkaloids. In a new, concise total synthesis of the variecolortides A and B, the natural racemates are obtained highly convergently and almost without protecting-group manipulations. The spirocyclic core is generated in a hetero-Diels-Alder reaction of a 1,4-anthraquinone with a didehydrodiketopiperazine.

Full text of DOI:10.1002/anie.201006154

Communications
DOI: 10.1002/anie.201006154
Natural Product Synthesis
Concise Total Syntheses of Variecolortides A and B through an
Unusual Hetero-Diels–Alder Reaction**
Christian A. Kuttruff, Hendrik Zipse,* and Dirk Trauner*
Speculations on the biosynthetic origin of natural products
continue to inspire synthetic chemists and have led to many
elegant and efficient total syntheses.[1] Many of these involve
cascade reactions, where a high-energy substrate is formed
and then spontaneously reacts further to create increasingly
more complex products.[2] As such, biomimetic strategies do
not rely on the use of sophisticated enzymes, which are
usually, but not always, required in a “real” biosynthesis.
We recently published a synthesis of the dopamine-
derived alkaloids exiguamine A and B (2a, b) that relied on
a biomimetic pericyclic reaction and oxidation cascade
(Scheme 1).[3] The exiguamines occur as racemates in nature
and feature a spirobicyclic N,O-acetal as a key structural
element. It was therefore unsurprising that the variecolortides
(1a–c), a newly disclosed family of racemic N,O-acetals,
piqued our interest.[4] These unusual fungal natural products
were isolated from a halotolerant strain of the fungus
Aspergillus variecolor and were shown to have modest
cytotoxic effects.[4]
Structurally, the variecolortides feature an unprecedented
9,10-anthraquinone methide moiety fused to a dihydropyran
(Scheme 1). This flat tetracyclic ring system is further linked
to a diketopiperazine, generating a spirocyclic N,O-acetal.
The central diketopiperazine is also coupled to an indole,
which is reverse-prenylated at C26. Variecolortide A (1a) is
the most complex member of the family, owing to the
additional prenyl group at C22 of the indole nucleus, which is
lacking in variecolortide B (1b) and its methyl ether varie-
colortide C (1c).
Biosynthetically, the variecolortides appear to stem from
a C16 polyketide, the amino acids serine and tryptophan, and
one or two equivalents of dimethylallyl pyrophosphate
(Scheme 2). As such, they represent a unique merger of
three major streams of biosynthesis: the shikimic acid path-
way (for the aromatic amino acid tryptophan), the type II
polyketide pathway (for the anthraquinone derivative), and
Scheme 1. Racemic natural products featuring N,O-acetals.
[*] C. A. Kuttruff, Prof. Dr. H. Zipse, Prof. D. Trauner
Department of Chemistry and Pharmacology
and Center of Integrated Protein Science (CIPSM)
Ludwig-Maximilians-Universitꢀt
Butenandtstrasse 5–13 (F4.086), 81377 Munich (Germany)
Fax: (+49)89-2180-77972.
E-mail: dirk.trauner@lmu.de
zipse@cup.uni-muenchen.de
[**] We thank the CIPSM for financial support. C.A.K. is grateful to the
Fonds der Chemischen Industrie for a PhD scholarship. We thank
undergraduate researchers Simon Geiger and Andreas Fetzer. We
also thank Dr. Eddie Myers for proofreading this manuscript and
Mesut Cakmak and Anastasia Hager for insightful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006154.
Scheme 2. Biosynthetic analysis of the variecolortides. PP=diphos-
phate.
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Angew. Chem. Int. Ed. 2011, 50, 1402 –1405

the terpenoid pathway (for the prenyl and reverse-prenyl side
chains). While many natural compounds are known that
integrate three or more different biosynthetic pathways, this
particular combination appears to be rare if not unknown.
Natural products that exhibit a subset of the structural
features of the variecolortides, however, are well known.
Several decades ago, Laatsch and Anke reported the struc-
tures of emodin anthrone (4) and hydroxyviocristin (5), an
anthrone and a 1,4-anthraquinone, respectively, that bear a
substitution pattern corresponding to the variecolortides
(Scheme 2).[5] These compounds were isolated from Asper-
gillus cristatus. The unsaturated diketopiperazine isoechinu-
lin B (3a)[6] was obtained from Aspergillus ruber, along with
several congeners that lack a prenyl moiety or in which the
exo-methylene moiety is oxidatively cleaved, hydrogenated,
or masked as a methanol adduct.[7] At the outset of our studies
it was not clear, however, how these natural products, which
have not been reported to occur in Aspergillus variecolor,
would come together to form the variecolortides. We now
report a concise total synthesis of 1a and 1b that offers a
surprisingly simple solution for the linkage of the anthraqui-
none and diketopiperazine components and incorporates a
new type of Diels–Alder reaction[8] that could be biosynthet-
ically relevant.
Our total synthesis of the variecolortides started with the
preparation of hydroxyviocristin (5; Scheme 3). Deprotona-
tion of the known orsellinic acid derived anhydride 6,[9]
Scheme 3. Total synthesis of hydroxyviocristin (5).
followed by addition of the resultant benzylic anion to
chloro para-benzoquinone 7 resulted in a conjugate addi-
tion/decarboxylation sequence to give the known 1,4-anthra-
quinone 8.[10] Subsequent demethylation of this material in
molten AlCl3/NaCl yielded hydroxyviocristin (5).
Scheme 4. Total synthesis of isoechinulin B (3a) and neoechinulin B
(3b). BBN=9-borabicyclo[3.3.1]nonane, Cbz=benzyloxycarbonyl,
DIPEA=N,N-diisopropylethylamine, EDCI=N’-(3-dimethylamino-
propyl)-N-ethylcarbodiimide, HOBt=1-hydroxybenzotriazole, NCS=
N-chlorosuccinimide.
The synthesis of the diketopiperazine–indole component
started with palladium-catalyzed cross-coupling of 5-bro-
moindole (9) with tributylallylstannane, followed by Grubbs
olefin cross-metathesis to give prenylated indole 11
(Scheme 4). Installation of the remaining prenyl group in
the reverse sense was achieved by subjecting 11 to Danishef-
sky conditions to give 12.[11] Intermediate 12 was then
formylated with the Vilsmeier reagent to give 13. In order
to incorporate the amino acid portion, carbobenzoxy-pro-
tected glycine 14 was condensed with serine methyl ester (15)
to afford dipeptide 16, which after removal of the Cbz group,
underwent cyclization and formal elimination of water to
afford exo-methylene diketopiperazine 18. Following a pro-
tocol developed by Kishi et al., we condensed this material
with formyl indole 13 to afford isoechinulin B (3a) as a single
isomer.[12] An analogous sequence starting from indole (19!
20!21) provided neoechinulin B (3b; Scheme 4).
With both hydroxyviocristin (5) and the echinulins (3a,
3b) in hand, we were in a position to investigate the critical
coupling of these components. Our original plan had called
for the use of nucleophilic or radical additions using either 5
or emodin anthrone (4) as the polyketide building block.
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Communications
However, this strategy was eventually abandoned because of
is slightly less stable than 8 by 16.7 kJmolꢀ1, but significantly
more reactive with respect to the Diels–Alder reaction with
dienophile 18. The reaction barrier now amounts to a mere
+ 74.2 kJmolꢀ1, and formation of the cycloaddition product
24 is exothermic by 49.8 kJmolꢀ1. Owing to the considerable
differences in reaction energetics and reaction barriers, it is
clear that only cycloaddition through tautomer 22 is relevant
under the experimental conditions.
the poor reactivity of exo-methylene diketopiperazines
towards anionic nucleophiles and our inability to control the
radical additions of emodin anthrone. We therefore turned
our attention to a cycloaddition strategy, which was initially
explored experimentally and computationally with a model
system (Scheme 5). We reasoned that hydroxyviocristin (5)
The calculated geometry of the transition state of this
reaction is depicted in Figure 1. Our calculations also show
that the exo-methylene diketopiperazine 18 functions as the
nucleophilic component in an asynchronous cycloaddition,
wherein the carbon–carbon bond is formed to a larger extent
than the carbon–oxygen bond in the transition state.
Figure 1. Calculated transition state of the asynchronous hetero-Diels–
Alder reaction involving 22 and 18. The lengths of the forming bonds
and the partial charge q(NPA) of the heterodienophile are indicated.
NPA=natural population analysis.
Scheme 5. A model system for the key cycloaddition. The relative
energies of key intermediates and activation barriers are indicated in
brackets (see text). o-DCB=ortho-dichlorobenzene, TS[4+2]=
transition state of the [4+2] cycloaddition.
In order to assess the feasibility of the proposed oxidation
of 24 to give the isolated reaction product 25, the stability of
ꢀ
the bisbenzylic C H bond indicated in Scheme 5 was
calculated using a series of reference compounds at the
G3B3 level of theory. Using appropriate isodesmic reactions
ꢀ
ꢀ
and its dimethyl ether 8 could undergo an intramolecular 1,5-
hydrogen shift to afford the quinone methide tautomer 22.
This reactive intermediate would be prone to undergo a
hetero-Diels–Alder cycloaddition with exo-methylene dike-
topiperazine 18 to afford spiro-N,O-acetal 24. Being an
anthrone, such an intermediate exhibits an extremely labile
the C H bond dissociation energy (BDE) for this C H bond
amounts to + 315.9 kJmolꢀ1 (see the Supporting Information
for details). This value is even less than that found in common
ꢀ
reducing agents such as 1,4-cyclohexadiene [BDE(C H) = +
318.0 kJmolꢀ1)], HSnBu3 [BDE(Sn H) = + 328.9 kJmol ],
ꢀ1
ꢀ
ꢀ1
ꢀ
and thiophenol [BDE(S H) = + 335.4 kJmol ] and thus in
ꢀ
benzylic C H bond and would undergo rapid oxidation in the
full support of the in situ oxidation pathway proposed
presence of air to yield the 9,10-anthraquinone methide 25.
Indeed, when 8 and 18 were heated together in a sealed tube
with an aerobic headspace, 25 was isolated as the only
identifiable product (Scheme 5). This compound corresponds
to the upper portion of the variecolortides and could even be
an intermediate in their synthesis.
above.[13]
Armed with these insights and having optimized our key
step with model compounds, we proceeded to complete the
syntheses of the variecolortides (Scheme 6). We anticipated
that the disubstituted exo-methylene moiety in the echinulins
would be the most reactive heterodienophile and that the
correct regioisomers would be formed. We were pleased to
find that heating of hydroxyviocristin (5) and isoechinulin B
(3a) in ortho-dichlorobenzene indeed afforded variecolorti-
de A in 48% yield. Similarly, variecolortide B was obtained
from building blocks 3b and 5. The modest yields of these key
reactions probably reflect the known instability of hydrox-
yviocristin.[5] The variecolortides were the only isomers
isolated under these conditions; no regioisomers and no
Our proposed Diels–Alder/oxidation mechanism is sup-
ported by density functional calculations performed at
B3LYP/6-31G(d) level (see Scheme 5 and Figure S1 in the
Supporting Information). According to these calculations,
tautomer 8 reacts with 18 through a concerted, asynchronous
pathway to yield cycloaddition product 23. This process is
considerably endothermic (by 55.1 kJmolꢀ1) and faces a large
reaction barrier of + 133.7 kJmolꢀ1. By contrast, tautomer 22
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calculations strongly support our hypothesis that this key
reaction proceeds through a concerted cycloaddition. Our
synthesis also raises questions about whether a similar step
could occur in nature and whether an enzyme is involved.
Answering these questions will require detailed biosyn-
thetic studies, which are beyond the scope of the present
work. Meanwhile, our laboratory synthesis provides ample
access to the variecolortides and allows for a full biological
exploration of these fascinating natural products.
Received: October 1, 2010
Revised: October 29, 2010
Published online: December 29, 2010
Keywords: alkaloids · biomimetic synthesis ·
.
cascade reactions · cycloaddition · total synthesis
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Scheme 6. The total synthesis of variecolortide A and B.
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reaction with other double bonds present in the echinulins
were observed.
While the conditions employed in our key reaction can
certainly not be deemed biomimetic, the concerted nature of
the reaction does raise some interesting biosynthetic ques-
tions. Preliminary experiments under more biological con-
ditions (aqueous phosphate buffer at ambient temperature)
have failed to yield any identifiable products. Given these
results, and the calculated activation barriers, it appears that
the variecolortides are not the products of an adventitious,
uncatalyzed Diels–Alder reaction occurring in the fungus.
This raises the interesting possibility that a “Diels–Alderase”
is involved.[14] Also, while it is entirely conceivable that 5 and
3a/3b are the true biosynthetic precursors of the variecolor-
tides, the possibility that the linkage takes place before the
polyketide moiety is released from a type II polyketides
synthase must also be considered. The exact sequence in
which the individual biosynthetic components of the varieco-
lortides come together, and the mechanism of their fusion
remain to be determined.
In summary, we have developed a concise total synthesis
of variecolortide A and B that provides the natural racemates
in seven and five steps, respectively (longest linear sequence).
Our synthesis is largely devoid of protecting-group opera-
tions[15] and is highly convergent. It incorporates an unprece-
dented hetero-Diels–Alder reaction of a 1,4-anthraquinone
with a didehydrodiketopiperazine to form the central spiro-
cyclic core of the natural products. Density functional theory
[15] I. S. Young, P. S. Baran, Nat. Chem. 2009, 1, 193 – 205.
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