Stabilizing effects in oxazolidin-2-ones-containing pseudopeptides

Article abstract of DOI:10.1016/j.tetasy.2004.03.033

Novel homo-oligomers of the Gly-L-Oxd moiety have been prepared and their preferential conformations analyzed by IR, ¹H NMR and CD spectroscopy, with the aim of determining whether these molecules were able to fold in ordered structures. In the

Full text of DOI:10.1016/j.tetasy.2004.03.033

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 1645–1650
Stabilizing effects in oxazolidin-2-ones-containing pseudopeptides
*
ꢀ
Gianluigi Luppi, Chiara Soffre and Claudia Tomasini
Dipartimento di Chimica ‘G. Ciamician’, Alma Mater Studiorum, Universitꢀa di Bologna, Via Selmi 2, 40126 Bologna, Italy
Received 16 March 2004; accepted 23 March 2004
Available online 22 April 2004
Abstract—Novel homo-oligomers of the Gly-L-Oxd moiety have been prepared and their preferential conformations analyzed by
IR, 1H NMR and CD spectroscopy, with the aim of determining whether these molecules were able to fold in ordered structures. In
these homo-oligomers two stabilizing effects are active: besides the trans conformation of the imide group, the formation of
C@Oꢀ ꢀ ꢀH–N hydrogen bonds takes place and is very sensitive to the pseudopeptide size.
ꢀ 2004 Elsevier Ltd. All rights reserved.
1. Introduction
O
OH
O
The members of the d-peptide family include isosteric
replacements of dipeptide units, such as depsipeptides,
thioesters or hetomethylene isoesters.1 As such, these
sequences have usually been introduced into polypeptide
chains to create b-turn mimics.2 However very little
work has been done on d-peptides though some pio-
neering work has been reported by Balaram et al.3 and
Toniolo et al.4 on Xaa-Pro peptides. In contrast, the
described d-peptides mostly involve carbapeptoid
backbones,5 which are homo-oligomers of sugar amino
acids and have been prepared from both furanose and
pyranose residues. Furthermore oligomers of a new
quinoline-derived d-amino acids have recently been
proposed as aromatic d-peptides.6
N
R
O
O
NH2
R'
Figure 1. General structure of Xaa-Oxd, a conformationally con-
strained d-amino acid. The preferential trans conformation of the
imidic bond, which accounts for the anomalous chemical shift of the
CH-a protons, is shown.
OH ¼ pyroglutamic acid) and high level DFT calcula-
tions proved a verification of this finding. On intro-
duction of our pseudoprolines into small polypeptide
chains,8 we could demonstrate that the imide group
always adopts the trans conformation, as was proven by
1H NMR spectroscopy: thus the Xaa-Oxd moiety is a
very rigid skeleton and can be considered as a single
conformationally constrained d-amino acid (Fig. 1).
Moreover this skeleton can easily be modified, by simply
changing the starting amino acids, which can belong to
Herein we report the synthesis and conformational
analysis of
L
-pyroglutamic acid and trans-(4S,5R)-4-
-Oxd) homo-oligo-
carboxy-5-methyl oxazolidin-2-one (
L
mers,7 up to the tetramer and pentamer level, respec-
tively, and we demonstrate that these short oligomers
fold in ordered structures similar to the polyproline II
conformation, even with only two or three units. This
peculiar property is due to the presence of a carbonyl on
the ring, nearby to the nitrogen, so that an imide bond is
formed. This latter group is characterized by a nitrogen
atom connected to an endo-cyclic carbonyl and an exo-
cyclic carbonyl, which tend to lie anti from one to the
other and thus adopt the trans conformation. X-ray
the
D or L series.
2. Results and discussion
Bearing this effect in mind, many possible Xaa-Oxd can
be simply prepared by varying the amino acid or the
oxazolidin-2-one moiety; for example, Oxd can be re-
placed by Oxac [(4S)-oxazolidineacetic acid, 2-oxo].9
Furthermore by alternating an L-amino acid and a
-Oxd group completely different sequences10 can be
diffraction analysis of Boc-(L-pGlu)2-OH (pGlu-
D
assembled. Herein, we report the results obtained for the
synthesis and the conformational analysis of short
* Corresponding author. Tel.: +39-051-2099511; fax: +39-051-20994-
56; e-mail: claudia.tomasini@unibo.it
0957-4166/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2004.03.033

1646
G. Luppi et al. / Tetrahedron: Asymmetry 15 (2004) 1645–1650
pseudopeptides, which have the general formula Boc-
(Gly- -Oxd)n-OBn and are the simplest oligomers that
can be prepared.
Then, in order to establish whether our molecules
formed ordered structures, we analyzed their behaviour
by IR, CD and H NMR.
L
1
The monomer Boc-Gly-
L
-Oxd-OBn 2 was obtained by
The IR absorption spectra of all the synthesized mole-
cules were obtained as 3 mM solutions in methylene
chloride: at this concentration the intramolecular
aggregation is usually unimportant. Figure 2 shows the
absorption bands of the NH stretching bonds (3500 and
3200 cmꢁ1) of the esters 2, 5, 7 and 9: the non-hydrogen-
bonded amide protons bands are above 3400 cmꢁ1 while
the hydrogen-bonded amide protons bands are below
acylation of trans-(4S,5R)-4-carboxybenzyl-5-methyl
oxazolidin-2-one (H- -Oxd-OBn) 1, which was achieved
by a straightforward method from
-threonine11 on
a multigram scale (Scheme 1). The synthesis of the
Boc-Gly- -Oxd-OBn 2 unit however is not trivial. Previ-
ously,8;9 we described the preparation of pentafluoro-
phenyl esters, such as Boc- -Oxd-OPfp or Boc- -Val-
OPfp under mild conditions,12 and their coupling with
H- -Oxd-OBn 1 in the presence of diisopropylethyl-
L
L
L
L
L
3400 cmꢁ1 14
Boc-Gly-L-Oxd-OBn 2 did not show any
.
L
sign of forming a C@Oꢀ ꢀ ꢀH–N hydrogen bond: this
outcome is in agreement with the tendency to form a
C@Oꢀ ꢀ ꢀH–C hydrogen bond, which would clash with
the formation of a C@Oꢀ ꢀ ꢀH–N hydrogen bond. Better
results were obtained with the longer chains, as there is a
continuous shift of the NH stretching bands up to
3395 cmꢁ1; these results suggest that a cooperative effect
takes place.
amine (DIEA) and 4-(dimethylamino)pyridine (DMAP)
in DMF. This coupling requires two steps and, on some
occasions, affords unsatisfactory results. As a result we
utilized as the coupling reagent HBTU, which has
recently been developed for the solid phase synthesis of
polypeptides. This compound is an activated form of
classical HOBt and, as Carpino et al.13 has recently
demonstrated, can exist in two forms, which are in
equilibrium: the uronium form (more active) and the
guanidinium form (less active). As the presence of tri-
ethylamine strongly favours the guanidinium form, it is
essential to add the tertiary base always after the two
reagents and after the HBTU in the formation of the
imide bond.
To further validate these results, the oligomers Boc-
1
(Gly-
L
-Oxd)n-OBn were analyzed by H NMR spectro-
scopy. We extensively demonstrated7;8 that the for-
mation of a C@Oꢀ ꢀ ꢀH–C bond can be identified simply
by checking the Xaa a-hydrogen chemical shift, as the
carbonyl proximity with the hydrogen involves a strong
deshielding of the hydrogen chemical shift. Table 1
Utilizing the same approach, the longer homo-oligomers
were synthesized by liquid phase synthesis in satisfactory
yields (Scheme 2).
shows the most interesting chemical shift of Boc-Gly-
L
L-
Oxd-OBn 2 and of Boc-(Gly- -Oxd)2-OBn 5, compared
with Boc-Gly-
L
-Pro-NHMe.
O
O
O
OBn
O
OBn
i, ii, iii
iv
85%
N
O
+
-threonine
L
NH
O
Cl3CO
OCCl3
O
NHBoc
-Oxd-OBn 2
O
Boc-Gly-
L
L
-Oxd-OBn 1
Scheme 1. Reagents and conditions: (i) 1 M NaOH, DMF; (ii) Cs2CO3, H2O; (iii) BnBr (1.1 equiv), DMF; (iv) (i) Boc-Gly-OH (1 equiv), HBTU
(1 equiv), Et3N (2 equiv), CH3CN, 30 min, rt.
i
H-Gly-L-Oxd-OBn
.CF3CO2H
quant.
3
iii
Boc-Gly-
L
-Oxd-OBn 2
Boc-(Gly-L-Oxd)2-OBn 5
75%
ii
Boc-Gly-
L
4
-Oxd-OH
quant.
ii
Boc-(Gly-
Boc-(Gly-
Boc-(Gly-
L
L
L
-Oxd)2-OBn 5
-Oxd)2-OH 6 + 4
-Oxd)3-OBn 7
Boc-(Gly-
Boc-(Gly-
L
-Oxd)2-OH 6
quant.
iii
55%
ii
L
-Oxd)3-OBn 7
Boc-(Gly-
L
-Oxd)3-OH 8
-Oxd)4-OBn 9
quant.
iii
40%
Boc-(Gly-
L
Boc-(Gly-
L-Oxd)3-OH 8 + 4
ii
Boc-(Gly-
L
-Oxd)4-OBn 9
Boc-(Gly-L-Oxd)4-OH 10
quant.
Scheme 2. Reagents and conditions: (i) TFA (18 equiv), CH2Cl2, 4 h, rt; (ii) H2, Pd/C, MeOH, 2 h, rt; (iii) HBTU (1 equiv), Et3N (3 equiv), CH3CN,
30 min, rt.

G. Luppi et al. / Tetrahedron: Asymmetry 15 (2004) 1645–1650
1647
Boc-Gly-L-Oxd-OBn 2
Boc-(Gly-L-Oxd) -OBn 5
Boc-(Gly-L-Oxd)2-OBn 7
Boc-(Gly-L-Oxd)34-OBn 9
O
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
O
H
N
H
O
HN
H
O
H
O
O
O
N
O
N
Ph
O
H
H
O
O
Figure 3. NOE enhancements of Boc-(Gly-L-Oxd)2-OBn 5 obtained
performing the NOESY 1D experiments on 10 mM solutions in CDCl3
and utilizing a mixing time of 1.000 s.
3500 3450 3400 3350 3300 3250 3200
ν (cm-1)
The IR analysis suggests that there is a regular increase
in the number of hydrogen bonds from the dimer 2 to
the octamer 9; this effect was also detected by investi-
gation of the DMSO-d6 dependence of the NH proton
chemical shift.16 This solvent has a strong hydrogen-
bonding acceptor character and, if bound to a free NH
proton, it would be expected to dramatically move its
chemical shift downfield. The evaluation of inaccessible
NH groups by 1H NMR was performed by adding
increasing amounts of DMSO-d6 to 1 mM solutions in
CDCl3. The results are reported in Figure 4. While the
variation of chemical shift on the increase of DMSO, d6
Figure 2. N–H stretch region FT-IR data for 3 mM samples of oligo-
mers 2, 5, 7 and 9 in pure CH2Cl2 at room temperature, after sub-
traction of the spectrum of pure CH2Cl2.
Table 1. Selected values of chemical shifts of Boc-Gly-
and 5 in 10ꢁ2 M solutions in CDCl3 at 25 ꢁC
L
-Pro-NHMe, 2
a-CH
Entry
Product
a-CH
1
2
Boc-Gly-
Boc-Gly-
L
L
-Pro-NHMe
-Oxd-OBn 2
3.89
4.37
4.66
4.48
4.84
––
––
––
percentage is very big for Boc-(Gly-
results are a little better for Boc-(Gly-
The titration of octamer Boc-(Gly-
L-Oxd)2-OBn 5, the
3
Boc-(Gly-
L
-Oxd)2-OBn 5
4.43
4.60
L
-Oxd)3-OBn 7.
-Oxd)4-OBn 9
L
proved to be in good agreement with the IR of the NH
region, with a small variation of the chemical shift of the
NH’s. This outcome suggests that a secondary structure
is forming, on prolonging the chain.
The chemical shift of the Boc-Gly-L-Pro-NHMe
a-hydrogens is reported in entry 1 and is below 4 ppm.15
In contrast, entries 2 and 3 show that the chemical shifts
of the Gly moieties a-hydrogens of Boc-Gly-
2 and of Boc-(Gly- -Oxd)2-OBn 5 are more deshielded
L
-Oxd-OBn
The oligomers Boc-(Gly-L-Oxd)n-OH 3, 6, 8 and 10 were
L
then investigated in MeOH by CD (Fig. 5). Although
this technique is intrinsically a low resolution method,17
it can furnish useful information on the formation of
hydrogen bond, which drives to the formation of sec-
ondary structures.18 The per-residue CD signal changes
from 3 to 10 and shows a dramatic absorbance increase
up to the octamer 10, thus showing that an extra effect is
present besides the chromophore absorption. This effect
can be attributed to the formation of hydrogen bonds
and of a secondary structure. This outcome is perfectly
in agreement with the results obtained from the analysis
by about 0.5–0.6 ppm. This implies that those molecules
assume a preferential conformation with the two imide
carbonyls in the trans conformation. The same effect is
observed for the longer oligomers 7 and 9, which show a
complex 1H NMR spectrum, but all the signals are
between 4.20 and 4.80 ppm, thus we can confirm that all
the imide groups assume a preferential conformation
with the two carbonyls in trans, as expected.
From NOESY 1D experiments (Fig. 3), we can see that
the Gly-NH hydrogen is near the a-hydrogen of the
1
of the IR and H NMR spectra, which furnished evi-
dence for the formation of hydrogen bonds.
adjacent
L
-Oxd group.
Boc-(Gly-L-Oxd)2-OBn 5
Boc-(Gly-L-Oxd)3-OBn 7
Boc-(Gly-L-Oxd)4-OBn 9
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
8.5
8.0
7.5
7.0
6.5
6.0
5.5
8.5
8.0
7.5
7.0
6.5
6.0
5.5
Gly1
Gly2
Gly3
Gly4
Gly1
Gly2
Gly3
Gly1
Gly2
5.0
0
2
4
6
8
10
12
0
2
4
6
8
10
0
2
4
6
8
10
% DMSO. d6
% DMSO. d6
% DMSO. d6
Figure 4. Variation of NH proton chemical shift (ppm) of 5, 7 and 9 a function of increasing percentages of DMSO-d6 to the CDCl3 solution (v/v)
(concentration: 1 mM).

1648
G. Luppi et al. / Tetrahedron: Asymmetry 15 (2004) 1645–1650
internal standard. Proton signals were assigned by
10000
5000
COSY spectra. Data for conformational analysis are
obtained with NOESY 1D spectra with typical mixing
times of 1.0 s.
0
-5000
-10000
-15000
-20000
-25000
-30000
-35000
The CD spectra were obtained on a Jasco J-810 spec-
tropolarimeter. Cylindrical fused quartz cells of 0.05 and
0.01 cm path length were used. The values are expressed
in terms of ½hꢂT , the total molar ellipticity (deg ·
cm2 · dmolꢁ1).
Boc-Gly-L-Oxd-OH
Boc-(Gly-L-Oxd)2-OH
Boc-(Gly-L-Oxd)3-OH
Boc-(Gly-L-Oxd)4-OH
200
220
240
260
280
300
4.1. Boc-Gly-
L
-Oxd-OBn 2
-Oxd-OBn 1 (4 mmol, 0.94 g),
Wavelength (nm)
Figure 5. Normalized per-residue CD spectra of Boc-(Gly-
OH (n ¼ 1, 2, 3, 4) in methanol.
L-Oxd)n-
To a stirred solution of
L
Boc-Gly-OH (4 mmol, 0.76 g) and HBTU (4 mmol,
1.52 g) in dry acetonitrile (20 mL) was added Et3N
(8 mmol, 1.16 mL) at room temperature. The mixture
was stirred for 30 min, then acetonitrile removed under
reduced pressure and replaced with ethyl acetate. The
mixture was washed with brine, 1 M aqueous HCl
(3 · 30 mL) and 5% aqueous NaHCO3 (1 · 30 mL), dried
over sodium sulphate and concentrated in vacuo.
Product 2 was obtained pure in 85% yield (1.33 g) as a
3. Conclusions
We have designed and prepared novel pseudopeptides
with the general formula Boc-(Gly- -Oxd)n-OBn, where
the monomer unit was obtained by reacting H- -Oxd-
OBn and Boc-Gly-OH. We have analyzed their pre-
ferred conformations by IR, 1H NMR and CD analysis,
with the aim of checking whether these molecules are
able to fold in ordered structures. We observed that a
C@Oꢀ ꢀ ꢀH–N hydrogen bond was formed only in longer
chains, while the trans conformation of the endo-cyclic
L
L
waxy solid after silica gel chromatography (cyclohexane/
20
ethyl acetate 8:2 as eluant). ½aꢂ ¼ ꢁ31:2 (c 1.0,
CH2Cl2); IR (CH2Cl2, 3 · 10ꢁ3 M): mD¼ 3443, 1792, 1752,
1712 cmꢁ1
;
1H NMR (CDCl3, 300 MHz): d ¼ 1:44 (s,
9H, t-Bu), 1.52 (d, 3H, J ¼ 6:3 Hz, OCH–CH3), 4.37
(dd, 1H, J ¼ 4:5, 19.1 Hz, NCHHCO), 4.51 (d, 1H,
J ¼ 4:8 Hz, CHN), 4.56 (dq, 1H, J ¼ 4:8, 6.3 Hz, CHO),
4.66 (dd, 1H, J ¼ 6:3, 19.1 Hz, NCHHCO), 5.13 (br s,
1H, NH), 5.20 (s, 2H, OCH2Ph), 7.29–7.38 (m, 5H, Ph);
13C NMR (CDCl3, 200 MHz): d ¼ 21:0, 28.1, 44.6, 61.3,
67.9, 74.1, 79.8, 128.2, 128.6, 134.4, 151.7, 155.8, 167.4,
169.9. C19H24N2O7 (392.40): calcd C 58.16, H 6.16, N
7.14; found C 58.19, H 6.13, N 7.10.
and hexocyclic carbonyls of the Gly-L-Oxd unit is
always present. Thus in these homo-oligomers two stabi-
lizing effects of the chain are active: besides the trans
conformation of the imide bond, which is a general effect
and has never been contradicted, the formation of
C@Oꢀ ꢀ ꢀH–N hydrogen bonds takes place and is very
sensitive to the pseudopeptide size. These results can be
very useful for the design of secondary ordered struc-
tures: for instance PNA analogues19 or especially
designed pseudopeptides for ion recognition.
.
-Oxd-OBn CF3CO2H, 3
4.2. H-Gly-
L
A solution of Boc-Gly-
L
-Oxd-OBn 2 (1 mmol, 0.39 g)
and TFA (18 mmol, 1.39 mL) in dry methylene chloride
(20 mL) was stirred for 4 h at room temperature, then
the volatiles removed under reduced pressure and the
product obtained pure in quantitative yield without any
further purification. 1H NMR (CDCl3, 200 MHz):
d ¼ 1:51 (d, 3H, J ¼ 5:8 Hz, OCH–CH3), 4.37–4.66 (m,
4H, NCH2CO+CHN+CHO), 5.19 (AB, 2H, J ¼
12:2 Hz, OCH2Ph), 6.90–8.15 (br s, 4H, COOH+NHþ3 ),
7.26–7.38 (m, 5H, Ph); NMR (CDCl3, 200 MHz):
d ¼ 20:6, 43.2, 61.1, 68.7, 75.3, 128.5, 128.9, 129.0,
134.4, 152.1, 167.0, 167.3.
4. Experimental
Routine NMR spectra were recorded with spectro-
meters at 400, 300 or 200 MHz (1H NMR) and at 100,
75 or 50 MHz (13C NMR). Chemical shifts are reported
in d values relative to the solvent peak of CHCl3, set at
7.27 ppm. Infrared spectra were recorded with an FT-IR
spectrometer. Melting points were determined in open
capillaries and are uncorrected.
High quality infrared spectra (64 scans) were obtained at
2 cmꢁ1 resolution using a 1 mm NaCl solution cell and a
Nicolet 210 FT-infrared spectrometer. All spectra were
obtained in 3 mM solutions in dry CH2Cl2 at 297 K. All
compounds were dried in vacuo and all the sample
preparations performed in a nitrogen atmosphere.
4.3. General method for the hydrogenolysis of the benzyl
esters
To a solution of Boc-(Gly-L-Oxd)n-OBn (1 mmol) in
methanol (20 mL) was added 10% palladium on char-
coal and the mixture stirred under hydrogen atmosphere
for 1 h. The catalyst was then filtered on a Celite pad
and the mixture concentrated. The corresponding acid
1
High quality H NMR spectra were recorded with a
Varian Inova 600. Measurements were carried out in
CDCl3 and in DMSO-d6 using tetramethylsilane as the

G. Luppi et al. / Tetrahedron: Asymmetry 15 (2004) 1645–1650
1649
Boc-(Gly-
tive yield without any further purification.
L
-Oxd)n-OH was obtained pure in quantita-
4.58 (m, 2H, 2 · NCHHCO), 4.61–4.83 (m, 4H,
2CHO+2CHN); 13C NMR (CD3OD, 200 MHz): d ¼
20:8, 21.1, 28.7, 44.4, 45.4, 63.6, 76.5, 77.0, 80.7, 154.1,
154.4, 170.0, 171.1, 171.5. C19H26N4O11 (486.43): calcd
C 46.91, H 5.39, N 11.52; found C 46.95, H 5.42, N
11.55.
4.4. General method for the homologation
To a stirred solution of Boc-(Gly-
and HBTU (1 mmol, 0.38 g) in dry acetonitrile (10 mL)
was added a mixture of H-Gly- -Oxd-OBnÆCF3CO2H
L-Oxd)n-OH (1 mmol)
L
4.8. Boc-(Gly-L-Oxd)3-OBn, 7
(1 mmol) and Et3N (3 mmol, 0.44 mL) in dry acetonitrile
(10 mL) at room temperature. The solution was stirred
for 40 min, then acetonitrile removed under reduced
pressure and was replaced with ethyl acetate. The mix-
ture was washed with brine, 1 M aqueous HCl
(3 · 30 mL) and with 5% aqueous NaHCO3 (1 · 30 mL),
dried over sodium sulphate and concentrated in vacuo.
The product was obtained pure after silica gel chroma-
tography (cyclohexane/ethyl acetate 8:2 as eluant).
20
Yield: 55%. Mp ¼ 128–129 ꢁC; ½aꢂ ¼ ꢁ49:5 (c 1.0,
CH2Cl2); IR (CH2Cl2, 3 · 10ꢁ3 M): m D¼ 3423, 1793, 1753,
1719, 1693 cmꢁ1; 1H NMR (CDCl3, 200 MHz): d ¼ 1:43
(s, 9H, t-Bu), 1.53 (d, 3H, J ¼ 7:2 Hz, OCH–CH3), 1.55
(d, 6H, J ¼ 7:2 Hz, 2 · OCH–CH3), 4.40–4.68 (m, 10H),
4.72–4.81 (m, 2H), 5.21 (AB, 2H, J ¼ 12:1 Hz,
OCH2Ph), 5.37 (br s, 1H, NH), 7.20–7.40 (m, 7H,
Ph+2NH); 13C NMR (CDCl3, 300 MHz): d ¼ 20:3, 20.5,
20.7, 20.9, 28.2, 43.6, 60.3, 61.3, 62.2, 68.1, 74.5, 75.5,
75.5, 80.0, 128.0, 128.2, 128.7, 134.5, 152.1, 152.7, 156.3,
167.7, 168.7, 169.3, 170.8, 171.1. C33H40N6O15 (760.26):
calcd C 52.10, H 5.30, N 11.05; found C 52.06, H 5.27,
N 11.01.
4.5. Boc-Gly-
L
-Oxd-OH, 4
20
Mp ¼ 52–53 ꢁC; ½aꢂ ¼ ꢁ15:5 (c 1.0, MeOH); IR
D
(CH2Cl2, 3 · 10ꢁ3 M): m ¼ 3397, 1790, 1769, 1734, 1718,
1697 cmꢁ1
;
1H NMR (CDCl3, 200 MHz): d ¼ 1:47 (s,
9H, t-Bu), 1.61 (d, 3H, J ¼ 6:2 Hz, OCH–CH3), 4.31–
4.41 (m, 1H, NCHHCO), 4.51 (d, 1H, J ¼ 4:8 Hz,
CHN), 4.54–4.78 (m, 2H, CHO+NCHHCO), 5.19 (br s,
1H, NH); 13C NMR (CDCl3, 200 MHz): d ¼ 21:8, 28.2,
44.7, 61.6, 74.7, 80.3, 152.3, 156.4, 170.2, 170.6.
C12H18N2O7 (302.28): calcd C 47.68, H 6.00, N 9.27;
found C 47.71, H 6.01, N 9.29.
4.9. Boc-(Gly-
L
-Oxd)3-OH, 8
20
Mp ¼ 189–191 ꢁC; ½aꢂ ¼ ꢁ93:8 (c 0.3, MeOH); IR
D
(CH2Cl2, 3 · 10ꢁ3 M): m ¼ 3412, 3340, 1787, 1738, 1716,
1710 cmꢁ1; H NMR (CD3OD, 200 MHz): d ¼ 1:45 (s,
1
9H, t-Bu), 1.53–1.59 (m, 9H, 3 · OCH–CH3), 4.30–4.83
(m, 12H, 3 · CHN+3 · CHO+3 · NCH2CO); 13C NMR
(CD3OD, 200 MHz): d ¼ 20:7, 20.8, 21.2, 28.7, 44.5,
45.5, 63.6, 76.6, 77.0, 77.1, 80.7, 154.1, 154.3, 154.4,
158.6, 170.0, 170.1, 171.1, 171.2, 171.6; C26H34N6O15
(670.58): calcd C 46.57, H 5.11, N 12.53; found C 46.60,
H 5.09, N 12.56.
4.6. Boc-(Gly-L-Oxd)2-OBn, 5
20
Yield: 75%. Mp ¼ 146–148 ꢁC; ½aꢂ ¼ ꢁ35:5 (c 1.0,
CH2Cl2); IR (CH2Cl2, 3 · 10ꢁ3 M): m D¼ 3430, 1793, 1753,
1713 cmꢁ1
;
1H NMR (CDCl3, 300 MHz): d ¼ 1:46 (s,
9H, t-Bu), 1.55 (d, 3H, J ¼ 6:6 Hz, OCH–CH3,
L
-Oxd1),
4.10. Boc-(Gly-L-Oxd)4-OBn, 9
1.56 (d, 3H, J ¼ 6:6 Hz, OCH–CH3,
L
-Oxd2), 4.43 (dd,
20
Yield: 40%. Mp ¼ 216–219 ꢁC; ½aꢂ ¼ ꢁ48:6 (c 1.0,
CH2Cl2); IR (CH2Cl2, 3 · 10ꢁ3 M): m D¼ 3390, 1792, 1718,
1H, J ¼ 4:2, 16.2 Hz, NCHHCO, Gly2), 4.44 (d, 1H,
J ¼ 4:2 Hz, CHN,
L
-Oxd2), 4.48 (dd, 1H, J ¼ 4:8,
18.6 Hz, NCHHCO, Gly1), 4.50 (d, 1H, J ¼ 4:2 Hz,
1693 cmꢁ1
;
1H NMR (CDCl3, 300 MHz): d ¼ 1:43 (s,
CHN,
L
-Oxd1), 4.60 (m, NCHHCO, Gly2), 4.60 (dq,
9H, t-Bu), 1.55 (m, 12H, 4 · OCH–CH3), 4.25–4.82
(16H, 4 · CHN+4 · CHO+4 · NCH2CO), 5.21 (AB, 2H,
J ¼ 12:0 Hz, OCH2Ph), 5.47 (br s, 1H, NH), 7.35–7.42
(m, 5H, Ph), 7.78 (br s, 3H, NH); 13C NMR (CDCl3,
300 MHz): d ¼ 20:5, 20.8, 28.3, 43.9, 52.0, 61.3, 62.3,
68.1, 74.5, 75.7, 80.1, 128.3, 128.7, 134.6, 152.2, 152.8,
156.2, 167.7, 168.7, 170.7; C40H48N8O19 (944.85): calcd
C 50.85, H 5.12, N 11.86; found C 50.82, H 5.07, N
11.81.
1H, J ¼ 4:2, 6.0 Hz, CHO,
L
-Oxd2), 4.84 (dd, 1H,
J ¼ 6:6, 19.2 Hz, NCHHCO, Gly1), 4.89 (dq, 1H,
J ¼ 4:8, 6.0 Hz, CHO,
L
-Oxd1), 5.12 (s, 1H, NH), 5.24
(AB, 2H, J ¼ 12:0 Hz, OCH2Ph), 6.90 (br s, 1H, NH),
7.27–7.42 (m, 5H, Ph); 13C NMR (CDCl3, 200 MHz):
d ¼ 20:3, 20.9, 28.1, 43.7, 60.3, 61.2, 62.3, 68.1, 74.5,
75.3, 80.1, 128.2, 128.6, 134.5, 152.1, 152.5, 156.3, 167.7,
168.5, 168.8, 170.8. C26H32N4O11 (576.55): calcd C
54.16, H 5.59, N 9.72; found C 54.11, H 5.55, N 9.68.
4.11. Boc-(Gly-
L
-Oxd)4-OH, 10
4.7. Boc-(Gly-
L
-Oxd)2-OH, 6
20
Mp ¼ 159–160 ꢁC (dec); ½aꢂ ¼ ꢁ55:2 (c 0.5, MeOH); IR
D
20
Mp ¼ 188–190 ꢁC; ½aꢂ ¼ ꢁ65:6 (c 1.0, MeOH); IR
(Nujol): m ¼ 3562, 3321, 1789, 1752, 1719, 1683 cmꢁ1; 1H
NMR (CD3OD, 400 MHz): d ¼ 1:44 (s, 9H, t-Bu), 1.53–
1.56 (m, 12H, 4 · OCH–CH3), 4.24–4.79 (16H,
4 · CHN+4 · CHO+4 · NCH2CO); 13C NMR (CD3OD,
400 MHz): d ¼ 20:7, 20.8, 21.2, 28.7, 44.5, 45.5, 62.8,
D
(Nujol): m ¼ 3390, 3238, 1792, 1719, 1701 cmꢁ1
;
1H
NMR (CD3OD, 200 MHz): d ¼ 1:45 (s, 9H, t-Bu), 1.54
(d, 3H, J ¼ 6:6 Hz, OCH–CH3), 1.55 (d, 3H, J ¼ 6:2 Hz,
OCH–CH3), 4.40–4.45 (m, 2H, 2 · NCHHCO), 4.50–

1650
G. Luppi et al. / Tetrahedron: Asymmetry 15 (2004) 1645–1650
(c) Smith, M. D.; Claridge, T. D. W.; Tranter, G. E.;
Sansom, M. S. P.; Fleet, G. W. J. Chem. Commun. 1998,
2041–2042; (d) Smith, M. D.; Long, D. D.; Martin, A.;
Marquess, D. G.; Claridge, T. D. W.; Fleet, G. W. J.
Tetrahedron Lett. 1999, 40, 2191–2194; (e) Long, D. D.;
Hungerford, N. L.; Smith, M. D.; Brittain, D. E. A.;
Marquess, D. G.; Claridge, T. D. W.; Fleet, G. W. J.
Tetrahedron Lett. 1999, 40, 2195–2198; (f) Claridge, T. D.
W.; Long, D. D.; Hungerford, N. L.; Aplin, R. T.; Smith,
M. D.; Marquess, D. G. P.; Fleet, G. W. J. Tetrahedron
Lett. 1999, 40, 2199–2202; (g) Suhara, Y.; Yamaguchi, Y.;
Collins, B.; Schnaar, R. L.; Yanagishita, M.; Hildreth, J.
E. K.; Shimada, I.; Ichikawa, Y. Bioor. Med. Chem. 2002,
10, 1999–2013; (h) Chakraborty, T. K.; Jayaprakash, S.;
Srinivasu, P.; Govardhana Chary, M.; Diwan, P. V.;
Nagaraj, R.; Ravi Sankar, A.; Kunwar, A. C. Tetrahedron
Lett. 2000, 41, 8167–8171.
63.5, 63.6, 76.5, 77.0, 77.1, 80.7, 154.1, 154.3, 154.4,
158.6, 170.0, 170.1, 171.1, 171.2, 171.6; C33H42N8O19
(854,73): calcd C, 46.37; H, 4.95; N, 13.11; found C,
46.33; H, 4.91; N, 13.14.
Acknowledgements
We thank MURST Cofin 2002 (Programmi di Ricerca
Scientifica di Rilevante Interesse Nazionale), FIRB and
60% (Ricerca Fondamentale Orientata), and the Uni-
versity of Bologna (funds for selected topics ‘Sintesi e
Caratterizzazione di Biomolecole per la Salute Umana’)
for financial support.
6. Jiang, H.; Leger, J.-M.; Huc, I. J. Am. Chem. Soc. 2003,
125, 3448–3449.
References and notes
7. (a) Bernardi, F.; Garavelli, M.; Scatizzi, M.; Tomasini, C.;
Trigari, V.; Crisma, M.; Formaggio, F.; Peggion, C.;
Toniolo, C. Chem. Eur. J. 2002, 8, 2516; (b) Lucarini, S.;
Tomasini, C. J. Org. Chem. 2001, 66, 727; (c) Tomasini,
C.; Trigari, V.; Lucarini, S.; Bernardi, F.; Garavelli, M.;
Peggion, C.; Formaggio, F.; Toniolo, C. Eur. J. Org.
Chem. 2003, 259–267.
8. (a) Luppi, G.; Lanci, D.; Trigari, V.; Garavelli, M.;
Garelli, A.; Tomasini, C. J. Org. Chem. 2003, 68, 1982–
1993; (b) Tomasini, C.; Villa, M. Tetrahedron Lett. 2001,
42, 5211–5214.
1. Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.;
Moore, J. S. Chem. Rev. 2001, 101, 3893–4011.
2. For some recent examples see: (a) Fisk, J. D.; Powell,
D. R.; Gellman, S. H. J. Am. Chem. Soc. 2000, 122, 5443;
(b) Langenhan, J. M.; Fisk, J. D.; Gellman, S. H. Org.
Lett. 2001, 3, 2559; (c) Chung, Y. J.; Huck, B. R.;
€
Christianson, L. A.; Stanger, H. E.; Krauthauser, S.;
Powell, D. R.; Gellman, S. H. J. Am. Chem. Soc. 2000,
122, 3995; (d) Woll, M. G.; Lai, J. R.; Guzei, I. A.; Taylor,
S. J. C.; Smith, M. E. B.; Gellman, S. H. J. Am. Chem.
Soc. 2001, 123, 11077; (e) Gennari, C.; Mielgo, A.;
Potenza, D.; Scolastico, C. Eur. J. Org. Chem. 1999,
379; (f) Belvisi, L.; Gennari, C.; Mielgo, A.; Potenza, D.;
Scolastico, C. Eur. J. Org. Chem. 1999, 389; (g) Belvisi, L.;
Gennari, C.; Madder, A.; Mielgo, A.; Potenza, D.;
Scolastico, C. Eur. J. Org. Chem. 2000, 695; (h) Nowick,
J. S. Acc. Chem. Res. 1999, 32, 287; (i) Nowick, J. S.; Tsai,
J. H.; Bui, Q.-C. D.; Maitra, S. Soc. J. Am. Chem. Soc.
1999, 121, 8409; (j) Holmes, D. L.; Smith, E. M.; Nowick,
J. S. J. Am. Chem. Soc. 1997, 119, 7665; (k) Nowick, J. S.;
Insaf, S. J. Am. Chem. Soc. 1997, 119, 10903; (l) Bolm, C.;
9. Luppi, G.; Villa, M.; Tomasini, C. Org. Biomol. Chem.
2003, 1, 247–250.
10. (a) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri,
M. R. Angew. Chem., Int. Ed. 2001, 40, 988–1011; (b)
Horne, W. S.; Stout, C. D.; Ghadiri, M. R. J. Am. Chem.
Soc. 2003, 125, 9372–9376, and references cited therein.
11. Falb, E.; Nudelman, A.; Hassner, A. Synth. Commun.
1993, 23, 2839–2844.
12. Green, M.; Berman, M. Tetrahedron Lett. 1990, 31, 5851–
5854.
13. Carpino, L. A.; Imazumi, H.; El-Faham, A.; Ferrer, F. J.;
Zhang, C.; Lee, Y.; Foxman, B. M.; Henklein, P.; Hanay,
C.; Mugge, C.; Wenschuh, H.; Klose, J.; Beyermann, M.;
Bienert, M. Angew. Chem., Int. Ed. 2002, 41, 441–445.
14. For an interpretation of N–H stretches in CH2Cl2, see: (a)
Gardner, R. R.; Liang, G.-B.; Gellman, S. H. J. Am.
Chem. Soc. 1995, 117, 3280; (b) Rao, C. P.; Nagaraj, R.;
Rao, C. N. R.; Balaram, P. Biochemistry 1980, 19, 425; (c)
Yang, J.; Christianson, L. A.; Gellman, S. H. Org. Lett.
1999, 1, 11.
€
Muller, D.; Hackenberger, C. P. R. Org. Lett. 2002, 893;
(m) Golebiowski, A.; Klopfenstein, S. R.; Shao, X.; Chen,
J. J.; Colson, A.-O.; Grieb, A. L.; Russell, A. F. Org. Lett.
2000, 2, 2615.
3. (a) Venkatachalapathi, Y. V.; Nair, C. M. K.; Vijayan, M.
Biopolymers 1981, 20, 1123–1145; (b) Venkataram, B. V.;
Balaram, P. Int. J. Biol. Macromol. 1982, 4, 99–102; (c)
Valle, G.; Bardi, R.; Piazzasi, A. M.; Crisma, M.; Toniolo,
C.; Cavicchioni, G.; Uma, K.; Balaram, P. Biopolymers
1991, 31, 1669–1676.
15. Boussard, G.; Marraud, M.; Aubry, A. Biopolymers 1979,
18, 1297–1331.
4. (a) Di Blasio, B.; Pavone, V.; Saviano, M.; Lombardi, A.;
Nastri, F.; Pedone, C.; Benedetti, E.; Crisma, M.; Anzolin,
M.; Toniolo, C. J. Am. Chem. Soc. 1992, 114, 6273–6278;
(b) Crisma, M.; Anzolin, M.; Bonora, G. M.; Toniolo, C.;
Benedetti, E.; Di Blasio, B.; Pavone, V.; Saviano, M.;
Lombardi, A.; Nastri, F.; Pedone, C. Gazz. Chim. It. 1992,
122, 239–244; (c) Moretto, V.; Valle, G.; Crisma, M.;
Bonora, G. M.; Toniolo, C. Int. J. Biol. Macromol. 1992,
14, 178–184; (d) Crisma, M.; Valle, G.; Bonora, G. M.;
Toniolo, C.; Cavicchioni, G. Int. J. Peptide Res. 1993, 41,
553–560.
5. (a) Mayes, B. A.; Stetz, R. J. E.; Watterson, M. P.;
Edwards, A. A.; Ansell, C. W. G.; Tranter, G. E.; Fleet,
G. W. J. Tetrahedron: Asymmetry 2004, 15, 627–638; (b)
Smith, M. D.; Long, D. D.; Marquess, D. G.; Claridge, T.
D. W.; Fleet, G. W. J. Chem. Commun. 1998, 2039–2040;
16. (a) Kopple, K. D.; Ohnishi, M.; Go, A. Biochemistry 1969,
8, 4087; (b) Martin, D.; Hauthal, G. Dimethyl Sulphoxide;
Van Nostrand-Reinhold: Wokingham, UK, 1975.
17. Glaettli, A.; Daura, X.; Seebach, D.; van Gunsteren, W. F.
J. Am. Chem. Soc. 2002, 124, 12972–12978.
18. See, for example: (a) Huck, B. R.; Fisk, J. D.; Guzei, I. A.;
Carlson, H. A.; Gellman, S. H. J. Am. Chem. Soc. 2003,
125, 9035–9037; (b) Park, J.-S.; Lee, H.-S.; Lai, J. R.; Kim,
B. M.; Gellman, S. H. J. Am. Chem. Soc. 2003, 125, 8539–
8545; (c) Raguse, T. L.; Lai, J. R.; Gellman, S. H. J. Am.
Chem. Soc. 2003, 125, 5592–5593; (d) Hart, S. A.;
Bahadoor, A. B. F.; Matthews, E. E.; Qiu, X. J.;
Schepartz, A. J. Am. Chem. Soc. 2003, 125, 4022–4023.
19. (a) Pulsh, A.; Tedeschi, T.; Nielsen, P. E. Org. Lett. 2000,
2, 4161–4163; (b) D’Costa, M.; Kumar, V. A.; Ganesh, K.
N. Org. Lett. 1999, 1, 1513–1516.