Chirality-driven folding of short β-lactam pseudopeptides

Article abstract of DOI:10.1021/jo302368y

Novel enantiopure pseudopeptide models containing a central -(β-lactam)-(Aa)- scaffold characterized by the combined presence of an α-alkyl-α-amino-β-lactam (i+1) residue and a α-substituted (i + 2) amino acid have been readily synthesized from α-alkyl serines. The conformational analysis of such β-lactam pseudopeptides conducted in CDCl₃ and DMSO-d₆ solutions using 1D- and 2D-NMR techniques revealed an equilibrium between β-II turn and γ-turn conformers, which was ultimately modulated by the relative configuration of the -(β-lactam)-(Aa)- residues. Long-range chiral effects on the α-lactam pseudopeptide conformers were also found when two (i) and (i + 3) chiral residues were attached to the termini of a central -(β-lactam)-(Aib)- segment. In such mimetics, heterochiral (i) and (i + 3) residues reinforced a β-II turn conformer, whereas homochiral corner residues stabilized an overlapped β-II/ β-I double turn motif. No β-hairpin nucleation was observed in any instance. In good agreement with the conformers found in solution, β-turned and open structures were also characterized by X-ray crystallography. Relative stabilities of the different conformers were estimated computationally at a B3LYP/6-31++G* calculation level, and finally, a conformation equilibrium model based on steric inter-residual interactions around the -(β-lactam)-(i + 2)- segment was proposed to account for the observed chiral effects.

Full text of DOI:10.1021/jo302368y

Featured Article
pubs.acs.org/joc
Chirality-Driven Folding of Short β‑Lactam Pseudopeptides
Jesus M. Aizpurua,*,‡ Claudio Palomo,*,† Eva Balentova,‡ Azucena Jimenez,‡ Elena Andreieff,‡
́
Maialen Sagartzazu-Aizpurua,‡ Jose Ignacio Miranda,‡ and Anthony Linden§
́
†Departamento de Química Organ
́
ica-I, Joxe Mari Korta R&D Center, Universidad del País Vasco UPV/EHU, Avda Tolosa-72,
20018 San Sebastian, Spain
‡Departamento de Química Organ
20018 San Sebastian, Spain
́
ica-I, Facultad de Química, Universidad del País Vasco UPV/EHU, Paseo Manuel Lardizabal-3,
§Institute of Organic Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
̈
̈
S
* Supporting Information
ABSTRACT: Novel enantiopure pseudopeptide models
containing a central -(β-lactam)-(Aa)- scaffold characterized
by the combined presence of an α-alkyl-α-amino-β-lactam (i
+1) residue and a α-substituted (i + 2) amino acid have been
readily synthesized from α-alkyl serines. The conformational
analysis of such β-lactam pseudopeptides conducted in CDCl3
and DMSO-d6 solutions using 1D- and 2D-NMR techniques
revealed an equilibrium between β-II turn and γ-turn
conformers, which was ultimately modulated by the relative configuration of the -(β-lactam)-(Aa)- residues. Long-range chiral
effects on the α-lactam pseudopeptide conformers were also found when two (i) and (i + 3) chiral residues were attached to the
termini of a central -(β-lactam)-(Aib)- segment. In such mimetics, heterochiral (i) and (i + 3) residues reinforced a β-II turn
conformer, whereas homochiral corner residues stabilized an overlapped β-II/ β-I double turn motif. No β-hairpin nucleation was
observed in any instance. In good agreement with the conformers found in solution, β-turned and open structures were also
characterized by X-ray crystallography. Relative stabilities of the different conformers were estimated computationally at a
B3LYP/6-31++G** calculation level, and finally, a conformation equilibrium model based on steric inter-residual interactions
around the -(β-lactam)-(i + 2)- segment was proposed to account for the observed chiral effects.
peptides9 were only of limited success because of their inherent
INTRODUCTION
■
ring-puckering. This makes it very difficult to interpret the
stereoelectronic interactions arising from the chirality effect of
residues neighboring the lactam moiety. Hence, a scaffold size
reduction strategy intended to design “minimal” short peptide
models becomes particularly attractive in terms of design
simplicity and folding interpretation.
Chirality is at the origin of the regular folding patterns
occurring in proteins and natural peptides. It is well-known that
short “all-L” peptides may experience dramatic changes in their
backbone conformation due to the replacement of a single
residue by an achiral unit or a D-configuration amino acid.1 This
phenomenon was first found in natural peptides and is now
widely used as a strategy to design peptidomimetics stabilizing
reverse turn motifs such as β-turns2 1 and γ-turns3 2 (Figure 1,
top). Indeed, changing the configuration of some backbone
residues in functional short peptides may result in increased
potency, altered selectivity,4 or improved proteolytic stability.
Furthermore, these effects can become amplified when
conformationally constrained lactam scaffolds 3 incorporating
interresidual bridges within contiguous CαH and NH positions
(Freidinger lactams)5 are embedded in the native peptide
backbone (Figure 1, bottom). Formation of more complex
structures, such as the doubly hydrogen-bonded β-hairpins 3
(antiparallel β-sheet nucleators), is also strongly dependent on
the configurations of the terminal residues (i) and (i + 3).6
Unfortunately, most of the (i + 1)-(i + 2) dipeptide lactam
surrogates used routinely as turn mimetics7 possess compara-
tively large external fragments, and their restraint elements
(cycles) and recognition groups (R1 and R2) are often crammed
in the scaffold.8 Alternatively, the studies carried out on proline
In our first accomplishment in this area,10 we introduced the
β-lactam peptides 4 containing a central α-alkyl-α-amino-β-
lactam ring placed as the (i+1) residue.11 This approach,
termed “β-lactam scaffold-assisted design” (β-LSAD), involved
the formal insertion of a single carbon atom (Cα-H + H-N →
Cα-CH2-N) in the native peptide to form “minimal” β-lactam
pseudopeptides 4, which displayed a set of interesting structural
features. First, the linear disposition of the Cα, N and Cα′
atoms within the β-lactam ring confers to peptidomimetics 4 a
high potential for β-hairpin nucleation. Second, they possess a
ψ(i+1) dihedral angle fixed at 120° by the β-lactam ring, which
prevents the formation of type-I and/or type-III β-turns (both
with ψ(i+1) near −30°) and favors only the β-II-turn/γ-turn
conformational equilibrium.12 Indeed, a preliminary conforma-
tional study of β-LSAD pseudopeptides containing a glycine (i
Received: October 28, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/jo302368y | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Featured Article
Figure 1. (Top) Peptides folded into β-turn (1) and γ-turn (2) motifs.
(Bottom) Freidinger’s lactam peptidomimetics (3) and β-LSAD
concept (β-lactam scaffold-assisted design) to design minimally
scaffolded β-lactam pseudopeptides 4, differing from the parent
peptides in a single carbon atom.
Figure 2. (Top) β-Lactam pseudopeptide models 5−7 selected to
study the chiral effect of residues (i), (i + 2), and (i + 3) on the
peptide folding. (Bottom) Synthesis of the β-lactam dipeptide scaffolds
8 and 9.
+ 2) residue (4; R2 = H) conducted in our laboratory
confirmed their strong bias toward forming type-II β-turns
which survive even in the presence of the highly coordinating
solvent DMSO.10a Finally, α-alkyl-α-amino-β-lactam peptides
are expected to be chemically much more stable than common
α-amino-β-lactam antibiotics, which undergo easy ring-opening
to β-amino acid derivatives. This transformation usually
involves an AdN nucleophilic attack onto the CO sp2 carbon
sented the smallest capped β-lactam dipeptide structures
containing three peripheral methyl groups, whereas mimetics
6 [Boc-(α-Bn-β-lactam)-Aa-Aib-OH] had a larger central
benzyl group and bulkier achiral N- and C-terminal groups
(Boc and Aib-OH). Finally, pseudopeptides 7 [Boc-Ala*-(α-
Bn-β-lactam)-Aib-Ala*-NH2] were designed to ascertain the
chiral effect of two additional stereocenters at the (i) and (i +
3) alanine residues on the possible β-hairpin nucleation.
The enantiopure scaffolds 8 and 9 required to form the
central -(i + 1)-(i + 2)- pseudopeptide segments were easily
obtainable from the corresponding α-alkylserinates17 10 and 11
according to an operationally very simple method developed in
our laboratory.18 Tables 1 and 2 show the details of the
transformation of the dipeptide scaffolds 8, 9, and 14 into the
target β-lactam pseudopeptide models.
As shown in Table 1, the synthesis of pseudopeptides 5 and 6
required the coupling of the C-terminal carboxylic acid group in
8 and 9 with methylamine and α-aminoisobutiric benzyl ester,
respectively. Formation of N-methylamides 12a−e from
nonhindered β-lactam carboxylic acid precursors was best
performed using the mixed anhydride method assisted by
isobutyl chloroformate and N-methylmorpholine, whereas
couplings involving hindered Aib residues were most efficiently
accomplished with EEDQ19 reagent (products 13, 15, 16, and
7a−d). Denosylation of compounds 12 and 13 proceeded
smoothly with thiophenol20 in the presence of potassium
carbonate to afford the intermediate α-amino-β-lactams, which
were trapped in situ with acetic anhydride or di-tert-butyl
dicarbonate to provide the desired mimetic models 5 or the
analogues 6 after hydrogenolytic deprotection of the C-terminal
benzyl ester. Stereochemical integrity was maintained in all
transformations and no epimerization of the α′ stereocenter
could be detected via NMR and HPLC analysis of the reaction
crudes. Following a similar protocol (Table 2), mimetics 7 were
obtained in good overall yields from the α′,α′-dimethyl-β-
lactam 14 after consecutive coupling with N-protected alanines
atom following the Burgi−Dunitz trajectory.13 In β-lactams 4,
̈
such a trajectory is hindered by the α-substituents at both sides
of the 2-azetidinone plane, thus making the ring-opening
reaction more difficult with respect to other α-monosubstituted
β-lactam analogues.14 Recently, the surprisingly high “amidic-
ity” of monocyclic β-lactams has also been invoked to account
for their chemical stability.15
As mentioned above, only the solution conformation of short
β-lactam pseudopeptides containing a central -(β-lactam)-
(Gly)- core has been studied so far, and the possible β-turn
stabilizing (or destabilizing) effect exerted by the chiral α-
amino acid residues neighboring the β-lactam nucleus remains
to be explored. Herein we report the synthesis and detailed
conformational analysis of three families of β-lactam
pseudopeptide models designed to show the chiral effects of
residues (i), (i + 2) and (i + 3) on the folding of short β-lactam
peptides 4 in solution and in the solid state. Finally, we further
examine the ability of this motif to nucleate a β-hairpin, the
shortest β-sheet structure.
RESULTS AND DISCUSSION
■
Design and Synthesis of β-Lactam Pseudopeptide
Models. Short pseudopeptide models 5−7 (Figure 2) with a
homogeneous (R) configuration at the Cα stereocenter of the
β-lactam ring were selected for development because they are
reminiscent of constrained natural L α-amino acid residues.16
Configurationally related β-lactam peptides 5 and 6 containing
a single additional stereocenter at residue (i + 2) were designed
to carry terminal groups with different steric demands.
Accordingly, mimetics 5 [Ac-(α-Me-β-lactam)-Aa-Me] repre-
B
dx.doi.org/10.1021/jo302368y | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Featured Article
Table 1. Preparation of β-Lactam Peptidomimetics 5 and 6
R1
R2
config Cα′
product
yield (%)
product
yield (%)
H
H
12a
12b
12c
12d
12e
12f
80
58
59
78
85
5a
5b
5c
5d
5e
5f
84
72
81
89
94
82
79
80
82
80
92
Me
H
H
R
S
Me
H
Ph
H
R
S
Ph
Me
H
a
Me
Me
H
75
R
S
13b
13c
13d
13e
79
70
55
66
6b
6c
6d
6e
Me
H
Ph
H
R
S
Ph
Me
b
Me
6f
a
b
EEDQ reagent was used as the amide formation reagent. The product was prepared from acid Boc-(β-lactam)-Aib-OH, which was coupled to H-
Aib-OBn and submitted to hydrogenolysis.
Table 2. Preparation of β-Lactam Peptidomimetics 7
a
R1
R2
config Cα″
R3
R4
config Cα″′
product
yield (%)
H
Me
Me
H
R
R
S
H
Me
H
R
S
7a
7b
7c
7d
80
65
73
77
H
Me
Me
H
Me
Me
H
S
H
S
Me
R
a
Overall yield from 15 and 16.
17 and alaninamides 18 using, respectively, the EEDQ and
HATU/HOAT21 reagents.
unambiguously assigned from DQF-COSY and HMBC spectra
(Figures S1−S10, Supporting Information). Chemical shift
variations upon solvent change and 1H NMR thermal
coefficients22 for the exchangeable amide protons of the
pseudopeptides in DMSO-d6 solutions were determined over
a temperature range of 300−325 K at 5 K intervals (Table 3).
An inspection of the data recorded immediately revealed a
totally different deshielding for the β-lactam N2H amide
protons and the N7H protons of the (i + 3) residue, suggesting
the formation of a turned conformer in most of the cases with
the N7H amide protons participating in the intramolecular
hydrogen bond network of the β-lactam peptides and the N2H
protons exposed to the solvent. This was evident from the
analysis of thermal coefficients in DMSO-d6 (see Table 3, two
right-hand columns) showing in the range −2.3 to −4.9 ppb/K
for N7H protons, whereas amides and carbamates N2H gave
larger absolute thermal coefficient values (up to −8.9 ppb/K).
Consistently, the solvent change from nonacceptor CDCl3 to
acceptor DMSO-d6 resulted in a very small chemical shift
variation for N7H amides in all peptides (0.1−1.1 ppm) but in a
significantly larger downfield shift (2.3 − 2.8 ppm) in N2H β-
lactam amides. While seeking more insight into the conforma-
Solution Conformation of β-Lactam Peptides 5 and 6
with a Chiral (i + 2) Residue. With the model β-lactam
peptides in hand, we first addressed the NMR conformational
analysis of the simplest families 5 [Ac-(α-Me-β-lactam)-Aa-Me]
and 6 [Boc-(α-Bn-β-lactam)-Aa-Aib-OH] in order to character-
ize the presumably more populated β-II turn conformers in
solution and to assess the possible stabilizing or destabilizing
effects exerted by the chiral Aa residues on the conformer
equilibrium. Although water was considered first as the most
biologically meaningful medium for the study, finally DMSO-d6
solvent was chosen for practical reasons to ensure solubility. In
the case of peptides 5a−f, however, the recurrent overlapping
of NH amide signals in dimethylsulfoxide precluded a
straightforward interpretation of the results and CDCl3 was
used as solvent. Both solvents (99.80% D) were used as
purchased and were not subjected to any particular pretreat-
ment.
Solutions of each peptide 5a−f and 6a−f (5 × 10−3 M)
1
yielded sharp, well-resolved H NMR spectra consisting of
single sets of signals in both solvents. All protons were
C
dx.doi.org/10.1021/jo302368y | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Featured Article
Table 3. Amide Proton 1H NMR Chemical Shifts (δ) and Thermal Coefficients (Δδ/ΔT) for β-Lactam Peptidomimetic Models
5a−f and 6a−f
a
a
b
b
entry
compd
R5
R6
config C α′
N2H(δ)
N7H(δ)
N2H(Δδ/ΔT)
N7H(Δδ/ΔT)
1
2
5a
5b
5c
5d
5e
5f
H
H
5.63 (+2.77)
5.60 (+2.72)
5.57 (+2.66)
5.64 (+2.77)
5.62 (+2.52)
5.55 (+2.81)
4.89 (+2.21)
4.76 (+2.84)
4.90 (+2.29)
4.85 (+2.69)
4.94 (+2.32)
4.84 (+2.73)
8.09 (+0.08)
8.08 (+0.18)
7.94 (+0.29)
8.41 (+0.16)
7.12 (+1.15)
8.19 (+0.16)
7.82 (+0.01)
7.93 (+0.03)
7.83 (+0.10)
8.18 (+0.14)
7.85 (+0.56)
7.76 (+0.10)
−5.3
−4.8
−4.5
−5.0
−4.5
−6.5
−8.3
−8.9
−9.0
−8.3
−8.9
−8.6
−3.2
−3.4
−3.9
−4.0
−4.9
−3.9
−2.3
−2.8
−3.0
−3.1
−4.2
−2.5
Me
H
H
R
S
3
Me
H
4
Ph
H
R
S
5
Ph
Me
H
6
Me
H
7
6a
6b
6c
6d
6e
6f
8
Me
H
H
R
S
9
Me
H
10
11
12
Ph
H
R
S
c
Ph
Me
Me
a
Chemical shifts (ppm) measured in CDCl3 at 300 K. Values in parentheses represent variations of the chemical shift when the CDCl3 solvent was
b
c
changed to DMSO-d6. Thermal coefficients in ppb/K measured in DMSO-d6 from 300 to 325 K at 5 K intervals. A mixture of conformers was
observed at room temperature in CDCl3, which turned to a single set of peaks in DMSO-d6.
tional differences between the β-lactam peptide diastereomers
with (i + 2) chiral α-amino acid residues (Table 3, entries 2−5
and 8−11), we noticed that the thermal coefficients of the N7H
protons where routinely larger for (i + 2)-(S) diastereomers
(5c, 5e and 6c, 6e) than for (i + 2)-(R) ones (5b, 5d and 6b,
6d), and furthermore, this effect was appreciably enhanced in
pseudopeptides containing the bulkier phenylglycine residues
(compare entries 4−5 and 10−11). This lower exposure of the
(R) diastereomers to the DMSO-d6 coordinating solvent was
consistent with the tight intramolecular N7H···OC hydrogen
bonding of β-turned β-lactam peptides 4 (Figure 1), whereas
the weaker hydrogen bonds of the (S) diastereomers (entries 3,
5, 9, and 11) suggested a partial folding of the peptide
backbone into γ-turned conformers or open conformers
possessing more exposed amide protons. Finally, the β-lactam
pseudopeptides with nonchiral Gly or Aib (i + 2) residues
(entries 1, 6, 7, and 12) exhibited only very shielded N7H
protons in DMSO-d6, consistent with their likely participation
in stable β-turns.
Next, relevant noncontiguous interproton distances for
peptides 5 and 6 were calculated following the ISPA (isolated
spin-pair approximation) method23 from the integration of key
NOESY or ROESY cross-peak signals recorded at 300 K (500
MHz) at mixing times of 200−400 ms (Tables S1 and S2,
Figures S15 and S16, Supporting Information).
Some expansions of the NOESY spectra of 5d,e (in CDCl3)
and 6d,e (in DMSO-d6), incorporating a chiral phenylglycine (i
+ 2) residue, are shown in Figure 3 as an example. The spectra
gave evidence for a number of NOE contacts, including the
excellent diagnostic correlation peaks provided by the pro-R
and pro-S diastereotopic C4 protons placed, respectively, at the
upper and lower faces of the β-lactam ring, as depicted in
Figure 3. Only the 4HR proton gave a characteristic strong
NOE contact (cross-peak a) with the 3Me singlet in peptides 5
and with the benzylic moiety 3/3Ar in peptides 6. Conversely,
the 4HS proton oriented to the lower face of the azetidin-2-one
ring plane was used to monitor the 4HS−N7H distances (cross-
peak b) and to estimate the preferred formation of canonical β-
II turns (≈ 2.5−2.8 Å), distorted γ-turns (≈ 3.8−4.0 Å) or
open conformers (>4 Å). Additionally, the orientation of the R5
and R6 substituents of the chiral (i + 2) residue relative to the
β-lactam ring could be established on the basis of the distances
of some of their key protons to 4HR and 4HS (cross-peaks c
and d). In the case of 6d, the abnormal shielding of 5Ar ortho
protons (6.52 ppm) arising from the diamagnetic effect of the
neighboring benzylic 3Ar ring provided a further indirect sign
of β-turn conformation. Finally, as none of the peptides 5 and 6
gave measurable 4HR/N7H correlation cross-peaks, a significant
participation in the conformational equilibrium of inverse γi-
turn β-lactam pseudopeptides with the (i + 2)-(i + 3) branch
oriented toward the top side of the azetidinone ring was ruled
out (see empty boxes in Figure 3).
From the interproton distances collected in Figure 3, a
regular chiral effect trend could be observed for the β-turn/γ-
turn equilibrium as the α′-(R) configuration of peptides 5d/6d
changed to α′-(S) in their diastereomers 5e/6e. Indeed, the
average 4HS −N7H distances (cross-peak b) were significantly
shorter for 5d and 6d (2.6 and 3.0 Å) than for their respective
isomers 5e and 6e (3.8 Å and >4 Å), suggesting that in the
latter cases β-turns were weakened or broken in favor of γ-
turns. This assumption was further supported by the close
distances of the CαH5 protons of the (S)-phenylglycine residue
to 4HR and 4HS in 5e/6e and also by the absence of a
comparable interaction for CαH6 in 5d/6d. In line with these
observations, β-lactam pseudopeptides 5b/6b and 5c/6c
derived, respectively from (R)-alanine and (S)-alanine, showed
similar β-turn destabilization trends upon (i + 2) α′
stereocenter configuration inversion, albeit in these cases the
chiral effect was less pronounced (Figures S15 and S16,
Supporting Information). Finally, all the β-lactam pseudopep-
tides with achiral (i + 2) residues of glycine (5a/6a) or α-
aminoisobutyric acid Aib (5f/6f) populated exclusively very
D
dx.doi.org/10.1021/jo302368y | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Featured Article
Figure 3. Expansion of the NOESY spectra of pseudopeptides 5d−e in CDCl3 (top) and 6d−e in DMSO-d6 (bottom) showing a preferred β-II turn
conformation (left-hand) and a γ-turn conformation (right-hand). Key NOE correlations with the upper (4HR) and lower (4HS) sides of the β-
lactam ring plane are highlighted inside boxes. Empty boxes show the expected position of alternative b cross-peaks with 4HR protons. Model figures
show energy-minimized SYBYL molecular mechanics structures restrained with NOESY interproton distances ( 0.3 Å) (Tables S1 and S2,
Supporting Information).
Table 4. Amide Proton 1H NMR Thermal Coefficients (Δδ/
stable β-turn conformations and showed no tendency to form
a
γ-turns.
ΔT) for β-Lactam Peptidomimetic 7
Solution Conformation of β-Lactam Peptides 7 with
Chiral (i) and (i + 3) Residues. Following the same NMR
methodology disclosed above, we next studied the conforma-
tional behavior of β-lactam peptidomimetics 7a−d in DMSO-d6
(2 × 10−3 M) to check the chiral effect of (i) and (i + 3)
alanine residues on the nucleation of β-sheets. The thermal
coefficient data collected in Table 4 show that all the (i + 3)
N8H amide protons participate in strong intramolecular
hydrogen bonds, likely forming stable β-II turns. Interestingly,
unlike the “heterochiral” diastereomers 7b and 7d, “homo-
chiral” diastereomers 7a (R,R) and 7c (S,S) showed solvent-
shielded N10H terminal trans-amide protons which were
expected to be accepted by the Boc groups to form β-hairpin
type secondary structures. A more detailed examination of the
interproton distances from ROESY spectra of 7a, however,
revealed that the actual acceptor of the N10H amide proton was
the β-lactam carbonyl oxygen, thus forming two overlapped β-
II/β-I turns (Figure 4). In particular, this assumption was
config
entry compd
(C3, C9)
N2H
N4H
N8H
N10H N11H
1
2
3
4
7a
7b
7c
7d
R,R
R,S
S,S
−7.4
−7.3
−6.3
−7.0
−6.6
−6.7
−5.8
−5.8
−2.3
−2.8
−3.5
−2.5
−2.4
−3.8
−2.5
−4.3
−5.2
−4.1
−5.2
−4.5
S,R
a
Thermal coefficients in ppb/K measured in DMSO-d6 from 300 to
325 K at 5 K intervals.
further supported by the long-range N8H−N10H NOE
diagnostic cross-peak h observed for the homochiral peptide
E
dx.doi.org/10.1021/jo302368y | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Featured Article
Figure 4. Expansion of the ROESY spectra of pseudopeptides 7a and 7b in DMSO-d6 showing two overlapped β-turns sharing the β-lactam ring and
the N8H−N10H close distance NOE correlation (h). The model figure shows the energy-minimized structure restrained with ROESY interproton
distances ( 0.3 Å) (Table S3, Supporting Information).
7a, but not for the heterochiral peptide 7b (for an analogous
behavior for diastereomers 7c/7d, see Figure S17, Supporting
Information).
Solid-State Conformation of β-Lactam Peptidomimet-
ics 5−7. Single crystals of the β-lactam pseudopeptides 5e, 5f,
7a, and 7d were grown from their solutions in MeOH and were
used for X-ray crystal structure determination at 160 K (Figure
5). While 5e and 5f are ansolvates and there is no free space
within the structures that might accommodate solvent
molecules, 7a crystallizes as a 1:1 methanol solvate and 7d
crystallizes as its monohydrate. A detailed analysis of some key
structural parameters, Table 5, revealed the formation of well-
defined β-II turn conformers in all instances, with the exception
of the (S)-phenylglycine-derived mimetic 5e, which showed an
extended backbone structure with the (i) acetamido group and
the (i + 3) phenylglycine N-methylcarbamoyl group lying on
opposite sides of the plane of the β-lactam ring. Consistent with
the extremely destabilized γ-turn conformation observed for
this pseudopeptide in solution by NMR, no intramolecular
hydrogen bond was detected in the solid state. Instead, each
NH amide group acted as a donor to form intermolecular
bifurcated hydrogen bonds with the CO oxygen atom of the
β-lactam ring.
Figure 5. ORTEP plots (50% probability ellipsoids) of the molecular
structures of β-lactam peptidomimetics 5e, 5f, 7a, and 7d in the solid
state. Compound 7a shows overlapping β-II/β-I turns.
Mimetic models 5f, 7a, and 7d containing the (i + 2) Aib
residue fulfilled all the structural criteria of canonical type-II β-
turns in terms of (i)CO...HN(i+3) hydrogen bond distances
(1.9−2.1 Å), O...H−N(i+3) angles (159−167°) and the δ
residues (i + 1)-(i + 2) with respect to the ideal values for a
type-II β-turn. On the other hand, none of the peptides 7a and
7d showed a second hydrogen bond between the terminal Boc
and NH2 groups denoting the formation of β-sheet structures.
In spite of that, the NHtrans terminal amide proton of peptide 7a
acted as a donor to the β-lactam carbonyl oxygen atom to form
(i)
pseudodihedral angle formed by the four Cα atoms (5−14°),
which was very close to the ideal null value. Only a slight
distortion (15−20°) caused by the coplanar arrangement of
N(i+1), Cα(i+1), N(i + 2) and Cα(i + 2) atoms around the central β-
lactam ring, was observed for the torsion angles ϕ−ψ in
F
dx.doi.org/10.1021/jo302368y | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Featured Article
a
Table 5. Selected Geometric Parameters for β-Lactam Peptidomimetic Models 5e, 5f, 7a, and 7d
b
c
compd
5e
(i)CO...HN(i+3) (Å)
ϕ(i+1); ψ(i+1) (deg)
ϕ(i+2); ψ(i+2) (deg)
δ
(deg)
(i)CO...H−N(i + 3) (deg)
>4.0
−39.0(4); 126.6(3)
−38.3(2); 125.4(2)
−39.7(3); 118.9(2)
47.0(4); 30.4(3)
−40.6(3); 123.9(2)
−60; 120
−95.2(4); 134.9(3)
90.8(2); −2.8(2)
47.0(4); 30.4(3)
67.2(3); 24.3(3)
95.4(3); −2.5(3)
80; 0
−157.0
14.4
−31.3
−77.3
15.9
0
5f
1.92(3)
2.07(3)
1.87(4)
1.97(3)
1.8−2.5
1.8−2.5
1.8−2.5
160(3)
167(2)
168(5)
159(3)
160
7a (β-II)
7a (β-I)
7d
β-II (ideal)
β-I (ideal)
γ (ideal)
60; 30
90; 0
0
160
−; −
50; −60
0
160
c
a
b
Measured at 170 K. Solvates: 7a·MeOH and 7d·H2O. Pseudodihedral angle formed by the four Cα atoms of the peptide models. O···H−N angle.
a quasi-canonical β-I turn partially overlapping the β-II turn. To
the best of our knowledge, this is the first example of such a
type of double motif in solid pseudopeptide structures.24
Finally, a rationale for the chiral effect caused by the (i + 2)
residue on the relative stability of β-II/ γ conformers in
pseudopeptides 5 was established on the basis of the steric
interactions of R1 and R2 groups with the carbonyl and
methylene groups of the β-lactam ring (i + 1). Accordingly, we
submitted structures 5a−f to a DFT conformational
optimization25 consisting of a Monte Carlo search (6-31G*),
followed by structure minimization at the B3LYP/6-31++G**
level26 (for details, see the Supporting Information, S39).
Energy values collected in Figure 6 show a general trend for the
preferred stabilization (2−3 kcal/mol) of type II β-turns, over
the normal γ-turns. As expected, no type-I β-turn could be
characterized as a stable energy minimum. In addition, a clear
chiral-effect destabilizing the β-II turn was apparent in the case
of heterochiral (R,S) mimetics 5c and 5e with respect to the
homochiral (R,R) mimetics 5b and 5d, and this energy-
difference increased when the size of R group was increased
from methyl to phenyl. In the latter case, a dramatic inversion
in the stability of the γ-turn is also observed. Finally, models 5a
and 5f bearing respectively achiral (i + 2) residues of Gly and
Aib displayed very similar relative stability patterns favoring the
type β-II conformer over the γ-turns, albeit in the latter case the
energy difference was lower.
Figure 6 also depicts the Newman projections of models 5
with the β-lactam nitrogen atom and Cα′ eclipsed. In
homochiral (R,R) isomers, the group R can adopt an alternative
disposition only when the β-turn-II conformation is reached,
leading to the more stable conformations. The exception to this
general bias only occurs when the (i + 2) residue is heterochiral
with respect to the β-lactam (i + 1) stereocenter and the size-
difference between R and the β-lactam α- group is large. For
instance, in model 5e with a (S)-phenylglycine (i + 2) residue,
conformer β-II is destabilized by the steric repulsion of the
phenyl group and the β-lactam carbonyl oxygen atom, resulting
in the stabilization of the γ-turn conformer.
CONCLUSIONS
■
The present work illustrates the intrinsic ability of short β-
lactam peptide chains to form β-II and γ-turn structures in
solution. It also emphasizes that “one carbon-scaffolded” β-
lactam pseudopeptides provide very precise information on the
H-bonding pattern in short peptide models, which comple-
ments prior quantum chemistry and gas-phase laser-desorption
experiments.25c 2D NMR experiments conducted in solution
have provided compelling evidence that β-lactam pseudopep-
tides 5 and 6 containing a central homochiral -[(R)-β-Lactam-
(R)-Aa]- segment behave as betagenic, whereas the heterochiral
analogues -[(R)-β-lactam-(S)-Aa]- destabilize β-turn conforma-
tions and favor γ-turned peptides, especially when the (i + 2)
Aa residue carries sterically demanding Cα backbone
substituent groups. In addition, pseudopeptides 7 containing
a central betagenic -[(R)-β-lactam-Aib]- nucleus flanked by
heterochiral (i) and (i + 3) α-amino acid residues do not show
a straightforward ability to stabilize β-hairpins. Instead, the
homochiral counterparts show a strong tendency to form
unprecedented β-II/β-I overlapped double turns around the
central β-lactam core.
In the light of the current results, it can be stated that
chirality-driven conformational effect may ultimately determine
the preferred type of turn stabilized by β-lactam pseudopep-
tides. Therefore, the β-LASD design model, consisting in the
incorporation of an intraresidual methylene bridge to frozen
type-II β-turns, must be applied cautiously to “all-L” peptide
natural segments when chiral residues neighboring the central
Figure 6. (Top) Relative zero energies (kcal·mol−1) of β-II turn and γ-
turn conformers for pseudopeptides 5a−f. (Bottom) Steric inter-
actions accounting for the chiral-effect and relative conformer
stabilities. The energy values for the isomer pairs 5b−c and 5d−e
are assigned to the lower energy conformer in each case.
G
dx.doi.org/10.1021/jo302368y | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Featured Article
(3R)-1-[(R)-1-Carboxyethyl]-3-methyl-3-(2-nitrobenzenesulfonyl-
amino)azetidin-2-one (8b). The general procedure was followed
(oxidation method A) from N-[(2R)-2-hydroxy-1-methylethyl]-β-
lactam (0.524 mmol, 0.180 g): yield 0.160 g (87%); yellowish solid;
β-lactam core are involved. Despite this limitation, the β-LASD
design method provides a simple steric model that opens up the
route for a better understanding of the steric interactions
driving the shallow β-/γ- transition in larger peptides.
mp 55 °C; [α]25 +7.7 (0.45, MeOH); IR (KBr) νmax 3320, 2944,
D
1747, 1718, 1545, 1169, 832 cm−1; 1H NMR (500 MHz, CD3OD) δH
8.22−7.78 (m, 4H), 4.09 (q, 1H, J = 7.2 Hz), 3.72 (d, 1H, J = 5.8 Hz),
3.35 (d, 1H, J = 5.8 Hz), 1.52 (s, 3H), 1.38 (d, 3H, J = 7.3 Hz); 13C
NMR (75 MHz, CD3OD) δ 177.7, 169.4, 149.4, 136.4, 135.0, 133.7,
131.9, 125.9, 66.5, 54.8, 54.3, 20.5, 16.5; MS m/z (ESI, negative
polarity) MS-1 355.9; MS2(355.9) 308.8, 200.7, 185.7, 137.8. Anal.
Calcd for C13H15N3O7S: C, 43.69; H, 4.23; N, 11.76. Found: C, 43.50;
H, 4.17; N, 11.49.
EXPERIMENTAL SECTION
■
General Experimental Procedures. All reactions were carried
out under an atmosphere of nitrogen in oven- or flame-dried glassware
with magnetic stirring. Solvents were distilled prior to use. Anhydrous
tetrahydrofuran (THF) and dichloromethane were distilled, respec-
tively, from sodium metal/benzophenone ketyl and from calcium
hydride. Reaction products were purified by flash chromatography
using silica gel 60 (230−400 mesh). Analytical TLC was performed on
0.25 mm silica gel 60-F plates and spots were visualized, either with
UV light or with the spray reagent phosphomolybdic acid-ammonium
cerium(IV) nitric-sulfuric acid−water, followed by heating. Melting
points are uncorrected. 1H NMR and 13C NMR spectra were recorded
at 500 and 125.7 MHz, respectively, and are reported as δ values
(ppm) relative to residual CDCl3 (δ 7.26), DMSO (δ 2.49), and
CDCl3 (δ 77.0), DMSO (δ 39.5) as internal standards. Mass spectra
were obtained from samples ionized under EI (70 eV) or CI
conditions and detected using a quadrupole or a TOC mass analyzer,
after direct injection (HRMS) or GC−MS coupling (column: fused
silica gel, 15 m, 0.25 mm, 0.25 nm phase SPB-5). The preparations and
spectroscopic data of compounds 6a, 8a, 9a−f, and 14 were reported
previously.10a,18
General Procedure for the Preparation of 1-Carboxyalkyl-3-
methyl-3-(2-nitrobenzenesulfonylamino)azetidin-2-ones 8b−
f.18 To a solution of (R)-methyl 2-methylserinate 10 (1.90 mmol,
0.32 g) and 2-nitrobenzenesulfonyl chloride (NsCl, 4.18 mmol, 0.935
g) in acetonitrile (30 mL) was added KHCO3 (9.50 mmol, 0.951 g),
and the suspension was stirred at reflux for 16 h. Then saturated
aqueous NaHCO3 (15 mL) was added, and the mixture was extracted
with EtOAc (3 × 15 mL). The organic layer was dried (MgSO4), and
the solvents were evaporated at reduced pressure to give pure (2R)-2-
methyl-2-methoxycarbonyl-1-(2-nitrobenzenesulfonyl)aziridine. A sol-
ution of the aziridine (1.51 mmol) and the corresponding β-(tert-
butyldimethylsilyloxy)ethylamine (1.51 mmol) in dry acetonitrile (30
mL) was stirred at room temperature for 20 h. Then saturated aqueous
NaHCO3 (15 mL) was added, the mixture was extracted with EtOAc
(3 × 15 mL), the organic fractions were dried (MgSO4) and filtered,
and solvents were evaporated in vacuo to give the corresponding β-
azaalanine, which was purified by column chromatography (hexanes/
EtOAc 4:1). To a stirred solution of the β-azaalanine (1 mmol) in dry
THF (15 mL) was added 2.5 mmol of LiHMDS in THF (1 M). After
being stirred for 12 h, the mixture was washed with a saturated
aqueous solution of NaHCO3 (3 × 10 mL), and the aqueous layer was
extracted with CH2Cl2. The solvent was evaporated under reduced
pressure, and the crude product was purified by column chromatog-
raphy (hexanes/EtOAc, 1:7). The intermediate obtained was oxidized
following two different procedures.
(3R)-1-[(S)-1-Carboxyethyl]-3-methyl-(2-nitrobenzenesulfonyl-
amino)azetidin-2-one (8c). The general procedure was followed
(oxidation method A) from N-hydroxy-(2S)-methylethyl-β-lactam
(0.350 mmol, 0.120 g): yield 0.110 g (88%); white solid; mp 58−60
°C; [α]25 −21.3 (0.5, MeOH); IR (KBr) νmax 3260, 2901, 1742,
D
1728, 1543, 1170, 820 cm−1; 1H NMR (500 MHz, CD3OD) δH 8.17−
7.83 (m, 4H), 4.23 (q, 1H, J = 7.5 Hz), 3.63 (d, 1H, J = 5.5 Hz), 3.50
(d, 1H, J = 5.5 Hz), 1.58 (s, 3H), 1.38 (d, 3H, J = 7.5 Hz); 13C NMR
(75 MHz, CD3OD) δ 176.1, 168.1, 147.9, 135.1, 133.6, 132.2, 130.1,
124.4, 65.0, 52.5, 51.5, 19.3, 14.9; MS m/z (ESI, negative polarity)
MS-1 356.0; MS2(356.0) 200.7, 137.8; MS3(200.7) 136.8. Anal. Calcd
for C13H15N3O7S: C, 43.69; H, 4.23; N, 11.76. Found: C, 43.54; H,
4.46; N, 11.47.
(3R)-3-Methyl-3-(2-nitrobenzenesulfonylamino)-1-[(R)-1-phenyl-
carboxymethyl]azetidin-2-one (8d). The general procedure was
followed (oxidation method B) from N-hydroxy-(2R)-phenylethyl-β-
lactam (1.43 mmol, 0.58 g): yield 0.330 g (55%); yellowish powder;
mp 115 °C; [α]25 56.1 (0.7, MeOH); IR (KBr) νmax 3050, 2934,
D
1749, 1725, 1530, 1156, 802 cm−1; 1H NMR (500 MHz, CD3OD) δH
8.24−7.32 (m, 9H), 5.37 (s, 1H), 3.97 (d, 1H, J = 6.0 Hz), 3.08 (d,
1H, J = 6.0 Hz), 1.41 (s, 3H); 13C NMR (75 MHz, CD3OD) δ 170.9,
166.4, 146.4, 134.2, 132.2, 130.9, 129.1, 126.9, 126.5, 126.4, 123.1,
63.8, 58.4, 52.5, 17.8; MS m/z (ESI, positive polarity) MS+23: 442.0,
420.0; MS2(420.1) 392.0; 204.9; MS3(392.0) 346.0, 205.0, 158.9.
Anal. Calcd for C18H17N3O7S: C, 51.55; H, 4.09; N, 10.02. Found: C,
51.78; H, 4.28; N, 9.76.
(3R)-3-Methyl-3-(2-nitrobenzenesulfonylamino)-1-[(S)-1-
phenylcarboxymethyl]azetidin-2-one (8e). The general procedure
was followed (oxidation method B) from N-hydroxy-(2S)-phenylethyl-
β-lactam (0.740 mmol, 0.300 g): yield 0.180 g (58%); yellowish
powder; mp 188−190 °C; [α]25D +66.3 (1.25, MeOH); IR (KBr) νmax
1
3360, 2944, 1741, 1736, 1536, 1167, 826 cm−1; H NMR (500 MHz,
CD3OD) δH 7.83−7.33 (m, 9H), 5.38 (s, 1H), 3.56 (d, 1H, J = 5.6
Hz), 3.23 (d, 1H, J = 5.6 Hz), 1.60 (s, 3H); 13C NMR (75 MHz,
CD3OD) δ 173.4, 168.1, 147.8, 135.6, 134.9, 133.6, 132.2, 130.0,
128.4, 128.3, 127.7, 124.5, 65.0, 59.8, 52.8, 19.7; MS m/z (ESI, positive
polarity) MS+23 442.0, 420.1; MS2(420.1) 392.0; MS3(392.0) 346.0,
205.0, 159.0. Anal. Calcd for C18H17N3O7S: C, 51.55; H, 4.09; N,
10.02. Found: C, 51.32; H, 3.82; N, 9.82.
Oxidation Method A.27 BAIB (1.069 mmol, 0.344 g), TEMPO
(0.243 mmol, 0.038 g), and the corresponding N-hydroxyethyl-β-
lactam (0.486 mmol) were combined in the reaction vessel, and to this
mixture was added 1:1 acetone−water solution (2 mL). The reaction
mixture was stirred for 3 h before evaporation of the solvents under
reduced pressure. The resulting product was purified by column
chromatography (hexanes/EtOAc, 1:10 or CH2Cl2/MeOH, 10:1).
Oxidation Method B. The Jones reagent28 (1.48 mmol) was added
dropwise to the solution of the corresponding N-hydroxyphenylethyl-
β-lactam (0.74 mmol) in acetone (10 mL) at 0 °C, and the mixture
was stirred at the same temperature for 2 h. The oxidant excess was
(3R) 1-(1-Carboxyisopropyl)-3-methyl-3-(2-nitrobenzenesulfonyl-
amino)azetidin-2-one (8f). To the solution of N-(methoxycarbonyl-
isopropyl)-β-lactam (0.259 mmol, 0.110 g) in THF (1 mL) was added
a solution of LiOH·H2O (1.299 mmol, 0.055 g) in H2O (1 mL). The
reaction mixture was stirred at room temperature for 2 h and then was
acidified with 2 M HCl and extracted with EtOAc (3 × 5 mL). The
organic layer was dried over MgSO4 and evaporated to give a product
which was purified by column chromatography (hexanes/EtOAc, 2:1):
yield 0.090 g (93%); yellow oil; [α]25D −33.0 (1.2, MeOH); IR (KBr)
1
νmax 3260, 2989, 1785, 1764, 1532, 1179, 816 cm−1; H NMR (500
MHz, CD3OD) δH 8.16−7.78 (m, 4H), 3.68 (d, 1H, J = 5.0 Hz), 3.34
(d, 1H, J = 5.0 Hz), 1.61 (s, 3H), 1.52 (s, 3H), 1.44 (s, 3H). 13C NMR
(75 MHz, CD3OD) δ 175.9, 169.1, 149.3, 136.4, 135.0, 133.6, 131.7,
125.9, 65.6, 60.2, 54.5, 24.3, 24.2, 20.9; MS m/z (ESI, negative
polarity) MS-1: 383.0; MS2(383.0) 200.7, 185.7, 137.8; MS3(137.8)
117.9. Anal. Calcd for C14H17N3O7S: C, 45.28; H, 4.61; N, 11.32.
Found: C, 44.97; H, 4.77; N, 11.53.
i
quenched with PrOH (10 mL), and the dark solid formed was
dissolved with H2O (4 mL). The mixture was extracted with EtOAc (3
× 10 mL), and the combined organic fractions were washed with brine
(5 mL). The organic solution was dried (MgSO4) and evaporated
under reduced pressure, and the resulting product was purified by
column chromatography (MeOH/CH2Cl2, 1:10).
H
dx.doi.org/10.1021/jo302368y | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Featured Article
General Procedure for the Preparation of 3-Methyl-1-[(N-
methylcarbamoyl)alkyl]-3-(2-nitrobenzenesulfonylamino)-
azetidin-2-ones (12a−f). To the solution of the corresponding β-
lactam 8 (0.175 mmol, 0.060 g) in THF (1.5 mL) were successively
added NMM (0.175 mmol, 0.020 mL) and IBCF29 (0.175 mmol,
0.022 mL) at −20 °C. After an activation period of 3 min, 40%
aqueous methylamine (0.350 mmol, 0.012 mL) was added, and the
resulting solution was stirred for 45 min at −20 °C. After the solvent
was evaporated under reduced pressure, the residue was directly
loaded on silica gel (hexanes/EtOAc, 1:7) to give the corresponding
N-methylcarbamoyl-β-lactam.
137.8, 131.8. Anal. Calcd for C19H20N4O6S: C, 52.77; H, 4.66; N,
12.96. Found: C, 52.91; H, 4.70; N, 12.61.
(3R)-3-Methyl-1-[(1S)-1-(N-methylcarbamoyl)-1-phenylmethyl]-
3-(2-nitrobenzenesulfonylamino)azetidin-2-one (12e). According to
the aforementioned procedure (compound 12d) product 12e was
prepared starting from β-lactam (8e) (0.31 mmol, 0.130 g) and
methylamine (2 M in THF; 0.465 mmol, 0.233 mL). An epimerization
took place, and the mixture of diastereisomers 12e and 12d (ratio
96:4) was separated by preparative TLC (hexanes/EtOAc, 1:5): yield
0.113 g (85%); yellowish powder; mp 148−150 °C; [α]25D +38.9 (0.7,
CH2Cl2); IR (KBr) νmax 3210, 2943, 1745, 1725, 1544, 1164, 821
cm−1; 1H NMR (500 MHz, CDCl3) δH 7.92−7.35 (m, 10H), 5.99 (br
s, 1H), 5.93 (br s, 1H), 5.38 (s, 1H), 3.62 (s, 2H), 2.79 (d, 3H, J = 4.7
Hz), 1.62 (s, 3H); 13C NMR (125 MHz, CDCl3) δC 168.7, 167.1,
147.5, 135.3, 133.9, 133.5, 132.9, 130.7, 129.2, 128.9, 128.4, 125.3,
65.9, 59.2, 54.8, 26.4, 21.3; MS m/z (ESI, negative polarity) MS-1:
431.0; MS2(431.0) 137.8, 188.8, 185.7. Anal. Calcd for C19H20N4O6S:
C, 52.77; H, 4.66; N, 12.96. Found: C, 53.00; H, 4.59; N, 12.58.
(3R)-3-Methyl-1-[1-(N-methylcarbamoyl)isopropyl]-3-(2-nitro-
benzenesulfonylamino)azetidin-2-one (12f). The β-lactam (8f)
(0.162 mmol, 0.160 g) was dissolved in CH2Cl2 (2.5 mL) and cooled
to 0 °C. To this solution was added methylamine (2 M THF) (0.323
mmol, 0.162 mL), and then the reaction mixture was cooled to −12
°C (ice/NaCl) and EEDQ (0.072g, 0.292 mmol) was added as a
CH2Cl2 solution (2.5 mL). The reaction was slowly warmed to room
temperature and stirred overnight. After this time, the reaction mixture
was concentrated under reduced pressure, and the crude product was
purified by preparative TLC (hexanes/EtOAc, 1:7) to afford pure N-
methylamide 12f as a white powder: yield 0.050 g (75%); mp 135 °C;
(3R)-3-Methyl-1-(N-methylcarbamoylmethyl)-3-(2-nitro-
benzenesulfonylamino)azetidin-2-one (12a). The general procedure
was followed from β-lactam (8a) (0.175 mmol, 0.060 g). The obtained
crude product was purified by preparative TLC (hexanes/EtOAc, 1:7):
yield 0.050 g (80%); white powder; mp 145−147 °C; [α]25 −87.3
D
(1.5, CH2Cl2); IR (KBr) νmax 3108, 2990, 1760, 1732, 1526, 1140, 825
cm−1; 1H NMR (500 MHz, CDCl3) δH 8.10−7.26 (m, 4H), 6.98 (br s,
1H), 5.95 (s, 1H), 4.35 (d, 1H, J = 17.0 Hz), 3.89 (d, 1H, J = 5.5 Hz),
3.57 (d, 1H, J = 17.0 Hz), 3.37 (d, 1H, J = 5.5 Hz), 2.90 (d, 3H, J = 5.5
Hz), 1.62 (s, 3H); 13C NMR (125 MHz, CDCl3) δC 167.4, 167.1,
147.6, 134.9, 133.9, 133.2, 130.8, 125.5, 67.9, 57.4, 45.6, 26.3, 21.8; MS
m/z (ESI, negative polarity) MS-1 354.9; MS2(354.9) 135.8 137.7
185.7, 193.7. Anal. Calcd for C13H16N4O6S: C, 43.82; H, 4.53; N,
15.72. Found: C, 43.80; H, 4.69; N, 15.47.
(3R)-3-Methyl-1-[(1R)-1-(N-methylcarbamoyl)ethyl]-3-(2-nitro-
benzenesulfonylamino)azetidin-2-one (12b). The general procedure
was followed from β-lactam (8b) (0.08 mmol, 0.030 g). The crude
product was purified by preparative TLC (hexanes/EtOAc, 1:7): yield
0.018 g (58%); white solid; mp 152 °C; [α]25D −67.3 (0.75, CH2Cl2);
[α]25 −46.2 (1.75, CH2Cl2); IR (KBr) νmax 3315, 2934, 1758, 1728,
D
1
1540, 1166, 798 cm−1; H NMR (500 MHz, CD3OD) δH 8.14−7.84
1
IR (KBr) νmax 3150, 2963, 1750, 1735, 1520, 1160, 825 cm−1; H
(m, 4H), 3.63 (d, 1H, J = 5.0 Hz), 3.41 (d, 1H, J = 5.0 Hz), 2.79 (s,
3H,), 1.57 (s, 3H), 1.54 (s, 3H), 1.45 (s, 3H); 13C NMR (125 MHz,
CD3OD) δC 174.5, 168.3, 148.2, 135.4, 134.1, 132.6, 130.2, 124.8,
64.8, 60.2, 53.8, 25.8, 23.4, 23.3, 20.1; MS m/z (ESI, negative polarity)
MS-1 383.0; MS2(383.0) 200.7, 185.7, 137.8; MS3(137.8) 117.9. Anal.
Calcd for C15H20N4O6S: C, 46.87; H, 5.24; N, 14.57. Found: C, 47.02;
H, 5.33; N, 14.36.
NMR (500 MHz, CDCl3) δH 8.09−7.74 (m, 4H), 7.08 (br s, 1H), 5.92
(s, 1H), 4.47 (q, 1H, J = 7.3 Hz), 3.74 (d, 1H, J = 5.5 Hz), 3.40 (d, 1H,
J = 5.5 Hz), 2.88 (d, 3H, J = 4.7 Hz), 1.60 (s, 3H), 1.42 (d, 3H, J = 7.3
Hz); 13C NMR (125 MHz, CDCl3) δC 170.1, 166.9, 147.6, 135.0,
133.9, 133.2, 130.8, 125.4, 66.5, 53.9, 50.3, 26.4, 21.9, 14.5; MS m/z
(ESI, negative polarity) MS-1 369.0; MS2(369.0) 185.7, 137.8, 126.8.
Anal. Calcd for C14H18N4O6S: C, 45.40; H, 4.90; N, 15.11. Found: C,
45.41; H, 4.78; N, 15.20.
General Procedure for the Preparation 3-Acetamido-3-
methyl-1-[(N-methylcarbamoyl)alkyl]azetidin-2-ones (5a−f).
To a stirred solution of the corresponding β-lactam 12 (0.067
mmol, 0.045 g) in dry CH3CN (5 mL) were added thiophenol (0.335
mmol, 0.034 g) and K2CO3 (0.268 mmol, 0.037 g). After the solution
(3R)-3-Methyl-1-[(1S)-1-(N-methylcarbamoyl)ethyl]-3-(2-nitro-
benzenesulfonylamino)azetidin-2-one (12c). The general procedure
was followed from β-lactam (8c) (0.078 mmol, 0.028 g). The crude
product was purified by preparative TLC (hexanes/EtOAc, 1:7): yield
1
was stirred at room temperature for 1 h (monitored by TLC and H
0.017 g (59%); white solid; mp 163−164 °C; [α]25 −69.3 (0.67,
D
NMR), acetic anhydride (0.402 mmol, 0.088 g) was added, and the
reaction was stirred overnight. The solvent was evaporated under
reduced pressure, and the crude was purified by preparative TLC
(hexanes/EtOAc, 1:7).
CH2Cl2); IR (KBr) νmax 3120, 2959, 1765, 1745, 1540, 1100, 808
cm−1; 1H NMR (500 MHz, CDCl3) δH 8.12−7.72 (m, 4H), 6.56 (br s,
1H), 5.96 (s, 1H), 4.06 (q, 1H, J = 7.3 Hz), 3.84 (d, 1H, J = 5.5 Hz),
3.39 (d, 1H, J = 5.5 Hz), 2.84 (d, 3H, J = 4.8 Hz), 1.58 (s, 3H), 1.57
(d, 3H, J = 7.3 Hz); 13C NMR (125 MHz, CDCl3) δC 171.1, 167.4,
148.0, 135.8, 134.2, 133.5, 131.2, 125.8, 66.5, 55.9, 53.9, 26.8, 21.9,
15.9; MS m/z (ESI, negative polarity) MS-1 369.0; MS2(369.0) 185.7,
137.8, 126.9; MS3(137.0) 121.8, 90.0. Anal. Calcd for C14H18N4O6S:
C, 45.40; H, 4.90; N, 15.11. Found: C, 45.04; H, 4.88; N, 15.23.
(3R)-3-Methyl-1-[(1R)-1-(N-methylcarbamoyl)-1-phenylmethyl]-
3-(2-nitrobenzenesulfonylamino)azetidin-2-one (12d). To a solu-
tion of β-lactam (8d) (0.24 mmol, 0.100 g) and methylamine (2 M in
THF; 0.357 mmol, 0.179 mL) in CH2Cl2 (5 mL) were added
sequentially DCC (0.334 mmol, 0.069 g) and HOBt (0.286 mmol,
0.039 g) at 0 °C. The reaction mixture was allowed to warm to rt and
stirred for 12 h. Then solvent was removed under reduced pressure,
and the resulting crude product was purified by preparative TLC
(hexanes/EtOAc, 1:5): yield 0.080 g (78%); yellowish powder; mp
(R)-3-Acetamido-3-methyl-1-[1-(N-methylcarbamoyl)methyl]-
azetidin-2-one (5a). The general procedure was followed from α-(2-
nosyl)-β-lactam 12a (0.084 mmol, 0.030 g). The crude product was
purified by preparative TLC (hexanes/EtOAc, 1:7): yield 0.015 g
(84%); yellowish oil; [α]25D +94.8 (0.5, CH2Cl2); IR (KBr) νmax 3210,
1
2965, 1740, 1728 cm−1; H NMR (500 MHz, CDCl3) δH 8.19 (br s,
1H), 6.16 (s, 1H), 4.44 (d, 1H, J = 17.6 Hz), 3.89 (d, 1H, J = 5.0 Hz),
3.57 (d, 1H, J = 17.6 Hz), 3.37 (d, 1H, J = 5.0 Hz), 2.90 (d, 3H, J = 4.6
Hz), 2.06 (s, 3H), 1.58 (s, 3H); 13C NMR (125 MHz, CDCl3) δC
170.7, 168.8, 168.0, 65.0, 53.5, 45.3, 26.3, 22.9, 20.5; MS m/z (ESI,
negative polarity) MS-1 211.8; MS2(212.0) 181.8, 169.8, MS3(169.8)
126.8, 112.8, 100.8, 96.0, 82.9. Anal. Calcd for C9H15N3O3: C, 50.69;
H, 7.09; N, 19.71. Found: C, 50.72; H, 7.35; N, 19.62.
(3R)-3-Acetamido-3-methyl-1-[1-(1R)-(N-methylcarbamoyl)-
ethyl]azetidin-2-one (5b). The general procedure was followed from
α-(2-nosyl)-β-lactam 12b (0.064 mmol, 0.024 g). The crude product
was purified by preparative TLC (hexanes/EtOAc, 1:7): yield 0.010 g
(72%); yellowish solid; mp 159−160 °C; [α]25D +58.2 (0.5, CH2Cl2);
145 °C; [α]25 −144.4 (0.75, CH2Cl2); IR (KBr) νmax 3320, 2984,
D
1
1768, 1732, 1540, 1158, 828 cm−1; H NMR (500 MHz, CDCl3) δH
8.14−7.25 (m, 10H), 5.52 (s, 1H), 3.80 (d, 1H, J = 5.8 Hz), 2.99 (d,
3H, J = 4.7 Hz), 2.94 (d, 1H, J = 5.8 Hz), 1.46 (s, 3H); 13C NMR (125
MHz, CDCl3) δC 169.1, 167.2, 147.9, 135.4, 134.3, 133.7, 131.3, 129.6,
129.2, 125.9, 66.7, 60.1, 55.3, 27.0, 21.8; MS m/z (ESI, negative
polarity) MS+1 431.0; MS2(431.0) 193.7, 188.8, 137.8; MS3(188.8)
1
IR (KBr) νmax 3125, 3060, 1782, 1768 cm−1; H NMR (500 MHz,
CDCl3) δH 8.13 (br s, 1H), 5.96 (s, 1H), 4.47 (q, 1H, J = 7.3 Hz), 3.73
(d, 1H, J = 5.1 Hz), 3.23 (d, 1H, J = 5.1 Hz), 2.84 (d, 3H, J = 4.6 Hz),
2.04 (s, 3H), 1.55 (s, 3H), 1.40 (d, 3H, J = 7.3 Hz); 13C NMR (125
I
dx.doi.org/10.1021/jo302368y | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Featured Article
MHz, CDCl3) δC 170.6, 168.6, 63.5, 50.2, 49.8, 26.5, 23.0, 20.7, 14.4;
MS m/z (ESI, negative polarity) MS-1 225.9; MS2(225.9) 183.8;
MS3(183.8) 126.8, 96.9. Anal. Calcd for C10H17N3O3: C, 52.85; H,
7.54; N, 18.49. Found: C, 52.86; H, 7.60; N, 18.53.
phases were dried (MgSO4) and evaporated. The product was purified
by flash column chromatography (hexanes/EtOAc, 2:1).
Compound 13b. The general procedure was followed from the N-
carboxyalkyl-β-lactam 9b (1 mmol, 0.433 g): yield 0.486 g (79%);
white solid; mp 216−218 °C; [α]25D +54.7 (0.42, CH21Cl2); IR (KBr)
νmax 3396, 3085, 1762, 1681, 1539, 1347, 1153 cm−1; H NMR (500
MHz, CDCl3) δH 8.15−7.72 (m, 4H, o-Ns), 7.37−7.21 (m, 10H, Ar),
7.17 (s, 1H, NHCMe2), 5.89 (s, 1H, NH-Ns), 5.21 (d, 1H, J = 12.3
Hz, OCH2Ph), 5.09 (d, 1H, J = 12.6 Hz, OCH2Ph), 4.32 (m, 1H,
NCHMe), 3.55 (d, 1H, J = 5.7 Hz, CH2NCO), 3.34 (d, 1H, J = 5.7
Hz, CH2NCO), 3.14 (d, 1H, J = 13.5 Hz, NCCH2Ph), 3.03 (d, 1H, J =
13.5 Hz NCCH2Ph), 1.59 (s, 3H, Me2C), 1.56 (s, 3H, Me2C), 0.83 (d,
3H, J = 6.9 Hz, MeCHNH); 13C NMR (125 MHz, CDCl3) δC 173.8,
168.8, 166.1, 147.4, 134.9, 133.8, 133.2, 132.6, 130.9, 130.2, 128.9,
128.5, 128.2, 128.1, 125.3, 70.1, 67.0, 56.6, 50.5, 49.5, 40.3, 25.4, 24.7,
13.4; MS m/z (ESI, positive polarity) MS+23: 264.0, 242.0;
MS2(242.0) 129.0; MS3(129.0) 101.1, 58.4. Anal. Calcd for
C30H32N4O8S: C, 59.20; H, 5.30; N, 9.20. Found: C, 58.86; H, 5.64;
N, 9.01.
(3R)-3-Acetamido-3-methyl-1-[1-(1R)-(N-methylcarbamoyl)-
ethyl]azetidin-2-one (5c). The general procedure was followed from
α-(2-nosyl)-β-lactam 12c (0.060 mmol, 0.022 g). The crude product
was purified by preparative TLC (hexanes/EtOAc, 1:7): yield 0.011 g
(81%); white powder; mp 115 °C; [α]25 +58.6 (0.5, CH2Cl2); IR
D
(KBr) νmax 3310, 3140, 1785, 1758 cm−1; 1H NMR (500 MHz,
CDCl3) δH 8.01 (br s, 1H), 5.96 (s, 1H), 3.80 (d, 1H, J = 5.0 Hz), 3.75
(q, 1H, J = 7.4 Hz), 3.17 (d, 1H, J = 5.0 Hz), 2.86 (d, 3H, J = 4.7 Hz),
2.03 (s, 3H), 1.73 (d, 3H, J = 7.5 Hz), 1.55 (s, 3H); 13C NMR (125
MHz, CDCl3) δC 171.4, 170.5, 168.6, 63.4, 56.2, 53.5, 26.4, 23.0, 20.6,
16.3; MS m/z (ESI, negative polarity) MS-1 225.9; MS2(225.9) 195.8,
183.8; MS3(183.8) 126.8, 96.9. Anal. Calcd for C10H17N3O3: C, 52.85;
H, 7.54; N, 18.49. Found: C, 52.59; H, 7.68; N, 18.84.
(3R)-3-Acetamido-3-methyl-1-[1-(1R)-(N-methylcarbamoyl)-
phenylmethyl]azetidin-2-one (5d). The general procedure was
followed from α-(2-nosyl)-β-lactam 12d (0.108 mmol, 0.047 g). The
crude product was purified by preparative TLC (CH2Cl2/MeOH
Compound 13c. The general procedure was followed from the N-
carboxyalkyl-β-lactam 9b (1 mmol, 0.433 g): yield 0.426 g (70%);
white solid; mp 216−218 °C; [α]25D +106.7 (0.3, CH2Cl2); IR (KBr)
10:1): yield 0.028 g (89%); white powder; mp 99 °C; [α]25 +21.2
D
1
νmax 3378, 3105, 1748, 1539, 1356, 1165 cm−1. H NMR (500 MHz,
(0.5, CH2Cl2); IR (KBr) νmax 3125, 3094, 1760, 1725 cm−1; 1H NMR
(500 MHz, CDCl3) δH 8.44 (br s, 1H), 7.37−7.22 (m, 5H), 5.99 (br s,
1H), 5.50 (s, 1H), 3.73 (d, 1H, J = 5.0 Hz), 2.95 (d, 3H, J = 4.6 Hz),
2.72 (d, 1H, J = 5.0 Hz), 2.00 (s, 3H), 1.38 (s, 3H); 13C NMR (125
MHz, CDCl3) δC 170.9, 169.2, 168.3, 134.3, 129.0, 128.9, 128.4, 63.4,
59.4, 51.3, 26.6, 22.9, 20.2; MS m/z (ESI, negative polarity) MS-1
288.1; MS2(288.0) 245.8, 230.8, 171.8, 126.8, 111.9; MS3(245.8)
126.8; MS4(126.8) 111.8, 98.9. Anal. Calcd for C15H19N3O3: C, 62.27;
H, 6.62; N, 14.52. Found: C, 62.40; H, 6.51; N, 14.75.
CDCl3) δH 8.14−7.74 (m, 4H, o-Ns), 7.40−7.25 (m, 10H, Ar), 6.93
(s, 1H, NHCMe2), 5.86 (s, 1H, NH-Ns), 5.18 (s, 2H, OCH2Ph), 3.89
(q, 1H, J = 1.6 Hz, NCH(Me)CO), 3.72 (d, 1H, J = 5.7 Hz,
CH2NCO), 3.47 (d, 1H, J = 5.7 Hz, CH2NCO), 3.14 (q, 2H, J = 1.6
Hz, CqCH2Ph), 1.59 (s, 3H, Me2C), 1.58 (s, 3H, Me2C), 1.46 (d, 3H, J
= 7.25 Hz, MeCHNH); 13C NMR (125 MHz, CDCl3) δC 173.9, 169.5,
166.5, 147.4, 133.6, 133.2, 133.1, 130.9, 130.0, 129.0, 128.5, 128.2,
128.1, 127.9, 125.3, 69.5, 67.2, 56.6, 53.8, 53.5, 40.6, 29.7, 24.9, 15.1;
MS m/z (TOF MS ES+) MS[+Na] 630.9, MS[+H] 608.8, MS: 562.8,
500.9, 472.9, 394.0, 359.9, 115.9. Anal. Calcd for C30H32N4O8S: C,
59.20; H, 5.30; N, 9.20. Found: C, 59.14; H, 5.55; N, 8.92.
(3R)-3-Acetamido-3-methyl-1-[1-(1S)-(N-methylcarbamoyl)-
phenylmethyl]azetidin-2-one (5e). The general procedure was
followed from α-(2-nosyl)-β-lactam 12e (0.144 mmol, 0.062 g). The
crude product was purified by preparative TLC (CH2Cl2/MeOH
10:1): yield 0.039 g (94%); white crystals; mp 198−199 °C; [α]25
+133.5 (0.75, MeOH); IR (KBr) νmax 3210, 2998, 1744, 1725 cm−1D;
1H NMR (500 MHz, CDCl3) δH 7.43−7.39 (m, 5H), 7.14 (br s, 1H),
5.66 (s, 1H), 5.77 (s, 1H), 3.77 (d, 1H, J = 5.0 Hz), 3.46 (d, 1H, J =
5.0 Hz), 2.93 (d, 3H, J = 4.7 Hz), 2.02 (s, 3H), 1.58 (s, 3H); 13C NMR
(125 MHz, CDCl3) δC 170.1, 168.9, 168.3, 134.6, 128.8, 128.7, 128.5,
76.6, 63.4, 62.3, 53.9, 26.5, 23.0, 20.4; MS m/z (ESI, negative polarity)
MS-1: 288.1; MS2(288.0) 245.8, 230.8, 188.8, 171.8, 126.8;
MS3(245.8) 126.8; MS4(126.8) 111.8, 58.3. Anal. Calcd for
C15H19N3O3: C, 62.27; H, 6.62; N, 14.52. Found: C, 62.67; H, 6.70;
N, 14.30.
Compound 13d. The general procedure was followed from the N-
carboxyalkyl-β-lactam 9d (1 mmol, 0.497 g): yield 0.368 g (55%);
white solid; mp 72−74 °C; [α]25 −83.8 (0.65, CH2Cl2); IR (KBr)
D
1
νmax 3378, 3028, 1768, 1742, 1677, 1534, 1148 cm−1; H NMR (500
MHz, CDCl3) δH 8.15−7.68 (m, 4H), 7.50−6.85 (m, 15H), 5.90 (br s,
1H), 5.32 (s, 1H), 5.20 (d, 1H, J = 12.5 Hz), 5.16 (d, 1H, J = 12.5 Hz),
3.69 (d, 1H, J = 6.0 Hz), 3.04 (d, 1H, J = 6.0 Hz), 3.00 (d, 1H, J = 14.0
Hz), 2.95 (d, 1H, J = 14.0 Hz), 1.67 (s, 3H), 1.64 (s, 3H); 13C NMR
(125 MHz, CDCl3) δC 173.8, 167.8, 165.9, 147.5, 136.0, 134.8, 134.0,
133.4, 133.3, 132.5, 131.1, 130.1, 129.1, 128.9, 128.7, 128.6, 128.5,
128.2, 128.1, 128.0, 125.5, 69.9, 67.2, 59.2, 55.9, 52.3, 40.3, 25.5, 24.9;
MS m/z (ESI, positive polarity) MS 693.2; MS2(693.2) 671.2, 666.1,
665.2, 478.3; MS3(665.2) 479.2, 478.2, 463.1. Anal. Calcd for
C35H34N4O8S: C, 62.67; H, 5.11; N, 8.35. Found: C, 62.60; H, 4.95;
N, 8.07.
(3R)-3-Acetamido-3-methyl-1-[1-(N-methylcarbamoyl)-
isopropyl]azetidin-2-one (5f). The general procedure was followed
from α-(2-nosyl)-β-lactam 12f (0.091 mmol, 0.035 g). The crude
product was purified by preparative TLC (hexanes/EtOAc, 1:7): yield
Compound 13e. The general was followed from the N-
carboxyalkyl-β-lactam 9e (0.42 mmol, 0.2 g) and α-aminoisobutyric
acid benzyl ester (0.44 mmol; 0.085 g): yield 0.023 g (66%); mp 64−
0.018 g (82%); yellowish powder; mp 216−218 °C; [α]25 +57.7
D
(0.75, CH2Cl2); IR (KBr) νmax 3320, 3180, 1769, 1748 cm−1; 1H NMR
(500 MHz, CDCl3) δH 8.28 (br s, 1H), 6.30 (s, 1H), 3.73 (d, 1H, J =
5.0 Hz), 3.20 (d, 1H, J = 5.0 Hz), 2.82 (d, 3H, J = 4.6 Hz), 2.02 (s,
3H), 1.71 (s, 3H), 1.52 (s, 3H), 1.36 (s, 3H); 13C NMR (125 MHz,
CDCl3) δC 173.7, 170.7, 168.3, 62.2, 60.3, 51.0, 26.6, 24.8, 24.3, 22.9,
20.7; MS m/z (ESI, positive polarity) MS+23 264.0, 242.0;
MS2(242.0) 129.0; MS3(129.0) 101.1, 58.4. Anal. Calcd for
C11H19N3O3: C, 54.76; H, 7.94; N, 17.42. Found: C, 54.50; H, 8.00;
N, 17.09.
65 °C; [α]25 +9.4 (0.65, CH2Cl2); IR (KBr) νmax 3350, 2928, 1744,
D
1726, 1682, 1536, 1148 cm−1; 1H NMR (500 MHz, CDCl3) δH 7.95−
7.61 (m, 4H), 7.37−7.26 (m, 15H), 6.44 (br s, 1H), 5.81 (br s, 1H),
5.33 (s, 1H), 5.19 (d, 1H, J = 12.5 Hz), 5.15 (d, 1H, J = 12.5 Hz), 3.76
(d, 1H, J = 5.5 Hz), 3.52 (d, 1H, J = 5.5 Hz), 3.25 (dd, 2H, J = 11.9
Hz), 1.59 (s, 3H), 1.56 (s, 3H); 13C NMR (125 MHz, CDCl3) δC
173.9, 167.4, 166.7, 147.3, 135.6, 135.3, 133.6, 133.5, 133.0, 131.1,
129.9, 129.2, 129.1, 129.0, 128.7, 128.5, 128.2, 127.9, 125.4, 69.3, 67.5,
59.7, 57.1, 53.6, 41.1, 25.0, 24.5; MS m/z (ESI, positive polarity) MS
693.3; MS2(693.2) 666.1, 665.2, 478.2, 361.2; MS3(665.2) 479.1,
478.2, 463.2. Anal. Calcd for C35H34N4O8S: C, 62.67; H, 5.11; N, 8.35.
Found C, 62.32; H, 5.06; N, 7.98.
General Procedure for the Preparation of Ns-(β-lactam)-Aa-
Aib-OBn Compounds (13b−e). To a stirred solution of the
corresponding N-carboxymethyl β-lactam 9b−e (1 mmol) in CH2Cl2
(20 mL) cooled to −0 °C was added H-Aib-OBn (1 mmol, 0.193 g)
dissolved in CH2Cl2 (9 mL). The solution was cooled to −12 °C,
EEDQ (1.2 mmol, 0.310 g) was added, and the reaction mixture was
slowly warmed to room temperature and stirred overnight. The
resulting solution was washed with 1 M HCl (3 × 10 mL), the
aqueous layer was extracted with CH2Cl2, and the combined organic
General Procedure for the Preparation of Boc-(β-lactam)-
Aa-Aib-OH Compounds 6b−e. Thiophenol (5.00 mmol, 0.469 mL)
and K2CO3 (4.00 mmol, 0.567 g) were added to a solution of the
corresponding Ns-(β-Lactam)-Aa-Aib-OBn 13b−e (1 mmol) in dry
MeCN (15 mL), and the suspension was stirred at room temperature
1
for 2 h (monitoried by TLC and H NMR). Upon completion, the
J
dx.doi.org/10.1021/jo302368y | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Featured Article
reaction mixture was filtered through a pad of Celite, evaporated in
vacuo, and quickly purified by column chromatography (CH2Cl2, then
MeOH). The corresponding intermediate amine H-(β-lactam)-Aa-
Aib-OBn was redissolved in MeCN (15 mL), di-tert-butyl dicarbonate
(0.402 mmol; 0.088 g) was added, and the solution was stirred
overnight at room temperature. The solution was evaporated, treated
with saturated aqueous NaHCO3, and extracted with CH2Cl2 (3 × 10
mL). The combined organic solutions were dried (MgSO4), and the
solvents were evaporated under reduced pressure to afford the
corresponding intermediate Boc-(β-lactam)-Aa-Aib-OBn, which was
purified by preparative TLC (hexanes/EtOAc, 2:1). This benzyl ester
was dissolved in ethanol (15 mL) containing 10% Pd/C (0.050 g), and
the suspension was stirred at room temperature under hydrogen for 16
h. After this time, the reaction mixture was filtered through a pad of
Celite, washed with ethanol (2 × 10 mL), and the combined organic
solution was evaporated under reduced pressure to afford the
corresponding carboxyl peptide product 6b−e.
167.3, 154.8, 136.3, 136.1, 130.5, 128.7, 128.5, 128.4, 128.1, 127.0,
79.2, 67.5, 58.4, 56.0, 51.4, 38.8, 28.6, 25.2, 25.0. HPLC-MS m/z (ESI,
negative polarity) 494.3; MS2(494.3) 420.0; MS3(420.0) 377.0, 334.0,
333.1, 291.0, 273.0, 260.9, 249.9, 248.0, 174.8, 127.9; MS4(377.0)
333.0, 275.9, 249.9, 247.9, 207.0, 159.9, 127.9 188.0, 187.0, 172.0,
170.0, 159.0, 130.9. Anal. Calcd for C27H33N3O6: C, 65.44; H, 6.71; N,
8.48. Found C, 65.14; H, 6.98; N, 8.20.
Preparation of 2-[[(3R)-3-Benzyl-3-(tert-butoxycarbonyl-
amino)2-oxoazetidin1-yl]dimethylacetamido]-2-methylpropa-
noic Acid (6f). (3R)-3-Benzyl-3-tert-butoxycarbonyl-1-[1-
(methoxycarbonyl)isopropyl]azetidin-2-one. Thiophenol (5 mmol,
0.469 mL) and K2CO3 (4 mmol, 0.567 g) were added to a solution of
(3R)-3-benzyl-3-(2-nitrobenzenesulfonylamino)-1-[1-(methoxy-
carbonyl)isopropyl]-azetidin-2-one1 (1 mmol, 0.461 g) in MeCN (5
mL). The suspension was stirred for 2 h at rt, the solvent was
evaporated in vacuo, and the residue was purified by column
chromatography (EtOAc, then MeOH) to afford (3R)-3-benzyl-3-
amino-1-[1-(methoxycarbonyl)isopropyl]azetidin-2-one in 90% yield
(0.248 g). To a solution of this material in CH2Cl2 (5 mL) was added
(Boc)2O (3 mmol, 0.654g, and the mixture was stirred at rt for 16 h.
After evaporation of the solvent, the crude was purified by column
chromatography hexanes/EtOAc, 5:1): overall yield 0.188 g (50%);
2-[2-[(3R)-3-Benzyl-3-(tert-butoxycarbonylamino)-2-oxoazetidin-
1-yl]-(R)-propanamido]-2-methylpropanoic Acid (6b). The general
procedure was followed from compound 13b (1 mmol, 0.608 g): yield
0.342 g (79%); oil; [α]25 −15.0 (0.2, CH2Cl2); IR (KBr) νmax 3320,
D
1
3180, 1769, 1748 cm−1. H NMR (500 MHz, CD3OD) δH 7.31−7.29
yellow oil; [α]25 +10.2 (0.6, CH2Cl2); IR (KBr) νmax 3318, 1760,
D
(m, 5H, Ar), 4.0 (m, 1H, CHMe), 3.67 (d, 1H, J = 4.8 Hz, CH2NCO),
3.35 (d, 1H, J = 4.8 Hz, CH2NCO), 3.07 (d, 1H, J = 13.3 Hz, CH2Ph),
2.97 (d, 1H, J = 13.3 Hz, CH2Ph), 1.47 (s, 9H, tBuO), 1.46 (s, 6H,
Me2CH), 0.58 (d, 3H, J = 7.1 Hz, MeCH); 13C NMR (125 MHz,
CD3OD) δC 181.2, 171.2, 171.0, 156.6, 135.6, 131.8, 129.4, 128.4,
81.0, 68.8, 59.5, 50.8, 39.1, 28.7, 26.9, 25.8, 13.9; HPLC-MS, MeOH/
HCO2H m/z (ESI, positive polarity) MS+23 264.0, 242.0;
MS2(242.0) 129.0; MS3(129.0) 101.1, 58.4. Anal. Calcd for
C22H31N3O6: C, 60.95; H, 7.21; N, 9.69. Found C, 61.33; H, 6.95;
N, 9.60.
1742, 1714 cm−1; 1H NMR (500 MHz, CDCl3) δH 7.38−7.25 (m, 5H,
Ph), 4.78 (s, 1H, NH), 3.66 (s, 3H, OCH3), 3.58 (d, 1H, J = 5.2 Hz,
CH2NCO), 3.38 (d, 1H, J = 5.2 Hz, CH2NCO), 3.11 (s, 2H, CH
CH2Ph), 1.48 (s, 9H, tBuO), 1.35 (s, 3H, CMe2), 1.21 (s, 3H, CMe2).
13C NMR (75 MHz, CDCl3) 173.2, 166.9, 153.4, 135.3, 130.3, 128.4,
127.1, 65.8, 58.8, 52.6, 49.9, 39.1, 28.2, 23.7, 23.6; MS m/z (ESI,
positive polarity) +MS2 343.1, +MS4 283.0. Anal. Calcd for
C20H28N2O5: C, 63.81; H, 7.50; N, 7.44. Found: C, 63.64; H, 7.36;
N, 7.19.
Compound 6f. To a solution of (3R)-3-benzyl-3-tert-butoxycar-
bonyl-1-[1-(methoxycarbonyl)isopropyl]azetidin-2-one (1 mmol,
0.376 g) in MeOH (20 mL) was added LiOH·.H2O (10 mmol,
0.485 g) in 10 mL of H2O, and the mixture was stirred at rt for 4 h.
Acidification of the resulting mixture with 1 M HCl and extraction
with CH2Cl2 (3 × 8 mL) provided an organic solution of (3R)-3-
benzyl-3-tert-butoxycarbonyl-1-[1-(carboxy)isopropyl]azetidin-2-one,
which was dried (Na2SO4) and filtered. H-Aib-OBn (1.3 mmol, 0.248
g) was added, the mixture was cooled to −12 °C, and EEDQ (1.3
mmol, 0.331 g) was added. The reaction mixture was stirred during 16
h, while rt was slowly reached. The resulting solution was washed with
1 M HCl (3 × 10 mL), the aqueous layer was extracted with CH2Cl2,
and the combined organic phases were dried (MgSO4) and
evaporated. The crude was purified by flash column chromatography
(hexanes/EtOAc, 5:1) to afford benzyl 2-[[(3R)-3-benzyl-3-(tert-
butoxycarbonylamino)2-oxoazetidin1-yl]dimethylacetamido]-2-meth-
ylpropanoate: white solid; 0.494 g (92%). A suspension of this benzyl
ester (0.92 mmol) and 20% Pd/C in EtOAc (35 mL) was stirred for 1
h at rt under H2 atmosphere. The mixture was filtered through a Celite
pad, and the filtrate was evaporated: yield 0.411 g (100%); white solid;
mp 213−215 °C; [α]25D +65.2 (0.48, CH2Cl2); IR (KBr) 3346, 1736,
1702, 1685 cm−1; 1H NMR (500 MHz, CDCl3) δH 7.87 (s, 1H,
NHCMe2), 7.38−7.27 (m, 5H, Ar), 4.84 (s, 1H, BocNH), 3.62 (d, 1H,
J = 5.1 Hz, CH2NCO), 3.35 (d, 1H, J = 5.1 Hz, CH2NCO), 3.14 (d,
1H, J = 13.4 Hz, CH2Ph), 2.89 (d, 1H, J = 13.4 Hz, CH2Ph), 1.58 (s,
3H, Me2C), 1.57 (s, 3H, Me2C), 1.45 (s, 9H, tBuO), 1.32 (s, 3H,
Me2C), 0.95 (s, 3H, Me2C); 13C NMR (75 MHz, CDCl3) 175.8, 174.5,
168.3, 154.7, 133.5, 130.8, 129.1, 128.2, 81.6, 66.4, 59.9, 57.9, 48.6,
39.5, 28.7, 25.9, 25.3, 24.4, 23.8; MS m/z (ESI, positive polarity) MS
446.2, MS2(446.2) 372.8, 372.1, MS3(372.1) 329.0, MS4(329.0)
329.1, 242.9, 201.9, 199.9, 172.0, 168.9, 153.9, 127.9, 111.0. Anal.
Calcd for C23H33N3O6: C, 61.73; H, 7.43; N, 9.39. Found: C, 61.90;
H, 7.26; N, 9.34.
2-[2-[(3R)-3-Benzyl-3-(tert-butoxycarbonylamino)-2-oxoazetidin-
1-yl]-(S)-propanamido]-2-methylpropanoic acid (6c). The general
procedure was followed from compound 13c (1 mmol, 0.608 g):
overall yield 0.346 g (80%); white solid; mp 216−218 °C; [α]25
D
−28.1 (0.36, CH2Cl2); IR (KBr) νmax 2974, 1739, 1698, 1583, 1363,
1166 cm−1; 1H NMR (500 MHz, CDCl3) δH 7.77 (s, 1H, NHCMe2),
7.39−7.19 (m, 5H, Ar), 4.9 (s, 1H, BocHN), 4.34 (m, 1H, CHMe),
3.65 (d, 1H, J = 4.4 Hz, CH2NCO), 3.38 (d, 1H, J = 5.4 Hz,
CH2NCO), 3.19 (d, 1H, J = 13.5 Hz, CH2Ph), 2.94 (d, 1H, J = 13.5
Hz, CH2Ph), 1.55 (s, 3H, Me2C), 1.55 (s, 3H, Me2C), 1.45 (s, 9H,
tBuO), 0.85 (d, 3H, J = 7.25 Hz, MeCH); 13C NMR (125 MHz,
CDCl3) δC 180.6, 170.2, 169.8, 155.5, 134.6, 130.7, 128.4, 127.4, 79.9,
67.7, 58.4, 49.8, 38.1, 29.7, 27.7, 25.8, 24.8, 12.9. HRMS (TOF CI) m/
z 433.2221, C22H31N3O6 requires 433.2216.
2-[[(3R)-3-Benzyl-3-(tert-butoxycarbonylamino)-2-oxoazetidin1-
yl]-(R)-phenylacetamido]-2-methylpropanoic Acid (6d). The general
procedure was applied from β-lactam 13d (1.00 mmol, 0.670 g): yield
0.355 g (82%); white solid; mp = 238 °C; [α]25 +35.8 (0.65,
D
MeOH); IR (KBr) νmax 3320, 2928, 2851, 1744, 1686, 1670, 1651
cm−1; 1H NMR (500 MHz, CD3OD) δH 7.32−6.64 (m, 10H), 5.03 (s,
1H), 3.58 (d, 1H, J = 5.5 Hz), 3.02 (d, 1H, J = 13.5 Hz), 2.85 (d, 1H, J
= 13.5 Hz), 2.83 (d, 1H, J = 5.5 Hz), 1.61 (s, 3H), 1.57 (s, 3H), 1.50
(s, 9H); 13C NMR (125 MHz, CD3OD) δC 176.8, 169.7, 168.9, 155.4,
133.7, 133.6, 130.3, 128.7, 128.3, 127.8, 127.2, 79.8, 67.6, 58.6, 56.8,
48.5, 27.3, 25.4, 23.7. HPLC-MS, MeOH/HCO2H m/z (ESI, positive
polarity) 566.4; MS2(566.4) 436.1, 435.0, 434.2, 374.1, 202.1;
MS3(434.2) 375.1, 374.1, 188.0, 187.0, 178.0, 173.0, 172.0, 159.0,
131.0, 130.0; MS4(374.1) 188.0, 187.0, 172.0, 170.0, 159.0, 130.9.
Anal. Calcd for C27H33N3O6: C, 65.44; H, 6.71; N, 8.48. Found C,
65.30; H, 6.49; N, 8.13.
2-[[(3R)-3-Benzyl-3-(tert-butoxycarbonylamino)-2-oxoazetidin1-
yl](S)-phenylacetamido]-2-methylpropanoic Acid (6e). The general
procedure was applied from β-lactam 13e (1.00 mmol, 0.670 g): yield
0.346 g (80%); mp 113−115 °C; [α]25 +44.5 (0.3, MeOH); IR
General Procedure for the Preparation of Boc-Ala-(α-Bn-β-
lactam)-Aib-OMe Compounds 15 and 16. Thiophenol (5 mmol,
0.469 mL) and K2CO3 (4 mmol, 0.567 g) were added to a stirred
solution of (3R)-3-benzyl-1-[1-(methoxycarbonyl)isopropyl]-3-(2-
nitrobenzenesulfonylamino)azetidin-2-one 14 (1 mmol, 0.461 g) in
D
(KBr) νmax 3326, 2980, 2916, 2853, 1744, 1698, 1674, 1660 cm−1; 1H
NMR (500 MHz, DMSO-d6) δH 8.41 (s, 1H), 7.30−7.22 (m, 10H),
5.29 (s, 1H), 3.48 (br s, 1H), 3.32 (br s, 1H), 3.17−3.10 (br s, 2H),
1.36 (br s, 15H). 13C NMR (75 MHz, DMSO-d6) δ 176.8, 168.1,
K
dx.doi.org/10.1021/jo302368y | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Featured Article
1
MeCN (5 mL). The mixture was stirred over 2 h, the solvent was
evaporated in vacuo, and the crude was purified by flash column
chromatography (EtOAc, then MeOH): yield 90%. A solution of the
resulting amine and the corresponding Boc-Ala-OH (1.3 mmol, 0.248
g) in anhydrous CH2Cl2 (20 mL) was cooled to −12 °C, and EEDQ
(1.3 mmol, 0.331 g) was added. The mixture was stirred during 16 h,
while rt was slowly reached. The resulting solution was washed with 1
M HCl (3 × 10 mL), the aqueous layer was extracted with CH2Cl2,
and the combined organic phases were dried (MgSO4) and
evaporated. The product was purified by flash column chromatography
(hexanes/EtOAc, 2:1).
νmax 3416, 3308, 2965, 1741, 1726, 1694, 1536, 691 cm−1; H NMR
(500 MHz, DMSO-d6) δH 8.3 (s, 1H, NHCCH2Ph), 8.16 (d, 1H, J =
6.9 Hz, NHCH(Me)CONH2), 7.33−7.28 (m, 5H, Ph), 7.1 (bs, 1H,
NH2), 6.87 (d, 1H, J = 8.1 Hz, BocNH), 6.83 (bs, 1H, NH2), 4.13 (m,
1H, BocHNCH(Me), 4.06 (m, 1H, CH(Me)CONH2), 3.58 (d, 1H, J
= 5.1 Hz, CH2NCMe2), 3.26 (d, 1H, J = 5.0 Hz, CH2NCMe2), 3.05 (d,
1H, J = 13.4 Hz, CH2Ph), 3.02 (d, 1H, J = 13.5 Hz, CH2Ph), 1.39 (s,
9H, tBuO), 1.29 (d, 3H, J = 7.2 Hz, CH(Me)CONH2), 1.20 (d, 3H, J
= 6.9 Hz, BocHNCH(Me), 1.12 (s, 3H, Me2CCO), 0.7 (s, 3H,
Me2CCO); 13C NMR (75 MHz, CDCl3) 175.0, 173.6, 173.2, 166.8,
156.1, 133.1, 130.3, 128.7, 127.8, 80.9, 77.2, 65.8, 59.8, 49.6, 38.9, 28.3,
24.3, 23.6, 15.9, 16.5; HPLC-MS, MeOH/HCO2H m/z (ESI, positive
polarity) 217.2 (19), 167.1 (33), 149.0 (100), 119.1 (26), 85.9 (29);
HRMS (TOF CI) m/z 503.2763, requires 503.2744. Anal. Calcd for
C25H37N5O6: C, 59.63; H, 7.41; N, 13.91. Found: C, 59.70; H, 7.01;
N, 13.91.
Methyl 2-[[(3R)-3-Benzyl-3-[2-(tert-butoxycarbonylamino)-(2R)-
propanamido]2-oxoazetidin1-yl]-2-methylpropanoate (15). The
general procedure was followed from compound 17 (1 mmol, 0.461
g) and Boc-D-Ala-OH (1.3 mmol, 0.248 g): yield 0.371 g (83%); white
solid; mp 135−137 °C; [α]25D +28.5 (0.2, CH2Cl2); IR (KBr) ν 3309,
1
3000, 1763, 1752, 1704, 1656, 1160, 672 cm−1; H NMR (500 MHz,
2-[2-[(3R)-Benzyl-3-[2-(tert-butoxycarbonylamino)-(2R)-
propanamido]2-oxoazetidin1-yl]2-methylpro panamido]-(2S)-
propanamide (7b). The general procedure was followed from methyl
2-[(3R)-3-benzyl-3-[2-(tert-butoxycarbonylamino)(2R)-propan-
amido]2-oxoazetidin1-yl]-2-dimethylpropanoate 15 (1 mmol, 0.447 g)
and (S)-alaninamide 18 (1.5 mmol, 0.131 g): yield 0.327 g (65%);
white solid; mp 202−204 °C; [α]25D −80.0 (0.50, CH2Cl2); IR (KBr)
CDCl3) δH 7.28−7.32 (m, 5H, Ar), 6.62 (bs, 1H, CONHCCH2Ph),
4.9 (bs, 1H, CONHCH(CH3)CO), 4.14 (m, 1H, NHCH(Me)CO),
3.71 (s, 3H, CO2Me), 3.63 (d, 1H, J = 5.5 Hz, NCH2CCH2Ph), 3.45
(d, 1H, J = 5.4 Hz, NCH2CCH2Ph), 3.21 (d, 1H, J = 14.2 Hz,
CH2Ph), 3.18 (d, 1H, J = 14.0 Hz, CH2Ph), 1.45 (s, 9H, tBuO), 1.43
(s, 3H, Me2CCO2Me), 1.36 (s, 3H, Me2CCO2Me), 1.31 (d, 3H, J = 7.1
Hz, CH(Me)CONH); 13C NMR (75 MHz, CDCl3) 173.1, 172.7,
166.8, 155.3, 134.8, 130.2, 128.4, 127.2, 80.2, 65.7, 58.9, 52.5, 49.4,
38.7, 28.3, 23.6, 18.3, 16.8; HPLC-MS, MeOH/HCO2H m/z (ESI,
positive polarity) 187.1 (35), 133.1 (99), 126.1 (62), 112.1 (41), 97.1
(41), 72.1 (85), 59.1 (100), 57.1 (48), 55.1 (53); HRMS (TOF CI)
m/z 447.2372, C23H33N3O6 requires 447.2369.
1
νmax 3412, 3213, 1733, 1730, 1681, 1629, 696, 734 cm−1; H NMR
(500 MHz, DMSO-d6) 8.41 (s, 1H, NHCCH2Ph), 8.30 (d, 1H, J = 6.7
Hz, NHCH(Me)CONH2), 7.33−7.28 (m, 5H, Ph), 7.10 (s, 1H,
NH2), 6.88 (d, 1H, J = 7.8 Hz, NHBoc), 6.84 (s, 1H, NH2), 4.13 (m,
1H, tBuOCONHCHMe), 4.06 (m, 1H, CH(Me)CONH2), 3.58 (d,
1H, J = 5.3 Hz, CH2NCMe2), 3.26 (d, 1H, J = 5.0 Hz, CH2NCMe2),
3.06 (d, 1H, J = 13.4 Hz, CH2Ph), 3.02 (d, 1H, J = 13.6 Hz, CH2Ph),
1.39 (s, 9H, tBuO), 1.29 (d, 3H, J = 7.1 Hz, CH(Me)CONH2), 1.20
(d, 3H, J = 6.9 Hz, BocHNCH(Me), 1.09 (s, 3H, Me2CCO), 0.75 (s,
3H, Me2CCO); 13C NMR (75 MHz, CDCl3) 175.2, 175.1, 173.5,
173.3, 167.0, 156.1, 133.1, 130.3, 128.7, 127.8, 80.8, 77.2, 65.8, 59.8,
49.7, 47.5, 38.8, 28.3, 24.3, 16.6; HPLC-MS, MeOH/HCO2H m/z
(ESI, positive polarity) 119.1 (88), 97.1 (60), 91.1 (72), 71.1 (67),
69.1 (63), 57.1 (100), 55.1 (90); HRMS (TOF CI) m/z 503.2727,
C25H37N5O6 requires 503.2744.
Methyl 2-[(3R)-3-Benzyl-3-[2-(tert-butoxycarbonylamino)(2S)-
propanamido]2-oxoazetidin-1-yl]-2-methylpropanoate (16). The
general procedure was followed from compound 17 (1 mmol, 0.461
g) and Boc-Ala-OH (1.3 mmol, 0.248 g): yield 0.380 g (85%);
yellowish solid; [α]25 −22.4 (0.5, CH2Cl2); IR (KBr) ν 3271, 2993,
D
1753, 1721, 1716, 1673, 1167, 740 cm−1; 1H NMR (500 MHz,
CDCl3) δH 7.32 (m, 5H, Ar), 4.08 (m, 1H, COCH(Me)NH), 3.65 (s,
3H, CO2Me), 3.59 (d, 1H, J = 5.5 Hz, NCH2C), 3.41 (d, 1H, J = 5.3
Hz, NCH2C), 3.15 (dd, 2H, J = 13.7 Hz, CH2Ph), 1.45 (s, 9H, tBuO),
1.33 (s, 3H, CMe2CO2Me), 1.29 (d, 1H, J = 6.9 Hz, CHMeNH), 1.23
(s, 3H, CMe2CO2Me); 13C NMR (75 MHz, CDCl3) 173.2, 172.9,
155.3, 134.9, 130.3, 128.4, 127.1, 79.6, 65.9, 58.9, 53.5, 52.5, 49.8, 38.5,
28.3, 23.4, 18.3; HPLC-MS, MeOH/HCO2H m/z (ESI, positive
polarity) 187.1 (32), 134.1 (28), 133.1 (100), 115.6 (27), 102.5 (21),
92.1 (33), 91.1 (41), 58 (56); HRMS (TOF CI) m/z 447.2348,
C23H33N3O6 requires 447.2369.
2-[2-[(3R)-Benzyl-3-[2-(tert-butoxycarbonylamino)(2S)-
propanamido]2-oxoazetidin1-yl]2-methylpro panamido]-(2S)-
propanamide (7c). The general procedure was followed from methyl
2-[(3R)-3-benzyl-3-[2-(tert-butoxycarbonylamino)(2S)-propan-
amido]2-oxoazetidin-1-yl]-2-dimethylpropanoate 16 (1 mmol, 0.447
g) and (S)-alaninamide 18 (1.5 mmol, 0.131 g): yield 0.326 g (73%);
yellowish solid; mp 160−162 °C; [α]25 +66.0 (0.20, CH2Cl2); IR
D
(KBr) νmax 3334, 2969, 1748, 1682, 1536, 1165 cm−1; H NMR (500
1
General Procedure for the Preparation of Boc-Ala-(β-
Lactam)-Aib-Ala-NH2 Compounds 7a−d. To a solution of the
corresponding methyl 2-[(3R)-3-benzyl-3-[2-(tert-butoxycarbonyl-
amino)2R-propanamido]2-oxoazetidin-1-yl]-2-dimethylpropanoate 15
or 16 (1 mmol, 0.447 g) in MeOH (20 mL) was added a solution of
LiOH·H2O (10 mmol, 0.485 g) in MeOH/H2O 10:5 mL. The mixture
was stirred for 2 h and acidified with 1 M HCl (5 mL), and the
aqueous phase was extracted with CH2Cl2 (3 × 15 mL) to afford the
intermediate carboxylic acid in 85% yield. To this crude material
dissolved in anhydrous DMF (15 mL) cooled to 0 °C was added
successively the corresponding alaninamide (1.5 mmol, 0.131 g),
HATU (1.5 mmol, 0.570 g), HOAT (1.4 mmol, 0.191 g), and KHCO3
(15 mmol, 1.5 g). The mixture was stirred during 16 h while slowly
reaching rt. The solvent was evaporated at low pressure, and the
residue was redissolved in CH2Cl2 and washed with 1 M HCl (3 × 10
mL) and satd NaHCO3 (3 × 10 mL). After evaporation of the organic
solvent, the crude product was purified by flash column chromatog-
raphy (hexanes/EtOAc, 5:1 or CH2Cl2/MeOH, 15:1).
MHz, DMSO-d6) δH 8.32 (s, 1H, NHCCH2Ph), 8.16 (d, 1H, J = 6.5
Hz, NHCH(Me)CONH2), 7.33−7.28 (m, 5H, Ph), 7.06 (bs, 1H,
NH2), 6.91 (d, 1H, J = 7.0 Hz, NHBoc), 6.77 (bs, 1H, NH2), 4.08 (m,
1H, BocNHCHMe), 4.02 (m, 1H, CH(Me)CONH2), 3.61 (d, 1H, J =
4.7 Hz, CH2NCMe2), 3.24 (d, 1H, J = 4.8 Hz, CH2NCMe2), 3.05 (d,
1H, J = 13.4 Hz, CH2Ph), 3.01 (d, 1H, J = 13.5 Hz, CH2Ph), 1.38 (s,
9H, tBuO), 1.31 (d, 3H, J = 7.2 Hz, CH(Me)CONH2), 1.20 (d, 3H, J
= 7.2 Hz, BocNHCHMe), 1.12 (s, 3H, Me2CCO), 0.7 (s, 3H,
Me2CCO); 13C NMR (75 MHz, CDCl3) 175.0, 173.6, 173.0, 166.7,
155.9, 133.0, 130.3, 128.7, 127.8, 80.9, 75.8, 65.9, 59.8, 49.6, 47.6, 38.9,
28.3, 24.3, 23.6, 16.5; HPLC-MS, MeOH/HCO2H m/z (ESI, positive
polarity) 187.1 (49), 133.1 (37), 91.1 (100), 70.1 (60), 57.1 (42).
HRMS (TOF CI) m/z 503.2771, requires 503.2744. Anal. Calcd for
C25H37N5O6: C, 59.63; H, 7.41; N, 13.91. Found: C, 59.85; H, 7.34;
N, 13.92.
2-[2-[(3R)-Benzyl-3-[2-(tert-butoxycarbonylamino)(2S)-
propanamido]2-oxoazetidin1-yl]2-methylpro panamido]-(2R)-
propanamide (7d). The general procedure was followed from methyl
2-[(3R)-3-benzyl-3-[2-(tert-butoxycarbonylamino)(2S)-propan-
amido]2-oxoazetidin1-yl]-2-dimethylpropanoate 16 (1 mmol, 0.447 g)
and (R)-alaninamide 18 (1.5 mmol, 0.131 g): yield 0.392 g (77%);
2-[2-[(3R)-Benzyl-3-[2-(tert-butoxycarbonylamino)-(2R)-
propanamido]2-oxoazetidin1-yl]-2-methylpro panamido]-(2R)-
propanamide (7a). The general procedure was followed from methyl
2-[(3R)-3-benzyl-3-[2-(tert-butoxycarbonylamino)(2R)-propan-
amido]2-oxoazetidin1-yl]-2-dimethylpropanoate 15 (1 mmol, 0.447 g)
and (R)-alaninamide 18 (1.5 mmol, 0.131 g): yield 0.629 g (80%);
white solid; mp 190−192 °C; [α]25D +36.0 (0.30, CH2Cl2); IR (KBr)
yellowish solid; mp 219−220 °C; [α]25 −15.9 (0.20, CH2Cl2); IR
D
(KBr) νmax 3413, 3303, 2978, 2914, 1744, 1663, 1542 cm−1; 1H NMR
(500 MHz, DMSO-d6) δH 8.53 (s, 1H, NHCCH2Ph), 8.37 (d, 1H, J =
L
dx.doi.org/10.1021/jo302368y | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Featured Article
7.3 Hz, NHCH(Me)CONH2), 7.33−7.28 (m, 5H, Ar), 7.03 (d, 1H, J
= 6.7 Hz, BocNH), 6.97 (s, 1H, NH2), 6.88 (s, 1H, NH2), 4.04 (m,
1H, BocNHCHMe), 4.03 (m, 1H, CH(Me)CONH2), 3.46 (d, 1H, J =
4.8 Hz, CH2NCMe2), 3.34 (d, 1H, J = 5.0 Hz, CH2NCMe2), 3.06 (d,
1H, J = 13.4 Hz, CH2Ph), 3.01 (d, 1H, J = 13.4 Hz, CH2Ph), 1.38 (s,
9H, tBuO), 1.31 (d, 3H, J = 7.2 Hz, CH(Me)CONH2), 1.19 (d, 3H, J
= 7.0 Hz, BocHNCHMe), 0.97 (s, 3H, Me2CCO), 0.79 (s, 3H,
Me2CCO); 13C NMR (75 MHz, CDCl3) 175.9, 173.4, 172.5, 167.7,
155.9, 132.7, 130.3, 128.8, 127.9, 80.8, 77.2, 65.6, 58.9, 49.5, 48.1, 38.8,
28.3, 23.7, 17.3; HPLC-MS, MeOH/HCO2H m/z (ESI, positive
polarity) 187.1 (44), 173.1 (28), 172.1 (26), 117.1 (37), 116 (25), 115
(26), 113.1 (35), 91 (100), 70.0 (47); HRMS (TOF CI) m/z
503.2742, requires 503.2744. Anal. Calcd for C25H37N5O6: C, 59.63;
H, 7.41; N, 13.91. Found: C, 59.67; H, 7.62; N, 13.88.
Blomberg, D.; Sethson, I.; Brickmann, K.; Ekholm, K.; Johansson, B.;
Nilsson, A.; Kihlberg, J. J. Med. Chem. 2002, 45, 2512−2519. (f) Chou,
K. C. Anal. Biochem. 2000, 286, 1−16.
(4) See, for example: (a) Conti, P.; Tamborini, L.; Pinto, A.; Blondel,
A.; Minoprio, P.; Mozzarelli, A.; De Micheli, C. Chem. Rev. 2011, 111,
6919−6946. (b) Nickl, C. K.; Raidas, S. K.; Zhao, H.; Sausbier, M.;
Ruth, P.; Tegge, W.; Brayden, J. E.; Dostmann, W. R. Biochim. Biophys.
Acta 2010, 1804, 524−532. (c) Schumacher, T. N. M.; Mayr, L. M.,
Jr.; D. L., M.; Milhollen, M. A.; Burgess, M. W.; Kim, P. S. Science
1996, 271, 1854−1857.
(5) (a) Freidinger, R. M. J. Med. Chem. 2003, 46, 5553−5566.
(b) Veber, D. F.; Freidinger, R. M.; Perlow, D. S.; Paleveda, W. J., Jr.;
Holly, F.; Strachan, R. G.; Nutt, R. F.; Arison, B. H.; Homnick, C.;
Randall, W. C.; Glitzer, M. S.; Saperstein, R.; Hirshmann, R. A. Nature
1981, 292, 55−58.
(6) (a) Robinson, J. A. Acc. Chem. Res. 2008, 41, 1278−1288.
(b) Haque, T. S.; Little, J. C.; Gellman, S. H. J. Am. Chem. Soc. 1996,
118, 6975−6985.
ASSOCIATED CONTENT
* Supporting Information
■
S
1H NMR spectra of compounds 5−7 and 13d−e. 13C NMR
spectra of compounds 5−13b,c, 15, and 16. NMR/ROESY
spectra and interproton distances of compounds 5−7. Gaussian
output data of structures 5a−f. X-ray data for 5e, 5f, 7a, and 7d
(CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
(7) (a) Synthesis of Peptides and Peptidomimetics. In Houben-Weyl,
Methods of Organic Chemistry; Felix, A., Moroder, L., Toniolo, C., Eds.;
Thieme: Stuttgart, 2003; Vol. E22c. (b) Burgess, K. Acc. Chem. Res.
2001, 34, 826−835. (c) MacDonald, M.; Aube, J. Curr. Org. Chem.
2001, 5, 417−438. (d) Souers, A. J.; Ellman, J. A. Tetrahedron 2001,
57, 7431−7448. (e) Peczuh, M. W.; Hamilton, A. D. Chem. Rev. 2000,
100, 2479−2494. (f) Hanessian, S.; McNaughton-Smith, G.; Lombart,
H.-G.; Lubell, W. D. Tetrahedron 1997, 53, 12789−12854. (g) Gante,
J. Angew. Chem., Int. Ed. Engl. 1994, 33, 1699−1720. (h) Giannis, A.;
Kolter, T. Angew. Chem., Int. Ed. Engl. 1993, 32, 1244−1267.
(8) For some recent selected examples, see: (a) Sacchetti, A.; Silvani,
A.; Lesma, G.; Pilati, T. J. Org. Chem. 2011, 76, 833−839.
(b) Larregola, M.; Lequin, O.; Karoyan, P.; Guianvarc’h, D.; Laville,
S. J. Pept. Sci. 2011, 17, 632−643. (c) Fustero, S.; Mateu, N.; Albert,
AUTHOR INFORMATION
Corresponding Author
*E-mail: jesusmaria.aizpurua@ehu.es, claudio.palomo@ehu.es.
■
Notes
The authors declare no competing financial interest.
X-ray crystallography was performed at the University of
Zurich.
L.; Acena, J. L. J. Org. Chem. 2009, 74, 4429−4432. (d) De Wachter,
̃
R.; Brans, L.; Ballet, S.; Van den Eynde, I.; Feytens, D.; Keresztes, A.;
ACKNOWLEDGMENTS
■
́
Toth, G.; Urbanczyk-Lipkowska, Z.; Tourwe, D. Tetrahedron 2009, 65,
We thank the Ministerio de Ciencia y Competitividad (MEC,
Spain) (Project No. CTQ2010-21625-C02-01), Gobierno
Vasco (ETORTEK-nanoIKER IE-11/304), and Universidad
2266−2278. (e) Bittermann, H.; Boeckler, F.; Einsiedel, J.; Gmeiner,
P. Chem.Eur. J. 2006, 12, 6315−6322. (f) Duggan, H. M. E.;
Hitchcock, P. B.; Young, D. W. Org. Biomol. Chem. 2005, 3, 2287−
2295.
́
del Pais Vasco UPV/EHU (UFI QOSYC 11/22) for financial
(9) (a) Lessing, J.; Roy, S.; Reppert, M.; Baer, M.; Marx, D.; La Cour
Jansen, T.; Knoester, J.; Tokmakoff, A. J. Am. Chem. Soc. 2012, 134,
5032−5035. (b) De Poli, M.; Moretto, A.; Crisma, M.; Peggion, C.;
Formaggio, F.; Kaptein, B.; Broxterman, Q. B.; Toniolo, C. Chem.
Eur. J. 2009, 15, 8015−8025. (c) Liang, G.-B.; Rito, C. J.; Gellman, S.
H. J. Am. Chem. Soc. 1992, 114, 4440−4442.
(10) (a) Palomo, C.; Aizpurua, J. M.; Benito, A.; Miranda, J. I.;
Fratila, R. M.; Matute, C.; Domercq, M.; Gago, F.; Martin-Santamaria,
S.; Linden, A. J. Am. Chem. Soc. 2003, 125, 16243−16260. (b) Palomo,
C.; Aizpurua, J. M.; Benito, A.; Galarza, R.; Khamrai, U. K.; Vazquez,
J.; de Pascual-Teresa, B.; Nieto, P. M.; Linden, A. Angew. Chem., Int.
Ed. Engl. 1996, 35, 1239−1241.
support and SGIker UPV/EHU for NMR facilities. Grants from
Gobierno Vasco to M.S.-A. and from UPV/EHU to E.A. are
acknowledged.
REFERENCES
■
(1) (a) Mahalakshmi, R.; Balaram, P. The Use of D-Amino Acids in
Peptide Design. In D-Amino Acids: A New Frontier in Amino Acid and
Protein Research − Practical Methods and Protocols; Konno, R.,
Bruckner, H., D’Aniello, A., Fischer, G. H., Fujii, N., Homma, H.,
̈
Eds.; Nova Science Publishers: New York, 2007; pp 415−430.
(b) Bruckner, H.; Fuji, N. D-Amino Acids in Chemistry, Life Sciences, and
̈
Biotechnology; Wiley: New York, 2010.
(11) For alternative β-lactams as reverse turn nucleators, see:
(a) Bari, S.; Bhalla, A. Top. Heterocycl. Chem. 2010, 22, 49−99.
(b) Bittermann, H.; Gmeiner., P. J. Org. Chem. 2006, 71, 97−102.
(c) Khasanov, A. B.; Ramirez-Weinhouse, M. M.; Webb, T. R.;
Thiruvazhi, M. J. Org. Chem. 2004, 69, 5766−5769. (d) Alonso, E.;
Lopez-Ortiz, F.; del-Pozo, C.; Peralta, E.; Macia, A.; Gonzalez, J. J. Org.
Chem. 2001, 66, 6333−6338.
(2) (a) Venkatachalam, M. Biopolymers 1968, 6, 1425−1436.
(b) Marcelino, A. M. C.; Gierasch, L. M. Biopolymers 2008, 89,
380−391 and references cited therein. (c) Rotondi, K. S.; Gierasch, L.
M. Biopolym. (Peptide Sci.) 2006, 84, 13−22. (d) Tyndall, J. D. A.;
Pfeiffer, B.; Abbenante, G.; Fairlie, D. P. Chem. Rev. 2005, 105, 793−
826. (e) Vass, E.; Hollosi, M.; Besson, F.; Buchet, R. Chem. Rev. 2003,
́
103, 1917−1954. (f) Ball, J. B.; Hughes, R. A.; Alewood, P. F.;
Andrews, P. R. Tetrahedron 1993, 49, 3467−3478.
(12) The most abundant natural form of β-turn is type III (ϕ(i+1)
=
−60/ ψ(i+1)= −30/ ϕ(i+2)= −60/ ψ(i+2)= −30), as part of 310-helices.
Type-I (ϕ(i+1)= −60/ ψ(i+1)= −30/ ϕ(i+2)= −90/ ψ(i+2)= 0) is the
second and type-II (ϕ(i+1)= −60/ ψ(i+1)= 120/ ϕ(i+2)= 80/ ψ(i+2)= 0) is
the third one. The sum of types I, II, and III accounts for more that
95% of all the β-turns assigned in globular proteins. As types III, I, and
II differ from each other by some 30° in the torsional angles ϕ(i+2) and
ψ(i+2), they are often not distinguished. Thus, when type preference of
β-turns is investigated as a function of their amino acid sequence, most
commonly only the ratio of type (I + III) to type II is determined. See:
(3) (a) Bystrov, V. F.; Portnova, S. L.; Tsetlin, V. I.; Ivanov, V. T.;
Ovchinnikov, Y. A. Tetrahedron 1969, 25, 493−515. (b) Zhu, Y.; Li,
T.; Li, D.; Zhang, Y.; Xiong, W.; Sun, J.; Tang, Z.; Chen, G. Amino
Acids 2012, 42, 1749−1755 and references cited therein. For selected
γ-turn mimetics, see: (c) Saavedra, C.; Boto, A.; Hernandez, R.;
Miranda, J. I.; Aizpurua, J. M. J. Org. Chem. 2012, 77, 5907−5913.
(d) 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. (e) Yuan, Z.;
M
dx.doi.org/10.1021/jo302368y | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Featured Article
́
Perczel, A.; Jakli, I.; Foxman, B. M.; Fasman, G. D. Biopolymers 1996,
38, 723−732.
(13) Burgi, H. B.; Dunitz, J. D.; E. Shefter, E. J. Am. Chem. Soc. 1973,
̈
95, 5065−5067.
(14) Tarbe, M.; Azcune, I.; Balentova, E.; Miles, J. J.; Edwards, E. E.;
Miles, K. M.; Do, P.; Baker, B. M.; Sewell, A. K.; Aizpurua, J. M.;
Douat-Casassus, C.; Quideau, S. Org. Biomol. Chem. 2010, 8, 5345−
5353.
(15) Glover, S. A.; Rosser, A. A. J. Org. Chem. 2012, 77, 5492−5502.
(16) The apparent configuration inversion at the azetidinone Cα
stereocenter was caused by a change in the group preference,
according to the Cahn−Ingold−Prelog rules.
(17) To prepare enantiopure α-alkylserinates from D-serine, see:
(a) Di Giacomo, M.; Vinci, V.; Serra, M.; Colombo, L. Tetrahedron:
Asymmetry 2008, 19, 247−257. (b) Seebach, D.; Aebi, J. D.; Gander-
Coquoz, M.; Naef, R. Helv. Chim. Acta 1987, 70, 1194−1216.
́ ́
(18) Palomo, C.; Aizpurua, J. M.; Balentova, E.; Jimenez, A.;
Oyarbide, J.; Fratila, R.; Miranda, J. I. Org. Lett. 2007, 9, 101−104.
(19) (a) Chan, T. Y.; Chen, A.; Allason, N.; Chen, R.; Liu, D.; Sofia,
M. J. Tetrahedron Lett. 1996, 37, 8097−8100. (b) Sipos, F.; Gaston, D.
W. Synthesis 1971, 321−322.
(20) Maligres, P. E.; See, M. M.; Askin, D.; Reider, P. J. Tetrahedron
Lett. 1997, 38, 5253−5256.
(21) (a) Tulla-Puche, J.; Marcucci, E.; Prats-Alfonso, E.; Bayo-Puxan,
N.; Albericio, F. J. Med. Chem. 2009, 52, 834−839. (b) Ghosh, A. K.;
Kulmaki, S. Org. Lett. 2008, 10, 3907−3909.
(22) For methods, see: (a) Troganis, A.; Gerothanassis, I. P.;
Athanassiou, Z.; Mavromoustakos, T.; Hawkes, G. E.; Sakarellos, C.
Biopolymers 2000, 53, 72−83. (b) Kessler, H. Angew. Chem., Int. Ed.
Engl. 1982, 21, 512−523. (c) Andersen, N. H.; Neidigh, J. W.; Harris,
S. M.; Lee, G. M.; Liu, Z.; Tong, Hui J. Am. Chem. Soc. 1997, 119,
8547−8561.
(23) (a) Neuhaus, D.; Williamson, M. The Nuclear Overhauser Effect
in Structural and Conformational Analysis; VCH Publishers: New York,
1989. (b) Thomas, P. D.; Basus, V. J.; James, T. L. Proc. Nat. Acad. Sci.
U.S.A. 1991, 88, 1237−1241.
(24) For related β-II/β-III′ overlapped peptides, see: (a) Lisowski,
M.; Jaremko, L.; Jaremko, M.; Mazur, A.; Latajka, R.; Makowski, M.
Biopolymers 2010, 93, 1055−1064. (b) Inai, Y.; Oshikawa, T.;
Yamashita, M.; Kato, I.; Hirabayashi, T. Chem. Lett. 2001, 472−473.
Two β-I overlapped peptides: (c) Lo, S.-C.; Li, X.; Henzl, M. T.;
Beamer, L. J.; Hannink, M. EMBO J. 2006, 25, 3605−3617. For a
review on multiple β-turns (not all overlapped), see: (d) Guruprasad,
K.; Prasad, M. S.; Kumar, G. R. J. Peptide. Res. 2000, 56, 250−263.
(25) For theoretical calculations on peptide conformation, see:
(a) Perczel, A.; Jak
́
li, I.; McAllister, M. A.; Csizmadia, I. G. Chem.
Eur. J. 2003, 9, 2551−2556 and references cited therein. (b) Sapse, A.
M. Molecular Orbital Calculations for Amino Acids and Peptides;
Birkhausen: New York, 2000. For gas-phase experimental studies
conducted on configurationally altered short peptides, see: (c) Brenner,
V.; Piuzzi, F.; Dimicoli, I.; Tardivel, B.; Mons, M. Angew. Chem., Int.
Ed. 2007, 46, 2463−2466. (d) Chin, W.; Dognon, J. P.; Piuzzi, F.;
Tardivel, B.; Dimicoli, I.; Mons, M. J. Am. Chem. Soc. 2005, 127, 707−
712.
(26) For hydrogen delocalization from the lone pair (n) of the CO
acceptor over the antibonding orbital (σ*) of the N−H donor, see:
Khaliullin, R. Z.; Cobar, E. A.; Lochan, R. C.; Bell, A. T.; Head-
Gordon, M. J. Phys. Chem. A 2007, 111, 8753−8765.
(27) Epp, J. E.; Widlanski, T. S J. Org. Chem. 1999, 64, 293−295.
(28) Rychnovsky, S. D.; Skalitzky, D. J. J. Am. Chem. Soc. 1992, 114,
671−677.
(29) Shendage, D. M.; Frohlich, R.; Haufe, G. Org. Lett. 2004, 6,
̈
3675−3678.
N
dx.doi.org/10.1021/jo302368y | J. Org. Chem. XXXX, XXX, XXX−XXX