Total synthesis of largamide H

Article abstract of DOI:10.1039/b921000h

Total synthesis of largamide H has been completed, utilising the oxidative elimination reaction of enantiomerically pure 2-amino-3-(phenylselenyl)butanoic acid residues to stereospecifically install both (Z)- and (E)-2,3-dehydro-2- aminobutanoic moieties.

Full text of DOI:10.1039/b921000h

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Total synthesis of largamide Hw
Shuo Liang,a Zhengshuang Xu*ab and Tao Ye*ab
Received (in College Park, MD, USA) 7th October 2009, Accepted 29th October 2009
First published as an Advance Article on the web 16th November 2009
DOI: 10.1039/b921000h
Total synthesis of largamide H has been completed, utilising the
oxidative elimination reaction of enantiomerically pure 2-amino-
3-(phenylselenyl)butanoic acid residues to stereospecifically
install both (Z)- and (E)-2,3-dehydro-2-aminobutanoic moieties.
strategy toward largamide H (1) as illustrated in Fig. 1. In
general, dehydroamino acids result in low peptide-coupling
yields because they are less reactive electrophiles toward
amide-bond formation and are also known as fairly reactive
Michael acceptors that react readily with soft nucleophiles.4
Therefore, we decided to employ 3-methyl-Se-phenylseleno-
cysteines to serve as the masked 2,3-dehydro-2-aminobutanoic
acid residues for incorporating in the macrocycle and to be
converted into the dehydroamino acids at a late stage,
preferably after global deprotection. The rationale behind this
decision is supported by the well-known syn stereochemistry of
selenoxide elimination.5 We also chose to construct the macro-
lactam ring via an intramolecular coupling between L-alanine
and glycine. Coupling reactions employing activated esters of
glycine residues are generally very efficient. In addition, since
glycine lacks an a-substituent, any possibility of epimerization
during the course of the activation/coupling sequence would
be avoided. Thus our target fragments for the assembly of the
macrocycle were peptides 3 and 4, incorporating threo- and
erythro-selenocysteine derivatives, respectively.
Secondary metabolites produced by cyanobacteria are a vital
source of potential target compounds for the pharmaceutical
industry. One particular species, Oscillatoria, is responsible for
the production of a considerable number of bioactive molecules.1
The compounds produced by these microorganisms possess a
wide array of structures, including cyclopeptides and cyclo-
depsipeptides that embody both polypeptide and polyketide
domains, displaying conformational flexibility. In addition,
the possibility of extracting these compounds from their
natural sources in significant quantities is very low. This,
together with their novel and complex structures, makes these
compounds a challenge to synthetic chemists. Indeed, many of
these compounds have already been produced synthetically.1
We have been interested for some time in marine cyclopeptides
and cyclodepsipeptides,2 and view their syntheses as a key
route to structural confirmation, structural modification and
subsequent activity control. Here we report the first total
synthesis of largamide H.
The synthesis commenced with the coupling of dipeptide
ester 56 with (2S,3S)-2-N-Boc-3-(phenylseleno)butanoic acid
(6)7 to afford tripeptide 7 in 59% yield (Scheme 1). The
synthesis of synthon 10 was achieved in four steps from the
chiral epoxide intermediate 8.8 Thus, treatment of alcohol
epoxide 8 with diisopropoxytitanium diazide under the
condition as described by Sharpless and co-workers9 led to
epoxide-opening products regioselectively (C-3 to C-2 ratio
of 10 : 1). Azido diol 9 was obtained as a single diastereomer in
76% yield after silica gel chromatography. The azide moiety
was readily reduced to the primary amine with hydrogen and
Pd/C followed by a PyAOP-mediated condensation10 with
Boc-L-Alanine to afford the corresponding amide in 81%
yield. Selective oxidation of the primary hydroxyl group by
the action of TEMPO/NaClO/NaClO211 produced the corres-
ponding acid 10 and set the stage for a fragment condensation.
Hence, removal of the N-Boc protecting group of 7 with TFA
produced the free amine which was coupled with hydroxy acid
10 to afford the key intermediate 3 in 54% yield. Hydrolysis of
the methyl ester was then carried out with n-Bu4NOH12 to give
the free acid 11. With acid 11 in hand, we next turned our
attention to the synthesis of the lower fragment 4, which is
composed of three subunits. N-terminal Fmoc protective
group of 1213 was cleanly removed with diethyl amine, and
the resulting free amine was coupled with (2R,3S)-2-N-Fmoc-
3-(phenylseleno)butanoic acid (13)14 to afford tripeptide 14 in
91% yield. N-Fmoc deprotection of 14 followed by a PyAOP-
mediated condensation with dipeptide 1515 furnished the
pentapeptide 4 in 79% yield. At this juncture, the time had
arrived to explore the assembly of two key fragments leading
Largamide H was isolated from the marine cyanobacterium
Oscillatoria sp. with the proposed stereochemistry as shown in
Fig. 1. Structurally, largamide H comprises ten stereogenic
centers, a novel 2,5-dihydroxylated b-amino acid moiety,
2-amino-5-(40-methoxyphenyl)pentanoic acid, and two resi-
dues of the nonstandard 2,3-dehydro-2-aminobutanoic acid
(Dab) inscribed in a 31-membered macrocycle. Its structure
and absolute configurations were determined by NMR, Mass
and Chiral HPLC techniques.3
There are three principal challenges associated with the
synthesis of largamide H: (1) the asymmetric construction of
a,d-dihydroxylated b-amino acid, (2) the formation of a
31-membered macrolactam, and (3) the formation and
incorporation of both (E)- and (Z)-Dab to the target molecule.
Bearing these challenges in mind, we devised a retrosynthetic
a Laboratory of Chemical Genomics, Peking University Shenzhen
Graduate School, University Town of Shenzhen, Xili,
Nanshan District, Shenzhen, China 518055.
E-mail: yet@szpku.edu.cn, xuzs@szpku.edu.cn;
Tel: +86 755 26032697
b Department of Applied Biology & Chemical Technology,
The Hong Kong Polytechnic University, Hung Hom, Kowloon,
Hong Kong, China. E-mail: bctaoye@inet.polyu.edu.hk;
Fax: +852 22641912; Tel: +852 34008722
w Electronic supplementary information (ESI) available: Full details
for experimental procedures for compounds 1, 3–5, 7–10, 12, 14, 15
and 17, and 1H and 13C NMR spectra for compounds 1, 3–4, 5b, 7–9,
10a, 12, 14, 15b and 17. See DOI: 10.1039/b921000h
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Fig. 1 Structure and retrosynthetic analysis of Largamide H.
to linear peptide precursor 17 for the final macrocyclization
Notes and references
reaction. Thus, treatment of 4 with diethylamine effected the
removal of the Fmoc protecting group to give 16 in 69% yield,
which underwent a HATU16-mediated coupling reaction
with fragment 11 to provide 17 in 58% yield. Simultaneous
deprotection of the tert-butyl ester, trityl and Boc-protecting
groups afforded the desired amino acid which was immediately
activated by HATU to afford cyclopeptide 2 in 46% overall
yield.17 Gratifyingly, upon treatment with sodium periodate,
both phenylselenide groups in 2 were converted into their
corresponding selenoxides that underwent concomitant
syn b-eliminations to afford largamide H in 60% yield.
The spectral data (1H and 13C NMR) of synthetic largamide
H ([a]2D0 ꢁ79.2, c 0.34, MeOH) were identical to that of natural
largamide H (lit. ([a]D20 ꢁ80.6, c 0.36, MeOH).3 With the
synthetic largamide H in hand, the screening of cytotoxic
activities toward cancer cell lines other than HCT-116 is
currently under investigation and will be reported in due
course.
1 A. Me
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In summary, we have developed a convergent synthesis of
largamide H. The key to success of our synthetic route was the
selection of L- and L-allo-threonine derived selenide precursors
and the use of oxidative elimination processes to control the
stereochemistry of 2,3-dehydro-2-aminobutanoic acid units
presented in the natural product.
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We acknowledge financial support from the Hong
Kong Research Grants Council (Project: PolyU 5407/06M)
and financial support from the Shenzhen Bureau of Science,
Technology and Information (08systs-01, JC200903160367A).
Z. S. X. would like to thank the support from Shenzhen
Foundation for R&D (SZKJ-2007011, SY2008063 00179A),
Nanshan Science & Technology (NANKEYUAN2007019)
and NSF of GuangDong Province (8451805704000656).
6 Dipeptide
5 was prepared from L-valine and L-2-amino-5-
(p-methoxyphenyl)pentanoic acid: P. N. Collier, A. D. Campbell,
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Scheme 1 Reagents and conditions: (a) 6, PyAOP, DIPEA, CH2Cl2, 59%; (b) TMSN3, Ti(OiPr)4, benzene, reflux, 76%; (c) Pd/C, H2; (d) Boc-L-
Ala-OH, PyAOP, DIPEA, 81% (two steps); (e) TEMPO, NaClO2, NaClO, phosphate buffer pH = 6.5, 35 1C; (f) 7, TFA, CH2Cl2; (g) 10, PyAOP,
DIPEA, CH2Cl2, 54% (three steps); (h) n-Bu4NOH, MeOH–THF–H2O, 100%; (i) Et2NH, MeCN, 100%; (j) 13, PyAOP, collidine, HOAt,
CH2Cl2, 91%; (k) Et2NH, MeCN, 100%; (l) 15, PyAOP, collidine, HOAt, CH2Cl2, 79%; (m) Et2NH, MeCN, 69%; (n) HATU, DIPEA,
DMF–CH2Cl2, 58%; (o) TFA, CH2Cl2; (p) HATU, DIPEA, DMF, 46%; (q) NaIO4, CH2Cl2–H2O, 60%.
M. Kohno, T. Goto, K. Suzuki, T. Mori, K. Hashimoto and
M. Nakata, Tetrahedron Lett., 2004, 45, 3707.
8 Epoxide alcohol 8 was prepared in six steps and 49% overall yield
from octanal (for details see ESIw).
9 M. Caron, P. R. Carlier and K. B. Sharpless, J. Org. Chem., 1988,
53, 5185.
10 F. Albericio, M. Cases, J. Alsina, S. A. Triolo, L. A. Carpino and
S. A. Kates, Tetrahedron Lett., 1997, 38, 4853.
14 (2R,3S)-2-N-Fmoc-3-(phenylseleno)butanoic acid was prepared
from (2R,3S)-2-N-Boc-3-(phenylseleno)butanoic acid, which was
in turn synthesized according to the procedure shown in
ref. 7.
15 Dipeptide acid 15 was prepared from N-(9-fluorenylmethoxy-
carbonyl)-L-threonine and N2-methyl-N2-[(2-nitrophenyl)sulfonyl]-
N-(triphenylmethyl)-L-asparagine methyl ester in three steps and
66% overall yield (for details see ESIw).
11 M. Zhao, J. Li, E. Mano, Z. Song, D. M. Tschaen, E. J. J.
Grabowski and P. J. Reider, J. Org. Chem., 1999, 64, 2564.
12 U. M. Reinscheid, B. D. Zlatopolskiy, C. Griesinger, A. Zeeck and
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and A. El-Faham, J. Org. Chem., 1995, 60, 3561.
17 Macrolactamization of the linear peptide (1–2 mM, DMF) using
HATU (2 equiv.) in the presence of DIPEA (10 equiv.) afforded 2
in 46% yield. Head-to-tail cyclizations were also performed using
DEPC and DPPA, respectively, but resulted in lower yields
(10–14%).
S. Hoger, Tetrahedron, 2003, 59, 9441.
¨
13 Dipeptide ester 12 was prepared in 87% yield by a condensation
between N2-Fmoc-N-(triphenylmethyl)-L-glutamine and tert-butyl
N-methylglycinate (for details see ESIw).
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Chem. Commun., 2010, 46, 153–155 | 155