An extension of the 'Bip method': Induced axial chirality in a series of dipeptides based on Bip/β2,2-HBip combined with Ala/β3-HAla

Article abstract of DOI:10.1016/j.tetasy.2006.01.001

In the search for an extension of the 'Bip method' for determining the absolute configuration of β-amino acids and β-peptides, dipeptides based on β^2,2-HBip/l(d)-Ala, Bip/l-β³-HAla, and β^2,2-HBip/l-β³-HAla were synthesized in solution and the induced circular dichroism (ICD) in their biphenyl core evaluated in comparison with the previously investigated Bip/l(d)-Ala series. Weak, poorly informative ICDs were observed in MeOH solution for the linear N-Boc protected dipeptide methyl esters based on β^2,2-HBip, as well as for those with Ala/β³-HAla at the N-terminus of Bip/β^2,2- HBip. However, a significant ICD was recorded for Boc-Bip-l-β³- HAla-OMe. These results were confirmed by low-temperature ¹H NMR spectroscopy studies of the dipeptides in CDCl₃ and CD₃OD solutions, showing two diastereoisomeric conformers in significantly different populations for Boc-Bip-l-β³-HAla-OMe in CD₃OD. In general, ICDs were found to be weaker for dipeptides containing β-amino acids as compared to those of their α-amino acid counterparts.

Full text of DOI:10.1016/j.tetasy.2006.01.001

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 17 (2006) 363–371
An extension of the ‘Bip method’: induced axial chirality in
2
,2
a series of dipeptides based on Bip/b -HBip combined
3
with Ala/b -HAla
a
a
a
a
Laurence Dutot, Anne Gaucher, Karen Wright, Michel Wakselman,
a,
b
b
*
Jean-Paul Mazaleyrat, Simona Oancea, Cristina Peggion,
b
b,
*
Fernando Formaggio and Claudio Toniolo
a
SIRCOB, UMR CNRS 8086, University of Versailles, F-78035 Versailles, France
b
Department of Chemistry, University of Padova, I-35131 Padova, Italy
Received 17 November 2005; accepted 3 January 2006
Available online 20 February 2006
Abstract—In the search for an extension of the ‘Bip method’ for determining the absolute configuration of b-amino acids and b-
2
,2
3
2,2
3
peptides, dipeptides based on b -HBip/L(D)-Ala, Bip/L-b -HAla, and b -HBip/L-b -HAla were synthesized in solution and the
induced circular dichroism (ICD) in their biphenyl core evaluated in comparison with the previously investigated Bip/L(D)-Ala ser-
ies. Weak, poorly informative ICDs were observed in MeOH solution for the linear N-Boc protected dipeptide methyl esters based
2
,2
3
2,2
on b -HBip, as well as for those with Ala/b -HAla at the N-terminus of Bip/b -HBip. However, a significant ICD was recorded
3
1
for Boc-Bip-L-b -HAla-OMe. These results were confirmed by low-temperature H NMR spectroscopy studies of the dipeptides in
CDCl and CD OD solutions, showing two diastereoisomeric conformers in significantly different populations for Boc-Bip-L-b -
3 3
3
HAla-OMe in CD OD. In general, ICDs were found to be weaker for dipeptides containing b-amino acids as compared to those
3
of their a-amino acid counterparts.
ꢀ
2006 Elsevier Ltd. All rights reserved.
*
1. Introduction
with the absolute configuration of Xaa . More specifi-
*
*
cally, the C-terminal L-Xaa and D-Xaa a-amino acid
residues preferentially induce negative and positive
Cotton effects at 250 nm and P and M torsions in the
biphenyl chromophore, respectively. This phenomenon
of induced circular dichroism (ICD) represents the basis
for the ‘Bip method’, an easy and fast configurational
assignment of chiral a-amino acids, organic acids,
amines, and alcohols currently being developed in our
laboratories. In the search for an extension of the Bip
method to the field of b-amino acids and b-peptides,
N-Boc protected dipeptide methyl esters based on Bip
We have previously reported that in peptides as short as
dimers, an axial chirality can be induced in the biphenyl
0
0
00 00
moiety of 2 ,1 :1,2;1 ,2 :3,4-dibenzcyclohepta-1,3-diene-
-amino-6-carboxylic acid (Bip), a conformationally
6
a
labile, atropoisomeric, turn/3 -helix inducer, C -tetra-
1
0
substituted a-amino acid previously investigated in our
1
–5
groups,
in the presence of a chiral a-amino acid
*
Xaa [Ala, Val, Leu, (aMe)Val and (aMe)Leu], resulting
in the onset of an equilibrium between two diastereoiso-
meric conformers with unequal populations, which can
be detected by CD and H NMR techniques. The
magnitude of this effect is particularly remarkable when
1
6,7
0 0 00 00
1 and 2 ,1 :1,2;1 ,2 :3,4-dibenzcyclohepta-1,3-diene-
2
,2
6-aminomethyl-6-carboxylic acid b -HBip 2, a con-
a
*
the chiral a-amino acid residue Xaa is positioned at the
formationally labile, atropoisomeric C -tetrasubstituted
8
–10
C-terminus of Bip, and signs of the CD bands correlate
b-amino acid analogue of Bip,
combined with either
0
3
L-Ala a, or D-Ala a , or L-b -HAla b at their C-terminus
0
0
1
,2/a,a ,b and their N-terminus a,a ,b/1,2 (Fig. 1) were
*
Corresponding authors. Tel.: +39 049 8275247; fax: +39 049 8275239
C.T.); e-mail addresses: jean-paul.mazaleyrat@chimie.uvsq.fr;
synthesized by solution methods. The induced axial
chirality in their biphenyl core was evaluated by H
1
(
claudio.toniolo@unipd.it
NMR and CD techniques.
0957-4166/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.01.001

364
L. Dutot et al. / Tetrahedron: Asymmetry 17 (2006) 363–371
O
H
O
H
O
H
N
O
O
H
N
N
H O
N
N
H O
N
O
tBuO
OMe
tBuO
N
H
OMe
tBuO
N
H
tBuO
H C
OMe
3
O
CH3
O
CH3
O
H C
OMe
3
O
3
2,2
Boc-β2,2-HBip-L-β -HAla-OMe 2b
3
Boc-Bip-L-Ala-OMe 1a
Boc-Bip-L-β -HAla-OMe 1b
Boc-β -HBip-L-Ala-OMe 2a
Boc-β2,2-HBip-D-Ala-OMe 2a'
O
tBuO
N
CH3
tBuO
O
H O
H
N
O
N
N
H O
OMe
tBuO
OMe
N
H
H
N
OMe
H O
O
O
CH3
O
O
H3C
3
Boc-L-Ala-β2,2-HBip-OMe a2
Boc-D-Ala-β2,2-HBip-OMe a'2
Boc-L-β -HAla-β2,2-HBip-OMe b2
Boc-L-Ala-Bip-OMe a1
Figure 1. Chemical structures of the N-Boc protected dipeptide methyl esters discussed in this work.
2
. Results and discussion
(S)
(R)
14 kcal/mol
2
.1. Synthesis
As for the previously synthesized Boc-Bip-L-Ala-OMe
7
X-HN
1
a, the terminally protected dipeptides Boc-Bip-L-
X-HN
3
2,2
n
CO-Y
n
CO-Y
b -HAla-OMe 1b (86%), Boc-b -HBip-L-Ala-OMe 2a
Bip (n=0)
2,2-HBip (n=1)
2
,2
0
(
45%), Boc-b -HBip-D-Ala-OMe 2a (79%), and Boc-
β
2
,2
3
3
b
-HBip-L-b -HAla-OMe 2b (85%), with the Ala/b -
2,2
Figure 2. Conformational equilibrium between the two diastereoiso-
meric conformers present in the dipeptides in which Bip or b -HBip is
combined with L-/D-Ala or L-b -HAla at its C-terminus 1a, 1b, 2a, 2a ,
HAla residue at the C-terminus of Bip/b -HBip, as well
2,2
7
2,2
as Boc-L-Ala-Bip-OMe a1, Boc-L-Ala-b -HBip-OMe
a2 (89%), Boc-D-Ala-b -HBip-OMe a 2 (98%), and
Boc-L-b -HAla-b -HBip-OMe b2 (79%) (Fig. 1), were
prepared in solution in high yields with EDC [N-
ethyl,N -(3-dimethylaminopropyl)-carbodiimide]/HOBt
3
0
2
,2
0
0
3
2,2
2b or at its N-terminus a1, a2, a 2, b2.
0
2,2
2,2
3
Boc-b -HBip-L-Ala-OMe 2a, and Boc-b -HBip-L-b -
HAla-OMe 2b in CD OD are shown as representative
examples (Fig. 3). In most cases (except for 2b), the
benzylic protons from Bip/b -HBip are resolved at
333 K into two different pairs of doublets reflecting the
inequivalency of both carbons and protons ArCH -
H –ArC H H , in both CDCl and CD OD solutions
1
1
(
1-hydroxy-1,2,3-benzotriazole)
or EDC/HOAt (7-
3
1
2
aza-1-hydroxy-1,2,3-benzotriazole) activation for cou-
pling at the C-terminus of Bip and at both N,C-termini
of b -HBip. These methods are known to be efficient
2
,2
2
,2
a
in difficult cases involving sterically demanding C -tetra-
A
1
3
0
substituted a-amino acids.
B
A
B
3
3
(
only in CDCl for b2). In analogy with the Bip/Ala ser-
3
1
.2. H NMR analysis
7
2
ies, the two doublets at a lower field are always better
resolved (sharper peaks) than the two doublets at higher
field, which remain broad, because of a higher coales-
cence temperature related to a higher difference in the
corresponding chemical shifts of the two diastereomeric
conformers. In the same manner, the patterns for the
1
The H NMR spectra of the dipeptides in CD OD,
3
CDCl , or CD CN solution at 233 K generally exhibit
3
3
two sets of signals corresponding to the presence of
two diastereoisomeric conformers (Fig. 2), exchanging
slowly on the NMR time scale, as previously observed
2
,2
NCH protons from b -HBip in 2a, a2, 2b, and b2
are quite well resolved at 333 K into either a singlet
2
7
for the Bip/Ala series. Fast-interconverting conditions,
resulting in the presence of only a single set of signals,
are reached at about 333 K, as expected from the rota-
(b2 in CD OD), or an AB quartet (a2 in CD OD), or
3
3
a doublet-like peak (in the other cases). Similarly, those
ꢀ
1
0
3
tional energy barrier of 14 kcal mol along the 1–1
for the CH CO protons from b -HAla in 1b, 2b, and b2
2
2
,5
bond of the biphenyl moiety.
are well resolved at 333 K into either an AB quartet (in
CD OD) or a doublet-like peak (in CDCl ). At 233 K,
slow exchange between the two conformers results in a
complex superposition of pairs of doublets or AB quar-
3
3
1
Evolution of the H NMR signals as a function of tem-
3
perature for the dipeptides Boc-Bip-L-b -HAla-OMe 1b,

L. Dutot et al. / Tetrahedron: Asymmetry 17 (2006) 363–371
365
1
3
2,2
2,2
3
Figure 3. H NMR signals (1.0–4.5 ppm) of the dipeptides Boc-Bip-L-b -HAla-OMe 1b, Boc-b -HBip-L-Ala-OMe 2a, and Boc-b -HBip-L-b -
3
HAla-OMe 2b in CD OD solution, as a function of temperature: 233 K (A) (the arrows indicate the separation of the –COOMe singlets in both
diastereoisomeric conformers), 263 K (B), 293 K (C), and 333 K (D).
tets for all of the above-mentioned protons. On the
other hand, the proton signals for the Boc, –COOMe,
HBip), with significant values in the case of Bip only,
6
,7
and (ii) as expected, on the C-terminal (higher dr) or
3
amide and/or carbamate NH groups, as well as
N-terminal (lower dr) position of the Ala or the b -HAla
3
the CH and CH groups from Ala or b -HAla, can
residue. Altogether, the diastereoisomeric ratios of
3
0
easily be identified at 333 K and 233 K (Table 1).
dipeptides 2a, 2b, a 1, a2, and b2 remained in the same,
poorly significant, low range of 50:50 to ca. 55:45 in
both CDCl and CD OD solutions. Apart from our pre-
The diastereoisomeric ratio (dr) was determined at
3
3
2
33 K by integration of the two singlets relative to the
viously reported ‘all-a’ dipeptide Boc-Bip-D-Ala-OMe
0
7
COOMe group, which are generally (except for a2 in
CDCl3 solution) well separated in both CDCl3 and
1a , the only compound of the present series showing
3
a significant dr value (68:32) is Boc-Bip-L-b -HAla-
CD OD and therefore especially suitable for a high-
OMe 1b (in CD OD).
3
3
accuracy calculation (Table 1). In most cases, the two
singlets relative to the Boc group are separated in CDCl3
2.3. CD analysis
solution (except for 2b), but not in CD OD (except for
a2). The amide and/or carbamate NH proton (in CDCl3
3
2,2
The biphenyl chromophore present in the Bip/b -HBip
and CD CN only), and the CH and CH protons from
Ala or b -HAla as well, are also more or less separated
residues is characterized by an intense electronic transi-
tion at about 240–250 nm, assigned to the A band, fol-
3
3
3
14
and could only be utilized occasionally.
lowed by a very intense transition at ca. 210–215 nm (C
band). The wavelength of the absorption maximum of
the A band is strongly dependent on h, the biphenyl
axial torsion angle. Several studies have established that
in the CD spectra of biphenyl-based chiral molecules
The diastereoisomeric ratios (Table 2) are dependent on
the nature of the solvent, with CD OD, also used in our
3
CD experiments (vide infra), being the solvent of choice
over CDCl and CD CN to observe higher dr values, as
a negative maximum corresponding to the A band is
3
3
6
,7
0
found in our previous studies. In CD OD, the dr val-
related to a P torsion of the C –C bond [(S)-configu-
3
Ar
Ar
2
,2
ues were found to be dependent (i) on the nature of the
residue containing the diphenyl core (either Bip or b
ration of Bip or b -HBip], and a positive maximum to
2
,2
15–17
-
an M torsion.

366
L. Dutot et al. / Tetrahedron: Asymmetry 17 (2006) 363–371
1
Table 1. Chemical shifts (ppm) of selected H NMR signals of the Bip dipeptides in CDCl
3
and CD
3
OD solutions at 233 and 333 K
NMR signals
CDCl
3
CD
3
OD
2
33 K
333 K
233 K
333 K
3
Boc-Bip-L-b -HAla-OMe 1b
3
NH b -HAla
Masked
5.06 and 4.93
4.43
3.70 and 3.66
1.44 and 1.43
1.27 and 1.21
6.95
4.82
4.38
3.68
1.49
1.27
—
—
—
—
4.30
3.69
1.49
1.24
NH Bip
3
CH b -HAla
4.36
3.72 and 3.65
1.49
COOMe
CH
CH
3
Boc
3
3
b -HAla
1.26 and 1.16
2,2
Boc-b -HBip-L-Ala-OMe 2a
NH Ala
NH b -HBip
CH Ala
6.18
5.61 and 5.49
4.48
3.80 and 3.77
1.45 and 1.42
1.41 and 1.35
6.13
5.30
4.55
3.76
1.46
1.38
—
—
—
—
4.43
3.73
1.45
1.41
2,2
4.43 and 4.34
3.74 and 3.72
1.47
COOMe
CH
CH
3
Boc
Ala
3
1.42
2
,2
3
Boc-b -HBip-L-b -HAla-OMe 2b
3
NH b -HAla
6.38 and 6.19
5.47 and 5.41
4.37
3.64 and 3.58
1.43
6.12
5.22
4.37
3.64
1.46
1.20
—
—
—
—
4.32
3.63
1.44
1.19
2
,2
NH b -HBip
3
CH b -HAla
4.33
3.68 and 3.58
1.46
COOMe
CH
CH
3
Boc
b -HAla
3
3
1.17
1.25 and 1.13
2,2
Boc-L-Ala-b -HBip-OMe a2
NH Ala
NH b -HBip
CH Ala
5.08 and 5.05
6.95 and 6.78
4.17 and 4.12
3.73
1.44 and 1.42
1.37
4.87
6.51
4.15
3.75
1.48
1.38
—
—
4.02
3.70 and 3.69
1.45 and 1.43
1.30 and 1.28
—
—
4.08
3.72
1.45
1.30
2,2
COOMe
CH
CH
3
Boc
Ala
3
3
2,2
Boc-L-b -HAla-b -HBip-OMe b2
3
NH b -HAla
5.55 and 5.44
6.63 and 6.55
4.01
3.75 and 3.72
1.40 and 1.39
1.23
5.07
6.08
3.97
3.75
1.45
1.26
—
—
—
—
3.93
3.72
1.41
1.18
2
,2
NH b -HBip
3
CH b -HAla
3.95
3.72 and 3.70
1.41
COOMe
CH
CH
3
Boc
b -HAla
3
3
1.15
Table 2. Dr values for the two diastereoisomeric conformers of title N-
Boc protected dipeptide methyl esters in CD OD, CDCl , and CD CN
solutions at 233 K, calculated by integration of the COOMe singlets
3
3
3
(see Section 4)
Dipeptides
CD
3
OD CDCl
3
CD
3
CN
0
7
7
7
7
Boc-Bip-D-Ala-OMe 1a
81:19
51:49
68:32
56:44
57:43
52:48
56:44
54:46
53:47
52:48
76:24
—
—
2,2
Boc-b -HBip-L-Ala-OMe 2a
Boc-Bip-L-b -HAla-OMe 1b
3
2,2
3
Boc-b -HBip-L-b -HAla-OMe 2b 54:46
56:44
—
0
7
7
7
Boc-D-Ala-Bip-OMe a 1
51:49
52:48
2,2
Boc-L-Ala-b -HBip-OMe a2
Boc-L-b -HAla-b -HBip-OMe b2 52:48
—
3
2,2
54:46
Our CD analysis in MeOH solution of the terminally
protected Bip linear dipeptides allowed us to draw the
following conclusions: (i) The previously reported ICD
in the biphenyl core of the Boc-Bip-L/D-Ala-OMe dipep-
tides, giving a clear information on the Ala configura-
7
Figure 4. CD spectra of Boc-Bip-L-Ala-OMe 1a (1) and Boc-Bip-L-
3
ꢀ3
b -HAla-OMe 1b (2) in MeOH solution (concentration 1 · 10 M).
7
3
tion, is also shown by the Boc-Bip-L-b -HAla-OMe
dipeptide 1b, in which the L-Ala residue at the C-termi-
nal position of Boc-Bip-L-Ala-OMe 1a is replaced by a
2
,2
Bip by b -HBip is always accompanied by a significant
decrease of the CD signal, resulting in a less informative
signature of the A band. That is the case for the dipep-
3
L-b -HAla residue, although with a weaker, but still
informative, CD signal (Fig. 4); (ii) the replacement of

L. Dutot et al. / Tetrahedron: Asymmetry 17 (2006) 363–371
367
1
3
C NMR spectra were recorded on a Bruker WM300
spectrometer operating at 300 and 77 MHz, respectively,
1
13
the solvent CDCl ( H: d = 7.27 ppm; C: d = 77.00 ppm)
3
1
1
or CD OD ( H: d = 3.31 ppm) or CD CN ( H:
3
3
d = 1.94 ppm) being used as the internal standard. Split-
ting patterns are abbreviated as follows: (s) singlet, (d)
doublet, (t) triplet, (q) quartet, and (m) multiplet. The
optical rotations were measured in a 1-dm thermostated
cell on a Perkin–Elmer 241 polarimeter, with an accuracy
of 0.3%. Elemental analyses were performed by the
CNRS Service of Microanalyses in Gif-sur-Yvette
(France). Analytical and/or preparative TLC and column
chromatography were performed on Kieselgel 60 F254
and Kieselgel 60 (0.040–0.063 mm) (Merck), respectively,
with the following eluant systems: 5% EtOAc (ethyl ace-
tate)–95% CH Cl (I); 10% EtOAc–90% CH Cl (II),
2
2
2
2
2
,2
20% EtOAc–80% CH Cl (III); 5% MeOH (methanol)–
95% CH Cl (IV). UV light (k = 254 nm) allowed visual-
2 2
ization of the spots after TLC runs for all compounds.
Except when stated, all starting materials and solvents
were obtained from commercial suppliers and were used
2 2
Figure 5. CD spectra of Boc-b -HBip-L-Ala-OMe 2a (1) and Boc-
2,2
0
b
4
-HBip-D-Ala-OMe 2a (2) in MeOH solution (concentration
ꢀ
3
· 10 M).
2
,2
tide Boc-b -HBip-L-Ala-OMe 2a (Fig. 5) compared to
a (Fig. 4). Still, it can be clearly observed that the spec-
tra of the enantiomeric Boc-b -HBip-L-Ala-OMe 2a
and Boc-b -HBip-D-Ala-OMe 2a dipeptides are mir-
ror images (Fig. 5), as expected; (iii) The CD curves of
as received. The syntheses and characterizations of the
1
2,2
2
10
2
,2
Bip derivatives Boc-Bip-OH, Boc-b -HBip-OH, and
2,2
1
0
2
,2
0
H-b -HBip-OMe
have been previously reported.
Boc-L-b -HAla-OH was purchased from Fluka.
3
0
2,2
the dipeptides 2b, b2, and a2/a 2, all involving b
-
HBip, present very weak, not informative, Cotton effects
in the A band region of the biphenyl absorption (not
shown); (iv) in dipeptides 1b, 2a, and 2a a P torsion
4.2. Peptide coupling: general procedure
0
A solution (or a suspension) of the N-Boc-protected
aminoacid, the amino ester (or its hydrochloride) and
HOAt (or HOBt) in CH Cl /THF was cooled to 0 ꢁC.
of the biphenyl axial bond [(S)-configuration of both
2
,2
Bip and b -Hbip] is preferentially induced by both L-
2
2
3
Ala and L-b -HAla C-terminal residues, and a M torsion
In case of an amino ester hydrochloride, NMM [N-
methyl morpholine] (or TEA [triethylamine]) was added.
This was followed by the addition of EDC. The reaction
mixture was allowed to warm up to rt, magnetically stir-
red for ca. 3 days and concentrated in vacuo. The resi-
due was dissolved in EtOAc (150 mL), the solution
was successively extracted with 0.5 M HCl (2 · 75 mL),
H O (100 mL), 5% NaHCO (2 · 75 mL), and H O
by a C-terminal D-Ala residue.
3. Conclusion
Taken together, the results of the present extension of the
Bip method’ have revealed that a substantial central-to-
‘
2
3
2
3
axial induction of chirality from a C-terminal L-b -HAla
to the pro-atropoisomeric, C -tetrasubstituted a-amino
(
2 · 100 mL), then dried over MgSO , filtered, and evap-
4
a
orated in vacuo at 40 ꢁC, to yield a crude product which
was purified by chromatography.
acid Bip residue occurs in simple linear dipeptides, and
results in a marked ICD effect, the negative maximum
of the A band of which is associated to an S configuration
of Bip. Conversely, a poor ICD is observed for the corre-
3
4
.3. HClÆH-L-b -HAla-OMe
2
,2
sponding b -HBip dipeptides. Therefore, the Bip (rather
3
2
,2
The N-protected amino acid Boc-L-b -HAla-OH
than b -HBip) residue is proposed to be used as a con-
venient CD probe for the determination of the absolute
configuration of not only a-amino acids, as previously
(0.150 g, 0.74 mmol) was dissolved in CH Cl
2 2
(7.5 mL), the solution cooled to 0 ꢁC and TFA (trifluo-
7
roacetic acid) (7.5 mL) added. The solution was magnet-
ically stirred at 0 ꢁC for 15 min and then at rt for 2.5 h.
The solution was evaporated in vacuo at 25 ꢁC and the
residue repeatedly co-evaporated in vacuo with CH Cl
2
reported, but of b-amino acids as well. A further exten-
sion of this study to a series of dipeptides, in which Bip
will be combined to other b-amino acids, for comparison
3
with b -HAla, is currently in progress in our laboratories.
2
3
at 40 ꢁC. The crude TFAÆH-L-b -HAla-OH obtained
was dissolved in MeOH (6 mL), the solution cooled to
4
. Experimental
0 ꢁC and SOCl (1.20 mL) added dropwise. The result-
2
ing solution was stirred at rt for 1 week, and evaporated
to dryness in vacuo. The obtained crude residue was
repeatedly co-evaporated in vacuo with MeOH at
4
.1. Synthesis of peptides
3
Melting points were measured on a Mettler apparatus
with a final temperature raise of 3 ꢁC/min or by means
of a capillary tube immersed in an oil bath (Tottoli appa-
40 ꢁC. The crude HClÆH-L-b -HAla-OMe obtained
1
(0.127 g) was pure by H NMR (CD OD): d 3.74 [s,
3
3H, OCH ], 3.69 [m (partly masked), 1H, CH bAla],
3
1
ratus, B u¨ chi) and are uncorrected. H NMR and
2.80 [dd, J = 6.8 and 17.1 Hz, 1H] and 2.73 [dd,

368
L. Dutot et al. / Tetrahedron: Asymmetry 17 (2006) 363–371
2
,2
J = 6.3 and 17.1 Hz, 1H] [CH bAla], and was used in
4.5. Boc-b -HBip-L-Ala-OMe 2a
2
the next coupling steps without further treatment.
The N-protected amino acid Boc-b -HBip-OH10
(0.092 g, 0.25 mmol) and the amino ester hydrochlo-
ride HClÆH-L-Ala-OMe (0.070 g, 0.50 mmol) were re-
acted with HOBt (0.067 g, 0.50 mmol), TEA
(0.070 mL; 0.50 mmol), and EDC (0.071 g, 0.37 mmol)
in THF (2.5 mL) and CH Cl (5 mL), according to the
2,2
3
4
.4. Boc-Bip-L-b -HAla-OMe 1b
The N-protected amino acid Boc-Bip-OH2 (0.261 g,
.74 mmol) and the amino ester hydrochloride
0
3
HClÆH-L-b -HAla-OMe (0.114 g, 0.74 mmol) were
2
2
reacted with HOAt (0.201 g, 1.48 mmol), NMM
general procedure of peptide coupling. The crude
product obtained after extraction was purified by col-
umn chromatography on silica gel eluted with eluant
(
0.164 mL, 1.48 mmol) and EDC (0.212 g, 1.11 mmol)
in THF (6 mL) and CH Cl (13 mL), according to the
2
2
general procedure of peptide coupling. The crude prod-
uct obtained after extraction was purified by column
chromatography on silica gel, and eluted successively
(II), to afford 0.051 g (45%) of pure dipeptide 2a as
1
a
solid. Mp = 75 ꢁC. R = 0.33 (II).
H
NMR
f
(293 K, CDCl ): d 7.5–7.2 [m, 8H, ArH bBip], 6.14
3
with eluants (I) and (II), to afford 0.288 g (86%) of pure
[br m, 1H, NH Ala], 5.41 [br m, 1H, NH bBip],
4.52 [dq, J ꢁ 7.2 Hz and 7.2 Hz, 1H, CH Ala], 3.76
[s, 3H, OCH ], 3.40 [m (br, d-like), 2H, NCH bBip],
1
dipeptide 1b as a solid. Mp = 197 ꢁC. R = 0.32 (II). H
f
NMR (293 K, CDCl ): d 7.5–7.2 [m, 8H, ArH Bip], 7.04
3
3
2
0
[
br m, 1H, NH bAla], 4.87 [s, 1H, NH Bip], 4.39 [m, 1H,
3.2–2.1 [br m, 4H, ArCH bBip and ArC H bBip],
2 2
CH bAla], 3.68 [s, 3H, OCH ], 3.3–2.1 [br m, 4H,
1.45 [s, 9H, CH Boc], 1.38 [d, J = 7.2 Hz, 3H, CH
3 3
3
0
1
ArCH2 Bip and ArC H Bip], 2.57 [m (d-like),
Ala]. H NMR (333 K, CDCl ): d 7.45–7.25 [m, 8H,
2
3
J = 5.3 Hz, 2H, CH2 bAla], 1.47 [s, 9H, CH Boc],
ArH bBip], 6.13 [br d, J ꢁ 6.2 Hz, 1H, NH Ala],
3
1
1
.26 [d, J = 6.8 Hz, 3H, CH bAla]. H NMR (333 K,
5.30 [br m, 1H, NH bBip], 4.55 [dq, J ꢁ 7.1 and
3
CDCl ): d 7.45–7.25 [m, 8H, ArH Bip], 6.95 [br d, 1H,
7.1 Hz, 1H, CH Ala], 3.76 [s, 3H, OCH ], 3.49 [m
3
3
NH bAla], 4.82 [s, 1H, NH Bip], 4.38 [m, 1H, CH bAla],
(d-like), J ꢁ 6.5 Hz, 2H, NCH2 bBip], 2.83 [d,
3
.68 [s, 3H, OCH ], 3.23 [d, J = 12.8 Hz, 1H] and 2.66
J ꢁ 12.5 Hz, 1H] and 2.51 [br d, J ꢁ 13.3 Hz, 1H]
3
[
br d, J ꢁ 13.0 Hz, 1H] [ArCH2 Bip], 3.18 [d,
[ArCH bBip], 2.79 [d, J ꢁ 13.0 Hz, 1H] and 2.57 [br
2
0
J = 13.0 Hz, 1H] and 2.66 [br d, J ꢁ 13.0 Hz, 1H]
d, J ꢁ 13.2 Hz, 1H] [ArC H bBip], 1.46 [s, 9H, CH
2
3
0
1
[
ArC H Bip], 2.57 [m (d-like), J = 5.3 Hz, 2H, CH
Boc], 1.38 [d, J = 7.1 Hz, 3H, CH Ala]. H NMR
2
2
3
bAla], 1.49 [s, 9H, CH Boc], 1.27 [d, J = 6.7 Hz, 3H,
(233 K, CDCl ): d 7.5–7.2 [m, 8H, ArH bBip], 6.18
3
3
1
CH bAla]. H NMR (233 K, CDCl ): d 7.6–7.1 [m,
[br m, 1H, NH Ala], 5.61 (ca. 57%) and 5.49 (ca.
43%) [br m, 1H, NH bBip], 4.48 [m, 1H, CH Ala],
3.80 (ca. 43%) and 3.77 (ca. 57%) [s, 3H, OCH3],
3
3
9
H, ArH Bip and NH bAla], 5.06 (ca. 52%) and 4.93
(
ca. 48%) [s, 1H, NH Bip], 4.43 [br m, 1H, CH bAla],
3
2
1
1
.70 (ca. 52%) and 3.66 (ca. 48%) [s, 3H, OCH ], 3.4–
3.42 and 3.33 [m, 2H, NCH bBip], 3.0–2.2 [m, 4H,
3
2
0
0
.2 [m, 6H, ArCH Bip, ArC H Bip and CH bAla],
ArCH and ArC H bBip], 1.45 (ca. 57%) and 1.42
2
2
2
2
2
.44 (ca. 50%) and 1.43 (ca. 50%) [s, 9H, CH Boc],
(ca. 43%) [s, 9H, CH Boc], 1.41 and 1.35 [d (partly
3
3
1
.27 (ca. 50%) and 1.21 (ca. 50%) [d, J ꢁ 6.7 Hz, 3H,
masked), J ꢁ 7.1 Hz, 3H, CH3 Ala].
H NMR
1
CH bAla]. H NMR (293 K, CD OD): d 7.5–7.1 [m,
(293 K, CD OD): d 7.5–7.1 [m, 8H, ArH bBip], 4.42
3
3
3
8
H, ArH Bip], 4.32 [m, 1H, CH bAla], 3.69 [s, 3H,
[m (q-like), 1H, CH Ala], 3.73 [s, 3H, OCH ], 3.34
3
0
OCH ], 3.15–2.30 [br m, 6H, ArCH Bip, ArC H Bip
and CH bAla], 1.49 [s, 9H, CH Boc], 1.24 [br m, 3H,
CH bAla]. H NMR (333 K, CD OD): d 7.45–7.10
[br m, 2H, NCH bBip], 3.2–1.9 [br m, 4H, ArCH
bBip and ArC H bBip], 1.46 [s, 9H, CH Boc], 1.45
2 3
1
3
2
2
2
2
0
2
3
1
[d (masked), 3H, CH3 Ala]. H NMR (333 K,
3
3
[
m, ArH, 8H], 4.30 [m, 1H, CH bAla], 3.69 [s, 3H,
CD OD): d 7.40–7.20 [m, 8H, ArH bBip], 4.43 [q,
3
OCH ], 3.03 [d, J = 13.6 Hz, 1H] and 2.71 [br d,
J = 7.2 Hz, 1H, CH Ala], 3.73 [s, 3H, OCH ], 3.37
3
3
J ꢁ 13.9 Hz, 1H] [ArCH Bip], 2.97 [d, J = 13.6 Hz,
[m (d-like), J ꢁ 4.1 Hz, 2H, NCH bBip], 2.88 [d,
2
2
0
1
2
H] and 2.77 [br d, J ꢁ 13.9 Hz, 1H] [ArC H Bip],
J ꢁ 13.8 Hz, 1H] and 2.40 [br d, 1H] [ArCH bBip],
2
2
.62 [dd, J = 6.1 and 15.4 Hz, 1H] and 2.52 [dd,
2.82 [d, J ꢁ 13.6 Hz, 1H] and 2.48 [br d, J ꢁ 13.4 Hz,
0
J = 6.4 and 15.4 Hz, 1H] [CH bAla], 1.49 [s, 9H, CH
1H] [ArC H bBip], 1.45 [s, 9H, CH Boc], 1.41 [d,
2
3
2
3
1
1
Boc], 1.24 [d, J = 6.7 Hz, 3H, CH bAla]. H NMR
J = 7.3 Hz, 3H, CH Ala]. H NMR (233 K, CD OD):
3
3
3
(
233 K, CD OD): d 7.5–7.1 [m, 8H, ArH Bip], 4.36 [br
d 7.6–7.1 [m, 8H, ArH bBip], 4.43 (ca. 50%) and 4.34
(ca. 50%) [br q, 1H, CH Ala], 3.74 (ca. 51%) and 3.72
(ca. 49%) [s, 3H, OCH ], 3.6–2.0 [m, 6H, NCH bBip,
3
m, 1H, CH bAla], 3.72 (ca. 68%) and 3.65 (ca. 32%)
0
[
s, 3H, OCH ], 3.2–2.4 [m, 6H, ArCH Bip, ArC H
3
2
2
3
2
0
Bip and CH bAla], 1.49 [s, 9H, CH Boc], 1.26 (ca.
3
ArCH2 bBip and ArC H bBip], 1.47 [s, 9H, CH
2
3
2
3
1
3
2%) and 1.16 (ca. 68%) [br d, J ꢁ 6.2 Hz, 3H, CH
Boc], 1.42 [m (partly masked), 3H, CH3 Ala].
C
3
1
3
bAla]. C NMR (293 K, CDCl ): d 172.3, 172.0 [C@O
NMR (293 K, CDCl ): d 175.4, 173.6 [C@O Ala and
3
3
bAla and Bip], 154.8 [C@O Boc], 140.7, 135.6, 130.2,
bBip], 156.4 [C@O Boc], 140.8, 130.5, 128.6, 127.9,
Ar
a
Ar
a
1
28.3, 127.8, 127.7 [C ], 77.9 [C–O Boc], 69.8 [C
127.8 [C ], 79.4 [C–O Boc], 58.3 [C bBip], 52.7
[OCH ], 48.6 [CH Ala], 45.8 [NCH bBip], 38.1 (br)
Bip], 51.7 [OCH3], 42.2 [CH bAla], 40.2–39.8
3
2
0
0
(
br) [ArCH and ArC H Bip], 40.0 [CH bAla], 28.4
[ArCH2 and ArC H
bBip], 2528.6 [CH3 Boc],
2
2
2
2
25
25
25
25
[
CH3 Boc], 20.1 (CH bAla). ½aꢂ ¼ ꢀ18:2; ½aꢂ
¼
18.0 (CH Ala). ½aꢂ ¼ ꢀ9:1; ½aꢂ ¼ ꢀ12:8; ½aꢂ
¼
3
589
578
3
589
578
546
0.1,
2
46
5
25
436
25
365
25
436
25
365
ꢀ
22:1; ½aꢂ5 ¼ ꢀ26:3; ½aꢂ ¼ ꢀ54:7; ½aꢂ ¼ ꢀ95:8
ꢀ14:6;
½aꢂ ¼ ꢀ41:1;
½aꢂ ¼ ꢀ69:4
(c
(
(
c
0.3, CH Cl ). Anal. Calcd for C H N O
CH Cl ). Anal. Calcd for C H N O (452.532): C,
2
2
26 32
2
5
2 2 26 32 2 5
452.532): C, 69.00; H, 7.13; N, 6.19. Found: C, 68.50;
69.00; H, 7.13; N, 6.19. Found: C, 68.84; H, 7.16;
N, 6.31.
H, 7.12; N, 5.83.

L. Dutot et al. / Tetrahedron: Asymmetry 17 (2006) 363–371
369
2
,2
0
1
4
.6. Boc-b -HBip-D-Ala-OMe 2a
CH Ala]. H NMR (233 K, CD OD): d 7.5–7.2 [m,
3 3
8
H, ArH bBip], 4.02 [br q, 1H, J ꢁ 6.9 Hz, CH Ala],
Prepared as 2a from Boc-b -HBip-OH10 (0.037 g,
.10 mmol), HClÆH-D-Ala-OMe (0.028 g, 0.20 mmol),
HOBt (0.027 g, 0.20 mmol), TEA (0.028 mL,
.20 mmol), and EDC (0.029 g, 0.15 mmol) in THF
1 mL) and CH Cl (2 mL), according to the general
2,2
3.70 (ca. 50%) and 3.69 (ca. 50%) [s, 3H, OCH ], 3.72
[d (partly masked), 0.5H]-3.35 [d (partly masked),
0.5H] (ca. 50%) and 3.57 [d, J = 13.4 Hz, 0.5H]-3.20
[d, J = 13.3 Hz, 0.5H] (ca. 50%) [NCH bBip], 3.1–2.1
3
0
0
(
2
0
[m, 4H, ArCH2 bBip and ArC H bBip], 1.45 (ca.
2
2
2
procedure of peptide coupling. The crude product
obtained after extraction was purified by column chro-
matography on silica gel eluted with eluant (II), to
48%) and 1.43 (ca. 52%) [s, 9H, CH Boc], 1.30 (ca.
3
50%) and 1.28 (ca. 50%) [d (superimposed), J ꢁ 6 Hz,
1
3
3H, CH Ala]. C NMR (293 K, CDCl ): d 175.5,
3
3
0
afford 0.036 g (79%) of pure dipeptide 2a as a solid.
173.0 [C@O Ala and bBip], 155.7 [C@O Boc], 140.8,
1
13
Ar
Mp = 82 ꢁC. H NMR (293 K, CDCl ) and C NMR
135.7, 130.2, 128.3, 127.7, 127.6 [C ], 80.2 [C–O Boc],
3
25
25
a
(
293 K, CDCl ): as 2a. ½aꢂ ¼ þ28:4; ½aꢂ ¼ þ28:4;
58.4 [C bBip], 52.4 [OCH3], 50.5 [CH Ala],
3
589
578
25
25
25
½
aꢂ ¼ þ24:2; ½aꢂ ¼ þ27:4; ½aꢂ ¼ þ54:7 (c 0.1,
44.1 [NCH2 bBip], 38.2 (br) [ArCH2 bBip],
5
46
436
365
0
CH Cl ). Anal. Calcd for C H N O Æ0.5H O
36.2 (br) [ArC H bBip], 28.5 [CH3 2B5 oc], 18.3
2
2
26 32
2
5
2
2
25
25
(
461.612): C, 67.65; H, 7.21; N, 6.07. Found: C, 67.75;
(CH Ala). ½aꢂ ¼ ꢀ6:7; ½aꢂ ¼ ꢀ14:3; ½aꢂ ¼ ꢀ14:3;
2 2
3
589
578
546
25
25
H, 7.41; N, 5.85.
½aꢂ ¼ ꢀ47:6; ½aꢂ ¼ ꢀ82:9 (c 0.1, CH Cl ). Anal.
436
365
Calcd for C H N O (452.532): C, 69.00; H, 7.13; N,
2
6
32
2
5
6.19. Found: C, 68.90; H, 7.20; N, 6.25.
2
,2
4
.7. Boc-L-Ala-b -HBip-OMe a2
2
,2
0
4
.8. Boc-D-Ala-b -HBip-OMe a 2
The N-protected amino acid Boc-L-Ala-OH (0.151 g,
2
,2
10
0
(
1
(
.80 mmol) and the amino ester H-b -HBip-OMe
0.112 g, 0.40 mmol) were reacted with HOBt (0.216 g,
.60 mmol) and EDC (0.230 g, 1.20 mmol) in THF
8 mL) and CH Cl (8 mL), according to the general
Prepared as a2 from Boc-D-Ala-OH (0.038 g,
2,2
1
0
0
.20 mmol), H-b -HBip-OMe (0.028 g, 0.10 mmol),
HOBt (0.054 g, 0.40 mmol) and EDC (0.057 g,
.30 mmol) in THF (2 mL) and CH Cl (2 mL), accord-
2
2
0
2
2
procedure of the peptide coupling. The crude product
obtained after extraction was purified by column chro-
matography on silica gel eluted with eluant (III) to
afford 0.161 g (89%) of pure dipeptide a2 as a solid.
ing to the general procedure of peptide coupling. The
crude product obtained after extraction was purified
by column chromatography on silica gel eluted with elu-
0
1
ant (III) to afford 0.045 g (98%) of pure dipeptide a 2 as
Mp = 86 ꢁC. R = 0.22 (III). H NMR (293 K, CDCl ):
1
13
f
3
a solid. Mp = 83 ꢁC. H NMR (293 K, CDCl ) and
C
3
d 7.5–7.1 [m, 8H, ArH bBip], 6.78 [br m, 1H, NH bBip],
25
25
NMR (293 K, CDCl ): as a2. ½aꢂ ¼ þ38:6; ½aꢂ
¼
3
589
578
5
.17 [d, J = 7.3 Hz, 1H, NH Ala], 4.16 [br m, 1H, CH
25
25
25
þ34:5; ½aꢂ ¼ þ38:6; ½aꢂ ¼ þ45:7; ½aꢂ ¼ þ70:0 (c
546
436
365
Ala], 3.72 [s, 3H, OCH ], 3.7–3.2 [br m, 2H, NCH
bBip], 3.2–2.1 [br m, 4H, ArCH2 bBip and ArC H
3
2
2
0
.1, CH Cl ). Anal. Calcd for C H N O Æ0.5H O
0
2 2 26 32 2 5 2
(461.612): C, 67.65; H, 7.21; N, 6.07. Found: C, 67.89;
bBip], 1.45 [s, 9H, CH Boc], 1.37 [d, J = 7.0 Hz, 3H,
3
1
H, 7.42; N, 5.62.
CH Ala]. H NMR (333 K, CDCl ): d 7.45–7.25 [m,
3
3
8
H, ArH bBip], 6.51 [br m, 1H, NH bBip], 4.87 [br d,
J ꢁ 5.7 Hz, 1H, NH Ala], 4.15 [dq, J ꢁ 7.1 and 7.1 Hz,
H, CH Ala], 3.75 [s, 3H, OCH ], 3.55 [m, 2H, NCH
2
,2
3
4.9. Boc-b -HBip-L-b -HAla-OMe 2b
1
3
2
2,2
The N-protected amino acid Boc-b -HBip-OH10
bBip], 2.91 [d, J ꢁ 13.8 Hz, 1H] and 2.47 [br d, 1H]
[
ArCH bBip], 2.89 [d, J ꢁ 13.8 Hz, 1H] and 2.51 [br
(0.110 g, 0.30 mmol) and the amino ester hydrochloride
2
0
3
d, 1H] [ArC H bBip], 1.48 [s, 9H, CH Boc], 1.38 [d,
HClÆH-L-b -HAla-OMe (0.092 g, 0.60 mmol) were
2
3
1
J = 7.1 Hz, 3H, CH Ala]. H NMR (233 K, CDCl ):
reacted with HOBt (0.081 g, 0.60 mmol), TEA
3
3
d 7.5–7.1 [m, 8H, ArH bBip], 6.95 (ca. 53%) and 6.78
ca. 47%) [br m, 1H, NH bBip], 5.08 (ca. 47%) and
.05 (ca. 53%) [br d, J ꢁ 7.9 Hz, 1H, NH Ala], 4.17
ca. 50%) and 4.12 (ca. 50%) [m, 1H, CH Ala], 3.73 [s,
(0.084 mL; 0.60 mmol), and EDC (0.086 g, 0.45 mmol)
in THF (3 mL) and CH Cl (6 mL), according to the
2 2
general procedure of peptide coupling. The crude prod-
uct obtained after extraction was purified by column
chromatography on silica gel eluted with eluant (II) to
(
5
(
3
3
4
H, OCH ], 3.64 [m, 1H] and 3.35 [m, 1H] [NCH bBip],
3 2
0
.0–2.2 [m, 4H, ArCH and ArC H bBip], 1.44 (ca.
afford 0.119 g (85%) of pure dipeptide 2b as a solid.
2
2
1
7%) and 1.42 (ca. 53%) [s, 9H, CH Boc], 1.37 [m
Mp = 64 ꢁC. R
(293 K, CDCl
m, 1H, NH bAla], 5.33 [br m, 1H, NH bBip], 4.36 [br
m, 1H, CH bAla], 3.62 [s, 3H, OCH ], 3.38 [br d,
f
= 0.30 (II), 0.75 (IV). H NMR
3
1
(
partly masked), 3H, CH3 Ala]. H NMR (293 K,
3
): d 7.5–7.2 [m, 8H, ArH bBip], 6.18 [br
CD OD): d 7.5–7.1 [m, 8H, ArH bBip], 4.07 [m (q-like),
3
1
H, CH Ala], 3.71 [s, 3H, OCH ], 3.59 [m, 1H] and 3.35
3
3
[
m, 1H] [NCH bBip], 3.2–1.9 [br m, 4H, ArCH bBip
J ꢁ 5.9 Hz, 2H, NCH
2
bBip], 3.1–2.2 [br m, 4H, ArCH
bBip], 2.49 [br m (d-like), 2H, CH
2
2
2
0
0
and ArC H bBip], 1.44 [s, 9H, CH Boc], 1.29 [d,
J = 7.2 Hz, 3H, CH Ala]. H NMR (333 K, CD OD):
d 7.40–7.20 [m, 8H, ArH bBip], 4.08 [q, J = 7.2 Hz,
bBip and ArC H
2
2
2
3
1
bAla], 1.45 [s, 9H, CH
Boc], 1.19 [d, J = 6.7 Hz, 3H,
3
3
3
1
CH
bAla]. H NMR (333 K, CDCl ): d 7.45–7.25 [m,
3
3
1
H, CH Ala], 3.72 [s, 3H, OCH ], 3.58 [d, J = 13.6 Hz,
8H, ArH bBip], 6.12 [br d, J ꢁ 7.1 Hz, 1H, NH bAla],
3
1
H] and 3.39 [d, J = 13.6 Hz, 1H] [NCH bBip], 2.87
5.22 [br m, 1H, NH bBip], 4.37 [m, 1H, CH bAla],
2
[
2
d, J ꢁ 13.5 Hz, 1H] and 2.45 [br d, 1H] [ArCH bBip],
3.64 [s, 3H, OCH ], 3.40 [d, J = 6.4 Hz, 2H, NCH
3 2
2
0
.85 [d, J ꢁ 13.5 Hz, 1H] and 2.45 [br d, 1H] [ArC H
bBip], 2.75 [d, J = 13.4 Hz, 2H] and 2.50 [br d,
2
0
bBip], 1.45 [s, 9H, CH Boc], 1.30 [d, J = 7.2 Hz, 3H,
J ꢁ 12.8 Hz, 2H] [ArCH
2
bBip and ArC H
2
bBip], 2.49
3

370
L. Dutot et al. / Tetrahedron: Asymmetry 17 (2006) 363–371
[
1
d, J = 5.6 Hz, 2H, CH bAla], 1.46 [s, 9H, CH Boc],
J = 13.5 Hz, 1H] and 2.48 [br d, 1H] [ArCH bBip],
2
3
2
1
0
.20 [d, J = 6.7 Hz, 3H, CH bAla]. H NMR (233 K,
2.91 [d, J = 13.6 Hz, 1H] and 2.48 [br d, 1H] [ArC H
3
2
CDCl ): d 7.5–7.1 [m, 8H, ArH bBip], 6.38 (ca. 44%)
bBip], 2.45 [dd, J = 5.7 and 14.4 Hz, 1H] and 2.39 [dd,
3
and 6.19 (ca. 56%) [br d, J ꢁ 8.3 Hz, 1H, NH bAla],
J = 5.0 Hz and 14.4 Hz, 1H] [CH bAla], 1.45 [s, 9H,
2
1
5
4
5
.47 (ca. 56%) and 5.41 (ca. 44%) [br m, 1H, NH bBip],
.37 [m br, 1H, CH bAla], 3.64 (ca. 44%) and 3.58 (ca.
6%) [s, 3H, OCH ], 3.37 [br d, J ꢁ 5.7 Hz, 2H, NCH
CH3 Boc], 1.26 [d, J = 6.8 Hz, 3H, CH3 bAla].
H
NMR (233 K, CDCl ): d 7.5–7.1 [m, 8H, ArH bBip],
3
6.63 (ca. 50%) and 6.55 (ca. 50%) [br m, 1H, NH bBip],
5.55 (ca. 50%) and 5.44 (ca. 50%) [d, J = 8.7 Hz, 1H,
NH bAla], 4.01 [m br, 1H, CH bAla], 3.75 (ca. 48%)
3
2
0
bBip], 2.9–2.2 [m, 6H, ArCH bBip, ArC H bBip and
2
2
CH bAla], 1.43 [s, 9H, CH Boc], 1.17 [d br, J ꢁ 5.3 Hz,
2
3
1
3
H, CH bAla]. H NMR (293 K, CD OD): d 7.5–7.1
and 3.72 (ca. 52%) [s, 3H, OCH ], 3.65 [m, 1H] and
3
3
3
[
m, 8H, ArH bBip], 4.34 [br q, J ꢁ 6.4 Hz, 1H, CH
3.39 [m, 1H] [NCH bBip], 3.1–2.1 [m, 6H, ArCH bBip,
2
2
0
bAla], 3.63 [br s, 3H, OCH ], ꢁ3.30 [m (masked),
ArC H bBip and CH bAla], 1.40 (ca. 52%) and 1.39
3
2 2
NCH bBip], 3.2–2.0 [br m, 4H, ArCH bBip and
(ca. 48%) [s, 9H, CH Boc], 1.23 [d br, J ꢁ 7.1 Hz, 3H,
2
2
3
0
1
ArC H bBip], 2.53 [m, 2H, CH bAla], 1.45 [s, 9H,
CH bAla]. H NMR (293 K, CD OD): d 7.5–7.2 [m,
2
2
3
3
1
CH3 Boc], 1.20 [br m, 3H, CH3 bAla]. H NMR
333 K, CD OD): d 7.45–7.25 [m, 8H, ArH bBip], 4.32
8H, ArH bBip], 3.93 [m, 1H, CH bAla], 3.72 [s, 3H,
(
OCH ], 3.47 [br m, 2H, NCH bBip], 3.2-2.1 [br m,
3
3
2
0
[
q, J = 6.6 Hz, 1H, CH bAla], 3.63 [s, 3H, OCH3],
4H, ArCH2 bBip and ArC H bBip], 2.41 [dd,
2
3
.36 [d, J = 14.1 Hz, 1H] and 3.29 [d (partly masked),
J = 7.1 Hz and 13.6 Hz, 1H] and 2.27 [dd, J = 7.1 and
1
H] [NCH bBip], 2.81 [d, J = 14.1 Hz, 2H] and 2.42
13.6 Hz, 1H] [CH bAla], 1.41 [s, 9H, CH Boc], 1.17
[d, J = 6.6 Hz, 3H, CH3 bAla]. H NMR (333 K,
2
2
3
0
1
[
br d, J ꢁ 13.6 Hz, 2H] [ArCH bBip and ArC H bBip],
2
2
2
.57 [dd, J = 6.9 and 15.3 Hz, 1H] and 2.49 [dd, J = 6.1
CD OD): d 7.45–7.25 [m, 8H, ArH bBip], 3.93 [q,
3
and 15.3 Hz, 1H] [CH bAla], 1.44 [s, 9H, CH Boc],
J = 6.7 Hz, 1H, CH bAla], 3.72 [s, 3H, OCH ], 3.48 [s,
2
3
3
1
1
.19 [d, J = 6.7 Hz, 3H, CH bAla]. H NMR (233 K,
2H, NCH2 bBip], 2.87 [br d, J ꢁ 13.3 Hz, 2H] and
3
CD OD): d 7.6–7.2 [m, 8H, ArH bBip], 4.33 [br m,
2.46 [br d (partly masked), 2H] [ArCH bBip and
3
2
0
1
H, CH bAla], 3.68 (ca. 46%) and 3.58 (ca. 54%) [s,
ArC H bBip], 2.42 [dd, J = 6.7 Hz and 14.1 Hz, 1H]
2
3
H, OCH ], 3.35 [d (partly masked), 1H] and 3.24 [d,
and 2.30 [dd, J = 6.6 and 14.1 Hz, 1H] [CH bAla],
3
2
J = 14.1 Hz, 1H] [NCH bBip], 3.1–2.0 [m, 6H, ArCH
1.41 [s, 9H, CH Boc], 1.18 [d, J = 6.6 Hz, 3H, CH
2
2
3
3
3
0
1
bBip, ArC H bBip and CH bAla], 1.46 [s, 9H, CH
bAla]. H NMR (233 K, CD OD): d 7.5–7.2 [m, 8H,
2
2
3
Boc], 1.25 (ca. 50%) and 1.13 (ca. 50%) [d, J ꢁ 6.4 Hz,
ArH bBip], 3.95 [br m, 1H, CH bAla], 3.72 (ca. 48%)
1
3
3
1
1
5
H, CH Ala]. C NMR (293 K, CDCl ): d 174.4,
and 3.70 (ca. 52%) [s, 3H, OCH ], 3.6–3.3 [m, 2H,
3
3
3
0
71.8 [C@O bAla and bBip], 156.2 [C@O Boc], 140.7,
NCH bBip], 3.1–2.1 [m, 6H, ArCH bBip, and ArC H
2
2
2
Ar
35.5, 130.1, 128.4, 127.6, 127.5 [C ], 79.3 [C–O Boc],
bBip and CH bAla], 1.41 [s, 9H, CH Boc], 1.15 [br m,
2 3
13
a
8.2 [C bBip], 51.9 [OCH ], 45.5 [NCH bBip], 42.3
3H, CH bAla]. C NMR (293 K, CDCl ): d 175.7,
3 3
3
2
[
CH bAla], 39.9 [CH bAla], 37.6 (br) [ArCH2
171.1 [C@O bAla and bBip], 156.7 [C@O Boc], 140.8,
2
0
Ar
bBip and ArC H bBip], 28.6 [CH3 Boc], 20.2
(
135.6, 130.2, 128.4, 127.8, 127.7 [C ], 79.6 [C–O Boc],
2
2
589
5
25
578
25
546
a
CH bAla). ½aꢂ ¼ þ9:9; ½aꢂ ¼ þ5:8; ½aꢂ ¼ þ3:3;
60.6 [C bBip], 52.5 [OCH ], 44.4 [NCH bBip], 43.2
3
3
2
2
4
5
25
½
aꢂ ¼ ꢀ16:5; ½aꢂ ¼ ꢀ45:5 (c 0.12, CH Cl ). Anal.
[CH bAla and [CH bAla], 38.1 (br) [ArCH bBip],
36
365
2
2
2
2
0
Calcd for C H N O Æ0.5H O (475.566): C, 68.19; H,
36.5 (br) [ArC H bBip], 28.6 [CH Boc], 225 0.8
2
7
34
2
5
2
2
3
25 25 25
7
.42; N, 5.89. Found: C, 67.90; H, 7.52; N, 5.83.
(CH3 bAla). ½aꢂ ¼ 0; ½aꢂ ¼ 0; ½aꢂ ¼ 0; ½aꢂ
¼
589
578
546
436
2
5
ꢀ
23:1; ½aꢂ ¼ ꢀ42:3 (c 0.1, CH Cl ). Anal. Calcd for
365
2
2
C H N O Æ0.5H O (475.566): C, 68.19; H, 7.42; N,
2
7 34 2 5 2
3
2,2
4
.10. Boc-L-b -HAla-b -HBip-OMe b2
5.89. Found: C, 68.50; H, 7.63; N, 5.58.
.11. Circular dichroism
3
The N-protected amino acid Boc-L-b -HAla-OH
(
4
2
,2
0.041 g, 0.20 mmol) and the amino ester H-b -HBip-
1
0
OMe (0.028 g, 0.10 mmol) were reacted with HOBt
0.027 g, 0.20 mmol) and EDC (0.029 g, 0.15 mmol) in
THF (1 mL) and CH Cl (2 mL), according to the gen-
The CD spectra were obtained on a Jasco J-710 dichro-
graph. Cylindrical, fused quartz cells of 10-, 1-, 0.2- and
(
2
2
0
.1-mm pathlengths (Hellma) were used. The values are
eral procedure of peptide coupling. The crude product
obtained after extraction was purified by column chro-
matography on silica gel eluted with eluant (IV) to
afford 0.036 g (76%) of pure dipeptide b2 as a solid.
Mp = 74 ꢁC. R = 0.50 (IV). H NMR (293 K, CDCl ):
d 7.5–7.1 [m, 8H, ArH bBip], 6.25 [br m, 1H, NH bBip],
expressed in terms of [H] , the total molar ellipticity
T
2
ꢀ1
(
deg cm dmol ). Spectrograde MeOH (Acros Organ-
ics) was used as the solvent. Peptide concentration was
ꢀ3
ꢀ3
from 1 · 10 to 4 · 10 M.
1
f
3
5
3
1
.26 [br m, 1H, NH bAla], 3.99 [br m, 1H, CH bAla],
References
.74 [s, 3H, OCH ], 3.63 [br m, 1H] and 3.44 [br m,
3
H] [NCH bBip], 3.1–2.1 [br m, 4H, ArCH bBip and
2
2
0
1. Mazaleyrat, J.-P.; Gaucher, A.; Wakselman, M.; Tcherta-
ArC H bBip], 2.42 [br m, 2H, CH bAla], 1.43 [s, 9H,
CH3 Boc], 1.25 [d, J = 6.6 Hz, 3H, CH3 bAla].
NMR (333 K, CDCl ): d 7.45–7.25 [m, 8H, ArH bBip],
2
2
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