2-Alkyl-2-carboxy-azetidines as scaffolds for the induction of γ-turns

Article abstract of DOI:10.1016/j.tetlet.2007.03.126

To investigate the ability of 2-alkyl-2-carboxy-azetidines (Azx) to induce reverse turns when incorporated into peptides, RCO-Azx-l-Ala-NHMe dipeptide derivatives were selected as simplified tetrapeptide models, in which the azetidine residue is incorpora

Full text of DOI:10.1016/j.tetlet.2007.03.126

Tetrahedron Letters 48 (2007) 3689–3693
2-Alkyl-2-carboxy-azetidines as scaffolds
for the induction of c-turns
a
´
´
Jose Luis Baeza, Guillermo Gerona-Navarro, M Jesu´s Perez de Vega,
a
*
´
´
´
´
´
M Teresa Garcıa-Lopez, Rosario Gonzalez-Muniz and Mercedes Martın-Martınez
˜
Instituto de Quı´mica Me´dica (CSIC), Juan de la Cierva 3, 28006 Madrid, Spain
Received 14 February 2007; revised 19 March 2007; accepted 21 March 2007
Available online 25 March 2007
Abstract—To investigate the ability of 2-alkyl-2-carboxy-azetidines (Azx) to induce reverse turns when incorporated into peptides,
RCO-Azx-L-Ala-NHMe dipeptide derivatives were selected as simplified tetrapeptide models, in which the azetidine residue is incor-
porated at the i + 1 position. Molecular modelling, 1H NMR and FTIR studies showed the high tendency of the model tetrapeptides
to adopt c-turn conformations, indicating that these azetidine-containing amino acids could serve as general c-turn promoters.
Ó 2007 Elsevier Ltd. All rights reserved.
The search for small molecules able to mimic or induce
specific aspects of peptide secondary structure is a valu-
able approach in the search of new chemical entities able
to interfere with peptide–protein and protein–protein
interactions.1,2 An approach to induce secondary struc-
ture into peptides is the replacement of proteinogenic
amino acids with non-proteinogenic counterparts with
secondary structure-promoting effects. In this sense, in
our ongoing search for small molecules able to induce
secondary structures we have focused our attention on
a series of 2-substituted azetidine-2-carboxylic acids
(Azx), that combines features of a,a-disubstituted amino
acids and a aCi–aNi cyclization.3
influences the protein architecture, and this residue has
been found quite often at the (i + 1) position of b-turn
structures.6,7 It has been shown that the a-alkylation
of Pro, as in a-Me-Pro, provides further stabilization
of b-turn conformations in linear peptides.8 On the
other hand, conformational studies have also been car-
ried out on the lower and upper homologues of Pro,
the natural occurring amino acids, azetidine-2-carbox-
ylic acid (Azg)9 and piperidine-2-carboxylic acid (Pip),
respectively. In this sense, NOESY experiments on tetra-
peptides Ac-Gly-Xaa-Leu-Gly-NMe2 (Xaa = Azg, Pro
or Pip) showed that all formed b-turn conformations,
although, the distances between both terminals of the
tetrapeptide are larger for Azg and Pip containing pep-
tides in comparison with Pro analogues.5
a,a-Disubstituted amino acids have been shown to
induce defined secondary structure elements in short
peptides, in particular, type III (III0) b-turn, 310/a-heli-
cal structures and fully extended (C5) conformation,
depending upon overall bulkiness and nature (e.g.,
whether acyclic or aCi–aCi cyclized) of their side
chains.4,5
It is worth noting that, to the best of our knowledge,
there are no studies on the effect of the a-alkylation of
the azetidine ring upon peptide conformation. The 2-
alkyl-2-carboxy azetidines possess a / dihedral angle re-
stricted to approximately ꢀ70° or 70° depending on the
absolute configuration of the asymmetric carbon. These
values are similar to those reported for the central resi-
dues of the main types of b- and c-turns, which involve
four and three consecutive residues, respectively.10–13
Quite frequently these turns are stabilized by an intra-
molecular H-bond between the backbone CO of the first
(i) residue and the backbone NH of the fourth (i + 3) (b-
turn) or the third (i + 2) (c-turns) residues.10,13,14 Since
reverse turns have been shown to be relevant in biomo-
lecular recognition events,10,15–17 it would be of interest
Among the proteinogenic amino acids, proline is unique
as its side chain is bonded to both the a-carbon and its
preceding a-amine nitrogen. Due to its cyclic nature Pro
Keywords: Azetidines; Restricted amino acids; Peptidomimetics;
Reverse turns.
*
Corresponding author. Tel.: +34 91 5622900; fax: +34 91 5644853;
e-mail: iqmm317@iqm.csic.es
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.03.126

3690
J. L. Baeza et al. / Tetrahedron Letters 48 (2007) 3689–3693
to ascertain whether the 2-alkyl-2-carboxy azetidines are
able to induce a particular turn conformation upon
incorporation into peptides.
A 45
40
d(COi-NHi+3)
d(COi-NHi+2)
d(COi+1-NHi+3)
35
30
25
20
15
10
5
As a first step, a simulated annealing molecular dynamic
procedure was carried out, to explore the conforma-
tional space available to the dipeptide MeCO-Aza-Ala-
NHMe using AMBER as the forcefield. This dipeptide
may be considered as a simplified tetrapeptide model
in which the N-terminal amino acid has been substituted
by an acetyl group and the C-terminal by an N-methyl
moiety.
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
Distances CO-NH
The analysis of the modeling results showed the presence
of cis/trans rotamers around the CO–Aza bond, with
the cis conformers having higher energy than the trans
ones. It is worth mentioning that in all the conformers
the azetidine ring is planar, in agreement with the crystal
structure of Boc-L-Azg-OH18 and the NMR structure of
Ac-L-Azg-OH.19 Then, the characteristic conforma-
tional parameters (distances, dihedral and pseudodi-
hedral angles) obtained for conformers with trans
amide bond were compared to those expected for the
main types of b- and c-turns. The analysis of the param-
eters revealed that around 30% of the conformers pre-
sented values of the distance between the a-carbon of
the first residue (in our model the CH3 from the N-ter-
minal acetyl group) and the fourth residue (C-terminal
CH3), as well as of the pseudodihedral angle s20 corre-
B
˚
Figure 1. (A) Distance distribution (A) corresponding to the intra-
molecular H-bonds for the trans conformers within a +3 Kcal/mol
window of the global minimum. (B) Representative minimal energy
conformers for the model tetrapeptide MeCO-Aza-Ala-NHMe.
sponding to the atomic sequence aCi–aCi+1–aCi+2
–
aCi+3, within those required for the formation of b-turns
(aCi–aCi+3 < 7 A, ꢀ90 6 s 6 90).10,13,14,20 However,
˚
less than 10% of conformers showed the characteristic
H-bond of this type of conformation (COi–
NHi+3 6 2.5 A) (Fig. 1A).
˚
R1
N
R1
N
H
On the other hand, an increased percentage of around
40% of the total conformers was able to form the H-
bond characteristic of c-turns (COi–NHi+2 and/or
CO2H
CO2
Φ
O
Φ
O
1
R1=Bn
COAux
1a
R =Me
2b
2a R1=Bn
COi+1–NHi+3 6 2.5 A). The global minimum is included
OBn
OBn
˚
in a family of conformers characterized by the coexis-
tence of two c-turns (Fig. 1B). It is worth mentioning
that most of the lower energy conformers display c-turn
arrangements between i and i + 2 residues, and that it
corresponds to an inverse c-turn, as it was expected
for azetidine derivatives of S configuration at the stereo-
genic centre. These theoretical data seem to indicate a
tendency of the 2,2-disubstituted azetidines to preferen-
tially induce c-turns, although b-turn-like conforma-
tions are also possible.
COAux
Ph2SiH2
N
N
RhH(CO)(PPh3)3
O
3a
4a
OMe
OMe
1) H2/Pd(OH)2
2) CbzCl,
propylene oxide
To ascertain experimentally the conformational prefer-
ences in solution, we prepared a series of model dipep-
tide derivatives that incorporate 2-methyl or 2-benzyl
substituted azetidine-2-carboxylic acid. The synthesis
of the designed dipeptides was performed following
classical procedures of peptide synthesis in solution,
starting from the appropriately protected azetidine
derivatives, 1a and 2ab. The enantiomerically pure 2-
methyl-azetidine derivative 1a was prepared from 3a
(4S configuration), following a enantioselective route,
previously developed by us, based on the use of the chi-
ral auxiliary (+)-10-(N,N-dicyclohexylsulfamoyl)-iso-
NaOH
CO2H
COAux
N
O
N
O
O
O
1a
5a
Scheme 1. Synthesis of enantiomerically pure azetidine 1a. Yields:
4a (42%), 5a (49%), 1a (83%), Aux = (+)-10-(N,N-dicyclohexyl-
sulfamoyl)isoborneol.

J. L. Baeza et al. / Tetrahedron Letters 48 (2007) 3689–3693
3691
borneol (Scheme 1).21,22 The chemoselective reduction
of azetidinone 3a afforded azetidine 4a in moderate
yield. Subsequent catalytic hydrogenation of the p-
(methoxybenzyl) group, followed by reaction with benz-
yl chloroformate led to azetidine 5a. Finally, removal of
the isoborneol ester by saponification provided optically
pure (S)-Z-Aza-OH (1a).23 Azetidines 2ab (2:1, S:R
ratio) were prepared following a procedure previously
developed in our laboratory.24
Ac-Azf-Ala-NHMe (9a)
Ac-Aza-Ala-NHMe (8a)
99
97
95
The coupling between azetidine 1a or 2ab and H-Ala-
NHMe, using BOP as the coupling reagent, led to dipep-
tides 6a, 7ab, respectively, in good yield (Scheme 2). Dia-
stereoisomeric mixture 7ab was chromatographically
resolved into diastereoisomers 7a and 7b. Subsequent
catalytic hydrogenation of 2(S)-azetidines 6a and 7a,
using Pd–C as the catalyst, followed by treatment with
acetyl chloride in the presence of TEA, resulted in the
isolation of products 8a and 9a.25
3500
3400
3300
Wavenumber (cm-1)
The structural features of dipeptides 6–9 in solution
were analyzed by FTIR and H NMR. The IR spectra
1
Figure 2. NH stretch region of FTIR spectra for Ac-Aza-Ala-NHMe
(8a) and Ac-Azf-Ala-NHMe (9a) in CHCl3 at room temperature.
of these compounds in chloroform showed, for all deriv-
atives except for dipeptide 6a, the presence of hydrogen-
bonded (3380–3360 cmꢀ1) and non-hydrogen-bonded
(3456 cmꢀ1) stretching NH bands,26–28 suggesting the
existence of hydrogen-bonding interactions for com-
pounds 7–9. In Figure 2 it is shown the data for com-
pound 8a and 9a.
Table 1. Percentage of cis rotamers in compounds 6–9
Compound
RCO
R1
% cis-CDCl3
% cis-DMSO
6a
7a
7b
8a
9a
Z
Z
Z
Ac
Ac
CH3
17
<5
<5
0
43
31
26
23
0
CH2Ph
CH2Ph
CH3
As it has been previously observed for peptides that
incorporate proline and azetidine residues,26,29–32 the
1H NMR spectra of all dipeptide derivatives showed
the presence of two set of homologous signals, which
indicated the existence of cis/trans isomers around the
amide bond CO–Azx. There is a higher population of
CO–Azx cis conformers when the polarity of the solvent
increases (Table 1), in agreement with the previous
reports on related Pro derivatives.33 On the other hand,
the ratio of the trans isomer increases with the volume of
the substituent attached to position 2 of the azetidine
ring, likely due to steric hindrance between the alkyl
group at this position in the azetidine ring and the cor-
CH2Ph
0
responding residue at the N-terminus, that destabilize
the cis isomer.
The 1H NMR studies also provide further evidence sup-
porting the presence of intramolecular H-bonds in the
trans isomers of compounds 7–9. In particular, the sol-
vent dependence of NH chemical shifts and the temper-
ature coefficients were analyzed. The values of these
parameters for the NH proton of the NH–Me moiety,
namely Dd/DT > 4 ppb/K, the chemical shift below
7 ppm in CDCl3, and variations of >1 ppm between
CDCl3 and DMSO-d6, indicated that this proton is
accessible to the solvent, and thus not involved in the
formation of an H-bond. On the contrary, it was ob-
served that the chemical shifts of the NH proton of
the Ala residue in compounds 7–9, were above 7 ppm
in CDCl3 and that there were small variations
(<0.2 ppm) when the solvent was change to DMSO-d6
(Table 2), which suggested the participation of these
protons in H-bonded conformations.15,27 Moreover,
the temperature coefficients of the NH proton of Ala
residue in 2-benzyl azetidine derivatives 7a, 7b and 9a
are equal or lower than 3 ppb/K, in absolute value,
indicative of the existence of an intramolecular H-bond
(COiꢁ ꢁ ꢁNHi+2). A similar behaviour is observed for
compound Ac-Aza-Ala-NHMe (8a), although with a
value of NH–Ala Dd/DT in the uncertainty range
(3 ppb/K > Dd/DT < 4 ppb/K). However, taking into
account the small variation of the chemical shift of this
1 O
R1
R
H
N
N
H
CO2H
O
2
O
H-Ala-NHMe
BOP
N
N
6a R1=Me (2S)
O
7a R1=Bn (2S)
R1=Bn (2R)
BnO
BnO
7b
1a R1=Me
1) H2 / Pd-C
2) ClCOMe
1
2ab
R =Bn
1 O
H
N
R
N
H
O
O
N
R1=Me
9a R1=Bn
8a
Me
Scheme 2. Synthesis of the model tetrapeptide derivatives. Yields: 6a
(90%), 7a (48%), 7b (12%), 8a (21%), 9a (23%).

3692
J. L. Baeza et al. / Tetrahedron Letters 48 (2007) 3689–3693
Table 2. Chemical shifts and temperature coefficients for the NH
protons of dipeptides 6–9
8. Welsh, J. H.; Zerbe, O.; von Philipsborn, W.; Robinson, J.
A. FEBS Lett. 1992, 297, 216.
9. A new nomenclature has been proposed for our azetidine-
containing derivatives: The first two letters (Az) indicate
the azetidine ring while the third letter is the one letter
symbol for the corresponding amino acid. Thus, Azg, Aza
and Azf refer to Gly, Ala and Phe-derived azetidines,
respectively. Whereas, Azx indicates any azetidine-con-
dNH–Ala (ppm)
dNH–Me (ppm) Dd/DTa
Dd/DTa
NH–Ala NH–Me
CDCl3 DMSO CDCl3 DMSO
6a 7.74
7a 8.18
7b 8.22
8a 8.51
9a 8.78
7.99
8.22
8.11
8.35
8.67
6.38
6.36
6.31
6.44
6.37
7.75
7.81
7.89
7.72
7.78
ꢀ4.9
ꢀ3.0
ꢀ2.7
ꢀ3.3
ꢀ2.8
ꢀ4.9
ꢀ5.0
ꢀ5.4
ꢀ4.4
ꢀ4.8
´
taining amino acid derivative. Bonache, M. A.; Garcıa-
´
´
´
Martınez, C.; Garcıa de Diego, L.; Carreno, C.; Perez de
˜
Vega, M. J.; Garcia-Lopez, M. T.; Ferrer-Montiel, A.;
a Values in ppb/K. Dd measured in DMSO-d6, 30–60 °C (each 5 °C for
a total of 7 points).
Gonzalez-Muniz, R. Chem. Med. Chem. 2006, 1, 429.
˜
10. Rose, G. D.; Gierasch, L. M.; Smith, J. A. Adv. Protein
Chem. 1985, 37, 1.
11. Chou, K. C. Anal. Biochem. 2000, 286, 1.
NH upon solvent change in 8a we might consider that
this proton is also involved in the formation of an intra-
molecular H-bond. Thus with the exception of 6a, all the
other dipeptide derivatives adopt H-bonded c-turn-like
12. Guruprasad, K.; Shukla, S. J. Pept. Res. 2003, 61, 159.
13. Guruprasad, K.; Shukla, S.; Adindla, S.; Guruprasad, L.
J. Pept. Res. 2003, 61, 243.
14. Toniolo, C. CRC Crit. Rev. Biochem. 1980, 9, 1.
15. Kessler, H. Angew. Chem., Int. Ed. Engl. 1982, 21, 512.
16. Che, Y.; Marshall, G. R. J. Med. Chem. 2006, 49, 111.
17. Brickmann, K.; Yuan, Z. Q.; Sethson, I.; Somfai, P.;
Kihlberg, J. Chem. Eur. J. 1999, 5, 2241.
conformation with a H-bond between COiꢁ ꢁ ꢁNHi+2
,
similar to the second conformer in Figure 1B, which
would imply that this is the most stable conformer in
solution.
18. Cesari, M.; D’Ilario, L.; Giglio, E.; Perego, G. Acta
Crystallogr., Sect. B 1975, 31, 49.
In conclusion, the results of theoretical, IR and NMR
conformational studies support that the 2-alkyl-2-car-
boxy azetidines represent an effective way for inducing
c-turns conformations in short peptides. It is worth
mentioning that, so far, there have been described less
scaffolds able to induce c-turns rather than b-turns.
Therefore, these constrained amino acids could be useful
as scaffolds for c-turn scan in higher peptides, through
the replacement of individual amino acids of the peptide
by the azetidine functionalized with the appropriate side
chain at position 2. This could provide valuable infor-
mation regarding the bioactive conformation of peptides
of biological interest, which in fact is critical in the
design of peptidomimetics.
19. Thomas, W. Q.; Williams, M. K. Org. Magn. 1972, 4, 145.
20. Perczel, A.; Mcallister, M. A.; Csaszar, P.; Csizmadia, I.
G. J. Am. Chem. Soc. 1993, 115, 4849.
´
´
21. Gerona-Navarro, G.; Garcıa-Lopez, M. T.; Gonzalez-
Muniz, R. J. Org. Chem. 2002, 67, 3953.
˜
22. Although several asymmetric synthesis of azetidine deriv-
atives have been described, none of them allow an easy
preparation of the 2,2-disubstituted analogues described
here (a) Couty, F.; Evano, G.; Prim, D. Mini Rev. Org.
Chem. 2004, 1, 133; (b) Enders, D.; Gries, J. Synthesis
2005, 3508; (c) de Figueiredo, R. M.; Fro¨hlich, R.;
Christmann, M. J. Org. Chem. 2006, 71, 4147.
23. Analytical and spectroscopic data of 2(S)-Z-Aza-OH (1a):
[a]D ꢀ7.6 (c, 0.09, CHCl3). 1H NMR (CDCl3): cis/trans
isomers ratio 1:4. d 8.65 (m, 1H, COOH), 7.30 (m, 5H,
Ph), 5.16 (s, 2H, CH2-Z), 3.91 (m, 2H, H-4), 2.76 and 2.44
(m, 1H, H-3), 2.14 (m, 1H, H-3), 1.74 and 1.67 (s, 3H, 2-
CH3). 13C NMR (CDCl3): d 174.6 (2-CO), 157.1 (CO-Z),
135.5 (C–Ph), 128.6, 128.2, 127.7 (CH–Ph), 69.1 (2-C),
67.7 and 66.8 (CH2-Z), 44.7 (4-C), 28.8 and 28.0 (3-C),
22.3 (2-CH3). ES-MS: 250.1 [M+1]+, 272.0 [M+Na]+.
24. Derivatives 2ab are described in: Gerona-Navarro, G.;
Acknowledgements
This work has been supported by CICYT (SAF 2003-
07207-C02 and SAF 2006-01205) and Comunidad de
Madrid (GR/SAL/0846/2004). J.L.B. is a predoctoral
fellow from the Spanish Ministry of Education and
Science.
´
´
´
Perez de Vega, M. J.; Garcıa-Lopez, M. T.; Andrei, G.;
´
Snoeck, R.; De Clercq, E.; Balzarini, J.; Gonzalez-Muniz,
˜
R. J. Med. Chem. 2005, 48, 2612.
25. Analytical and spectroscopic data of selected compounds:
2(S)-Ac-Aza-L-Ala-NHMe (8a): [a]D ꢀ127.74 (c 1.01,
1
CHCl3). H NMR (CDCl3): d 8.51 (d, 1H, J = 7.2, NH–
References and notes
Ala), 6.44 (br s, 1H, NH–CH3), 4.36 (q, 1H, J = 7.2, a-H,
Ala), 3.98 (m, 2H, H-4), 2.78 (d, 3H, J = 4.9, CH3–
NH), 2.76 (m, 1H, H-3), 2.05 (m, 1H, H-3), 1.90 (s, 3H,
CH3–CO), 1.73 (s, 3H, 2-CH3), 1.37 (d, 3H, J = 7.2, b-H,
1. Yin, H.; Hamilton, A. D. Angew. Chem., Int. Ed. 2005, 44,
4130.
2. Peczuh, M. W.; Hamilton, A. D. Chem. Rev. 2000, 100,
1
Ala). H NMR (DMSO-d6): cis/trans isomers ratio 1:3.3.
2479.
Trans isomer: d 8.35 (d, 1H, J = 7.3, NH–Ala), 7.72 (m,
1H, NH–CH3), 4.21 (q, 1H, J = 7.3, a-H, Ala), 4.05 (m,
1H, H-4), 3.93 (m, 1H, H-4), 2.56 (d, 3H, J = 4.4, CH3–
NH), 2.42 (m, 1H, H-3), 1.94 (m, 1H, H-3), 1.79 (s, 3H,
CH3–CO), 1.54 (s, 3H, 2-CH3), 1.19 (d, 3H, J = 7.3, b-H,
Ala), cis isomer: d 7.99 (d, 1H, J = 7.5, NH–Ala), 7.82 (m,
1H, NH–CH3), 4.21 (m, 1H, a-H, Ala), 4.05 (m, 1H, H-4),
3.93 (m, 1H, H-4), 2.56 (d, 3H, CH3–NH), 2.42 (m, 1H, H-
3), 1.94 (m, 1H, H-3), 1.71 (s, 3H, CH3–CO), 1.62 (s, 3H,
2-CH3), 1.21 (d, 3H, J = 7.3, b-H, Ala). 13C NMR
(CDCl3): d 174.1 (2-CO), 172.5 (CO–Ala), 172.0 (CO–
´
´
3. Gerona-Navarro, G.; Bonache, M. A.; Alıas, M.; Perez de
´
´
´
Vega, M. J.; Garcıa-Lopez, M. T.; Lopez, P.; Cativiela, C.;
Gonzalez-Muniz, R. Tetrahedron. Lett. 2004, 45, 2193.
´
˜
4. Toniolo, C.; Crisma, M.; Formaggio, F.; Peggion, C.
Biopolymers 2001, 60, 396, and references cited therein.
5. Crisma, M.; Moretto, A.; De Zotti, M.; Formaggio, F.;
Kaptein, B.; Broxterman, Q. B.; Toniolo, C. Biopolymers
2005, 80, 279, and references therein.
6. Chou, P. Y.; Fasman, G. D. J. Mol. Biol. 1977, 115, 135.
7. Hayashi, T.; Asai, T.; Ogoshi, H. Tetrahedron Lett. 1997,
38, 3039.

J. L. Baeza et al. / Tetrahedron Letters 48 (2007) 3689–3693
3693
CH3), 71.1 (2-C), 49.1 (a-C, Ala), 45.9 (4-C), 26.4 (3-C),
26.3 (CH3–NH), 23.0 (2-CH3), 19.7 (CH3–CO), 17.1 (b-C,
Ala). ES-MS: 242.1 [M+1]+, 264.0 [M+Na]+, 505.2
[2M+Na]+. Anal. Calcd for C11H19N3O3: C, 54.76; H,
7.94; N, 17.41. Found: C, 54.58; H, 8.20; N, 17.46. 2(S)-
Ac-Azf-L-Ala-NHMe (9a): [a]D ꢀ5.62 (c 0.64, CHCl3). 1H
NMR (CDCl3): d 8.78 (d, 1H, J = 7.1, NH–Ala), 7.28–7.14
(m, 5H, Ph), 6.37 (m, 1H, NH–CH3), 4.33 (q, 1H, J = 7.1,
a-H, Ala), 3.56 (m, 1H, H-4), 3.53 (d, 1H, J = 13.8, 2-
CH2), 2.95 (d, 1H, J = 13.8, 2-CH2), 2.92 (m, 1H, H-4),
2.72 (d, 3H, J = 4.9, CH3–NH), 2.60 (m, 1H, H-3), 2.16
(m, 1H, H-3), 1.77 (s, 3H, CH3–CO), 1.35 (d, 3H, J = 7.1,
Ala), 46.2 (4-C), 39.3 (2-CH2), 26.3 (CH3–NH), 22.4 (3-C),
19.7 (CH3–CO), 17.1 (b-C, Ala). ES-MS: 318.0 [M+1]+,
240.0 [M+Na]+. Anal. Calcd for C17H23N3O3: C,
64.33; H, 7.30; N, 13.24. Found: C, 64.26; H, 7.42; N,
13.36.
26. Ishimoto, B.; Tonan, K.; Ikawa, S. Spectrochim. Acta,
Part A 2000, 56A, 201.
27. Belvisi, L.; Gennari, C.; Mielgo, A.; Potenza, D.; Scolas-
tico, C. Eur. J. Org. Chem. 1999, 389.
28. Vass, E.; Hollosi, M.; Besson, F.; Buchet, R. Chem. Rev.
2003, 103, 1917.
29. Schubert, M.; Labudde, D.; Oschkinat, H.; Schmieder, P.
J. Biomol. NMR 2002, 24, 149.
1
b-H, Ala). H NMR (DMSO-d6): d 8.67 (d, 1H, J = 7.3,
NH–Ala), 7.78 (m, 1H, NH–CH3), 7.35–7.19 (m, 5H, Ph),
4.26 (q, 1H, J = 7.3, a-H, Ala), 3.69 (m, 1H, H-4), 3.37 (d,
1H, J = 13.4, 2-CH2), 2.99 (d, 1H, J = 13.4, 2-CH2), 2.91
(m, 1H, H-4), 2.60 (d, 3H, J = 4.6, CH3–NH), 2.34 (m,
1H, H-3), 2.04 (m, 1H, H-3), 1.74 (s, 3H, CH3–CO),
1.22 (d, 3H, J = 7.3, b-H, Ala). 13C NMR (CDCl3): d
173.7, 172.6, 172.5 (CO–Ala, 2-CO, CO–CH3), 134.7 (C–
Ph), 130.4, 130.3, 128.4 (CH–Ph), 74.8 (2-C), 49.3 (a-C,
30. Montelione, G. T.; Hughes, P.; Clardy, J.; Scheraga, H. A.
J. Am. Chem. Soc. 1986, 108, 6765.
31. Zagari, A.; Nemethy, G.; Scherag, H. A. Biopolymers
1990, 30, 951.
32. Couty, F.; Evano, G.; Rabasso, N. Tetrahedron: Asym-
metry 2003, 14, 2407.
33. Madison, V.; Kopple, K. D. J. Am. Chem. Soc. 1980, 102,
4855.