Total synthesis of grassypeptolide

Article abstract of DOI:10.1039/c0cc01200a

The first total synthesis of grassypeptolide, an anticancer cyclodepsipeptide isolated from marine cyanobacteria, has been achieved in 17 steps and an overall 11.3% yield.

Full text of DOI:10.1039/c0cc01200a

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Chemical Communications
www.rsc.org/chemcomm
Volume 46 | Number 40 | 28 October 2010 | Pages 7445–7640
ISSN 1359-7345
FEATURE ARTICLE
James Stephen Harvey and
Véronique Gouverneur
Catalytic enantioselective synthesis
of P-stereogenic compounds
COMMUNICATION
Zhengshuang Xu and Tao Ye et al.
Total synthesis of grassypeptolide

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COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Total synthesis of grassypeptolidew
Hui Liu,a Yuqing Liu,b Xiangyou Xing,a Zhengshuang Xu*ab and Tao Ye*ab
Received 30th April 2010, Accepted 6th July 2010
DOI: 10.1039/c0cc01200a
The first total synthesis of grassypeptolide, an anticancer cyclo-
depsipeptide isolated from marine cyanobacteria, has been
achieved in 17 steps and an overall 11.3% yield.
by a coupling with hydroxy acid 94 using bis(2-oxo-3-
oxazolidinyl)phosphinic chloride (BOPCl)5 in the presence of
triethylamine furnished the corresponding tripeptide which
was deacetylated to afford 10 in 95% yield. 2-Methyl-3-
aminobutyric acid (11) was prepared from N-Cbz-D-alanine
by a published route involving diazoketone formation,6 Wolff
rearrangement6,7 and a diastereoselective methylation.2m,8
Condensation of acid 11 with alcohol 10, using various
coupling agents including EDCI–DMAP,9 DCC–DMAP,9
and Mukaiyama reagent10 always resulted in incomplete
conversion and low yield presumably due to the congested
steric environment and the reduced reactivity of the a-hydroxy
amide. After extensive experimentation, we found that the acyl
chloride, derived from acid 11, was superior for this difficult
coupling, and the key coupling product 4 was obtained in
90% yield.
Grassypeptolide (1), isolated from an extract of Lyngbya
confervoides collected off Grassy Key in Florida, is a potential
anticancer cyclodepsipeptide.1 Grassypeptolide inhibited
cancer cell growth with IC50 values from 1.0 to 4.2 mM. The
gross chemical structure of grassypeptolide was established
using a combination of chemical and spectral techniques. The
absolute configuration of the thiazolines was elucidated through
X-ray analysis. Grassypeptolide possesses a 31-membered
macrolactone which is composed of unique, non-proteinogenic
amino acid residues such as 2-methyl-3-aminobutyric acid
(Maba), 2-aminobutyric acid (Aba), several D- and N-methylated
amino acids and a tandem thiazoline-ring moiety.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 sub-
sequent activity control. Here we report the first total synthesis
of grassypeptolide.
With the key intermediate 4 in hand, we next turned our
attention to the synthesis of the lower fragment 5, which
contains two thioamide bonds (Scheme 2). Thus, D-allo-threonine
methyl ester (12) was reacted with N-Boc-N-methyl-D-leucine
(13) using EDCI9 with HOBt9 to afford dipeptide 14 in 93%
yield. The Boc group in 14 was removed with trimethylsilyl
trifluoromethanesulfonate (TMSOTf) in the presence of
2,6-lutidine,11 and the resulting free amine underwent a
BOPCl-mediated coupling reaction5 with N-Cbz-O-TBS-L-
Ser-OH to afford tripeptide 15 in 89% yield.12 Hydrogenolysis
of the Cbz group of tripeptide 15 provided the corresponding
free amine, which was coupled with the thioacylating agent 16,
prepared using the method described by Rapoport,13 to afford
the thioamide 17 in 88% yield. To continue the linear chain
elongation, Boc group was selectively removed with TMSOTf/
2,6-lutidine11 to afford the corresponding amine, which was
The principal synthetic challenges associated with the prepa-
ration of grassypeptolide are the efficient formation of the
delicate macrocyclic depsipeptide and assembly of the sensitive
thiazoline3 heterocycles in the presence of a secondary hydroxyl
group. In our retrosynthetic strategy, we envisioned macro-
cyclization and late-stage introduction of the labile tandem
thiazoline-ring moiety (Fig. 1). Thus, grassypeptolide would
be derived from the macrocycle 2 by cyclodehydration of
b-hydroxy thioamides. It was decided to close the macrocycle
at the proline (C37–41)/N-Me-Phe-thn-ca (C24–36) peptide
bond, despite the known problem of slower acylation of
secondary amines. Further retrosynthetic analysis reveals that
the linear precursor 3 may be most conveniently constructed
by the assembly of the two fragments 4 and 5 with approxi-
mately equal complexity.
The synthesis of fragment 4 commenced with the coupling
of L-proline tert-butyl ester (6) with N-Cbz-N-methyl-L-valine
(7) to afford dipeptide 8 in 84% yield (Scheme 1). Hydro-
genolysis removal of the N-Cbz protecting group in 8 followed
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
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;
Tel: +852 34008722
w Electronic supplementary information (ESI) available: Full details
for experimental procedures, 1H and 13C NMR spectra for compounds
1, 4, 5, 21. See DOI: 10.1039/c0cc01200a
Fig. 1 Retrosynthetic analysis of grassypeptolide.
c
7486 Chem. Commun., 2010, 46, 7486–7488
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Scheme 1 (a) EDCI, HOBt, DIPEA, CH2Cl2, 0 1C to RT, 84%;
(b) 10% Pd/C, H2, MeOH–EtOAc; (c) 9, BOPCl, Et3N, CH2Cl2, 0 1C
to RT, 92% from 8; (d) K2CO3, MeOH, 0 1C, 95%; (e) 11, (COCl)2,
cat. DMF, CH2Cl2, 0 1C to RT, then 10, cat. DMAP, NMM, CH2Cl2,
0 1C to RT, 90%.
condensed with N-Boc-O-TBS-L-Ser-OH employing various
coupling reagents. Only when PyAOP14 was employed as a
coupling agent, the reaction afforded amide 18 in high yield.
To our surprise, the condensation resulted generally in sluggish
reactions and low yields when other coupling reagents such as
EDCI–HOAt,9 HATU,9 BOPCl,5 and Mukaiyama reagent10
were used. Intermediate 18 was then elaborated to the key
fragment 5 in 82% yield12 by an identical strategy as described
for 17, including Boc deprotection and thiolation of the
resulting free amine with thioacylating agent 19. Hydrolysis
of the methyl ester was then carried out with lithium hydroxide
to give free acid 20.
Scheme 3 (a) 10% Pd/C, H2, EtOAc, RT; (b) PyAOP, DIPEA,
CH2Cl2, 0 1C to RT, 81%; (c) 1. TMSOTf, 2,6-lutidine, CH2Cl2,
RT; 2. BOPCl, 2,6-lutidine, CH2Cl2 (0.001 M), 0 1C to RT, 72%; (d)
TrocCl, Py, CH2Cl2, 0 1C; (e) TBAF/HOAc, THF, 0 1C to RT; (f)
DAST, CH2Cl2, À78 1C to À50 1C; (g) Zn, 1 M NH4OAc, THF, 0 1C,
37% from 21.
With the two key fragments in hand, the stage was now set
for their assembly and elaboration into grassypeptolide
(Scheme 3). Thus, hydrogenolysis of the Cbz protecting group
in 4 afforded the corresponding amine, which underwent a
PyAOP-mediated coupling reaction14 with acid 20 to provide
the linear precursor 3 in 81% yield. Simultaneous removal of
the tert-butyl ester15 and Boc-protecting group was achieved
by treatment of 3 with TMSOTf/2,6-lutidine at room tempera-
ture to produce the desired amino acid which was immediately
activated by BOPCl5 in the presence of 2,6-lutidine to afford
cyclodepsipeptide 21 in 72% yield16 and without noticeable
epimerization evident by NMR analysis. The secondary
hydroxyl group of 21 was protected as Troc ester,17 and the
existing two primary tert-butyldimethylsilyl protecting groups
were selectively removed using acetic acid buffered tetrabutyl-
ammonium fluoride to give the corresponding b-hydroxy
thioamide 2. Activation of the primary hydroxyl groups of
thioamide 2 with diethylaminosulfur trifluoride (DAST)18 in
CH2Cl2 at À78 1C led to the cyclized product 22 which was
then treated with activated zinc and aqueous NH4OAc in THF
to afford the fully synthetic grassypeptolide (1), in 37% yield
(Scheme 3).
The spectral data for the synthetic material (1H NMR,
13C NMR, and HRMS) were identical to those published
for the natural product and the optical rotation [a]2D0 +87
(c 0.12, CH2Cl2) was in close agreement with values reported
in the literature for natural grassypeptolide, [a]2D0 +76
(c 0.1, CH2Cl2). Furthermore, biological evaluation using cell
proliferation assay confirmed that the inhibitory effect of
synthetic grassypeptolide (1) on cancer cell proliferation is
Scheme 2 (a) EDCI, HOBt, Et3N, CH2Cl2, 0 1C to RT, 93%; (b) 1.
TMSOTf, 2,6-lutidine, CH2Cl2, RT; 2. N-Cbz-O-TBS-L-Ser-OH,
BOPCl, DIPEA, CH2Cl2, 0 1C to RT, 89%; (c) 1. Pd/C, H2, MeOH,
RT; 2. 16, THF, 0 1C, 88%; (d) 1. TMSOTf, 2,6-lutidine, CH2Cl2, RT;
2. N-Boc-O-TBS-L-Ser-OH, PyAOP, DIPEA, CH2Cl2, 0 1C to RT,
88%; (e) 1. TMSOTf, 2,6-lutidine, CH2Cl2, RT; 2. 19, THF, 0 1C, then
acidic work-up, 82%; (f) 1 N LiOH, tert-BuOH/THF, 0 1C.
c
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similar to that of naturally occurring grassypeptolide. The
average IC50 values of Hela and of HT29 were 1.9 and 2.3 mM,
respectively. The detailed biological studies, including the
biological mode of action, are currently under way and will
be reported in due course.
(b) J. Cabre and A. L. Palomo, Synthesis, 1984, 413–417;
(c) R. D. Tung and D. H. Rich, J. Am. Chem. Soc., 1985, 107,
4342–4343; (d) R. Baker and J. Castro, J. Chem. Soc., Chem.
Commun., 1989, 378–381; (e) Y. Hamada, Y. Kondo, M. Shibata
and T. Shioiri, J. Am. Chem. Soc., 1989, 111, 669–673;
(f) W. J. Colucci, R. D. Tung, J. A. Petri and D. H. Rich,
J. Org. Chem., 1990, 55, 2895–2903.
In summary, we have developed a convergent synthesis of
grassypeptolide. Key strategic elements of the approach include
the late stage introduction of the sensitive tandem thiazoline
heterocycles which are potentially highly sensitive and prone to
epimerization at no less than four of its stereogenic sites, and
31-membered macrocyclization conducted at the sterically con-
gested secondary amide site in superb conversion (72% yield).
Pre-organization of the cyclization precursor resulting in
more favorable cyclization kinetics could be attributed to the
efficiency of this transformation.
6 (a) M. P. Doyle, M. A. McKervey and T. Ye, Modern Catalytic
Methods for Organic Synthesis with Diazocompounds, Wiley,
New York, 1998; (b) T. Ye and M. A. McKervey, Chem. Rev.,
1994, 94, 1091.
7 W. Kirmse, Eur. J. Org. Chem., 2002, 2193–2256.
8 (a) H. Estermann and D. Seebach, Helv. Chim. Acta, 1988, 71,
1824–1839; (b) J. Podlech and D. Seebach, Liebigs Ann., 1995,
1217–1228.
9 Abbreviations: EDC
=
1-[3-(dimethylamino)propyl]-3-ethyl-
carbodiimide hydrochloride; HOBt = 1-hydroxybenzotriazole;
HOAt = 1-hydroxy-7-azabenzotriazole; HATU = 2-(1H-7-aza-
benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate;
PyAOP = (7-azabenzotriazol-1-yloxy)tripyrrolidinophosphonium
hexafluorophosphate); BEP = 2-bromo-1-ethylpyridinium tetra-
fluoroborate.
We acknowledge financial support from the Hong Kong
Research Grants Council (Projects: PolyU5407/06M and
PolyU5638/07M); financial support from the Shenzhen
10 T. Mukaiyama, K. Saigo, E. Shimada and M. Usui, Chem. Lett.,
1975, 1045–1048.
Bureau
of
Science,
Technology
&
Information
11 M. Sakaitani and Y. Ohfune, J. Org. Chem., 1990, 55, 870–876.
12 Presumably, the secondary hydroxyl group in intermediates 15, 18,
5, 21 has been converted into the corresponding trimethylsilyl ether
during the removal of Boc group with TMSOTf. The respective
coupling product was exposed to mild acidic work-up and purified
by chromatography on silica gel, which effected hydrolysis of
trimethylsilyl ether. Therefore, the isolated product was characterized
as its free secondary alcohol.
(JC200903160367A, ZD200806180051A), Shenzhen Founda-
tion for R&D (SY200806300179A, JC200903160372A), and
Nanshan Science & Technology (NANKEYUAN2009083,
NANKEPING2007005).
Notes and references
1 J. C. Kwan, J. R. Rocca, K. A. Abboud, V. J. Paul and H. Luesch,
Org. Lett., 2008, 10, 789–792.
13 M. A. Shalaby, C. W. Grote and H. Rapoport, J. Org. Chem.,
1996, 61, 9045–9048.
2 (a) Z. Chen, L. Song, Z. S. Xu and T. Ye, Org. Lett., 2010, 12,
2036–2039; (b) Y. Jin, Y. Q. Liu, Z. Wang, S. Kwong, Z. S. Xu and
T. Ye, Org. Lett., 2010, 12, 1100–1103; (c) B. Chen, L. Dai,
H. Zhang, W. Tan, Z. S. Xu and T. Ye, Chem. Commun., 2010,
46, 574–576; (d) S. Liang, Z. S. Xu and T. Ye, Chem. Commun.,
2010, 46, 153–155; (e) S. Li, S. Liang, W. F. Tan, Z. S. Xu and
T. Ye, Tetrahedron, 2009, 65, 2695–2702; (f) Q. Ren, L. Dai,
H. Zhang, W. F. Tan, Z. S. Xu and T. Ye, Synlett, 2008,
2379–2383; (g) S. Li, S. Liang, Z. S. Xu and T. Ye, Synlett,
2008, 569–574; (h) Z. Chen and T. Ye, New J. Chem., 2006, 30,
518–520; (i) H. W. Pang, Z. S. Xu, Z. Y. Chen and T. Ye, Lett.
Org. Chem., 2005, 2, 699–702; (j) Y. G. Peng, H. W. Pang, Z. S. Xu
and T. Ye, Lett. Org. Chem., 2005, 2, 703–706; (k) H. L. Chen,
Z. S. Xu and T. Ye, Tetrahedron, 2005, 61, 11132–11140;
(l) Z. Y. Chen and T. Ye, Synlett, 2005, 2781–2785;
(m) Y. G. Peng, H. W. Pang and T. Ye, Org. Lett., 2004, 6,
3781–3784; (n) Z. Y. Chen, J. G. Deng and T. Ye, ARKIVOC,
2003, (Part VII), 268–285; (o) Z. S. Xu, Y. G. Peng and T. Ye, Org.
Lett., 2003, 5, 2821–2824.
14 F. Albericio, M. Cases, J. Alsina, S. A. Triolo, L. A. Carpino and
S. A. Kates, Tetrahedron Lett., 1997, 38, 4853–4856.
15 (a) D. A. Evans, H. P. Ng and D. L. Rieger, J. Am. Chem. Soc.,
1993, 115, 11446–11459; (b) A. Trzeciak and W. Bannwarth,
Synthesis, 1996, 1433–1434.
16 Macrolactamization of the linear depsipeptide using BEP or
PyAOP was also performed, but resulted in only trace product.
17 (a) B. E. Maryanoff, M. N. Greco, H.-C. Zhang, P. Andrade-
Gordon, J. A. Kauffman, K. C. Nicolaou, A. Liu and
P. H. Brungs, J. Am. Chem. Soc., 1995, 117, 1225–1239;
(b) K. Morihira, R. Hara, S. Kawahara, T. Nishimori,
N. Nakamura, H. Kusama and I. Kuwajima, J. Am. Chem. Soc.,
1998, 120, 12980–12981; (c) H. Kusama, R. Hara, S. Kawahara,
T. Nishimori, H. Kashima, N. Nakamura, K. Morihira and
I. Kuwajima, J. Am. Chem. Soc., 2000, 122, 3811–3820;
(d) C. B. Lee, Z. Wu, F. Zhang, M. D. Chappell, S. J. Stachel,
T.-C. Chou, Y. B. Guan and S. J. Danishefsky, J. Am. Chem. Soc.,
2001, 123, 5249–5259; (e) T. Doi, Y. Numajiri, A. Munakata and
T. Takahashi, Org. Lett., 2006, 8, 531–534; (f) H. Tanaka,
Y. Nishiura and T. Takahashi, J. Am. Chem. Soc., 2008, 130,
17244–17245; (g) B. M. Andresen and J. Du Bois, J. Am. Chem.
Soc., 2009, 131, 12524–12525.
3 (a) P. Wipf and P. Fritch, Tetrahedron Lett., 1994, 35, 5397–5400;
(b) C. D. J. Boden, G. Pattenden and T. Ye, Synlett, 1995, 417–419.
4 For the preparation of 9, please see: A. B. Hughes and
M. M. Sleebs, J. Org. Chem., 2005, 70, 3079–3088.
5 (a) J. Diago-Meseguer, A. L. Palomo-Coll, J. R. Fernandez-
Lizarbe and A. Zugaza-Bilbao, Synthesis, 1980, 547–551;
18 (a) A. L. Johnson, J. Org. Chem., 1982, 47, 5220–5222;
(b) P. Lafargue, P. Guenot and J. P. Lellouche, Synlett, 1995,
171–172.
c
7488 Chem. Commun., 2010, 46, 7486–7488
This journal is The Royal Society of Chemistry 2010