Enantioselective total synthesis of plectosphaeroic acid B

Article abstract of DOI:10.1021/ja401423j

The first total synthesis of a member of the plectosphaeroic acid family of fungal natural products is reported. Key steps include the late-stage formation of the hindered N6-C9″ bond and stereoselective introduction of the two methylthio substituents.

Full text of DOI:10.1021/ja401423j

Communication
pubs.acs.org/JACS
Enantioselective Total Synthesis of Plectosphaeroic Acid B
Salman Y. Jabri and Larry E. Overman*
Department of Chemistry, 1102 Natural Sciences II, University of California, Irvine, California 92697-2025, United States
S
* Supporting Information
disclosed that 1−3 were equipotent (IC50 = 2 μM) against
ABSTRACT: The first total synthesis of a member of the
plectosphaeroic acid family of fungal natural products is
reported. Key steps include the late-stage formation of the
hindered N6−C9″ bond and stereoselective introduction
of the two methylthio substituents.
indoleamine 2,3-dioxygenase (IDO), a recently identified
molecular target for the potential treatment of cancer. Although
it was found that only the phenoxazinone subunit is required
for this activity,8 we were intrigued by the potential utility of
molecules that contain both anticancer motifs,9 as well as the
considerable synthetic challenges10−12 posed by the complexity
of 1−3. Herein, we describe our recent efforts that culminated
in the first total synthesis of (+)-plectosphaeroic acid B (2).
Our plan for preparing plectosphaeroic acids A−C (1−3) is
outlined in Scheme 1. We envisaged stereoselectively
n 2009, Mauk, Andersen, and co-workers reported the
I
isolation of plectosphaeroic acids A−C (1−3) from cultured
extracts of the fungus Plectosphaerella cucumerina collected in
Barkley Sound, British Columbia, Canada (Figure 1).1 These
Scheme 1
introducing the labile thioethers or bridging trisulfide
functionalities of 1−3 after late-stage construction of the
critical N6−C9″ bond between the indoline nitrogen atom of
fragment 12 and a halogenated congener 13 of cinnabarinic
acid. Although efficient transition metal-catalyzed methods are
known, the hindered nature and structural complexity of these
intermediates would make the successful joining of fragments
12 and 13 one of the most challenging applications of C−N
cross-coupling to date.13,14 Fragment 12 was seen arising from
the simpler trioxopiperazine alkaloid (+)-gliocladin C
(14)4b,e,10a,15 using a general sequence our group disclosed in
2011 for the total synthesis of the ETP natural product
(+)-gliocladine C (5).10 Halide 13 was envisioned originating
from the biomimetic, oxidative dimerization of 6-halo-3-
hydroxyanthranilic acid 15.16
Figure 1. The plectosphaeroic acids and related metabolites.
highly functionalized, secondary metabolites are defined by the
union of a tryptophan-derived epitrithiodioxopiperazine (or
methylthio analogue)2 and 2-aminophenoxazin-3-one frag-
ments,3 structural motifs commonly found in natural products
but never before seen in conjunction. A number of
epipolythiodioxopiperazine (ETP) alkaloids and their methyl-
thio congeners (e.g., 5−11) share a polycyclic core that is
homologous to the northern fragments of 1−3.4,5 Naturally
occurring cinnabarinic acid (4) comprises the southern
fragment of 1−3. Molecules comprised of either structural
motif display a broad spectrum of biological properties (e.g.,
antimicrobial, antiviral, antifungal, immunosuppressive, and
anticancer activities),2,3 with numerous studies focusing on
chemotherapeutic applications.6,7 In the isolation report, it was
In order to assess the feasibility ofand identify conditions
forthe critical C−N bond-forming step, we began our efforts
by examining N-arylation of a structurally simplified indoline
fragment (Scheme 2). The di-(tert-butoxycarbonyl) derivative
16 of (+)-gliocladin C, which is available on multigram scale by
Received: February 7, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja401423j | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Scheme 2
Scheme 3
chemical synthesis,10a was exposed to a catalytic amount of
Sc(OTf)3, resulting in the selective deprotection of the indoline
nitrogen atom. After a considerable screening effort,17 joining
of the two fragments was realized in 67% yield when indoline
17 was allowed to react with 2.3 equiv of iodide 18,18 3.0 equiv
of copper(I) thiophene-2-carboxylate (CuTC),19 and excess
K2CO3 in toluene at 90 °C. Other Cu(I) salts and ligand
combinations that were screened gave poor conversions to 19
(0−20% yields). Reducing the amount of CuTC also resulted
in low yields of 19. Additionally, it was found that double
protection of the 2-amino group of iodide 18 was critical,20 and
that N-arylation of the corresponding bromide was much less
efficient (8% yield of 19). Although the excess of iodide 18 was
recoverable, minor amounts of the byproduct arising from
undesired hydrodehalogenation of 18 were observed as well.
Having discovered conditions to successfully unite the two
fragments, we focused our attention on the synthesis of
plectosphaeroic acid B (2).
The total synthesis of (+)-plectosphaeroic acid B (2)
commenced with deprotection of the indoline nitrogen atoms
of the individual epimers 20a and 20b, intermediates previously
prepared en route to (+)-gliocladine C (5) (Scheme 3).10
Attempts to chemoselectively remove the Boc group of 20a or
20b with Lewis or protic acids proved challenging because of
the other acid-labile functionalities that were present, including
the C3- and C12-N,O-acetals. For this reason, a two-step
(single-pot) procedure was developed. Thermolytic cleavage of
both Boc groups of 20a or 20b, followed by selective
reprotection of the indole nitrogen atom by reaction with 1
equiv of Boc2O and a catalytic amount of DMAP, afforded
intermediates 21a and 21b in 60−80% yields. Complete
inversion of the C12-stereocenter occurred during the
thermolytic deprotection step.21
The potential to form an N-acyliminium ion by loss of the
oxygen substituents at C3 or C12 also complicated the ensuing
copper-mediated C−N cross-coupling reaction. In preliminary
experiments, treatment of epimer 21a with iodide 18, CuTC,
and K2CO3 resulted in inefficient conversion to the coupled
product 22a (10−30% yields). In these reactions, some
formation of the thiophene-2-carboxylate adduct 23 was
observed. As in situ activation of the angular N,O-acetal
appeared unavoidable, we explored substituting CuTC with
CuOAc22 in order to minimize the formation of byproducts.
After some optimization, exposure of 21a or 21b to 3.0 equiv of
iodide 18 and 6.0 equiv of CuOAc in toluene at 90 °C delivered
22a and 22b in 50−58% yields.
of 22a and 22b by exposure to excess BF3·OEt2 and MeSH in
CH2Cl2 at −78 °C with slow warming to room temperature led
to the generation of a 1.3:1.0 mixture of di(methylthio)ethers
24 and C3-epi-24 in high yield (79% from 22a, 92% from 22b).
Alternatively, it was found that transforming 22a or 22b by
reaction with H2S and BF3·OEt2, then methylation with MeI
and K2CO3, provided cis-di(methylthio)ether 24 in 80−90%
yield as virtually a single stereoisomer. The difference in
stereochemical outcomes for these sulfenylation procedures
warrants further comment. Substantial precedent suggests that
introduction of the sulfur nucleophile would occur first at C12,
with high stereoselectivity from the concave face.2c,e,10,11 The
factors governing the facial selectivity of the subsequent
addition of the sulfur nucleophiles at C3 are less certain. The
greater stereoselectivity we observe in forming 2 by the two-
We turned our attention to the stereoselective installation of
the methylthio substituents of 2.23 Activation of the N,O-acetals
B
dx.doi.org/10.1021/ja401423j | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(2) For recent reviews, see: (a) Gardiner, D. M.; Waring, P.; Howlett,
B. J. Microbiology 2005, 151, 1021−1032. (b) Jiang, C.-S.; Guo, Y.-W.
Mini-Rev. Med. Chem. 2011, 11, 728−745. (c) Iwasa, E.; Hamashima,
Y.; Sodeoka, M. Isr. J. Chem. 2011, 51, 420−433. (d) Jiang, C.-S.;
step sequence could reflect the difference between directly
forming configurationally stable thioether products and
proceeding via configurationally less stable hemithioaminal
intermediates, which could be equilibrating under the
sulfenylation or methylation conditions to the more stable cis
product.24−26
Muller, W. E. G.; Schroder, H. C.; Guo, Y.-W. Chem. Rev. 2012, 112,
̈
̈
2179−2207. For a recent report of in vitro antitumor activity of a large
selection of synthetic ETPs, see: (e) Boyer, N. C.; Morrison, K. C.;
Kim, J.; Hergenrother, P. J.; Movassaghi, M. Chem. Sci. 2013, 4, 1646−
1657.
(3) (a) Hollstein, U. Chem. Rev. 1974, 74, 625−652. (b) Graves, D.
E. In Sequence-specific DNA Binding Agents; Waring, M. J., Ed.; RSC
Publishing: Cambridge, 2006; pp 109−129. (c) Bolognese, A.;
Correale, G.; Manfra, M.; Lavecchia, A.; Novellino, E.; Pepe, S. J.
Med. Chem. 2006, 49, 5110−5118. (d) Le Roes-Hill, M.; Goodwin, C.;
Burton, S. Trends Biotechnol. 2009, 27, 248−258 and references
therein.
(4) (a) Feng, Y.; Blunt, J. W.; Cole, A. L. J.; Munro, M. H. G. J. Nat.
Prod. 2004, 67, 2090−2092. (b) Usami, Y.; Yamaguchi, J.; Numata, A.
Heterocycles 2004, 63, 1123−1129. (c) Dong, J.-Y.; He, H.-P.; Shen, Y.-
M.; Zhang, K.-Q. J. Nat. Prod. 2005, 68, 1510−1513. (d) Zheng, C.-J.;
Kim, C.-J.; Bae, K. S.; Kim, Y.-H.; Kim, W.-G. J. Nat. Prod. 2006, 69,
1816−1819. (e) Bertinetti, B. V.; Rodriguez, M. A.; Godeas, A. M.;
Cabrera, G. M. J. Antibiot. 2010, 63, 681−683. (f) Li, L.; Li, D.; Luan,
Y.; Gu, Q.; Zhu, T. J. Nat. Prod. 2012, 75, 920−927. (g) Wang, F.-Z.;
Huang, Z.; Shi, X.-F.; Chen, Y.-C.; Zhang, W.-M.; Tian, X.-P. Bioorg.
Med. Chem. Lett. 2012, 22, 7265−7267.
Completion of the synthesis of (+)-plectosphaeroic acid B
(2) required careful cleavage of the three remaining ester
groups.27 Methanolysis of the C11-acetate of 24 was achieved
using excess La(OTf)3 and 1 equiv of DMAP at 50 °C.
Conversion of the methyl esters of the phenoxazinone subunit
to carboxylic acids by the use of LiI in pyridine at 90 °C gave
(+)-plectosphaeroic acid B (2) in 65% yield over two steps after
23
HPLC purification. The optical rotation of synthetic 2, [α]D
+228 (c 0.08, MeOH), was considerably higher than the value
reported for the natural sample, [α]D23 +69.8 (c 0.27, MeOH).
However, all other spectroscopic data, including CD spectra,
compared well.
In conclusion, the first total synthesis of (+)-plectosphaeroic
acid B (2) was achieved in seven steps from the known
intermediates 20a and 20b. This total synthesis confirms the
unique structure and absolute configuration of plectosphaeroic
acid B, which had been assigned on the basis of NMR, MS, and
CD data.1 Introduction of the highly congested, central C−N
bond of 2 by a late-stage copper-mediated process provides one
of the most demanding examples of C−N cross-coupling
reported to date. The convergence of this synthesis strategy
should enable the synthesis of the remaining plectosphaeroic
acids and analogues and allow for the pharmacological
evaluation of these and related molecules containing multiple
anticancer motifs.
(5) Many ETPs possess a hydroxyl substituent adjacent to the
quaternary center in the pyrrolidine ring, as found in 1−3. Molecules
of this type readily undergo fragmentation when exposed to basic or
acidic conditions or triphenylphosphine, see: Overman, L. E.; Shin, Y.
Org. Lett. 2007, 9, 339−341 and references therein.
(6) For selected studies reporting in vivo efficiacy of ETPs, see:
(a) Waring, P.; Eichner, R. D.; Mullbacher, A. Med. Res. Rev. 1988, 8,
̈
499−524. (b) Vigushin, D. M.; Mirsaidi, N.; Brooke, G.; Sun, C.; Pace,
P.; Inman, L.; Moody, C. J.; Coombes, R. C. Med. Oncol. 2004, 21,
21−30. (c) Kung, A. L.; et al. Cancer Cell 2004, 6, 33−43. (d) Isham,
C. R.; Tibodeau, J. D.; Jin, W.; Xu, R.; Timm, M. M.; Bible, K. C. Blood
2007, 109, 2579−2588. (e) Lee, Y.-M.; Lim, J.-H.; Yoon, H.; Chun, Y.-
S.; Park, J.-W. Hepatology 2011, 53, 171−180. (f) Chaib, H.; Nebbioso,
A.; Preber, T.; Castellano, R.; Garbit, S.; Restouin, A.; Vey, N.; Altucci,
L.; Collette, Y. Leukemia 2012, 26, 662−674.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details, characterization data, H and 13C NMR
1
spectra of new compounds, complete ref 6c, and CIF file for
21a. This material is available free of charge via the Internet at
http://pubs.acs.org.
(7) For aminophenoxazinones, see ref 3 and Estlin, E. J.; Veal, G. J.
Cancer Treat. Rev. 2003, 29, 253−273.
(8) T988 A (7), an ETP co-isolated with 1−3, was inactive against
IDO, whereas synthetic cinnabarinic acid (4) was equipotent to 1−3;
see ref 1.
(9) For an example of portmanteau inhibitors, see: Wang, Z.;
Bennett, E. M.; Wilson, D. J.; Salomon, C.; Vince, R. J. Med. Chem.
2007, 50, 3416−3419.
AUTHOR INFORMATION
Corresponding Author
leoverma@uci.edu
■
Notes
The authors declare no competing financial interest.
(10) For total syntheses of (+)-gliocladin C (14) and (+)-gliocladine
C (5), see: (a) DeLorbe, J. E.; Jabri, S. Y.; Mennen, S. M.; Overman, L.
E.; Zhang, F.-L. J. Am. Chem. Soc. 2011, 133, 6549−6552. For total
syntheses of (+)-T988 C (6), (+)-gliocladin A (8) and structurally
related ETPs, see: (b) DeLorbe, J. E.; Horne, D.; Jove, R.; Mennen, S.
M.; Nam, S.; Zhang, F.-L.; Overman, L. E. J. Am. Chem. Soc. 2013, 135,
No. DOI: 10.1021/ja400315y.
ACKNOWLEDGMENTS
■
Support was provided by the National Institute of General
Medical Sciences of NIH (R01GM-30859). S.Y.J. thanks Eli
Lilly and Co. and Bristol-Myers Squibb Co. for graduate
fellowships. Computational studies were performed on
hardware purchased with funding from CRIF (CHE-
0840513). NMR and mass spectra were determined at UC
Irvine using instruments purchased with the assistance of NSF
and NIH shared instrumentation grants. We thank Dr. Joseph
Ziller and Dr. John Greaves, Department of Chemistry, UC
Irvine, for their assistance with X-ray and mass spectrometric
analyses, and Dr. Nathan Crawford for helpful discussion with
computational studies.
(11) For the total synthesis and stereochemical confirmation of
(+)-gliocladin B (11), see: Boyer, N.; Movassaghi, M. Chem. Sci. 2012,
3, 1798−1803.
(12) For a review on total syntheses of ETPs, see ref 2c. For recent
̀
syntheses, see: (a) Nicolaou, K. C.; Totokotsopoulos, S.; Giguere, D.;
Sun, Y.-P.; Sarlah, D. J. Am. Chem. Soc. 2011, 133, 8150−8153.
(b) Codelli, J. A.; Puchlopek, A. L. A.; Reisman, S. E. J. Am. Chem. Soc.
2012, 134, 1930−1933. (c) Nicolaou, K. C.; Lu, M.; Totokotsopoulos,
̀
S.; Heretsch, P.; Giguere, D.; Sun, Y.-P.; Sarlah, D.; Nguyen, T. H.;
Wolf, I. C.; Smee, D. F.; Day, C. W.; Bopp, S.; Winzeler, E. A. J. Am.
Chem. Soc. 2012, 134, 17320−17332.
REFERENCES
■
(1) Carr, G.; Tay, W.; Bottriell, H.; Andersen, S. K.; Mauk, A. G.;
Andersen, R. J. Org. Lett. 2009, 11, 2996−2999.
(13) For recent reviews, see: (a) Ley, S. V.; Thomas, A. W. Angew.
Chem., Int. Ed. 2003, 42, 5400−5449. (b) Evano, G.; Blanchard, N.;
C
dx.doi.org/10.1021/ja401423j | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Toumi, M. Chem. Rev. 2008, 108, 3054−3131. (c) Hartwig, J. F. Acc.
Chem. Res. 2008, 41, 1534−1544. (d) Monnier, F.; Taillefer, M. Angew.
Chem., Int. Ed. 2009, 48, 6954−6971. (e) Surry, D. S.; Buchwald, S. L.
Chem. Sci. 2010, 1, 13−31. (f) Surry, D. S.; Buchwald, S. L. Chem. Sci.
2011, 2, 27−50. (g) Fischer, C.; Koenig, B. Beilstein J. Org. Chem.
2011, 7, 59−74. (h) Rosen, B. M.; Quasdorf, K. W.; Wilson, D. A.;
Zhang, N.; Resmerita, A. M.; Garg, N. K.; Percec, V. Chem. Rev. 2011,
111, 1346−1416.
(14) For C−N cross-coupling of complex pyrrolidinoindolines, see:
́
(a) Richard, D. J.; Schiavi, B.; Joullie, M. M. Proc. Natl. Acad. Sci. U.S.A.
2004, 101, 11971−11976. (b) Foo, K.; Newhouse, T.; Mori, I.;
Takayama, H.; Baran, P. S. Angew. Chem., Int. Ed. 2011, 50, 2716−
2719.
(15) For other syntheses of 14, see refs 5, 11, and (a) Furst, L.;
Narayanam, J. M. R.; Stephenson, C. R. J. Angew. Chem., Int. Ed. 2011,
50, 9655−9659. (b) Sun, M.; Hao, X.-Y.; Liu, S.; Hao, X.-J.
Tetrahedron Lett. 2013, 54, 692−694.
(16) (a) Ruan, J. W.; Huang, Z. S.; Huang, J. F.; Du, C. J.; Huang, S.
L.; Shi, Z.; Fu, L. W.; Gu, C. Q. Chin. Chem. Lett. 2006, 17, 1141−
1144. (b) Giurg, M.; Piekielska, K.; Gębala, M.; Ditkowski, B.;
Wolanski, M.; Peczynska−Czoch, W.; Mlochowski, J. Synth. Commun.
́ ́
2007, 37, 1779−1789 and references therein.
(17) Our early investigations focused on the N-arylation of indoline
17 with o-bromo(or iodo)benzoic acids and derivatives with Cu(I) and
Pd(0) catalysts.13 Successful N-arylation was observed only using
Cu(I) catalysts.
(18) See Supporting Information for details of the preparation of 18.
(19) For an early example of CuTC in cross-couplings, see:
(a) Allred, G. D.; Liebeskind, L. S. J. Am. Chem. Soc. 1996, 118,
2748−2749. For CuTC in a related C−N cross-coupling, see: (b) Li,
G.; Padwa, A. Org. Lett. 2011, 13, 3767−3769.
(20) For example, the corresponding mono-Boc derivative of 18 was
cross-coupled in less than 20% yield.
(21) The relative and absolute configuration of 21a was secured by
single crystal X-ray diffraction. These data have been deposited at The
Cambridge Crystallographic Data Centre as entry CCDC 922842 and
can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif.
(22) For previous use of CuOAc in C−N cross-couplings, see:
(a) Bellina, F.; Calandri, C.; Cauteruccio, S.; Rossi, R. Eur. J. Org.
Chem. 2007, 2147−2151. (b) Kubo, T.; Katoh, C.; Yamada, K.;
Okano, K.; Tokuyama, H.; Fukuyama, T. Tetrahedron 2008, 64,
11230−11236.
(23) For recent examples of direct introduction of methylthio
substituents, at the α and α′-positions of dioxopiperazines, see refs 2e,
̀
10b, 11, 12a, c, and Nicolaou, K. C.; Giguere, D.; Totokotsopoulos, S.;
Sun, Y.-P. Angew. Chem., Int. Ed. 2012, 51, 728−732.
(24) As observed computationally (and likely experimentally) for
simple 2,5-dialkyl-3,6-di-(p-methoxybenzylthio)dioxopiperazines,25 the
lowest energy cis-disulfated stereoisomers of a are calculated (B3-LYP/
def2-TZVP)26 to be more stable by 2.5 kcal/mol (R = H) and 0.9
kcal/mol (R = Me).
(25) Aliev, A. E; Hilton, S. T.; Motherwell, W. B.; Selwood, D. L.
Tetrahedron Lett. 2006, 47, 2387−2390.
(26) (a) TURBOMOLE, V6.3; Turbomole GmbH: Karlsruhe,
Germany, 2011; http://www.turbomole.com. (b) Treutler, O.;
Ahlrichs, R. J. Chem. Phys. 1995, 102, 346−354.
(27) See ref 5, and for the lability of aminophenoxazinones, see:
Bolognese, A.; Piscitelli, C.; Scherillo, G. J. Org. Chem. 1983, 48,
3649−3652.
D
dx.doi.org/10.1021/ja401423j | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX