The first enantioselective synthesis of cytotoxic marine natural product palau'imide and assignment of its C-20 stereochemistry

Article abstract of DOI:10.1039/c0cc00452a

Methyl tetramate derivative 6 has been developed as a new building block for the flexible and racemization-free synthesis of methyl 5-benzyl-3- methyltetramate via alkylation, and used in the first asymmetric synthesis of palau'imide (1). This allowed the establishment of the hitherto unknown stereochemistry at the C-20 of palau'imide as S.

Full text of DOI:10.1039/c0cc00452a

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The first enantioselective synthesis of cytotoxic marine natural product
palau’imide and assignment of its C-20 stereochemistryw
Hong-Qiao Lan,a Yuan-Ping Ruana and Pei-Qiang Huang*ab
Received 17th March 2010, Accepted 24th May 2010
First published as an Advance Article on the web 16th June 2010
DOI: 10.1039/c0cc00452a
Methyl tetramate derivative 6 has been developed as a new
building block for the flexible and racemization-free synthesis of
methyl 5-benzyl-3-methyltetramate via alkylation, and used in
the first asymmetric synthesis of palau’imide (1). This allowed
the establishment of the hitherto unknown stereochemistry at the
C-20 of palau’imide as S.
at the C-20 was unknown. Second, palau’imide (1) possesses a
hitherto unknown 3-methyltetramate, which is rare in naturally
occurring tetramic acids and tetramates.2 In addition, to date,
the conversion of 5-alkyltetramic acids to the corresponding
methyl 5-alkyltetramates remains the main entrance to the
latter sub-class of compounds,9,10 and this route may suffer
from partial racemization.4,10 A racemization-free method
developed by Tønder and co-workers was only demonstrated
by the synthesis of unnatural ethyl tetramates.11 Thus, there
is a need for the development of a direct enantioselective
entrance to methyl 5-alkyltetramates. Recently, Schobert and
co-workers reported a highly efficient route to 5-alkyltetra-
mates by addition of a-amino esters to polystyrene-bound
cumulated ylide Ph3PCCO.12 In view of a SAR study, more
flexible methods would be those allowing a flexible and direct
introduction of substituents at the C-5 of a methyl tetramate
derivative.
One of the contemporary trends in drug discovery is the
investigation of marine natural products as anticancer drugs.1
A growing number of marine originated bioactive natural
products possessing a methyl 5-alkyltetramate substructure
have been isolated.2 For example, antineoplastic dolastatin 15
(Fig. 1) was isolated in a minute amount from the Indian
Ocean sea hare Dolabella auricularia, which displayed extra-
ordinary cytotoxicity to cancer cells (IC50 value 2.9 nM);3
mirabimide E was isolated from the terrestrial blue green alga
Scytonema mirabile (Dillwyn) Bornet UH strain BY-8-1,4
which displayed murine solid tumor selective cytotoxicity in
the Corbett assay; sintokamides A to E were isolated recently
from the sponge Dysidea sp., which inhibited transactivation
of the N-terminus of the androgen receptor in prostate cancer
cells.5 Marine cyanobacteria, in particular, have afforded
numerous bioactive natural products.6 Palau’imide (1) is a
cytotoxic constituent of the apratoxin-producing marine
cyanobacterium Lyngbya sp. isolated from Palau in 2002.7 It
exhibited IC50 values of 1.4 and 0.36 mM for in vitro cytotoxicity
against KB and LoVo cells, respectively. The configuration at
the C-4 and C-5 of palau’imide has been determined, while the
stereochemistry at the C-20 of the 2-methylhexanoic acid
(Mha) residue remains unknown due to a lack of sufficient
material from natural sources. Because the third generation
dolastatin 15 analogue ILX-651 (or tasidotin) is an antitumor
agent currently undergoing Phase II trials,6e,8 the discovery of
palau’imide as a novel dolastatin 15 analogue contributes both
to SAR studies and to the development of potential drug leads
as anticancer agents.6d As such, the flexible total synthesis of
palau’imide (1) is highly desirable.
Previously, we have developed a flexible and direct approach
to (S)-5-alkyltetramic acid derivatives,13a and two asymmetric
approaches to N-protected (S)-5-alkyltetramate derivatives,
with the latter based on the use of (S)-phenylglycinol as a
chiral auxiliary.13b,c The racemization nature of the 5-alkyl-
tetramate systems under acidic, basic or reductive conditions,
required for removal of the chiral auxiliary, prompted us to
develop a new chiral auxiliary that is cleavable under mild
racemization-free conditions. We now report the use of tetra-
mate derivative 6, bearing an oxidatively cleavable14 chiral
auxiliary, as a precursor of methyl tetramate 5, and its applica-
tion to the first asymmetric synthesis of natural palau’imide (1).
Palau’imide (1) consists of a Mha residue, a valine residue
with an uncommon R-configuration, and a hitherto unknown
methyl 3-methyltetramate residue. Although the configuration
of the Mha residue was unknown, analogous lipid chains also
The challenges associated with the synthesis of natural
palau’imide (1) have been twofold. First, the stereochemistry
a Department of Chemistry and The Key Laboratory for Chemical
Biology of Fujian Province, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen, Fujian 361005,
P. R. China
b State Key Laboratory of Bioorganic and Natural Products
Chemistry, 354 Fenglin Lu, Shanghai 200032, P. R. China.
E-mail: pqhuang@xmu.edu.cn; Fax: (+) 86-592-2186400
w Electronic supplementary information (ESI) available: Experimental
procedures, NMR spectra and X-ray structure of 11. CCDC 756363.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c0cc00452a
Fig. 1 Naturally occurring antineoplastic/cytotoxic agents dolastatin
15 and palau’imide (1).
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Chem. Commun., 2010, 46, 5319–5321 | 5319

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yield from 85% to 95% (cf. ESIw). In such a manner, the chiral
auxiliary (S)-10 was synthesized from (S)-p-hydroxyphenyl-
glycine in four steps with an overall yield of 84%. Heating an
acetonitrile solution of bromide ester 9 and b-amino alcohol
(S)-10 in the presence of triethylamine at 80 1C for 6 h
produced tetramate derivative 6 in 78% yield. A chiral HPLC
analysis showed that the e.e. of building block 6 was 99.0%.
For the introduction of the C-5 benzyl group, methyl
tetramate derivative 6 was deprotonated with t-BuLi, and
the resulting enolate was allowed to react with benzyl bromide
at ꢁ78 1C for 7 h, which gave the C-5/C-3 benzylation
products (11 and 12 versus 13) in a 4 : 1 ratio (Scheme 3).
The diastereoselectivity of the C-5 regioisomer (11 : 12) was
6 : 1 in favor of the desired diastereomer (5S,10S)-11. The
stereochemistry of the newly formed stereocenter of the major
diastereomer 11 was tentatively deduced as S by analogy with
our previous results,13b which was confirmed by a single
crystal X-ray crystallographic analysis (cf. ESIw). The chiral
auxiliary was cleaved by treatment of tetramate derivative 11
with ceric ammonium nitrate (CAN) in MeCN–H2O, which
gave the desired tetramate 5 in 80% yield. A chiral HPLC
analysis showed that the e.e. of methyl-5-benzyl-3-methyl-
tetramate 5 was 99.0%.
Scheme 1 Retrosynthetic analysis of palau’imide (1).
exist in other cyanobacterial secondary metabolites such
as malevamide A (with a Mha residue),15 apramides A, D
(a 2-methyl-7-octynoic acid residue: Moya), apramides C, F
(a 2-methyl-7-octenoic acid residue: Moea),16 and dragon-
amides A, B (with a 2-methyloctanoic acid residue).17a Only
the stereochemistry of the 2-methyloctanoic acid residue presented
in dragonamide A was determined (as S).17b We envisioned
that the 2-methylhexanoic acid residue in palau’imide (1)
might also have the S-configuration. On the basis of these
considerations, we decided to synthesize palau’imide with an
S-configuration at the Mha residue.
For the synthesis of mixed imide 3, we envisioned the use of
Andrus’ pentafluorophenyl ester method.23 Thus Boc-(R)-
valine (R-14) was converted to the pentafluorophenyl ester 4
in 92% yield by Steglich’s method24 (DCC, C6F5OH, EtOAc)
(Scheme 4). Successive treatment of methyl tetramate 5 with
n-BuLi and activated amino ester 4 at ꢁ78 1C afforded mixed
imide 15 in 85% yield. Treatment of 15 with TFA (20% in
CH2Cl2) at 0 1C for 2 h afforded imide 3 chemoselectively in
93% yield. Finally, coupling of imide 3 with (S)-2 was
achieved by using the coupling system HOBt/EDCI/NMM
in DMF, which furnished palau’imide (1) in 72% yield. The
physical {[a]2D0 + 51 (c 0.32, MeOH), lit.7 [a]2D5 +50 (c 0.33,
MeOH)} and spectroscopic data of the synthetic compound
were identical with those reported for the natural product.7
These results revealed the configuration at the C-20 of
palau’imide (1) as S, and thus established that the natural
palau’imide (1) possess a (S,R,S)-configuration.
Our retrosynthetic analysis is displayed in Scheme 1. The
key feature of our approach resides on the flexible synthesis
of methyl tetramate derivative 5 from the new oxidatively
cleavable chiral auxiliary-bearing tetramate derivative 6 by
alkylation (benzylation).13a,b
The known (S)-2-methylhexanoic acid18 {2: [a]D20 + 22.6
(c 5.1, Et2O)} was synthesized by Evans’ method (cf. ESIw).19
The synthesis of the g-bromoester derivative 9, required for
the synthesis of tetramate derivative 6, is outlined in Scheme 2.
Reaction of methyl 2-methylacetoacetate (7) with trimethyl
orthoester in the presence of a catalytic amount of concentrated
sulfuric acid, followed by thermolysis of the resultant acetal at
130 1C,20 provided compound 8 in about 80% yield over two
steps. For the monobromination of the latter, a chemoselective
`
reaction vis-a-vis the two allylic methyl groups was required.
Gratifyingly, treatment of 8 with AIBN/NBS21 in THF at
60 1C gave (E)-4-bromo-3-methoxy-2-methyl-2-butenoate 9 in
86% yield (cf. ESIw).
To conclude, tetramate derivative 6 has been shown to be a
suitable building block for the direct synthesis of methyl
5-benzyl-3-methyltetramate (5) in high enantioselectivity. In
light of our previous results,13a,b different alkyl groups might
The chiral auxiliary 10 was prepared from commercially
available (S)-p-hydroxyphenylglycine by analogy with Ciufolini’s
procedure.22 Modification of the chemoselective reduction
step (LAH, THF, ꢁ20 to 0 1C) allowed improvement of the
Scheme 2 Synthesis of the new tetramate chiral building block 6.
5320 | Chem. Commun., 2010, 46, 5319–5321
Scheme 3 Regio- and diastereo-selective benzylation of 6.
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Scheme 4 Synthesis of palau’imide (1).
be introduced at the C-5 position that renders the method
flexible for the synthesis of other methyl 5-alkyltetramates. On
the basis of this method, the first asymmetric synthesis of
palau’imide (1) has been achieved in 9 steps starting from
compound 7 with an overall yield of 15%. This allowed the
confirmation of our assumption on the hitherto unknown
stereochemistry at the C-20 of palau’imide (2-methylhexanoic
acid residue) as S. The result also gave an indication about the
stereochemistry at the 2-methylhexanoic acid residue of the
cyanobacterial secondary metabolite malevamide A, which is
still unknown.
The authors are grateful to the NSF of China (20832005)
and the National Basic Research Program (973 Program) of
China (Grant No. 2010CB833200) for financial support.
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Chem. Commun., 2010, 46, 5319–5321 | 5321