Total synthesis of aeruginosin 98B

Article abstract of DOI:10.1021/ja309947n

The first total synthesis of aeruginosin 98B was accomplished. The key step is a highly diastereoselective Pd-catalyzed intramolecular asymmetric allylic alkylation reaction of a diastereomeric mixture of allylic carbonates that is enabled by the use of racemic phosphine ligand L1.

Full text of DOI:10.1021/ja309947n

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Communication
Total Synthesis of Aeruginosin 98B
Barry M. Trost, Toshiyuki Kaneko, Neil G Andersen, Christoph Tappertzhofen, and Bruce T. Fahr
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 01 Nov 2012
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Total Synthesis of Aeruginosin 98B
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Barry M. Trost,* Toshiyuki Kaneko, Neil G. Andersen, Christoph Tappertzhofen and Bruce Fahr
Department of Chemistry, Stanford University, Stanford, California 94305-5080, United States
Supporting Information Placeholder
inhibit the serine proteases thrombin, plasmin, and trypsin
with IC50 values of 10.0, 7.0 and 0.6 ꢀ
g/mL, respectively.1 The
ABSTRACT: The first total synthesis of aeruginosin 98B (1)
was accomplished. The key step includes a highly diastereose-
lective Pd-catalyzed intramolecular asymmetric allylic alkyla-
tion (AAA) reaction of a diastereomeric mixture of allylic
carbonates, which is enabled by the use of racemic phosphine
ligand L1.
trypsin selectivity could be attributed to its unique 2-carboxy-
6-hydroxyoctahydroindole sulfate (Choi sulfate) structure. The
sulfonic acid group could interact with Tyr96 of trypsin
through a H2O-mediated hydrogen bonding.2 However, other
aeruginosins, aeruginosin 298A (2)6 and oscillarin (3)7 for
example, are known to interact with Asp102 of thrombin. The
negative interaction between the sulfonyl group and Asp102
may explain the IC50 value of aeruginosin 98B, which is 33
times smaller than that of aeruginosin 298. For these reasons,
SAR studies of aeruginosin family were continued vigorous-
ly.3c, 8 Herein, we describe the first total synthesis of aeru-
ginosin 98B.
Aeruginosin 98B (1) is a tetrapeptide with an unnatural
amino acid, isolated from a blue green algae Microcystis ae-
ruginosa by Murakami and co-workers (Figure 1).1 Its struc-
ture was determined by 2D-NMR analysis, and the absolute
configurations of the seven stereogenic centers were deter-
mined by X-ray crystallographic analysis of its cocrystal with
trypsin.2 Aeruginosin 298A (2), another member of aeru-
ginosin family, was synthesized by several groups.3 Oscillarin
(3) was also isolated by Boehringer Mannheim GmbH.4 Re-
cently, Hanessian and co-workers reported a total synthesis of
aeruginosin 205B (4),5 which has a unique sulfonic sugar in
the aeruginosin family.
A major goal in developing a new strategy is to enable fac-
ile structural diversity to examine structure-function relation-
ships. Aeruginosin 98B can be dissected into four, suitably
protected building blocks: 4-hydroxyphenyl lactic acid deriva-
tive (Hpla, 5), bis-protected agmatine (6), 2-carboxyl-6-
hydroxyoctahydroindole core (Choi, 7), and -allo-isoleucine
D
(8) (Figure 2). It was further reasoned that the octahydroindole
subunit could be prepared in a highly diastereoselective fash-
ion by employing an intramolecular Pd-catalyzed AAA reac-
tion9 on allylic carbonate 9a and 9b. Since the palladium-
catalyzed allylic alkylation reaction is a stereospecific double
inversion process, it is only necessary to control the relative
stereochemistry between the protected hydroxyl and carbonate
functional groups during the preparation of the AAA precur-
sors 9a and/or 9b. An important component of developing a
route was to avoid the use of tyrosine as a precursor to both
Hpla and Choi fragments as was commonly done in the syn-
thesis of the other aeruginosins, in order to allow more facile
modification of those subunits.
H
CO2R
OR
N
NHR
H2N
NR
RO
RO
Figure 1. Structure of Aeruginosin 98B (1) and related
compounds.
Hpla fragment(5)
agmatine fragment (6)
1
Aeruginosin 98B ( )
H
CO2R
Aeruginosin family has gathered attention because of their
promising biological activities. Aeruginosins exhibit inhibitory
activities against serine proteases, especially thrombin and
trypsin, and their activity profiles can be explained by a high
degree of pharmacophoric and structural homology within the
family.5 This array of structural and functional features is re-
sponsible for their high affinity to the catalytic binding pocket
of trypsin, thrombin, and other serine proteases involved in the
blood coagulation cascade. Aeruginosin 98B (1) was shown to
N
RHN
CO2R
H
H
Choi fragment (7)
D-allo-Ile fragment (8)
Pd-AAA
OTIPS
OCO2Me
Negishi
CO2Me
CO2Me
CO2Me
IZn
NH2
NHBoc
TIPSO
OTES
10
NH2
OCO2Me
TIPSO
X
9a
9b
11
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ined. Using dppp as ligand with [(η3-C3H5)PdCl]2 in THF at
Figure 2. Retrosynthetic strategy of Aeruginosin 98B (1).
60oC, no reaction was obtained (Table 1, entry 1).14 On the
other hand, using the (R, R)-chiral ligand L1, a 4.4:1 ratio of
19a:19b in a combined yield of 70% (Table 1, entry 2) was
obtained. Surprisingly, switching to the (S, S) enantiomer of
L1, a 3.8:1 ratio still favoring 19a was obtained in 63% yield
(Table 1, entry 3). Gratifyingly, switching to the racemic mix-
ture of L1 as ligand, this ratio favoring 19a jumped to 11:1 in
a 96% combined yield (Table 1, entry 4). Thus, a new role has
been established for this family of ligands.
The allyl carbonates 9a and 9b were prepared from 4-
methoxyphenol (12). Birch reduction11 followed by TIPS si-
lylation of the resultant alcohol provided methyl enol ether in
97% overall yield. Dihydroxylation of methyl enol ether pro-
vided a 2:1 mixture of α-hydroxyketones 13a and 13b. The
direct hydroxylation of the TIPS ether of 4-
hydroxylcyclohexanone with MoOPH gave 13a selectively in
72% yield. The anti-diastereomer 13a was treated with TESCl
under standard conditions to afford bis-silyl ether 14 in 97%
yield. The enolate of ketone 14 was then allowed to react with
Comins reagent12 to furnish vinyl triflate 15 in 88% yield.
Gratifyingly, compound 15 could be efficiently coupled with
alkylzinc reagent 1113 to give 16a and 16b as a 1:1 diastereo-
meric mixture. Amino esters 16a and 16b were treated with
HF-pyridine complex to remove the TES protecting group
selectively. The resulting allylic alcohols were easily separable
by flash column chromatography and thus each diastereomer
could be transformed separately toward the desired heterocy-
cle. Treating 17a and 17b with methyl chloroformate and
catalytic DMAP in CH2Cl2 furnished allylic carbonates 18a
and 18b in 98% yield. Subsequent removal of the Boc pro-
tecting group under standard conditions, gave aminoesters 9a
and 9b in 93% and 97% yield, respectively.
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Completion of the synthesis of the Choi subunit required di-
astereoselective reduction of the olefin. Since the simple ami-
no group proved unstable to TBAF conditions, this amino
group was benzylated and subsequently treated with TBAF in
the same pot to give alcohol 20 in 89% yield whose hydro-
genation then directly led to amino alcohol 21 in 92% yield.
The relative stereochemistry of 21 was assigned on the basis
of nOe analysis (see Supporting Information). More im-
portantly, the spectroscopic properties of its TFA salt exactly
matched those previously reported by Bonjoch and co-
workers.3a
Table 1. Asymmetric allylic alkylation of diastereomeric
mixture 9a and 9b.
Scheme 1. Synthesis of Choi fragment.a
OTIPS
OTIPS
OH
OTIPS
OTIPS
a
b
c
OTES
OTES
OH
13a
OH
13b
O
OTf
OMe
O
O
12
14
15
OTIPS
OTIPS
OTIPS
OTIPS
e
d
HO
OH
CO2Me
TESO
OTES
CO2Me
CO2Me
CO2Me
The synthetic routes to Hpla and agmatine fragments are
shown in Scheme 2. The cinnamic ester 22a was subjected to
Sharpless asymmetric dihydroxylation to provide diol 23a.15
The product was shown to have >99%ee by chiral HPLC anal-
ysis. Removal of the benzylic hydroxyl group was effected
using triethylsilane and TFA in CH2Cl2 to afford α-
hydroxyester 24a. The hydroxyl group of 24a was protected by
TIPS by treatment with TIPSCl and imidazole. The same route
starting from cinnamate 22b (X=Cl) allows access to the hy-
droxylated ester 25b, the intermediate required for the synthe-
sis of aeruginosin 98A. Agmatine fragment 2816 was prepared
from N-Boc-1, 4-diaminobutane 26, according to Rich and
NHBoc
17a
NHBoc
17b
NHBoc
16a
NHBoc
16b
OTIPS
OTIPS
OTIPS
OTIPS
g
f
MeO2CO
OCO2Me
CO2Me
MeO2CO
OCO2Me
CO2Me
CO2Me
NH2
CO2Me
NH2
9b
NHBoc
18a
NHBoc
9a
18b
OTIPS
OH
OH
h
i
j
O
O
H
H
H
NH
HN
NH
NBn
NH
H
co-workers.17
Treatment
of
26
with
N,
N'-
PPh2 Ph2P
CO2Me
CO2Me
20
CO2Me
21
L1
19a
bis(benzyloxycarbonyl)-1H-pyrazole-1-carboxamidine in THF
smoothly provided 27 in 65% yield. The Boc group was re-
a(a) i) Li, NH3, EtOH, -78oC, ii) TIPSOTf, 2, 6-lutidine, CH2Cl2,
0oC, iii) OsO4, NMO, THF/H2O=3/1, 0oC, 96%, 13a/13b=2/1. (b)
TESCl, imidazole, DMAP, CH2Cl2, r.t., 97%. (c) LHMDS,
Comins reagent, -78oC to -40oC, 88%. (d) N-Boc-3-iodoalanine
methyl ester, Zn, TMSCl, Pd(PPh3)4, LiCl, DMA, 60oC, 87%. (e)
HF-pyridine, CH2Cl2, 0oC, 93%. (f) Methyl chloroformate, pyri-
dine, DMAP, CH2Cl2, 0oC, 98%. (g) TFA, CH2Cl2, -10oC, 93-
97%. (h) [(η3-C3H5)PdCl]2, racemic L1, TFA, THF, 60oC, 96%,
19a/19b=11/1. (i) BnBr, Et3N, CH3CN, r.t., then TBAF, 89%. (j)
5% Pd/C, H2, MeOH, r.t., 92%.
moved under standard conditions in 95% yield.
D-allo-Ile me-
thyl ester (29) was prepared by an esterification of
D
-allo-Ile,
which was obtained by an optical resolution of DL-allo-Ile.18
Analytical data of 29 matched with that previously reported.19
Scheme 2. Synthesis of other fragments.a
With the key intermediates 9a and 9b in hand, the diastere-
oselectivity of the Pd catalyzed allylic alkylation was exam-
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28, WSC, HOBt, Et3N, DMF, 0oC-r.t., 60% in 2 steps. (g) SO3-
pyridine, pyridine, 40oC, 75%. (h) HF-pyridine, CH3CN, 0oC-r.t.,
57%, 91%brsm. (i) Pd(OH)2, H2, MeOH, r.t., 98%.
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In summary, we achieved the first total synthesis of aeru-
ginosin 98B in eight steps from the four fragments (Choi, -
D
allo-Ile, Hpla and agmatine). After the construction of aeru-
ginosin core, sulfonic acid was directly introduced by treat-
ment with SO3-pyridine complex. In the preparation of Choi
fragment, a Pd-catalyzed intramolecular AAA reaction of a
diastereomeric mixture of allyl carbonates 9a and 9b using
racemic ligand L1 provided hexahydroindole derivative 19a in
high diastreo- and enantioselectivity wherein the asymmetry
derived from 3-iodo-S-alanine. Curiously, higher reactivity of
the racemic mixture of the chiral ligands suggests that each
enantiomer of the racemic ligand might have recognized the
matched diastereomer of 9a and 9b imparting better kinetics
for ionization but the substrate controls the diastereoselectivity.
We are currently exploring the precise reaction mechanism
that explains the observed diastereo- and enantioselectivity.
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13
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a(a) AD-mix α, MeSO2NH2, t-BuOH/H2O=1/1. 0oC, 23a; 86%,
>99%ee, 23b; 91%, 98%ee. (b) Et3SiH, TFA, CH2Cl2, r.t., 24a;
80%, 24b; 82%. (c) 25a; TIPSCl, imidazole, DMAP, CH2Cl2, r.t.,
79%, 25b; BnBr, Ag2O, TBAI, PhH, reflux, >63%. (d) N, N'-
bis(benzyloxycarbonyl)-1H-pyrazole-1-carboxamidine, THF, r.t.,
65%. (e) TFA, CH2Cl2, 0oC, 95%.
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Ethyl ester 25a was hydrolyzed with 1N LiOH aq, and the
resulting carboxylic acid was coupled with -allo-Ile methyl
D
ASSOCIATED CONTENT
Supporting Information
Experimental procedures and analytical data for all new com-
pounds. This material is available free of charge via the Internet
at http://pubs.acs.org.
ester (29) to afford dipeptide 30 in 73% yield (Scheme 3).
Subsequent hydrolysis of the ester of 30 and the following
peptide coupling with Choi fragment 21 provided tripeptide
31 in 69% yield. The tetrapeptide 32 was prepared in the same
manner as the synthesis of tripeptide 31, using agmatine frag-
ment 28. For the introduction of the sulfonic acid group, sulfur
trioxide pyridine complex was used to afford sulfonic acid 33
in 75% yield. Removal of TIPS group was accomplished by
treatment with HF-pyridine complex in acetonitrile to provide
access to the precursor 34. The removal of Bn and Cbz groups
provided aeruginosin 98B (1) in quantitive yield. The crude
material was purified by reverse phase HPLC to afford pure
aeruginosin 98B (1) in 98% yield as a colorless solid. The 1H-
and 13C-NMR spectra matched those of the natural product.
Surprisingly, the optical rotation of synthetic 1 was higher (-
11.24, c 0.25, H2O) than the natural product (-5.24, c 0.25,
H2O).
AUTHOR INFORMATION
Corresponding Author
Barry M. Trost
bmtrost@stanford.edu
ACKNOWLEDGMENT
We thank the National Institutes of Health (GM 033049) and
Teijin Pharma Ltd for their generous support of our program.
REFERENCES
Scheme 3. Total synthesis of Aeruginosin 98B.a
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