Azetidine-derived amino acids versus proline derivatives. Alternative trends in reverse turn induction

Article abstract of DOI:10.1021/jo701746w

(Figure Presented) The influence of 2-alkyl-2-carboxyazetidines (Aze) on the 3D structure of model tetrapeptides R₂CO-2-R¹Aze-L- Ala-NHMe has been analyzed by molecular modeling, ¹H NMR, and FT-IR studies. The conformation

Full text of DOI:10.1021/jo701746w

Azetidine-Derived Amino Acids versus Proline Derivatives.
Alternative Trends in Reverse Turn Induction
Jose´ Luis Baeza, Guillermo Gerona-Navarro, M. Jesu´s Pe´rez de Vega,
M. Teresa Garc´ıa-Lo´pez, Rosario Gonza´lez-Mun˜iz, and Mercedes Mart´ın-Mart´ınez*
Instituto de Qu´ımica Me´dica (CSIC), Juan de la CierVa 3, 28006 Madrid, Spain
iqmm317@iqm.csic.es
ReceiVed August 27, 2007
The influence of 2-alkyl-2-carboxyazetidines (Aze) on the 3D structure of model tetrapeptides R2CO-
1
2-R1Aze-L-Ala-NHMe has been analyzed by molecular modeling, H NMR, and FT-IR studies. The
conformational constraints introduced by the four-membered ring resulted in an effective way to stabilize
γ-turn-like conformations in these short peptides. The conformational preferences of these Aze-containing
peptides have been compared to those of the corresponding peptide analogues containing Pro or R-MePro
in the place of 2-alkyl-Aze residue. In the model studied, both Pro and Aze derivatives are able to induce
reverse turns, but the nature of the turn is different as a function of the ring size. While the five-membered
ring of Pro tends to induce â-turns, as previously suggested by different authors, the four-membered ring
of Aze residues forces the peptide to preferentially adopt γ-turn conformations. In both cases, the presence
of an alkyl group at the R-position of Pro or the azetidine-2-carboxylate ring enhances significantly the
turn-inducing ability. These results might open the opportunity of using 2-alkyl-Aze residues as versatile
tools in defining the role of γ-turn structures within the bioactive conformation of selected peptides, and
represent an alternative to Pro derivatives as turn inducers.
Introduction
to their frequent localization in the exposed surface of peptides
and proteins.4-7 Thus, turn-like hot-spots have been recognized
within several protein-protein interaction surfaces,6,8 and it has
recently been described that over one hundred G protein coupled
receptors bind to their respective peptide/protein ligands through
turn motifs.9 The most prevalent reverse turn in peptides and
proteins is the â-turn, which is defined as any tetrapeptide
Protein-protein and peptide-protein interactions play key
roles in most biological processes, and therefore represent
valuable targets for drug discovery. As these interactions require
a high degree of surface complementarity and precision in
binding site docking, a valuable approach in the search of new
chemical entities able to interfere with these interactions is the
use of small compounds able to mimic or to induce precise
aspects of specific peptide secondary structures.1-3 Among the
secondary structure elements, reverse turns have been shown
to be relevant in biomolecular recognition events, mainly due
(4) Kessler, H. Angew. Chem., Int. Ed. 1982, 21, 512-523.
(5) Che, Y.; Marshall, G. R. J. Med. Chem. 2006, 49, 111-124.
(6) Rose, G. D.; Gierasch, L. M.; Smith, J. A. AdV. Protein Chem. 1985,
37, 1-109.
(7) Brickmann, K.; Yuan, Z. Q.; Sethson, I.; Somfai, P.; Kihlberg, J.
Chem.-Eur. J. 1999, 5, 2241-2253.
(1) Yin, H.; Hamilton, A. D. Angew. Chem., Int. Ed. 2005, 44, 4130.
(2) Peczuh, M. W.; Hamilton, A. D. Chem. ReV. 2000, 100, 2479.
(3) Pe´rez de Vega, M. J.; Mart´ın-Mart´ınez, M.; Gonza´lez-Mun˜iz, R. Curr.
Top. Med. Chem. 2007, 7, 33-62.
(8) Panasik, N., Jr.; Fleming, P. J.; Rose, G. D. Protein Sci. 2005, 14,
2910-2914.
(9) Tyndall, J. D. A.; Pfeiffer, B.; Abbenante, G.; Fairlie, D. P. Chem.
ReV. 2005, 105, 793-826.
10.1021/jo701746w CCC: $40.75 © 2008 American Chemical Society
Published on Web 01/25/2008
1704
J. Org. Chem. 2008, 73, 1704-1715

AlternatiVe Trends in ReVerse Turn Induction
sequence in a nonhelical region with a distance from the
R-carbons of the first and fourth residues (RCi-RCi+3) lower
than 7 Å.6,10-12 The γ-turn is the second most characterized
and commonly found turn, after the â-turn. γ-Turns, consisting
of three consecutive amino acids structured in a seven-membered
pseudocycle, have also been implicated in several biological
events.6,10,11,13 Structurally, â- and γ-turns are frequently
stabilized by an intramolecular H-bond between the backbone
CO of the first residue (i) and the backbone NH of the fourth
(i + 3) and the third (i + 2) residue, respectively.6,11,14
It is known that proline serves as a turn inducer in natural
peptides and proteins, with a high frequency of occurrence in
the (i + 1) position of â-turn motifs.15,16 Moreover, it has been
shown that the R-alkylation of Pro, as in R-MePro, provides
further stabilization of â-turn conformations in linear peptides.17
Conformational studies carried out on its lower homologue, the
natural amino acid azetidine-2-carboxylic acid (Aze),15,18-22
have pointed out the propensity for â-bend formation of the
Aze-containing peptides, but with higher flexibility compared
to Pro analogues. We have recently performed a conformational
analysis of 2-alkyl-2-carboxy azetidines, when incorporated at
the i + 1 position of simplified model tetrapeptides, and showed
that these non-proteinogenic amino acids possess an unexpected
ability to induce γ-turn conformations.23 Thus, contrary to Pro,
R-MePro, and Aze that tend to fix â-turns, the R-alkylation of
azetidine-containing amino acids seems to change the confor-
mational preferences, favoring γ-turns. However, the encoun-
tered discrepancies might be due to the different peptide models
used in the reported studies, or to the different parameters
measured to determine the conformation preferences in each
case.
(Xaa ) Pro, R-MePro, Aze, 2-MeAze, 2-BnAze) have now been
synthesized and studied for their conformational preferences by
means of molecular modeling, IR-FT, and NMR. These dipep-
tides could be considered as simplified tetrapeptide models, in
which the N-terminal amino acid has been substituted by
t
different acyl groups (R ) OBn, Me, Bu) and the C-terminal
residue by an N-methyl moiety. The present results fully agree
with our preliminary conclusions and provide strong support
for Aze amino acid derivatives as complementary tools to Pro
analogues for inducing reverse turns.
Results and Discussion
Synthesis. Noncommercially available Z-R-MePro-OH (1b)
was obtained from H-R-L-MePro-OH by acylation with ben-
zyloxycarbonyl chloride. Boc-Aze-OH (2a) was from com-
mercial sources, and starting azetidine derivatives Z-2-MeAze-
OH ((R,S)-3b), Piv-2-MeAze-OH ((R,S)-4b), and Z-2-BnAze-
OH ((R,S)-3c) were prepared following a synthetic procedure
previously developed in our laboratory (see the Supporting
Information for details).24-26
Two main approaches, following standard solution peptide
coupling procedures, were initially investigated for the synthesis
of model peptide derivatives (Scheme 1). Approach A consisted
of the formation of the dipeptide methyl ester, transformation
into the corresponding N-methyl amide, removal of the Z group,
and incorporation of the acetyl or pivaloyl moiety. Using the
reverse sequence of reactions, in Approach B the N-Boc-
protected dipeptide 6a was treated with HCl, to remove the Boc
group, followed by the incorporation of the selected acyl
moieties (Z, Ac, Piv), and final formation of the N-methyl amide.
The reaction of proline or azetidine derivatives 1b, 2a, 3b,
and H-Ala-OMe, using BOP as coupling reagent, led to
dipeptides 5b, 6a, and 7b, in good yield, while dipeptide 5a
was purchased from commercial sources. Since the starting
azetidine (R,S)-3b is racemic, dipeptide (R,S)-7b was obtained
as a mixture of two diastereoisomers which could not be
separated. Following approach A the subsequent treatment of
esters 5a and 5b with MeNH2‚EtOH afforded model peptides
8a and 8b in good yield. On the contrary, a similar reaction of
dipeptide (R,S)-7b led to compound 9b in low yield (14%) due
to the formation of an unexpected product that will be discussed
later in Scheme 2. It is interesting to note that only one
diastereoisomer of 9b was obtained from the diastereoisomeric
mixture (R,S)-7b. This isomer was identified as the one having
2(S)-configuration at the C-2 position of the azetidine ring by
comparison with an authentic sample of compound Z-2(S)-
MeAze-Ala-NHMe (9b), prepared from enantiomerically pure
Z-2(S)-MeAze-OH.23
We are now interested in further exploring the effects of the
alkylation at position 2 of the azetidine ring on peptide
conformation, and in comparing the results with those obtained
for the non-alkylated amino acid (Aze) and their higher
homologues Pro and R-MePro. As it is known that the amino
acids appended to these residues might influence their secondary
structure propensities, a unique peptide model, and the same
conformational studies have to be used for reaching consistent
conclusions. Thus, a series of dipeptides RCO-Xaa-Ala-NHMe
(10) Toniolo, C. CRC Crit. ReV. Biochem. 1980, 9, 1-44.
(11) Chou, K. C. Anal. Biochem. 2000, 286, 1-16.
(12) Guruprasad, K.; Shukla, S. J. Pept. Res. 2003, 61, 159-162.
(13) (a) Rosenstroem, U.; Skoeld, C.; Plouffe, B.; Beaudry, H.; Lindeberg,
G.; Botros, M.; Nyberg, F.; Wolf, G.; Karlen, A.; Gallo-Payet, N.; Hallberg,
A. J. Med. Chem. 2005, 48, 4009-4024. (b) Yuan, Z.; Blomberg, D.;
Sethson, I.; Brickmann, K.; Ekholm, K.; Johansson, B.; Nilsson, A.;
Kihlberg, J. J. Med. Chem. 2002, 45, 2512-2519.
(14) Guruprasad, K.; Shukla, S.; Adindla, S.; Guruprasad, L. J. Pept.
Res. 2003, 61, 243-251.
Following the alternative approach B, dipeptide 6a was
deprotected by treatment with saturated HCl in EtOAc, and then
converted into compounds 7a, 10a, and 11a by incorporation
of benzyloxycarbonyl, acetyl, or pivaloyl groups, respectively,
at N-terminus. The conversion of esters 7a, 10a, and 11a into
their corresponding amides 9a, 14a, and 15a was achieved again
by reaction with MeNH2‚EtOH.
(15) Hayashi, T.; Asai T.; Ogoshi, H. Tetrahedron Lett. 1997, 38, 3039-
3042.
(16) Chou, P. Y.; Fasman, G. D. J. Mol. Biol. 1977, 115, 135-175.
(17) Welsh, J. H.; Zerbe, O.; von Philipsborn, W.; Robinson, J. A. FEBS
Lett. 1992, 297, 216-220.
(18) Boni, R.; Blasi, D.; Verdini, A. S. Macromolecules 1974, 8, 140-
145.
(19) Kern, D.; Schutkowski, M.; Drakenberg, T. J. Am. Chem. Soc. 1997,
119, 8403-8408.
(20) Zagari, A.; Nemethy, G.; Scheraga, H. A. Biopolymers 1990, 30,
951-959.
(24) Gerona-Navarro, G.; Bonache, M. A.; Herranz, R.; Garc´ıa-Lo´pez,
M. T.; Gonza´lez-Mun˜iz, R. J. Org. Chem. 2001, 66, 3538-3547.
(25) Gerona-Navarro, G.; Bonache, M. A.; Alias, M.; Pe´rez de Vega,
M. J.; Garc´ıa-Lo´pez, M. T.; Lo´pez, P.; Cativiela, C.; Gonza´lez-Mun˜iz, R.
Tetrahedron Lett. 2004, 45, 2193-2196.
(26) Compound 5 has been described in: Gerona-Navarro, G.; Pe´rez de
Vega, M. J.; Garc´ıa-Lo´pez, M. T.; Andrei, G.; Snoeck, R.; De Clercq, E.;
Balzarini, J.; Gonza´lez-Mun˜iz, R. J. Med. Chem. 2005, 48, 2612-2621.
(21) Zagari, A.; Nemethy, G.; Scheraga, H. A. Biopolymers 1990, 30,
961-966.
(22) Deming, T. J.; Fournier, M. J.; Mason, T. L.; Tirrell, D. A.
Macromolecules 1996, 29, 1442-1444.
(23) Baeza, J. L.; Gerona-Navarro, G.; Pe´rez de Vega, M. J.; Garc´ıa-
Lo´pez, M. T.; Gonza´lez-Mun˜iz, R.; Mart´ın-Mart´ınez M. Tetrahedron Lett.
2007, 48, 3689-3693.
J. Org. Chem, Vol. 73, No. 5, 2008 1705

Baeza et al.
SCHEME 1a,b
a a, R1 ) H; b, R1 ) Me; c, R1 ) Bn. b The stereochemistry on C-2 is (S) unless otherwise indicated, except for 9c and 15b that are a mixture of two
diastereoisomers on C-2 (R,S).
SCHEME 2
Due to the problems encountered in the aminolysis of (R,S)-
7b to 9b, an alternative route (C) was explored for the
preparation of (R,S)-9c and (R,S)-15b. The new method implied
the direct coupling of (R,S)-3c and (R,S)-4b, respectively, with
H-Ala-NHMe to afford the expected compounds in good yield.24
Diastereoisomeric dipeptides (R,S)-15b could not be chromato-
graphically resolved, but the comparison of the 1H and 13C NMR
of the mixture with those of dipeptide Piv-2(S)-MeAze-Ala-
NHMe ((S)-15b), obtained from enantiopure Z-2(S)-MeAze-
Ala-NHMe (9b), allowed the unequivocal assignment of the
signals of both diastereoisomers.
Finally, the catalytic hydrogenation of dipeptide derivatives
8a,b and 9b,c using Pd-C as catalyst, followed by N-acylation
with acetyl or pivaloyl chloride in the presence of TEA resulted
in the isolation of compounds 12a,b, 13a,b, 14b,c, and 15b,c.
The yield of the latter reactions varied according to the R2 group,
from moderate to good for the acylation with pivaloyl chloride,
to low for the reactions with acetyl chloride.
unexpected product, which was identified as the urea derivative
(R,S)-16 (Scheme 2), a diastereoisomeric mixture that could not
be resolved. Compound (R,S)-16 is the result of the nucleophilic
intermolecular attack of the nitrogen of the methylamine to the
carbonyl group of the benzyloxycarbonyl moiety of (R,S)-9b.
This nucleophilic attack should occur with kinetic resolution,
since diastereoisomer (R)-9b was completely converted into (R)-
16, whereas (S)-9b was only partially transformed. It is worth
noticing that in similar reactions with Z-protected dipeptides
containing Aze or Pro the formation of the urea secondary
product was not appreciated, even after long reaction periods.
The only exception was the aminolysis of Z-R-MePro-Ala-OMe
(5b) with methylamine, which afforded, along with the expected
compound 8b, a secondary product identified as the tetra-
hydro-1,3-dioxopyrrolo[1,2-e]imidazole 17 (14%). Among the
1H and 13C NMR experiments carried out with 17, the analysis
of the bidimensional HMBC spectrum, showing correlations
between the R Ala proton and the three carbonyl groups of the
molecule, was essential to establish the indicated bicyclic
structure.
As previously mentioned, the reaction of 2(R,S)-Z-MeAze-
Ala-OMe ((R,S)-7b) with methylamine resulted in a major
1706 J. Org. Chem., Vol. 73, No. 5, 2008

AlternatiVe Trends in ReVerse Turn Induction
FIGURE 1. Distance distribution (Å) corresponding to the RCi-RCi+3 and intramolecular H-bonds for Ac-Xaa-Ala-NHMe trans conformers (Xaa
as indicated) within a +3 kcal/mol window of the global minimum.
Molecular Modeling. Theoretical studies were performed
with the simplest dipeptide derivatives Ac-Xaa-Ala-NHMe (Xaa
) Pro, R-MePro, Aze, 2-MeAze). The conformational studies
on these dipeptides were carried out using AMBER as the force
field, implemented in InsightII (version 2000.1, Byosym Tech,
San Diego, CA). To carry out the molecular dynamic studies,
different initial conformations of each peptide were used. The
different conformers obtained within +3 kcal mol-1 from the
global minimum were then grouped into families according to
the torsion angles of the peptide skeleton (φ and ψ). The dihedral
angle (ω) of the N-terminal amide was analyzed to study the
possible presence of cis/trans isomerism around this bond. In
all cases, the cis conformers were shown to have higher energy
compared to the trans isomers. Next, the minimum energy
conformations with an N-terminal trans amide bond were
evaluated for their turn propensity by analyzing a series of
characteristic parameters of â- and γ-turns. In this particular
case, the distances between the R-carbon of the first residue (in
our model the CH3 of the N-terminal acetyl group) and the fourth
residue (N-CH3 at C-terminal) (RCi-RCi+3 < 7 Å) and the
distance between the oxygen of the carbonyl group of the first
residue and the amide proton of the fourth (COi-NHi+3 e 2.5
Å) were measured to study the possible existence of â-turns in
these simple peptide models. On the other hand, the distances
between the carbonyl oxygen of the first residue and the amide
proton of the third one (COi-NHi+2 e 2.5 Å) and between the
carbonyl oxygen of the second residue and the NH of the fourth
(COi+1-NHi+3 e 2.5 Å) were analyzed to study the possible
existence of γ-turns. Finally, for the classification of the type
of â- and γ-turns the values of the torsion angles of central
residues of the turn were also measured (see Tables 1-4 within
the Supporting Information for details).
FIGURE 2. Minimal energy conformers of dipeptides MeCO-Xaa-
Ala-NHMe (A, Xaa ) Pro; B,: Xaa ) R-MePro; C, Xaa ) Aze; D,
Xaa ) 2-MeAze).
showed that about 40% of conformers of the Pro-containing
dipeptide 12a fulfill the required RCi-RCi+3 distance of â-turn-
like conformations, although none of them have the character-
istic COi-NHi+3 H-bond. In this case the minimum energy
conformer corresponds to an open â-turn (Figure 2A). Dipeptide
derivatives with an R-MePro residue have the highest tendency
to adopt â-turn conformations. In fact, more than 70% of the
conformers showed values of the distance between RCi-RCi+3
The values of the characteristic distances indicated above are
collected in the histograms depicted in Figure 1. These data
J. Org. Chem, Vol. 73, No. 5, 2008 1707

Baeza et al.
FIGURE 3. Representative conformers for the model tetrapeptide MeCO-2-MeAze-Ala-NHMe (14b).
and of the pseudodihedral angle τ within those expected for
â-turns, and around 20% of them have the expected COi-NHi+3
H-bond of this reverse turn type. The minimum energy
conformer of the R-MePro derivative 12b is a hydrogen-bonded
type I â-turn (Figure 2B), while some other families are
stabilized type II â-turns. It is interesting to note that molecular
modeling studies also predict a significant number of γ-turn-
like conformations for Pro- and R-MePro-containing dipeptides
(40% and 30%, respectively).
conformations are normally overestimated in theoretical calcula-
tions, it is necessary to undertake alternative studies to elucidate
if the conformational behavior imposed by Pro and azetidine
derivatives when incorporated into peptides is so different as
suggested by the molecular modeling study or not.
Conformational Studies in Solution. To experimentally
validate whether the theoretically predicted reverse turns are
still conserved in solution, the conformational behavior of model
peptides 8, 9, and 12-15 was analyzed using FT-IR and NMR
spectroscopy. First, the FT-IR spectra were examined for
H-bonded NH stretch bands, which are generally observed at
about 3300-3380 cm-1, while free NH bands should appear
Azetidine-containing dipeptides showed lower tendency to
adopt â-turn-like conformations than Pro analogues, but a high
propensity to γ-turn arrangements. Thus, more than 40% of the
Ac-Aze-Ala-NHMe (14a) conformers have a COi-NHi+2 H-
bond, characteristic of a γ-turn centered at the azetidine residue.
This percentage increase to 50% for the 2,2-disubstituted
azetidine derivative 14b, with the most frequent H-bond between
i and i + 2 residues, which correspond to an inverse γ-turn (φ2
around -68°). In both cases, the molecular modeling studies
also suggested the possible existence of another γ-turn between
i + 1 and i + 3 residues. In fact, the global minima of
compounds 14a and 14b show two consecutive γ-turns centered
at the second and third residues of the simplified model
tetrapeptides (Figure 2C,D). Finally, for the model peptide Ac-
2-MeAze-Ala-NHMe (14b) there is a family of conformers, 1
kcal/mol above the global minimum, which show the charac-
teristic COi-NHi+3 H-bond and dihedral angles of a type I
â-turn (Figure 3F). As can be appreciated in Figure 3, the
C-terminal amino acid of compound 14b shows a remarkable
conformational flexibility while keeping a γ-turn-like structure
at the N-terminus in most cases. This is in agreement with
previous conformational energy calculations that indicated a
larger number of low-energy conformers, and hence a higher
flexibility, for Aze-containing dipeptides and Aze-copolymers
than for the corresponding Pro analogues.20,21
between 3410 and 3460 cm-1 27-29
FT-IR experiments were
.
carried out at concentrations of 2 and 20 mM, using CHCl3 as
solvent. The form and frequency of the observed bands were
independent of the concentration used, thus excluding intermo-
lecular aggregation effects, and indicating that if the H-bonded
NH stretch band appears it should be due to intramolecular
hydrogen bonds. All dipeptides display non-hydrogen- (3410-
3456 cm-1) and hydrogen-bonded states (3346-3377 cm-1),
suggesting a certain contribution of H-bonded species (Figure
4). The intensity of the absorption bands corresponding to the
NH involved in H-bonds was lower in the Z-protected dipeptides
with respect to acetyl and pivaloyl analogues. A greater extent
of intramolecular H-bonding was also detected for R-MePro
compared to Pro, and for 2-MeAze with respect to Aze (Figure
4), which is consistent with the larger number of H-bonded
conformers for the R-alkylated derivatives predicted by the
molecular modeling studies.
Further information on the conformational preferences of
1
dipeptides 8, 9, and 12-15 in solution was obtained from H
NMR experiments. In most cases, two distinct sets of signals
(27) Belvisi, L.; Gennari, C.; Mielgo, A.; Potenza, D.; Scolastico, C.
Eur. J. Org. Chem. 1999, 389-400.
In general, the molecular modeling analysis reported here
suggests that Pro derivatives, especially R-MePro, are effective
as â-turn inducers, whereas the lower azetidine homologues are
more predisposed to induce γ-turns. However, as γ-turn
(28) Ishimoto, B.; Tonan, K.; Ikawa, S. Spectrochim. Acta, Part A 2000,
56A, 201-209.
(29) Vass, E.; Hollosi, M.; Besson, F.; Buchet, R. Chem. ReV. 2003,
103, 1917-1954.
1708 J. Org. Chem., Vol. 73, No. 5, 2008

AlternatiVe Trends in ReVerse Turn Induction
FIGURE 4. NH stretch region of FT-IR spectra of Piv-Xaa-Ala-NHMe at room temperature.
TABLE 1. Ratio of Cis/Trans Conformers in Compounds 8a-15c
conformations rose, in general, with the volume of the sub-
stituent attached at position 2 of the azetidine or proline ring,
likely due to steric clashes of the alkyl groups at this position
with the corresponding residue at the N-terminal, which
destabilizes the cis isomer. On the contrary, dipeptide derivatives
with a Z group at N-terminus showed greater population of cis
rotamer than the corresponding dipeptides with an acetyl or
pivaloyl moiety at this position. Although it has been shown
that an aromatic aminoacid N-terminal to Pro derivatives
increased the cis isomer population through the interaction
between the δ(+) nitrogen of Pro and the aromatic ring of the
preceding amino acid,33,34 molecular dynamic studies on dipep-
tide derivatives Z-Xaa-Ala-NHMe (Xaa ) Pro, R-MePro, Aze,
2-MeAze) have shown that this interaction is not present within
the minimum energy conformers (+3 kcal/mol window from
the global minimum, data not shown). These theoretical studies
have suggested other possible extra stabilizations of the cis
conformers: one through an intramolecular H-bond between
the Ala amide NH of the dipeptide and the sp3 oxygen of the
urethane group,35,36 a second one due to an H-bond between
the Ala amide NH and the aromatic ring of the Z group,37 and
finally an interaction between the oxygen of the CO (Ala) and
the edge of the Z aromatic group.38,39
compd
n
R1
H
R2
% cis (CDCl3)
% cis (DMSO)
8a
12a
13a
2
OBn
Me
28
14
0
54
29
0
tBu
8b
Me
H
OBn
Me
10
0
0
20
0
0
<5
43
<5
43
23
0
12b
13b
9a
14a
15a
tBu
1
OBn
Me
0
<5
<5
17
<5
0
tBu
9b
Me
OBn
Me
14b
(S)-15b
(R)-15b
(S)-9c
(R)-9c
14c
tBu
tBu
0
0
Bn
OBn
OBn
Me
<5
<5
0
31
26
0
15c
tBu
0
0
1
With the only exception of compound 15a, the H NMR
1
were observed in the one-dimensional H spectra, indicating
the existence of cis/trans isomers around the amide bond RCO-
Xaa (Xaa ) Pro, R-MePro, Aze, 2-MeAze, 2-BnAze).19,20,28,30,31
The assignment of the cis/trans conformations was made
considering a series of criteria that have been widely applied
for Xaa-Pro dipeptides:4,28,30,31 (i) the NOE effect between
protons of the R2-CO moiety and the R-H (Pro) or H-2 (Aze)
(cis isomer), and δ-H (Pro) or H-4 (Aze) protons (trans isomer)
and (ii) the anisotropic effect of the R2-CO carbonyl group
upon the R-Pro or 2-Aze protons and carbons (trans isomer) or
δ-Pro or 4-Aze protons and carbons (cis isomer). Depending
on the 1H NMR data, one or more of these criteria were applied
for our derivatives. Since for dipeptides 9a and 9c none of these
criteria could be applied, the mayor isomer was tentatively
assigned as trans, considering the high preference toward this
rotamer in the other derivatives.
spectra of pivaloyl dipeptide derivatives exhibited only one set
of resonances, the trans conformation, independent of the
N-terminal amino acid (Pro or Aze derivative) and the polarity
of the solvent used for the NMR analysis.
To explore whether a reverse turn structure was present in
model peptide derivatives, we examined the possible involve-
ment of the amide NH groups of compounds 8, 9, and 12-15
in the intramolecular H-bonds characteristic of â- and γ-turns.
The solvent and temperature dependence of the 1H NMR
chemical shifts of amide protons evidence their hydrogen-
bonding state. It is described that values of the chemical shifts
above 7 ppm in CDCl3, together with small variations of these
shifts when the solvent is changed to DMSO, are indicative of
a possible participation of the corresponding amide proton in
an H-bond.6,27 Application of these criteria to our compounds
indicates that the C-terminal NHMe proton of R-MePro-
The cis/trans relative populations for the different peptides
were determined by integration of separated signals in each 1H
NMR spectrum (Table 1). As expected, the population of the
less abundant species, cis conformers, increased with the polarity
of the solvent, in agreement with previous reports on related
Pro derivatives.32 On the other hand, the percentage of trans
(33) Do¨rr, A.; Lubell, W. D. Biopolymers (Pept. Sci.) 2007, 88, 290-
299.
(34) Halab, L.; Lubell, W. D. J. Am. Chem. Soc. 2002, 124, 2474-2484.
(35) Moraczewski, A. L.; Banaszynski, L. A.; From, A. M.; White, C.
E.; Smith, B. D. J. Org. Chem. 1998, 63, 7258-7262.
(36) Molcanov, K.; Kojic-Prodic, B.; Raos, N. Acta Crystallogr. B 2004,
60, 424-432.
(30) Schubert, M.; Labudde, D.; Oschkinat, H.; Schmieder, P. J. Biomol
NMR 2002, 24, 149-154.
(37) Meyer, E. A.; Castellano, R. K.; Diederich, F. Angew. Chem., Int.
Ed. 2003, 42, 1210-1250.
(31) Montelione, G. T.; Hughes, P.; Clardy, J.; Scheraga, H. A. J. Am.
Chem. Soc. 1986, 108, 6765-6773.
(38) Thomas, K. A.; Smith, G. M.; Thomas, T. B.; Feldmann, R. J. Proc.
Natl. Acad. Sci. 1982, 79, 4843-4847.
(32) Madison, V.; Kopple, K. D. J. Am. Chem. Soc. 1980, 102, 4855-
4863.
(39) Jain, A.; Purohit, C. S.; Verma, S.; Sankararamakrishnan, R. J. Phys.
Chem. Lett. B 2007, 111, 8680-8683.
J. Org. Chem, Vol. 73, No. 5, 2008 1709

Baeza et al.
TABLE 2. Chemical Shifts and Temperature Coefficients for the NH Amide Protons of Compounds 8, 9, and 12-15.
δNH (Ala, ppm) δNH-Me (ppm)
∆δ/∆Ta
NH (Ala)
∆δ/∆Ta
NH-Me
compd
no.
CDCl3
DMSO
∆δDMSO-CDCl
CDCl3
DMSO
∆δDMSO-CDCl
3
3
Z-Pro-Ala-NHMe
Ac-Pro-Ala-NHMe
Piv-Pro-Ala-NHMe
Z-R-MePro-Ala-NHMe
Ac-R-MePro-Ala-NHMe
Piv-R-MePro-Ala-NHMe
Z-Aze-Ala-NHMe
Ac-Aze-Ala-NHMe
Piv-Aze-Ala-NHMe
Z-2-MeAze-Ala-NHMe
Ac-2-MeAze-Ala-NHMe
Piv-2-MeAze-Ala-NHMe
Piv-2-MeAze-Ala-NHMe
Z-2-BnAze-Ala-NHMe
Z-2-BnAze-Ala-NHMe
Ac-2-BnAze-Ala-NHMe
Piv-2-BnAze-Ala-NHMe
8a
12a
13a
8b
12b
13b
9a
14a
15a
9b
14b
(S)-15b
(R)-15b
(S)-9c
(R)-9c
14c
6.95
7.09
6.67
6.23
6.44
5.71
7.26
7.91
7.60
7.74
8.51
8.18
7.88
8.18
8.22
8.78
8.55
8.02
7.88
7.72
7.91
7.76
7.28
8.09
8.13
7.99
7.99
8.35
8.07
7.95
8.22
8.11
8.67
8.35
1.07
0.79
1.05
1.68
1.32
1.57
0.83
0.32
0.39
0.25
0.16
0.11
0.07
0.04
0.11
0.11
0.20
6.86
6.53
6.90
7.02
7.04
7.22
6.71
6.79
6.80
6.38
6.44
6.55
6.72
6.36
6.31
6.37
6.49
7.71
7.56
7.64
7.25
7.50
7.46
7.78
7.74
7.75
7.75
7.72
7.71
7.82
7.81
7.89
7.78
7.78
0.85
1.03
0.74
0.23
0.46
0.24
1.07
1.05
0.95
1.37
1.28
1.16
1.10
1.45
1.58
1.41
1.29
-6.0
-5.0
-5.3
-6.5
-5.1
-5.9
-5.3
-3.9
-4.1
-4.9
-3.3
-2.7
-2.7
-3.0
-2.7
-2.8
-1.8
-6.3
-3.5
-4.3
nd
-2.2
-2.1
-4.9
-4.3
-4.6
-4.9
-4.4
-4.0
-4.4
-5.0
-5.4
-4.8
-3.9
15c
a Values in ppb/K. ∆δ measured in DMSO-d6, 30-60 °C (each 5 °C for a total of 7 points). nd: not determined.
containing dipeptides and the Ala NH proton of the Aze
analogues are solvent-shielded, suggesting their involvement in
intramolecular H-bonds (Table 2). Variable-temperature studies,
carried out to determine the temperature coefficients of the
amide proton, corroborated this assumption. The spectra were
recorded between 30 and 60 °C (at 5 deg intervals) in DMSO-
d6, and the temperature coefficients (∆δ/∆T) were determined
from the slopes of the linear regression lines obtained from the
chemical shifts versus temperature plots. It is assumed that
temperature coefficients equal or less than 3 ppb/K (in absolute
value) are indicative of a solvent-shielded NH.6,27 In small
peptides, these data provide information about the NH involve-
ment in an intramolecular hydrogen bond. On the contrary,
values above 4 ppb/K show that the NH proton is accessible to
the solvent, whereas values of the coefficient in the interval
3-4 ppb/K are not conclusive. The temperature coefficients
calculated for the R-MePro-containing peptides 12b and 13b
were indicative of the existence of an intramolecular H-bond
for the NHMe amide proton (Table 2). These figures, in perfect
accordance with the results of molecular dynamic simulation
described here, and the data reported by other authors,19 indicate
that R-MePro dipeptide derivatives stabilize a â-turn-like
conformation in solution. It is worth mentioning the failure of
the Pro itself to form the characteristic H-bond of â-turns in
these short dipeptides, also in agreement with the results from
the molecular modeling studies. In the case of Ac-Pro derivative
12a, the temperature coefficient for the NHMe amide proton
could suggest an open â-turn-like conformation.
Regarding the Aze-containing peptides, we have to distinguish
among compounds with azetidine-2-carboxylate and those
incorporating 2-methyl- and 2-benzyl-substituted analogues (2-
MeAze and 2-BnAze). The NH (Ala) chemical shifts for Aze
derivatives 9a, 14a, and 15a, lacking the side chain at position
2, were above 7 ppm in CDCl3, and in 14a and 15a there were
small variations upon solvent change to DMSO. So, it can be
assumed that, at least in the acetyl and pivaloyl derivatives, this
NH could be engaged in the formation of an intramolecular
H-bond stabilizing a γ-turn-like conformation. However, this
assumption was not corroborated by the temperature coefficient
values obtained for Aze-containing compounds. On the contrary,
most of the dipeptide derivatives incorporating the 2-alkyl-2-
carboxy azetidines induce γ-turn-like conformations, as inferred
from the chemical shifts (higher than 7 ppm), scarce influence
of solvent change (∆δDMSO-CDCl ) 0.04-0.25), and inacces-
3
sibility to solvent (∆δ/∆T < 3 ppb/K) of the Ala amide NH
proton in 9c, 14b,c, and 15b,c (Table 2). At the same time, the
NMR parameters for the C-terminal NHMe amide proton of
these 2-MeAze and 2-BnAze derivatives did not support the
incidence of any significant contribution of â-turn-like confor-
mations. The ability of dipeptide models to adopt a γ-turn
conformation centered at the azetidine residue mainly depends
on the nature of the N-terminal acyl moiety (Piv > Ac > Z)
and the 2-alkyl group (Bn > Me . H). It has been suggested
for Pro-Xaa dipeptides that an N-terminal pivaloyl group could
disfavor the γ-turn formation, deduced from the lower chemical
shift of the amide proton of the Xaa amino acid in pivaloyl
derivatives compared to acetylated compounds.40 Although this
trend in chemical shifts is also followed by our azetidine
derivatives, the absolute values of temperature coefficients for
model tetrapeptides containing 2-MeAze and 2-BnAze indicated
just the contrary, an increased propensity to form γ-turns for
Piv-subtituted peptides compared to acetylated derivatives. In
short, the difference in chemical shift of amide protons alone
should be cautiously taken as an indicator of turn propensity.
Finally it is interesting to note that both diastereoisomers of
peptides 9c and 15b are able to adopt γ-turn-like conformations
(Table 2). Therefore, the absolute configuration of the asym-
metric carbon of the azetidine ring does not affect the capacity
of these non-proteinogenic amino acids to induce reverse turns,
but influences the type of γ-turn, inverse for (S)-9c and (S)-
15b, and classical for (R)-9c and (R)-15b.
From our theoretical and experimental conformational studies
the general trend of the Aze amino acids to induce γ-turns can
be deduced, as well as the positive influence of the R-alkylation
upon the capacity to promote this particular conformation.
However, previous studies (based on NOESY spectra in CD2-
Cl2) on small linear peptides containing Aze have been
interpreted as indicative of the existence of â-turns.15,20-22 To
investigate these divergences NOESY experiments have also
been recorded on our model peptides, both in CDCl3 and in
DMSO-d6. The inspection of these spectra showed the existence
(40) Liang, G. B.; Rito, C. M.; Gellman, S. H. Biopolymers 1992, 32,
293-301.
1710 J. Org. Chem., Vol. 73, No. 5, 2008

AlternatiVe Trends in ReVerse Turn Induction
of a weak NOE between the methyl groups at the N- and
C-termini of derivatives with an acetyl or pivaloyl N-terminal
group (14a, 14b, 15a, (S)-15b, 15c), except for (S)-15b and
15c in DMSO-d6. In the absence of other experimental evidence,
this Me-Me NOE might be understood as indicative of â-turn
conformations. However, in our case, the chemical shifts and
the temperature coefficients of the NH amide protons clearly
corroborate a preference of Aze-dipeptides for γ-turn-like
arrangements. These discrepancies can be easily reconciled if
we consider that several conformations might be present in
solution, mainly due to the conformational flexibility of the
C-terminal residue in R2CO-R1Aze-Ala-NHMe dipeptide de-
rivatives (Figure 3). The minimum energy conformers D and
F, having γ- and â-turn-like structures, respectively, have
distances between the above indicated methyl groups (around
5 Å) that are within the NOE distances. On the other hand,
NOE difference studies have been carried out on those signals
that can provide clues regarding the population of â-turn-like
conformations. A weak NOE was observed between C-terminal
N-Me protons and the Me- or tBu-CO for Ac- and Piv-R-MePro-
containing peptides, 12b (0.1%) and 13b (0.6%), while this NOE
was absent for Piv-2-MeAze-Ala-NHMe, (S)-15b, and only a
tiny signal is observed for Ac-2-MeAze-Ala-NHMe, 14b. In a
similar way, a significant NOE between the C-terminal NH and
the N-terminal Me- or tBu-groups was exclusively observed for
MePro derivatives, 12b (0.6%) and 13b (1.1%), but not for Aze
analogues. Additionally, medium-strong NOEs between NH-
Ala and NH-Me amide protons were measured for compounds
12b (2.7%) and 13b (4.0%), whereas the intensity of these NOEs
decreases for Aze-containing peptides 14b (1.3%) and (S)-15b
(2.1%). Thus, NOE differences are also in agreement with a
higher population of â-turn conformations for Pro-containing
peptides. On the whole, the experimental data obtained with
our peptide models provide evidence that Aze derivatives have
a higher tendency than Pro analogues to induce γ-turns, although
the conformational flexibility at the C-terminus might also allow
the adoption of an open â-turn-like arrangement in a much lesser
extent.
new foldamers,41 which could compete for a variety of protein-
protein interactions. Further studies are underway in this respect.
Experimental Section
Analytical and spectroscopic data of compounds 14b and 14c
were described in a preliminary short communication.23
General Procedure for the Preparation of RCO-Xaa-Ala-
OMe Derivatives. Method A: A solution of the corresponding
aminoacid (2.92 mmol) in dry CH2Cl2 (8 mL) was treated with
H-Ala-OMe‚HCl (571 mg, 4.09 mmol), BOP (1.8 g, 4.09 mmol),
and TEA (0.97 mL, 7.01 mmol). After being stirred for 24 h at
room temperature, the solution was washed successively with citric
acid (10%), NaHCO3 (10%), H2O, and brine. The organic phase
was dried over Na2SO4 and evaporated to dryness. The residue was
purified by flash chromatography, using the system of eluents
indicated in each case.
Method B: Compound 6a (143 mg, 0.50 mmol) was dissolved
in a solution of HCl/EtOAc (12 mL, 3.2 M) and the solution was
stirred at room temperature for 4 h. After evaporation of the solvent,
the crude was dissolved in dry CH2Cl2 (4 mL), cooled at 0 °C, and
propylene oxide (0.53 mL, 7.50 mmol) and benzyl chloroformate
(0.11 mL, 0.80 mmol) were added. After the solution was stirred
for 2 h at room temperature, the organic phase was dried over Na2-
SO4 and evaporated to dryness. The residue was washed succes-
sively with citric acid (10%), NaHCO3 (10%), H2O, and brine, and
subsequently purified by preparative radial chromatography, using
a gradient from 20% to 66% of EtOAc in hexane.
Method C: Compound 6a (143 mg, 0.50 mmol) was stirred in
a solution of HCl/AcOEt (12 mL, 3.2 M) at room temperature for
4 h. After evaporation of the solvent, the crude was dissolved in
dry CH2Cl2 (4 mL) and cooled at 0 °C under argon atmosphere.
To this solution was added TEA (0.22 mL, 1.55 mmol) and the
corresponding acyl chloride (0.80 mmol). After being stirred for 2
h at room temperature the solution was washed successively with
citric acid (10%), NaHCO3 (10%), H2O, and brine. The organic
phase was dried over Na2SO4 and evaporated to dryness. The
residue was purified by flash chromatography, using the system of
eluents indicated in each case.
Z-L-R-MePro-L-Ala-OMe (5b): Syrup, 82% yield (from 1b,
Method A). Gradient from 25% to 33% of EtAcO in hexane.
HPLC: tR ) 4.69 min (A:B ) 30:70). [R]D -55.58 (c 1.46, CHCl3).
1H NMR (CDCl3): cis/trans isomers ratio: 1:1.7. Trans isomer: δ
7.51 (br s, 1H, NH-Ala), 7.30 (m, 5H, Ph), 5.18 (d, 1H, J ) 12.5
Hz, CH2-Z), 5.11 (d, 1H, J ) 12.5 Hz, CH2-Z), 4.46 (m, 1H, R-H,
Ala), 3.73 (s, 3H, OMe), 3.60 (m, 2H, δ-H, Pro), 2.64 (m, 1H,
â-H, Pro), 1.81 (m, 3H, â-H, γ-H, Pro), 1.68 (s, 3H, R-CH3, Pro),
1.36 (m, 3H, â-H, Ala). Cis isomer: δ 7.51 (br s, 1H, NH-Ala),
7.30 (m, 5H, Ph), 5.16 (s, 2H, CH2-Z), 4.46 (m, 1H, R-H, Ala),
3.65 (s, 3H, OMe), 3.60 (m, 2H, δ-H, Pro), 2.27 (m, 1H, â-H,
Pro), 1.81 (m, 3H, â-H, γ-H, Pro), 1.47 (s, 3H, R-CH3, Pro), 1.27
(m, 3H, â-H, Ala). 13C NMR (CDCl3): trans isomer: δ 173.9
(CON), 173.3 (COO), 155.1 (OCON), 136.5 (C-Ph), 128.4, 127.9,
127.7 (CH-Ph), 67.1 (R-C, Pro), 66.9 (CH2-Z), 52.2 (OMe), 48.5
(δ-C, Pro), 48.3 (R-C, Ala), 38.7 (â-C, Pro), 22.6 (γ-C, Pro), 22.5
(R-CH3, Pro), 17.9 (â-C, Ala). ES-MS: 349.2 [M + 1]+, 371.2
[M + Na]+. Anal. Calcd for C18H24N2O5: C 62.05, H 6.94, N 8.04.
Found: C 62.00, H 6.92, N 8.09.
Conclusions
Theoretical and experimental conformational studies by
molecular dynamics, NMR, and IR-FT support that the presence
of either Pro or Aze amino acid derivatives in short peptides,
RCO-Xaa-Ala-NHMe, biased their conformational preferences
toward the adoption of particular reverse turns. The ring size
of these RCi-RNi cyclized amino acids is crucial to determine
the type of reverse turn structure, with the five-membered ring
of proline derivatives inducing â-turns, and the four-membered
ring of azetidine analogues predisposing the peptides to adopt
γ-turn conformations. The incorporation of a substituent at
position R plays a decisive role for conferring enough turn
inducing capacities to both Pro and Aze derivatives. In conclu-
sion, the 2-alkyl-2-carboxyazetidine amino acids represent a way
of stabilizing γ-turn conformations in short peptides, and are a
complementary way to proline derivatives for the induction of
reverse turns. Thus, they can be used for â- or γ-turn scan, by
permutation of the amino acids of peptides of interest by the
corresponding proline or azetidine non-proteinogenic analogue,
respectively, to provide information regarding the bioactive
backbone conformation of these peptides. The 2-alkylazetidine
derivatives might open the possibility of achieving suitable
templates for the design of biologically active molecules and
2(S)-Boc-Aze-L-Ala-OMe (6a): Syrup, 93% yield (from 2a,
Method A). Eluent: gradient from 33% to 66% of EtAcO in hexane.
[R]D -117.16 (c 1.38, CHCl3). 1H NMR (CDCl3): δ 7.40 (m, 1H,
NH-Ala), 4.66-4.53 (m, 2H, R-H, Ala, H-2), 3.84 (m, 2H, H-4),
3.72 (s, 3H, OMe), 2.40 (m, 2H, H-3), 1.44 (s, 9H, CH3-tBu), 1.41
(d, 3H, J ) 7.1 Hz, â-H, Ala). 13C NMR (CDCl3): δ 173.0 (COO),
171.2 (CON), 157.1 (OCON), 81.1 (C-tBu), 62.2 (2-C), 52.4 (OMe),
(41) Baruah, P. K.; Sreedevi, N. K.; Gonnade, R.; Ravindranathan, S.;
Damodaran, K.; Hofmann, H.-J.; Sanjayan, G. J. J. Org. Chem. 2007, 72,
636-639.
J. Org. Chem, Vol. 73, No. 5, 2008 1711

Baeza et al.
48.0 (R-C, Ala), 47.4 (4-C), 28.3 (CH3-tBu), 19.9 (3-C), 18.4 (â-
C, Ala). ES-MS: 309.2 [M + Na]+, 595.2 [2M + Na]+. Anal.
Calcd for C13H22N2O5: C 54.53, H 7.74, N 9.78. Found: C 54.47,
H 7.80, N 9.85.
CH3-tBu). 13C NMR (CDCl3): δ 179.6 (CO-tBu), 172.8 (COO),
170.5 (CON), 62.4 (2-C), 52.1 (OMe), 51.7 (4-C), 48.0 (R-C, Ala),
38.4 (C-tBu), 26.8 (CH3-tBu), 18.8 (3-C), 17.7 (â-C, Ala). ES-MS:
271.3 [M + 1]+, 293.2 [M + Na]+, 563.3 [2M + Na]+. Anal. Calcd
for C13H22N2O4: C 57.76, H 8.20, N 10.36. Found: C 57.88, H
8.12, N 10.41.
General Procedures for the Preparation of RCO-Xaa-Ala-
NHMe Derivatives. Method A: The corresponding ester (4.22
mmol) was dissolved in MeNH2‚EtOH (10 mL, 8M) and reacted
over 2-12 h at room temperature. When necessary, the reaction
was purified by flash chromatography or centrifugal circular thin-
layer chromatography, using the system of eluents indicated in each
case.
Method B: The corresponding azetidine (2.92 mmol) was
dissolved in dry CH2Cl2 (8 mL) and treated with H-Ala-NHMe‚
HCl (567 mg, 4.09 mmol), BOP (1.81 g, 4.09 mmol), and TEA
(0.97 mL, 7.01 mmol). After being stirred for 24 h at room
temperature, the solution was washed successively with citric acid
(10%), NaHCO3 (10%), H2O, and brine. The organic layer was
dried over Na2SO4 and evaporated to dryness. The residue was
purified by flash chromatography, using as eluent EtOAc:hexane
(1:4).
Method C: To a solution of the corresponding Z-protected
dipeptide NHMe amide (0.34 mmol) in MeOH (13 mL), at 0 °C,
was added 10% Pd-C (15% w/w) and the solution was hydroge-
nated at 15 psi and room temperature. After 1 h the catalyst was
filtered and the solvent was evaporated to dryness. The resulting
residue was dissolved in dry CH2Cl2 (4 mL), cooled to 0 °C under
argon atmosphere, and treated with TEA (0.15 mL, 1.05 mmol)
and the corresponding acyl chloride (0.54 mmol). After being stirred
for 2 h at room temperature, the solution was washed successively
with citric acid (10%), NaHCO3 (10%), H2O, and brine. The organic
phase was dried over Na2SO4 and evaporated to dryness. The
residue was purified by flash chromatography, using the system of
eluents indicated in each case.
2(S)-Z-Aze-L-Ala-OMe (7a). Syrup, 61% yield (Method B).
Eluent: gradient from 20% to 66% of EtAcO in hexane. HPLC:
1
tR ) 8.59 min (A:B ) 20:80). [R]D -77.98 (c 1.06, CHCl3). H
NMR (CDCl3): δ 7.35 (m, 6H, NH-Ala, Ph), 5.18 (d, 1H, J )
12.2 Hz, CH2-Z), 5.12 (d, 1H, J ) 12.2 Hz, CH2-Z), 4.73 (t, 1H,
J ) 8.0 Hz, H-2), 4.56 (q, 1H, J ) 7.2 Hz, R-H, Ala), 3.95 (m,
2H, H-4), 3.73 (s, 3H, OMe), 2.47 (m, 2H, H-3), 1.39 (d, 3H, J )
1
7.2 Hz, â-H, Ala). H NMR (DMSO-d6): δ 8.40 (m, 1H, NH-
Ala), 7.32 (m, 5H, Ph), 5.00 (m, 2H, CH2-Z), 4.60 (dd, 1H, J )
8.7, 5.1 Hz, H-2), 4.28 (q, 1H, J ) 7.5 Hz, R-H, Ala), 3.88 (m,
2H, H-4), 3.60 (s, 3H, OMe), 2.44 (m, 1H, H-3), 2.00 (m, 1H,
H-3), 1.24 (d, 3H, J ) 7.5 Hz, â-H, Ala). 13C NMR (CDCl3): δ
172.7 (COO), 170.5 (CON), 157.3 (OCON), 135.9 (C-Ph), 128.4,
128.1, 127.9 (CH-Ph), 67.1 (CH2-Z), 62.1 (2-C), 52.2 (OMe), 47.9
(R-C, Ala), 47.2 (4-C), 19.9 (3-C), 18.0 (â-C, Ala). ES-MS: 321.2
[M + 1]+, 343.2 [M + Na]+. Anal. Calcd for C16H20N2O5: C 59.99,
H 6.29, N 8.74. Found: C 60.03, H 6.21, N 8.62.
2(R,S)-Z-MeAze-L-Ala-OMe ((R,S)-7b): Syrup, 89% yield
(from (R,S)-3b, Method A). Eluent: MeOH:CH2Cl2 (1:90).
HPLC: tR ) 8.30 min (A:B ) 25:75). 1H NMR (CDCl3): δ 8.23-
7.98 (br s, 1H, NH-Ala), 7.37 (m, 5H, Ph), 5.17 (br s, 2H, CH2-Z),
4.53 (q, 1H, J ) 7.2 Hz, R-H, Ala), 4.00-3.60 (m, 2H, H-4), 3.75
(s, 3H, OMe), 2.80-2.44 (m, 1H, H-3), 2.05 (m, 1H, H-3), 1.80-
1.56 (br s, 3H, 2-CH3), 1.50-1.15 (m, 3H, â-H, Ala). 13C NMR
(CDCl3): diastereoisomer A: δ 174.1 (CON), 173.0 (COO), 156.7
(OCON), 136.1 (C-Ph), 128.5, 128.1, 127.9 (CH-Ph), 70.0 (1-C),
66.9 (CH2-Z), 52.2 (OMe), 48.0 (R-C, Ala), 44.4 (4-C), 27.3 (3-
C), 22.5 (2-CH3), 17.8 (â-H, Ala); diastereoisomer B: δ 174.1
(CON), 173.0 (COO), 156.7 (OCON), 136.1 (C-Ph), 128.5, 128.1,
127.9 (CH-Ph), 70.0 (1-C), 66.9 (CH2-Z), 52.2 (OMe), 48.0 (R-C,
Ala), 44.4 (4-C), 27.6 (3-C), 22.5 (2-CH3), 17.8 (â-C, Ala). ES-
MS: 335.2 [M + 1]+, 357.2 [M + Na]+. Anal. Calcd for
C17H22N2O5: C 61.07, H 6.63, N 8.38. Found: C 61.27, H 6.60, N
8.47.
Z-L-Pro-L-Ala-NHMe (8a):42 Solid (mp 129-131 °C), 100%
yield (from 5a, Method A). HPLC: tR) 9.55 min (trans isomer)
and 8.21 min (cis isomer) (A:B ) 15:85). [R]D -89.12 (c 1, CHCl3).
1H NMR (CDCl3): cis/trans isomers ratio: 1:2.6. Trans isomer: δ
7.25 (m, 5H, Ph), 6.95 (d, 1H, J ) 7.2 Hz, NH-Ala), 6.86 (br s,
1H, NH-CH3), 5.05 (br s, 2H, CH2-Z), 4.38 (q, 1H, J ) 7.2 Hz,
R-H, Ala), 4.25 (m, 1H, R-H, Pro), 3.45 (m, 2H, δ-H, Pro), 2.65
(m, 3H, CH3-NH), 2.04 (m, 2H, â-H, Pro), 1.83 (m, 2H, γ-H, Pro),
1.25 (d, 3H, J ) 7.2 Hz, â-H, Ala); cis isomer: δ 7.25 (m, 5H,
Ph), 6.95 (m, 1H, NH-Ala), 6.86 (br s, 1H, NH-CH3), 5.05 (br s,
2H, CH2-Z), 4.38 (m, 1H, R-H, Ala), 4.25 (m, 1H, R-H, Pro), 3.45
(m, 2H, δ-H, Pro), 2.58 (br s, 3H, CH3-NH), 2.04 (m, 2H, â-H,
2(S)-Ac-Aze-L-Ala-OMe (10a): Oil, 79% yield (Method C).
Eluent: MeOH:CH2Cl2 (1:10) (centrifugal circular thin-layer chro-
1
matography). [R]D -119.28 (c 0.52, CHCl3). H NMR (CDCl3):
δ 8.28 (m, 1H, NH-Ala), 4.88 (dd, 1H, J ) 9.5, 6.8 Hz, H-2), 4.50
(q, 1H, J ) 7.1 Hz, R-H, Ala), 4.06 (t, 2H, J ) 7.8 Hz, H-4), 3.73
(s, 3H, OMe), 2.70 (m, 1H, H-3), 2.45 (m, 1H, H-3), 1.92 (s, 3H,
1
CH3-CO), 1.41 (d, 3H, J ) 7.1 Hz, â-H, Ala). H NMR (DMSO-
d6): cis/trans isomers ratio: 1:1. Isomer A: δ 8.58 (d, 1H, J ) 7.1
Hz, NH-Ala), 4.78 (m, 1H, H-2), 4.35 (q, 1H, J ) 7.1 Hz, R-H,
Ala), 4.02 (t, 2H, J ) 7.6 Hz, H-4), 3.30 (s, 3H, OMe), 2.36 (m,
1H, H-3), 2.02 (m, 1H, H-3), 1.76 (s, 3H, CH3-CO), 1.30 (d, 3H,
J ) 7.1 Hz, â-H, Ala); isomer B: δ 8.37 (d, 1H, J ) 7.1 Hz,
NH-Ala), 4.59 (m, 1H, H-2), 4.27 (q, 1H, J ) 7.1 Hz, R-H, Ala),
3.75 (t, 2H, J ) 7.2 Hz, H-4), 3.30 (s, 3H, OMe), 2.36 (m, 1H,
H-3), 2.02 (m, 1H, H-3), 1.64 (s, 3H, CH3-CO), 1.27 (d, 3H, J )
7.1 Hz, â-H, Ala). 13C NMR (CDCl3): δ 172.7 (COO), 172.5 (CO-
CH3), 170.3 (CON), 61.6 (2-C), 52.1 (OMe), 50.6 (4-C), 48.5 (R-
C, Ala), 18.7 (CH3-CO), 18.0 (3-C), 17.6 (â-C, Ala). ES-MS: 229.2
[M + 1]+, 251.2 [M + Na]+, 479.2 [2M + Na]+. Anal. Calcd for
C10H16N2O4: C 52.62, H 7.07, N 12.27. Found: C 52.60, H 7.10,
N 12.31.
1
Pro), 1.83 (m, 2H, γ-H, Pro), 1.15 (br s, 3H, â-H, Ala). H NMR
(DMSO-d6): cis/trans isomers ratio: 1:1.15. Trans isomer: δ 8.02
(m, 1H, NH-Ala), 7.71 (m, 1H, NH-CH3), 7.33 (m, 5H, Ph), 5.06
(d, 1H, J ) 12.8 Hz, CH2-Z), 4.94 (d, 1H, J ) 12.8 Hz, CH2-Z),
4.25 (m, 1H, R-H, Pro), 4.19 (q, 1H, J ) 7.2 Hz, R-H, Ala), 3.40
(m, 2H, δ-H, Pro), 2.55 (br s, 3H, CH3-NH), 2.10 (m, 1H, â-H,
Pro), 1.82 (m, 3H, â-H, Pro, γ-H, Pro), 1.06 (d, 3H, J ) 7.2 Hz,
â-H, Ala); cis isomer: δ 8.02 (m, 1H, NH-Ala), 7.62 (m, 1H, NH-
CH3), 7.33 (m, 5H, Ph), 5.05 (s, 2H, CH2-Z), 4.19 (m, 1H, J ) 7.2
Hz, R-H, Ala), 4.17 (m, 1H, R-H, Pro), 3.40 (m, 2H, δ-H, Pro),
2.55 (br s, 3H, CH3-NH), 2.10 (m, 1H, â-H, Pro), 1.82 (m, 3H,
â-H, Pro, γ-H, Pro), 1.18 (d, 3H, J ) 7.2 Hz, â-H, Ala). 13C NMR
(CDCl3): trans isomer: δ 172.5 and 171.6 (CON), 155.8 (OCON),
136.1 (C-Ph), 128.3, 128.0, 127.6 (CH-Ph), 67.2 (CH2-Z), 60.8 (R-
C, Pro), 48.7 (R-C, Ala), 47.0 (δ-C, Pro), 29.3 (â-C, Pro), 26.0
(CH3-NH), 24.4 (γ-C, Pro), 17.8 (â-C, Ala); cis isomer: δ 172.1
and 171.6 (CON), 154.6 (OCON), 136.1 (C-Ph), 128.3, 128.2, 127.6
(CH-Ph), 67.2 (CH2-Z), 60.4 (R-C, Pro), 48.7 (R-C, Ala), 47.3 (δ-
C, Pro), 31.1 (â-C, Pro), 26.0 (CH3-NH), 23.5 (γ-C, Pro), 18.2
2(S)-Piv-Aze-L-Ala-OMe (11a): Syrup, 82% yield (Method C).
Eluent: MeOH:CH2Cl2 (1:10) (centrifugal circular thin-layer chro-
1
matography). [R]D -98.38 (c 0.62, CHCl3). H NMR (CDCl3): δ
7.87 (m, 1H, NH-Ala), 4.82 (dd, 1H, J ) 9.5, 6.0 Hz, H-2), 4.41
(q, 1H, J ) 7.2 Hz, R-H, Ala), 4.26 (t, 2H, J ) 8.4 Hz, H-4), 3.64
(s, 3H, OMe), 2.51 (m, 1H, H-3), 2.35 (m, 1H, H-3), 1.30 (d, 3H,
1
J ) 7.2 Hz, â-H, Ala), 1.12 (s, 9H, CH3-tBu). H NMR (DMSO-
d6): δ 8.31 (br s, 1H, NH-Ala), 4.60 (m, 1H, H-2), 4.42-4.05 (m,
3H, R-H, Ala, H-4), 3.62 (s, 3H, OMe), 2.39 (m, 1H, H-3), 1.95
(m, 1H, H-3), 1.27 (d, 3H, J ) 7.2 Hz, â-H, Ala), 1.08 (s, 9H,
(42) Partial characterization details can be found in: Ohler, E.; Schmidt,
U. Chem. Ber. 1977, 110, 921-941.
1712 J. Org. Chem., Vol. 73, No. 5, 2008

AlternatiVe Trends in ReVerse Turn Induction
(â-C, Ala). ES-MS: 334.0 [M + 1]+, 356.0 [M + Na]+. Anal.
Calcd for C17H23N3O4: C 61.25, H 6.95, N 12.60. Found: C 61.44,
H 6.88, N 12.80.
cis/trans isomers ratio: 1:1.3. Trans isomer: δ 7.99 (d, 1H, J )
7.1 Hz, NH-Ala), 7.75 (m, 1H, NH-CH3), 7.35 (m, 5H, Ph), 5.07
(s, 2H, CH2-Z), 4.23 (q, 1H, J ) 7.1 Hz, R-H, Ala), 3.78 (m, 2H,
H-4), 2.58 (d, 3H, J ) 3.9 Hz, CH3-NH), 2.33 (m, 1H, H-3), 2.02
(m, 1H, H-3), 1.55 (s, 3H, 2-CH3), 1.19 (d, 3H, J ) 7.1 Hz, â-H,
Ala); cis isomer: δ 7.75 (m, 2H, NH-Ala, NH-CH3), 7.35 (m, 5H,
Ph), 5.06 (d, 1H, J ) 12.7 Hz, CH2-Z), 4.99 (d, 1H, J ) 12.7 Hz,
CH2-Z), 4.23 (m, 1H, R-H, Ala), 3.92 (m, 2H, H-4), 2.54 (d, 3H,
J ) 3.9 Hz, CH3-NH), 2.33 (m, 1H, H-3), 2.02 (m, 1H, H-3), 1.55
(s, 3H, 2-CH3), 1.19 (m, 3H, â-H, Ala). 13C NMR (CDCl3): δ 174.0
and 172.4 (CON), 156.2 (OCON), 136.0 (C-Ph), 128.5, 128.3, 128.0
(CH-Ph), 69.9 (2-C), 67.1 (CH2-Z), 48.9 (R-C, Ala), 44.6 (4-C),
29.6 (3-C), 26.2 (CH3-NH), 22.0 (2-CH3), 17.9 (â-C, Ala). ES-
MS: 335.2 [M + 1]+, 357.2 [M + Na]+. Anal. Calcd for
C17H23N3O4: C 61.25, H 6.95, N 12.60, Found: C 61.08, H 6.77,
N 12.79.
Z-L-R-MePro-L-Ala-NHMe (8b): Syrup, 82% yield (from 5b,
Method A). Eluent: MeOH:CH2Cl2 (1:20). HPLC: tR ) 11.47 min
1
(A:B ) 20:80). [R]D -31.14 (c 0.97, CHCl3). H NMR (CDCl3):
cis/trans isomers ratio: 1:6.1. Trans isomer: δ 7.34 (m, 5H, Ph),
7.02 (br s, 1H, NH-CH3), 6.23 (d, 1H, J ) 7.4 Hz, NH-Ala), 5.18
(d, 1H, J ) 12.4 Hz, CH2-Z), 5.08 (d, 1H, J ) 12.4 Hz, CH2-Z),
4.42 (q, 1H, J ) 7.4 Hz, R-H, Ala), 3.63 (m, 2H, δ-H, Pro), 2.71
(d, 3H, J ) 4.5 Hz, CH3-NH), 2.22 (m, 1H, â-H, Pro), 1.98-1.86
(m, 3H, â-H, Pro, γ-H, Pro), 1.58 (s, 3H, R-CH3, Pro), 1.30 (d,
3H, J ) 7.4 Hz, â-H, Ala); cis isomer: δ 7.34 (m, 5H, Ph), 6.44
(br s, 1H, NH-CH3), 6.23 (d, 2H, J ) 7.4 Hz, NH-Ala), 5.17 (s,
2H, CH2-Z), 4.42 (m, 1H, R-H, Ala), 3.63 (m, 2H, δ-H, Pro), 2.71
(m, 3H, CH3-NH), 2.22 (m, 1H, â-H, Pro), 1.98-1.86 (m, 3H, â-H,
Pro, γ-H, Pro), 1.58 (s, 3H, R-CH3, Pro), 1.30 (m, 3H, â-H, Ala).
1H NMR (DMSO-d6): cis/trans isomers ratio: 1:2.9. Trans
isomer: δ 7.91 (d, 1H, J ) 7.5 Hz, NH-Ala), 7.42-7.20 (m, 6H,
NH-CH3, Ph), 5.12 (d, 1H, J ) 12.9 Hz, CH2-Z), 5.05 (d, 1H, J )
12.9 Hz, CH2-Z), 4.19 (q, 1H, J ) 7.5 Hz, R-H, Ala), 3.63 (m, 1H,
δ-H, Pro), 3.51 (m, 1H, δ-H, Pro), 2.51 (d, 3H, J ) 4.6 Hz, CH3-
NH), 2.10 (m, 1H, â-H, Pro), 1.95-1.70 (m, 3H, â-H, Pro, γ-H,
Pro), 1.40 (s, 3H, R-CH3, Pro), 1.21 (d, 3H, J ) 7.5 Hz, â-H, Ala);
cis isomer: δ 7.60 (m, 1H, NH-CH3), 7.54 (d, 1H, J ) 7.4 Hz,
NH-Ala), 7.42-7.20 (m, 5H, Ph), 5.05 (d, 1H, J ) 12.9 Hz, CH2-
Z), 4.89 (d, 1H, J ) 12.9 Hz, CH2-Z), 4.19 (m, 1H, R-H, Ala),
3.51 (m, 1H, δ-H, Pro), 3.20 (m, 1H, δ-H, Pro), 2.51 (m, 3H, CH3-
NH), 2.10 (m, 1H, â-H, Pro), 1.95-1.70 (m, 3H, â-H, Pro, γ-H,
Pro), 1.46 (s, 3H, R-CH3, Pro), 1.13 (d, 3H, J ) 7.4 Hz, â-H, Ala).
13C NMR (CDCl3): trans isomer: δ 173.4 and 172.5 (CON), 155.0
(OCON), 136.3 (C-Ph), 128.6, 128.3, 127.7 (CH-Ph), 68.7 (R-C,
Pro), 67.3 (CH2-Z), 49.1 (R-C, Ala), 47.8 (δ-C, Pro), 39.3 (â-C,
Pro), 26.2 (CH3-NH), 23.0 (γ-C, Pro), 21.1 (R-CH3, Pro), 17.8 (â-
C, Ala); cis isomer: δ 173.4 and 172.5 (CON), 155.0 (OCON),
136.3 (C-Ph), 128.6, 128.3, 127.7 (CH-Ph), 68.7 (R-C, Pro), 67.0
(CH2-Z), 49.1 (R-C, Ala), 47.8 (δ-C, Pro), 39.3 (â-C, Pro), 26.2
(CH3-NH), 23.0 (γ-C, Pro), 21.1 (R-CH3, Pro), 17.8 (â-C, Ala).
ES-MS: 348.0 [M + 1]+, 370.0 [M + Na]+. Anal. Calcd for
C18H25N3O4: C 62.23, H 7.25, N 12.10. Found: C 62.42, H 7.22,
N 12.33.
2(S)-Z-BnAze-L-Ala-NHMe ((S)-9c): Solid (mp 132-134 °C),
48% yield (from (R,S)-3c, Method B). Eluent: EtOAc:hexane (1:
4). HPLC: tR ) 19.93 min (A:B ) 30:70). [R]D +12.3 (c 0.34,
1
CHCl3). H NMR (CDCl3): δ 8.18 (d, 1H, J ) 7.2 Hz, NH-Ala),
7.46-7.07 (m, 10H, Ph), 6.36 (br s, 1H, NH-CH3), 5.28 (d, 1H, J
) 12.2 Hz, CH2-Z), 5.09 (d, 1H, J ) 12.2 Hz, CH2-Z), 4.42 (q,
1H, J ) 7.2 Hz, R-H, Ala), 3.51 (m, 1H, H-4), 3.49 (d, 1H, J )
13.9 Hz, 2-CH2), 3.02 (d, 1H, J ) 13.9 Hz, 2-CH2), 2.96 (m, 1H,
H-4), 2.79 (d, 3H, J ) 4.9 Hz, CH3-NH), 2.60 (m, 1H, H-3), 2.19
(m, 1H, H-3), 1.40 (d, 3H, J ) 7.2 Hz, â-H, Ala). 1H NMR (DMSO-
d6): cis/trans isomers ratio: 1:2.25. Trans isomer: δ 8.23 (d, 1H,
J ) 7.4 Hz, NH-Ala), 7.81 (m, 1H, NH-CH3), 7.39-7.12 (m, 10H,
Ph), 5.24 (d, 1H, J ) 12.3 Hz, CH2-Z), 5.04 (d, 1H, J ) 12.3 Hz,
CH2-Z), 4.28 (m, 1H, R-H, Ala), 3.53 (m, 1H, H-4), 3.31 (d, 1H,
J ) 13.8 Hz, 2-CH2), 3.00 (d, 1H, J ) 13.8 Hz, 2-CH2), 2.82 (m,
1H, H-4), 2.61 (d, 3H, J ) 3.8 Hz, CH3-NH), 2.32 (m, 1H, H-3),
2.10 (m, 1H, H-3), 1.23 (d, 3H, J ) 6.5 Hz, â-H, Ala); cis isomer:
δ 7.93 (d, 1H, J ) 7.5 Hz, NH-Ala), 7.75 (m, 1H, NH-CH3), 7.39-
7.12 (m, 10H, Ph), 5.24 (m, 2H, CH2-Z), 4.28 (m, 1H, R-H, Ala),
3.53 (m, 1H, H-4), 3.13 (s, 2H, 2-CH2), 2.79 (m, 1H, H-4), 2.57
(d, 3H, J ) 3.8 Hz, CH3-NH), 2.32 (m, 1H, H-3), 2.07 (m, 1H,
H-3), 1.23 (d, 3H, J ) 6.5 Hz, â-H, Ala). 13C NMR (50 MHz,
CDCl3): δ 173.8 and 172.4 (CON), 156.3 (OCON), 136.0, 134.6
(C-Ph), 130.3, 128.5, 128.3, 127.1 (CH-Ph), 73.3 (2-C), 67.1 (CH2-
Z), 49.1 (R-C-Ala), 44.8 (4-C), 39.0 (2-CH2), 26.3 (CH3-NH), 23.6
(3-C), 17.3 (â-C, Ala). ES-MS: 410.2 [M + 1]+, 432.1 [M + Na]+.
Anal. Calcd for C23H27N3O4: C 67.46, H 6.65, N 10.26, Found: C
67.22, H 6.84, N 10.01.
2(S)-Z-Aze-L-Ala-NHMe (9a): White solid (mp 172-174 °C),
99% yield (from 7a, Method A). HPLC: tR ) 6.93 min (A:B )
15:85). [R]D -102.25 (c 0.40, CHCl3). 1H NMR (CDCl3): δ 7.26
(m, 6H, NH-Ala, Ph), 6.71 (m, 1H, NH-CH3), 5.07 (d, 1H, J )
12.3 Hz, CH2-Z), 5.01 (d, 1H, J ) 12.3 Hz, CH2-Z), 4.63 (t, 1H,
J ) 7.7 Hz, H-2), 4.41 (q, 1H, J ) 7.0 Hz, R-H, Ala), 3.85 (m,
2H, H-4), 2.65 (d, 3H, J ) 4.3 Hz, CH3-NH), 2.35 (m, 2H, H-3),
1.28 (d, 3H, J ) 7.0 Hz, â-H, Ala). 1H NMR (DMSO-d6): δ 8.09
(d, 1H, J ) 7.2 Hz, NH-Ala), 7.78 (m, 1H, NH-CH3), 7.32 (m,
5H, Ph), 5.01 (m, 2H, CH2-Z), 4.63 (dd, 1H, J ) 8.4, 5.2 Hz, H-2),
4.24 (q, 1H, J ) 7.2 Hz, R-H, Ala), 3.87 (m, 2H, H-4), 2.56 (d,
3H, J ) 4.6 Hz, CH3-NH), 2.40 (m, 1H, H-3), 2.04 (m, 1H, H-3),
1.15 (m, 3H, â-H, Ala). 13C NMR (CDCl3): δ 172.3 and 170.7
(CON), 157.3 (OCON), 135.7 (C-Ph), 128.4, 128.2, 127.9 (CH-
Ph), 67.2 (CH2-Z), 62.1 (2-C), 48.7 (R-C, Ala), 47.3 (4-C), 25.8
(CH3-NH), 20.2 (3-C), 18.0 (â-C, Ala). ES-MS: 320.0 [M + 1]+,
342.0 [M + Na]+. Anal. Calcd for C16H21N3O4: C 60.17, H 6.63,
N 13.16. Found: C 60.26, H 6.44, N 13.06.
2(R)-Z-BnAze-L-Ala-NHMe ((R)-9c): Foam, 12% yield (from
(R,S)-3c, Method B). Eluent: EtOAc:hexane (1:4). HPLC: tR
)
1
17.56 min (A:B ) 30:70). [R]D -40.2 (c 0.31, CHCl3). H NMR
(CDCl3): δ 8.22 (d, 1H, J ) 7.0 Hz, NH-Ala), 7.45-7.10 (m, 10H,
Ph), 6.31 (br s, 1H, NH-CH3), 5.33 (d, 1H, J ) 12.1 Hz, CH2-Z),
5.04 (d, 1H, J ) 12.1 Hz, CH2-Z), 4.43 (q, 1H, J ) 7.0 Hz, R-H,
Ala), 3.51 (d, 1H, J ) 13.9 Hz, 2-CH2), 3.49 (m, 1H, H-4), 2.94
(d, 1H, J ) 13.9 Hz, 2-CH2), 2.84 (m, 1H, H-4), 2.81 (d, 3H, J )
4.9 Hz, CH3-NH), 2.57 (m, 1H, H-3), 2.18 (m, 1H, H-3), 1.45 (d,
3H, J ) 7.0 Hz, â-H, Ala). 1H NMR (DMSO-d6): cis/trans isomers
ratio: 1:2.8. Trans isomer: δ 8.11 (d, 1H, J ) 7.4 Hz, NH-Ala),
7.90 (m, 1H, NH-CH3), 7.43-7.12 (m, 10H, Ph), 5.24 (d, 1H, J )
12.5 Hz, CH2-Z), 5.03 (d, 1H, J ) 12.5 Hz, CH2-Z), 4.29 (m, 1H,
R-H, Ala), 3.53 (m, 1H, H-4), 3.32 (d, 1H, J ) 13.7 Hz, 2-CH2),
2.99 (d, 1H, J ) 13.7 Hz, 2-CH2), 2.81 (m, 1H, H-4), 2.60 (d, 3H,
J ) 4.6 Hz, CH3-NH), 2.33 (m, 1H, H-3), 2.06 (m, 1H, H-3), 1.25
(d, 3H, J ) 7.1 Hz, â-H, Ala); cis isomer: δ 8.22 (d, 1H, J ) 7.5
Hz, NH-Ala), 7.81 (m, 1H, NH-CH3), 7.43-7.12 (m, 10H, Ph),
5.16 (m, 2H, CH2-Z), 4.29 (m, 1H, R-H, Ala), 3.53 (m, 1H, H-4),
3.12 (m, 2H, 2-CH2), 2.73 (m, 1H, H-4), 2.60 (d, 3H, J ) 4.6 Hz,
CH3-NH), 2.19 (m, 1H, H-3), 2.06 (m, 1H, H-3), 1.15 (d, 3H, J )
7.1 Hz, â-H, Ala). 13C NMR (CDCl3): δ 174.3 and 172.4 (CON),
156.4 (OCON), 136.1, 134.7 (C-Ph), 130.4, 130.3, 128.6, 128.4,
127.1 (CH-Ph), 73.4 (2-C), 67.0 (CH2-Z), 49.1 (R-C, Ala), 44.8
(4-C), 39.0 (2-CH2), 26.3 (CH3-NH), 23.5 (3-C), 17.3 (â-C, Ala).
2(S)-Z-MeAze-L-Ala-NHMe (9b): Syrup, 14% yield (from
(R,S)-7b, Method A) and 90% yield (from 3b, Method B). Eluent:
MeOH:CH2Cl2 (1:60). HPLC: tR ) 16.03 min (cis isomer) and
18.75 min (trans isomer) (A:B ) 15:85). [R]D -113.13 (c 0.98,
1
CHCl3). H NMR (CDCl3): cis/trans isomers ratio: 1:5. Trans
isomer: δ 7.74 (m, 1H, NH-Ala), 7.30 (m, 5H, Ph), 6.38 (m, 1H,
NH-CH3), 5.08 (s, 2H, CH2-Z), 4.34 (q, 1H, J ) 7.1 Hz, R-H, Ala),
3.89 (m, 1H, H-4), 3.74 (m, 1H, H-4), 2.72 (d, 3H, J ) 3.8 Hz,
CH3-NH), 2.60 (m, 1H, H-3), 1.98 (m, 1H, H-3), 1.64 (s, 3H,
2-CH3), 1.30 (d, 3H, J ) 7.1 Hz, â-H, Ala). 1H NMR (DMSO-d6):
J. Org. Chem, Vol. 73, No. 5, 2008 1713

Baeza et al.
ES-MS: 410.2 [M + 1]+, 432.1 [M + Na]+. Anal. Calcd for
C23H27N3O4: C 67.46, H 6.65, N 10.26, Found: C 67.25, H 6.72,
N 10.10.
NH), 2.07-1.87 (m, 4H, â-H, Pro, γ-H, Pro), 1.22 (br s, 3H, â-H,
Ala), 1.16 (s, 9H, CH3-tBu). 13C NMR (CDCl3): δ 178.0 (CO-t-
Bu), 172.6 and 172.0 (CON), 62.4 (R-C, Pro), 48.7 (R-C, Ala),
48.6 (δ-C, Pro), 39.0 (C-tBu), 27.6 (â-C, Pro), 27.4 (CH3-tBu), 26.1
(CH3-NH), 25.9 (γ-C, Pro), 17.9 (â-C, Ala). ES-MS: 284.0 [M +
1]+, 306.0 [M + Na]+, 589.3 [2M + Na]+. Anal. Calcd for
C14H25N3O3: C 59.34, H 8.89, N 14.83. Found: C 59.56, H 8.62,
N 14.77.
Ac-L-Pro-L-Ala-NHMe (12a):43 Syrup, 11% yield (from 8a,
Method C). Eluent: MeOH:CH2Cl2 (1:10). [R]D -60.71 (c 0.54,
1
CHCl3). H NMR (CDCl3): cis/trans isomers ratio: 1:6. Trans
isomer: δ 7.09 (d, 1H, J ) 7.1 Hz, NH-Ala), 6.53 (br s, 1H, NH-
CH3), 4.48 (m, 1H, R-H, Pro), 4.37 (q, 1H, J ) 7.1 Hz, R-H, Ala),
3.60 (m, 1H, δ-H, Pro), 3.48 (m, 1H, δ-H, Pro), 2.78 (d, 3H, J )
4.8 Hz, CH3-NH), 2.28 (m, 1H, â-H, Pro), 2.13 (s, 3H, CH3-CO),
2.03 (m, 3H, â-H, Pro, γ-H, Pro), 1.36 (d, 3H, J ) 7.1 Hz, â-H,
Ala); cis isomer: δ 6.75 (d, 1H, J ) 7.1 Hz, NH-Ala), 6.16 (br s,
1H, NH-CH3), 4.48 (m, 1H, R-H, Pro), 4.37 (m, 1H, R-H, Ala),
3.60 (m, 1H, δ-H, Pro), 3.48 (m, 1H, δ-H, Pro), 2.78 (d, 3H, J )
4.8 Hz, CH3-NH), 2.28 (m, 1H, â-H, Pro), 2.13 (s, 3H, CH3-CO),
2.03 (m, 3H, â-H, Pro, γ-H, Pro), 1.36 (d, 3H, J ) 7.1 Hz, â-H,
Piv-L-R-MePro-L-Ala-NHMe (13b): Amorphous solid, 63%
yield (from 8b, Method C). Eluent: MeOH:CH2Cl2 (1:20). [R]D
1
-2.72 (c 1, CHCl3). H NMR (CDCl3): δ 7.22 (br s, 1H, NH-
CH3), 5.71 (d, 1H, J ) 7.5 Hz, NH-Ala), 4.45 (q, 1H, J ) 7.5 Hz,
R-H, Ala), 3.78 (m, 2H, δ-H, Pro), 2.75 (d, 3H, J ) 4.7 Hz, CH3-
NH), 2.06-1.80 (m, 4H, â-H, Pro, γ-H, Pro), 1.56 (s, 3H, R-CH3,
Pro), 1.36 (d, 3H, J ) 7.5 Hz, â-H, Ala) 1.26 (s, 9H, CH3 Bu). 1H
t
NMR (DMSO-d6): δ 7.46 (m, 1H, NH-CH3), 7.28 (m, 1H, J )
7.3 Hz, NH-Ala), 4.17 (q, 1H, J ) 7.3 Hz, R-H, Ala), 3.86 (m,
1H, δ-H, Pro), 3.75 (m, 1H, δ-H, Pro), 2.54 (d, 3H, J ) 4.6 Hz,
CH3-NH), 1.96 (m, 2H, γ-H, Pro), 1.86 (m, 1H, â-H, Pro), 1.72
(m, 1H, â-H, Pro), 1.32 (s, 3H, R-CH3, Pro), 1.21 (d, 3H, J ) 7.3
1
Ala). H NMR (DMSO-d6): cis/trans isomers ratio: 1:2.4. Trans
isomer: δ 7.88 (d, 1H, J ) 7.5 Hz, NH-Ala), 7.56 (m, 1H, NH-
CH3), 4.22 (m, 1H, R-H, Pro), 4.15 (m, 1H, R-H, Ala), 3.48 (m,
2H, δ-H, Pro), 2.55 (d, 3H, J ) 4.6 Hz, CH3-NH), 2.03 (m, 2H,
â-H, Pro), 1.98 (s, 3H, CH3-CO), 1.83 (m, 2H, γ-H, Pro), 1.18 (d,
3H, J ) 7.5 Hz, â-H, Ala); cis isomer: δ 8.16 (d, 1H, J ) 7.5 Hz,
NH-Ala), 7.82 (m, 1H, NH-CH3), 4.35 (m, 1H, R-H, Pro), 4.28
(m, 1H, R-H, Ala), 3.48 (m, 2H, δ-H, Pro), 2.55 (d, 3H, J ) 4.6
Hz, CH3-NH), 2.15 (m, 2H, â-H, Pro), 1.98 (s, 3H, CH3-CO), 1.83
(m, 2H, γ-H, Pro), 1.18 (d, 3H, J ) 7.5 Hz, â-H, Ala). 13C NMR
(CDCl3): trans isomer: δ 172.5 and 171.30 (CON), 171.33 (CO-
CH3), 60.2 (R-C, Pro), 49.1 (R-C, Ala), 48.5 (δ-C, Pro), 28.1 (â-
C, Pro), 26.3 (CH3-NH), 25.0 (γ-C, Pro), 22.6 (CH3-CO), 17.6 (â-
C, Ala); cis isomer: δ 172.5 and 171.30 (CON), 171.33 (CO-CH3),
62.0 (R-C, Pro), 49.1 (R-C, Ala), 46.8 (δ-C, Pro), 29.7 (â-C, Pro),
26.3 (CH3-NH), 25.0 (γ-C, Pro), 22.9 (CH3-CO), 18.4 (â-C, Ala).
ES-MS: 242.1 [M + 1]+, 264.0 [M + Na]+. Anal. Calcd for
C11H19N3O3: C 54.76, H 7.94, N 17.41, Found: C 54.72, H 7.92,
N 17.36.
t
Hz, â-H, Ala), 1.18 (s, 9H, CH3 Bu). 13C NMR (CDCl3): δ 177.4
(CO-tBu), 173.6 and 172.7 (CON), 69.0 (R-C, Pro), 49.0 (δ-C, Pro),
48.9 (R-C, Ala), 39.4 (C-tBu), 37.8 (â-C, Pro), 27.0 (CH3-tBu),
26.5 (CH3-NH), 24.8 (γ-C, Pro), 20.8 (R-CH3, Pro), 17.9 (â-C, Ala).
ES-MS: 298.0 [M + 1]+, 320.0 [M + Na]+, 617.3 [2M + Na]+.
Anal. Calcd for C15H27N3O3: C 60.58, H 9.15, N 14.13. Found: C
60.87, H 9.03, N 14.40.
2(S)-Ac-Aze-L-Ala-NHMe (14a): Solid (mp 152-154 °C), 64%
yield (from 10a, Method A). Eluent: MeOH:CH2Cl2 (1:10)
(centrifugal circular thin-layer chromatography). [R]D -197.35 (c
1
1, CHCl3). H NMR (CDCl3): δ 7.91 (d, 1H, J ) 7.1 Hz, NH-
Ala), 6.79 (m, 1H, NH-CH3), 4.79 (t, 1H, J ) 7.3 Hz, H-2), 4.36
(q, 1H, J ) 7.1 Hz, R-H, Ala), 4.04 (t, 2H, J ) 7.4 Hz, H-4), 2.71
(d, 3H, J ) 4.6 Hz, CH3-NH), 2.50 (m, 1H, H-3), 2.42 (m, 1H,
H-3), 1.87 (s, 3H, CH3-CO), 1.31 (d, 3H, J ) 7.1 Hz, â-H, Ala).
1H NMR (DMSO-d6): cis/trans isomers ratio: 1:1.3. Trans
isomer: δ 8.13 (d, 1H, J ) 7.3 Hz, NH-Ala), 7.74 (m, 1H, NH-
CH3), 4.60 (dd, 1H, J ) 9.0, 5.9 Hz, H-2), 4.21 (q, 1H, J ) 7.3
Hz, R-H, Ala), 4.02 (t, 2H, J ) 7.7 Hz, H-4), 2.56 (m, 3H, CH3-
NH), 2.38 (m, 1H, H-3), 2.04 (m, 1H, H-3), 1.77 (s, 3H, CH3-
CO), 1.18 (d, 3H, J ) 7.3 Hz, â-H, Ala); cis isomer: δ 8.28 (d,
1H, J ) 7.3 Hz, NH-Ala), 7.86 (m, 1H, NH-CH3), 4.80 (dd, 1H, J
) 9.0, 5.1 Hz, H-2), 4.29 (q, 1H, J ) 7.3 Hz, R-H, Ala), 3.73 (t,
2H, J ) 7.6 Hz, H-4), 2.56 (m, 3H, CH3-NH), 2.38 (m, 1H, H-3),
2.04 (m, 1H, H-3), 1.62 (s, 3H, CH3-CO), 1.20 (d, 3H, J ) 7.3 Hz,
â-H, Ala). 13C NMR (CDCl3): trans isomer: δ 172.7 (CO-CH3),
172.5 and 170.5 (CON), 61.6 (2-C), 49.0 (R-C, Ala), 48.5 (4-C),
26.1 (CH3-NH), 18.8 (CH3-CO), 18.4 (3-C), 17.6 (â-C, Ala); cis
isomer: δ 172.7 (CO-CH3), 172.5 and 171.0 (CON), 63.0 (2-C),
51.3 (R-C, Ala), 46.2 (4-C), 26.1 (CH3-NH), 18.8 (CH3-CO), 18.6
(3-C), 17.9 (â-C, Ala). ES-MS: 228.1 [M + 1]+, 250.1 [M + Na]+,
477.2 [2M + Na]+. Anal. Calcd for C10H17N3O3: C 52.85, H 7.54,
N 18.49. Found: C 52.94, H 7.61, N 18.37.
Ac-L-R-MePro-L-Ala-NHMe (12b): Syrup, 32% yield (from 8b,
Method C). Eluent: MeOH:CH2Cl2 (1:15). [R]D -48.30 (c 1.37,
1
CHCl3). H NMR (CDCl3): δ 7.04 (br s, 1H, NH-CH3), 6.44 (m,
1H, NH-Ala), 4.37 (q, 1H, J ) 7.5 Hz, R-H, Ala), 3.65 (m, 2H,
δ-H, Pro), 2.77 (d, 3H, J ) 4.6 Hz, CH3-NH), 2.28 (m, 1H, â-H,
Pro), 2.13 (s, 3H, CH3-CO), 2.09-1.88 (m, 3H, â-H, Pro, γ-H,
Pro), 1.60 (s, 3H, R-CH3, Pro), 1.39 (d, 3H, J ) 7.5 Hz, â-H, Ala).
1H NMR (DMSO-d6): δ 7.76 (d, 1H, J ) 7.5 Hz, NH-Ala), 7.50
(m, 1H, NH-CH3), 4.16 (q, 1H, J ) 7.5 Hz, R-H, Ala), 3.67 (m,
1H, δ-H, Pro), 3.57 (m, 1H, δ-H, Pro), 2.54 (d, 3H, J ) 4.5 Hz,
CH3-NH), 1.99 (s, 3H, CH3-CO), 1.97-1.78 (m, 4H, â-H, Pro, γ-H,
Pro), 1.35 (s, 3H, R-CH3, Pro), 1.22 (d, 3H, J ) 7.5 Hz, â-H, Ala).
13C NMR (CDCl3): δ 173.4 and 172.8 (CON), 170.5 (CO-CH3),
67.4 (R-C, Pro), 49.4 (δ-C, Pro), 49.2 (R-C, Ala), 39.1 (â-C, Pro),
26.2 (CH3-NH), 23.8 (CH3-CO), 23.5 (γ-C, Pro), 21.1 (R-CH3, Pro),
17.7 (â-C, Ala). ES-MS: 256.0 [M + 1]+, 278.0 [M + Na]+, 533.3
[2M + Na]+. Anal. Calcd for C12H21N3O3: C 56.45, H 8.29, N
16.46. Found: C 56.56, H 8.25, N 16.57.
Piv-L-Pro-L-Ala-NHMe (13a):44 White solid (mp 85-87 °C),
55% yield (from 8a, Method C). Eluent: MeOH:CH2Cl2 (1:20).
2(S)-Piv-Aze-L-Ala-NHMe (15a): White solid (mp 125-127
°C), 93% yield (from 11a, Method A). Eluent: MeOH:CH2Cl2 (1:
1
1
20). [R]D -153.25 (c 1, CHCl3). H NMR (CDCl3): δ 7.60 (m,
[R]D -56.45 (c 0.86, CHCl3). H NMR (CDCl3): δ 6.90 (m, 1H,
1H, NH-Ala), 6.80 (m, 1H, NH-CH3), 4.78 (m, 1H, H-2), 4.36 (q,
1H, J ) 7.1 Hz, R-H, Ala), 4.28 (t, 1H, J ) 7.8 Hz, H-4), 2.81 (d,
3H, J ) 4.8 Hz, CH3-NH), 2.55 (m, 2H, H-3), 1.40 (d, 3H, J )
7.1 Hz, â-H, Ala), 1.24 (s, 9H, CH3-tBu). 1H NMR (DMSO-d6): δ
7.99 (m, 1H, NH-Ala), 7.75 (m, 1H, NH-CH3), 4.59 (m, 1H, H-2),
4.24 (m, 3H, R-H, Ala, H-4), 2.56 (d, 3H, J ) 4.6 Hz, CH3-NH),
2.38 (m, 1H, H-3), 2.05 (m, 1H, H-3), 1.18 (d, 3H, J ) 7.1 Hz,
â-H, Ala), 1.08 (s, 9H, CH3-tBu). 13C NMR (CDCl3): δ 179.8 (CO-
tBu), 172.4 and 170.8 (CON), 62.4 (2-C), 51.8 (4-C), 48.5 (R-C,
Ala), 38.6 (C-tBu), 29.5 (CH3-NH), 26.9 (CH3-tBu), 19.3 (3-C),
17.6 (â-C, Ala). ES-MS: 270.0 [M + 1]+, 292.0 [M + Na]+, 561.3
[2M + Na]+. Anal. Calcd for C13H23N3O3: C 57.97, H 8.61, N
15.60. Found: C 58.04, H 8.91, N 15.87.
NH-CH3), 6.67 (d, 1H, J ) 7.3 Hz, NH-Ala), 4.45 (m, 1H, R-H,
Pro), 4.37 (q, 1H, J ) 7.3 Hz, R-H, Ala), 3.71 (m, 2H, δ-H, Pro),
2.71 (d, 3H, J ) 4.7 Hz, CH3-NH), 2.07-1.87 (m, 4H, â-H, Pro,
γ-H, Pro), 1.30 (d, 3H, J ) 7.3 Hz, â-H, Ala), 1.24 (s, 9H, CH3-
tBu). 1H NMR (DMSO-d6): δ 7.72 (br s, 1H, NH-Ala), 7.64 (br s,
1H, NH-CH3), 4.30 (m, 1H, R-H, Pro), 4.13 (q, 1H, J ) 7.2 Hz,
R-H, Ala), 3.63 (m, 2H, δ-H, Pro), 2.55 (d, 3H, J ) 4.6 Hz, CH3-
(43) Partial characterization details can be found in: Liang, G.-B.; Rito,
C. J.; Gellman, S. H. J. Am. Chem. Soc. 1992, 114, 4440-4442.
(44) Partial characterization details can be found in: (a) Aubry, A.; Cung,
M. T.; Maraud, M. J. Am. Chem. Soc. 1985, 107, 7640-7647. (b) Boussard,
G.; Maraud, M. Biopolymers 1979, 18, 1297-1331.
1714 J. Org. Chem., Vol. 73, No. 5, 2008

AlternatiVe Trends in ReVerse Turn Induction
2(S)-Piv-MeAze-L-Ala-NHMe ((S)-15b): Syrup, 63% yield
(from 9b, Method C). Eluent: MeOH:CH2Cl2 (1:30). [R]D -150.68
(DMSO-d6): δ 8.35 (d, 1H, J ) 7.2 Hz, NH-Ala), 7.78 (m, 1H,
NH-CH3), 7.34-7.20 (m, 5H, Ph), 4.27 (q, 1H, J ) 7.2 Hz, R-H,
Ala), 4.04 (m, 1H, H-4), 3.46 (d, 1H, J ) 13.3 Hz, 2-CH2), 3.24
(m, 1H, H-4), 2.97 (d, 1H, J ) 13.3 Hz, 2-CH2), 2.60 (d, 3H, J )
4.7 Hz, CH3-NH), 2.34 (m, 1H, H-3), 2.10 (m, 1H, H-3), 1.23 (d,
3H, J ) 7.2 Hz, â-H, Ala), 1.06 (s, 9H, CH3-tBu). 13C NMR
(CDCl3): δ 179.7 (CO-tBu), 173.9 and 172.5 (CON), 135.1 (C-
Ph), 130.6, 130.0, 128.3 (CH-Ph), 75.5 (2-C), 49.6 (4-C), 49.1 (R-
C, Ala), 39.1 (C-tBu), 38.7 (2-CH2), 26.9 (CH3-tBu), 26.3 (CH3-
NH), 23.5 (3-C), 17.2 (â-C, Ala). ES-MS: 382.0 [M + Na]+. Anal.
Calcd for C20H29N3O3: C 66.83, H 8.13, N 11.69. Found: C 66.76,
H 8.01, N 11.59.
Molecular Modeling Studies. All calculations were run on an
SGI workstation (Fuel, RP14000, 500 MB RAM) under an Irix
6.5 operating system. The initial conformation was built using the
library of fragments available in the molecular modeling program
Insight II (version 2000.1, Biosym Technologies, San Diego, CA).
The calculations were carried out within the molecular mechanics
using the AMBER force field implemented in DISCOVER. They
were conducted under vacuum with a distance dependent dielectric
constant (4r) and a cutoff of 16 Å. The structure is heated to 1000
K and equilibrated during 100 ps. The structure is then cooled
slowly to 300 K in steps; in each step the temperature was lowered
by 100 deg, and the system was allowed to stay at the new
temperature for 100 ps. After cooling to 300 K, the final conforma-
tion obtained is energy-refined using a conjugated gradient algo-
rithm with a final gradient of 0.001 kcal/mol as the convergence
criteria. The conformer is stored and used to start a new simulation
at high temperature. This procedure produced samples of 100
energy-minimized conformations, which were compared to each
other to eliminate the identical ones. The protocol was run three
times, employing different starting structures. Independently of the
starting structure, the percentage of the different families of
conformers was equivalent.
1
(c 0.52, CHCl3). H NMR (CDCl3): δ 8.18 (d, 1H, J ) 7.3 Hz,
NH-Ala), 6.55 (br s, 1H, NH-CH3), 4.40 (q, 1H, J ) 7.3 Hz, R-H,
Ala), 4.28 (m, 2H, H-4), 2.77 (d, 3H, J ) 4.9 Hz, CH3-NH), 2.07
(m, 2H, H-3), 1.74 (s, 3H, 2-CH3), 1.37 (d, 3H, J ) 7.3 Hz, â-H,
Ala), 1.20 (s, 9H, CH3-tBu). 1H NMR (DMSO-d6): δ 8.07 (d, 1H,
J ) 7.2 Hz, NH-Ala), 7.71 (m, 1H, NH-CH3), 4.29 (m, 2H, H-4),
4.21 (q, 1H, J ) 7.2 Hz, R-H, Ala), 2.57 (d, 3H, J ) 4.6 Hz, CH3-
NH), 2.40 (m, 1H, H-3), 1.90 (m, 1H, H-3), 1.55 (s, 3H, 2-CH3),
1.21 (d, 3H, J ) 7.2 Hz, â-H, Ala), 1.12 (s, 9H, CH3-tBu). 13C
NMR (CDCl3): δ 179.6 (CO-tBu), 174.1 and 172.6 (CON), 72.0
(2-C), 49.4 (4-C), 49.1 (R-C, Ala), 38.9 (C-tBu), 27.7 (3-C), 26.7
(CH3-tBu), 26.3 (CH3-NH), 22.5 (2-CH3), 17.4 (â-C, Ala). ES-
MS: 284.0 [M + 1]+, 306.0 [M + Na]+, 589.3 [2M + Na]+. Anal.
Calcd for C14H25N3O3: C 59.34, H 8.89, N 14.83. Found: C 59.17,
H 8.91, N 14.71.
2(R,S)-Piv-MeAze-L-Ala-NHMe ((R,S)-15b): Syrup, 88% yield
(from (R,S)-4b, Method B). Eluent: MeOH:CH2Cl2 (1:30). 1H NMR
(CDCl3): diastereoisomers ratio: 1.2:1. Major diastereoisomer: (S)-
15b (described above); minor diastereoisomer: δ 7.88 (d, 1H, J )
7.3 Hz, NH-Ala), 6.72 (br s, 1H, NH-CH3), 4.40 (q, 1H, J ) 7.3
Hz, R-H, Ala), 4.30 (m, 2H, H-4), 2.77 (d, 3H, J ) 4.9 Hz, CH3-
NH), 2.04 (m, 2H, H-3), 1.74 (s, 3H, 2-CH3), 1.40 (d, 3H, J ) 7.3
Hz, â-H, Ala), 1.19 (s, 9H, CH3-tBu). 1H NMR (DMSO-d6): Major
diastereoisomer: (S)-15b (described above); minor diastereoiso-
mer: δ 7.95 (d, 1H, J ) 7.2 Hz, NH-Ala), 7.82 (m, 1H, NH-CH3),
4.29 (m, 2H, H-4), 4.21 (q, 1H, J ) 7.2 Hz, R-H, Ala), 2.57 (d,
3H, J ) 4.6 Hz, CH3-NH), 2.40 (m, 1H, H-3), 1.90 (m, 1H, H-3),
1.60 (s, 3H, 2-CH3), 1.21 (d, 3H, J ) 7.2 Hz, â-H, Ala), 1.12 (s,
9H, CH3-tBu). 13C NMR (CDCl3): major diastereoisomer: (S)-
15b (described above); minor diastereoisomer: δ 179.5 (CO-tBu),
173.6 and 172.6 (CON), 71.4 (2-C), 49.8 (4-C), 49.3 (R-C, Ala),
38.8 (C-tBu), 27.4 (3-C), 26.7 (CH3-tBu), 26.3 (CH3-NH), 22.4 (2-
CH3), 17.3 (â-C, Ala). ES-MS: 284.0 [M + 1]+, 306.0 [M + Na]+,
589.3 [2M + Na]+. Anal. Calcd for C14H25N3O3: C 59.34, H 8.89,
N 14.83. Found: C 59.03, H 9.01, N 14.59.
Acknowledgment. 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.
2(S)-Piv-BnAze-L-Ala-NHMe (15c): Syrup, 90% yield (from
9a, Method C). Eluent: MeOH:CH2Cl2 (1:30). HPLC: tR ) 3.77
min (A:B ) 40:60). [R]D -5.17 (c 0.97, CHCl3). 1H NMR
(CDCl3): δ 8.55 (d, 1H, J ) 7.2 Hz, NH-Ala), 7.35-7.20 (m, 5H,
Ph), 6.49 (br s, 1H, NH-CH3), 4.41 (q, 1H, J ) 7.2 Hz, R-H, Ala),
3.95 (ddd, 1H, J ) 9.8, 8.3, 5.3 Hz, H-4), 3.68 (d, 1H, J ) 13.9
Hz, 2-CH2), 3.38 (m, 1H, H-4), 3.04 (d, 1H, J ) 13.9 Hz, 2-CH2),
2.77 (d, 3H, J ) 4.9 Hz, CH3-NH), 2.66 (ddd, 1H, J ) 12.1, 9.8,
7.2 Hz, H-3), 2.26 (ddd, 1H, J ) 12.1, 9.4, 5.3 Hz, H-3), 1.39 (d,
3H, J ) 7.2 Hz, â-H, Ala), 1.13 (s, 9H, CH3-tBu). 1H NMR
Supporting Information Available: Experimental procedures
and analytical and spectroscopic data of compounds 1, 3-4, 28,
16-23, 25, and 26, relevant conformational parameters of dipeptide
derivatives 12a, 12b, 14a, and 14b, and NMR spectra. This material
is available free of charge via the Internet at http://pubs.acs.org.
JO701746W
J. Org. Chem, Vol. 73, No. 5, 2008 1715