Stereoselective and N-terminal selective α-alkylation of peptides using a pyridoxal model compound as a chiral N-terminal activator

Article abstract of DOI:10.1039/a805596c

Stereoselective and N-terminal selective α-alkylation of peptides is achieved using a pyridoxal model compound as an N-terminal activator which also functions as a chiral auxiliary.

Full text of DOI:10.1039/a805596c

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Stereoselective and N-terminal selective a-alkylation of peptides using a
pyridoxal model compound as a chiral N-terminal activator
Kazuyuki Miyashita, Hiroshi Iwaki, Kuninori Tai, Hidenobu Murafuji and Takeshi Imanishi*†
Graduate School of Pharamceutical Sciences, Osaka University, 1-6 Yamadaoka, Suita, Osaka 565-0871, Japan
Stereoselective and N-terminal selective a-alkylation of
peptides is achieved using a pyridoxal model compound as
an N-terminal activator which also functions as a chiral
auxiliary.
peptides using a chiral pyridoxal model compound bearing an
ionophore function.
Taking into account the application of the reaction not only to
liquid-phase synthesis but also to solid-phase synthesis, we
investigated the benzylation of the aldimines prepared from
l-Ala-NHBn and some chiral pyridoxal models under various
conditions.‡ As a consequence, we found that the reaction with
pyridoxal model 1 having a chiral ansa-structure§ in the
Peptides containing unnatural amino acid(s) are gathering much
attention because of their biochemical and medicinal properties.
Although such peptides are generally synthesized by sequential
coupling of the respective amido acids prepared independently,1
several methods for the direct modification of peptides have
also been reported.2–4 Most of these methods, however, deal
with the alkylation of peptides at sites other than the N-terminal
position and seem to lack generality. In addition, their
stereoselectivities depend on the stereochemistries of the
peptides employed and are not always sufficient.2,3 In contrast,
direct N-terminal modification of peptides appears to be of great
utility for the synthesis of peptides including unnatural amino
acids, particularly where applied to combinatorial chemistry, as
both liquid- and solid-phase peptide syntheses are generally
achieved by sequential coupling from a C-terminal amino acid.
O’Donnell and co-workers recently reported an interesting
method utilizing this idea: direct N-terminal selective
a-alkyltion of peptides via imine formation.4 The only serious
problem with their method is the lack of stereoselectivity. If the
a-alkylation takes place with predictable stereoselectivity, this
method would be more useful and versatile. In order for this
method to be applicable to the synthesis of various peptides with
predictable stereochemistry, it is desirable that the stereo-
selectivity of the a-alkylation is induced only by an external
chiral auxiliary, and is not influenced by the neighbouring chiral
centre on the peptide sequence. Here we report a useful method
for N-terminal selective and stereoselective a-alkylation of
5
presence of LiClO4 and DBU was the most effective for the
present purpose and stereoselectively afforded the a-benzylated
product (R)-4 (X = NHBn) and recovered 1 (79%) after an
acidic treatment (run 1 in Table 1 and Scheme 1).
These conditions were employed for the alkylation of
peptides 2 (X = AA-OBn) and the results are summarized in
Table 1.¶ The peptide-aldimine 3 (X = l-Ala-OBn) prepared
from l-Ala-l-Ala-OBn and 1 was also stereoselectively
benzylated at the N-terminal position without any detectable
racemisation at the C-terminal a-position via these sequential
reactions (run 2).∑ As expected, the stereochemistry at the
N-terminal a-position was not related to the stereoselectivity at
all (run 3). In order to examine the influence of the stereogenic
centre at the neighbouring C-terminal a-position on the
stereoselectivity of the alkylation, dipeptides l-Ala-Gly-OBn,
l-Ala-l-Val-OBn, l-Ala-d-Ala-OBn, and l-Ala-d-Val-OBn
were chosen as substrates. The reaction of peptides without a
stereogenic centre or with a bulkier alkyl group at the
C-terminal position similarly gave good R-stereoselectivity
(runs 4 and 5). Moreover, in the reactions of the peptides having
a d-amino acid at the C-terminal position, the same predom-
inantly R-configuration was gained with slightly lower stereo-
selectivity (runs 2 vs. 6 and 5 vs. 7). It is noteworthy and quite
significant that neither the stereochemistry nor the size of the
Table 1 Alkylation of aldimines 3 with RBr
Substrate 2
Product 4
Run
R1
R2
X
R
M+
t/h
Yield (%)a
R:Sb
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Me
Me
H
H
H
Me
H
H
H
H
H
H
H
H
H
H
H
NHBn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Li
Li
Li
Li
Li
Li
Li
Li
Li
Li
Li
none
Na
K
2.5
4
4
4
4.5
4
5
4
4.5
4.5
5
60
51
53
46
50
49
51
50
48
56
48
38
38
32
83:17
86:14
86:14
83:17
88:12
74:26
73:27
85:15
73:27
84:16
86:14
21:79
26:74
23:77
l-Ala-OBn
l-Ala-OBn
Gly-OBn
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
l-Val-OBn
d-Ala-OBn
d-Val-OBn
l-Ala-OBn
l-Ala-OBn
l-Ala-OBn
l-Ala-l-Ala-OBn
l-Ala-OBn
l-Ala-OBn
l-Ala-OBn
4-O2NC6H4CH2
CH2NCHCH2
CH·CCH2
Bn
Bn
Bn
Bn
7
5
7
a Isolated yield based on substrate 2. b The stereochemistries of the products (R)- and (S)-4 (X = l-Ala-OBn) were confirmed by comparing with an authentic
sample (S)-4 (X = l-Ala-OBn) prepared from l-Ala and (S)-(a-Bn) Ala-OBn, which had been obtained using our previous method (ref. 5). The
1
stereochemistries of other products 4 were assigned as shown from their (R)-MTPA amides by comparing their H and 19F NMR data with those of the
1
(R)-MTPA amide of authentic (S)-(a-Bn)Ala-l-Ala-OBn and (S)-(a-Bn)Ala-OBn. Excepting runs 1 and 4, the ratio was determined from the H NMR
1
spectra. In the runs 1 and 4, the ratios were determined from the H and 19F NMR spectra of the corresponding (R)-MTPA amides.
Chem. Commun., 1998
1987

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In the present study, we have demonstrated the first example
of stereoselective and N-terminal selective a-alkylation of
peptides using a chiral pyridoxal model as an N-terminal
activator which also functions as a chiral auxiliary. This
a-alkylation reaction could be incorporated into standard
sequential peptide syntheses, and could provide a novel method
for the stereoselective synthesis of unnatural peptides, in
particular, for construction of unnatural peptide libraries.††
This research was financially supported in part by the
Houansha Foundation (HOUANSHA) and by a Grant-in-Aid
for General Scientific Research (09672282) from the Ministry
of Education, Science, Sports and Culture of Japan.
Notes and References
† E-mail: imanish@phs.osaka-u.ac.jp
‡ Although we first applied the pyridoxal model compound and the reaction
conditions which had previously been effective for the asymmetric
alkylation of a-amino esters (ref. 6) to this reaction, the desired
stereoselectivity was not obtained. Hence, reactions with pyridoxal models
having a chiral ionophore chain at C-3 and/or a chiral ansa-structure in the
presence of various organic bases and metal ions were examined. Details
will be reported in a full article.
§ The pyridoxal derivative 1 was synthesized from the 3-hydroxy derivative
(ref. 7) according to the literature procedure (refs. 5, 6).
Scheme 1 Reagents and conditions: i, CH2Cl2, room temp.; ii, RBr, MClO4,
¶ General procedure: The peptide-aldimine was prepared according to the
previously described procedure (ref. 5). To a stirred solution of peptide-
aldimine 3 (0.10 mmol) and LiClO4 (32.2 mg, 0.30 mmol) in MeCN (1 ml)
was added DBU (29.9 ml, 0.20 mmol) at 0 °C. After stirring for 5 min at the
same temperature, BnBr (13.2 ml, 0.11 mmol) was added and the mixture
was stirred at 0 °C for the period indicated in Table 1. The reaction mixture
was diluted with AcOEt (10 ml) and washed with cold water and cold brine.
To the organic layer, TsOH·H2O (38.6 mg, 0.20 mmol) was added and the
mixture was stirred for 30 min at room temperature and partitioned with
Et2O and water. The Et2O phase was worked-up as usual and the residue
was purified by SiO2 column chromatography (AcOEt–hexane = 1:2) to
give recovered 1 (70–80%). The aqueous phase was basified with NaHCO3
and extracted with AcOEt. Usual work-up and purification with SiO2
column chromatography (AcOEt) yielded the benzylated peptide 4.
∑ This was confirmed by the fact that the (R)-MTPA amide derived from the
benzylated dipeptide was shown to be a mixture of only two diastereomers
based on the N-terminal a-position by 1H and 19F NMR analyses.
** The ansa-loop could push the other substituents out of the side of the
ansa-loop and consequently make the other side crowded, which might
allow the electrophile to approach from the same side of the ansa-loop. See
also ref. 8.
DBU, MeCN, 0 °C; iii, TsOH·H2O, AcOEt, room temp., 30 min
substituent at the C-terminal a-position of the peptides affected
the stereoselectivity. In addition, alkylation with other alkyl
bromides proceeded successfully with similar stereoselectiv-
ities (runs 8–10). Tripeptide l-Ala-l-Ala-l-Ala-OBn was also
stereoselectively benzylated under the same conditions (run 11).
These findings show that compound 1 can work as an external
chiral auxiliary as well as an N-terminal activator, at least in the
synthesis of peptides with neutral amino acids at the neighbour-
ing position.
Interestingly, the reaction without Li+ or with other alkali
metal ions was found to show the reverse stereoselectivity (runs
12–14). Concerning the reaction mechanism, 1H NMR analysis
of the peptide-aldimine 3 in the absence and presence of Li+
revealed that the rotation of the C4–C4A bond shown in Fig. 1
was induced only by the addition of Li+.5 The stereoselectivities
obtained in the absence and presence of Li+ appear to be
attributable to these preferred conformations. Although the
detailed mechanism has yet to be determined, predominant
attack of the electrophile on the enolates from the side of the
ansa-loop (i.e. upper side in Fig. 1) in the respective conforma-
tions could explain the stereoselectivities.**
†† Further extensions to the synthesis of longer peptides and to solid-phase
synthesis are in progress.
1 R. M. Williams, Synthesis of Optically Active a-Amino Acids, Pergamon,
Oxford, 1989; H. Heimgartner, Angew. Chem., Int. Ed. Engl., 1991, 30,
238; T. Wirth, Angew. Chem., Int. Ed. Engl., 1997, 36, 225.
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S. L. Griffiths and D. Seebach, Helv. Chim. Acta, 1993, 76, 563;
D. Seebach, A. K. Beck, H. G. Bossler, C. Gerber, S. Y. Ko,
C. W. Murtiashaw, R. Naef, S. Shoda, A. Thaler, M. Krieger and
R. Wenger, Helv. Chim. Acta, 1993, 76, 1564; H. G. Bossler and
D. Seebach, Helv. Chim. Acta, 1994, 77, 1124; H. G. Bossler,
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439.
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Commun., 1996, 1073.
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M. Ando, Y. Tachibana and H. Kuzuhara, Bull. Chem. Soc. Jpn., 1982,
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Fig. 1 Selected NOE data for 3 in the absence and presence of Li+
Received in Cambridge, UK, 20th July 1998; 8/05596C
1988
Chem. Commun., 1998