First total syntheses of aeruginosin 298-A and aeruginosin 298-B, based on a stereocontrolled route to the new amino acid 6-hydroxyoctahydroindole-2-carboxylic acid

Article abstract of DOI:10.1002/1521-3765(20010817)7:16<3446::AID-CHEM3446>3.0.CO;2-0

The first total syntheses of aeruginosin 298-A (1) and aeruginosin 298-B (3) are described. The syntheses of the alternative putative structures 2 and 4 were also accomplished. The key common strategic element is the stereocontrolled synthesis of (2S,3aS,6R,7aS)-6-hydroxyoctahydroindole-2-carboxylic acid (L-Choi, 5) from L-tyrosine. The synthesis of this new bicyclic α-amino acid, which is the core of aeruginosins, involves Birch reduction of O-methyl-L-tyrosine (6) and aminocyclization of the resulting dihydroanisole 7 in acid medium, followed by N-benzylation to give the diastereoisomers 12 and 13. Upon acid treatment with HCl-MeOH, the last two produce an equilibrium mixture in which the endo isomer 13 significantly predominates. Hydrogenation of 13 in the presence of (Boc)₂O gives 16, which on reduction with LS-Selectride furnishes the alcohol 22, a protected L-Choi. Successive couplings of 22 with D-leucine, protected (R)-(4-hydroxyphenyl)lactic acid, and L-arginine fragments, followed by reduction to the argininol level and a deprotection end step complete the synthetic sequence to produce aeruginosin 298-A (1). Spectral comparison showed that peptide 2, with the structure previously proposed for aeruginosin 298-A, was different from the natural product. However, synthetic 1 was found to be identical to the isolated natural sample of aeruginosin 298-A. These results unequivocally establish that the absolute stereochemistry of aeruginosin 298-A, formerly assigned incorrectly, is D-Hpla-D-Leu-L-Choi-L-Argol, as shown by structure 1. Aeruginosin 298-B was also synthesized and shown to be a mixture of rotamers of D-Hpla-D-Leu-L-ChoiNH₂ (3), rather than an epimeric mixture of 3 and the L-Leu-incorporating 4.

Full text of DOI:10.1002/1521-3765(20010817)7:16<3446::AID-CHEM3446>3.0.CO;2-0

FULL PAPER
First Total Syntheses of Aeruginosin 298-A and Aeruginosin 298-B, Based on
a Stereocontrolled Route to the New Amino Acid 6-Hydroxyoctahydroindole-
2-carboxylic Acid
Nativitat Valls, Meritxell LoÂpez-Canet, MerceÁ Vallribera, and Josep Bonjoch*[a]
Abstract: The first total syntheses of
aeruginosin 298-A (1) and aerugino-
sin 298-B (3) are described. The synthe-
ses of the alternative putative structures
2 and 4 were also accomplished. The key
common strategic element is the stereo-
controlled synthesis of (2S,3aS,6R,7aS)-
6-hydroxyoctahydroindole-2-carboxylic
acid (l-Choi, 5) from l-tyrosine. The
synthesis of this new bicyclic a-amino
acid, which is the core of aeruginosins,
involves Birch reduction of O-methyl-l-
tyrosine (6) and aminocyclization of the
resulting dihydroanisole 7 in acid me-
dium, followed by N-benzylation to give
the diastereoisomers 12 and 13. Upon
acid treatment with HCl-MeOH, the last
two produce an equilibrium mixture in
which the endo isomer 13 significantly
predominates. Hydrogenation of 13 in
the presence of (Boc)2O gives 16, which
on reduction with LS-Selectride fur-
nishes the alcohol 22, a protected l-
Choi. Successive couplings of 22 with d-
leucine, protected (R)-(4-hydroxyphe-
nyl)lactic acid, and l-arginine fragments,
followed by reduction to the argininol
level and a deprotection end step com-
plete the synthetic sequence to produce
aeruginosin 298-A (1). Spectral compar-
ison showed that peptide 2, with the
structure previously proposed for aeru-
ginosin 298-A, was different from the
natural product. However, synthetic 1
was found to be identical to the isolated
natural sample of aeruginosin 298-A.
These results unequivocally establish
that the absolute stereochemistry of
aeruginosin 298-A, formerly assigned
incorrectly, is d-Hpla-d-Leu-l-Choi-l-
Argol, as shown by structure 1. Aerugi-
nosin 298-B was also synthesized and
shown to be a mixture of rotamers of d-
Hpla-d-Leu-l-ChoiNH2 (3), rather than
an epimeric mixture of 3 and the l-Leu-
incorporating 4.
Keywords: aeruginosins
´
amino
acids ´ peptides ´ total synthesis ´
thrombin inhibitor
(d-Hpla) moiety or derivative at the N-terminus, and iii) a
guanidine-containing unit at the C-terminus (except for
aeruginosin 298-B), in which the arginine is present in a
reduced (Argol or Argal) or decarboxylated (Agmatin) form.
In addition to the thirteen aeruginosins isolated by Murakami
since 1994[5] (see Scheme 1), Carmelli has reported the
isolation of microcin SF608, a closely related peptide, from
Microcystis sp. waterbloom.[6]
Aeruginosin 298-Awas the first member of this family to be
reported and our initial synthetic target. It was isolated from
the toxic strain of M. aeruginosa NIES-298 and found to
inhibit thrombin and trypsin.[2a] The absolute stereochemistry
of aeruginosin 298-A was determined in 1998 by single-crystal
X-ray diffraction analysis of the ternary complex of aerugi-
nosin 298-A bound to hirugen-thrombin,[7] but the putative
structure 2 assigned then should be revised in the light of the
data presented in this work.[8]
Introduction
Cyanobacteria (blue-green algae) are prokaryotic, photo-
synthetic microorganisms that have been shown to be a rich
source of a variety of structurally novel, bioactive nitrogen
compounds.[1] Among the wide range of serine protease
inhibitors found in cyanobacteria, aeruginosins are a group of
linear peptides that have in common a new, nonproteinogenic
a-amino acid that contains a cis-perhydroindole ring. The
aeruginosins isolated from Microcystis species contain the
new bicyclic amino acid 2-carboxy-6-hydroxyoctahydroindole
(Choi),[2] while those found in Oscillatoria agardhii incorpo-
rate a 6-chloro analog (Ccoi).[3] Other common structural
trends in aeruginosins are: i) a d-amino acid unit linked to the
Choi nitrogen atom,[4] ii) an (R)-(4-hydroxyphenyl)lactic acid
Â
[a] Prof. Dr. J. Bonjoch, Dr. N. Valls, Dr. M. Lopez-Canet, M. Vallribera
Laboratory of Organic Chemistry
Faculty of Pharmacy, University of Barcelona
Av. Joan XXIII s/n, 08028-Barcelona (Spain)
Fax : (‡34)93-4021896
In this paper, we describe the first synthesis of the amino
acid (2S,3aS,6R,7aS)-6-hydroxy-octahydroindole-2-carboxylic
acid (5, l-Choi), which is the common structural motif shared
by almost all aeruginosins, and we report the use of Choi
derivatives for the total syntheses of aeruginosin 298-A (1)
and the peptide 2, the structure of which is the same as that
E-mail: bonjoch@farmacia.far.ub.es
Supporting information for this article is available on the WWW under
http://wiley-vch.de/home/chemistry/ or from the author.
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Chem. Eur. J. 2001, 7, No. 16

3446±3460
previously proposed for aeruginosin 298-A.[2a, 7] Our results
have led to a revision of the structure originally assigned to
the natural product, which we have concluded to be 1 rather
than 2. We also describe the syntheses of aeruginosin 298-B
(3) and its epimer 4 (Scheme 2), thus clarifying that this
natural product exists as a mixture of conformational isomers
and not a mixture of Leu epimers as has been reported.[2e]
The construction of the whole carbon skeletons for these
peptides involves the synthesis of an appropriately function-
alized and stereochemically pure octahydroindole (Choi
core), and its stepwise elongation with the other fragments,
the unit linked at the C-terminus being incorporated in the
last step.
Results and Discussion
The first stage: synthesis of azabicyclic ketone 13: Our
approach to the synthesis of the amino acid l-Choi (5)
involved the stereoselective formation of ketone 13,[9, 10]
which could be obtained by Birch reduction of tyrosine
derivative 6, followed by an aminocyclization onto the
masked enone present in the Birch reduction compound
(Scheme 3). This synthetic route to octahydroindol-6-ones
takes advantage of the presence of the b-amino ketone moiety
for the formation of the last bond, which involves an
Abstract in Spanish: Se describen las síntesis totales de las
Â
aeruginosinas 298-A (1) y 298-B (3). Tambien se han llevado a
cabo las síntesis de las estructuras putativas 2 y 4. El elemento
Â
Â
Â
estrategico comun es la síntesis estereocontrolada del aci-
do (2S,3aS,6R,7aS)-6-hidroxioctahidroindol-2-carboxílico (l-
Choi, 5) a partir de la l-tirosina. La síntesis de este nuevo
Scheme 1. Structure of aeruginosins. [a] Phenol ring sulfated and o-
substituted by
a chloro atom. l-Argal unit has hemiaminal form.
Aeruginosin 89-B has a d-Argal unit. [b] Aeruginosins 98-A and 98-C
have a chloro and a bromo atom, respectively, as o-substituents in the
phenol ring. Aeruginosin 101 is the o,o-dichloro derivative. The Choi
6-hydroxyl is sulfated in all cases. [c] Sulfated phenol ring. l-Argal unit has
hemiaminal form. Aeruginosin 102-B has a d-Argal unit. [d] Hemiaminal
cyclic form (O-ethyl). [e] Aeruginosin 298-B has a carboxamido group at
Choi C(2).
Â
Â
aminoacido bicíclico, nucleo central de las aeruginosinas,
implica la reduccion de Birch de la O-metil-l-tirosina (6),
seguida de aminociclacion en medio acido del dihidroanisol 7
resultante y subsiguiente N-bencilacion, proporcionando los
Â
Â
Â
Â
azabiciclos 12 y 13 que posteriormente se equilibran bajo
Â
Â
tratamiento acido (HCl-MeOH). La hidrogenacion del mayo-
Â
ritario isomero endo 13 en presencia de (Boc)2O proporciona
Â
16, que por reduccion con LS-Selectide rinde el alcohol 22,
una forma protegida de l-Choi. Sucesivos acoplamientos a
partir de 22 con fragmentos de d-Leucina y formas protegidas
Â
Â
del acido (R)-hidroxifenillactico y l-arginina, seguidos de una
Â
reduccion al grado de argininol y una etapa final de
Â
Â
desproteccion, completan la secuencia sintetica para la obten-
Â
cion de la aeruginosina 298-A.
Â
Â
Â
La comparacion de los datos espectroscopicos mostro que el
Â
peptido 2, con la estructura propuesta para la aeruginosi-
na 298-A, era diferente al producto natural. Sin embargo, el
Â
Â
Â
compuesto sintetico 1 mostro ser identico a la aeruginosi-
na 298-A natural. Estos resultados establecen de forma inam-
Â
bigua que la configuracion absoluta de la aeruginosina 298-A,
incorrectamente asignada anteriormente, es d-Hpla-d-Leu-l-
Choi-l-Argol.
Â
La aeruginosina 298-B tambien fue sintetizada y su estructura
Â
es una mezcla de rotameros de d-Hpla-d-Leu-l-ChoiNH2 (3)
Â
en vez de la anteriormente propuesta de una mezcla epimerica
de 3 y 4, que incorpora una l-Leu.
Scheme 2. Previously proposed and revised structures for aerugino-
sins 298-A and 298-B.
Chem. Eur. J. 2001, 7, No. 16
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3447

J. Bonjoch et al.
FULL PAPER
could be removed by chromatographic separation and then
reused. The equilibration of these b-amino ketones[15] may
occur through acid-mediated b-elimination, vinylogous enol-
ization, and intramolecular Michael readdition.
H
H
HO
N
H
CO2H
O
N
CO2Me
CO2H
H
H
Bn
At that point, we thought we might improve the synthesis of
azabicyclo 13 by carrying out the cyclization of dihydroanisole
7 under the same reaction conditions as used for the
equilibration (12 ! 13) so as at least to avoid the trans-
esterification process (Scheme 4, path b). Thus, dihydroani-
sole 7 was treated with a methanolic solution of hydrogen
chloride to give a mixture of octahydroindoles 9 (exo/endo),
which was benzylated to furnish a diastereomeric mixture of
exo and endo isomers 12 and 13 in a 1.8:1 ratio, in 52% overall
yield. The probable explanation for this improvement (milder
reaction conditions and better yields) is that cyclization takes
place much more readily when a-amino esters are used
instead of a-amino acids for the intramolecular 1,4-addi-
tion.[9c] After the acid-mediated equilibration of the mixture
of diastereoisomers 12 and 13, enantiopure octahydroindol-
one 13 could be obtained in multigram amounts, the overall
yield for these four initial steps being 44%. The stereo-
chemical integrity of 12 and 13 was confirmed by 1H NMR by
using the Pirkle alcohol procedure.[16]
5
13
H
NH2
MeO
O
H2N
CO2R
6
Scheme 3. Retrosynthesic analysis of l-Choi.
intramolecular 1,4-addition of an amino group across a
cyclohexenone intermediate.[11]
Our studies began with the Birch reduction of O-methyl-
tyrosine (6), by using lithium in ammonia and ethanol.[12] In
our initial studies,[13] the resulting dihydroanisole 7 was
treated in aqueous acid medium (3n HCl) to give a mixture
of cis compounds 8 (exo and endo isomers),[14] which was
submitted to a benzylation process (BnBr, EtOH, NaHCO3)
to give the two octahydroindoles 10 and 11 in a 1:1 ratio and
44% overall yield from 6 (Scheme 4). Both 10 and 11 were
The above results indicate that the secondary amine exo-9 is
slightly more stable than the isomer endo-9, but that, after N-
benzylation, 13 (the endo isomer) is thermodynamically more
stable than 12 (the exo isomer). This stability change agrees
with molecular mechanics calculations,[17] which showed the
hydrochloride of 13 to be at least 2.65 kcalmol 1 more stable
than the hydrochloride of 12, probably because only the endo
isomer 13 can incorporate an intramolecular hydrogen bond
between the carbonyl of the ester group at C(2) and the
proton of the amine salt.
CO2H
NH2
CO2Li
NH2
Li, NH3
EtOH or
tBuOH
MeO
MeO
7
6
H
H
H
H
HCl
+
path a H2O
path b MeOH
O
N
CO2R
O
N
CO2R
H
H
BnBr,
NaHCO3
path a
path b
exo-8
exo-9
R = H
R = Me
endo-8
endo-9
H
H
H
H
NMR structural analysis of octahydroindolones: Two NMR
features clearly differentiate the exo and endo series of cis-6-
oxo-octahydroindole-2-carboxylic acid alkyl esters: i) the
1H NMR coupling constant pattern of H-2 [which is a doublet
in the exo series (10, 12, and 14) but a doublet of doublets, in
fact a pseudo triplet, in the endo series (11, 13, 15, and 16][18, 19]
and the zero values of the 3J coupling between H-2 and H-3b
in exo compoundsÐindicating that the pyrrolidine ring adopts
a conformation in which C(2) is puckered in toward the exo
surface of the bicyclic system, and ii) the chemical shifts of
H-3a and H-7a, which are more deshielded in the exo
derivatives than in the endo compounds due to the syn
relationship of the alkoxycarbonyl group with both protons
(Figure 1).
NMR studies also allowed us to assign the conformational
preference of cis-6-oxo-octahydroindole-2-carboxylic acid
alkyl ester derivatives, including the positions of the nitrogen
lone-pair electrons, when present. As shown in Figure 2, c1
and c2 refer to the N-outside and N-inside conformers of the
conformationally mobile cis-octahydroindole system,[20] re-
spectively (H-7a axial and H-7a equatorial with respect to the
carbocyclic ring). N-Benzylated compounds (10 ± 13) have
deformed c2 conformations (half-boat in the cyclohexanone
ring), rather than the flip form c2 (chair). The H7 ± H7a
coupling constants (both 5 Hz) are consistent with gauche
+
O
N
CO2R
O
N
CO2R
Bn
Bn
11
13
10
12
path a R = Bn
path b R = Me
Dowex
MeOH
Dowex
MeOH
MeOH-HCl
Scheme 4. Synthesis of cis-6-oxo-octahydroindole-2-carboxylic acid deriv-
atives.
converted by a transesterification process into the corre-
sponding methyl esters 12 and 13. Although the configura-
tions of 10 ± 13 were unequivocally assigned by NMR experi-
ments (see below), both 12 and 13 were transformed into the
corresponding N-acetyl derivatives 14 and 15 for analytical
purposes, by hydrogenation in an acetic anhydride medium, in
order to check their NMR data against those corresponding to
the endo derivative (rac-15), the X-ray structure of which had
been reported previously.[9b]
Taking into account that only the ketone 13 (endo isomer)
can be considered to be a precursor of the target Choi, we
were pleased to find that treatment of exo isomer 12 with
methanolic HCl produced the desired endo isomer 13 in 87%
yield, accompanied by about 11% of starting material, which
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Chem. Eur. J. 2001, 7, No. 16

Syntheses of Aeruginosin 298-A and -B
3446±3460
shown in Scheme 4, we needed to produce the alcohol with
the 6R configuration. All attempts to satisfactorily obtain the
alcohol 18 from ketone 13 were fruitless, because under all the
conditions used (NaBH4, L-Selectride, LS-Selectride, and so
on) the 6S alcohol 17 was the major product (Scheme 5,
endo series
exo series
H
H
triplet
doublet
H
H
O
N
CO2R
O
N
H
R´
CO2R
H
R´
10 R = Bn; R' = Bn
12 R = Me; R' = Bn
14 R = Me; R' = Ac
11 R = Bn; R' = Bn
13 R = Me; R' = Bn
15 R = Me; R' = Ac
16 R = Me; R' = Boc
H
13 R = Bn
15 R = Ac
16 R = Boc
H-3b
CO2R
CO2R
H-3b
O
N
R
CO2Me
H
C-3a
C-7a
Bn
90°
150°
H
N
H-2a
H
H
N
30°
C-7a
Bn
30°
H-3a
H-3a
C-3a
H-2a
HO
N
R
CO2Me
HO
N
R
CO2Me
H
Figure 1. NMR behavior of exo and endo cis-octahydroindole-2-carboxylic
acid derivatives.
18
20
22
17
19
21
R = Bn
R = Ac
R = Boc
H
H
H
H
H
Scheme 5. Reduction of octahydroindolones (cf. Table 1).
H
N
H
H
H
H
H
3a
7a
H
3a
H
H
H
H
H
N
H
H
H
H
7a
H
Table 1). However, we decided to optimize the synthesis of
alcohol 17 in order to subject it to the Mitsunobu inversion
and so achieve the 6R configuration. When the reduction of 13
was carried out with NaBH4 in MeOH at 238C with addition
H
H
H
H
H
H
c1 conformation
c2 conformation
Figure 2. Conformational equilibria in octahydroindoles.
couplings; this indicates that the H-7a proton is equatorially
situated with respect to the carbocyclic ring. NMR NOESY
data and molecular mechanics calculations[18] agree with the
twisted conformation depicted in Figure 3 for azabicyclic
compound 13.
Table 1. Yields and diastereoselectivities obtained in reductions of azabi-
cyclic ketones 13, 15, and 16 as shown in Scheme 5.
Substrate
R
Reagent
Compounds (yield)
[a]
1
2
3
4
5
6
13
13
15
15
16
16
Bn
Bn
Ac
Ac
Boc
Boc
NaBH4, CeCl3
17 (75%)
17 (51%)
19 (88%)
19 (9%)
21 (91%)
21 (7%)
18 (10%)
18 (14%)
20 (5%)
20 (56%)
22 (3%)
22 (56%)
LS-Selectride[b]
[a]
NaBH4
LS-Selectride[b]
equatorial attack
bulky H-
H
MeO2C
[a]
CO2Me
H
NaBH4, CeCl3
LS-Selectride[b]
CH2Ph
N
H
H
H
H
O
H
H
H
H
H
[a] at 238C in MeOH. [b] at 788C in THF.
H
N
H
H
13
Boc
16
O
H
twisted c2
c1
H-
small and bulky reagents
small H-
axial attack
of CeCl3, the process occurred in 85% yield, the ratio of
alcohols 17 and 18 being 7.5:1. By submitting pure alcohol 17
to Mitsunobu conditions (BzOH 1.1 equiv, PPh3 1.5 equiv,
diethyl-azodicarboxylate (DEAD) 1.1 equiv, THF, RT, 17 h)
we isolated the benzoate 23 in 59% yield;[22] this represents a
triprotected derivative of target 5 (Scheme 6). It is noteworthy
that benzoylation of the alcohol 18, with the correct config-
uration at its four stereocenters, gave 23 in poor yield; this
indicates the low reactivity of its hydroxyl group.
At the same time, we investigated ketone reduction in the
N-acetyl derivative 15. The most interesting finding was that
the reduction took place with a different stereoselectivity to
that observed in the endo derivative 13, which incorporates an
N-benzyl substituent. Thus, when ketone 13 was reduced with
L-Selectride, it gave alcohol 20 as the major product, with the
correct configuration at C(6). When NaBH4 was used, the
epimeric alcohol 19 was formed almost exclusively, as
expected.
Figure 3. Different behavior in reduction processes involving ketones 13
and 16.
In contrast, N-acetyl derivatives 14 and 15, together with N-
Boc compound 16, had c1 as the preferred conformation
(compare, for example, the H7ax ± H7a coupling of 11 Hz).
These derivatives have a strong preference for a c1 conforma-
tion to avoid the A1,3 strain between the N-substituent and the
1
C(7) methylene group.[21] Whereas the 500 MHz H NMR
spectrum of the N-Boc derivative 16 at 208C is difficult to
interpret, owing to the hindered rotation around the carbon ±
nitrogen bond of the carbamate moiety, all protons are well
resolved in the spectrum at 808C, and the proton H-2 appears
as a broadened triplet (J ˆ 8.5 Hz) at d ˆ 4.26.
Synthesis of l-Choi (5): To reach the target 5 from ketone 13,
which is available in multigram quantities following path b
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J. Bonjoch et al.
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H
H
H
H
H
H
H
H2, Pd(OH)2
Boc2O
NaBH4,
CeCl3
O
CO2Me
13
N
H
C6H5
AcO
HO
N
N
CO2Me
CO2Me
N
O
N
CO2Me
H
H
Bn
Ac
Bn
Boc
16
24
29
17
BzOH, PPh3
DEAD
LS-Selectride
H
H
hydrolysis was confirmed by the conversion of 5 into 29, and
comparison of its 1H NMR data (500 MHz) with those
described for the same compound prepared from the natural
source by bisacetylation and coupling with (S)-methoxy-
phenylglycine.[2e]
HO
N
BzO
N
CO2Me
CO2Me
H
H
Boc
Bn
22
23
H2, Pd(OH)2
Boc2O for 26
a 0.1N LiOH
b 2N HCl
NMR structural analysis of l-Choi derivatives: The 6-hydroxy-
octahydroindoles 5 and 17 ± 29 show different conformational
behavior depending on the substituent on the nitrogen atom,
as occurs in the ketone derivative series. Compounds with a
benzyl substituent at the nitrogen atom adopt the inside c2
conformation, whereas N-acetyl and N-Boc compounds
prefer the outside c1 conformation (Figure 4). As a conse-
H
H
H
H
6N HCl
HO
N
H
CO2H
BzO
N
R
CO2Me
5
L-Choi
25 R = H
26 R = Boc
via 16 3 steps, 44%
via 17 4 steps, 40%
H
H
H
H
MeO2C
MeO2C
2
2
TFA
LiOH
CO2H
Bn
Bn
N
22
N
3H
H
H
H
H
3
HO
N
HO
N
H
CO2Me
H
4
7a
H
7a
H
4
OH
H
H
Boc
H
3a
3a
H
H
7
5
H
H
7
5
H
OH
27
28
H
H
c2 conformation
Scheme 6. Synthesis of l-Choi and protected derivatives.
17
18
CO2Me
With this information, and taking into account the same
preferred conformation of N-acetyl compound 15 and the N-
Boc derivative 16 (vide supra), we decided to study the
reduction of this latter ketone to avoid the extra step of
inversion at C(6) required when reducing the N-benzyl ketone
13. Ketone 16, which is available from 13 (H2, Pd C, Boc2O)
(Scheme 6), was reduced with LS-Selectride to afford a
chromatographically separable 8:1 mixture of alcohols 22
and 21. The different reaction course taken by the basic
nitrogen azabicyclo 13 and the N-acyl and N-Boc derivatives
15 and 16 can be explained by their different preferred
conformations, as shown in Figure 3.[23]
Although the isolated yield of the desired 6R alcohol 22
could not be improved to more than 56%, the simplicity of the
procedure enabled us to prepare this valuable protected a-
amino acid in amounts sufficient to achieve the target Choi
and also to carry out the peptide synthesis in the aeruginosin
field. The new amino acid Choi was prepared from both 22
and 23. Debenzylation of 23 was sluggish, and the secondary
amine 25 was accompanied by N-alkylation products from the
alcohol used as solvent.[24] However, 23 was easily converted
to the N-Boc compound 26 in 95% yield by catalytic,
hydrogenolytic debenzylation in the presence of (Boc)2O.
The three protecting groups of 26 underwent solvolysis in 6n
HCl to give Choi in its hydrochloride form. The target Choi,
as well as its monoprotected derivatives 27 and 28, was also
obtained from 22, as outlined in Scheme 6.
CO2Me
H
H
H
3
H
H
3
H
H
H
3a
3a
5
H
5
H
4
4
H
N
N
HO
H
7
H
7
7a
H
7a
Boc
Boc
H
H
OH
H
c1 conformation
22
NOESY
21
Figure 4. Conformations of 6-hydroxy-octahydroindole-2-carboxylic acid
derivatives.
quence, in compound 23, with the correct 6R configuration,
the benzoyloxy substituent at C(6) is equatorial (d ˆ 5.37, tt,
J ˆ 10.5 Hz and 4 Hz for H-6), whereas in compound 22, with
the same configuration, the hydroxyl group is located axially
(d ˆ 4.11, brs for H-6). NMR data show that the secondary
amine 28 exists as a mixture of conformers c1 and c2 and the
preferred conformation of the hydrochloride of Choi (5) is c1.
The disposition of the hydroxyl group can also be diagnosed
by IR spectra, since the C O bond appears at longer
wavelengths for alcohols with an axial hydroxyl group than
for those with an equatorial one. For instance, absorptions
were observed at 1017 cm 1 in 22 and 1066 cm 1 in 21.
The synthesis of the putative aeruginosin 298-A (2): Among
several possible scenarios for the use of the Choi derivatives in
the preparation of aeruginosins, the strategy used to produce
d-Hpla-l-Leu-l-Choi-l-Argol (2), thought at the time to be
natural aeruginosin 298-A, was centered around an initial
coupling with l-Leu, followed by incorporation of d-Hpla
The identity of our synthetic l-Choi 5 and the natural
compound isolated by Murakami from aeruginosins by acid
3450
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Chem. Eur. J. 2001, 7, No. 16

Syntheses of Aeruginosin 298-A and -B
3446±3460
and, finally, a unit of l-Arg (Scheme 7). Later on, the same
reaction sequence was successfully used for the synthesis of
the real aeruginosin 298-A (1), by incorporating d-Leu.
Having successfully achieved the preparation of Choi, we
had at our disposal two derivatives, 22 and 26, suitable for the
methodology recently described by Wang [( )-DIP-Cl, THF,
Et3N],[26] resulted in a 92% yield of the a-hydroxy acid 31 (d-
Hpla) in 86% ee.[27] After sequential protection of both
hydroxyl groups and recrystallization with (‡)-1-phenylethyl-
amine, we obtained the required 34 in >98% ee.
Removal of the Boc group from 30, followed by coupling
with the d-Hpla derivative 34 by using BOP, gave compound
35.[25] After saponification of the methyl ester in 35, which
simultaneously induced deprotection of the acetate group in
the Hpla fragment, coupling to the desired tetramer unit
system was achieved by treatment of acid 36 with arginine
(Nw-NO2) methyl ester in DMF medium, by using BOP.
At this point, only two operations were now required to
complete the synthesis: reduction of the ester moiety of the
arginine unit in compound 37 and removal of phenol and
guanidino protecting groups. LiBH4 reduction[28] of 37
(1.75 mmol for each mmol of ester)[29] in THF brought about
the transformation of the arginine moiety methyl ester to the
hydroxymethyl group. Finally, treatment of the resulting
compound 38 with palladium on charcoal in acid medium
effected the cleavage of the benzyl group and the depro-
tection of the guanidine moiety to give the target 2.
Purification by reversed-phase HPLC provided pure 2
HO
H
H
a
H
HO
H
N
N
CO2Me
N
H
Boc
CO2Me
Boc
O
22
30
HO
H
O
b
c
O
H
O
N
N
H
OMe
OAc
HO
BnO
35a R = H
35b R = COCF3
H
e
2
O
H
NH
NHNO2
H
N
N
N
H
N
H
O
OH
BnO
O
R
1
37 R = CO2Me
38 R = CH2OH
(positive Sakaguchi test) in 63% yield. However, the H and
d
13C NMR spectra of our synthetic aeruginosin 298-A (2)
unexpectedly showed some chemical shifts different from
those reported in the literature for the natural product,
although the patterns of the peaks were quite similar. The
most significant differences in the NMR data of 2 in
comparison with those reported for the natural compound
correspond to the signals attributable (COSY, HSQC) to H-7a
(d ˆ 4.4) and H-7eq (d ˆ 1.83) of Choi and the methyl groups
of Leu (d ˆ 0.84 and 0.85), as well as to C(7) of Choi (d ˆ 34.4)
and C(1) of Leu (d ˆ 170.5). For detailed NMR data of 2, see
Tables 2 and 3.
Scheme 7. Synthesis of the putative aeruginosin 298-A (2): a) TFA,
CH2Cl2, 08C, 45 min; Boc-l-Leu, BOP, NMM, CH2Cl2, 08C, 30 min, then
22 h, 43% from 22; b) TFA, CH2Cl2, 08C, 45 min; 34, BOP, NMM, CH2Cl2,
08C, 30 min, then 18 h, 60%; c) 0.1n LiOH, THF, 20 h, 92%; l-Arg-
(NO2)OMe ´ HCl, BOP, NMM, DMF, 08C, 30 min, then 24 h 46%;
d) LiBH4 (2m in THF), THF, 3 h, 50%; e) H2, Pd-C 10%, EtOAc-MeOH,
6n HCl, 1 atm, 8 h, 70%.
aeruginosin synthesis. Preliminary studies showed that the
hydroxyl group at C(6) did not require protection against
unwanted coupling reactions, and consequently we carried out
the synthetic sequence using the N-Boc derivative 22, which
has a free hydroxyl group. Removal of the Boc group from
Choi derivative 22, followed by coupling to Boc-l-Leu, by
using benzotriazol-1-yloxytris(dimethylamino)phosphonium
hexafluorophosphate (BOP), gave dipeptide 30.[25]
At this stage, we concluded that the original structure
elucidation[2a, 7] of aeruginosin 298-Awas incorrect and should
be revised. After careful analyses of NMR data of 2 and
several aeruginosins, as well as microcin SF608, which has an
l-amino acid linked to the Choi nucleus, we suspected that the
correct configuration for aeruginosin 298-A should include a
d-Leu unit. Working from this perspective, stereoselective
synthesis of 1, an aeruginosin incorporating a d-Leu instead of
an l-Leu, was carried out.
As summarized in Scheme 8, the protected D-Hpla 34 was
synthesized in a straightforward sequence. Enantioselective
reduction of (4-hydroxyphenyl)pyruvic acid by using the
The synthesis and structural reassignment of aeruginosin 298-
A (1): Working as described for the synthesis of 2, but
coupling the Choi derivative 22 with Boc-d-Leu, we prepared
the new peptide d-Leu-l-Choi 39. In this series, the Choi
hydroxyl group was more reactive in the coupling processes
than it had been in the above series, undergoing trifluoro-
acetylation more easily. Thus, peptides 39 and 40 were isolated
as a mixture of compounds with both free and trifluoroacety-
lated Choi hydroxyl groups. Although this lengthened the
analytical process, it did not constitute a synthetic problem,
since the trifluoroacetate was removed in the saponification
step to provide 41. More importantly, the higher solubility of
the trifluoroacetylated intermediates enhanced the yields of
the coupling steps. Thus, following the sequence depicted in
CO2H
(–)-DIP-Cl, TEA
THF, –20 °C
CO2H
OH
31 (D-Hpla)
O
HO
HO
RT, 8 h (92%)
BnBr, K2CO3
CO2R
AcCl, CH2Cl2
RT, 16 h (87%)
CHCl3, MeOH
OH
60 ºC, 6 h (70%)
BnO
32 R = H
33 R = Bn
0.1N LiOH,
THF, RT, 24 h
CO2H
ee>98% after
crystallization with
OAc
(R)-phenylethylamine
BnO
34
Scheme 8. Synthesis of d-Hpla and protected derivatives.
Chem. Eur. J. 2001, 7, No. 16
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3451

J. Bonjoch et al.
FULL PAPER
Table 2. 1H NMR data of natural, putative, and synthetic aeruginosins 298-A and 298-B[a]
298-A
1 (d-Leu)
trans cis
2 (l-Leu)
298-B[b]
minor
3 (d-Leu)
cis
4 (l-Leu)
[c]
trans
cis
major
trans
trans
cis
Hpla 2
4.04 m
4.04 m
4.01 ddd
(8, 4, 5)
4.01 m
4.00 m
4.01 m 4.00 m
4.01 m
3.95 m
3
2.64 dd
(14, 7.5)
2.85 dd
(14, 3.6)
2.64 dd 2.55 dd 2.55 dd
(14, 7.5) (14, 7.5) (14, 8)
2.85 dd 2.80 dd 2.78 d
(14, 3.6) (14, 3.6) (14, 4.5)
2.63 dd
(13.3, 7.7)
2.84 dd
(14.1, 3.9)
6.98 dd
(8.6, 1.7)
6.62 dd
2.61 dd
2.63 dd 2.61 dd
2.53 dd
2.45 dd
(14.4, 7.3) (14, 7.5) (14, 7)
2.81 dd 2.85 dd 2.81 d
(13.7, 4.3) (14, 3.5) (14, 4)
6.95 dd 6.98 d 6.95 d
(8.6, 1.7) (8.5) (8.5)
6.60 dd 6.62 d 6.60 d
(8.6, 1.7) (8.5) (8.5)
(13.8, 4.8)
2.78 dd
(13.8, 4.8)
6.95 d
(8.4)
6.60 d
(8.4)
5.32 d
(6.0)
(13.8, 7.8)
2.87 dd
(13.8, 4.8)
6.98 d
(8.4)
6.64 d
(6.0)
5.53 d
(6.0)
5, 9 6.98 d
(8.5)
6, 8 6.60
(8.5)
6.98 d 6.94 d
(8.5) (8.5)
6.62 d 6.61 d
(8.5) (8.5)
5.72 d 5.46 d
6.94 d
(8.5)
6.60 d
(8.5)
5.40 d
(6.5)
(8.6, 1.7)
2OH
5.76 br 5.5 br
(5.5)
(6.0)
7OH
2
9.12 brd
4.51 m
9.16 s
4.51 ddd
(11, 9.3, 3.5)
1.2 ± 1.3 m
9.14 s
4.19 ddd 4.52 t
(10, 9, 3) (9.0)
9.10 s
4.50 br
Leu
4.51 ddd
(10.1, 8.3, 3.5)
1.20 t
4.53 t
(9, 0)
4.19 ddd
(13.5, 12.5, 3.5)
3
1.20 t
(3)
1.40 m 1.60
1.35 m
0.81d
(6.0)
0.87 d 0.77 d
(6.5) (6.0)
7.46 d 7.50 d
(6.0)
4.17 t
(8.7)
1.80 td
(13, 9)
2.0 m
1.40
1.20 ddd
(13.3, 9, 4.7)
1.33 m
1.32 m
0.80 d
(6.4)
0.87 d
(6.4)
7.40 d
(8.5)
1.24 m
1.20 m 1.24 m
1.25 m
(3.5)
1.40 m
1.35 m
0.82 d
(6.4)
0.88 d
(6.5)
1.4 ± 1.6 m
1.4 ± 1.6 m
0.84 d
(6.5)
0.85 d
(6.5)
7.69 d
(9.0)
1.41 m
1.28 m
0.76 d
(6.0)
0.87 d
(6.0)
7.33 d
(9.0)
4.64 t
(8.6)
1.34 m 1.41 m
1.32 m 1.27 m
0.80 d 0.71 d
(6.4)
0.87 d 0.76 d
(6.0) (6.0)
7.40 d 7.35 d
(9.0)
4.12 t
(9.0)
1.80 m 1.85 m
1.99 m 2.22 m
2.25 m 2.22 m
1.49 m
1.49 m
0.84 d
(6.6)
0.85 d
(6.6)
7.62 d
(8.4)
4.19 dd
(10.2, 1.8)
1.80 m
4
5
0.68 d
(6.0)
0.80 d
(7.0)
0.88 d
(7.0)
0.82 d
(6.6)
0.91 d
(6.6)
(6.0)
NH 7.45 d
(8.3)
Choi 2
(6.0)
4.63
(9.0)
4.64 t
(9.0)
4.16 t
(8.9)
1.80 td
(12.7, 10.1)
2.0 m
4.23 dd
(9.5, 8.5)
1.83 m
4.29 dd 4.12 t
(9.5, 8.5) (8.0)
4.24 m
3b
1.80 ddd
(14.2, 13.3, 8)
1.98 m
1.85 m
3a
3a
4
1.96 ddd
(12, 7, 7)
2.2 ddddd
(13, 7, 7, 7, 0.5)
1.4 ± 1.6 m
2.04 m
2.22 m
2.22 m
1.99 m
2.27 m
2.27 m
2.25 m
2.18 ddddd
(13, 7, 6.5, 6.5, 1)
1.43 m
1.99 m
1.43 m
1.43 m
2.02 m
1.43 m
1.43 m
2.00 m
1.43 m
1.41 m
2.02 m
1.42 m
1.45 m
2.02 m
1.39 m
1.98 m
3.84 br
1.43 m
1.42 m 1.45 m
2.02 m 2.20 m
1.42 m 1.39 m
1.98 m
3.91 brs 3.84 brs
1.65 m 1.43 m
5
1.2 ± 1.3 m
6
3.92 s
3.91 s
1.65 t
(11)
3.81 s
3.91 s
1.72 t
(12)
3.84 s
3.92 br
3.90 brs
1.70 t
(12)
1.80 m
4.39 ddd
(11, 6, 6)
6.80 br
7.25 br
3.85 brs
4.24 m
7ax 1.65 t
(11.8)
7eq 2.02 m
1.65ddd
(14, 11.6, 2)
2.00 m
2.00 m
1.83 m
1.85 m
2.00 m 1.85 m
4.27 ddd 4.02 m 4.27 ddd
7a
4.04 m
4.04 m 4.28 ddd 4.40 ddd
(11, 6, 6) (11, 6, 6)
4.05 ddd 4.02 m
(11, 6, 6)
(11.5, 6, 6)
7.10 br
7.60 br
(11.5, 6, 6)
6.84 br 7.10 br
7.23 br 7.60 br
NH2
1
6.83 br
7.22 br
Arg
3.21dd
3.21 dd
(10.5, 6)
3.31 dd
(10.5, 4)
3.60 m
1.30 m
1.60 m
1.50 m
3.07 m
4.63 t
3.24 dd
(11.5, 5)
3.31 dd
(10.5, 5)
3.67 m
1.2 ± 1.3 m
1.4 ± 1.6 m
1.4 ± 1.6 m
3.06 m
(10.7, 6.2)
3.31 dd
(10.7, 5.1)
3.64 m
1.30 m
1.60 m
2
3
3.42 m
4
5
1.50 m
3.07 m
1OH
4.70 t
(6.1)
4.70 t
(5.5)
4.74 t
(5.5)
(6.1)
2NH 7.54 s
5NH 7.52 s
7.58 d 7.80 d
(8.5)
7.55 t
(5.5)
(8.0)
7.60 t
(5.5)
7.42 brs
[a] All spectra were recorded in [D6]DMSO at 500 MHz (1 ± 3) or 600 MHz (4) and the peak assignments are derived from COSY and NOESY experiments.
[b] Described as a d-Leu and l-Leu mixture of epimers. [c] trans/cis rotamer ratio 6:1 for 1, 5:1 for 2; 3:1 for 3, and 7:1 for 4.
3452
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Chem. Eur. J. 2001, 7, No. 16

Syntheses of Aeruginosin 298-A and -B
3446±3460
Table 3. 13C NMR Chemical shifts of Natural, Putative, and Synthetic Aeruginosins 298-A and 298-B[a]
298-A
1
2
298-B
3
4
trans
trans
major[b]
minor
trans[c]
cis[c]
trans
Hpla
1
2
3
172.8
72.1
39.3
128.1
130.4
114.7
155.7
169.8
48.1
41.8
23.9
23.3
21.4
171.3
59.9
30.7
36.0
19.0
26.0
63.9
33.4
54.0
63.3
50.2
28.0
25.0
40.8
156.7
172.7
72.0
39.2
128.0
130.4
114.6
155.7
169.6
48.0
42.0
23.9
23.4
21.4
171.2
59.9
30.5
36.0
19.0
26.0
63.8
33.4
54.0
63.2
50.2
27.8
25.1
40.7
157.0
173.1
72.4
39.4
126.2
130.3
114.8
155.7
170.5
47.9
41.6
24.3
23.6
21.7
171.4
60.4
30.4
36.6
19.2
26.1
64.1
34.3
54.5
63.5
50.1
28.3
25.1
40.9
156.8
172.3
71.8
39.0
127.9
130.0
114.4
155.4
169.4
47.4
41.8
23.6
22.9
21.4
173.0
59.4
30.1
35.8
18.8
25.9
63.7
33.2
53.6
172.3
72.0
39.0
125.2
129.9
114.4
155.4
169.8
46.9
41.7
23.7
23.0
21.3
173.3
59.1
30.1
35.9
18.7
25.9
63.7
33.4
53.9
172.5
72.1
39.2
128.1
130.4
114.6
155.6
169.5
47.8
42.2
23.8
23.3
21.5
173.3
59.5
30.4
36.0
19.0
26.0
63.8
33.5
53.9
172.5
72.1
39.2
125.0
130.3
114.6
155.6
169.7
47.8
42.1
23.8
23.4
21.0
173.4
59.4
30.4
36.1
19.0
25.9
63.8
33.5
54.0
173.3
72.4
40.0
129.0
130.6
115.0
156.0
170.6
48.1
42.1
24.5
23.9
22.0
173.6
59.5
30.7
37.0
19.4
26.3
64.3
34.6
54.7
4
5, 9
6, 8
7
1
2
3
4
5
5'
1
2
3
3a
4
5
6
7
7a
1
2
Leu
Choi
Argol
3
4
5
ˆ
C N
[a] All spectra were recorded in [D6]DMSO at 75 MHz and the assignments were aided by HSQC experiments. [b] These data correspond to the major and
minor components described for the natural product. [c] trans-cis rotamers of the d-Leu derivative.
Scheme 9, we synthesized 1, which had NMR spectral data
matching those reported by Murakami when he isolated
aeruginosin 298-A.[2a] Thus, we concluded that the stereo-
structure of the natural product corresponds to that of 1.[30]
In fact, aeruginosin 298-A (1) in DMSO solution appears as
a 6:1 mixture of trans and cis rotamers about the Choi Leu
peptide bond. The relative populations of the amide cis and
trans isomers were measured in the 1H NMR spectra by
integration of the isomeric methyl proton signals of Leu. The
assigned isomer geometry was based on NOESY experiments
and the chemical shift values for the signals of H-2 and H-7a
of Choi. In the 1H NMR spectra of 1 and its precursors, which
also exhibit this phenomenon,[31] the H-2 and H-7a protons
appear more shielded in the trans isomer than in the cis isomer
(see Table 2). The NMR of the l-Leu epimer peptide 2 and its
precursors (Scheme 8) also show this rotational isomerism,
although the change in the chemical shifts of H-2 and H-7a
follows a different rule: in the l-Leu compounds, the H-2
signal of the trans isomer appears upfield to that of the cis
isomer, but the H-7a signal of the trans isomer appears
downfield from that of the cis isomer[32] (see Table 2).
of 298-A)/thrombin complex reported by Clardy,[33] the
configuration of the allo-Ile unit corresponds to d on the
basis of its van der Waals interactions with Trp215 and Leu99
and neither an l-Ile nor l-allo-Ile side chain could be
accommodated by thrombin. In addition, the d-configuration
of the P3 substituent is consistent with the structure ± activity
relationship of d-Phe-l-Pro-arginine inhibitors of thrombin[34]
and recently described peptidomimetics.[35]
The synthesis of aeruginosin 298-B (3). Our next concern was
to synthesize aeruginosin 298-B (3) by utilizing the same
advanced intermediate 41 as in the above synthesis of
aeruginosin 298-A (1). At the same time, we also decided to
synthesize the epimer 4, which incorporates an l-Leu, since
the natural product isolated from microcystis aeruginosa in
1999[2e] had been reported to be a 3:1 mixture of 3 and 4 (see
Scheme 2).
From the d-Hpla-d-Leu-l-Choi derivative 41, a two-step
sequence consisting of coupling with ammonium hydroxide by
using PyBOP to give the amide 44 and a deprotecting
debenzylation gave aeruginosin 298-B 3 (Scheme 10).
Aeruginosin 298-B (3) showed minor NMR signals with
approximately 20% of the intensity of the major signals.
Virtually all signals were doubled. In fact, the 1H and
13C NMR data of 3 matched the duplicate signals reported
by Murakami for the natural product. It was therefore
concluded that the dichotomy of the signals observed in 3,
and hence in the natural aeruginosin, was due to geometrical
With hindsight, the d-configuration of the Leu subunit is
consistent with the absolute configuration of related members
of the aeruginosin family that are also potent inhibitors of
thrombin. Assuming a related biosynthetic pathway and
function for other aeruginosins, it would have been surprising
to find that 298-A is an exception. Furthermore, from the
X-ray co-crystal structure of the aeruginosin 98-B (a relative
Chem. Eur. J. 2001, 7, No. 16
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3453

J. Bonjoch et al.
FULL PAPER
H
HO
CO2H
H
a-d
e,f
NH4OH
PyBOP
NH2
O
HO
MeO
O
N
H
O
CO2Me
Scheme 4
Scheme 6
H
N
H
N
6
Bn
OH
O
13
H
OH
H
N
BnO
O
41
N
NH2
H
RO
H
O
OH
RO
g
O
H
HO
39a R = H
39b R = COCF3
44 R = Bn
3 R = H
N
CO2Me
H
O
HO
H
H2, Pd/C
H
N
Boc
N
H
CO2Me
Boc
O
NH4OH
22
H
N
HO
O
N
RO
PyBOP
OH
H
H
O
H
O
OH
h
BnO
O
40a R = H
40b R = COCF3
O
36
H
O
H
O
N
N
N
H
N
H
NH2
OMe
OH
OAc
RO
O
BnO
45 R = Bn
HO
H2, Pd/C
k
4 R = H
H
i
O
Scheme 10. Syntheses of aeruginosin 298-B (3) and its epimer 4.
H
O
NH
H
N
N
N
N
H
NHNO2
H
OH
BnO
O
R
j
Conclusion
42 R = CO2Me
43 R = CH2OH
In conclusion, we have accomplished the first total syntheses
of aeruginosins 298-A (11 steps, 3.1% yield) and 298-B (10
steps, 2.6% yield); this allows the unequivocal reassignment
of the absolute configuration of both natural aquatic peptides.
Interestingly, this fact shows that all aeruginosins[2, 3] have the
d (R) configuration in the a-amino acid attached to the
perhydroindole nitrogen.
HO
H
O
H
H
N
NH
NH2
N
N
N
H
H
O
OH
HO
O
CH2OH
aeruginosin 298-A (1)
Since our previous communication[9] and the writing of this
manuscript, another synthesis of aeruginosin 298-A has been
published.[36] The synthesis, reported by Wipf, is conceptually
different from that reported here, the main differences being:
i) l-Choi was prepared by an oxidative process rather than a
reductive one, ii) d-Hpla was synthesized from (R)-benzyl-
glycidol instead of from a phenylpyruvic acid derivative,
iii) orthogonal protecting groups requiring two deprotection
steps were used instead of one-step removal protecting
groups, and iv) a different coupling sequence was used.
Scheme 9. Synthesis of aeruginosin 298-A (1): a) Li, NH3, THF-tBuOH
(2.5:1), 788C; b) MeOH, 8n HCl, 358C, 48 h; c) BnBr, NaHCO3, EtOH,
708C, 6 h; d) MeOH, 9n HCl, 658C, 17 h (44% from 6); e) H2, Pd(OH)2
20%, Boc2O, EtOAc, 48 h, 79%, f) LS-Selectride (1m in THF, 1.3 equiv),
THF, 788C, 53% for isomer 22; g) TFA, CH2Cl2, 08C, 45 min; Boc-d-
Leu, BOP, NMM, CH2Cl2, 08C, 30 min, then 22 h, 73% from 22; h) TFA,
CH2Cl2, 08C, 45 min; 34, BOP, NMM, CH2Cl2, 08C, 30 min, then 18 h,
60%; i) 0.1n LiOH, THF, 20 h, 85%; l-Arg(NO2)OMe ´ HCl, PyBOP,
NMM, DMF, 08C, 30 min, then 22 h, 90%; j) LiBH4 (2m in THF), THF, 3 h,
53%; k) H2, Pd-C 10%, EtOAc/MeOH (1:1), 6n HCl (2 drops), 1 atm, 8 h,
95%.
isomerism in the Choi Leu peptide bond and not to a mixture
of d- and l-Leu residues. The 1H chemical shifts of the major
peaks, based on NOESY studies, corresponded to attributable
signals for the trans isomer (Choi, H-2 d ˆ 4.12; H-7a d ˆ 4.02;
Leu, H-2 d ˆ 4.52), while the minor peaks (Choi, H-2 d ˆ 4.64;
H-7a d ˆ 4.27; Leu, H-2 d ˆ 4.19) corresponded to those of the
cis isomer. Thus, as had been observed in the aeruginosin 298-
A series, both the H-2 and H-7a protons of Choi appeared
further upfield in the trans rotamer than in the cis rotamer.
As expected, the NMR data of 4 (Table 2)Ðthe l-Leu
epimer of aeruginosin 298-B (3), prepared by starting from
acid 36 and following the same procedure as used for the
synthesis of 3 (Scheme 10)Ðare clearly different from the
minor signals described for the natural product and errone-
ously attributed to 4 when aeruginosin 298-B was isolated. In
fact, duplicate signals again appear in NMR spectra of
compound 4, due to cis-trans amide isomerism, but they do
not correspond to those reported for the natural aeruginosin.
Experimental Section
General: All reactions were carried out under argon atmosphere with dry,
freshly distilled solvents under anhydrous conditions. Unless otherwise
noted, analytical thin layer chromatography was performed on SiO2 (silica
gel 60 F254, Merck), and spots were located with iodoplatinate reagent
(compounds 5 ± 30), 1% aqueous KMnO4 (compounds 3, 4, 31 ± 44) or the
Sakaguchi reagent (compounds 1 and 2). Chromatography refers to flash
chromatography and was carried out on SiO2 (silica gel 60, SDS, 230 ± 240
mesh ASTM). Drying of organic extracts during workup of reactions was
performed over anhydrous Na2SO4. Evaporation of solvent was accom-
plished with a rotary evaporator. Optical rotations were recorded with a
PerkinÐElmer241 polarimeter. 1H and 13C NMR spectra were recorded
with a Varian300 or a VarianVXR-500 instrument. Chemical shifts are
reported in ppm downfield (d) from Me4Si. IR spectra were recorded on a
Nicolet205 FT-IR spectrophotometer, and only noteworthy absorptions
are listed. Melting points were determined in a capillary tube. Micro-
Â
analyses and HRMS were performed by the Centro de Investigacion y
Desarrollo (CSIC), Barcelona. Abbreviations: BOP, benzotriazol-1-yloxy-
3454
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Chem. Eur. J. 2001, 7, No. 16

Syntheses of Aeruginosin 298-A and -B
3446±3460
tris(dimethylamino)phosphonium hexafluorophosphate; NMM, (N-meth-
ylmorpholine); PyBOP (benzotriazol-1-yloxy)tripyrrolidinophosphonium
hexafluorophosphate
4.2 g (84%) of ketone 13, with 350 mg of the starting material being
recovered.
This process can obviously be applied to mixtures of exo-endo derivatives
(12 and 13), thus avoiding some of the chromatographic process in the
aforementioned synthetic procedure.
O-Methyl-l-tyrosine hydrochloride (6): This compound was prepared
following the previously reported procedure,[37] either from N-Boc-l-
tyrosine (Fluka) or N-acetyl-l-tyrosine (Fluka) on 82 mmol and 112 mmol
scales (90% and 77% overall yield, respectively) in two steps (i, Me2SO4,
aq NaOH; ii, 4n HCl).
Methyl (2S,3aS,7aS) 1-tert-butoxycarbonyl-6-oxo-octahydroindole-2-carb-
oxylate (16): A suspension of 13 (712 mg, 2.48 mmol), Boc2O (704 mg,
3.2 mmol), and Pd(OH)2/C (20%, 142 mg) in EtOAc (5 mL) was stirred
under hydrogen (atmospheric pressure) at room temperature for 48 h. The
catalyst was then removed by filtration through a short pad of Celite, which
was washed with CHCl3 (30 mL), EtOAc (30 mL), and MeOH (30 mL).
The organic filtrates were concentrated and chromatographed (Al2O3,
hexane/EtOAc 1:1) to furnish carbamate 16 as a yellow oil. Yield: 658 mg,
89%; Rf ˆ 0.54 (hexane/EtOAc); [a]2D0 ˆ ‡30.2 (c ˆ 3.6 in CHCl3); 1H NMR
(500 MHz, CDCl3, COSY, NOESY) (1:1 mixture of trans and cis rotamers):
d ˆ 1.37 and 1.42 (2  s, 9 H; CH3), 1.88 (brs, 1H; H-4ax), 1.96 ± 2.03 (m, 2H;
H-3, H-4eq), 2.12 ± 2.18 (m, 1H; H-5ax), 2.34 ± 2.44 (m, 2H; H-3, H-5eq),
2.50 ± 2.60 (m, 1H; H-3a), 2.64 ± 2.70 (brs, 1H; H-7ax), 2.84 and 2.96 (2 Â
brs, 1H; H-7eq), 3.72 (s, 3H; OCH3), 4.11 and 4.22 (2 Â brs, 1H; H-7a), 4.28
and 4.37 (2  brs, 1H; H-2); (300 MHz, [D6]DMSO, 808C): d ˆ 1.37 (s, 9H;
CH3), 1.80 (dddd, J ˆ 13.5, 9, 5.5, 5.5 Hz, 1H; H-4ax), 1.89 ± 2.02 (m, 2H;
H-3a, H-4eq), 2.11 (dddd, J ˆ 15.9, 7.8, 5.4, 0.9 Hz, 1H; H-5eq), 2.36 (ddd, J ˆ
15.9, 9, 5.4 Hz, 1H; H-5ax), 2.40 (dt, J ˆ 13, 8 Hz, H-3b), 2.53 (masked,
H-3a), 2.54 (dd, J ˆ 15.3, 9.9 Hz, 1H; H-7ax), 2.63 (ddd, J ˆ 15.3, 6.6, 0.9 Hz,
1H; H-7eq), 3.67 (s, 3H; OCH3), 4.11 (ddd, J ˆ 9.6, 7.5, 6.6 Hz, 1H; H-7a),
4.26 (t, J ˆ 8.5 Hz, 1H; H-2); 13C NMR (75 MHz, CDCl3, HMQC): d ˆ (cis
rotamer) 24.5 (C(4)), 28.3 (CH3), 33.1 (C(3)), 35.5 (C(3a)), 36.5 (C(5)), 44.0
(C(7)), 52.2 (OCH3), 57.4 (C(7a)), 59.3 (C(2)), 80.6 (OCMe3), 153.0 (NCO),
173.6 (COOMe), 210.1 (CO); (trans rotamer) 24.3 (C(4)), 28.3 (CH3), 34.1
(C(3)), 35.5 (C(3a)), 36.5 (C(5)), 43.0 (C(7)), 52.2 (OCH3), 57.2 (C(7a)),
59.7 (C(2)), 80.5 (CMe3), 153.5 (NCO), 173.6 (COOMe), 209.7 (CO);
13C NMR (75 MHz, [D6]DMSO, 808C): d ˆ 23.7 (C(4)), 27.8 (C(CH3)3),
33.1 (C(3)), 35.2 (C(3a)), 36.0 (C(5)), 42.9 (C(7)), 51.5 (OCH3), 56.9
(C(7a)), 59.2 (C(2)), 79.2 (C(CH3)3), 152.5 (NCO2), 172.8 (CO2Me), 209.0
(C(6)); IR (film): nÄ ˆ 1749, 1716, 1698 cm 1; MS (EI) m/z: 297, 238, 196,
182, 138 (100), 80, 57; elemental analysis calcd (%) for C15H23NO5 (297.33):
C 60.59, H 7.80, N, 4.71; found C, 60.39, H 8.05, N 4.55.
Methyl (2S,3aR,7aR)- and (2S,3aS,7aS)-1-benzyl-6-oxo-octahydroindole-
2-carboxylate (12 and 13): Ammonia (200 mL) was added to a cooled
(
788C) solution of 6 ´ HCl (13.85 g, 60 mmol) in a mixture of THF/tBuOH
(2.5:1) (250 mL). Small chips of lithium (2.35 g, 0.34 mmol) were added
with vigorous stirring until the solution was a persistent deep blue, and
stirring was maintained for 90 min. The cooling bath was removed, the
ammonia was allowed to evaporate overnight, and the reaction mixture was
concentrated to give lithium (2S)-2-amino-3-(2,5-dihydro-4-methoxyphen-
yl)propanonate (7) as a white solid: 1H NMR (200 MHz, D2O): d ˆ 1.8 ± 2.2
(m, 2H), 2.45 (brs, 4H), 3.03 ± 3.07 (m, 1H), 3.29 (s, 3H), 4.56 (brs, 1H),
5.24 (brs, 1H); 13C NMR (50 MHz, D2O): d ˆ 31.0 (t), 31.2 (t), 45.1 (t), 56.6
(d and q), 94.8 (d), 123.5 (d), 134.8 (s), 154.9 (s), 185.6 (s). The lithium salt 7
was dissolved in a freshly prepared 8.5n solution of HCl in MeOH (70 mL),
and the solution was stirred at 358C for 48 h. The mixture was concentrated
to dryness, and the residue[38] was dissolved in EtOH (100 mL). NaHCO3
(35 g, 0.42 mol) and benzyl bromide (25 mL, 0.21 mmol) were added, and
the mixture was stirred at 708C until the disappearance of the starting
material 9 was observed by TLC (7 h). If necessary, additional NaHCO3
(1 equiv) and benzyl bromide (1 equiv) were added. The reaction mixture
was filtered, the solvent removed, and the residue taken up with CH2Cl2
(25 mL) and extracted with 2n HCl (4 Â 10 mL). The aqueous extracts
were basified with Na2CO3 to pH 10 ± 11 and extracted with CH2Cl2 (6 Â
50 mL). Purification of the latter organic extracts by chromatography
(CH2Cl2) provided a partially separated 1.8:1 mixture of compounds 12 and
13. The first purification gave 3.6 g of 12, 2.52 g of an exo-endo isomer
mixture, and 2.57 g of 13.[39, 40] Further careful chromatographic treatment
allowed the separation of the fractions containing mixtures of exo and endo
isomers. After the overall process, 5.47 g (32%) of 12 and 3.10 g (18%) of
13 were isolated as colorless oils (combined yield 50% from 6). On
standing, 13 solidified as colorless needles
Alternatively, the chromatography can be avoided if the reaction mixture is
treated with a pH 3 buffer of AcOH/NaOAc (20 mL) at room temperature
for 48 h. The aqueous solution was basified with Na2CO3 and extracted with
CH2Cl2 and CHCl3. The organic extracts were concentrated to give pure
carbamate 16 with the same yield as above, operating on a 6.6 mmol scale
(1.67 g of 16).
Exo isomer 12: Rf ˆ 0.64 (hexane/CH2Cl2/EtOAc 6:9:2); [a]D20
ˆ
43.4 (c ˆ
3.6 in CHCl3); 1H NMR (300 MHz, CDCl3, COSY, NOESY): d ˆ 1.72 (dq,
J ˆ 14, 5 Hz, 1H; H-4eq), 1.85 (dt, J ˆ 14, 7.5 Hz, 1H; H-3a), 1.98 ± 2.09 (m,
2H; H-3b and H-4ax), 2.21 (dt, J ˆ 14, 5.7 Hz, 1H; H-5eq), 2.41 (ddd, J ˆ 14,
10.4, 5 Hz, 1H; H-5ax), 2.54 (d, J ˆ 4.7 Hz, 2H; H-7), 2.80 (m, 1H; H-3a),
3.54 (d, J ˆ 7.5 Hz, 1H; H-2), 3.66 (s, 3H; OCH3), 3.72 (dt, J ˆ 9, 4.5 Hz,
1H; H-7a), 3.55 and 3.90 (2d, J ˆ 13.5 Hz, 1H each; CH2Ph), 7.20 ± 7.27 (m,
5H; Ph); 13C NMR (50 MHz, CDCl3): d ˆ 25.4 (C(4)), 33.3 (C(3)), 34.2
(C(3a)), 35.3 (C(5)), 42.1 (C(7)), 51.0 (OCH3), 52.2 (CH2Ph), 59.2 (C(7a)),
61.8 (C(2)), 127.0, 128.2, 128.6 and 138.6 (Ph), 174.2 (COOMe), 212.3 (CO);
IR (film): nÄ ˆ 1732, 1717 cm 1; elemental analysis calcd (%) for C17H21NO3
(287.4): C 71.08, H 7.32, N 4.88; found C 71.03, H 7.34, N 4.82.
Methyl (2S,3aS,6S,7aS) 1-benzyl-6-hydroxy-octahydroindole-2-carboxy-
late (17): CeCl3 (296 mg, 1.2 mmol) was added to a solution of 13 (2.28 g,
7.94 mmol) in MeOH (60 mL). The reaction was stirred at room temper-
ature for 20 min, then NaBH4 (1 g, 27.8 mmol) was added. The mixture was
stirred at 238C for 6 h. After removal of MeOH, water (100 mL) was
added, and the solution was extracted with EtOAc (2 Â 100 mL) and CHCl3
(3 Â 100 mL). The dried organic extracts were concentrated to give an 8:1
mixture of alcohols 17 and 18, which was purified by chromatography
(Al2O3, hexane/EtOAc 1:2). The first eluate gave 17 (1.72 g, 75%): Rf ˆ
0.77 (Al2O3, hexane/EtOAc, 1:2); m.p. 49 ± 518C; [a]2D0 ˆ ‡11.8 (c ˆ 1 in
CH3OH); 1H NMR (500 MHz, CDCl3, COSY, NOESY): d ˆ 1.40 ± 1.46 (m,
2H; H-4eq, H-5ax), 1.54 (dd, J ˆ 13, 3.5 Hz, 1H; H-3b), 1.60 (dt, J ˆ 15,
3.5 Hz, 1H; H-7ax), 1.85 (m, 1H; H-5eq), 1.90 (m, 1H; H-4ax), 2.04 (ddddd,
J ˆ 12.5, 6, 6, 5.5, 5.5 Hz, 1H; H-3a), 2.20 (td, J ˆ 13, 6.5 Hz, 1H; H-3a), 2.39
(dd, J ˆ 15, 2.5 Hz, 1H; H-7eq), 2.93 (s, n1/2 ˆ 3 Hz, 1H; H-7a), 3.31 (s, 3H;
OCH3), 3.32 (masked, 1H; H-2), 3.33 and 4.24 (2  d, J ˆ 13 Hz, 1H each;
NCH2Ph), 4.04 (s, n1/2 ˆ 2 Hz, 1H; H-6eq), 5.40 (br, 1H; OH), 7.24 (m, 5H;
PhH); 13C NMR (50 MHz, CDCl3): d ˆ 27.5 (C(4)), 33.3 (C(3)), 34.8 (C(5)),
35.5 (C(7)), 37.0 (C(3a)), 51.5 (OCH3), 57.0 (CH2Ar), 64.5 (C(7a)), 65.1
(C(2)), 66.3 (C(6)), 127.0 (p-C), 127.9 (o-C), 129.3 (m-C), 137.8 (ipso-C),
175.2 (CO2Me); IR (film): nÄ ˆ 3550, 1748 cm 1; MS (EI) m/z: 289, 230
(100), 212, 170, 122, 91; elemental analysis calcd (%) for C17H23NO3
(289.35): C 70.59, H 7.96, N 4.84; found C 70.78, H 8.03, N 4.84. The
second eluate gave 229 mg (10%) of 6R isomer 18.[40]
Endo isomer 13: Rf ˆ 0.54 (hexane/CH2Cl2/EtOAc 6:9:2); m.p. 688C;
[a]2D0
ˆ
50.5 (c ˆ 3.3 in CHCl3); 1H NMR (500 MHz, CDCl3, COSY,
NOESY): d ˆ 1.71 (ddd, J ˆ 12.5, 9, 6.5 Hz, 1H; H-3a), 1.83 ± 1.96 (m, 2H;
H-4), 2.17 (ddd, J ˆ 17.5, 8.5, 4.5 Hz, 1H; H-5), 2.31 (ddd, J ˆ 12.5, 8.5, 8 Hz,
1H; H-3b), 2.43 (m, 1H; H-3a), 2.44 (dd, J ˆ 15.5, 4.8 Hz, 1H; H-7ax), 2.54
(ddd, J ˆ 17.5, 8, 5 Hz, 1H; H-5), 2.58 (dd, J ˆ 15.5, 5.5 Hz, 1H; H-7eq), 3.05
(dt, J ˆ 9, 5 Hz, 1H; H-7a), 3.26 (t, J ˆ 8.5 Hz, 1H; H-2), 3.41 (s, 3H;
OCH3), 3.57 and 3.83 (2d, J ˆ 14 Hz, 1H each; CH2Ar), 7.21 ± 7.24 (m, 5H;
Ar); 13C NMR (50 MHz, CDCl3, HMQC): d ˆ 26.6 (C(4)), 34.6 (C(3a)),
34.8 (C(3)), 36.6 (C(5)), 42.1 (C(7)), 51.6 (OCH3), 56.2 (CH2Ar), 61.5
(C(7a)), 65.6 (C(2)), 127.1, 127.5, 127.9 and 136.6 (Ar), 173.8 (COOMe),
211.7 (CO); IR (film): nÄ ˆ 1746, 1716 cm 1; elemental analysis calcd (%) for
C17H21NO3 (287.4): C 71.08, H 7.32, N 4.88; found C 71.01, H 7.38, N 4.85.
Epimerization of exo isomer 12 to endo isomer 13: A solution of ketone 12
(5 g) in MeOH/HCl (9n, 500 mL) was heated at 658C overnight. MeOH
was evaporated, the residue was taken up with HCl (2n, 50 mL) and THF
(6 mL), and the mixture was stirred at room temperature for 20 min. Solid
Na2CO3 was added, and the basic solution was extracted with EtOAc and
CHCl3. The dried extracts were concentrated to give a 1:7 mixture of 12 and
13, respectively. Careful separation by chromatography (CH2Cl2) afforded
Methyl (2S,3aS,6R,7aS) 1-tert-butoxycarbonyl-6-hydroxy-octahydroin-
dole-2-carboxylate (22): A solution of carbamate 16 (219 mg, 0.73 mmol)
in THF (5 mL) was added to a cooled ( 788C) solution of LS-SelectrideꢀR
Chem. Eur. J. 2001, 7, No. 16
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0716-3455 $ 17.50+.50/0
3455

J. Bonjoch et al.
FULL PAPER
(1m in THF, 0.95 mL) in THF (5 mL). The mixture was stirred at 788C for
2 h and then allowed to warm to room temperature. After addition of 5%
NaHSO4 (7 mL) and continued stirring for 2 h, the mixture was basified
with solid Na2CO3 to pH 8 and extracted with CH2Cl2 (3 Â 5 mL) and
CHCl3/iPrOH (4:1, 4 Â 15 mL). The dried organic extracts gave a mixture
of alcohols 21 and 22 in 1:8 ratio. Purification by chromatography (Al2O3,
hexane/EtOAc 1:1 to EtOAc) furnished the alcohol 22 (97 mg, 44%) and
then a mixture of 21[40] and 22 (40 mg).
trifluoroacetate salt of 28 was obtained quantitatively (115 mg). An
analytical sample was obtained by trituration with CHCl3: Rf ˆ 0.57
(EtOAc/EtOH 1:1); m.p. 126 ± 1288C (CHCl3); [a]D20
ˆ
25.7 (c ˆ 0.7 in
CH3OH); 1H NMR (500 MHz, CDCl3 ‡ CD3OD, COSY): d ˆ 1.30 (br, 1H;
H-4ax), 1.39 (m, 1H; H-5eq), 1.53 (tm, J ˆ 10 Hz, 1H; H-5ax), 1.66 (br, 1H;
H-3), 1.89 (brm, 3H; H-3, H-4eq and H-7), 2.35 (brs, 2H; H-3a, H-7), 3.74
(s, 3H; OCH3), 3.91 (brs, 2H; H-2, H-6), 4.37 (brs, n1/2 ˆ 24 Hz, 1H; H-7a);
13C NMR (75 MHz, CDCl3 ‡ CD3OD): d ˆ 20.4 (C(4)), 27.5 (C(5)), 31.1
(C(3)), 31.5 (C(7)), 36.2 (C(3a)), 53.5 (OCH3), 57.3 (C(7a)), 57.6 (C(2)), 64.1
(C(6)), 170.4 (COOMe); IR (KBr): nÄ ˆ 3403, 2937, 1748, 1677, 1019;
elemental analysis calcd (%) for C12H18F3NO5 ´ 1ꢁ2 H2O (322.26): C 44.72, H
5.90, N 4.35; found C 44.81, H 5.98, N, 4.43. A sample of 28 ´ TFA was
dissolved in a saturated aqueous Na2CO3 solution (2 mL) and extracted
with a mixture of CHCl3/iPrOH (4:1, 5 Â 2 mL). The concentrated dried
organic extracts gave 28: Rf ˆ 0.18 (EtOAc/EtOH 3:1); m.p. 83 ± 858C;
Alcohol 22: Overall yield: 56%; Rf ˆ 0.45 (Al2O3, EtOAc); m.p. 160 ±
1628C; [a]2D0
ˆ
31.1 (c ˆ 2.4 in CH3OH); 1H NMR (500 MHz, CDCl3,
COSY, NOESY) (mixture of conformers, 60% of trans and 40% of cis):
d ˆ 1.37 and 1.40 (2  s, 9 H; CH3), 1.47 (masked, 1H; H-4), 1.56 (br, 2H;
H-5), 1.67 (brt, J ˆ 12.5 Hz, 1H; H-7ax), 1.92 (br, 1H; H-3a), 2.08 ± 2.14 (m,
2H; H-3b, H-4), 2.23 (br, 1H; H-7eq), 2.32 (br, 1H; H-3a), 3.70 (s, 3H;
OCH3), 4.11 (brs n1/2 ˆ 10 Hz, 1H; H-6eq), 4.18 (br, 1H; H-7a), 4.25 (br,
1H; H-2); 13C NMR, (75 MHz, CDCl3, HMQC): d ˆ 19.4 (C(4)), 26.3
(C(5)), 28.2 (C(CH3)3), 31.6 and 32.4 (C(3)), 33.6 (C(7)), 35.8 and 36.4
(C(3a)), 51.9 (OCH3), 53.4 (C(7a)), 58.8 and 59.3 (C(2)), 65.8 and 66.1
(C(6)), 79.8 (C(CH3)3), 153.2 (NCO2), 174.0 (CO2Me); MS (EI) m/z: 371,
312, 270, 256, 212(100), 196, 182, 166, 122, 80, 57; IR (KBr): nÄ ˆ 3487, 1754,
1673, 1017 (OHax) cm 1; elemental analysis calcd (%) for C15H25NO5 ´
1ꢁ4 H2O (303.85): C 59.29, H 8.46, N. 4.61; found C 59.29, H 8.20, N 4.23.
[a]2D0
ˆ
26.2 (c ˆ 0.3 in CH3OH); 1H NMR (500 MHz, CDCl3, COSY,
NOESY): d ˆ 1.20 ± 1.30 (m, 2H; H-4, H-5ax), 1.55 ± 1.61 (m, 2H; H-4,
H-7ax), 1.67 (ddd, J ˆ 13, 4.5, 2.5 Hz, 1H; H-3), 1.81 (m, 1H; H-5eq), 1.92 (br,
2H; H-3a, NH), 2.12 (brd, J ˆ 13 Hz, 1H; H-7eq), 2.23 (ddd, J ˆ 13, 11,
7 Hz, 1H; H-3), 3.32 (brs n1/2 ˆ 20 Hz, 1H; H-7a), 3.72 (s, 3H; OCH3), 3.77
(br, 1H; H-2), 3.92 (dddd, J ˆ 8, 8, 4, 4 Hz, 1H; H-6eq); 13C NMR, 75 MHz,
CDCl3): d ˆ 25.8 (C(4)), 33.7 (C(5)), 36.0 (C(3)), 36.2 (C(7)), 37.1 (C(3a)),
52.2 (OCH3), 58.2 (C(7a)), 59.0 (C(2)), 66.3 (C(6)), 176.0 (CO2Me); IR
Methyl (2S,3aS,6R,7aS)-6-benzoyloxy-1-benzyl-octahydroindole-2-carb-
oxylate (23): Benzoic acid (250 mg, 2 mmol) and triphenylphosphine
(710 mg, 2.7 mmol) were added to a cooled (08C) solution of alcohol 17
(515 mg, 1.8 mmol) in THF (16 mL). Diethyl azodicarboxylate (0.43 mL,
2.7 mmol) was added slowly, and the mixture was stirred at 08C for 1 h and
then overnight at room temperature. The solvent was evaporated, and the
residue was partitioned between saturated Na2CO3 (150 mL) and CH2Cl2
(100 mL). The dried organic extracts were concentrated and chromato-
graphed (SiO2, hexane/EtOAc 3:1) to furnish 392 mg (56%) of 23 and
29 mg (6%) of alkene 24.[40] After further careful chromatography of
slightly impure 23 (CH2Cl2), and trituration of the resulting solid with
hexane, 23 was obtained as white crystals. Rf ˆ 0.68 (hexane/EtOAc 3:1);
1
(KBr): nÄ ˆ 3420, 2931, 1724, 1079 cm
.
(2S,3aS,6R,7aS)-6-Hydroxy-octahydroindole-2-carboxylic acid (5): From
22: LiOH (0.1n, 3.2 mL) was added to a cooled (08C) solution of 22 (35 mg,
0.12 mmol) in THF (3 mL), and the mixture was stirred at room temper-
ature for 20 h. The mixture was extracted with Et2O (1 Â 5 mL), and the
aqueous phase was acidified with 5% aqueous NaHSO4 to pH 4. Extraction
of aqueous layer with CH2Cl2 (3 Â 10 mL) and CHCl3/iPrOH (4:1, 3 Â
10 mL) gave an organic extract which provided (2S,3aS,6R,7aS) 1-tert-
butoxycarbonyl-6-hydroxy-octahydroindole-2-carboxylic acid (27) as
a
white solid: Yield: 31 mg, 91%; 1H NMR (300 MHz, CDCl3): d ˆ 1.43 (s,
9H), 1.50 ± 1.80 (br, 4H), 1.98 ± 2.20 (br, 3H), 2.35 (br, 2H), 4.20 (brs, 1H),
4.15 ± 4.32 (br, 2H), 5.60 (br, 2H); 13C NMR, (75 MHz, CDCl3): 19.5 (C(4)),
26.4 (C(5)), 28.3 (C(CH3)3), 29.6 (C(3)), 33.6 (C(7)), 35.9 (C(3a)), 53.6
(C(7a)), 59.2 (C(2)), 65.9 (C(6)), 80.6 (C(CH3)3); IR (KBr): nÄ ˆ 3446, 2932,
1733, 1681, 1011 (OHax) cm 1. A solution of 27 (31 mg, 0.12 mmol) in HCl
(1n, 10 mL) was stirred at room temperature for 2 h. After concentration
and drying, the hydrochloride salt of 5 (26.5 mg, 100%) was isolated as an
m.p. 79 ± 808C; [a]D20
ˆ
41.7 (c ˆ 1.5 in CHCl3); 1H NMR (300 MHz,
CDCl3, COSY): d ˆ 1.49 ± 1.62 (m, 2H; H-3 and H-5), 1.64 (m, 1H; H-4),
1.69 (ddd, 13.8, 10.5, 8 Hz, 1H; H-7ax), 1.95 (m, 1H; H-4), 2.05 (m, 1H;
H-3a), 2.10 (m, 1H; H-3), 2.15 (m, 1H; H-5), 2.38 (dm, J ˆ 13.8 Hz, 1H;
H-7eq), 2.95 (dd, J ˆ 8, 3.5 Hz, 1H; H-7a), 3.28 (dd, J ˆ 11, 4.8 Hz, 1H; H-2),
3.53 and 4.07 (2  d, J ˆ 13.5 Hz, 1H each; NCH2Ph), 5.37 (tt, J ˆ 10.5,
4 Hz, 1H; H-6ax), 7.20 ± 7.35 (m, 5H; PhH), 7.43 (tt, J ˆ 7.5, 1.5 Hz, 2H; m-
Ar), 7.54 (tt, J ˆ 7.5, 1.5 Hz, 1H; p-Ar), 8.02 (dd, J ˆ 8, 1.5 Hz, 2H; o-Ar);
13C NMR, (50 MHz, CDCl3, HMQC): d ˆ 26.9 (C(4)), 30.8 (C(5)), 31.8
(C(7)), 33.8 (C(3)), 36.8 (C(3a)), 51.5 (OCH3), 56.5 (CH2Ar), 63.3 (C(7a)),
64.8 (C(2)), 70.4 (C(6)), 127.0 (C p-Bn), 127.9 (C m-Bz), 128.2 (C m-Bn),
hygroscopic solid: Rf ˆ 0.57 (BuOH/H2O/AcOH 4:1:1); [a]D20
ˆ
20.9 (c ˆ
0.9 in CH3OH); 1H NMR (300 MHz, CD3OD): d ˆ 1.40 ± 1.49 (m, 2H; H-3,
H-4), 1.65 ± 1.85 (m 3H; H-3, 2H-5), 1.90 ± 2.10 (m, 2H. H-4, H-7), 2.49
(brs, 2H; H-3a, H7), 3.40 ± 4.10 (m, 2H; H-2, H-6), 4.45 (brs, 1H; H-7a);
13C NMR, (75 MHz, CD3OD): d ˆ 21.8 (C(4)), 29.0 (C(3)), 32.1 (C(5)), 32.5
(C(7)), 38.0 (C(3a)), 59.3 (C(7a)), 59.5 (C(2)), 65.7 (C(6)), 172.4 (CO2H);
129.4 (C o-Bz), 129.6 (C o-Bn), 130.9 (C ipso-Bz), 132.6 (C p-Bz), 137.5 (C
1
1
IR (KBr): nÄ ˆ 3409, 1734 cm
; elemental analysis calcd (%) for
ipso-Bn), 165.9 (OCOPh), 175.1 (CO2Me); IR (film): nÄ ˆ 1747, 1715 cm
;
C9H16ClNO3 ´ 1ꢁ4 CHCl3 (251.6): C 44.15, H 6.52, N 5.56; found C 43.84, H
elemental analysis calcd (%) for C24H27NO4 (393.5): C 73.26, H. 6.91, N.
3.56; found C 72.90, H 6.94, N 3.55.
6.64, N 5.48.
From 25: A solution of 25 ´ HCl (47 mg, 0.15 mmol) in HCl (6n, 4.2 mL)
was heated at 858C for 8 h. The reaction mixture was cooled to room
temperature and washed with CH2Cl2 (7 Â 5 mL). The aqueous solution
was concentrated to give 5 ´ HCl as a hygroscopic solid (31 mg, 100%).
Methyl (2S,3aS,6R,7aS)-6-benzoyloxy-1-tert-butoxycarbonyl-octahydro-
indole-2-carboxylate (26): Pd(OH)2/C (20%, 20 mg) and Boc2O (72 mg,
0.33 mmol) were added to a solution of 23 (100 mg, 0.25 mmol) in EtOAc
(4 mL), and the mixture was stirred under atmospheric hydrogen pressure
for 15 h. The catalyst was removed by filtration through a short pad of
Celite, which was washed with EtOAc and CHCl3 (100 mL). The filtrate
and washings were combined and treated with a buffer solution AcOH/
NaAcO (pH 3) for 24 h. The mixture was basified with solid Na2CO3 to
pH 9 and extracted with CH2Cl2 (3 Â 15 mL) and CHCl3 (3 Â 15 mL). The
dried organic extracts were concentrated to give pure 26. Yield: 101 mg,
(R)-b-(4-Hydroxyphenyl)lactic acid [d-Hpla] (31): This compound was
obtained by following Wangꢂs method.[26] A solution of b-(4-hydroxyphen-
yl)pyruvic acid (Aldrich, 1.5 g, 8.3 mmol) in THF (30 mL) was treated with
triethylamine (1.15 mL, 8.3 mmol) at 208C. After 5 min at this temper-
ature, a solution of ( )-DIP-Cl (3.2 g, 9.96 mmol) in THF (10 mL) was
added, and the resulting solution was stirred at room temperature for 8 h.
The reaction mixture was quenched with 2n aqueous NaOH (10 mL) and
H2O (5 mL). The aqueous layer was washed with tert-butylmethylether
(2 Â 20 mL), and the organic layer with H2O (2 Â 30 mL). The combined
aqueous phases were acidified with 2n HCl to pH 1 and extracted with
EtOAc (4 Â 70 mL). The dried and concentrated organic extracts gave d-
Hpla (31) as a white solid. Yield: 1.4 g, 92%; Rf ˆ 0.42 (EtOAc/MeOH 1:1);
m.p. 164 ± 1668C; [a]2D0 ˆ ‡10.8 (c ˆ 0.52 in MeOH);[41] 1H NMR
(200 MHz, CDCl3 ‡ CD3OD): d ˆ 2.90 (dd, J ˆ 14, 8 Hz, 1H; H-3), 3.10
(dd, J ˆ 14.4, 4.4 Hz, 1H; H-3), 3.4 (brs, 1H; OH), 4.34 (dd, J ˆ 7.8, 4.4 Hz,
1H; H-2), 6.75 (d, J ˆ 8.4 Hz, 2H; H-6, H-8), 7.10 (d, J ˆ 8.8 Hz, 2H; H-5,
H-9); 13C NMR (50 MHz, CDCl3 ‡ CD3OD): d ˆ 40.7 (C(3)), 73.0 (C(2)),
116.0 (C(6), C(8)), 129.5 (C(4)), 131.5 (C(5), C(9)), 157.0 (C(7)), 177.5
99%; Rf ˆ 0.85 (hexane/EtOAc 1:2); [a]D20
ˆ
99.5 (c ˆ 0.5 in CHCl3);
1H NMR (200 MHz, CDCl3): d ˆ 1.40 (s, 9H), 1.6 ± 2.3 (m, 7H), 2.4 ± 2.6 (m,
2H), 3.76 (s, 3H), 4.28 (m, 2H), 5.35 (brs, n1/2 ˆ 14 Hz, 1H), 7.44 (m, 3H),
8.04 (dd, J ˆ 8, 1.2, 2H); 13C NMR (50 MHz, CDCl3): d ˆ 20.0 (C(4)), 23.6
(C(5)), 28.2 (C(CH3)3), 30.8 (C(3)), 32.3 (C(7)), 35.6 (C(3a)), 51.0 (C(7a)),
53.6 (OCH3), 59.3 (C(2)), 69.8 (C(6)), 79.9 (OCCH3), 128.2 (m-C), 129.3 (o-
C), 130.4 (ipso-C), 132.8 (p-C), 153.8 (NCO2), 165.6 (OCOPh), 174.0
1
(CO2Me); IR (KBr): nÄ ˆ 1750, 1705, 1700 cm
.
Methyl (2S,3aS,6R,7aS) 6-hydroxy-octahydroindole-2-carboxylate (28):
Trifluoroacetic acid (TFA; 1.4 mL, 18.2 mmol) was added to a solution of
22 (110 mg, 0.37 mmol) in CH2Cl2 (3 mL). The reaction mixture was stirred
at 08C for 45 min. After evaporation of the solvent and excess reagent, the
3456
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0716-3456 $ 17.50+.50/0
Chem. Eur. J. 2001, 7, No. 16

Syntheses of Aeruginosin 298-A and -B
3446±3460
(C(1)); IR (KBr): nÄ ˆ 3251, 1737 cm 1; elemental analysis calcd (%) for
C9H10O4 ´ 1ꢁ2 H2O (190.95): C 56.53, H 5.76; found C 56.56, H 6.02.
H-5 Choi, H-4 Leu), 1.78 (ddd, J ˆ 13, 13, 1.5 Hz, 1H; H-7 Choi) 1.93 (ddd,
J ˆ 12.5, 12.5, 10 Hz, 1H; H-3 Choi), 2.16 (m, 2H; H-3 Choi, H-4 Choi),
2.23 (m, 1H; H-7 Choi), 2.40 (ddddd, J ˆ 12, 6, 6, 6, 6 Hz, 1H; H-3a Choi);
3.73 (s, 3H; OCH3), 4.14 (brs, 1H; H-6 Choi), 4.20 (ddd, J ˆ 12.6, 6 Hz, 1H;
H-7a Choi), 4.31 (dd, J ˆ 9, 2 Hz, 1H; H-2 Choi), 4.47 (ddd, J ˆ 9.5, 9.5,
4.5 Hz, 1H; H-2 Leu), 5.2 (d, J ˆ 9 Hz, 1H; NH); cis rotamer: 0.85 and 0.90
(6H; H-5, H-5' Leu), 4.01 (1H; H-2 Leu), 4.07 (1H; H-6 Choi), 4.56 (1H;
H-7a Choi), 4.92 (1H; H-2 Choi), 5.04 (1H; NH Leu); 13C NMR (75 MHz,
CDCl3, HSQC): d ˆ 19.2(C(4) Choi), 21.9 and 24.6 (C(5), C(5') Leu), 23.5
(C(4) Leu), 25.7 (C(5) Choi), 28.3 (C(CH3)3), 30.4 (C(3) Choi), 34.1 (C(7)
Choi), 37.0 (C(3a) Choi), 43.5 (C(3) Leu), 49.2 (C(2) Leu), 52.2 (OCH3),
54.3 (C(7a) Choi), 59.0 (C(2) Choi), 65.4 (C(6) Choi), 79.5 (C(CH3)3), 155.2
(NCO2), 170.7 (C(1) Leu), 173.0 (CO2Me); IR (KBr): nÄ ˆ 3425, 1748, 1702,
1638, 1173, 1017 cm 1; elemental analysis calcd (%) for C21H36N2O6(412.5):
C 61.14, H 8.80, N 6.79; found C 60.89, H 8.97, N 6.64.
(R)-b-[4-(Benzyloxy)phenyl]lactic acid [HO-d-Hpla(Bn)-OH] (32):
K2CO3 (516 mg, 3.3 mmol) was added to a solution of hydroxy acid 31
(152 mg, 0.83 mmol) in CHCl3/MeOH (2:1, 6 mL). After the mixture had
been stirred for 15 min at 608C, benzyl bromide (0.25 mL, 2.10 mmol) was
added dropwise, and the mixture was stirred at this temperature for 6 h.
The solvent was removed, H2O (20 mL) was added, and the mixture was
extracted with EtOAc (3 Â 20 mL). The dried organic extracts were
concentrated to give the dibenzylated compound 33 and benzyl alcohol.
The basic aqueous phase was acidified with 2n HCl to pH 1 and extracted
with CHCl3/iPrOH (4:1, 4 Â 30 mL). After being washed with brine, dried,
and concentrated, the organic extract yielded 95 mg (42%) of 32 as a white
solid.
HO-d-Hpla(Bn)-OBn (33): Rf ˆ 0.69; (EtOAc/MeOH 1:1); 1H NMR
(200 MHz, CDCl3): d ˆ 2.60 (brs, 1H; OH), 2.89 (dd, J ˆ 14.0, 6.2 Hz,
1H; H-3), 3.10 (dd, J ˆ 14.0, 4.4 Hz, 1H; H-3), 3.72 (s, 2H; CO2CH2Ar),
4.38 (dd, J ˆ 6.2, 4.4 Hz, 1H; H-2), 5.00 (s, 2H; OCH2Ar), 6.89 (d, J ˆ
8.4 Hz, 2H; H-6, H-8), 7.10 (d, J ˆ 8.4 Hz, 2H; H-5, H-9), 7.33 (m, 10H;
ArH).
Compound 39b: Rf ˆ 0.60 (EtOAc); [a]D:
36.5 (c ˆ 1.3 in CHCl3);
1H NMR (200 MHz, CDCl3) trans-cis rotamer 5:1 ratio, trans-rotamer: d ˆ
0.94 and 0.99 (2  d, J ˆ 6.6 Hz, 6H; H-5, H-5' Leu), 1.42 (s, 9H; C(CH3)3),
1.42 (masked, 1H; H-3 Leu), 1.60 ± 2.16 (m, 5H; 2H-5 Choi, H-4 Choi, H-3
L, H-4 Leu); 2.20 ± 2.65 (m, 6H; 2H-3 Choi, H-4 Choi, 2H-7 Choi, H-3a
Choi); 3.75 (s, 3H; OCH3), 4.15 (m, 1H; H-7a Choi); 4.21 (2H; H-2 Choi,
H-2 Leu), 5.16 (d, J ˆ 8.7 Hz, 1H; NH), 5.39 (brs, 1H; H-6 Choi); 13C NMR
(200 MHz, CDCl3) trans rotamer: d ˆ 19.3 (C(4) Choi), 21.0 and 24.4 (C(5)
L, C(5') Leu), 22.8 (C(5) Choi), 23.4 (C(4) Leu), 28.2 (C(CH3)3), 30.2 (C(3)
Choi), 30.8 (C(7) Choi), 36.6 (C(3a) Choi), 43.5 (C(3) Leu), 50.0 (C(2)
Leu), 52.3 (CO2Me), 53.5 (C(7a)), 59.1 (C(2) Choi), 74.4 (C(6) Choi), 80
(C(CH3)3), 155.6 (NCO2), 172 (C(1) Leu), 173 (CO2Me).
LiOH (0.1n, 9.6 mL, 0.96 mmol) was added to a cooled (08C) solution of
the above mixture of 33 and benzyl alcohol (228 mg) in THF (2 mL), and
the comnbined mixture was stirred at room temperature for 24 h. The
solution was extracted with EtOAc (2 Â 8 mL) and acidified with 2n HCl to
pH 1. This aqueous phase was extracted with CHCl3/iPrOH (4:1, 4 Â
20 mL). The organic extracts were washed with brine, dried, and
concentrated to give an additional 63 mg of acid 32 (70% overall yield
from 31): Rf ˆ 0.42 (EtOAc/MeOH 1:1); m.p. 140 ± 1428C; [a]2D0: ‡14 (c ˆ
0.61 in MeOH);[41] 1H NMR (200 MHz, CDCl3 ‡ CD3OD) d ˆ 2.87 (dd, J ˆ
13.8, 7.0 Hz, 1H; H-3), 3.08 (dd, J ˆ 14.2, 4.0 Hz, 1H; H-3), 4.36 (dd, J ˆ 7.0,
4.4 Hz, 1H; H-2), 5.04 (s, 2H; OCH2Ar), 6.91 (d, J ˆ 8.8 Hz, 2H; H-6, H-8),
7.18 (d, J ˆ 8.8 Hz, 2H; H-5, H-9), 7.41 (m, 5H; ArH); 13C NMR (50 MHz,
CDCl3 ‡ CD3OD) d ˆ 40.7 (C(3), 70.8 (OCH2Ar), 72.8 (C(2)), 115.6 (C(6),
C(8)), 128.5 (p-C), 128.7 (m-C), 129.4 (o-C), 138.8 (ipso-C), 131.0 (C(4)),
131.5 (C(5), C(9)), 158.9 (C(7)), 177.2 (C(1)); IR (KBr): nÄ ˆ 3448 ± 2928,
AcO-d-Hpla(Bn)-d-Leu-l-Choi-OMe (40): TFA (1.42 mL, 18.5 mmol) was
added to a cooled (08C) solution of 39b (257 mg, 0.51 mmol) in CH2Cl2
(7 mL), and the mixture was stirred for 45 min and concentrated. A
solution of the resulting material in CH2Cl2 (2.8 mL) with NMM (86 mL,
0.77 mmol) was added to a cooled (08C) solution of the d-Hpla derivative
34 (157.2 mg, 0.51 mmol) in CH2Cl2 (2.8 mL), which had been previously
stirred at this temperature with BOP (226 mg, 0.51 mmol) and NMM
(120 mL, 1.02 mmol) for 30 min. The mixture was stirred at room temper-
ature for 16 h. CH2Cl2 (14 mL) was added, and the organic layer was
successively washed with HCl (1n, 3 Â 15 mL), saturated NaHCO3 solution
(3 Â 15 mL), and brine (1 Â 10 mL). The concentrated dried extract was
chromatographed (hexane/EtOAc 2:1 to EtOAc) to give 40b (199 mg,
0.28 mmol) and later 40a (16 mg, 0.026 mmol), the combined yield being
61%.
1
1727 cm ; elemental analysis calcd (%) for C16H16O4 ´ 1ꢁ2 H2O (280.95): C
68.30, H 6.05; found C 68.22, H 6.24.
(R)-2-Acetyloxy-3-(4-benzyloxyphenyl)propanoic acid [AcO-d-Hpla(Bn)-
OH] (34): Acetyl chloride (4 mL, 56 mmol) was added dropwise to a cooled
(08C) solution of hydroxy acid 32 (1 g, 3.67 mmol) in CH2Cl2 (7 mL). The
solution was stirred at room temperature for 16 h and concentrated.
Purification by chromatography (SiO2, CH2Cl2/AcOH 1%) gave 34 as a
white solid. Yield: 1 g, 87%; Rf ˆ 0.50 (hexane/CH2Cl2/AcOH 3:6:1); m.p.
109Ð1108C; [a]D: ‡7.4 (c ˆ 0.7, CHCl3); ee ꢁ98%, after crystallization
(see Supporting Information); 1H NMR (200 MHz, CDCl3): d ˆ 2.10 (s,
3H; CH3CO), 3.06 (dd, J ˆ 14.6, 8.4 Hz, 1H; H-3), 3.20 (dd, J ˆ 14.4, 4 Hz,
1H; H-3), 5.04 (s, 2H; OCH2Ar), 5.20 (dd, J ˆ 8.6, 4.2 Hz, 1H; H-2), 6.92
(d, J ˆ 8.4 Hz, 2H; H-6, H-8), 7.16 (d, J ˆ 8.8 Hz, 2H; H-5, H-9), 7.34 (m,
5H; ArH), 10.0 (brs, 1H; COOH); 13C NMR (50 MHz, CDCl3): d ˆ 20.5
(CH3CO), 36.2 (C(3)), 70.0 (OCH2Ar), 79.0 (C(2)), 114.8 (C(6), C(8)),
127.5 (C p-Ar), 128.0 (C m-Ar), 128.5 (C o-Ar), 137.0 (C i-Ar), 128.0 (C(4)),
130.3 (C(5), C(9)), 158.0 (C(7)), 170.6 (C(1)), 174.8 (CH3CO2); IR (KBr):
nÄ ˆ 3213 ± 2926, 1760, 1706 cm 1; elemental analysis calcd (%) for C18H18O5
(314.32): C 68.77, H 5.71; found C 68.38, H 5.81.
Compound 40a: Rf ˆ 0.40 (EtOAc); [a]D:
10.7 (c ˆ 1.2 in CDCl3);
1H NMR (500 MHz, CDCl3, COSY, NOESY) trans-cis rotamer 4:1 ratio,
trans-rotamer: d ˆ 0.83 and 0.91 (2  d, J ˆ 6Hz, 6H; H-5, H-5' Leu), 1.26 ±
1.32 (m, 2H; H-3 Leu, H-4 Leu), 1.39 (t, J ˆ 9 Hz, 1H; H-3 Leu), 1.47 ± 1.55
(m, 2H; H-4 Choi, H-5 Choi), 1.63 (m, 1H; H-5 Choi), 1.71 (ddd, J ˆ 14, 13,
5 Hz, 1H; H-7 Choi), 1.94 (ddd, J ˆ 12, 12, 8 Hz, 2H; H-3 Choi), 2.10 (s,
3H; CH3O), 2.16 (m, 3H; H-3, H-4, H-7 Choi), 2.39 (ddddd, J ˆ 13, 6.5, 6.5,
6.5, 6 Hz, 1H; H-3a Choi), 3.08 (m, 2H; H-3 Hpla), 3.70 (s, 3H; OCH3),
4.14 (s, 1H; H-6 Choi), 4.19 (ddd, J ˆ 12, 6, 6 Hz, 1H; H-7a Choi), 4.30 (dd,
J ˆ 10, 8 Hz, 1H; H-2 Choi), 4.80 (td, J ˆ 9.5, 4.5 Hz, 1H; H-2 Leu), 5.00 (s,
2H; OCH2Ar), 5.31 (dd, J ˆ 6.5, 5 Hz, 1H; H-2 Hpla), 6.60 (d, J ˆ 9.5 Hz,
1H; NH), 6.85 (d, J ˆ 8.5 Hz, 2H; H-6, H-8 Hpla); 7.06 (d, J ˆ 85 Hz, 2H;
H-5, H-9 Hpla), 7.30 (t, J ˆ 7.5 Hz, 1H; H p-Ar), 7.36 (dd, J ˆ 7.5, 2H; H m-
Ar), 7.40 (d, J ˆ 7.5 Hz, 2H; H o-Ar); cis rotamer: d ˆ 2.01 (OAc), 2.95 (H-3
Hpla), 4.06 (H-6 Choi), 4.52 (H-7a Choi), 4.76 (H-2 Leu), 4.90 (H-2 Choi),
5.28 (H-2 Hpla), 6.63 (NH Leu); 13C NMR (50 MHz, CDCl3, HSQC) trans
rotamer: d ˆ 19.2 (C(4) Choi), 20.8 and 24.3 (C(5), C(5') Leu), 22.0
(CH3CO), 22.1 (C(4) Leu), 25.6 (C(5) Choi), 30.4 (C(3) Choi), 34.0 (C(7)
Choi), 36.6 (C(3) Hpla), 37.0 (C(3a) Hpla), 42.7 (C(3) Leu), 48.3 (C(2)
Leu), 52.3 (OCH3), 54.4 (C(7a) Choi), 59.0 (C(2) Choi), 65.5 (C(6) Choi),
70 (OCH2Ar), 74.2 (C(2) Hpla), 114.6 (C(6), C(8) Hpla), 127.5 (o-C), 128.0
(p-C), 128.5 (m-C), 130.6 (C(5), C(9) Hpla), 137.0 (ipso-C), 157.7 (C(7)
Boc-d-Leu-l-Choi-OMe (39): BOP (644 mg, 1.46 mmol) and NMM
(0.5 mL, 3.92 mmol) were added to a cooled (08C) solution of 28 ´ TFA
(350 mg, 1.12 mmol) and Boc-d-Leu (314 mg, 1.36 mmol) in CH2Cl2
(23 mL). After the mixture had been stirred at 08C for 30 min, the
temperature was allowed to rise to room temperature, and stirring was
continued for 22 h. The solution was diluted with CH2Cl2(53 mL) and
washed successively with aqueous NaHSO4 (5%, 3 Â 60 mL), saturated
aqueous NaHCO3 (2 Â 60 mL), and brine (1 Â 50 mL). The organic portion
was dried and concentrated, and the crude product was purified by
chromatography (hexane/EtOAc 1:1) to give alcohol 39a (114 mg) and
trifluoroacetate 39b (335 mg) in a 1:2 ratio and 73% combined yield.
Compound 39a: Rf ˆ 0.80 (EtOAc); [a]2D0 ˆ ‡26.0 (c ˆ 3.3 in CHCl3);
1H NMR (500 MHz, CDCl3, COSY, NOESY) mixture of 85% of trans and
15% of cis conformers; trans rotamer: d ˆ 0.89 and 0.96 (2  d, J ˆ 6.5 Hz,
6H; H-5 and H-5' Leu), 1.32 (ddd, J ˆ 13.5, 9, 4 Hz, 1H; H-3 Leu), 1.40 (s,
9H; (C(CH3)3), 1.50 (m, 3H; H-4 Choi, H-5 Choi, H-3 Leu), 1.63 (m, 2H;
Hpla), 168.5 (C(1) Hpla), 169.3 (C(1) Leu), 170.0 (CH3CO2), 172.4
1
(CO2Me); IR (NaCl): nÄ ˆ 3425 ± 3350, 1747, 1636, 1250, 1017, 801, 752 cm
.
Compound 40b: Rf ˆ 0.50 (EtOAc); [a]D: 20 (c ˆ 2 in CDCl3); 1H NMR
(500 MHz, CDCl3, COSY, NOESY) trans-cis rotamer 4:1 ratio, trans
rotamer: d ˆ 0.83 and 0.91 (2  d, J ˆ 6.5 Hz, 6H; H-5, H-5' Leu), 1.14 ± 1.32
(m, 2H; H-3 Leu, H-4 Leu), 1.43 (t, J ˆ 9 Hz, 1H; H-3 Leu), 1.60 ± 1.71 (m,
2H; H-4 Choi, H-5 Choi), 1.90 ± 1.98 (m, 4H; H-4 Choi, H-5 Choi, H-7
Choi, H-3 Choi), 2.24 (ddd, J ˆ 13, 7.5, 6.5 Hz, 1H; H-3 Choi), 2.44 ± 2.54
Chem. Eur. J. 2001, 7, No. 16
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3457

J. Bonjoch et al.
FULL PAPER
(m, 2H; H-3a Choi, H-7 Choi), 3.10 (m, 2H; H-3 Hpla), 3.71 (s, 3H;
OCH3), 4.07 (ddd, J ˆ 12, 6, 6 Hz, 1H; H-7a Choi), 4.34 (dd, J ˆ 9.5, 8 Hz,
1H; H-2 Choi), 4.73 (ddd, J ˆ 10, 9.5, 3.5 Hz, 1H; H-2 Leu), 5.00 (s, 2H;
OCH2Ar), 5.30 ± 5.34 (m, 2H; H-2 Hpla, H-6 Choi), 6.51 (d, J ˆ 9.5 Hz, 1H;
NH), 6.85 (d, J ˆ 8.5 Hz, 2H; H-6, H-8 Hpla), 7.06 (d, J ˆ 8.5 Hz, 2H; H-5,
H-9 Hpla), 7.30 (t, J ˆ 7.5 Hz, 1H; H p-Ar), 7.36 (dd, J ˆ 7.5 Hz, 2H; H m-
Ar), 7.40 (d, J ˆ 7.5 Hz, 2H; H o-Ar); 13C NMR (300 MHz, CDCl3, HSQC)
trans rotamer: d ˆ 19.3 (C(4) Choi), 21.0 (CH3CO), 21.0 and 23.2 (C(5),
C(5') Leu), 22.7 (C(5) Choi), 24.0 (C(4) Leu), 30.2 (C(3) Choi), 30.6 (C(7)
Choi), 36.3 (C(3) Hpla), 36.4 (H-3a Choi), 42.8 (C(3) Leu), 48.1 (C(2) Leu),
52.4 (CO2CH3), 53.7 (C(7a)), 59.0 (C(2) Choi), 69.7 (OCH2Ar), 74.1 (C(2)
Hpla), 74.3 (C(6) Choi), 114.5 (C(6), C(8)), 127.5 (o-C), 128.0 (C(4) Hpla),
128.0 (m-C), 128.5 (p-C), 130.6 (C(5) , C(9 Hpla), 137.0 (ipso-C), 157.7 (C(7)
Hpla), 168.5 (C(1) Hpla), 169 (C(1) Leu), 170.3 (CH3CO2), 172.4 (CO2Me).
Leu, H-5 Choi, 2H-3 Argol, H-4 Choi), 1.70 (brs, 2H; H-5 Choi, H-4
Argol), 1.82 (brt, J ˆ 12 Hz, 1H; H-7ax Choi), 2.00 ± 2.20 (m, 3H; H-7eq
Choi, 2H-3 Choi), 2.22 ± 2.42 (m, 2H; H-4 Choi, H-3a Choi), 2.85 (dd, J ˆ
14, 6.5 Hz, 1H; H-3 Hpla), 3.00 (dd, J ˆ 14, 4 Hz, 1H; H-3 Hpla), 3.27 (m,
2H; 2H-5 Argol), 3.48 (d, J ˆ 5.5 Hz, 2H; 2H-1 Argol), 3.87 (brd, 1H; H-6
Choi), 4.24 (m, 1H; H-7a Choi), 4.26 ± 4.34 (m, 2H; H-2 Hpla, H-2 Choi),
4.63 (dd, J ˆ 10, 4 Hz, 1H; H-2 Leu), 5.02 (s, 2H; OCH2Ar), 6.88 (d, J ˆ
8.5 Hz, 2H; H-6, H-8 Hpla), 7.16 (d, J ˆ 8.5 Hz, 2H; H-5, H-9 Hpla), 7.34
(m, 5H; 1H p-Ar, 2H m-Ar, 2H o-Ar); 13C NMR (75 MHz, CD3OD,
408C): d ˆ 20.2 (C(4) Choi), 22.1 and 23.8 (C(5), C(5') Leu), 25.6 (C(4)
Leu), 26.8 (C(3) Argol), 29.4 (C(5) Choi, C(4) Argol), 31.8 (C(3) Choi),
34.7 (C(7) Choi), 38.1 (C(3a) Choi), 40.6 (C(3) Hpla), 42.1 (C(3) Leu), 42.6
(C(5) Argol), 50.3 (C(2) Leu), 52.1 (C(2) Argol), 56.3 (C(7a) Choi), 62.2
(C(2) Choi), 65.1 (C(1) Argol), 66.5 (C(6) Choi), 71.0 (OCH2Ar), 73.7
(C(2) Hpla), 115.6 (C(6), C(8) Hpla), 128.4 (o-C), 128.7 (p-C), 129.4 (m-C),
131.0 (C(4) Hpla), 131.8 (C(5), C(9) Hpla), 138.8 (ipso-C), 158.9 (C(7)
HO-d-Hpla(Bn)-d-Leu-l-Choi-l-Arg(NO2)-OMe (42): A cooled solution
of LiOH (0.1n, 10 mL, 1 mmol) was added dropwise to a cooled (08C)
solution of peptide 40 (197 mg, 0.28 mmol) in THF (10 mL). After having
been stirred at room temperature for 20 h, the solution was washed with
Et2O (2 Â 15 mL) and acidified to pH 1 with 1n HCl. The aqueous solution
was extracted with EtOAc (4 Â 30 mL) and CHCl3/iPrOH (4:1, 5 Â 30 mL).
The combined extracts were washed with brine, dried, and concentrated to
give acid 41. Yield: 120 mg, 80%; 1H NMR (200 MHz, CDCl3): d ˆ 0.82 (t,
J ˆ 6 Hz, 6H; H-5, H-5' Leu), 1.25 ± 1.80 (br, 6H), 1.80 ± 2.40 (br, 5H), 2.80
(br, 1H; H-3 Hpla), 3.11 (br, 1H; H-3 Hpla), 4.1 ± 4.3 (m, 4H; H-2, H-6,
H-7a Choi, H-2 Hpla), 4.82 (brt, 1H; H-2 Leu), 4.97 (s, 2H; OCH2Ar), 6.84
(d, J ˆ 8.4 Hz, 2H; H-6, H-8 Hpla), 7.0 (brs, 4H; NH, OH, COOH), 7.2 (d,
J ˆ 8.4 Hz, 2H; H-5, H-9 Hpla), 7.29 ± 7.38 (m, 5H; Ar); 13C NMR
(50 MHz, CDCl3): d ˆ 19.1 (C(4) Choi), 21.5 and 23.3 (C(5), C(5') Leu),
24.2 (C(4) Leu), 25.0 (C(5) Choi), 30.0 (C(3) Choi), 33.3 (C(7) Choi), 37.0
(C(3a) Choi), 39.0 (C(3) Hpla), 41.7 (C(3) Leu), 48.3 (C(2) Leu), 59.2 (C(2)
Choi), 65.5 (C(6) Choi), 70.0 (OCH2Ar), 72.3 (C(2) Hpla), 114.3 (C(6) and
C(8) Hpla), 127 (o-C), 128.0 (m-C), 128.4 (p-C), 130.7 (C(5) and C(9)
Hpla), 137.0 (ipso-C), 157.5 (C(7) Hpla), 171.0 (CO2H), 174.0 and 174.4
(C(1) Hpla and C(1) Leu).
ˆ
Hpla), 160.9 (C NH), 174.2 (CON), 175.7 (C(1) Hpla).
Pd/C (10%, 69 mg) was added to a solution of 43 (69 mg, 0.093 mmol) in
EtOAc/MeOH (1:1, 4.5 mL) and HCl (two drops, 6n), and the mixture was
stirred under hydrogen at atmospheric pressure for 8 h, the course of the
reaction being monitored by TLC. Upon complete conversion, the catalyst
was removed by filtration with Celite, which was washed with EtOAc/
MeOH (1:1, 50 mL) and MeOH (50 mL). The organic solution was
concentrated to give 59 mg (95%) of the hydrochloride of aeruginosin 298-
A (1) as a white powder; HPLC: Waters Nova-Park C-18 analytical; A:
H2O/TFA 0.05%, B: CH3CN/TFA 0.05%; isocratic elution with A/B 68:32
over 45 min; tr ˆ 1.40 min; Rf ˆ 0.60 (BuOH/AcOH/H2O 4:1:1); [a]D:
72.7 (c ˆ 0.23 in H2O); 1H NMR: see Table 2; 13C NMR: see Table 3;
‡
HRFABMS: calcd for C30H49N6O7, [M‡H] m/z: 605.3665; found 605.3663.
d-Hpla-d-Leu-l-Choi-NH2 [Aeruginosin 298-B] (3): NH4OH (33% NH3,
20 mL, 1 mmol), PyBOP (51.5 mg, 0.1 mmol), and NMM (30 mL,
0.26 mmol) were added sequentially to a cooled (08C) solution of acid 41
(42 mg, 0.076 mmol) in CH2Cl2 (1.5 mL). The mixture was stirred for
30 min at this temperature and at room temperature for 22 h. CHCl3 was
added, and the organic solution was washed successively with NaHSO4
(5%, 3 Â 10 mL), saturated NaHCO3 solution (3 Â 10 mL), and brine (1 Â
10 mL). The organic layer was dried and concentrated. Chromatography
(EtOAc/MeOH 1 ± 2%) of the residue gave HO-d-Hpla(Bn)-d-Leu-l-
Choi-NH2 (44). Yield: 16 mg, 39%; Rf ˆ 0.60; (EtOAc/MeOH 1:1);
1H NMR major rotamer (200 MHz, CDCl3): d ˆ 0.89 (m, 9H; H-5, H-5'
Leu), 1.2 ± 2.4 (m), 2.8 (dd, J ˆ 14, 6.5 Hz, 1H; H-3 Hpla), 3.1 (dd, J ˆ 14,
4 Hz, 1H; H-3' Hpla), 4.1 (br, 1H; H-6 Choi), 4.2 (br, 1H; H-2 Hpla), 4.4
(m, 2H; H-2 Choi, H-7a Choi), 4.6 (m, 1H; H-2 Leu), 5.0 (s, 2H; OCH2Ar),
6.2 (br, NH Leu), 6.7 (br, NH2 Choi), 6.9 (d, J ˆ 8.4 Hz, 2H; H-6, H-8
PyBOP (109.3 mg, 0.21 mmol) and NMM (82 mL, 0.73 mmol) were added
to a cooled (08C) solution of acid 41 (111 mg, 0.21 mmol) and the methyl
ester of Nw-nitro-arginine ´ HCl (Fluka, 56.64 mg, 0.21 mmol) in DMF
(2.3 mL). After the solution had been stirred for 30 min at 08C and 22 h at
room temperature, EtOAc (80 mL) was added, and the solution was
washed with saturated aqueous NaHCO3 (1 Â 60 mL) and H2O (5 Â
60 mL). The organic layer was dried and concentrated, and the residue
was purified by chromatography (EtOAc) to give 42 (147 mg, 91%) as a
white foam. Rf ˆ 0.36 (EtOAc/MeOH 1:1); [a]D: ‡20.8 (c ˆ 6.5 in CHCl3);
1H NMR (200 MHz, CDCl3): d ˆ 0.87 (t, 6.6 Hz, 6H; H-5, H-5' Leu), 1.25
(br, 2H; H-3 Leu, H-3 Arg), 1.60 (br, 6H; H-4, H-5 Choi, H-3, H-4 Leu,
2H-4 Arg), 1.80 ± 2.00 (br, 2H; H-3 Choi, H-7 Choi), 3.73 (s, 3H; OCH3),
4.13 (brs, 2H; H-7a Choi, H-6 Choi), 4.23 (br, 2H; H-2 Hpla, H-2 Choi),
4.55 (br, 1H; H-2 Arg), 4.86 (br, 1H; H-2 Leu), 4.99 (s, 2H; OCH2Ar), 6.88
(d, J ˆ 8.8 Hz, 2H; H-6, H-8 Hpla), 7.17 (d, J ˆ 8.8 Hz, 2H; H-5, H-9 Hpla),
7.27 (t, J ˆ 7 Hz, 1H; H p-Ar), 7.35 (t, J ˆ 7 Hz, 2H; H m-Ar), 7.41 (t, J ˆ
7Hz, 2H; H o-Ar); 13C NMR (50 MHz, CDCl3): d ˆ 19.2 (C(4) Choi), 21.6
and 23.4 (C(5), C(5') Leu), 24.5 (C(4) Leu), 25.7 (C(4) Arg), 29.7 (C(5)
Choi, C(3) Choi, C(3) Arg), 33.5 (C(7) Choi), 36.8 (C(3a) Choi), 39.6 (C(3)
Hpla), 40.7 and 41.7 (C(5) Arg, C(3) Leu), 48.9 (C(2) Arg), 51.5 (C(2) Leu),
52.7 (OCH3), 55.1 (C(7a) Choi), 60.4 (C(2) Choi), 65.5 (C(6) Choi), 69.9
(OCH2Ar), 72.6 (C(2) Hpla), 114.5 and 114.6 (C(6), C(8) Hpla), 127.4 (o-
C), 127.8 (p-C), 128.5 (m-C), 129.1 (C(4) Hpla), 130.7 (C(5), C(9) Hpla),
Hpla), 7.2 (d, J ˆ 8.4 Hz, 2H; H-5, H-9 Hpla), 7.4 (m, 5H; Ar); IR (NaCl):
1
nÄ ˆ 3392 ± 3210, 1682, 1639, 1014, 753, 582 cm
.
Pd/C (10%, 4.8 mg) was added to a solution of peptide 44 (16 mg,
0.028 mmol) in EtOAc/MeOH (2%, 1 mL), and the mixture was stirred
under hydrogen at atmospheric pressure for 20 h. The catalyst was removed
by filtration through Celite and washed with MeOH (45 mL). Evaporation
of the solvent gave aeruginosin 298-B (3). Yield: 13 mg, 97%; Rf ˆ 0.56
(EtOAc/MeOH 1:1); 1H NMR: see Table 2; 13C NMR: see Table 3;
HRMS: calcd for C24H35N3O6 461.2526; found 461.2533.
Acknowledgements
Financial support for this research was provided by the DGES, Spain
(Project PB97 ± 0877). Thanks are also due to the DURSI (Generalitat de
Catalunya) for Grant 1999SGR-00078. M.L.-C. and M.V. thank DURSI
and DGES, respectively, for doctoral fellowships. We thank Dr. Victor
Segarra (AlmirallProdesfarma) for carrying out the molecular mechanics
ˆ
137.0 (ipso-C), 157.7 (C(7) Hpla), 159.1 (C NH), 171.5 (C(1) Leu), 172.2
(CON), 172.3 (C(1) Arg), 174.0 (C(1) Hpla); IR (NaCl): nÄ ˆ 3360 ± 3317,
1
1741, 1670, 1625, 1261, 1014, 757, 688 cm
.
d-Hpla-d-Leu-l-Choi-l-Argol
[Aeruginosin 298-A]
(1):
LiBH4
(0.34 mmol, 170 mL of 2m solution in THF) was added dropwise to a
cooled (08C) solution of 42 (147 mg, 0.19 mmol) in THF (15 mL). The
resulting solution was stirred at room temperature for 3 h, and cooled again
to 08C. A saturated NaHCO3 solution (74 mL) was added, and the resulting
mixture stirred for 10 min. The aqueous layer was extracted with Et2O (1 Â
60 mL) and EtOAc (5 Â 60 mL). The dried organic extracts were concen-
trated to give a solid, which was crystallized (EtOAc/MeOH 95:5) to give
75 mg (53%) of 43: Rf ˆ 0.57 (EtOAc/MeOH 1:1); [a]D: ˆ ‡5.2 (c ˆ 0.45 in
MeOH); 1H NMR (300 MHz, CD3OD, 408C): d ˆ 0.86 and 0.89 (2  d, J ˆ
6.3 Hz, 6H; H-5, H-5' Leu), 1.10 ± 1.60 (m, 8H; H-4 Argol, 2H-3 Leu, H-4
Á
calculations, Dr. Ernesto Nicolas (UB) for his assistance in the purification
of the putative aeruginosin 298-A and Prof. Ernest Giralt (UB) for his
helpful comments. We also thank Dr. Juanlo Catena and Carles Traus for
experimental contributions.
[1] M. Namikoshi, K. L. Rinehart, J. Ind. Microbiol. 1996, 17, 373 ± 384.
[2] a) Aeruginosin 298-A: M. Murakami, Y. Okita, H. Matsuda, T. Okino,
K. Yamaguchi, Tetrahedron Lett. 1994, 35, 3129 ± 3132; b) Aerugino-
3458
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0716-3458 $ 17.50+.50/0
Chem. Eur. J. 2001, 7, No. 16

Syntheses of Aeruginosin 298-A and -B
3446±3460
sins 98-A and 98-B: M. Murakami, K. Ishida, T. Okino, H. Matsuda,
K. Yamaguchi, Tetrahedron Lett. 1995, 36, 2785 ± 2788; c) Aerugino-
sins 102-A and 102-B: H. Matsuda, T. Okino, M. Murakami, K.
Yamaguchi, Tetrahedron 1996, 52, 14501 ± 14506; d) Aeruginosin 103-
A: S. Kodani, K. Ishida, M. Murakami, J. Nat. Prod. 1998, 61, 1046 ±
1048; e) Aeruginosins 298-B, 89-A and B, 98-C and 101: K. Ishida, H.
Okita, H. Matsuda, T. Okino, M. Murakami, Tetrahedron 1999, 55,
10971 ± 10988.
with GNORM ˆ 0.1 were used in the optimization step. Conformation
analysis was performed with full AM1 optimization. All calculations
were performed on a Digital Alpha Station 300.
[18] This diagnostic rule fits with the NMR data of all N-substituted
derivatives synthesized in this work. N-Tosylated cis-2-carboxyocta-
hydroindoles follow the same pattern: A. Nuhrich, J. Moulines,
Tetrahedron 1991, 47, 3075 ± 3088.
[19] N-unsubstituted cis-2-carboxyoctahydroindoles follow an inverse rule,
showing the exo derivatives as triplets and endo compounds as
doublets of doublets (ꢂ10, 4 Hz) for H-2: a) R. Henning, H. Urbach,
Tetrahedron Lett. 1983, 24, 5339 ± 5342; b) M. Vincent, B. Marchand,
[3] Aeruginosins 205-A and B: H. J. Shin, H. Matsuda, M. Murakami, K.
Yamaguchi, J. Org. Chem. 1997, 62, 1810 ± 1813.
[4] Although it has been claimed[2a] that aeruginosin 298-A has an l-Leu
unit, in this work we conclude that a d-Leu is present in this natural
product.
[5] For recent work on the isolation of other bioactive peptides from
Microcyctis aeruginosa, see: a) R. Ishida, M. Murakami, J. Org. Chem.
2000, 65, 5898 ± 5900; b) K. Ishida, T. Kato, M. Murakami, M.
Watanabe, M. F. Watanabe, Tetrahedron 2000, 56, 8643 ± 8656.
[6] Microcin SF-608: R. Banker, S. Carmeli, Tetrahedron 1999, 55,
10835 ± 10844.
Â
G. Remond, S. Jaguelin-Guinamant, G. Damien, B. Portevin, J.-Y.
Baumal, J.-P. Volland, J.-P. Bouchet, J.-H. Lambert, B. Serkiz, W.
Luitjen, M. Laubie, P. Schiaci, Drug Des. and Discovery 1992, 9, 11 ±
28; c) S. G. Pyne, A. Javidan, B. W. Skelton, A. H. White, Tetrahedron
1995, 51, 5157 ± 5168.
[20] M. Mokotoff, S. T. Hill, J. Heterocycl. Chem. 1988, 25, 65 ± 71.
[21] a) The same conformational change has been observed in decahy-
droquinolines,[21b] and octahydropyrrolo[3,2-c]carbazoles;[11a]b) See
inter alia:H. Booth, D. V. Griffiths, J. Chem. Soc. Perkin Trans. 2
1975, 111 ± 118; A. I. Meyers, G. Milot, J. Am. Chem. Soc. 1993, 115,
6652 ± 6660; M. Naruse, S. Aoyagi, C. Kibayashi, J. Chem. Soc. Perkin
Trans. 1 1996, 1113 ± 1124.
[7] J.-L. Rios Steiner, M. Murakami, A. Tulinsky, J. Am. Chem. Soc. 1998,
120, 597 ± 598.
[8] For a preliminary communication of part of this work, see: N. Valls, M.
Â
Lopez-Canet, M. Vallribera, J. Bonjoch, J. Am. Chem. Soc. 2000, 122,
11248 ± 11249.
[22] Alkene 24 is formed as a by-product.
[9] References to 6-oxo-octahydroindole-2-carboxylic acids are scarceÐ
all of them are in the field of tetaine dipeptide synthesis, usually as
undesired products: a) P. Sawlewicz, I. Barska, M. Smulkowski, J.
Gumieniak, T. Czarnomska, M. Dzieduszycka, E. Borowski, Rocz.
Chem. 1976, 50, 1005 ± 1008; b) M. Souchet, J. Guilhem, F. Le Goffic,
Tetrahedron Lett. 1987, 28, 2371 ± 2374; c) H. Wild, J. Org. Chem. 1994,
59, 2748 ± 2761.
[10] For syntheses of 3a-oxygenated hexahydro derivatives from tyrosine
derivatives by oxidative processes, see: a) P. Wipf, Y. Kim, D. M.
Golstein, J. Am. Chem. Soc. 1995, 117, 11106 ± 11112; b) M. Cavazza,
G. Guella, L. Nucci, F. Pergola, N. Bicchierini, F. Pietra, J. Chem. Soc.
Perkin Trans. 1 1993, 3117 ± 3120; c) F. Jin, F, J. Leitich, C. von Sonn-
tag, J. Chem. Soc. Perkin Trans. 2 1993, 1583 ± 1588.
[23] For some recent contributions about facial diastereoselection in
cyclohexanones, see: a) S. Tomoda, T. Senju, Tetrahedron 1999, 55,
3871 ± 3882; b) T. Laube, J. Org. Chem. 1999, 64, 8177 ± 8182; c) A. S.
Cieplak, Chem. Rev. 1999, 99, 1265 ± 1336.
[24] For a detailed mechanism for this type of alkylation processes, see: X.
Fu, J. M. Cook, J. Org. Chem. 1993, 58, 661 ± 672.
[25] In some runs, this compound appears to be partially trifluoroacety-
lated at the Choi oxygen, but this is not a synthetic problem since the
trifluoroacetate is cleaved before the last coupling, in the saponifica-
tion step.
[26] Z. Wang, B. La, J. M. Fortunak, X.-J Meng, G. W. Kabalka,
Tetrahedron Lett. 1998, 39, 5501 ± 5504.
[27] An enantiopure sample of (R)-(4-hydroxyphenyl)lactic (d-Hpla) (see
Supporting Information) has [a]2D0 ˆ ‡10.8 (c ˆ 0.52 in MeOH). This
compound was prepared by Murakami[2e] by diazotation of 4-amino-d-
phenylalanine in 31% yield.
[11] We have used this approach in the synthesis of related azabicyclic
compounds: a) J. Bonjoch, J. Catena, N. Valls, J. Org. Chem. 1996, 61,
Á
7106 ± 7115. b) J. Bonjoch, J. Catena, D. Terricabras, J.-C. Fernandez,
Â
M. Lopez-Canet, N. Valls, Tetrahedron: Asymmetry 1997, 8, 3143 ±
3151.
[28] Y. Hamada, T. Shiori, Chem. Pharm. Bull. 1982, 30, 1921Ð1924. Using
this reagent in aprotic solvent, Murakami has synthesized the l-
Argol.[2e]
[29] A slight excess of the reducing agent induces the cleavage of the
Choi Leu amide bond. See: K. Soai, A. Ookawa, J. Org. Chem. 1986,
51, 4000 ± 4005.
[12] B. C. Laguzza, B. Ganem, Tetrahedron Lett. 1981, 22, 1483 ± 1486.
[13] For the first generation synthesis of 13, see:J. Bonjoch, J. Catena, E.
Â
Â
Isabal, M. Lopez-Canet, N. Valls, Tetrahedron: Asymmetry 1996, 7,
1899 ± 1902.
[14] Compounds 8 are mixtures of keto and hydrate forms, as the 13C NMR
data of the reaction mixture shows. The tendency of secondary b-
amino ketones toward hydration is well known: a) W. Wysocka,
Heterocycles 1982, 19, 1 ± 5; b) C. Herdeis, W. Engel, Arch. Pharm.
1992, 325, 419 ± 423.
[30] Due to a dearth of natural product, we were unable to obtain an
authentic sample of aeruginosin 298-A. However, comparisons were
performed with the 1H NMR and 13C NMR spectra of natural
aeruginosin 298-A. The optical rotation of synthetic 1, [a]D25
ˆ
73, is
very different from the reported value for the natural 1, [a]2D5 ˆ ‡22.3.
We are unable to comment on the significance of this discrepancy,
except to note that our sample was a hydrochloride salt while the
natural product had been isolated as a trifluoroacetate salt.
[31] For the role of proline cis-trans isomerization, see: T. P. Curran, L.A.
Marcaurelle, K. OꢂSullivan, Org. Lett. 1999, 1, 1225 ± 1228 and
references therein.
[15] For equilibration of b-amino ketones based on Mannich or retro-
Â
Michael processes, see inter alia: a) P. Slosse, C. Hootele, Tetrahedron
Á
1981, 37, 4287 ± 4292; b) J. Bonjoch, N. Casamitjana, J. Gracia, J.
Bosch, Tetrahedron Lett. 1989, 30, 5655 ± 5658; c) W. Deng, L. E.
Overman, J. Am. Chem. Soc. 1994, 116, 11241 ± 11250; d) R. W. Scott,
J. Epperson, C. H. Heathcock, J. Org. Chem. 1998, 63, 5001 ± 5012;
Á
e) D. Compere, C. Marazano, B. C. Das, J. Org. Chem. 1999, 64, 4528 ±
4532.
[32] For steric effects on the amide isomer equilibrium of prolyl peptides,
see: E. Beausoleil, W. D. Lubell, J. Am. Chem. Soc. 1996, 118, 12902 ±
12908.
[33] B. Sandler, M. Murakami, J. Clardy, J. Am. Chem. Soc. 1998, 120, 595 ± 596.
[34] a) S. Bajusz, E. Szell, D. Bagdy, E. Barabas, G. Horvath, M. Dioszegi,
Z. Fittler, G. Szabo, A. Juhasz, E. Tomori, G. Szilagyi, J. Med. Chem.
1990, 33, 1729 ± 1735; b) M. R. Wiley, N. Y. Chirgadze, D. K. Clawson,
T. J. Craft, D. S. Gifford-Moore, N. D. Jones, J. L. Olkowski, A. L.
Schacht, L. C. Weir, G. F. Smith, Bioorg. Med. Chem. Lett. 1995, 5,
2835 ± 2840.
[35] S. Hanessian, E. Balaux, D. Musil, L. L. Olsson, I. Nilsson, Bioorg.
Med. Chem. Lett. 2000, 10, 243 ± 247.
[36] P. Wipf, J.-L. Methot, Org. Lett. 2000, 2, 4213 ± 4216.
[37] a) B. Wünsch, M. Zott, Liebigs Ann. Chem. 1992, 39 ± 45; b) W. Siedel,
K. Sturm, R. Geiger, Chem. Ber. 1963, 96, 1636 ± 1440.
[16] The spectra of these compounds and of the corresponding racemic
derivatives were recorded on fairly concentrated samples (ca. 0.15m in
CDCl3) in the presence of 15 mg of (S)-(‡)-2,2,2-trifluoro-1-(9-
anthryl)ethanol. For this analytical procedure, see: a) W. H. Pirkle,
D. L. Sikkenga, J. Org. Chem. 1977, 42, 1370 ± 1374; b) M. Majewski,
R. Lazny, J. Org. Chem. 1995, 60, 5825 ± 5830
[17] Structures were set up with standard bond lengths and angles by using
the Chem-X molecular modeling package, version Jan97. All struc-
tures were initially optimized by using the steepest descents and
conjugate gradient methods. After semiempirical optimization with
the MOPAC program (version 6.0), charge distributions were
calculated with the aid of the AM1 method. The PULAY keyword
and the eigenvector following the (EF) routine for minimum search
Chem. Eur. J. 2001, 7, No. 16
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3459

J. Bonjoch et al.
FULL PAPER
[38] Mixtures of exo- and endo-9 and their corresponding dimethyl acetals
were observed by NMR during this stage.
[41] The optical rotation value corresponds to that of a sample obtained
from the recrystallyzed Hpla derivative 34 (ee >98%).
[39] In some runs, on elution with CH2Cl2/EtOAc (9:1), small quantities
(<3% yield) of a trans isomer were isolated.
[40] For analytical data see Supporting Information.
Received: February 12, 2001 [F3065]
3460
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Chem. Eur. J. 2001, 7, No. 16