Total synthesis and reassignment of configuration of aeruginosin 298-A [12]

Full text of DOI:10.1021/ja002341i

11248
J. Am. Chem. Soc. 2000, 122, 11248-11249
Scheme 1. Synthesis of the Putative Aeruginosin 298-A (1a)a
Total Synthesis and Reassignment of Configuration
of Aeruginosin 298-A
Nativitat Valls, Meritxell Lo´pez-Canet, Merce` Vallribera, and
Josep Bonjoch*
Laboratory of Organic Chemistry, Faculty of Pharmacy
UniVersity of Barcelona, 08028-Barcelona, Spain
ReceiVed June 28, 2000
Aeruginosin 298-A (1) is a peptidic active-site protease inhibitor
containing nonstandard amino acids produced by a blue-green
alga, reported for the first time in 1994,1 whose structural
elucidation was not effected until 1998.2 At that time, the X-ray
crystallographic structure of the ternary complex of 298-A bound
to hirugen-thrombin was reported, revealing several unexpected
interactions that may be useful and of importance for structure-
based drug design.
The structure 1a depicted in Figure 1 was assigned to
aeruginosin 298-A in which the core ring consisting of a
2-carboxy-6-hydroxyoctahydroindole (choi) is unprecedented
among natural or synthetic products. Until now, fourteen members
of the aeruginosin family have been identified,3,4 all of which
share the aforementioned new bicyclic R-amino acid choi or a
closely related derivative.5
a Reagents and conditions: (i) Li, NH3, THF-tBuOH (2.5:1), -78 °C;
(ii) MeOH-7.5 N HCl, 35 °C, 48 h; (iii) BnBr, NaHCO3, EtOH, 70 °C,
6 h; (iv) MeOH-8 N HCl, 65 °C, 17 h (44% from 2), (v), H2, Pd(OH)2
20%, Boc2O, EtOAc, room temperature, 48 h, 79%, (vi) LS-selectride
(1 M in THF, 1.3 equiv), THF, -78 °C, 53% for isomer 9, (vii) TFA,
CH2Cl2, 0 °C, 45 min; (viii) Boc-L-Leu, BOP, NMM, CH2Cl2, 0 °C, 30
min, then room temperature, 22 h, 43% from 9; (ix) TFA, CH2Cl2, 0 °C,
45 min; (x) 12, BOP, NMM, CH2Cl2, 0 °C, 30 min, then room
temperature, 18 h, 60% from 11; (xi) 0.1 N LiOH, THF, room
temperature, 20 h, 92%; (xii) L-Arg(NO2)OMe.HCl, BOP, NMM, DMF,
0 °C, 30 min, then room temperature, 24 h 46%; (xiii) LiBH4 (2 M in
THF), THF, room temperature, 3 h, 50%; (xiv) H2, Pd-C 10%, EtOAc-
MeOH, 6 N HCl, 1 atm, room temperature, 8 h, 70%.
Figure 1. Configuration at *S putative aeruginosin 298-A (1a) and
configuration at *R Aeruginosin 298-A (1b).
In this paper, we described the first total synthesis of aerugi-
nosin 298-A and in turn clarify its configuration, since we also
report that structure 1a does not correspond to natural aeruginosin
298-A. We give synthetic evidence that compound 1b, incorporat-
ing a D-Leu (not L-Leu), corresponds to the natural structure. The
strategy we have developed for assembling the four units of the
peptide involves the construction of an appropriately function-
alized and stereochemically pure octahydroindole (choi core) and
the coupling with the other fragments.
Initially, we synthesized the putative aeruginosin 298-A (1a).
The starting material for our synthesis was the protected L-tyrosine
2, which was submitted to the Birch reduction conditions followed
by treatment of the resulting dihydroanisole 3 with a methanol
solution of hydrogen chloride to give a mixture of octahydroindol-
6-ones,6 which were benzylated to furnish a diastereomeric
mixture of exo and endo isomers in a 1.8:1 ratio. After
chromatographic separation of both isomers, the exo isomer 4
was transformed to the desired isomer endo 5 by warming in
methanol containing hydrogen chloride, taking advantage of the
â-amino ketone moiety. The equilibration process took place in
more than 90% of extension in favor of the endo isomer. After
these operations octahydroindolone 5 with the adequate relation-
ship between its three stereocenters was available in multigrams
amounts, the overall yield for these four initial steps being 44%.
To achieve the functionalization and stereochemistry adequate
at C-6, ketone 5 was subjected to reduction under several
conditions, but in all cases the undesired isomer 6 was the major
product irrespective of the hydride used (i.e. NaBH4 or LS-
selectride). This fact is probably due to the conformation of
(1) Murakami, M.; Okita, Y.; Matsuda, H.; Okino, T.; Yamaguchi, K.
Tetrahedron Lett. 1994, 35, 3129.
(2) Rios Steiner, J. L.; Murakami, M.; Tulinsky, A. J. Am. Chem. Soc.
1998, 120, 597.
(3) (a) Aeruginosins 98-A and B: Murakami, M.; Ishida, K.; Okino, T.;
Okita, Y.; Matsuda, H.; Yamaguchi, K. Tetrahedron Lett. 1995, 36, 2785.
Sandler, B.; Murakami, M.; Clardy, J. J. Am. Chem. Soc. 1998, 120, 596. (b)
Aeruginosins 102-A and B: Matsuda, H.; Okino, T.; Murakami, M.;
Yamaguchi, K. Tetrahedron 1996, 52, 14501 (c) Aeruginosin 103-A: Kodani,
S.; Ishida, K.; Murakami, M. J. Nat. Prod. 1998, 61, 1046. (d) Aeuginosins
89-A and B, 98-C, 101 and 298-B: Ishida, K.; Okita, H.; Matsuda, H.; Okino,
T.; Murakami, M. Tetrahedron 1999, 55, 10971.
(4) Microcin SF608, isolated in the Red Sea, has the same structural
pattern: Banker, R.; Carmeli, S. Tetrahedron 1999, 55, 10835.
(5) Aeruginosins 205-A and 205-B show a 6-chloro substituent in the
octahydroindole ring: Shin, H. J.; Matsuda, H.; Murakami, M.; Yamaguchi,
K. J. Org. Chem. 1997, 62, 1810.
(6) For the use of tyrosine compounds as building blocks for the synthesis
of octahydroindolone derivatives, see: (a) Bonjoch, J.; Catena, J.; Isa´bal, E.;
Lo´pez-Canet, M.; Valls, N. Tetrahedron: Asymmetry 1996, 7, 1899. (b)
Bonjoch, J.; Catena, J.; Terricabras, D.; Ferna`ndez, J.-C.; Lo´pez-Canet, M.;
Valls, N. Tetrahedron: Asymmetry 1997, 8, 3143.
10.1021/ja002341i CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/01/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 45, 2000 11249
cyclohexanone ring in 5, which adopts a half-boat form that
precludes the attack from the â-face. Although we have explored
a route involving a Mitsunobu process that converts alcohol 67
into R-benzoate 7, from the synthetic standpoint, the best results
were obtained when we transformed the N-benzyl derivative 5
into the corresponding N-Boc derivative 8 and the resulting ketone
was reduced with LS-selectride to afford R-alcohol 9 and its C-6
epimer in a ratio of 8:1. The change of the stereochemical course
in the reduction of ketones 5 and 8 lies in that both show a
different conformational behavior.8 The N-Boc derivative 8 adopts
a chair conformation for the cyclohexanone ring, which allows
the preferential equatorial attack of the bulky hydride reagent,
thus achieving the configuration of the natural target in the choi
nucleus. The identity of our synthetic choi with the natural one,
isolated by Murakami from aeruginosins by acid hydrolysis, was
for the natural product although the patterns of the peaks were
quite similar.11
After careful analyses of NMR data of 1a, as well as other
aeruginosins and microcin incorporating D- and L-amino acids
linked to the choi nucleus, we suspected that the proposed
configuration for aeruginosin 298-A was incorrect and should be
revised. Building on this point of view, stereoselective synthesis
of 1b, an aeruginosin incorporating a D-Leu instead of an L-Leu,
was carried out.
Scheme 2. Synthesis of Aeruginosin 298-A (1b)a
1
confirmed by conversion of 9 into 10 and comparison of its H
NMR data (500 MHz) with that described for the same compound
prepared from the natural source by bisacetylation and coupling
with (S)-methoxyphenylglycine.3d
a Reagents and conditions: (i) TFA, CH2Cl2, 0 °C, 45 min; (ii) Boc-
D-Leu, BOP, NMM, CH2Cl2, 0 °C, 30 min, then room temperature, 22
h, 73% from 9; (iii) TFA, CH2Cl2, 0 °C, 45 min; (iv) 12, BOP, NMM,
CH2Cl2, 0 °C, 30 min, then room temperature, 18 h, 60%; (v) 0.1 N
LiOH, THF, room temperature, 20 h, 85%; vi, L-Arg(NO2)OMe‚HCl,
PyBOP, NMM, DMF, 0 °C, 30 min, then room temperature, 22 h 90%;
(vii) LiBH4 (2 M in THF), THF, room temperature, 3 h, 53%; (viii) H2,
Pd-C 10%, 1:1 EtOAc-MeOH, 6 N HCl (2 drops), 1 atm, room
temperature, 8 h, 95%.
Deprotection of N-Boc derivative 9 followed by coupling with
Boc-L-Leu using BOP as the condensating agent furnished the
dipeptide 11 in 53% yield (2 steps), it being noteworthy that the
C-6 hydroxyl group in choi does not need protection due to its
low reactivity. The peptide chain was further elongated by
coupling with (R)-O-benzylphenyllactic (hpla moiety) protected
as its acetate 12, which was synthesized from 4-hydroxyphen-
ylpyruvic acid by enantioselective reduction employing (+)-B-
chlorodiisopinocampheylborane [(+)-DIP-Cl],9 followed by ben-
zylation of the phenol group and acetylation of the R-hydroxy
acid generated. After saponification of methyl ester in compound
13, which simultaneously induces deprotection of the acetate
group of the hpla fragment, coupling to the desired tetraunit
system was achieved by treatment of tripeptide with arginine (NG-
NO2) methyl ester in DMF medium and using BOP.
Working as described for the synthesis of 1a but coupling choi
derivative 9 with Boc-D-Leu and following the sequence depicted
in Scheme 2 we synthesized 1b, which showed NMR spectral
data matching those reported by Murakami, when he isolated the
aeruginosin 298-A. Thus, we concluded that the stereostructure
of the natural product corresponds to that of 1b.
In summary, the first synthetic entry to aeruginosins has been
achieved, after developing an efficient synthesis of the new
R-amino acid choi. The synthesis of the putative aeruginosin
298-A (1a) and the aeruginosin 298-A itself (1b) clarifies the
real structure of this natural peptide.12
Acknowledgment. Financial support from the DGES, Spain (project
PB97-0877), is gratefully acknowledged. Thanks are also due to the
DURSI, Catalonia for Grant SGR99-0078. M.L.-C. and M.V. thank the
DURSI (Catalonia) and the MEC (Spain), respectively, for fellowships.
LiBH4 reduction of 14 (1.75 mmol for each mmol of ester)
induces the transformation of the methyl ester to the hydroxym-
ethyl group in the arginine moiety. Finally, treatment of the
resulting compound with palladium on charcoal in acid medium
caused the cleavage of the benzyl group and the deprotection of
the guanidine moiety to give the target 1a. Purification by
reversed-phase HPLC provided pure 1a (a positive Sakaguchi test)
Supporting Information Available: NMR data of compounds 1a,
1b, 5, 8, 9, 11, and 15 (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org..
JA002341I
1
in 63% yield. However, the H and 13C NMR spectra of our
(10) The most significant differences in the NMR data of 1a as compared
with that reported for the natural compound1 correspond to the signals
attributable (COSY, HSQC) to H-7a (δ 4.4) and H-7eq (δ 1.83) of choi and
the methyl groups of Leu (δ 0.84 and 0.85), as well as to C-7 of choi (δ 34.4)
and C-1 of Leu (δ 170.5). For detailed NMR data of 1a and 1b, see Supporting
Information.
synthetic aeruginosin 298-A (1a) unexpectedly showed some
different chemical shifts10 from those reported in the literature
(7) The reduction process of 5 using NaBH4-CeCl3 at -23 °C gave 6
(80% yield) and its C-6 epimer in a 7.5:1 ratio.
(8) The different preferred conformation of 5 and 8 was ascertained by
NOESY and coupling constant analyses and was supported by molecular
modeling studies (AM1 method). The downfield C-4 chemical shift (δ 26.6)
is diagnostic for the half-boat conformation of 5 (δ 23.7 for 8).
(9) Wang, Z.; La, B.; Fortunak, J. M.; Meng, X.-J.; Kabalka, G. W.
Tetrahedron Lett. 1998, 39, 5501.
(11) Unfortunately, a sample of natural aeruginosin 298-A was unavailable
for chromatographic comparisons.
(12) Interestingly, this fact shows that all aeruginosins1,3,5 have the D(R)
configuration for the R-amino acid attached to the perhydroindole nitrogen,
while the related microcin SF6084 is the only compound with the
configuration for the R-amino acid linked to choi.
L