Total synthesis of celogentin C

Full text of DOI:10.1002/anie.200902425

Communications
DOI: 10.1002/anie.200902425
Natural Product Synthesis
Total Synthesis of Celogentin C**
Bing Ma, Dmitry N. Litvinov, Liwen He, Biplab Banerjee, and Steven L. Castle*
[7a]
À
The bicyclic octapeptide celogentin C (1, Figure 1) was
isolated by Kobayashi and co-workers from the seeds of
Celosia argentea.[1] Other structurally similar natural products
macrolactamization at the Pro Arg site.
We envisioned
utilizing our radical conjugate addition methodology[12,13] to
prepare the b-substituted a-amino acid moiety resulting from
union of the Leu and Trp side chains. The radical acceptor
would be fashioned by means of a Knoevenagel condensation.
After the radical conjugate addition, formation of the left-
hand ring of 1 was to be accomplished by macrolactamization
À
at the Val Trp site. The approach was designed to be flexible
in terms of the order of ring closures; however, attempts to
append the left-hand ring onto the right-hand ring were
unsuccessful.[14] Consequently, we embarked upon a left-to-
right-sequence.
The first key reaction encountered in this route was the
Knoevenagel condensation. The preparation of the conden-
sation partners is detailed in Scheme 1. Tryptophan 2,
Figure 1. Celogentin C and synthetic plan.
include the bicyclic peptides moroidin,[2] celogentins A–H,[3]
and celogentin J,[3] as well as the monocyclic peptides
celogentin K[4] and stephanotic acid.[5] Some of these com-
pounds inhibit tubulin polymerization,[6] with 1 ranking as the
most potent antimitotic agent of this natural product family.
The unusual structure of 1 is derived from two cross-links
between amino acid side chains. A bond between the leucine
b-carbon atom and the indole C6 of tryptophan forms the left-
hand ring of 1, whereas the right-hand macrocycle contains a
À
C N linkage between the indole C2 and the imidazole N1.
The resultant heterobiaryl axis introduces the potential of
atropisomer stereochemistry. The combination of useful
biological activity and intriguing architecture has prompted
numerous synthetic efforts targeting 1 and related com-
pounds.[7–10] However, a total synthesis of one of the bicyclic
members of the celogentin family has not yet been
reported.[11] Herein, we describe our efforts which have
culminated in the synthesis of celogentin C.
Scheme 1. Synthesis of Knoevenagel condensation partners. Reagents
and conditions: a) AcOH/THF/H2O 3:2:2; b) DDQ, CH2Cl2, 08C;
c) TsOH (0.1 equiv), MeCN, reflux; d) HgCl2, MeCN/H2O 3:1.
Cbz=benzyloxycarbonyl, TBS=tert-butyldimethylsilyl, TES=triethyl-
silyl, DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone, TsOH=p-tol-
uenesulfonic acid.
Our synthetic plan is outlined in Figure 1. We previously
constructed the right-hand ring of 1 by using an intermolec-
ular indole–imidazole oxidative coupling and subsequent
possessing a silyloxymethyl group at C6 of the indole, was
constructed previously by using a combination of phase-
transfer-catalyzed asymmetric alkylation and Pd-catalyzed
heteroannulation.[7c] Silyl ether cleavage and oxidation of the
resulting benzylic alcohol with DDQ afforded aldehyde 3 in
[*] B. Ma, D. N. Litvinov, Dr. L. He, Dr. B. Banerjee, Prof. S. L. Castle
Department of Chemistry and Biochemistry
Brigham Young University, Provo, UT 84602 (USA)
Fax: (+1)801-422-0153
E-mail: scastle@chem.byu.edu
Homepage: http://people.chem.byu.edu/scastle
good yield. The nitroacetamide coupling partner of 3 was
[15]
À
fashioned from the dipeptide Leu Val-OBn (4) by means
[**] We thank Brigham Young University and the National Institutes of
Health (GM70483) for support of this work. We also thank Prof.
Hiroshi Morita of Hoshi University for providing an analytical
sample of celogentin C and Joshua W. Robinson for assistance in
preparing compound 2.
of Rajappaꢀs methodology.[16] Thus, condensation of 4 with
commercially available dithioketene acetal 5 in the presence
of catalytic TsOH provided vinyl sulfide 6 as a single alkene
isomer of undetermined configuration. The nitroacetamide
moiety of 7 was then revealed by exposure of 6 to HgCl2 in
aqueous MeCN.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200902425.
6104
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 6104 –6107

Angewandte
Chemie
The condensation of 3 and 7 occurred in the presence of
TiCl4 and NMM,[17] affording a,b-unsaturated a-nitro amide 8
as a single alkene isomer (Scheme 2).[18] Optimal yields in this
reaction were obtained with a 2:1 mixture of THFand Et2O as
solvent. Based on previous studies with model substrates, we
were hopeful that Mg-DBFOX (DBFOX = 4,6-dibenzofur-
andiyl-2,2’-bisoxazoline) chiral Lewis acids would promote a
stereoselective radical conjugate addition to 8, but neither
DBFOX/Ph[12b] nor our second-generation DBFOX cata-
lysts[12a] afforded the adducts with any degree of selectivity. In
fact, the best diastereomeric ratio, albeit low (1.0:2.9:2.0:1.2),
was acquired by employing substrate stereocontrol in con-
junction with the achiral Lewis acid Zn(OTf)2. Although the
stereoselectivity of the radical conjugate addition was modest,
the yield was excellent, as a mixture of amines 9a–d was
obtained in 90% yield after nitro group reduction by SmI2.
The two minor isomers 9a and 9d could be removed at this
stage, leaving a 1.5:1 mixture of 9b and 9c. Since the yield of
9b was a reasonable 36% over these two steps, we felt that
this protocol would enable us to synthesize 1 provided the
configuration of 9b at the two newly formed stereocenters
matched the natural product. Accordingly, we resolved to
convert 9b into a species that could be compared spectro-
scopically to a known compound.
transfer hydrogenolysis conditions afforded a separable
mixture of unprotected peptides 11b and 11c. Despite the
low diastereoselectivity of the radical conjugate addition,
diastereomerically pure 11b was obtained in 31% yield from
Knoevenagel adduct 8 due to the excellent yields of the four
intervening steps.
In light of the considerable epimerization encountered by
Moody and co-workers in a related macrolactamization,[11] we
were relieved to find that HOBt/HBTU-mediated cyclization
of 11b delivered 12b as a single detectable diastereomer in
91% yield. We attribute this difference to the fact that Moody
and co-workers formed their macrocycle at a site correspond-
À
ing to the Leu Val peptide bond in 12b, whereas our
À
cyclization occurs at the Val Trp peptide bond. Then,
simultaneous removal of the tert-butyl ester and triethylsilyl
groups was accomplished by the action of B-bromocatechol-
borane,[19] and subsequent methyl esterification provided 14b.
This compound is closely related to stephanotic acid methyl
ester, a natural product derivative previously synthesized by
the Moody group[11] with identical configuration to the left-
1
hand ring of 1. By comparison of H NMR data, particularly
for hydrogen atoms directly attached to the macrocycle, we
tried to determine whether or not 14b possessed the requisite
configuration for conversion into 1. We discovered that the
1H NMR data of 14b and stephanotic acid methyl ester
matched extremely well,[20] thereby giving us confidence that
the major isomer obtained from the radical conjugate
addition was of identical configuration to 1.
Coupling of the mixture of amines 9b and 9c to
pyroglutamic acid provided peptides 10b and 10c in 96%
yield, but separation was not possible at this stage. Fortu-
nately, cleavage of the Cbz and benzyl ester moieties under
Scheme 2. Synthesis of left-hand ring. Reagents and conditions: a) TiCl4, NMM, THF/Et2O 2:1; b) Et3B, O2, Zn(OTf)2, iPrI, Bu3SnH, CH2Cl2,
À788C; c) SmI2, MeOH; d) pyroglutamic acid, EDCI, HOBt, THF, 08C to RT; e) 10% Pd/C, HCO2NH4, MeOH/H2O 5:1; f) HOBt, HBTU, DMF,
08C to RT; g) BCB, CH2Cl2; h) SOCl2, MeOH. NMM=N-methylmorpholine, THF=tetrahydrofuran, EDCI=N-(3-dimethylaminopropyl)-N’-ethyl-
carbodiimide hydrochloride, HOBt=1-hydroxybenzotriazole, HBTU=O-(Benzotriazol-1-yl)-N,N,N’,N’-tetramethyluronium hexafluorophosphate,
DMF=N,N-dimethylformamide, BCB=B-bromocatecholborane.
Angew. Chem. Int. Ed. 2009, 48, 6104 –6107
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6105

Communications
À
The stereochemical assignment of 14b was confirmed by
the indole and the Trp Pro tertiary amide), forming a
conversion of acid 13b into celogentin C as illustrated in
Scheme 3. Thus, coupling of 13b with Pro-OBn afforded
hexapeptide 15, the substrate for the crucial oxidative
dichlorinated intermediate. We believe that the rates of
both chlorinations are very similar, as no monochlorinated
species could be detected by mass spectrometry in reactions
conducted without Pro-
OBn. Then, the chlorine
atom at the undesired
site could be transferred
to Pro-OBn, affording
chlorinated amine 19
along with
a mono-
chlorinated intermedi-
ate which evolves into
product upon addition
of dipeptide 16 to the
mixture.[22] Elimination
of HCl from 19 would
produce imine 20 and
sequester the chlorine
atom as an HCl salt of
the base (1,4-dimethyl-
piperazine, or possibly
another equivalent of
Pro-OBn). In support
Scheme 3. Completion of the synthesis of 1. Reagents and conditions: a) Pro-OBn, EDCI, HOBt, THF, 08C to RT;
b) Pro-OBn (2 equiv), NCS (3 equiv), 1,4-dimethylpiperazine, CH2Cl2 then 16 (5 equiv); c) 10% Pd/C, HCO2NH4,
MeOH/H2O 5:1; d) HOBt, HBTU, DMF; e) TFA/H2O 9:1. Pbf=2,2,4,6,7-pentamethyldihydrobenzofuran-5-sulfonyl,
NCS=N-chlorosuccinimide, TFA=trifluoroacetic acid.
of this hypothesis,
dichlorinated inter-
mediate, a monochlori-
a
coupling reaction.[21] In contrast to previous results with
simpler substrates,[7a] the oxidative coupling of 15 and Arg–
His dipeptide 16[7a] lead to formation of a byproduct, and the
desired product was not detected. Mass spectrometry data
indicated the presence of an additional chlorine atom in the
undesired compound, and 1H NMR spectra of crude reaction
mixtures showed significant changes in the chemical shifts of
the proline hydrogen atoms. This suggested that the unwanted
chlorination was taking place on the proline residue. Fortu-
nately, a serendipitous discovery demonstrated the effective-
ness of the oxidative coupling when Pro-OBn was present in
the reaction mixture. Optimized conditions enlisted 2 equiv of
Pro-OBn in conjunction with 3 equiv of NCS, and an excess of
16 (5 equiv) was required to ensure a satisfactory reaction
rate. Separation of the oxidative coupling product from
unreacted 16 was most easily accomplished after Cbz and
OBn deprotection. In this way, octapeptide 17 could be
formed in 64% yield over two steps from 15.
nated intermediate, chloroamine 19, and imine 20 were all
detected by mass spectrometry in oxidative coupling reactions
with added Pro-OBn. Nonetheless, additional studies are
required to determine the precise role of this additive. Finally,
our ability to conduct successful indole–imidazole oxidative
couplings without Pro-OBn in the synthesis of the model
right-hand ring of 1[7a] can be understood by recognizing that
the indole moiety in the prior substrate was less hindered and
therefore more reactive than the indole of macrocycle 15.
Consequently, the desired chlorination of the indole was
significantly faster than the undesired chlorination, and only
monochlorinated intermediates were formed in the presence
of 1 equiv of NCS.
Consistent with our observations in the right-hand ring
model system,[7a] macrolactamization of 17 promoted by
HOBt/HBTU provided bicyclic peptide 18 in high yield
(83%) with no evidence of epimerization. Then, exposure of
18 to TFA caused scission of both the Pbf and tert-butyl ester
protecting groups, delivering 1 in 88% yield. Notably, and in
agreement with prior studies, the Pbf moiety could be
removed cleanly without complications arising from indole
alkylation that have been observed with the related Pmc and
Mtr groups (Pmc = 2,2,5,7,8-pentamethylchroman-6-sulfonyl,
One possible explanation for the role of Pro-OBn in the
oxidative coupling reaction is provided in Scheme 4. Com-
pound 15 reacts with NCS at two different sites (presumably
Mtr= 2,3,6-trimethyl-4-methoxybenzenesulfonyl).[23]
Fur-
thermore, no chromatographic purification of 1 was required
as long as its immediate precursor 18 was carefully purified on
SiO2.
1
The vast majority of signals in the H NMR spectrum of
synthetic 1 matched the spectrum of the natural product, and
NOE correlations (indole NH/imidazole H2 and Trp b-H/
imidazole H5) demonstrated that our synthetic material
possessed the correct configuration about the heterobiaryl
Scheme 4. Possible role of Pro-OBn in oxidative coupling.
6106 www.angewandte.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 6104 –6107

Angewandte
Chemie
axis.[24] However, the chemical shift of imidazole H2 differed
significantly from the natural sample. Further investigations
established the concentration, temperature, and pH depend-
ence of this signal.[25] Specifically, the imidazole H2 peak
shifted upfield as the sample of 1 was diluted or as the
temperature of the sample was increased. In contrast, this
signal shifted downfield if TFA was added to the solution. We
have observed this peak anywhere from d = 9.53 to 8.04 ppm.
Significantly, this range encompasses the reported chemical
shift of imidazole H2 in the natural sample (d = 9.41 ppm).[1]
Smaller, but analogous variations were observed with the
imidazole H5 signal (d = 7.83–7.40 range, 7.79 ppm in natural
sample). It is likely that the imidazole N3 atom of 1 is involved
in intermolecular hydrogen bonding and/or acid–base reac-
tions, thereby perturbing the chemical shifts of neighboring
atoms. Finally, an analytical sample of natural 1 was shown to
be identical to our synthetic material by reverse-phase
HPLC.[26]
[7] a) L. He, L. Yang, S. L. Castle, Org. Lett. 2006, 8, 1165; b) B. Ma,
D. N. Litvinov, G. S. C. Srikanth, S. L. Castle, Synthesis 2006,
3291; c) S. L. Castle, G. S. C. Srikanth, Org. Lett. 2003, 5, 3611.
[8] a) D. J. Bentley, C. J. Moody, Org. Biomol. Chem. 2004, 2, 3545;
b) J. R. Harrison, C. J. Moody, Tetrahedron Lett. 2003, 44, 5189;
c) M. F. Comber, C. J. Moody, Synthesis 1992, 731.
[9] A. K. L. Yuen, K. A. Jolliffe, C. A. Hutton, Aust. J. Chem. 2006,
59, 819.
[10] J. Michaux, P. Retailleau, J.-M. Campagne, Synlett 2008, 1532.
[11] For a total synthesis of the monocyclic peptide stephanotic acid
methyl ester, see: D. J. Bentley, A. M. Z. Slawin, C. J. Moody,
Org. Lett. 2006, 8, 1975.
[12] a) B. Banerjee, S. G. Capps, J. Kang, J. W. Robinson, S. L. Castle,
J. Org. Chem. 2008, 73, 8973; b) L. He, G. S. C. Srikanth, S. L.
Castle, J. Org. Chem. 2005, 70, 8140; c) G. S. C. Srikanth, S. L.
Castle, Org. Lett. 2004, 6, 449.
[13] For a review of radical conjugate additions, see: G. S. C.
Srikanth, S. L. Castle, Tetrahedron 2005, 61, 10377.
[14] Details of this unsuccessful approach will be provided in a
forthcoming full paper.
[15] Synthesized by the coupling of Boc-Leu and Val-OBn (EDCI,
HOBt, THF, 93%) and subsequent Boc deprotection (TFA,
CH2Cl2, quant.).
[16] a) S. G. Manjunatha, P. Chittari, S. Rajappa, Helv. Chim. Acta
1991, 74, 1071; b) S. G. Manjunatha, K. V. Reddy, S. Rajappa,
Tetrahedron Lett. 1990, 31, 1327.
[17] R. S. Fornicola, E. Oblinger, J. Montgomery, J. Org. Chem. 1998,
63, 3528.
[18] The configuration of the alkene in 8 was assigned based on the
X-ray crystal structure of a model compound: see referen-
ce [12b].
In conclusion, we have completed the synthesis of
celogentin C. This work constitutes the first total synthesis
of a member of the celogentin/moroidin family of bicyclic
antimitotic peptides. The two unusual side chain cross-links
were constructed by a Knoevenagel condensation–radical
À
conjugate addition sequence (Leu Trp linkage) and an
À
indole–imidazole oxidative coupling (Trp His linkage). The
latter reaction was successful only upon use of Pro-OBn as an
additive. The effect of Pro-OBn on the oxidative coupling is
quite intriguing, and further studies are in progress to more
clearly elucidate its role in the reaction. Additionally, we
discovered an unusual dependence of the chemical shifts of
the His imidazole hydrogen atoms of 1 on concentration,
temperature, and pH. Our route to 1 should provide access to
other members of the celogentin/moroidin family, and their
syntheses and anticancer activities will be the subjects of
future investigations.
[19] R. K. Boeckman, Jr., J. C. Potenza, Tetrahedron Lett. 1985, 26,
1411.
[20] See Supporting Information for details. Minor product 9d was
carried through a sequence identical to that shown in Scheme 2,
and the 1H NMR spectrum of the resulting product 14d matched
well with the 2,3-anti diastereomer of stephanotic acid methyl
ester prepared by the Moody group (compound diast-6 in
reference [11]). Details of this work will be provided in a
forthcoming full paper.
[21] a) K. I. Booker-Milburn, M. Fedouloff, S. J. Paknoham, J. B.
Strachan, J. L. Melville, M. Voyle, Tetrahedron Lett. 2000, 41,
4657; b) J. Bergman, R. Engqvist, C. Stꢁlhandske, H. Wallberg,
Tetrahedron 2003, 59, 1033; c) R. Engqvist, J. Bergman, Tetrahe-
dron 2003, 59, 9649.
[22] Dipeptide 16 is added to the reaction mixture 6 h after the other
reagents; see Supporting Information for detailed procedure.
[23] C. G. Fields, G. B. Fields, Tetrahedron Lett. 1993, 34, 6661.
[24] Regarding the NOE correlations observed with natural celo-
gentin C, Figure 4 and the accompanying text of reference [1]
are in error. The correct data can be located in Table 3 and on
the NOESY spectrum given in Supporting Information. This
discrepancy caused us previously to erroneously conclude that
our model right-hand ring of celogentin C was the non-natural
atropisomer (see reference [7a]).
Received: May 7, 2009
Published online: June 24, 2009
Keywords: natural products · oxidative coupling · peptides ·
.
radical reactions · total synthesis
[1] J. Kobayashi, H. Suzuki, K. Shimbo, K. Takeya, H. Morita, J.
Org. Chem. 2001, 66, 6626.
[2] a) T.-W. C. Leung, D. H. Williams, J. C. J. Barna, S. Foti, P. B.
Oelrichs, Tetrahedron 1986, 42, 3333; b) S. D. Kahn, P. M. Booth,
J. P. Waltho, D. H. Williams, J. Org. Chem. 1989, 54, 1901.
[3] a) Celogentins A–C: reference [1]; b) Celogentins D–H and J:
H. Suzuki, H. Morita, S. Iwasaki, J. Kobayashi, Tetrahedron 2003,
59, 5307.
[25] See Supporting Information for spectra.
[26] We were only able to obtain ca. 0.1 mg of natural 1. The presence
of contaminants in this material precluded us from acquiring an
informative 1H NMR spectrum.
[4] H. Suzuki, H. Morita, M. Shiro, J. Kobayashi, Tetrahedron 2004,
60, 2489.
[5] K. Yoshikawa, S. Tao, S. Arihara, J. Nat. Prod. 2000, 63, 540.
[6] a) H. Morita, K. Shimbo, H. Shigemori, J. Kobayashi, Bioorg.
Med. Chem. Lett. 2000, 10, 469; b) see also references [1] and [3].
Angew. Chem. Int. Ed. 2009, 48, 6104 –6107
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6107