Total synthesis of sandramycin and its analogues via a multicomponent assemblage

Article abstract of DOI:10.1021/ol403319m

The total synthesis of sandramycin has been accomplished by using a Staudinger/aza-Wittig/diastereoselective Ugi three-component reaction sequence as a key step to obtain a linear pentadepsipeptide. Subsequent [5 + 5] coupling of the penptapeptide, macrolactamization, and introduction of the quinaldin chromophores afforded sandramycin. Dihydroxy and diacetoxy analogues were also prepared, and the cytotoxic activity of these analogues against a range of human cancer cell lines was evaluated.

Full text of DOI:10.1021/ol403319m

Letter
pubs.acs.org/OrgLett
Total Synthesis of Sandramycin and Its Analogues via a
Multicomponent Assemblage
Katsushi Katayama, Koji Nakagawa, Hiroshi Takeda, Akira Matsuda, and Satoshi Ichikawa*
Faculty of Pharmaceutical Sciences, Hokkaido University, Kita-12, Nishi-6, Kita-ku, Sapporo 060-0812, Japan
S
* Supporting Information
ABSTRACT: The total synthesis of sandramycin has been accomplished by using a Staudinger/aza-Wittig/diastereoselective
Ugi three-component reaction sequence as a key step to obtain a linear pentadepsipeptide. Subsequent [5 + 5] coupling of the
penptapeptide, macrolactamization, and introduction of the quinaldin chromophores afforded sandramycin. Dihydroxy and
diacetoxy analogues were also prepared, and the cytotoxic activity of these analogues against a range of human cancer cell lines
was evaluated.
andramycin (1),1 which was isolated from the culture broth
of Norcardioides sp. (ATCC 39419), constitutes one of the
reaction (U3CR) with a cyclic imine,11,12 which can be
applicable to the synthesis of a range of analogues at the
peptide framework. We retrosynthetically disconnected the
macrocycle 2 and the linear decapeptide 3 at the N-
methylamide bonds linking Sar and N-Me-L-Val residues
(Scheme 1). This is advantageous because Sar has no
substituent at the α-position and the peptide coupling is free
from racemization in the [5 + 5] coupling and the
macrolactamization. Our key step to the approach to the
synthesis of the pentadepsipeptide 4 is a sequential Staudinger/
S
members of a class of C2-symmetric cyclic decadepsipeptides2
that also include the luzopeptins3 and quinoxapeptins (Figure
1).4 Extensive efforts to elucidate the mode of action of 1 by
Scheme 1
Figure 1. Structure of sandramycin.
Boger’s group revealed that it binds to the minor groove of
double-stranded DNA (dsDNA), preferentially to the regions
containing 5′-AT dinucleotides, with bisintercalation of its
hydroxyquinaldic acid chromophores.5 Sandramycin has excep-
tionally potent activity against mouse leukemia L1210 and
mouse melanoma B16 cells in vitro with IC50 values of 0.02 and
0.07 nM, respectively. Furthermore, in vivo antitumor activity
tested against mouse lymphocytic leukemia P388 in mice is
promising. Therefore, sandramycin and its analogues5 display
potential for therapeutic use in anticancer chemotherapy. The
cyclic decadepsipeptide framework of 1 constitutes Gly,
sarcocine (Sar), N-Me-L-Val, L-pipecolic acid (L-Pip), and D-
Ser. Total synthesis of 15,7 as well as luzopeptins8,9 and
quinoxapeptins10 has been accomplished via a sequential
peptide coupling approach. Herein we describe the total
synthesis of 1 and its analogues via an Ugi three-component
Received: November 16, 2013
Published: December 17, 2013
© 2013 American Chemical Society
428
dx.doi.org/10.1021/ol403319m | Org. Lett. 2014, 16, 428−431

Organic Letters
Letter
aza-Wittig/diastereoselective U3CR13 of the azidoaldehyde 8
with the isonitrile 5 and the carboxylic acid 6 to obtain 4. This
strategy constructs a nonproteinogenic amino acid, L-Pip
residue, with simultaneous linking to the two dipeptides 5
and 6 at the C- and N-termini. The silyloxy substituent was
introduced at the α-position to the cyclic imine 7 in order to
control the stereoselectivity at the newly formed stereogenic
center.14−25
diastereomer was isolated in 59% yield by silica gel column
chromatography. The stereochemistry of the α-position of the
newly constructed Pip residue in the major product was
determined by conventional amino acid analysis29 (see
Supporting Information) of the deoxygenated derivative 22,
which was obtained from 4 after deprotection of the TIPS
group followed by radical deoxygenation30 of 20 (PhC(S)Cl,
Et3N, CH2Cl2, then Bu3SnH, AIBN, C6F5OH, CF3CH2OH,
reflux, 47% over two steps). These results unambiguously
determined the absolute stereochemistry of the Pip residue and
confirmed the C−C bond was formed trans to the protected
hydroxyl group of the cyclic imine 7 in the U3CR as expected.
Deprotection of either the Boc group or the allyl group of 22
gave the amine 24a (25% HCl, dioxane) or the carboxylic acid
25a (Pd(PPh3)4, morpholine, THF, 87%) (Scheme 4). The [5
Each key component to the U3CR was prepared as shown in
Scheme 2. Suitably protected Gly-Sar 11 prepared from 9 was
Scheme 2
Scheme 4
dehydrated to give the isonitrile 5 (triphosgene, Et3N, CH2Cl2,
84%). The carboxylic acid component 6 was prepared by the
coupling of the appropriately protected D-Ser derivative 1226
and N-Boc-N-Me-L-Val (13)27 (EDCI, DMAP, CH2Cl2, 65%)
followed by deprotection of the Tce group (Zn, THF,
phosphate buffer (pH 6.7), 86%). The known homoallyl
alcohol 1528 was protected with a TIPS group (TIPSOTf, 2,6-
lutidine, CH2Cl2, 91%) to give 16, which was converted to the
alcohol 17 by hydroboration followed by oxidation (BH3, THF,
then H2O2, aq NaOH, 79%). The alcohol 17 was converted to
the azide 18, and deprotection of the Tr group gave 19 (ZnBr2,
CH2Cl2, 92%). The resulting primary hydroxyl group was
oxidized to afford the aldehyde 8 (SO3·pyridine, Et3N, DMSO),
which was directly used for the following Staudinger/aza-
Wittig/U3CR sequence (Scheme 3).
Treatment of 8 with PEt3 in THF resulted in a clean
conversion to the corresponding cyclic imine 7, which was
subsequently reacted with the isonitrile 5 and the carboxylic
acid 6 at 70 °C. As a result, the U3CR proceeded in a
diastereoselective manner, and the desired pentadepsipeptide 4
was obtained in 68% yield (S/R = 85/15). The major
Scheme 3
+ 5] assemblage of 24a and 25a afforded the protected
decadepsipeptide 26 (3-(diethoxyphosphoryloxy)-1,2,3-benzo-
triazin-4(3H)-one (DEPBT),31 NaHCO3, CH2Cl2−DMF,
82%). Deprotection of the Boc and the allyl groups of 26
under the same conditions used for the preparation of 24a and
25a gave the free linear decadepsipeptide 4 (Scheme 1), which
was then cyclized by treatment with DEPBT in CH2Cl2−DMF
429
dx.doi.org/10.1021/ol403319m | Org. Lett. 2014, 16, 428−431

Organic Letters
Letter
Table 1. DNA Binding Properties of 1, 34, and 35
sequence
5′-GCATGC-3′
3′-CGTACG-5′
5′-GCGCGC-3′
3′-CGCGCG-5′
5′-GCTAGC-3′
3′-CGATCG-5′
5′-GCCGGC-3′
3′-CGGCCG-5′
a
1
Kb (107 M−1
ΔG° (kcal/mol)
Kb (107 M−1
ΔG° (kcal/mol)
Kb (107 M−1
ΔG° (kcal/mol)
)
23.0
14.5
8.5
8.0
−11.4
3.03
−11.0
6.07
−10.8
2.95
−8.8
2.88
−8.8
−10.8
5.01
−9.1
3.27
−8.9
34
35
)
−8.8
4.37
−9.2
3.33
)
−9.0
−8.9
Data from reported.8b
a
to afford the cyclic decadepsipeptide 2 in 48% yield over three
steps from 3. Dimerization/cyclization of the free peptapeptide
prepared by deprotection of both N- and C-terminal protecting
groups of the pentapeptide 22 was also investigated to directly
obtain the macrocycle 2. However, those efforts were
unsuccessful and only the cyclic pentapeptide was obtained
(data not shown).
Table 2. Cytotoxic Activity of 1 and Its Analogues against
Human Cancer Cell Lines
a
IC50 (nM)
HCT-
118
SU-
DHL6
SU-
DHL10
RPMI8226
A431
RKO
1
0.8
1000
390
3.8
2200
530
3.1
1500
630
1.3
1100
390
5.9
1500
330
3.3
1500
370
34
35
Finally, the Cbz groups of 2 were removed by hydrogenolysis
(H2, Pd(OH)2, MeOH), and the liberated amines were coupled
with 3-hydroxyquilaldic acids 2932 to complete the total
a
HCT-118 and RKO: human colon cancer cells. A431: human
i
epidermal cancer cells. RPMI8226: human myeloma cells. SU-DHL6
and SU-DHL10: human diffuse large B-cell lymphoma cells.
synthesis of 1 (HATU, Pr2NEt, DMF, 44% over two steps).
The analytical data of the synthetic 1 were in good agreement
with those reported.5,7 Dihydroxy analogue 34 and diacetoxy
analogue 35 were also prepared from 4 and 30 in a manner
similar to the synthesis of 1.
Echinomycin,33 which is another C2-symmetric cyclic
octadecadepsipeptide bisintercalator,34 is known to inhibit the
binding of transcription factor HIF-1α to the cis-element in
genome DNA.35 The ability of 1 to inhibit HIF-1α was
preliminarily investigated by using an HIF-1 reporter gene assay
in HEK293 cells (Figures S3 and S4). Different from
echinomycin, 1 does not inhibit the transcription activity of
HIF-1α and β. These data suggest that the mode of action of 1
is different from that of echinomycin.
In conclusion, the total synthesis of sandramycin (1) as well
as its analogues 34 and 35 of the macrocycle moiety was
accomplished. Central to our approach to the synthesis of the
pentadepsipeptide 4 is a key Staudinger/aza-Wittig/diaster-
eoselective U3CR sequence and racemization-free [5 + 5]
coupling and the macrolactamization. This strategy can be
applicable to other congeners of this class of natural products.
The binding properties of 34 and 35 were investigated
(Figure S2), and the results are summarized in Table 1. The
binding affinity and sequence selectivity of synthesized cyclic
decapeptides were evaluated by fluorescence quenching
experiment5,8 using four octamer double stranded oligodeox-
ynucleotides (dsODNs). Compared to 1, analogues 34 and 35
exhibited a reduced binding affinity to dsODNs and their
sequence selectivity was subtly altered compared to 1 but still
with high DNA binding potency. Treatment of negatively
supercoiled ϕX174 DNA with 34 and 35 successfully caused
the unwinding to form relaxed DNA, and a further increase in
the amount of agents successfully formed positive supercoils
(Figure 2). The cytotoxic activity of 1, 34, and 35 was evaluated
ASSOCIATED CONTENT
* Supporting Information
■
S
Complete experimental procedure and characterization data of
all the new compounds, amino acid analysis of 22, DNA
binding constant measurement, DNA unwinding experiments,
and HIF-1 inhibition assay. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
Figure 2. DNA unwinding experiments of 34 and 35.
*E-mail: ichikawa@pharm.hokudai.ac.jp.
Notes
against human cancer cells (Table 2). Their DNA binding
properties and cytotoxic activity are correlated. Namely,
synthetic 1 exhibits strong cytotoxic activity against a range
of cell lines with IC50 values in the range 0.8−5.9 nM. On the
other hand, the activity of 34 and 35 was reduced by 2−3
orders of magnitude. Introducing the hydroxyl and acetoxy
group at the L-Pip residue caused a decrease in DNA binding.
As a result, the cytotoxic activity was decreased.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by JSPS Grant-in-Aid for
Challenging Exploratory Research (SI, Grant Number
22659020), Scientific Research on Innovative Areas “Chemical
Biology of Natural Products” (SI, Grant Number 24102502),
430
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Organic Letters
Letter
(24) Gulevich, A. V.; Shevchenko, N. F.; Balenkova, E. S.;
Roeschenthaler, G. V.; Nenajdenko, V. G. Synlett 2009, 403−406.
(25) Mehta, V. P.; Modha, S. G.; Ruijter, E.; Van Hecke, K.; Van
Meervelt, L.; Pannecouque, C.; Balzarini, J.; Orru, R. V. A.; Van der
Eycken, E. J. Org. Chem. 2011, 76, 2828−2839.
and Scientific Research (B) (SI, Grant Number 25293026). We
thank Ms. S. Oka and Ms. A. Tokumitsu (Center for
Instrumental Analysis, Hokkaido University) for measurement
of the mass spectra and Taiho Pharmaceutical Co., Ltd. for
evaluation of cytotoxicity.
(26) (a) Lorenz, K. B.; Diederichsen, U. J. Org. Chem. 2004, 69,
3917−3927. (b) Shin, M.; Inouye, K.; Otsuka, H. Bull. Chem. Soc. Jpn.
1984, 57, 2203−2210.
REFERENCES
■
(27) Faul, M. M.; Winneroski, L. L.; Krumrich, C. A.; Sullivan, K. A.;
Gllig, J. R.; Neel, D. A.; Rito, C. J.; Jirousek, M. R. J. Org. Chem. 1998,
63, 1961−1973.
(28) Yu, S.; Pan, X.; Ma, D. Chem.Eur. J. 2006, 12, 6572−6584.
(29) The pentapeptide 22 was heated under reflux in 6 M aq. HCl for
24 h, and the resulting mixture was treated with Marfey’s reagent. The
reaction mixture was analyzed by reversed phase HPLC (ODS, 10−
60% MeCN−H2O linear gradient containing 0.1% TFA) with
authentic material 23 derived from L- or D-Pip. For details, see
Supporting Information.
(30) As for the radical reduction step, the use of CF3CH2OH as a
solvent was essential. Extensive β-elimination was observed in toluene,
and the desired 22 was not obtained at all.
(31) Jiang, H.; Li, X.; Fan, Y.; Ye, C.; Romoff, T.; Goodman, M. Org.
Lett. 1999, 1, 91−93.
(1) Matson, J. A.; Bush, J. A. J. Antibiot. 1989, 42, 1763−1767.
(2) For review, see: Dawson, S.; Malkinson, J. P.; Paumier, D.;
Searcey, M. Nat. Prod. Rep. 2007, 24, 109−126.
(3) (a) Ohkuma, H.; Sakai, F.; Nishiyama, Y.; Ohbayashi, M.;
Imanishi, H.; Konishi, M.; Miyaki, T.; Koshiyama, H.; Kawaguchi, H. J.
Antibiot. 1980, 33, 1087−1097. (b) Tomita, K.; Hoshino, Y.; Sasahira,
T.; Kawaguchi, H. J. Antibiot. 1980, 33, 1098−1102.
(4) Lingham, R. B.; Hsu, A. H. M.; O’Brien, J. A.; Sigmund, J. M.;
Sanchez, M.; Gagliardi, M. M.; Heimbuch, B. K.; Genilloud, O.;
Martin, I.; Diez, M. T.; Hirsch, C. F.; Zink, C. D. L.; Liesch, J. M.;
Koch, G. E.; Gartner, S. E.; Garrity; Tsou, G. M. N. N.; Salituro, G. M.
J. Antibiot. 1996, 49, 253−259.
(5) Boger, D. L.; Chen, J.; W. Saitonz, K. J. Am. Chem. Soc. 1996, 118,
1629−1644.
(6) (a) Boger, D. L.; Chen, J.-H.; Saionz, K. W.; Jin, Q. Bioorg. Med.
Chem. 1998, 6, 85−102. (b) Boger, D. L.; Chen, J.-H. Bioorg. Med.
Chem. Lett. 1997, 7, 919−922. (c) Boger, D. L.; Saionz, K. W. Bioorg.
Med. Chem. 1999, 7, 315−322.
(32) Boger, D. L.; Chen, J.-H. J. Org. Chem. 1995, 60, 7369−7371.
(33) Kong, D.; Park, E. J.; Stephen, A. G.; Calvani, M.; Cardellina, J.
H.; Monks, A.; Fisher, R. J.; Shoemaker, R. H.; Melillo, G. Cancer Res.
2005, 65, 9047−9055.
(7) Boger, D. L.; Chen, J. J. Am. Chem. Soc. 1993, 115, 11624−11625.
(8) (a) Boger, D. L.; Ledeboer, M. W.; Kume, M. J. Am. Chem. Soc.
1999, 121, 1098−1099. (b) Boger, D. L.; Ledeboer, M. W.; Kume, M.;
Searcey, M.; Jin, Q. J. Am. Chem. Soc. 1999, 121, 11375−11383.
(9) (a) Ciufolini, M. A.; Valognes, D.; Xi, N. Angew. Chem., Int. Ed.
2000, 39, 2493−2495. (b) Valognes, D.; Belmont, P.; Xi, N.; Ciufolini,
M. A. Tetrahedron Lett. 2001, 42, 1907−1909.
(34) For other representative cyclic octadepsipeptides, see the
following. Triostin A; isolation: (a) Shoji, J.; Katagiri, K. J. Antibiot.,
Sect. A 1961, 14, 335−339. Total synthesis: (b) Chakravarty, P. K.;
Olsen, R. K. Tetrahedron Lett. 1978, 19, 1613−1616. (c) Shin, M.;
Inouye, K.; Otsuka, H. Bull. Chem. Soc. Jpn. 1984, 57, 2203−2210.
Thiocoraline; isolation: (d) Romero, F.; Espliego, F.; Per
́
ez, B. J.; de
García, Q. T.; Gravalos, D.; de la Calle, F.; Fernandez-Puentes, J. L. J.
́
́
(10) Boger, D. L.; Ledeboer, M. W.; Kume, M.; Jin, Q. Angew. Chem.,
Int. Ed. 1999, 38, 2424−2426.
Antibiot. 1997, 50, 743−747. Total synthesis: (e) Boger, D. L.;
Ichikawa, S. J. Am. Chem. Soc. 2000, 122, 2956−2957. (f) Boger, D. L.;
Ichikawa, S.; Tse, W. C.; Hedrick, M. P.; Jin, Q. J. Am. Chem. Soc.
2001, 123, 561−568.
(11) Review: Domling, A.; Ugi, I. Angew. Chem., Int. Ed. 2000, 39,
3168−3210.
(35) Formica, J. V.; Waring, M. J. Antimicrob. Agents Chemother.
1983, 24, 735−741.
(12) (a) Nutt, R. F.; Joullie,
́
M. M. J. Am. Chem. Soc. 1982, 104,
M. M. J. Chem.
5852−5853. (b) Bowers, M. M.; Carroll, P.; Joullie,
́
Soc., Perkin Trans. 1 1989, 857−865.
(13) Bonger, K. M.; Wennekes, T.; Filippov, D. V.; Lodder, G.; van
der Marel, G.; Overkleeft, H. S. Eur. J. Org. Chem. 2008, 3678−3688.
(14) Znabet, A.; Ruijter, E.; de Kanter, F. J. J.; Kohler, V.; Helliwell,
̈
M.; Turner, N. J.; Orru, R. V. A. Angew. Chem., Int. Ed. 2010, 49,
5289−5292.
(15) Chapman, T. M.; Davies, I. G.; Gu, B.; Block, T. M.; Scopes, D.
I. C.; Hay, P. A.; Courtney, S. M.; McNeill, L. A.; Schofield, C. J.;
Davis, B. G. J. Am. Chem. Soc. 2005, 127, 506−507.
(16) Timmer, M. S. M.; Risseeuw, M. D. P.; Verdoes, M.; Filippov,
D. V.; Plaisier, J. R.; van der Marel, G. A.; Overkleeft, H. S.; van Boom,
J. H. Tetrahedron: Asymmetry 2005, 6, 177−185.
(17) Maison, W.; Lutzen, A.; Kosten, M.; Schlemminger, I.;
̈
Westerhoff, O.; Saak, W.; Martens, J. J. Chem. Soc., Perkin Trans. 1
2000, 1867−1871.
(18) Sperger, C. A.; Mayer, P.; Wanner, K. T. Tetrahedron 2009, 65,
10463−10469.
(19) (a) Zhu, D.; Chen, R.; Liang, H.; Li, S.; Pan, L.; Chen, X. Synlett
2010, 897−900. (b) Zhu, D.; Xia, L.; Pan, L.; Li, S.; Chen, R.; Mou, Y.;
Chen, X. J. Org. Chem. 2012, 77, 1386−1395. (c) Xia, L.; Li, S.; Chen,
R.; Liu, K.; Chen, X. J. Org. Chem. 2013, 78, 3120−3131.
(20) El Kaïm, L.; Grimaud, L.; Oble, J.; Wagschal, S. Tetrahedron Lett.
2009, 50, 1741−1743.
(21) Groger, H.; Hatam, M.; Martens, J. Tetrahedron 1995, 51,
̈
7173−7180.
(22) Banfi, L.; Basso, A.; Guanti, G.; Merlo, S.; Repetto, C.; Riva, R.
Tetrahedron 2008, 64, 1114−1134.
(23) Iizuka, T.; Takiguchi, S.; Kumakura, Y.; Tsukioka, N.; Higuchi,
K.; Kawasaki, T. Tetrahedron Lett. 2010, 51, 6003−6005.
431
dx.doi.org/10.1021/ol403319m | Org. Lett. 2014, 16, 428−431