Total synthesis of salinamide A: A potent anti-inflammatory bicyclic depsipeptide

Article abstract of DOI:10.1002/anie.200800397

It's swell! The first total synthesis of potent anti-inflammatory agent salinamide A was achieved. This synthesis features a concise elaboration of the phenylglycine-derived epoxide fragment and the identification of two possible macrolactamization sites (see scheme). (Chemical Equation Presented)

Full text of DOI:10.1002/anie.200800397

Communications
DOI: 10.1002/anie.200800397
Natural Products
Total Synthesis of Salinamide A: A Potent Anti-Inflammatory
Bicyclic Depsipeptide**
Li Tan and Dawei Ma*
Salinamide A (1; Figure 1) is a bicyclic hexadepsipeptide that
was first isolated from Streptomyces sp. CNB-091, which is an
actinomycete found on the surface of jellyfish collected in the
anti-inflammatory drugs imply that these natural anti-inflam-
matory agents might have novel modes of action. A total
synthesis and structure-activity relationship (SAR) studies of
these depsipeptides would be helpful for fully addressing this
question, thereby providing the opportunity to develop a new
generation of anti-inflammatory drugs. Whereas cyclomar-
ins[5] and halipeptins[6] have become attractive targets for the
synthetic community, salinamide A has not yet been syntheti-
cally accessible. Herein, we disclose the first total synthesis of
this natural product.
Upon close examination of the structure of salinamide A,
which was established by X-ray analysis of its chlorohydrin
derivative,[1] we retrosynthetically disconnected the a,b-
unsaturated amide bond first to convert the rigid bicyclic
molecule into a monocyclic hexadepsipeptide that is similar to
the proposed biosynthetic intermediate.[1b,7] After additional
disconnections it was evident that the molecule is comprised
of several natural and unnatural amino acid residues. There
are two fragments with unprecendented structural features
among natural products:[2] the b-hydroxy acid unit and the
complex phenylglycine-derived epoxide fragment (2). The
high sensitivity of this epoxide fragment required careful
consideration in terms of deprotection strategies that were
mild and orthogonal to ring opening. Thus, the development
of an effective protocol for the assembly of this moiety
became our first task. We planned to employ an epoxide ring-
opening strategy to create the aryl/alkyl ether bond.
Figure 1. Structure of salinamide A and its retrosynthetic analysis.
Florida Keys.[1] It was also found in an edaphic Streptomyces
strain (NRRL 21611) isolated from a soil sample.[2] Biological
evaluations revealed that salinamide A exhibits strong inhib-
itory activity (IC50 = 0.5 mm) against bacterial RNA poly-
merases without affecting human RNA polymerases;[2] it was
also shown to have potent topical anti-inflammatory activity
by using the phorbol ester-induced mouse ear edema assay.[1]
A similarly potent anti-inflammatory activity was also
observed in some members of cyclomarins[3] and halipeptins,[4]
two classes of depsipeptides that were recently isolated from
marine microorganisms and sponges. Considerable structural
differences between these depsipeptides and clinically used
The synthetic route to requisite epoxide 5 is illustrated in
Scheme 1. The known alcohol (3)[8] was protected with a p-
methoxybenzyl (PMB) group under acidic conditions, and
then reduced with DIBAL-H to afford allyl alcohol 4.
Subjecting 4 to the Sharpless asymmetric epoxidation[9]
provided 5 in 88% yield and with greater than 95% ee.
=
Scheme 1. Reagents and conditions: a) PMBO(C NH)CCl3, CSA,
CH2Cl2, 08C–RT; b) diisobutylaluminum hydride, THF, À788C, 90% for
2 steps; c) Ti(OiPr)4, (+)-DIPT, tBuOOH, CH2Cl2, À208C, 88% yield,
>95% ee. PMB=p-methoxybenzyl; CSA=camphersulphonic acid;
DIPT=diisopropyl tartrate.
[*] L. Tan, Prof. Dr. D. Ma
State Key Laboratory of Bioorganic & Natural Products Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
354 Fenglin Lu, Shanghai 200032 (China)
Fax: (+86)21-6416-6128
E-mail: madw@mail.sioc.ac.cn
The ring opening of 5 with a suitable phenylglycine
derivative proved to be a major challenge in the course of our
total synthesis. Initially, ester 6a was utilized as a coupling
partner (Scheme 2), but no reaction occurred under Lewis
acid mediated conditions (Ti(OiPr)4).[10a] Basic conditions,[10b]
[**] The authors are grateful to the Chinese Academy of Sciences and the
National Natural Science Foundation of China (grant 20632050 &
20621062) for their financial support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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in which DMF was the solvent and either CsF, NaH, or K2CO3
was the base, gave rise to complex product mixtures in all
cases. However, the desired coupling product 7a could be
the aldehyde, and further oxidation by using NaClO2 fur-
nished acid 11. Compound 11, which was activated as mixed
acid anhydride, was coupled to amino ester 12 to give
dipeptide 13.[12] Any attempt to couple a more advanced di- or
tripeptide to 11 resulted in very low yields or significant
epimerization; a variety of other coupling reagents and
conditions did not give any satisfactory results.
The peptide chain elongation of 13 is depicted in
Scheme 3. Esterification of threonine derivative 14a afforded
Scheme 3. Reagents and conditions: a) FmOH, MNBA, NEt3, DMAP,
88%; b) HCl, MeOH; c) Liberated acid from 13 by treatment with Et3N
in aqueous THF, HATU, HOAt, (iPr)2NEt, CH2Cl2, DMF, 90% yield
from 13; d) H COH ; e)N-Boc-d-allo-isoleucine, HATU, HOAt,
2
Scheme 2. Reagents and conditions: a) K2CO3, iPrOH, reflux, 20%
yield for 7a, 95% yield for 7b; b) MsCl, Et3N, THF, 08C; c) K2CO3,
DMF, 92% yield for 2 steps; d) DDQ, CH2Cl2, pH7 buffer, 89%;
e) Dess–Martin oxidation; f) (EtO)2P(O)CH2CO2Et, NaH, THF, 89%
yield for 2 steps; g) LiOH, H2O, MeOH, THF; h) TMSEOH, EDC,
DMAP, CH2Cl2, 65% yield for 2 steps; i) PPTS, MeOH, 95%; j) Dess–
Martin oxidation; k) NaClO2, NaH2PO4, 2-methyl-2-butene, tBuOH,
H2O; l) ClCO2iBu, NMM, CH2Cl2, 50% yield from 10. DDQ=2,3-
dichloro-5,6-dicyano-l,4-benzoquinone; TMSE=trimethylethyl;
EDC=1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride;
DMAP=4-(dimethylamino)pyridine; PPTS=pyridinium p-toluenesulfo-
nate; NMM=N-methylmorpholine.
(iPr)2NEt, CH2Cl2, DMF, 68% yield for 2 steps. Fm=9-fluorenylmethyl;
MNBA=2-methyl-6-nitrobenzoic acid anhydride; HATU=2-(7-Aza-1H-
benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate;
HOAT=1-hydroxy-7-azabenzotriazole.
15a in 88% yield and was subsequently exposed to meth-
anolic hydrogen chloride to cleave the ketal and remove the
tert-butoxycarbonyl (Boc) protecting group to give an amino
ester. This amino ester was condensed with the deprotected
acid of dipeptide 13 to provide tripeptide 16. After treatment
of 16 with formic acid, the resultant free amine was reacted
with N-Boc-d-allo-isoleucine to produce tetrapeptide 17 in
68% yield. Notably, the unsaturated epoxide moiety was
remarkably sensitive towards some deprotection conditions.
Allyl ester deprotection under various conditions led to
decomposition (observed in an earlier synthetic route that
was later abandoned) and dilute piperidine or other secon-
dary amines in moderately polar solvents led to competitive
side reactions.
The elaboration of the desired ester fragment of salina-
mide A is outlined in Scheme 4. Required TBS-protected b-
hydroxy acid 19 was obtained by Oppolzerꢀs anti-aldol
reaction by using TiCl4 as a catalyst,[13] and then it was
subsequently coupled to an amino ester generated from 15b
to provide amide 20. EDC-mediated esterification of hin-
dered secondary alcohol 20 with acid 21 produced ester 22a at
low temperatures; only subtle epimerization was detected and
the epimer could be readily separated by using flash
chromatography. Remarkably, 22 was both highly sensitive
isolated in 20% yield after refluxing a mixture of 5, 6a, and
K2CO3 in iPrOH.[11] We were unable to improve this yield by
optimizing the reaction conditions, and therefore decided to
switch the coupling partner to 6b. To our delight, ring opening
proceeded smoothly to afford 7b in 95% yield. This result was
rationalized by the fact that changing the ester group to a
protected hydroxymethylene group not only enhanced the
substrate stability, but also increased the nucleophilicity of the
phenol. Next, diol 7b was converted into a mesylate and then
into epoxide 8 in 92% overall yield. Oxidative removal of the
PMBgroup in 8 by using DDQ and subsequent Dess–Martin
oxidation of the liberated primary alcohol furnished an
aldehyde; this aldehyde was then subjected to a Horner–
Wadsworth–Emmons (HWE) reaction to afford a,b-unsatu-
rated ester 9. To facilitate cleavage of the ester in the later
stages of the synthesis, the ethyl ester in 9 was converted into
a trimethylsilylethyl ester to yield 10. Finally, treatment of 10
with PPTS in methanol, oxidation of the liberated alcohol to
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Scheme 4. Reagents and conditions: a) Et2BOTf, (iPr)2NEt, then TiCl4,
iPrCHO; b) TBSOTf, 2,6-lutidine, CH2Cl2; c) LiOH, 30% H2O2, THF,
80% yield for 3 steps; d) 15b, HCl, MeOH, then 19, HATU, HOAt,
(iPr)2NEt, CH2Cl2, DMF, 87%; e) 21, EDC, DMAP, CH2Cl2, À58C, 80%;
f) PPTS, MeOH, 90%.
to base and easily saponified. Deprotection of 22 with PPTS
in methanol afforded diol 22b.
The connection of tetrapeptide 17 to ester 22b and the
completion of the synthesis are depicted in Scheme 5. Treat-
ment of 17 with triethylamine in dry THF afforded an acid
which was then coupled to the amine liberated from 22b to
provide linear peptide 23 in 80% yield. After the C terminus
of 23 was carefully unmasked by the action of diethylamine in
CH3CN, cleavage of the N-terminal Boc protecting group
gave a linear precursor for the first macrolactamization.
Treatment of the amino acid with HATU/HOAt for 3 days
provided cyclic depsipeptide 24a in 34% yield. In this case a
racemization product was not isolated and the low yield could
possibly result from the degradation of the starting material as
some decomposed products were identified.
Scheme 5. Reagents and conditions: a) Et3N, THF; b) TFA, CH2Cl2,
c) HATU, HOAt, (iPr)2NEt, CH2Cl2, DMF, 80% yield from 17;
d) HNEt2, MeCN; e) TFA; f) HATU, HOAt, (iPr)2NEt, CH2Cl2, DMF,
34% yield for 3 steps; g) N-Boc-Gly-OH, EDC, DMAP, 82% yield
based on 39% recovery of 23; h) TAS-F, DMF, 87% yield based on
47% recovery of 24b; i) TFA, j) HATU, HOAt, (iPr)2NEt, DMF, 40%
yield for 2 steps. TFA=trifluoroacetic acid; TAS-F=tris(dimethyl-
amino)sulfonium difluorotrimethylsilicate.
To set the stage for the final macrocyclization, N-
protected glycine was coupled selectively to the primary
hydroxy group of the serine residue of 24a to afford
depsipeptide 24b, completing all the required building
blocks. Cautious removal of the TMSE protecting group
with tris(dimethylamino)sulfonium difluorotrimethylsilicate
(TAS-F)[14] furnished an acid in moderate yield without any
detectable b elimination. Then, the N-terminus of the side
chain was revealed to afford the final precursor for macro-
lactamization. According to the proposed biosynthesis[1b,7] the
presence of the first macrocycle might provide a favorable
conformation for the facilitating the second cyclization; our
monocyclic precursor was very similar to the biosynthetic
intermediate. Indeed, treatment with HATU/HOAt in DMF
for 3 days did afford the rigid bicyclic target compound,
salinamide A (1), in 40% yield (5.6 mg) over 2 steps, and all
analytical data were identical to those previously reported.[1b]
In conclusion, we have achieved the first total synthesis of
salinamide A (1% overall yield for 28 linear steps), which
features a concise elaboration of its phenylglycine-derived
epoxide fragment 2, and the identification of two possible
macrolactamization sites. This concise synthetic route could
provide access to salinamide A analogues to be used to
investigate the source of its anti-inflammatory properties.
Additional studies in this direction are actively being pursued
in our laboratory, and will be reported in due course.
Received: January 25, 2008
Published online: March 28, 2008
Keywords: anti-inflammatory agent · depsipeptide · epoxides ·
.
macrocyclization · total synthesis
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