First total synthesis of epicoccarine A via O- to C-acyl rearrangement strategy

Article abstract of DOI:10.1021/ol3024506

The first total synthesis of antibacterial epicoccarine A isolated from a fungus Epicoccum sp. has been accomplished in 10 steps along with synthetic elaboration of its C5-epimer, highlighting the utility of O- to C-acyl rearrangement of a 4-O-acyltetramic acid derivative. Comparison of spectroscopic properties and specific optical rotations of the synthetic samples with those reported for authentic material has clearly indicated the unspecified absolute stereochemistry of this natural product to be 5S.

Full text of DOI:10.1021/ol3024506

ORGANIC
LETTERS
2012
Vol. 14, No. 19
5142–5145
First Total Synthesis of Epicoccarine A via
O- to C-Acyl Rearrangement Strategy
Yasuaki Ujihara, Ken Nakayama, Tetsuya Sengoku, Masaki Takahashi, and
Hidemi Yoda*
Department of Materials Science, Faculty of Engineering, Shizuoka University,
3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8561, Japan
tchyoda@ipc.shizuoka.ac.jp
Received September 4, 2012
ABSTRACT
The first total synthesis of antibacterial epicoccarine A isolated from a fungus Epicoccum sp. has been accomplished in 10 steps along with
synthetic elaboration of its C5-epimer, highlighting the utility of O- to C-acyl rearrangement of a 4-O-acyltetramic acid derivative. Comparison of
spectroscopic properties and specific optical rotations of the synthetic samples with those reported for authentic material has clearly indicated
the unspecified absolute stereochemistry of this natural product to be 5S.
Microorganisms associated with other organisms are
a rich source of novel metabolites with unique and potent
biological activities.1 Epicoccum species are well-known
widespread fungi of symbiotic microorganisms, and one
constituent member of this family growing within the fruit
body of the tree fungus Pholiota squarrosa has shown to
produce structurally diverse compounds with intriguing
biological activities, as exemplified in Figure 1 by epipyr-
idone (antibacterial activity),2a epicoccalone (chymotrypsin
inhibitory activity),2b epicoccamides AꢀD (cytotoxicity),2c,d
and two types of epicoccarines. Epicoccarine A has been
reported as a minor component of the metabolites isolated
from this fungus and proved to be a more potent and
selective antibacterial agent against Gram positivebacteria
(Mycobacterium vaccae, MIC = 6.25 μg/mL) than epicoc-
carine B, making it a potential candidate as a new lead
antibacterial agent. These two epicoccarines consist of a
tyrosine-derived 3-acyltetramic acid core and a stereode-
Figure 1. Structures of medicinally important isolates from
Epicoccum sp.
fined alkenyl side chain, whose relative stereochemistry at
the two stereogenic centers (C8 and C10) has been estab-
lished as depicted in Figure 1.2a
Despite the simple structure and promising biological
properties of epicoccarine A, synthetic elaboration of
this natural product remains a significant challenge and,
to our knowledge, no reports have appeared previously
(1) (a) Piel, J. Nat. Prod. Rep. 2004, 21, 519. (b) Piel, J. Curr. Med.
Chem. 2006, 13, 39. (c) Konig, G. M.; Kehraus, S.; Seibert, S. F.; Abdel-
Lateff, A.; Muller, D. ChemBioChem 2006, 7, 229.
(2) (a) Kemami Wangun, H. V.; Hertweck, C. Org. Biomol. Chem.
€
2007, 5, 1702. (b) Kemami Wangun, H. V.; Ishida, K.; Hertweck, C. Eur.
€
J. Org. Chem. 2008, 3781. (c) Wright, A. D.; Osterhage, C.; Konig, G. M.
Org. Biomol. Chem. 2003, 1, 507. (d) Kemami Wangun, H. V.; Dahse,
H.-M.; Hertweck, C. J. Nat. Prod. 2007, 70, 1800.
r
10.1021/ol3024506
Published on Web 09/25/2012
2012 American Chemical Society

describing success in the completion of a total synthesis,
possibly due to the fact that mild and efficient methods for
manipulating chiral tetramate elements, without erosion of
their optical quality, are limited. In this regard, a modified
O- to C-acyl rearrangement of 4-O-acyl tetramates, in
which metal salts such as calcium chloride are used to
improve the reaction yield,3 was developed to extend the
carbon skeleta of C3-unsubstituted tetramic acids without
loss of stereochemical integrity of the chiral tetramate
systems. In addition to successful implementation of this
method for the synthesis of two natural products,3,4 the
accumulated results from our experiments suggested that
this approach would provide synthetic versatility for in-
troduction of more chemically sensitive chiral segments
into the tetramate systems. Thus, to test this possibility
and demonstrate more general utility of our synthetic
methodology, we set out to develop a stereocontrolled
synthesis of epicoccarine A through a strategy involving a
modified O- toC-acyl rearrangement. However, thetarget-
directed synthesis was complicated by the lack of stereo-
chemical knowledge of the C5 center on the tetramic
acid core. Accordingly, the objective of this work was to
synthesize the two possible candidates 1a and 1b from
L- and D-tyrosine, respectively, employing an O- to C-acyl
rearrangement strategy and to determine which C5-stereo-
isomer could be assigned to this natural product on the
basis of spectroscopic evidence.
Scheme 1. Retrosynthetic Analysis of Epicoccarine A
Scheme 2. Synthesis of 2a and 2b
For the construction of 1a and 1b, we planned to use the
retrosynthetic strategy outlined in Scheme 1, which would
involve the modified O- to C-acyl rearrangement of 1a0
and 1b0 as key steps, respectively. We envisaged that these
compounds could be prepared by introducing the alkenyl
side chain onto the enolic oxygen at C4 positions of 2a and
2b through dehydration with the relevant chiral carboxylic
acid 3. The synthesis started with functional group protec-
tion of L- and D-tyrosines through sequential treatment
with CbzCl and TBSCl (Scheme 2).5 The reactions pro-
ceeded smoothly to furnish 4a and 4b in 72 and 51% yields
respectively, which were then subjected to EDCI coupling
with Meldrum’s acid followed by thermal decarboxylation
and concomitant cyclization to generate the corresponding
Cbz-protected tetramic acids.6 The Cbz groups of these
substrates underwent facile deprotection upon hydro-
genolysis in EtOH, using catalytic Pd/C, and both tetramic
acids 2a and 2b were obtained in enantiomerically pure
form in 57 and 52% yields over three steps, respectively.
Our next task was to perform an asymmetric synthesis
of 3, which would be achieved using Oppolzer’s N-propio-
nylsultam 5 as a chiral source (Scheme 3).7 Indeed,
alkylation of 5 with (E)-1-bromo-2-methyl-2-pentene8
using NaHMDS and HMPA led to extremely high stereo-
chemical control, giving rise to 6 in 69% yield as a single
stereoisomer. The chiral substituent of this product was
removed by treatment with LiEt3BH in THF to provide
the alkenyl alcohol, which was then converted via the
Appel reaction9 to the corresponding iodide 7 in 81% yield
in two steps. Next, we attempted carbon chain elongation
of 7 by the Evans protocol10 to obtain alkenyl amide 8, in
which the absolute configuration at the C2 stereocenter
should be S. Accordingly, the alkenyl chain end of 7 was
elongated with D-prolinol N-propionamide using LDA
and HMPA at elevated temperatures (from ꢀ100 to
ꢀ40 °C) to afford 8 in 45% yield, in diastereomerically
enriched form, as a 94:6 mixture of 2S- and 2R-diaster-
eomers as determined by the 1H NMR. Then, we examined
acidic hydrolysis of this product to gain direct access to 3.
Exposure to 1 N aqueous HCl at 80 °C in 1,4-dioxane
solution was problematic, with carbonꢀcarbon double
bond isomerization occurring during the conversion, re-
sulting in the formation of complex mixtures of olefinic
(3) For a detailed investigation on the modified O- to C-acyl re-
arrangement and total synthesis of penicillenol A2, see: Sengoku, T.;
Nagae, Y.; Ujihara, Y.; Takahashi, M.; Yoda, H. J. Org. Chem. 2012,
77, 4391.
(4) For a total synthesis of penicillenol A1, see: Sengoku, T.;
Wierzejska, J.; Takahashi, M.; Yoda, H. Synlett 2010, 2944.
(5) Kato, E.; Kumagai, T.; Ueda, M. Tetrahedron Lett. 2005, 46,
4865.
(6) Jouin, P.; Castro, B.; Nisato, D. J. Chem. Soc., Perkin Trans. 1
1987, 1177.
(7) Brabander, J. D.; Oppolzer, W. Tetrahedron 1997, 53, 9169.
(8) Ziegler, F. E.; Becker, M. R. J. Org. Chem. 1990, 55, 2800.
(9) Appel, R. Angew. Chem., Int. Ed. Engl. 1975, 14, 801.
(10) (a) Evans, D. A.; Dow, R. L.; Shih, T. L.; Takacs, J. M.; Zahler,
R. J. Am. Chem. Soc. 1990, 112, 5290. (b) Evans, D. A.; Dow, R. L.
Tetrahedron Lett. 1986, 27, 1007.
Org. Lett., Vol. 14, No. 19, 2012
5143

isomers. Due to the loss of structural integrity, we next
explored a different route to 3. As in the case of 6, the chiral
substituent of 8 underwent slow reduction upon treatment
with 7.5 equiv of LiEt3BH at 40 °C for 3 days to afford the
desired alkenyl alcohol 9 in 72% yield as a regiochemically
and geometrically single isomer.11 Conversion of 9 to 3
was achieved via two sequential oxidation processes using
TPAP12 and NaClO2,13 which provided the desired car-
boxylic acid in 62% yield in two steps without loss of
stereochemistry.
Scheme 4. Completion of Total Synthesis of 1a and 1b
Scheme 3. Synthesis of 3
first synthesis of L- and D-tyrosine-derived epicoccarines 1a
and 1b in 59 and 50% yields in two steps, respectively. The
purity of these products was established by elemental
analysis and other characterization means, which proved
to be in good agreement with the expectations.
Figure 2 depicts the δ 2.5ꢀ3.4 ppm region of the
300 MHz 1H NMR spectra for 1a and 1b in CDCl3. As
anticipated from earlier observations, where various types
of 3-acyltetramic acids spontaneously equilibrate to mix-
tures of the keto and enol tautomers on the time scale of
NMR experiments,14 these materials underwent valence
tautomerization at room temperature in CDCl3 to result
in geometric isomerization at the C3,7-double bonds to
82:18 mixtures of Z- and E-enol tautomers (Figure 3).
Remarkably, the spectral shape observed for 1a perfectly
matched that given in the literature,2a where relative signal
intensities of each enol tautomer are entirely consistent
with the reported data. On the other hand, 1b showed
noticeable differences in chemical shift values for the
protons at the C6 position (H6 and H60) of the Z-enol
tautomer. Indeed, the H6 resonances were significantly
shifted upfield from δ 3.19 to 3.17 ppm, while the H60
proton exhibited a pronounced downfield shift from δ
2.66 to 2.68 ppm.15 From these observations, it is evident
that 1a can be assigned as natural epicoccarine A and 1b
Having succeeded in the efficient stereocontrolled synthe-
ses of 2 and 3, we turned our attention to their condensa-
tion and subsequent elaboration into epicoccarine A and
its C5-epimer (Scheme 4). The initial esterification of 3 of
a 94:6 mixture of 2S- and 2R-diastereomers was conducted
individually with 2a and 2b using EDCI and DMAP to
afford the respective O-acyl tetramates. Fortunately, both
products could be purified by means of silica gel column
chromatography to yield 1a0 and 1b0 as enantiomerically
pure materials in 65 and 68% yields, respectively. In the
final stages of the total synthesis, 1a0 and 1b0 were subjected
to the O- to C-acyl rearrangement reactions, which can
be facilitated by addition of metal salts.3 We found that
calcium chloride markedly effected the reactions of these
substrates in CH2Cl2 at room temperature, in the presence
of Et3N and DMAP, to form the respective 3-acyltetramic
acids without epimerization of the C5 stereogenic center.3,4
Finally, the TBS protective groups of these products could
be removed by treatment with TBAF to accomplish the
(11) In this case, carbonꢀcarbon double bond isomerization did not
occur as indicated by a simple 1H NMR spectrum of 9. Additionally, the
1,3-cis configuration of 9 was confirmed by the 1H NMR analysis,
revealing that the chemical shifts for the methine protons at the C2 and
C4 positions closely resemble those of a related molecular system; see:
Boeckman, R. K., Jr.; Pero, J. E.; Boehmler, D. J. J. Am. Chem. Soc.
2006, 128, 11032.
(12) Griffith, W. P.; Ley, S. V.; Whitcombe, G. P.; White, A. D.
J. Chem. Soc., Chem. Commun. 1987, 1625.
(13) Bal, B. S.; Childers, W. E., Jr.; Pinnick, H. W. Tetrahedron 1981,
37, 2091.
(14) (a) Yamaguchi, T.; Saito, K.; Tsujimoto, T.; Yuki, H. Bull.
Chem. Soc. Jpn. 1976, 49, 1161. (b) Saito, K.; Yamaguchi, T. Bull. Chem.
Soc. Jpn. 1978, 51, 651. (c) Steyn, P. S.; Wessels, P. L. Tetrahedron Lett.
1978, 19, 4707. (d) Nolte, M. J.; Steyn, P. S.; Wessels, P. L. J. Chem. Soc.,
Perkin Trans. 1 1980, 1057.
(15) Larger differences in the chemical shift values were observed for
the E-enol tautomer of 1b, where the resonances attributed to H6 and
H60 protons were shifted from δ 3.16 to 3.20 ppm and from δ 2.75 to 2.67
ppm, respectively.
(16) In the 13C NMR spectra, no significant difference that allowed
us to distinguish between 1a and 1b was observed.
5144
Org. Lett., Vol. 14, No. 19, 2012

Figure 2. Comparison of 1H NMR spectra on H6 and H60 of 1a
and 1b (300 MHz, CDCl3, rt).
should be its C5-epimer.16 The above structural assign-
ments to the synthetic samples were further secured by
comparable analyses of specific optical rotations mea-
sured in MeOH solution according to the reference
Figure 3. Geometric isomerization of 1a and 1b.
In conclusion, we have successfully achieved the first total
synthesis of antibacterial epicoccarine A and its C5-epimer
through a longest linear sequence of 10 steps. The syntheses
proceed in 4.3 and 3.8% overall yields from 5, respectively.
The work allowed the unspecified absolute stereochemistry
of this natural product to be assigned as 5S, with convincing
evidence furnished by comparison of the spectroscopic
properties and the specific optical rotations. The strategy
used an O- to C-acyl rearrangement to complete the total
synthesis. This serves as a mild and efficient method for
preparing 3-acyltetramic acids with highly labile alkenyl side
chains, without loss of structural and stereochemical quality.
Further extension of this O- to C-acyl rearrangement strat-
egy in the synthesis of various other 3-acyltetramic acid-type
of natural products will be the subject of future work.
([R]D ꢀ45.8 for natural epicoccarine A, c 0.15).2a In fact,
25
26
1a was found to exhibit an optical rotation of [R]D
ꢀ161.8 (c 0.15, MeOH). Although it was of the same sign,
it had an unexpectedly large magnitude in comparison to
the reported value, whereas 1b showed an opposite optical
21
rotation of [R]D þ97.5 (c 0.15, MeOH). Considering the
chemical integrity of the synthetic products, ensured by a
range of analytical experiments, the observed difference
in absolute values between the optical rotation of 1a and
that reported in the literature may be possibly due to low
purity of the extracted sample obtained from the natural
source, which would cause significant underestimation of
its own chiral properties.17,18 Nevertheless, it was advisable
at this stage to determine the absolute stereochemistry
on the basis of comparison of the signs of the measured
optical rotations, since the signs for 1a and 1b were
distinguishable from each other. The agreement of the sign
for 1a with that for the naturally occurring material
provides definitive support for the assignments mentioned
above.
Acknowledgment. We thank Mr. Kazuya Ashizawa
(Shizuoka University) for partial support of this research.
This work was supported in part by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Cul-
ture, Sports, Science and Technology, Japan.
(17) In view of the 1H NMR spectrum reported in the literature, we
found unassignable signals that would be attributed to residual impu-
rities included in the extractive sample; see ref 2a.
(18) The difference in optical rotation might be due to the conditions
of the samples, such as sodium salt, in strong acids as in the case of
tetramic acids. The synthetic samples are sure to be free acids from the
elemental analysis. However, that of the natural product was not
reported.
Supporting Information Available. Experimental de-
tails and characterization data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 19, 2012
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