Total synthesis of complestatin (chloropeptin II)

Article abstract of DOI:10.1002/anie.200906797

(Chemical Equation Presented) Substrate-dependent atropostereoselectivity: Cyclization of a linear DEFG tripeptide by an intramolecular Suzuki-Miyaura reaction afforded the 16-membered DEFG ring with complete atroposelectivity. Intramolecular SNAr reaction of a hexapeptide that contained a prebuilt DEFG ring afforded the bismacrocycle ABCDEFG that was converted to complestatin.

Full text of DOI:10.1002/anie.200906797

Communications
DOI: 10.1002/anie.200906797
Total Synthesis
Total Synthesis of Complestatin (Chloropeptin II)**
Zhihui Wang, Michꢀle Bois-Choussy, Yanxing Jia, and Jieping Zhu*
Complestatin (1) was first isolated in 1980 from the mycelium
of Streptomyces lavendulae as an inhibitor of alternative
pathways to the complement cascade;[1] its planar structure
was elucidated by Seto and co-workers in 1989.[2] Chloropep-
tin I (2)[3] was isolated in 1994 from Streptomyces sp. WK-
3419. The absolute configurations of the amino acid constit-
uents of 1[2,4] and 2 (Figure 1)[5] were elucidated through
detailed NMR spectroscopy, computational, and degradation
studies. Structurally, complestatin (1) and chloropeptin I (2)
differ only at the position of the substituted phenyl–indole
ring junction and it has been demonstrated that the former is
readily isomerized to the latter under mild acidic conditions.[6]
Biogenetically, both natural products are linear non-riboso-
mal peptides that have undergone oxidative phenolic cou-
pling to produce the rigid cross-linked architecture.[7] Inter-
estingly, although the shape of the rings and central strand in 1
and 2 strongly resembles that of the vancomycin family of
glycopeptide antibiotics,[8] the biological activities of 1 and 2
are completely different to that of vancomycin. Indeed, both 1
and 2 are inactive against Gram-positive bacteria, but display
potent activities against HIV-1-induced cytopathicity, syncyc-
tium formation in CD-4 lymphocytes, and inhibit HIV
replication by inhibition of gp 120-CD-4 binding at a low-
micromolar level (IC50 = 3.3 and 2.0 mm, respectively).[4,9] It
has also been demonstrated that complestatin blocks both
NMDA and AMPA neurotoxicity in a noncompetitive and
reversible manner. Therefore, it could also be potentially
useful for preventing excitotoxicity under certain pathological
conditions.[10]
The complex molecular architecture and important bio-
Figure 1. Structures of complestatin (1), chloropeptin I (2), and iso-
complestatin (3).
logical activities of 1 and 2 have attracted much attention, and
have provided chemists with impetus for the development of
new synthetic strategies. To develop a successful synthesis,
two key issues need to be addressed: 1) epimerization during
and after peptide coupling must be avoided as complestatin
contains four aryl glycine units that are extremely prone to
racemization, even under mild basic conditions;[11] 2) the
construction of strained bismacrocycles with defined atro-
postereochemistry.[12] The group of Snapper and Hoveyda
accomplished the first total synthesis of chloropeptin I (2)[13]
and an atropisomer of complestatin named isocomplestatin
(3, Figure 1).[14] These total syntheses have also allowed
Snapper and Hoveyda to assign the aR configuration to the
axial chirality of both natural products 1 and 2. Very recently,
Boger and co-workers published the first total synthesis of
complestatin.[15]
[*] Dr. Z. H. Wang, Dr. M. Bois-Choussy, Dr. Y. Jia, Dr. J. Zhu
Centre de Recherche de Gif
Previous studies by our group on the synthesis of
complestatin have demonstrated that the atroposelectivity
of the intramolecular Suzuki–Miyaura reaction[16] for the
construction of the DEFG ring is highly substrate dependent.
Thus, building the DEFG ring onto the preformed ABCD
macrocycle (4) afforded the isocomplestatin skeleton with an
aS configuration at the biaryl axis,[14,17] whereas the cycliza-
tion of linear peptide DEFG, which was suitably functional-
ized to afford the fragment of complestatin (1), furnished the
DEFG ring 5 with an aR configuration, identical to comple-
statin (1).[18] Based on these results, our revised synthesis
Institut de Chimie des Substances Naturelles, CNRS
91198 Gif-sur-Yvette Cedex (France)
Fax: (+33)1-6907-7247
E-mail: zhu@icsn.cnrs-gif.fr
Homepage: http://www.icsn.cnrs-gif.fr/article.php3?id_
article=122
[**] We gratefully acknowledge financial support from the CNRS and the
Institut de Chimie des Substances Naturelles.
Supporting information for this article, including experimental
procedures, product characterization, and 1H and 13C NMR spectra
of the synthetic compounds, is available on the WWW under http://
dx.doi.org/10.1002/anie.200906797.
2018
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required an intramolecular Suzuki–Miyaura reaction to close
the DEFG ring, followed by an intramolecular SNAr reac-
tion[19] to build the entire bismacrocycle of complestatin (1).
To this end, we initially synthesized hexapeptide 6 from
compound 5.[20] However, our inability to deprotect the
isopropyl ethers in 6, as well as those in 5, without destruction
of these molecules forced us to reconsider the protective
group of central amino acid D. Herein, we report the
realization of a convergent total synthesis of complestatin (1).
Scheme 1. a) SOCl2, MeOH; b) BCl3, CH2Cl2,; c) d-N-Boc-3,5-dichloro-4-
hydroxyphenylglycine (9), DEPBT, NaHCO3, THF; d) TBSCl (9 equiv),
imidazole (18 equiv), DMF, 408C, 65% yield over 4 steps; e) 12m HCl
in MeCN (v/v=0.07); f) HATU, 2,5-lutidine, CH2Cl2/THF, 80% yield;
g) [PdCl2(dppf)]·CH2Cl2 (1 equiv), K2CO3 (10 equiv), dioxane/H2O
(15:1), 908C, 66% yield. HATU=O-(7-azabenzotriazol-1-yl)-N,N,N’,N’-
tetramethyluronium hexafluorophosphate; TBS=tert-butyldimethylsilyl;
DMF=N,N-dimethylformamide; DEPBT=3-(diethoxyphosphoryloxy)-
1,2,3-benzotriazin-4(3H)-one; dppf=1,1’-bis(diphenylphosphino)ferro-
cene; Boc=tert-butyloxycarbonyl.
Our synthesis started from the construction of TBS-
protected DEFG ring 15 (Scheme 1). Treatment of a meth-
anol solution of d-N-Boc-3-iodo-4,5-diisopropyloxy phenyl-
glycine (7)[18] with thionyl chloride afforded a one-pot N-Boc
deprotection and carboxylic acid esterification to provide
amino ester 8, after a BCl3-mediated deprotection of the
isopropyl ether,[21] in quantitative yield. Coupling of 8 with d-
N-Boc-3,5-dichloro-4-hydroxyphenylglycine (9) under opti-
mized conditions (DEPBT, NaHCO3, THF) furnished the
dipeptide 10 in 86% yield; the three free hydroxy groups were
then silylated to afford 11. Selective removal of the N-Boc
protecting group was realized under carefully controlled
conditions using a solution of concentrated HCl in acetoni-
trile (v/v = 0.07) to afford 12, which was employed directly in
the following step. Other conditions including that of
Sakaitani and Ohfune (TBSOTf, 2,6-lutidine, CH2Cl2)[22]
provided a low yield of the desired product. HATU-mediated
coupling of a crude sample of 12 with tryptophan derivative
13a provided the tripeptide 14a in 80% yield. Unfortunately,
attempts to cyclize 14a (P1 = Boc) under a variety of
conditions failed to produce the desired macrocycle. Consid-
ering that the presence of an N-Boc group may lead to a
conformation that disfavors the ring closure, we prepared 14b
by coupling of 12 with 13b (P1 = H). To our delight, the
intramolecular Suzuki–Miyaura reaction[23] of 14b (P1 = H)
under our previously developed conditions ([PdCl2-
(dppf)]·CH2Cl2 (1.0 equiv), K2CO3 (10.0 equiv), dioxane/
H2O (v/v = 15:1), 908C) took place smoothly, in spite of the
bulkiness of the TBS group, to afford the 16-membered
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cyclophane 15, together with a small amount of partially O-
desilylated compound 16, in 66% yield. The axial chirality of
15 was determined to be aR by detailed NOE studies (see the
Supporting Information) as well as from the characteristically
upfield shift of the a proton of amino acid F (d = 4.2 ppm).
This assignment was ultimately confirmed by subsequent
conversion of 15 into the natural product.
The synthesis of bicyclic compound 22 is shown in
Scheme 2. Protection of the indole nitrogen in 15 [Boc2O,
DMAP, MeCN/water (v/v = 1000:1)] afforded 17, in which the
TBS ether of ring E was concurrently converted into tert-
butoxycarbonate. The presence of a small amount of water in
the reaction mixture is of utmost importance. In its absence,
the reaction afforded a complex mixture of products, pre-
sumably owing to the tert-butoxylcarbonylation of backbone
amides.[24] The lability of the TBS ether on the E ring might be
due to the presence of two ortho chlorine atoms that caused
the phenoxide to be a better leaving group. Prolonged heating
of a 1,2-dichloroethane solution of 17 in the presence of
trimethyltin hydroxide effected the hydrolysis of both the
methyl ester and the tert-butoxycarbonate on the E ring to
afford carboxylic acid 18 in 68% yield.[25] Coupling of 18 with
tripeptide 19b, which was obtained from the acid-mediated
deprotection of 19a, in the presence of HATU, provided the
hexapeptide 20 in 60% yield. The incorporation of a base into
the reaction mixture caused side reactions and should be
avoided. Deprotection of the two TBS ethers was not trivial;
after a number of unsuccessful trials, phenol 21 was obtained
using an HBr-buffered DMF solution of potassium fluoride
(pH ꢀ 6). A stirred solution of the crude 21 in freshly distilled
DMSO in the presence of K2CO3 and 4 ꢀ molecular sieves at
308C provided bicyclic compound 22 in 62% overall yield
from an intramolecular SNAr reaction.[15,26] The presence of
water in the reaction mixture was detrimental as it led to a
complex mixture of products.
Although of no consequence, this intramolecular SNAr
reaction turned out to be highly atropostereoselective to
provide a single atropisomer whose configuration was deter-
mined to be pR on the basis of NOE studies (see the
Supporting Information). More importantly, the cyclization
was regioselective as the alternative 14-membered, 17-mem-
bered, and 20-membered cyclophanes that would result from
nucleophilic attack of the other hydroxy groups on rings A, C,
D, and E onto the fluoro nitro aromatic system were not
observed.[27] Detailed NMR spectroscopic studies indicated
that the tertiary amide of residue B existed exclusively as the
cis conformer, as is found in the natural product. Interestingly,
the cyclization of pentapeptide 23, which lacks the C-terminal
4-hydroxy phenylglycine unit afforded the pS atropisomer of
bismacrocycle 24 (Scheme 3), wherein the tertiary amide of
residue B existed preferentially as a trans isomer (character-
istic NOE cross-peaks: Ha–Hb, Hb–Hd, Hc–He, Ha–NMe,
NMe–Hf, dHa = 4.08 ppm versus 5.00 ppm in 22). The crucial
role of the C-terminal amino acid on the tertiary amide
configuration has been noted previously by the Smith
group[12e] and by ourselves.[17]
Scheme 2. a) Boc2O (10 equiv), DMAP (0.4 equiv), MeCN/H2O (v/
v=1000:1, c=0.003m), RT, overnight; b) Me3SnOH, ClCH2CH2Cl,
808C, 3 days, 48% yield over 2 steps; c) tripeptide 19b, HATU
(1.5 equiv), THF, 60% yield; d) KF (20 equiv), conc. aqueous HBr in
DMF (0.03m, 3 equiv), RT, 2 h; e) K2CO3, DMSO, 4 ꢀ M.S., 308C, 20 h,
62% yield over 2 steps.
reaction conditions, the reduction proceeded successfully
using SnCl2 as the reductant in MeOH. To guarantee a good
yield of aniline 25, the reaction had to be quenched as soon as
it went to completion. We found it convenient to perform this
reaction in [D4]methanol and to monitor the progress of the
reaction by 1H NMR spectroscopy. Treatment of a THF
solution of aniline 25 with tBuONO and H3PO2 afforded the
The completion of the synthesis of complestatin is
detailed in Scheme 4. Reduction of the nitro group in
compound 22 was problematic. After an extensive survey of
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In conclusion, we have developed an efficient synthesis of
complestatin (1). Intramolecular Suzuki–Miyaura and SNAr
reactions were employed for the construction of two macro-
cycles by the formation of aryl–aryl and aryl–aryl ether bonds,
respectively. A [3+3] segment coupling was used for the
construction of the hexapeptide backbone, which makes this
synthesis highly convergent.
Received: December 2, 2009
Published online: February 10, 2010
Keywords: Chloropeptin I · complestatin · macrocycles ·
.
peptides · total synthesis
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