Synthesis of tripeptide hydrolysate from papuamide A: Determination of absolute stereostructure of β-methoxytyrosine

Article abstract of DOI:10.1016/j.tetlet.2005.08.023

Four diastereomers, (2R, 3R), (2S, 3S), (2S, 3R) and (2R, 3S) at β-methoxytyrosine (β-OMeTyr), of the tripeptide hydrolysate, H-(S)-N-MeThr-β-OMeTyr-(S)-Hpr-OH, from papuamide A have been synthesized. Comparison of the ¹H NMR data of the natura

Full text of DOI:10.1016/j.tetlet.2005.08.023

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 6827–6830
Synthesis of tripeptide hydrolysate from papuamide A:
determination of absolute stereostructure of b-methoxytyrosine
Kazuishi Makino, Eri Nagata and Yasumasa Hamada*
Graduate School of Pharmaceutical Sciences, Chiba University, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
Received 15 July 2005; revised 3 August 2005; accepted 4 August 2005
Available online 19 August 2005
Abstract—Four diastereomers, (2R,3R), (2S,3S), (2S,3R) and (2R,3S) at b-methoxytyrosine (b-OMeTyr), of the tripeptide hydro-
1
lysate, H-(S)-N-MeThr-b-OMeTyr-(S)-Hpr-OH, from papuamide A have been synthesized. Comparison of the H NMR data of
the natural hydrolysate with the four synthetic diastereomers unambiguously establishes the relative and absolute stereochemistry
of the methoxytyrosine as 2R,3R.
Ó 2005 Elsevier Ltd. All rights reserved.
Papuamides A (1) and B are a new family of novel
cyclodepsipeptides isolated from the marine sponge
genus Theonella mirabilis and Thenella swinhoei col-
lected at Papua New Guinea by Boyd and co-workers.1
Papuamides are known to strongly inhibit the infection
of human T-lymphoblastoid cells by HIV-1RF and also
exhibit potent cytotoxicity against a number of human
cancer cell lines. These cyclic heptadepsipeptides have
a unique structure containing (4Z,6E)-2,3-dihydroxy-
2,6,8-trimethyldecadienoic acid (Dhtda) and unusual
amino acid residues, such as (2S,3S,4R)-3,4-dimethyl-
glutamine (3,4-DiMeGln), (2R,3R)-3-hydroxyleucine
(3-OHLeu) and b-methoxytyrosine (b-OMeTyr), as
shown in Figure 1. The stereochemistry of papuamides
remains to be determined because of the uncertainty
regarding the stereochemistry in the b-OMeTyr and
the Dhtda parts. Interestingly, two unusual amino acids,
b-OMeTyr and 3,4-DiMeGln, are known as common
components of the cyclodepsipeptides callipeltin A2
and neamphamide A,3 which show anti-HIV and anti-
fungal activities. The intriguing structures as well as
the unique biological activities led others and us to syn-
thesize these cyclodepsipeptides.4,5 Herein, we report
synthesis of the tripeptide hydrolysate from papuamide
A and structural determination of the b-methoxytyro-
sine in papuamides by comparison of the four synthetic
tripeptides with the natural fragment.
OH
O
OH
H
N
*
*
*
N
H
HO
O
O
O
Dhtda
HN
H2N
3,4-DiMeGln
NH2
O
HN
HN
(S)-Hpr
O
3-OHLeu
MeO
O
O
N
OMe
*
-OMeTyr
OH
NH
NH
O
O
*
O
O
NH
O
H
N
Papuamide A (1) : R = Me
Papuamide B : R = H
N
R
O
HO
HO
N
OMe
*
5 % TEAaq
49 ˚C, 16 h
O
*
O
O
Papuamide A (1)
NH
OH
Me
1H-NMR
HN
Me
OH
0.32 ppm
2
Figure 1.
In order to accomplish total synthesis as well as struc-
tural determination of papuamides, configurational
assignment of two stereo-undefined components,
b-OMeTyr and Dhtda, is necessary. Boyd and co-work-
ers have reported an interesting experiment, in which
the hydrolysate, H-N-MeThr-b-OMeTyr-Hpr-OH (2)
Keywords: Papuamide A; b-Methoxytyrosine; Absolute stereochemis-
try; Relative stereochemistry.
*
Corresponding author. Tel./fax: +81 432902987; e-mail: hamada@
p.chiba-u.ac.jp
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.08.023

6828
K. Makino et al. / Tetrahedron Letters 46 (2005) 6827–6830
(Hpr: homoproline (pipecolic acid)), derived from
hydrolysis of papuamide A with 5% aqueous triethyl-
amine shows the anomalous high-field shift (0.32ppm,
doublet) at the methyl proton of the N-MeThr residue
in the NMR spectrum (Fig. 1).1 The experiment encour-
aged us to synthesize the four tripeptides with different
stereochemistry at the b-OMeTyr residue. The required
four diastereomers of the b-methoxytyrosine were pre-
pared from (S)- and (R)-Garner aldehydes6 by a concise
route developed by us.4e N-Methylthreonine was pre-
pared by use of BenoitonÕs method7 from L-threonine
because the known methods8 were tedious or gave low
overall yield. Thus, commercially available Cbz-Thr-OH
was protected with tert-butyldimethylchlorosilane and
imidazole in dimethylformamide and after workup
the obtained Cbz-Thr(TBS)-OH was N-methylated with
an excess amount of sodium hydride and iodomethane
in tetrahydrofuran to give Cbz-N-MeThr(TBS)-OH in
85% overall yield (Scheme 1).
The four tripeptides 7a–d required for structural deter-
mination of the b-OMeTyr were prepared from Boc-
(S)-Hpr-OBn by stepwise coupling with Boc-OMe-
Tyr(Bn)-OH and Cbz-N-MeThr(TBS)-OH using the
HATU/DIPEA and EDCI/HOBt methods, shown in
Scheme 2. Sequential deprotection of the TBS group
with HF-acetonitrile and the Cbz group by hydrogenol-
ysis furnished the free tripeptides 7a–d after purification
by ODS column chromatography using a mixture of
0.05% aqueous trifluoroacetic acid–acetonitrile (8:2) as
1
an eluant. The H NMR spectra showed that 7a and
7d each have an exclusive conformer containing a minor
one in a ratio of >9:1, while 7b and 7c are a mixture of
an equal amount of two conformers. It has been re-
1
ported that the H NMR spectrum of natural hydroly-
sate shows it to be a mixture of two conformers and
the major conformer is >90% excess, which is quite sim-
ilar to the conformerÕs ratio of 7a and 7d. Comparisons
H
H
NaH
MeI
OTBS
CO2H
H
TBSCl
OH
OTBS
CO2H
imidazole
1
of the H NMR data reported for the natural hydroly-
Me
N
THF
0 ˚C to rt
21 h
DMF
0 ˚C to rt
22 h
CbzHN
CO2H
sate with those of the synthetic four tripeptides 7a–d
showed that the natural fragment is consistent with the
(2R,3R)-isomer 7a as shown in Table 1. The chemical
shifts of the other three diastereomers at the methyl
CbzHN
Cbz
85% (2 steps)
Scheme 1.
1. TFA
1. 4 N HCl, dioxane
2. HATU, DIEA, DMF
2. A: EDC, HOBt, NMM
N
OMe
or B: HATU, DIEA BnO2C
O
N
OMe
OH OMe
BnO2C
N
O
OH
OTBS
BnO2C
Boc
O
NH
OBn
O
O
Me
3
N
Z
BocHN
BocHN
OBn
ZN
Me
OBn
(2R, 3R)-4a: 75%
(2S, 3S)-4b: 86%
(2S, 3R)-4c: 94%
(2R, 3S)-4d: 72%
OTBS
(2R, 3R)-5a: 54% (condition A)
(2S, 3S)-5b: 27% (condition A)
(2S, 3R)-5c: 42% (condition B)
(2R, 3S)-5d: 41% (condition A)
N
OMe
HF
5% Pd-C, H2
MeOH
BnO2C
(2R, 3R)-7a: 96%
(2S, 3S)-7b: 87%
(2S, 3R)-7c: 86%
(2R, 3S)-7d: 90%
CH3CN
O
NH
O
OBn
16-28 h
ZN
Me
OH
(2R, 3R)-6a: 83%
(2S, 3S)-6b: 45%
(2S, 3R)-6c: 91%
(2R, 3S)-6d: 36%
HO
HO
OH
HO
HO
N
OMe
N
OMe
N
OMe
N
OMe
O
O
O
O
O
O
O
O
O
O
O
O
NH
NH
NH
NH
OH
OH
OH
HN
Me OH
HN
HN
HN
Me OH
Me OH
Me OH
(2R, 3R)-7a
(2S, 3S)-7b
(2S, 3R)-7c
(2R, 3S)-7d
Scheme 2.

K. Makino et al. / Tetrahedron Letters 46 (2005) 6827–6830
6829
Table 1. Comparisons of the 1H NMR chemical shifts of synthetic four tripeptides with the natural hydrolysate
3.01
3.00
HO
OMe
HO
N
OMe
N
O
O
*
O
O
*
O
O
NH
NH
OH
OH
HN
Me OH
HN
CH3 OH
0.35
0.32
2.27
natural
(2R, 3R)-7a
2.28
3.02
3.03
3.08
3.12
3.09
HO
HO
N
OMe
HO
N
OMe
N
OMe
O
O
O
O
O
O
O
O
NH
O
NH
OH
NH
OH
OH
HN
Me OH
HN
0.98
1.00
HN
Me OH
1.13
Me OH
1.03
1.09
2.35
2.07
2.15
1.59
1.65
(2R, 3S)-7d
(2S, 3R)-7c
(2S, 3S)-7b
Figure 2.
proton of the N-MeThr residue differed largely from the
0.32ppm of the naturally derived one ( Fig. 2). In
addition, chemical shifts of the other parts except that
described above are in good accordance with those of
the natural hydrolysate. Accordingly, the relative
and absolute stereochemistry of the b-OMeTyr residue
in papuamide A was unambiguously established as
2R,3R. Incidentally, efficient synthesis of Boc-(2R,3R)-
b-OMeTyr(Bn)-OH, a building block for natural papua-
mides, has been reported by use of asymmetric hydroge-
nation using the chiral iridium catalyst developed by
us.4h
In conclusion, four diastereomers of the tripeptides
relating to the hydrolysate from papuamide A have been
synthesized from four diastereomers of the b-methoxy-
1
tyrosines. Comparison of the H NMR data of the nat-
ural hydrolysate with those of the four synthetic

6830
K. Makino et al. / Tetrahedron Letters 46 (2005) 6827–6830
K.; Hamada, Y. J. Org. Chem. 2002, 67, 9210–9215; (f)
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882–884; (h) Makino, K.; Hiroki, Y.; Hamada, Y. J. Am.
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absolute stereostructure of the b-OMeTyr residue in
papuamides as 2R,3R. Further investigation directed
towards total synthesis of papuamides is underway in
this laboratory.
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