The ant-pro reverse-turn motif. Structural features and conformational characteristics

Article abstract of DOI:10.1002/ejoc.201201739

This article details the characteristic conformational features of the Ant-Pro reverse turn - a folded pseudo β-turn motif that displays a closed nine-membered-ring hydrogen-bonded network involving just two amino acid residues, namely anthranilic acid (A

Full text of DOI:10.1002/ejoc.201201739

FULL PAPER
DOI: 10.1002/ejoc.201201739
The Ant-Pro Reverse-Turn Motif. Structural Features and Conformational
Characteristics
Vijaykumar H. Thorat,[a] Tukaram S. Ingole,[a] Kuruppanthara N. Vijayadas,[a]
Roshna V. Nair,[a] Sangram S. Kale,[a] Veera V. E. Ramesh,[a] Hilda C. Davis,[b]
Panchami Prabhakaran,[c] Rajesh G. Gonnade,[d] Rupesh L. Gawade,[d]
Vedavati G. Puranik,[d] Pattuparambil R. Rajamohanan,*[b] and Gangadhar J. Sanjayan*[a]
Dedicated to Prof. K. N. Ganesh on the occasion of his 60th birthday
Keywords: Peptides / Peptidomimetics / Protein structures / Protein folding
This article details the characteristic conformational features
of the Ant-Pro reverse turn Ϫ a folded pseudo β-turn motif
quires a coplanar disposition of the two amino acid residues,
as clearly evident from investigation of several crystal struc-
tures. The closed hydrogen-bonded network observed in the
Ant-Pro reverse turn motif, formed in the forward direction
of the sequence (1Ǟ2 amino acid interactions) involving only
two amino acid residues, is in stark contrast to the native
β-turns that involve four residues to form hydrogen-bonded
network featuring backward 1ǟ4 amino acid interactions.
The readily available two-residue Ant-Pro motif raises the
possibility of a practical utility, particularly in the application
of rigidifying flexible peptide backbones by inserting the ro-
bust Ant-Pro reverse turn motifs into their backbone.
that displays
a closed nine-membered-ring hydrogen-
bonded network involving just two amino acid residues,
namely anthranilic acid (Ant; a constrained β-amino acid),
and proline (Pro; a constrained α-amino acid). The results
from the extensive investigation of ten crystal structures and
their NMR conformations in the solution state provide a clear
idea about the conformational characteristics of the Ant-Pro
reverse turn. The Ant and Pro residues, which form the turn
segment, maintain
a perfect antiperiplanar orientation
throughout, leaving little possibility for the formation of the
otherwise possible six-membered hydrogen-bonding that re-
view.[3,4] There is now a growing body of evidence to suggest
that over one hundred peptide-activated GPCRs (G pro-
tein-coupled receptors) recognize ligands with “turn” con-
formations, as evidenced by structure–activity relationships
in their native ligands, and modified peptide fragments.[5]
Being conformationally flexible, native peptide ligands for
GPCRs provide limited clues to the design of nonpeptidic
ligands that are useful for drug development, because they
tend to bind in different ways to, or at different sites on, a
GPCR. In addition to conformational rigidity, which has
direct implications in modulating the receptor selectivity of
the ligands,[6] proteolytic stability has also been a central
issue concerning the development of successful peptide mi-
metics.[7] Thus, there is ever-increasing demand for the de-
velopment of novel, conformationally rigid and proteolit-
ically stable synthetic “turn” mimetics[8–13] with possibilities
for extensive periphery modification that might be useful
in the reverse-turn-based pharmacophore design that has
become a central topic in medicinal chemistry in recent
years.[14–16] The Hamilton group first described the confor-
mational preferences of anthranilc acid (Ant), a constrained
aromatic β-amino acid, in its homo and hetero oligo-
Introduction
Peptide reverse turns, defined as sites at which a peptide
chain reverses its overall direction, are common motifs in
protein structures.[1] Reverse turns are generally categorized
as γ-, β-, α-, and π-turns, which are formed by three, four,
five, and six amino acid residues, respectively (Figure 1).[1,2]
According to the folding mode and backbone dihedral
angles, each turn can be further classified into several sub-
types.[2] Reverse turns play an important role in globular
proteins, from both the structural and functional points of
[a] Division of Organic Chemistry, National Chemical Laboratory,
Dr. Homi Bhabha Road, Pune 411008, India
Fax: +91-20-25902629
E-mail: gj.sanjayan@ncl.res.in
Homepage: http://www.ncl.org.in/sanjayan/
[b] Central NMR Facility, National Chemical Laboratory,
Pune, India
[c] Department of Chemistry & Biology, University of Leeds,
LS2 9JT, Leeds, UK
[d] Centre for Material Characterization, National Chemical
Laboratory,
Pune, India
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201739.
Eur. J. Org. Chem. 2013, 3529–3542
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3529

P. R. Rajamohanan, G. J. Sanjayan et al.
FULL PAPER
mers.[17] In its homo oligomers, Ant, featuring six-mem-
bered-ring hydrogen-bonding,[18] promotes sheet-like sec-
ondary structures, as evident from extensive structural in-
vestigations, both in solution and the solid state.[17a,17b] In
its hetero oligomers involving all-aromatic amino acid resi-
dues, various structural architectures including “crescent”
conformations have been observed.[17e] In contrast, the nat-
urally occurring constrained α-amino acid proline (Pro),
with constrained φ and ψ angles, is known to induce a heli-
cal conformation, described as polyproline helices, in its
homo oligomers.[19] It is noteworthy that extensive work
with homo- and hetero-chiral Pro-Xaa building blocks have
yielded diverse sets of short peptide motifs that can stabilize
various peptide secondary structures, as independently
demonstrated by the groups of Balaram[20] and Gellman.[21]
While working on a programme on hetero foldamers,[22] we
inserted Ant periodically into an oligo Pro sequence (1:1
amino acid residue ratio) and showed that such hetero
oligomers display robust right-handed helical structural
architectures that are distinct from the structural architec-
ture of their homo oligomer counterparts.[22g] The helical
conformation displayed repeating segments of nine-mem-
Figure 2. Comparison of a four-residue ideal β-turn [C10 structure
(left)] with the two-residue Ant-Pro turn [C9 structure (right)]. The
contrasting hydrogen-bonding directions in the ideal β-turn and
Ant-Pro turn are highlighted with shaded arrows, and the distance:
d(Cα–Cα) is indicated by a double-headed arrow. Note: The
Cαi–1···Cαi+2 distance range shown for the Ant-Pro reverse turn
(right) is taken from the ten crystal structures reported herein (see
below). The dihedral angle θ has been shown for Ant, which is a
constrained aromatic β-amino acid.[24]
The conformational characteristics of the Ant-Pro re-
bered-ring hydrogen-bonded networked turns that we verse turn has been elucidated by carrying out extensive
termed Ant-Pro reverse turns (Figure 2).
structural modulations around the turn segment of N- and
C-terminal amidated peptides, including altering chiralities
of the amino acid residues,[25] modulating substitution
pattern[26] (introducing different amino acid residues), and
swapping of carboxamides with sulfonamides.[27] The re-
sults obtained from the investigation of ten crystal struc-
tures and their NMR conformations in the solution state
strongly suggest that the Ant-Pro assumes a folded reverse-
turn-like structure, raising the possibility of practical utility,
particularly in rigidifying peptide backbones.[28] Surpris-
ingly, the strong six-membered-ring hydrogen-bonding,
which is otherwise shown to clearly persist in N-acylated
anthranilamides (whether in its homo or its hetero aromatic
oligomers)[17a,17b] due to the planarity of the amide hydro-
gen-bonding codes, is absent in all these cases, as clearly
evident from crystal structures. In lieu of the six-membered
hydrogen-bonding, the newly formed nine-membered hy-
drogen-bonded network, with an antiperiplanar orientation
of the Ant and Pro rings, has been shown to persist in all
these cases, regardless of the structural modulations around
the turn segment. As mentioned previously, another notable
feature of the Ant-Pro reverse-turn structure is its reversed
hydrogen-bonding interaction – an arrangement that might
Figure 1. Schematic representation of intramolecular H-bonding
interactions that stabilize reverse turns such as the seven-mem-
bered-ring H-bonded γ-turn (i+2Ǟi), 10-membered-ring H-bonded
β-turn (i+3Ǟi), 13-membered-ring H-bonded α-turn (i+4Ǟi), and
the 15-membered-ring H-bonded π-turn (i+5Ǟi) in a peptide
chain.
In this article, we describe the characteristic conforma- offer improved resistance to proteolytic degradation.[29] In-
tional features of the robust Ant-Pro reverse-turn, which deed, reversal of peptide hydrogen-bonding has been shown
has Ant and Pro residues in the turn segment. A striking to be associated with high proteolytic stability, as demon-
feature of this reverse turn is that it forms a folded pseudo strated in a well-known class of peptidomimetics termed
β-turn conformation, [d(Cαi–1-Cαi+2) = 5–7 Å],[23] involving retro-inverso peptides, originally developed by the group of
only two residues (between the adjacent Ant and Pro resi- the eminent peptide chemist, (late) Murray Goodman.[29a]
dues) displaying the closed hydrogen-bonded network, in In a retro-inverso peptide, which contains NH–CO bonds
contrast to the native β-turn, which involves four residues instead of the CO–NH peptide bond, all of the natural l-
to form the closed hydrogen-bonded network.[2] Further- amino acids in the native peptide are replaced with the
more, the hydrogen-bonding direction is in the forward di- d-enantiomer, with concurrent chain reversal (N–C
rection (1Ǟ2)-type, rather than the native β-turns, which termini)[30] – a structural feature that challenges the proteo-
feature backward hydrogen-bonding direction (1ǟ4)-type.
lytic enzymes for the recognition process that may lead to
3530
www.eurjoc.org
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 3529–3542

Characterizing the Ant-Pro Reverse-Turn Motif
proteolysis. The remarkable proteolytic stability of such finally culminated in the development of an efficient proto-
peptidomimetics has a bearing in practical utility and has col that involved first the generation and isolation of the
been diligently exemplified in a number of cases.[29]
benzoxazinone intermediates and further subjecting these
intermediates, without purification, to nucleophilic ring-
opening with the amines aided by 1,8-diazabicyclo[5.4.0]un-
dec-7-ene (DBU) in N,N-dimethylformamide (DMF) con-
taining 4 Å molecular sieves. Thus, cyclodehydration of
Boc-Pro-Ant-OH 1b and 2b, aided by ethyl chloroformate/
triethylamine combination in dichloromethane, furnished
the benzoxazinone intermediates 1c and 2c (see the Sup-
porting Information, S23 and S24 for the MS spectra),
which, without further purification, was subjected to DBU-
mediated ring opening by amines H-Pro-anilide 3b and 4b
to afford the expected tripeptides 5a–8a in good yields. To
simplify NMR studies, the N-Boc substituted tripeptides
5a–8a were converted into their pivaloyl analogues 5b–8b,
because N-Boc prolines are known to undergo cis-trans
isomerization[32] and complicate NMR studies owing to rot-
amer formation.[33]
Results and Discussion
Synthesis
The functionalized tripeptides 5–8, required for the pres-
ent study, were synthesized in a multistep synthetic strategy,
as depicted in Scheme 1. Functionalization of the C-ter-
minus with 4(Br)-anilide was undertaken to simplify the
task of crystallization because of the relative ease with
which bromo compounds can form crystals.[22d,22f]
The Ant-Pro reverse-turn analogues featuring Xaa-Ant-
LPro-NH-Ph(4-Br) (Xaa ϶ Pro) were accessed by the direct
coupling of Xaa-Ant-OH with H-LPro-NH-Ph(4-Br), aided
by coupling agents, as described in Scheme 2. It should be
noted that oxazinone formation did not seriously affect the
yield of the coupling products 18–26, although this was not
the case when Xaa was Pro, as described in Scheme 1 (see
above). However, it is noteworthy that nonpolar products,
presumably associated with benzoxazinones, and other im-
purities showed up in the direct coupling reaction. Fortu-
nately, pure coupled products could be isolated without
problem. In these cases also, the C-terminus was amidated
as 4-bromo anilides, with the aim of obtaining crystals that
were suitable for crystal structure investigations.[22d,22f]
Scheme 1. Reagents and conditions: (i) aq. LiOH, MeOH, room
temp., 4 h; (ii) ethyl chloroformate, Et3N, CH2Cl2, room temp., 4 h;
(iii) TFA/CH2Cl2 (1:1), room temp., 2 h; (iv) DBU, DMF, 4 Å mo-
lecular sieves, (L/D) H-Pro-(4-Br)-anilide (3b/4b), room temp., 3 h;
(v) a. TFA/CH2Cl2 (1:1), room temp., 2 h; b. Piv-Cl, Et3N, CH2Cl2,
room temp., 4 h.
Although one would expect a straightforward direct cou-
pling strategy between Boc-Pro-Ant-OH 1b and 2b with H-
Pro-anilide 3b and 4b to access the functionalized Pro-Ant-
Pro tripeptides 5–8, the synthesis proved to be more com-
plex owing to the formation of the oxazinone intermediates
1c and 2c. Unfortunately, repeated attempts to isolate the
oxazinones in pure form did not succeed. Indeed, previous
studies have shown that benzoxazinones are sensitive to
Scheme 2. Reagents and conditions: (i) aq. LiOH, MeOH, room
temp., 4 h; (ii) 3b, coupling reagent (see experimental section for
specific coupling reagents and conditions); (iii) a. TFA/CH2Cl2,
room temp. 2 h; b. Piv-Cl, Et3N, CH2Cl2, room temp., 4 h; (iv) ICl,
hydrolysis; curiously in the solid state.[31] Extensive efforts DMAP, MeCN, room temp., 2 h.
Eur. J. Org. Chem. 2013, 3529–3542
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3531

P. R. Rajamohanan, G. J. Sanjayan et al.
FULL PAPER
However, in the case of 23a, the Ant ring needed to be reverse turns, featuring different amino acid residues at the
modified to 23b, by introducing iodine, to facilitate crystal N-terminus. It is noteworthy that modulating the chiralities
formation.[34]
and substitution pattern of the amino acid residues in the
turn segment has been shown to affect the hydrogen-bond-
ing interactions and, hence, the stability of the secondary
structure, and this has been demonstrated in a number of
cases.[25–27] The ability of LD-segments in natural peptide
sequences, in comparison to LL-segments, to nucleate turns
was theoretically predicted by Ramachandran and co-
workers, as early as 1972,[35] and has subsequently been
widely exploited in the design of synthetic sequences. Inde-
pendent studies from the laboratories of Balaram[20] and
Gellman[21] comparing (D)Pro-Xxx and (L)Pro-Xxx con-
taining sequences have shown that (D)Pro-Xxx is clearly
superior to its enantiomeric isomer in nucleating hairpin
formation in peptide sequences involving α-amino acids.
The latest observation from the laboratory of Balaram re-
garding the strong propensity of the heterochiral Pro-Pro
segment to stabilize turns when compared to its homochiral
segment further attests to the importance of configuration
modulations in short peptide segments.[20a] The conse-
quences of configuration alteration in the conformation of
reverse-turn segments has been independently verified by
the Yale group led by Scott J. Miller during their arduous
campaign towards developing β-hairpin templates[36] for ef-
fecting stereospecific, enzyme-like asymmetric catalysis.[37]
Their studies, which also lend support to the earlier findings
of the Balaram group, suggest that (D)Pro-Gly, in contrast
to (L)Pro-Gly, effectively stabilizes β-hairpins.[8d,20b] In fact,
short model peptide sequences carrying the (L)Pro-Gly mo-
Crystal Structure Investigations
Extensive efforts to crystallize compounds containing the
Ant-Pro reverse turns resulted in the crystallization of ten
C-terminal amidated 4-(Br)-anilides 5a, 6a, 7a, 8a, 17, 18,
19, 22, 23b (Figure 3) and 24 (shown in the Supporting In-
formation, S101). As mentioned above, the C-terminus was
functionalized with 4(Br)-anilide to aid crystal forma-
tion.[22d,22f] It may be noted that there is clear literature pre-
cedence, including from our group, that bromo- and iodo-
substituted compounds crystallize relatively easily.[22d,22f,34]
To reveal the structural changes of the Ant-Pro reverse turn
backbone upon configuration modulation of the Pro resi-
dues around the turn segment, all four stereoisomeric ana-
logues of the Pro-Ant-Pro tripeptides were constructed. The
four reverse-turn mimics 5a–8a, feature configuration-
altered proline in their backbone. The crystal structures of
all four configuration-altered combinations [LL (5a), LD
(6a), DD (7a), and DL (8a)] are presented. It may be noted
that only the analogues that yielded crystal structures are
mentioned herein, although several of their analogues were
made and subjected to crystallization. Our general observa-
tion, which also has literature precedence,[22d,22f,34] suggests
that short oligomers featuring p-bromo-anilide moiety at
the C-termini crystallize relatively easily. The crystal struc-
tures of 17–19 represent substitution-modulated Ant-Pro
Figure 3. Crystal structures of compounds containing the Ant-Pro reverse turns described in this article. Hydrogen atoms, other than the
polar amide hydrogen atoms, have been removed for clarity.
3532
www.eurjoc.org
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 3529–3542

Characterizing the Ant-Pro Reverse-Turn Motif
tif show little evidence of β-hairpin formation.[20b] The crys- dent from their short D–H···A distances when compared
tal structures of 23b and 24 (see the Supporting Infor- with others. The hydrogen-bonding distances [d(N–H···
mation, page S101) represent two examples that reveal the O=C)] of all the Ant-Pro reverse turns were found to be
structural changes of the Ant-Pro reverse turn backbone relatively short, ranging from 2.024 Å (23b) up to 2.379 Å
upon swapping carboxamide with sulfonamide at the N- (6a), suggesting that the C9 H-bonded network is robust.
terminus. It is noteworthy that sulfonamides, with distinctly The shortest hydrogen-bonding distance [d(N–H···O=C)]
different structural geometry (twisted when compared to found for the sulfonamide 23b is clearly due to the acidic
planar carboxamide) and hydrogen-bonding preferences nature of the tosyl amide N–H, which makes the hydrogen-
(two hydrogen-bond acceptor oxygen atoms compared with bonding network stronger; this is in agreement with litera-
one acceptor oxygen of carboxamide), are known to form ture reports.[27] While comparing the hydrogen-bonding dis-
unusual hydrogen-bonding interactions.[27]
tances [d(N–H···O=C)], we found an intriguing trend
Investigation of the ten crystal structures reveals that the among all the four stereoisomers of the Pro-Ant-Pro tripept-
nine-membered-ring Ant-Pro reverse-turn hydrogen-bond- ides 5a–8a. The hydrogen-bonding distances [d(N–H···
ing is clearly preserved in all cases, irrespective of the di- O=C)] of the homochiral tripeptides 5a (2.143 Å) and 7a
verse structural changes around the turn segment. The Ant (2.138 Å) have been found to be slightly shorter than their
and the Pro rings evidently maintain an antiperiplanar ar- heterochiral analogues 6a (2.379 Å) and 8a (2.376 Å); an
rangement throughout, causing the otherwise strong six- effect that we are unable to explain at the moment. This
membered-ring hydrogen-bonding to collapse and give way anomalous effect is reflected in their Cα–Cα distances and
to the formation of the nine-membered-ring hydrogen- also in the hydrogen-bonding angles, as noted below. The
bonding, eventually facilitating a folded conformation. It Cα–Cα distance [d(Cαi–1–Cαi+2)] of the homochiral tripept-
should be noted that the usually observed strong C-6 hydro- ide 5a is 5.567 Å, whereas that of heterochiral tripeptide 8a
gen-bonding would have been preferred over the C-9 hydro- is comparatively high, with a value of 6.187 Å. It should be
gen-bond had the Ant and Pro rings retained a coplanar noted that the shorter the hydrogen-bonding, the larger
disposition, which is actually not possible in the present would be the hydrogen-bonding angle [Δ(C=O···N)], which
case due to a steric clash between the two rings.[22g]
is a commonly observed factor.[38] Indeed, this trend is evi-
Analysis of the torsion angles and hydrogen-bonding dent when comparing, for instance, the hydrogen-bonding
parameters of the ten crystal structures reveals the follow- angles of homochiral tripeptide 5a (129.16°) with that of
ing characteristic features regarding the Ant-Pro two-resi- heterochiral tripeptide 8a (126.28°). The torsion angles of
due reverse-turn, as detailed in Table 1. The Cαi–1–Cαi+3 the Ant-Pro reverse turn segments are as follows: The Ant
distance of an ideal β-turn has been described to be less φ ranges from a minimum value of 150.95° (24) to a maxi-
than 7 Å;[23] typical reverse-turn mimics are expected to mum value of 179.39° (22), which is usually observed in
have Cα–Cα distances in the range 5–7 Å (Figure 2).[23] The oligomers featuring Ant residues. However, substantial dif-
Ant-Pro crystal structures described herein show, in gene- ferences in Ant ψ values have been noted in the Ant-Pro
ral, the Cα–Cα distance [d(Cαi–1–Cαi+1)] to be in the range reverse-turns reported herein, when compared with the
5.5–6.2 Å. Exceptions are noted in the cases of the N-sul- oligomers containing Ant reported elsewhere.[17a] In the
fonamide analogues 23b and 24, wherein the Cα–Cα dis- present Ant-Pro reverse-turn, Ant displays ψ values ranging
tance cross over to 7.286 and 7.297 Å, respectively, mainly from 63.36 (24) to 95.10° (18). In stark contrast, the ψ value
due to the twisted conformation of the N-terminal sulfon- differs substantially (Ϯ 150°) in many of the reported oligo-
amide group.[27] However, this does not prevent these sys- mers featuring Ant, which is presumably due to the antiper-
tems from adopting a folded conformation featuring a C9 iplanar predisposition of the Ant and Pro rings. The torsion
H-bonded network, which is intriguingly stronger, as evi- angle θ of the constrained β-amino acid Ant is close to
Table 1. Backbone torsion angles and hydrogen-bonding parameters observed in crystals.
Torsion (°)[b]
Hydrogen-bonding parameters[b]
Distances (Å) Angles (°)
Com-
pound[a]
Ant
Pro
φ
θ
ψ
φ
ψ
NH···O=C d(Cαi–1–Cαi+2) O···H N···O C=O···H C=O···N NH···O
5a (LL)
6a (DL)
7a (DD)
8a (LD)
17 (LL)
18 (DL)
19 (DL)
22 (L)
155.59
176.98
–156.01
–177.01
158.70
–168.65
–172.23
179.39
172.07
150.95
0.71
2.38
–1.49
–2.59
7.43
2.09
2.93
6.63
–0.08
3.09
–69.30 –56.31 165.17 –74.73
–70.77 –52.45 163.63 144.67
5.567
6.192
5.540
6.187
5.761
5.539
5.472
5.786
7.286
7.297
2.143 2.965 130.87
2.379 3.225 123.74
2.138 2.966 131.10
2.376 3.223 123.77
2.124 2.972 130.29
2.134 2.987 132.76
2.109 2.956 131.67
2.344 3.193 125.19
2.024 2.873 133.51
2.112 2.956 130.56
129.16
126.24
129.53
126.28
130.52
135.12
134.90
128.10
134.99
128.41
160.01
168.31
161.29
168.25
168.89
171.67
168.15
169.09
168.69
167.15
69.71
71.02
56.92
52.39
163.41 75.16
163.49 –144.36
–75.63 –53.44 156.66 –95.55
–95.10 –52.36 160.27 171.06
–86.56 –54.39 159.98 161.20
–85.77 –65.01 162.76 –174.99
–76.03 –55.23 161.97 –117.62
–63.36 –57.44 158.58 –55.82
23b (L)
24 (L)
[a] The chiralities of the amino acid residues are indicated in parentheses. [b] The amino acid torsion angles and Cα–Cα distances are
shown in Figure 2.
Eur. J. Org. Chem. 2013, 3529–3542
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3533

P. R. Rajamohanan, G. J. Sanjayan et al.
FULL PAPER
zero[24] [ranging from 0.08 (23b) to 7.43° (17)], which is tion at the N-terminal carbamate carbonyls, which is a well-
clearly due to the cis-disposition of the carboxyl and amino known feature in peptides with an N-terminal Pro resi-
groups appended on the rigid aromatic framework. It is due,[20b,33] presumably caused by the free rotation of the
noteworthy that the distinctive cyclic structure imparts to amide bond connecting the pyrrolidine ring and the Boc
proline an exceptional conformational rigidity with a re- group. However, the cis-trans isomerism could be effectively
stricted N–Cα torsion angle φ of aboutϮ60°, allowing it to arrested by capping their N-terminal Pro residues with piv-
seek spaces in the “turn” regions of proteins.[22g] In the Ant- aloyl groups (Piv),[20b] as demonstrated in the cases of 5b,
Pro reverse turns reported herein, Pro adopts values that 6b, 7b and 8b. It is noteworthy that our previous studies
range from 52.36 (18) to 65.01° (22), and torsion ψ ranging with Ant-Pro foldamers have confirmed the stability and
from 156.66 (17) to 165.17° (5a). Incidentally, the observed robustness of the Ant-Pro reverse turn motifs featuring the
Pro φ and ψ dihedral angles in the Ant-Pro reverse-turn C9-turn hydrogen-bonded network, as could be gauged
motif described herein are typical of the poly-proline II from dimethyl sulfoxide (DMSO)-titration [Δδ (NH)
semi-extended conformation.[19]
Ͻ 0.1 ppm], CDCl3 dilution (Δδ NH: Ͻ 0.15 ppm), variable
temperature NMR analysis (Δδ/ΔT = –4 ppb/deg K), and
H/D NH exchange experiments (in [D4]MeOH).[22g]
A
Conformational Investigation of Ant-Pro Reverse Turns in
the Solution State
1
closer look at the H NMR chemical shifts of the Ant-Pro
reverse-turn structures, which require the Ant and Pro resi-
dues to be in an antiperiplanar orientation for its formation
(see above), reveals that the Ant amide NHs involved in the
nine-membered ring hydrogen-bonded network generally
appear at δ = 9–9.5 ppm (see the Supporting Information,
pp S40–S53). In contrast, the Ant amide NHs that partici-
pate in six-membered hydrogen-bonding within the N-acyl-
To investigate the solution-state conformation of the
Ant-Pro two-residue reverse turns, we performed 2D
NOESY NMR studies on some of the representative sys-
tems (8b, 19, 24 and 18) in solution (CDCl3, 400/500 MHz,
Figure 4). The signal assignments were done by using a
combination of 2D COSY, NOESY, and TOCSY analyses.
1
1
anthranilimides generally have H NMR chemical shifts in
Inspection of the H NMR spectra of the N-Boc systems
the range δ = 10–13 ppm.[17a,17b,22g] However, it should be
5a, 6a, 7a and 8a (see the Supporting Information, pp S40,
S41, S42 and S43, respectively) revealed cis-trans isomeriza-
noted that six-membered hydrogen-bonding requires the
Figure 4. Selected NOE extracts from the 2D NOESY data of 8b (C6D6), 19, 24 and 18 (CDCl3, 400 MHz). (a) PδH vs. NH; (b) NH1
vs. Ar-H; (c) PδH vs. NH; (d) Boc vs. Ar-H; (e) PδH vs. NH; (f) Mesityl vs. Ar-H; (g) PδH vs. NH; (h) Boc and VγH vs. Ar-H regions.
Note: the structure of 8b (N-Piv) is the PyMOL generated model, based on the crystal structure of 8a (N-Boc).
3534
www.eurjoc.org
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 3529–3542

Characterizing the Ant-Pro Reverse-Turn Motif
Ant and Pro residues to be in a coplanar orientation, which borne out from the terminal characteristic NOE signals
is an arrangement that would have clearly derailed the Ant- C14H vs. C19H (Figure 4, f, 24), despite the twisted confor-
Pro reverse turns in the present case owing to the persistent mation of the sulfonamide group at the N-terminus of the
antiperiplanar arrangement of the rings. This observation Ant-Pro unit.
suggests that the characteristic chemical shifts of the Ant
amide NHs involved in the nine-membered-ring hydrogen-
bonded network (δ = 9–9.5 ppm) can serve as a good refer-
ence to distinguish between the possibilities of nine-mem-
bered-ring hydrogen-bonding and six-membered-ring hy-
Circular Dichroism (CD) Spectra of the Ant-Pro Reverse
Turns
drogen-bonding in a system that contains Ant-Pro residues.
CD spectra provide a characteristic signature for the con-
Analysis of the crystal structures of the Ant-Pro reverse formational features of ordered chiral oligomers.[39] The
turns suggested that one of the most diagnostic NOE sig- CD spectrum of 5a, which is composed of homochiral pro-
nals that would support the folded backbone conformation line residues, originates with a positive absorption at
would be the dipolar couplings between the Ant amide NH 190 nm, zero-crosses at 198 nm and displays strong minima
and the δ-protons of the succeeding Pro residue (Ant-NH at 208 nm, following which a small positive cotton effect is
vs. PδH, Figure 4, a, c, e and g), as these two residues share observed at about 219 nm (Figure 5). A strong negative cot-
an antiperiplanar geometry, allowing the protons of the ton effect is observed around 234 nm followed by an intense
aforesaid groups to come in close proximity. The average negative absorbance peak at around 260 nm, which is segre-
distance [dav(Ant amide NH–PδH)] between these protons gated by a shoulder observed at about 244 nm. Compound
from the ten crystal structures was found to be 2.47 Å, 7a, with opposite configuration at proline residues with re-
which clearly suggested that strong NOE signals should be spect to 5a, features an exact mirror image of the absorp-
observed for systems in the solution state that display such tion peaks. In the case of heterochiral derivatives, both 6a
a conformation. The characteristic strong NOE signals that (featuring LPro-Ant-DPro sequence) and 8a (featuring
were observed in this category are: NH1 vs. P2δH (Figure 4, DPro-Ant-LPro sequence) exhibit positive and negative cot-
a, 8b), NH2 vs. PδH (Figure 4, c, 19), NH1 vs. PδH (Fig- ton effects of opposite mirror image signatures. A positive
ure 4, e, 24), and NH2 vs. PδH (Figure 4, g, 18). Closer absorption peak of 6a is displayed at around 218 nm. A
investigation of the crystal structures revealed that the N- strong negative absorption band, followed by zero crossing
and C-terminal substituents come in close proximity, as a at 223 nm, is observed at around 240–260 nm due to the
consequence of the folded conformation induced by the backbone aromatic groups/aromatic electronic transitions
Ant-Pro reverse-turn motif.[22g] Indeed, the observed long- in oligomers.[39] The Ant-Pro reverse turn structures 17, 18,
range NOE signals emanating from dipolar couplings be- 19, 20, 21a and 22 that do not feature Pro at the N-terminal
tween the N- and C-terminal substituents strongly support displayed maxima at around 195 nm, zero crossing at
the folded conformation, as observed in the solid state. Rep- 200 nm, and minima at 209 nm. A strong negative cotton
resentative examples of such characteristic NOE signals effect was also observed (second minima) around 240 nm,
that further support the folded conformation are: NH1 vs. consistently in all the cases, including 24, owing to the
C24H (Figure 4, b, 8b), C24H/C21H, and C24H/C22H backbone aromatic groups/aromatic electronic transitions
(Figure 4, d, 19), and C23H/C26H, and C24H/C26H (Fig- in the oligomers.[39] It is noteworthy that the typical cotton
ure 4, h, 18). This trend is also noted in the case of the effects observed for these systems suggest the onset of heli-
sulfonamides, wherein the folded conformation is readily cal architecture.[39]
Figure 5. CD spectra of the Ant-Pro reverse-turns. Pro-Ant-Pro tripeptides (5a, 6a, 7a and 8a; left), and (17, 18, 19, 20, 21a, 22 and 24;
right). Note: the spectra have been shown separately to minimize crowding and to facilitate comparison of the characteristic CD signatures.
Eur. J. Org. Chem. 2013, 3529–3542
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3535

P. R. Rajamohanan, G. J. Sanjayan et al.
FULL PAPER
quentially with satd. KHSO4, satd. NaHCO3, water, and brine. The
organic layer was dried with anhydrous Na2SO4 and evaporated
under reduced pressure to obtain the crude product, which, on pu-
rification by column chromatography (petroleum ether/ethyl acet-
ate, 80:20; Rf = 0.3) afforded 2a as a viscous liquid (5.6 g, 87%).
Conclusions
This article describes the conformational characteristics
of the Ant-Pro reverse-turn, which assumes a folded pseudo
β-turn conformation [d(Cαi–1–Cαi+2) = 5–7 Å],[23] involving
just two residues displaying a closed nine-membered-ring
hydrogen-bonded network, formed in the forward direction
of the sequence by 1Ǟ2 amino acid interactions (between
the adjacent Ant and Pro residues). This is in contrast to
native β-turns, which involve four residues to form the
closed hydrogen-bonded network. Furthermore, the hydro-
gen-bonding is seen to be in the forward direction (1Ǟ2
amino acid interactions), in contrast to native β-turns,
which feature backward hydrogen-bonding direction (1ǟ4
amino acid interactions). The effect of structural alterations
on this Ant-Pro reverse turn has been investigated by carry-
ing out selective structural modifications around the turn
segment of the N- and C-terminal amidated peptides. This
included altering chiralities of the amino acid residues,
modulating the substitution pattern (use of different amino
acid residues), and substituting carboxamides with sulfon-
amides. Investigation of several crystal structures[40] and
their NMR conformations in the solution state suggests
that the Ant-Pro reverse turn features a robust nine-mem-
bered-ring hydrogen-bonded network. The strong six-mem-
bered-ring hydrogen-bonding, which has been shown to
clearly persist in N-acylated anthranilamides,[17a,17b] is ab-
sent in all these cases, as clearly evident from the crystal
structures. In lieu of the six-membered hydrogen-bonding
that would have required the coplanar arrangement of the
Ant and Pro residues, the nine-membered-ring hydrogen-
bonded network, with a perfect antiperiplanar orientation
of the adjacent Ant and Pro rings, was found to exist, re-
gardless of the structural modulations around the turn seg-
ment. The characteristic chemical shifts of the Ant amide
NHs involved in the nine-membered-ring hydrogen-bonded
network (δ = 9–9.5 ppm) provide a simple clue for the ready
identification of an Ant-Pro reverse turn in a system, which
can be clearly distinguished from possibly competing six-
membered-ring hydrogen-bonding within the N-acyl-
anthranilimides (10 Ͻ δ Ͻ 13 ppm), as noted earlier.[17,22g]
The robustness of the Ant-Pro reverse turn raises the pros-
pect of practical applications, particularly in rigidifying
peptide backbones by inserting the Ant-Pro reverse turn
motifs into their backbone – a work that is in progress in
our laboratory.
[α]2D6 = 126.93 (c = 1, CHCl ). IR (CHCl ): ν = 3269, 2984, 2941,
˜
3
3
2908, 2878, 1738, 1703, 1587, 1529, 1450, 1371, 1240, 1047 cm–1.
1H NMR (200 MHz, CDCl3): δ = 11.58–11.48 (br. s, 1 H), 8.79–
8.75 (d, J = 8.37 Hz, 1 H), 8.0–8.01 (d, J = 7.71 Hz, 1 H), 7.59–
7.51 (m, 1 H), 7.14–7.07 (m, 1 H), 4.45–4.28 (m, 1 H), 3.91 (s, 3
H), 3.74–3.47 (m, 2 H), 2.38–1.96 (m, 4 H), 1.51–1.35 (m, 9
H) ppm. 13C NMR (50 MHz, CDCl3): δ = 171.5, 167.4, 153.3,
140.2, 133.7, 130.1, 121.8, 119.2, 114.5, 79.3, 61.8, 51.5, 46.0, 30.7,
29.7, 27.4, 23.0 ppm. MS (ESI): m/z = 349.3947 [M + H]+, 371.3945
[M + Na]+, 387.3800 [M + K]+. C18H24N2O5 (348.40): calcd. C
62.05, H 6.94, N 8.04; found C 62.07, H 7.21, N 7.77.
General Method for Methyl Ester Hydrolysis: To a solution of ester
(1a, 2a or 9a–16a; 10 mmol) in methanol (15 mL), LiOH·H2O
(40 mmol) was added in water (12 mL) at 0 °C, and the reaction
mixture was stirred for 4 h. After complete consumption of the
starting material, the solvent was evaporated under reduced pres-
sure, and the free acid was liberated by treatment with satd. KHSO4
solution followed by extraction with CH2Cl2 (2ϫ 25 mL). The cor-
responding crude acid derivatives (1b, 2b or 9b–16b) obtained after
evaporation of the solvent under reduced pressure were carried for-
ward for the next reaction without further purification.
General Method for Boc Deprotection: A solution containing the
Boc-peptide 3a/4a (3 mmol) was subjected to deprotection using
CH2Cl2/TFA (50%, 10 mL) at 0 °C. After completion of the reac-
tion (ca. 2 h), the solvent was removed and the residue was neutral-
ized with saturated sodium hydrogen carbonate solution, diluted
with CH2Cl2 and the product was extracted into CH2Cl2 (3ϫ
10 mL). The organic layer was dried with anhydrous Na2SO4 and
the crude product (3b/4b), obtained after evaporating the solvent
under reduced pressure, was used for the next step without further
purification.
(S)-tert-Butyl 2-(4-Bromophenylcarbamoyl)pyrrolidine-1-carboxyl-
ate (3a): To a solution of Boc-LProline (5 g, 23.2 mmol) in anhy-
drous MeCN, 4-bromoaniline (4.39 g, 25.5 mmol), EDC·HCl
(5.34 g, 27.9 mmol), DMAP (3.4 g, 27.9 mmol), and a catalytic
amount of HOBt were added at 0 °C and the mixture was stirred
for 15 min, then at room temperature for 12 h. The solvent was
evaporated and the residue was dissolved in CH2Cl2 (25 mL),
washed with satd. NaHCO3 solution and satd. KHSO4 solution,
water and brine. The organic layer was dried with anhydrous
Na2SO4 and evaporated under reduced pressure to obtain the crude
product, which, on purification by column chromatography (petro-
leum ether/ethyl acetate, 80:20; Rf = 0.6) afforded 3a as a white
solid (7.2 g, 84%), m.p. 197–198 °C. [α]2D5 = –94 (c = 1, CHCl3). IR
(CHCl ): ν = 3318, 3122, 2977, 1668, 1591, 1539, 1490, 1397, 1366,
˜
3
1302, 1289, 1247, 1162, 1127, 1073 cm–1. 1H NMR (200 MHz,
CDCl3): δ = 9.66 (s, 1 H), 7.42–7.38 (m, 4 H), 4.47 (br. s, 1 H),
3.45 (br. s, 2 H), 2.46–1.93 (m, 1 H), 1.49 (m, 1 H) ppm. 13C NMR
(50 MHz, CDCl3): δ = 170.4, 156.0, 137.5, 131.5, 120.8, 116.0, 80.7,
60.3, 47.1, 28.3, 24.4 ppm. MS (ESI): m/z = 369.2641 [M + H]+,
391.2549 [M + Na]+, 409.2371 [M + K]+. C16H21BrN2O3 (369.26):
calcd. C 52.04, H 5.73, N 7.59; found C 52.14, H 5.77, N 7.45.
Experimental Section
Compound 1a was synthesized by the reported procedure.[22g]
(R)-tert-Butyl 2-[2-(Methoxycarbonyl)phenylcarbamoyl]pyrrolidine-
1-carboxylate (2a):
A
solution containing Boc-DProline (4 g,
18.6 mmol) and H-Ant-OMe (2.8 g, 18.6 mmol) in anhydrous
MeCN (30 mL) was cooled to 0 °C. TBTU (7.16 g, 22.4 mmol) and
DIEA (4.8 mL, 27.8 mmol) were added to the reaction mixture,
which was stirred at 0 °C for 10 min and then at room temperature
for 12 h. The solvent was removed under reduced pressure, the resi-
due was taken in CH2Cl2, and the organic layer was washed se-
(R)-tert-Butyl 2-(4-Bromophenylcarbamoyl)pyrrolidine-1-carboxyl-
ate (4a): Synthesized as described for 3a. White solid (76%), m.p.
196–197 °C. [α]2D5 = 96 (c = 1, CHCl ). IR (CHCl ): ν = 3318, 3195,
˜
3
3
2977, 1668, 1591, 1540, 1490, 1397, 1367, 1302, 1289, 1247, 1163,
1
1129, 1073 cm–1. H NMR (200 MHz, CDCl3): δ = 9.68 (s, 1 H),
3536
www.eurjoc.org
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 3529–3542

Characterizing the Ant-Pro Reverse-Turn Motif
7.37 (m, 4 H), 4.47 (br. s, 1 H), 3.47 (br. s, 2 H), 2.43–1.94 (m, 1
H), 1.49 (m, 1 H) ppm. 13C NMR (50 MHz, CDCl3): δ = 170.4,
156.0, 137.4, 131.5, 120.8, 116.1, 80.7, 60.5, 47.1, 28.3, 24.4 ppm.
MS (ESI): m/z = 369.5459 [M + H]+, 391.5499 [M + Na]+, 393.5525
[M + Na]+. C16H21BrN2O3 (369.26): calcd. C 52.04, H 5.73, N
7.59; found C 52.23, H 5.84, N 7.29.
9.54 (br. s, 1 H), 8.5–8.2 (m, 1 H), 7.45–7.27 (m, 6 H), 7.14–7.11
(t, J = 7.28 Hz, 1 H), 4.98–4.80 (m, 1 H), 4.48–4.34 (m, 1 H), 3.66–
3.39 (m, 4 H), 2.32–1.77 (m, 8 H), 1.46–1.39 (m, 9 H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 172.0, 170.4, 169.4, 154.6, 137.7,
137.0, 135.5, 131.7, 127.5, 126.9, 124.0, 123.7, 123.1, 122.7, 121.4,
116.7, 80.4, 62.4, 61.4, 50.8, 47.0, 31.6, 30.1, 29.3, 28.6, 25.4,
24.0 ppm. MS (ESI): m/z = 609.2386 [M + Na]+, 607.5495 [M +
Na]+. C28H33BrN4O5 (585.50): calcd. C 54.44, H 5.68, N 9.57;
found C 57.44, H 5.44, N 9.69.
General Procedure for the Synthesis of 5a–8a
(S)-tert-Butyl 2-{2-[(S)-2-(4-Bromophenylcarbamoyl)pyrrolidine-1-
carbonyl]phenylcarbamoyl}pyrrolidine-1-carboxylate (5a); Typical
Procedure: To a solution of 1b (0.5 g, 1.49 mmol) in anhydrous
THF (5 mL) was added TEA (1.79 mmol) under an N2 atmosphere
at 0 °C. Ethyl chloroformate (0.21 mL, 2.24 mmol) was added
dropwise over a period of 10 min at 0 °C then the reaction mixture
was stirred for 15 min and the progress of the reaction was moni-
tored by TLC. The solvent was removed under reduced pressure,
diluted with water (10 mL) and extracted with CH2Cl2 (3ϫ 10 mL).
The organic layer was dried with anhydrous Na2SO4 and concen-
trated to obtain the crude product 1c, which was carried forward
to the next reaction without further purification.
(R)-tert-Butyl 2-{2-[(S)-2-(4-Bromophenylcarbamoyl)pyrrolidine-1-
carbonyl]phenylcarbamoyl}pyrrolidine-1-carboxylate (8a): The prod-
uct 8a was obtained as a white solid (81%), m.p. 142–143 °C.
[α]2D4 = –128.47 (c = 0.5, CHCl ). IR (CHCl ): ν = 3275, 3120, 2977,
˜
3
3
1681, 1619, 1542, 1490, 1455, 1397, 1366, 1302, 1249, 1163,
1
1124 cm–1. H NMR (400 MHz, CDCl3): δ = 9.76–9.67 (m, 1 H),
9.60–9.42 (m, 1 H), 8.39–8.05 (m, 1 H), 7.51–7.36 (m, 6 H), 7.18–
7.13 (t, J = 7.53 Hz, 1 H), 4.91 (m, 1 H), 4.40–4.34 (m, 1 H), 3.56–
3.42 (m, 4 H), 2.48–1.89 (m, 8 H), 1.48–1.39 (m, 9 H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 172.2, 171.4, 170.5, 169.4, 154.6,
137.4, 136.5, 135.9, 132.0, 131.4, 128.0, 124.0, 121.6, 116.9, 80.8,
62.2, 61.0, 51.1, 50.9, 47.4, 47.2, 31.6, 29.7, 28.6, 27.9, 25.7, 24.7,
24.1 ppm. MS (ESI): m/z = 609.0935 [M + Na]+, 607.1043 [M +
Na]+. C28H33BrN4O5 (585.50): calcd. C 54.44, H 5.68, N 9.57;
found C 57.22, H 5.86, N 9.66.
To a solution containing the crude material 1c, amine 3b (0.484 g,
1.806 mmol) and 4 Å MS (0.5 g) in anhydrous DMF (5 mL) was
added DBU (0.424 mL, 3.01 mmol) at 0 °C. The solution was
stirred at room temperature for 4 h, then the reaction mixture was
quenched with satd. KHSO4 solution and diluted with ethyl acetate
(20 mL). The organic layer was repeatedly washed with water fol-
lowed by brine. The organic layer was dried with anhydrous
Na2SO4 and evaporated under reduced pressure to obtain the crude
product, which, on further purification by flash column chromatog-
raphy (petroleum ether/acetone, 70:30, Rf = 0.4) afforded 5a as a
white solid (0.75 g, 85%), m.p. 228–229 °C. [α]2D4 = –206.54 (c =
General Procedure for Synthesis of 5b–8b
(S)-N-(4-Bromophenyl)-1-{2-[(S)-1-pivaloylpyrrolidine-2-carbox-
amido]benzoyl}pyrrolidine-2-carboxamide (5b); Typical Procedure:
A solution containing 5a (0.3 g, 0.3 mmol) in CH2Cl2 (3 mL) was
subjected to Boc deprotection using CH2Cl2/TFA (50%, 3 mL). Af-
ter completion of the reaction (1 h), the solvent was removed and
the reaction mixture was neutralized with saturated sodium hydro-
gen carbonate solution, diluted with CH2Cl2 (10 mL), and the
product was extracted into the organic layer (2ϫ 15 mL). The or-
ganic layer was dried with anhydrous Na2SO4, evaporated, and
concentrated, and the residue was taken in anhydrous CH2Cl2
(5 mL) and cooled to 0 °C. Pivaloyl chloride (0.05 mL, 0.4 mmol)
was added followed by Et3N (0.08 mL, 0.4 mmol), then the reaction
mixture was stirred at room temperature for 4 h. The reaction mix-
ture was diluted with CH2Cl2 (10 mL) and washed sequentially
with satd. KHSO4 solution, satd. NaHCO3 solution, water and
brine. The organic layer was dried with anhydrous Na2SO4, and
the crude product obtained after removal of solvent under reduced
pressure was subjected to column purification (pet ether/acetone,
60:40; Rf = 0.4) to afford 5b (0.21 g, 72%) as a fluffy solid, m.p.
0.5, CHCl ). IR (CHCl ): ν = 3274, 3210, 2977, 1680, 1614, 1591,
˜
3
3
1541, 1490, 1455, 1397, 1367, 1302, 1249, 1162, 1123 cm–1. 1H
NMR (400 MHz, CDCl3): δ = 10.13 (br. s, 1 H), 9.60–9.48 (m, 1
H), 8.49–8.17 (m, 1 H), 7.47–7.31 (m, 6 H), 7.14–7.11 (t, J =
7.28 Hz, 1 H), 4.96–4.78 (m, 1 H), 4.45–4.31 (m, 1 H), 3.69–3.37
(m, 4 H), 2.28–2.15 (m, 4 H), 1.93–1.79 (m, 4 H), 1.47–1.38 (m, 9
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 171.4, 169.7, 168.9,
154.0, 137.0, 131.2, 127.1, 122.5, 120.5, 116.1, 79.8, 61.9, 60.5, 50.3,
46.3, 30.9, 30.5, 28.6, 27.9, 24.8, 23.4 ppm. MS (ESI): m/z =
607.6302 [M + Na]+. C28H33BrN4O5 (585.50): calcd. C 54.44, H
5.68, N 9.57; found C 57.08, H 5.84, N 9.77.
(S)-tert-Butyl 2-{2-[(R)-2-(4-Bromophenylcarbamoyl)pyrrolidine-1-
carbonyl]phenylcarbamoyl}pyrrolidine-1-carboxylate (6a): The prod-
uct 6a was obtained as a white solid (80%), m.p. 129–131 °C.
132–133 °C. [α]2D5 = –127.20 (c = 0.5, CHCl ). IR (CHCl ): ν =
[α]2D5 = 127.34 (c = 0.5, CHCl ). IR (CHCl ): ν = 3275, 2977, 3119,
˜
˜
3
3
3
3
3308, 3275, 2976, 1675, 1616, 1541, 1489, 1456, 1411, 1301, 1249,
1679, 1617, 1542, 1489, 1455, 1397, 1366, 1301, 1248, 1162,
1
1171 cm–1. H NMR (400 MHz, CDCl3): δ = 9.48 (s, 2 H), 8.26–
1
1123 cm–1. H NMR (400 MHz, CDCl3): δ = 9.70–9.56 (m, 1 H),
8.24 (d, J = 8.03 Hz, 1 H), 7.40–7.37 (m, 3 H), 7.29–7.27 (m, 3 H),
7.13–7.09 (t, J = 7.28 Hz, 1 H), 4.89 (m, 1 H), 4.72–4.70 (m, 1 H),
3.76–3.67 (m, 3 H), 3.43 (m, 1 H), 2.33–2.08 (m, 4 H), 1.90–1.78
(m, 4 H), 1.28 (m, 9 H) ppm. 13C NMR (100 MHz, CDCl3): δ =
177.6, 171.8, 170.5, 169.8, 137.6, 131.7, 131.1, 126.9, 123.9, 122.4,
121.6, 116.4, 63.2, 61.5, 50.3, 48.9, 39.2, 29.2, 27.7, 25.5 ppm. MS
(ESI): m/z (%) = 592.0862 [M + Na]+, 594.0412 [M + Na]+,
608.1041 [M + K]+, 610.0546 [M + K]+. C28H33BrN4O4 (569.50):
calcd. C 59.05, H 5.84, N 9.84; found C 59.28, H 5.65, N 9.77.
9.47–9.38 (m, 1 H), 8.32–8.12 (m, 1 H), 7.43–7.35 (m, 6 H), 7.16–
7.12 (t, J = 7.53 Hz, 3 H), 4.91 (m, 1 H), 4.40–4.34 (m, 1 H), 3.56–
3.42 (m, 4 H), 2.47–1.91 (m, 8 H), 1.48–1.26 (m, 9 H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 171.6, 170.8, 169.7, 168.8, 154.9,
153.9, 136.8, 135.7, 131.3, 127.2, 126.7, 124.2, 123.2, 122.2, 120.9,
116.2, 79.8, 61.5, 60.4, 50.4, 46.5, 31.0, 29.1, 27.9, 27.5, 23.5 ppm.
MS (ESI): m/z = 585.1435 [M + H]+, 609.2490 [M + Na]+, 607.2803
[M + Na]+. C28H33BrN4O5 (585.50): calcd. C 54.44, H 5.68, N
9.57; found C 57.71, H 5.55, N 9.39.
(R)-tert-Butyl 2-{2-[(R)-2-(4-Bromophenylcarbamoyl)pyrrolidine-1- (R)-N-(4-Bromophenyl)-1-{2-[(S)-1-pivaloylpyrrolidine-2-carbox-
carbonyl]phenylcarbamoyl}pyrrolidine-1-carboxylate (7a): The prod-
uct 7a was obtained as a white solid (78%), m.p. 227–228 °C.
amido]benzoyl}pyrrolidine-2-carboxamide (6b): The product 6b was
obtained as a white solid (70%), m.p. 108–110 °C. [α]2D5 = 113.96 (c
[α]2D4 = 205.92 (c = 0.5, CHCl ). IR (CHCl ): ν = 3275, 2977, 1679,
= 0.5, CHCl ). IR (CHCl ): ν = 3308, 3275, 2976, 1679, 1610, 1541,
˜
˜
3
3
3
3
1614, 1590, 1542, 1490, 1455, 1397, 1366, 1301, 1248, 1162,
1489, 1456, 1419, 1301, 1249, 1171 cm–1
.
1H NMR (400 MHz,
1
1123 cm–1. H NMR (400 MHz, CDCl3): δ = 10.07 (s, 1 H), 9.61– CDCl3): δ = 9.64 (br. s, 1 H), 9.48 (s, 1 H), 8.27–8.19 (d, J =
3537
Eur. J. Org. Chem. 2013, 3529–3542
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org

P. R. Rajamohanan, G. J. Sanjayan et al.
FULL PAPER
8.03 Hz, 1 H), 7.43–7.35 (m, 3 H), 7.32–7.28 (m, 4 H), 7.14–7.09
(t, J = 8.53 Hz, 1 H), 5.01–4.95 (m, 1 H), 4.69–4.62 (m, 1 H), 3.82–
2.27 (m, 2 H), 1.48 (s, 9 H), 1.07–1.03 (d, J = 6.83 Hz, 3 H), 0.97–
0.94 (d, J = 6.83 Hz, 3 H) ppm. 13C NMR (50 MHz, CDCl3): δ =
3.75 (m, 2 H), 3.59–3.46 (m, 2 H), 2.45–1.84 (m, 8 H), 1.30 (m, 9 170.8, 168.4, 155.7, 140.9, 134.5, 130.8, 122.7, 120.2, 115.2, 79.8,
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 176.9, 170.9, 169.7,
169.2, 136.9, 135.5, 131.2, 131.1, 130.5, 126.5, 125.1, 123.0, 121.9,
120.8, 116.1, 62.6, 60.4, 50.1, 48.2, 38.6, 27.8, 27.0, 25.5, 24.7 ppm.
MS (ESI): m/z = 592.1166 [M + Na]+, 594.0514 [M + Na]+,
607.8689 [M + K]+, 609.8799 [M + K]+. C28H33BrN4O4 (569.50):
calcd. C 59.05, H 5.84, N 9.84; found C 58.6, H 6.17, N 9.95.
60.9, 52.3, 30.9, 28.2, 19.3, 17.3 ppm. MS (ESI): m/z = 351.4256
[M + H]+, 373.4204 [M + Na]+. C18H26N2O5 (350.41): calcd. C
61.70, H 7.48, N 7.99; found C 61.59, H 7.62, N 7.84.
Methyl
2-(2-tert-Butoxycarbonylamino-3-methyl-butyrylamino)-
benzoate (10a): The product 10a was obtained as a white solid
(80%), m.p. 97–98 °C. [α]2D7 = 27.50 (c = 1, CHCl ). IR (CHCl ): ν
˜
3
3
= 3442, 3019, 2935, 2400, 1698, 1650, 1590, 1524, 1501, 1395, 1394,
(R)-N-(4-Bromophenyl)-1-{2-[(R)-1-pivaloylpyrrolidine-2-carbox-
amido]benzoyl}pyrrolidine-2-carboxamide (7b): The product 7b was
obtained as a white solid (78%), m.p. 131–132 °C. [α]2D5 = 113.47 (c
1
1215, 1045 cm–1. H NMR (200 MHz, CDCl3): δ = 11.5 (s, 1 H),
8.76–8.72 (d, J = 8.05 Hz, 1 H), 8.05–8.01 (dd, J = 8.04, 0.93 Hz,
1 H), 7.57–7.50 (m, 1 H), 7.13–7.05 (m, 1 H), 5.22–5.18 (m, 1 H),
4.28–4.2 (m, 1 H), 3.92 (s, 3 H), 2.39–2.30 (m, 1 H), 1.47 (s, 9 H),
1.06–1.03 (d, J = 6.88 Hz, 3 H), 0.97–0.93 (d, J = 6.92 Hz, 3
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 170.8, 168.4, 155.7,
140.9, 134.5, 130.8, 122.7, 120.2, 115.2, 79.8, 60.9, 52.3, 30.9, 28.3,
19.4, 17.3 ppm. MS (ESI): m/z = 351.2807 [M + H]+, 373.2709 [M
+ Na]+, 389.2405 [M + K]+. C18H26N2O5 (350.41): calcd. C 61.70,
H 7.48, N 7.99; found C 62.32, H 7.37, N 7.89.
= 0.5, CHCl ). IR (CHCl ): ν = 3308, 3275, 2974, 1675, 1616, 1541,
˜
3
3
1489, 1456, 1411, 1301, 1249, 1171 cm–1 1H NMR (400 MHz,
.
C6D6): δ = 10.20–10.07 (br. s, 2 H), 9.04–9.02 (m, 1 H), 7.57–7.53
(m, 2 H), 7.24–7.18 (m, 4 H), 6.85–6.95 (m, 1 H), 5.07 (br. s, 1 H),
3.97–3.68 (m, 1 H), 3.67–3.44 (m, 1 H), 3.38–3.11 (m, 1 H), 2.01–
2.84 (m, 1 H), 2.00–1.95 (m, 6 H), 1.57–1.45 (m, 1 H), 1.40–1.32
(m, 1 H), 1.25–1.21 (m, 9 H) ppm. 13C NMR (100 MHz, C6D6): δ
= 176.3, 172.0, 171.0, 169.0, 138.0, 131.4, 130.7, 123.0, 121.3, 116.2,
62.9, 61.4, 49.3, 48.4, 38.6, 29.6, 27.2, 25.1 ppm. MS (ESI): m/z =
569.7105 [M + H]+, 591.7623 [M + Na]+, 593.7574 [M + Na]+,
607.7841 [M + K]+, 609.7361 [M + K]+. C28H33BrN4O4 (569.50):
calcd. C 59.05, H 5.84, N 9.84; found C 59.42, H 5.63, N 9.68.
(R)-Methyl 2-[2-(tert-Butoxycarbonylamino)-4-methylpentanamido]-
benzoate (13a): The product 13a was obtained as a solid (92%),
m.p. 123–125 °C. [α]2D5 = 9.30 (c = 1.04, MeOH). IR (CHCl ): ν =
˜
3
3342, 3020, 2956, 1714, 1666, 1519, 1504, 1276, 1166, 1091 cm–1.
1H NMR (400 MHz, CDCl3): δ = 11.56 (s, 1 H), 8.74–8.71 (d, J =
8.26 Hz, 1 H), 8.03–8.01 (d, J = 8.25 Hz, 1 H), 7.55–7.51 (t, J =
7.02 Hz, 1 H), 7.10–7.06 (t, J = 7.25 Hz, 1 H), 5.07–5.06 (m, 1 H),
4.32 (m, 1 H), 3.91 (m, 3 H), 1.81–1.76 (m, 2 H), 1.61–1.55 (m, 1
H), 1.47 (s, 9 H), 1.00–0.97 (m, 6 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 171.9, 168.3, 155.4, 141.0, 134.5, 130.7, 122.6, 120.6,
115.2, 54.6, 52.6, 52.2, 41.7, 28.2, 24.8, 22.9, 21.7 ppm. MS (ESI):
m/z = 365.3166 [M + H]+, 387.3058 [M + Na]+, 403.2866 [M +
K]+. C19H28N2O5 (364.44): calcd. C 62.62, H 7.74, N 7.69; found
C 62.78, H 7.61, N 7.52.
(S)-N-(4-Bromophenyl)-1-{2-[(R)-1-pivaloylpyrrolidine-2-carbox-
amido]benzoyl}pyrrolidine-2-carboxamide (8b): The product 8b was
obtained as a white solid (69%), m.p. 128–129 °C. [α]2D5 = –130.03
(c = 0.5, CHCl ). IR (CHCl ): ν = 3308, 3275, 2974, 1675, 1616,
˜
3
3
1541, 1489, 1456, 1411, 1301, 1249, 1171 cm–1. 1H NMR
(400 MHz, C6D6): δ = 10.23–10.11 (br. s, 1 H), 9.80–9.71 (br. s, 1
H), 9.00–8.87 (m, 1 H), 7.69–7.62 (d, J = 8.89 Hz, 2 H), 7.42–7.36
(d, J = 8.34 Hz, 2 H), 7.18–7.06 (m, 1 H), 6.88–6.80 (m, 1 H), 5.24–
5.14 (m, 1 H), 5.00–4.99 (m, 1 H), 3.58–3.47 (m, 1 H), 3.38–3.28
(m, 1 H), 3.13–3.06 (m, 1 H), 2.90–2.82 (m, 1 H), 2.10–1.85 (m, 6
H), 1.62–1.45 (m, 2 H), 1.33 (s, 9 H) ppm. 13C NMR (100 MHz,
C6D6): δ = 176.7, 171.8, 170.5, 169.7, 138.3, 136.5, 131.9, 132.0,
130.5, 126.1, 123.0, 122.0, 121.8, 116.6, 63.4, 61.1, 49.7, 48.6, 38.8,
29.3, 27.5, 24.8 ppm. MS (ESI): m/z = 569.7502 [M + H]+, 591.7826
[M + Na]+, 593.7574 [M + Na]+, 607.6528 [M + K]+, 609.7977 [M
+ K]+. C28H33BrN4O4 (569.50): calcd. C 59.05, H 5.84, N 9.84;
found C 58.76, H 6.01, N 10.03.
General Procedure for the Synthesis of 11a, 12a and 14a
(R)-Methyl 2-[2-(tert-Butoxycarbonylamino)propanamido]benzoate
(11a): To an ice-cold solution of Boc-DAla-OH (4.5 g, 23.8 mmol)
and Ant-OMe (3 g, 19.8 mmol) in anhydrous MeCN (30 mL), and
HBTU (9.03 g, 23.8 mmol), DIEA (4.40 mL, 25.7 mmol) was
added. The reaction mixture was stirred at 0 °C for 10 min and at
room temperature for 12 h. After removal of solvent under reduced
pressure, the residue was dissolved in ethyl acetate and washed se-
quentially with satd. NaHCO3 solution followed by satd. KHSO4
solution and water. The organic layer was dried with anhydrous
Na2SO4 and the crude product was subjected to column purifica-
tion (20% ethyl acetate/petroleum ether; Rf = 0.5) to afford 11a as
white solid (4.15 g, 65%), m.p. 118–120 °C. [α]2D5 = 79.91 (c = 1,
General Procedure for the Synthesis of 9a, 10a and 13a
Methyl
2-(2-tert-Butoxycarbonylamino-3-methylbutyrylamino)-
benzoate (9a); Typical Procedure: A solution containing Boc-LVa-
line (4 g, 18.34 mmol) and 2-methyl anthranilate (2.22 mL,
14.67 mmol) in CH2Cl2 was cooled to 0 °C. DCC (4.54 g, 22 mmol)
and a catalytic amount of HOBt were added and the reaction mix-
ture was stirred at 0 °C for 10 min and then at room temperature
for 12 h. The solvent was removed under reduced pressure and the
residue was taken in ether and filtered to remove the insoluble
DCU. The organic layer was washed sequentially with satd.
NaHCO3 solution followed by satd. KHSO4 solution, water and
brine. The organic layer was dried with anhydrous Na2SO4, and
the crude product obtained on removal of solvent under reduced
pressure was purified by column chromatography (petroleum ether/
ethyl acetate 93:7; Rf = 0.3) to afford 9a (4.85 g, 75%) as a white
solid, m.p. 146–147 °C. [α]2D7 = –36.396 (c = 1, CHCl3). IR (CHCl3):
MeOH). IR (CHCl ): ν = 3304, 1722, 1693, 1681, 1604, 1587, 1537,
˜
3
1519, 1454, 1269, 1165 cm–1. 1H NMR (500 MHz, CDCl3): δ =
11.53 (s, 1 H), 8.70–8.68 (d, J = 8.53 Hz, 1 H), 7.99–7.97 (m, 1 H),
7.51–7.48 (t, J = 7.70 Hz, 1 H), 7.06–7.03 (t, J = 7.50 Hz, 1 H),
5.29 (br. s, 1 H), 4.37–4.34 (t, J = 6.60 Hz, 1 H), 3.88 (s, 3 H),
1.48–1.46 (m, 12 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 171.8,
168.2, 155.1, 140.9, 134.4, 130.7, 122.5, 120.1, 115.1, 52.1, 51.5,
28.2, 18.6 ppm. MS (ESI): m/z = 323.8735 [M + H]+, 345.9242 [M
+ Na]+, 361.9238 [M + K]+. C16H22N2O5 (322.36): calcd. C 59.61,
H 6.88, N 8.69; found C 59.70, H 6.72, N 8.54.
ν = 3442, 3019, 2935, 2400, 1698, 1650, 1590, 1524, 1501, 1395,
(S)-Methyl 2-[2-(tert-Butoxycarbonylamino)-4-methylpentamido]-
benzoate (12a): The product 12a was obtained as a white solid
(72%), m.p. 118–119 °C. [α]2D5 = –41.46 (c = 1.01, MeOH). IR
˜
1
1394, 1215, 1045 cm–1. H NMR (200 MHz, CDCl3): δ = 11.51 (s,
1 H), 8.77–8.72 (dd, J = 8.59, 0.89 Hz, 1 H), 8.06–8.01 (dd, J =
7.96, 1.51 Hz, 1 H), 7.59–7.5 (m, 1 H), 7.14–7.06 (m, 1 H), 5.22–
5.17 (d, J = 8.33 Hz, 1 H), 4.29–4.20 (m, 1 H), 3.93 (s, 3 H), 2.43–
(CHCl ): ν = 3310, 1756, 1698, 1651, 1604, 1587, 1537, 1519, 1405,
˜
3
1272, 1123 cm–1. H NMR (200 MHz, CDCl3): δ = 11.56 (s, 1 H),
1
3538
www.eurjoc.org
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 3529–3542

Characterizing the Ant-Pro Reverse-Turn Motif
8.73–8.69 (d, J = 8.46 Hz, 1 H), 8.01–7.98 (d, J = 7.70 Hz, 1 H),
7.55–7.47 (t, J = 7.70 Hz, 1 H), 7.10–7.02 (t, J = 7.60 Hz, 1 H),
5.13–5.09 (m, 1 H), 4.31–4.26 (m, 1 H), 3.89 (s, 3 H), 1.81–1.51 (m,
tion of 9b (0.1 g, 29.7 mmol) and 3b (0.079 g, 0.2976 mmol) in
CH2Cl2 maintained at 0 °C, DIEA (0.052 mL, 0.3571 mmol) and
HBTU (0.135 g, 0.3571 mmol) were added. The reaction mixture
2 H), 1.62–1.51 (m, 1 H), 1.45 (s, 9 H), 0.98–0.95 (m, 6 H) ppm. was stirred at 0 °C for 10 min and then at room temperature for
13C NMR (50 MHz, CDCl3): δ = 171.8, 168.2, 155.3, 140.9, 134.4, 12 h. The organic layer was washed sequentially with satd.
130.7, 122.5, 120.2, 115.2, 54.6, 52.1, 41.7, 28.2, 24.8, 22.8, 22.9,
21.6 ppm. MS (ESI): m/z = 365.6184 [M + H]+, 387.6166 [M +
Na]+, 403.6037 [M + K]+. C19H28N2O5 (364.44): calcd. C 62.62, H
7.74, N 7.69; found C 62.51, H 7.55, N 7.80.
NaHCO3 solution followed by satd. KHSO4 solution, water and
brine. The organic layer was dried with anhydrous Na2SO4 and
the crude product was subjected to column purification (petroleum
ether/ethyl acetate, 60:40; Rf = 0.3) to furnish 17 as a white solid
(0.104 g, 60%), m.p. 198–199 °C. [α]2D5 = –285.68 (c = 1, CHCl3).
Methyl 2-[2-(tert-Butoxycarbonylamino)acetamido]benzoate (14a):
The product 14a was obtained as a white solid (83%), m.p. 117–
IR (CHCl ): ν = 3303, 3121, 2932, 2876, 1676, 1617, 1543, 1490,
˜
3
1427, 1397, 1366, 1301, 248, 1163, 1163 cm–1. 1H NMR (200 MHz,
CDCl3): δ = 9.71 (s, 1 H), 9.57 (s, 1 H), 8.52–8.48 (d, J = 8.21 Hz,
1 H), 7.49–7.4 (m, 1 H), 7.35–7.29 (m, 3 H), 7.19–7.12 (m, 3 H),
5.33–5.29 (d, J = 9.22 Hz, 1 H), 4.99–4.92 (m, 1 H), 4.51–4.44 (m,
1 H), 3.66–3.54 (m, 1 H), 3.42–3.31 (m, 1 H), 2.41–1.82 (m, 7 H),
1.40 (s, 9 H), 1.08–1.05 (d, J = 6.69 Hz, 3 H), 1.0–0.96 (d, J =
6.70 Hz, 3 H) ppm. 13C NMR (50 MHz, CDCl3): δ = 171.8, 170.4,
168.7, 165.7, 155.8, 137.1, 135.0, 131.3, 130.8, 126.4, 126, 123.8,
121.4, 121.2, 116.4, 60.9, 59.9, 31.4, 29.7, 28.2, 25.1, 19.4,
17.5 ppm. MS (ESI): m/z = 609.8739 [M + Na]+. C28H35BrN4O5
(587.51): calcd. C 57.24, H 6.00, N 9.54; found C 57.39; H, 5.88;
N, 9.76.
118 °C. IR (CHCl ): ν = 3313, 2978, 1712, 1693, 1681, 1589, 1504,
˜
3
1454, 1269, 1165, 756 cm–1. 1H NMR (200 MHz, CDCl3): δ = 11.53
(s, 1 H), 8.67–8.63 (d, J = 8.22 Hz, 1 H), 7.97–7.92 (m, 1 H), 7.51–
7.43 (m, 1 H), 7.07–6.99 (t, J = 7.58 Hz, 1 H), 5.50–5.48 (m, 1 H),
3.99–3.96 (d, J = 6.07 Hz, 2 H), 3.86 (s, 3 H), 1.47 (s, 9 H) ppm.
13C NMR (50 MHz, CDCl3): δ = 168.7, 168.2, 155.7, 140.6, 134.3,
130.6, 122.6, 120.0, 115.0, 52.1, 45.3, 28.1 ppm. MS (ESI): m/z =
309.4865 [M + H]+, 331.4735 [M + Na]+, 347.4612 [M + K]+.
C15H20N2O5 (308.33): calcd. C 58.43, H 6.54, N 9.09; found C
58.29, H 6.43, N 9.24.
Methyl 2-(Tosylamino)benzoate (15a): To a solution of H-Ant-OMe
(3.0 g, 18.9 mmol) in anhydrous CH2Cl2 (20 mL) maintained at
0 °C, anhydrous Et3N (8.3 mL, 59.55 mol) and tosyl chloride
tert-Butyl
(1-{2-[2-(4-Bromophenylcarbamoyl)pyrrolidine-1-carb-
(4.94 g, 25.8 mmol) were added. The reaction mixture was stirred onyl]phenylcarbamoyl}-2-methylpropyl)carbamate (18): Compound
at 0 °C for 10 min and at room temp for 12 h. The reaction mixture
was diluted with CH2Cl2 (10 mL) and the organic layer was washed
sequentially with satd. NaHCO3 solution followed by satd. KHSO4
solution, water and brine. The organic layer was dried with anhy-
drous Na2SO4 and the crude product was subjected to column puri-
fication (petroleum ether/ethyl acetate, 80:20; Rf = 0.5) to afford
18 was synthsized as described for 17. White solid (56%); mp: 197–
198 °C; [α]2D5 = –105.3 (c = 0.11, CHCl ). IR (CHCl ): ν = 3303,
˜
3
3
3121, 2932, 2876, 1676, 1617, 1543, 1490, 1427, 1397, 1366, 1301,
1
248, 1163, 1163 cm–1. H NMR (200 MHz, CDCl3): δ = 9.78 (s, 1
H), 9.29 (s, 1 H), 8.54–8.50 (d, J = 8.21 Hz, 1 H), 7.47–7.39 (m, 4
H), 7.18–7.14 (m, 3 H), 5.9–5.86 (d, J = 9.1 Hz, 1 H), 4.98–4.91
15a as a white solid (5.0 g, 83%), m.p. 109–111 °C. IR (CHCl ): ν (m, 1 H), 4.44–4.36 (t, J = 7.58 Hz, 1 H), 3.57–3.42 (m, 2 H), 2.44–
˜
3
= 3209, 3020, 1869, 1844, 1791, 1772, 1732, 1716, 1697, 1653, 1315,
1.79 (m, 7 H), 1.41 (s, 9 H), 1.04–0.98 (m, 6 H) ppm. 13C NMR
1215, 1089 cm–1. H NMR (CDCl3, 500 MHz): δ = 10.63 (s, 1 H), (50 MHz, CDCl3): δ = 171.5, 170.4, 168.6, 165.7, 155.6, 136.8,
1
7.90–7.91 (dd, J = 7.98, 1.38 Hz, 1 H), 7.75 (br. s, 1 H), 7.73 (br.
s, 1 H), 7.67–7.69 (d, J = 8.25 Hz, 1 H), 7.44 (t, J = 7.44 Hz, 1 H),
7.20–7.22 (d, J = 8.26 Hz, 2 H), 7.02 (t, J = 7.44 Hz, 1 H), 3.87 (s,
3 H), 2.35 (s, 3 H) ppm. 13C NMR (CDCl3, 125 MHz): δ = 168.3,
143.9, 140.5, 136.4, 134.4, 131.1, 129.6, 127.2, 122.8, 118.9, 115.8,
52.4, 21.5 ppm. MS (ESI): m/z = 328.2499 [M + Na]+, 344.2352
[M + K]+. C15H15NO4S (305.35): calcd. C 59.00, H 4.95, N 4.59;
found C 58.67, H 5.25, N 4.19.
134.7, 131.3, 130.6, 125.9, 125.8, 123.7, 121.1, 116.6, 79.4, 61.0,
60.8, 49.5, 38.5, 31.3, 29.9, 28.2, 24.9, 19.2, 17.9 ppm. MS (ESI):
m/z = 588.0416 [M + H]+, 590.0733 [M + H]+, 610.0925 [M +
Na]+, 612.0887 [M + Na]+. C28H35BrN4O5 (587.51): calcd. C 57.24,
H 6.00, N 9.54; found C 57.38, H 6.21, N 9.23.
tert-Butyl {(2R)-1-[(2-{2-[(4-Bromophenyl)carbamoyl]pyrrolidine-1-
carbonyl}phenyl)amino]-1-oxopropan-2-yl}carbamate (19): To an
ice-cold solution of 11b (0.93 g, 3.08 mmol) and 3b (0.89 g,
3.31 mmol) in anhydrous MeCN (20 mL), HBTU (1.37 g,
3.62 mmol) and DIEA (0.67 mL, 3.92 mmol) were added. The reac-
tion mixture was stirred at 0 °C for 10 min and then at room tem-
perature for 12 h. After removal of the solvent under reduced pres-
sure, the reaction mixture was dissolved in ethyl acetate and the
organic layer was washed sequentially with satd. NaHCO3 solution
followed by satd. KHSO4 solution, water and brine. The organic
layer was dried with anhydrous Na2SO4 and the crude product was
subjected to column purification (30% acetone/ petroleum ether,
Rf = 0.5) to furnish 19 as a white solid (0.997 g, 59%), m.p. 208–
Methyl 2-(Mesylamino)benzoate (16a): To a solution of H-Ant-
OMe (2.0 g, 13.2 mmol) in anhydrous CH2Cl2 (10 mL) maintained
at 0 °C, Et3N (5.5 mL, 39.6 mmol) and methanesulfonyl chloride
(1.6 mL, 19.8 mmol) were added. The reaction mixture was stirred
at 0 °C for 10 min and then at room temperature for 2 h. The reac-
tion mixture was diluted with CH2Cl2 (5 mL) and the organic layer
was washed sequentially with satd. NaHCO3 solution followed by
satd. KHSO4 solution, water, and brine. The organic layer was
dried with anhydrous Na2SO4 and the crude product was subjected
to column purification (petroleum ether/ethyl acetate, 80:20; Rf =
0.5) to afford 16a as a white solid (2.81 g, 93%), m.p. 87–89 °C. IR
210 °C. [α]2D5 = –126.95 (c = 1, MeOH). IR (nujol): ν = 3425, 2955,
˜
(CHCl ): ν = 3203, 3026, 2956, 2850, 1838, 1786, 1693, 1683 cm–1.
2854, 1714, 1681, 1593, 1454, 1377, 1155, 1045 cm–1. 1H NMR
(500 MHz, CDCl3): δ = 9.45 (s, 1 H), 9.35 (s, 1 H), 8.33–8.32 (d, J
= 6.71 Hz, 1 H), 7.46–7.43 (t, J = 7.63 Hz, 1 H), 7.35–7.34 (m, 3
H), 7.28–7.26 (m, 2 H), 7.17–7.14 (t, J = 7.45 Hz, 1 H), 5.57–5.55
(d, J = 6.11 Hz, 1 H), 4.95 (br. s, 1 H), 4.49 (br. s, 1 H), 3.60–3.55
(m, 1 H), 3.51–3.50 (m, 1 H), 2.38–2.31 (m, 2 H), 2.12–2.07 (m, 1
H), 1.96–1.90 (m, 1 H), 1.46–1.44 (m, 12 H) ppm. 13C NMR
(125 MHz, CDCl3): δ = 172.1, 169.8, 169.6, 155.2, 136.9, 135.4,
131.6, 131.1, 126.8, 125.5, 123.7, 122.0, 121.2, 116.8, 51.2, 50.1,
29.6, 28.6, 28.3, 25.2, 18.8 ppm. MS (ESI): m/z = 581.3935 [M +
˜
3
1H NMR (CDCl3, 500 MHz): δ = 10.46 (s, 1 H), 8.04–8.05 (dd, J
= 7.98, 1.37 Hz, 1 H), 7.71–7.72 (d, J = 7.98 Hz, 1 H), 7.54 (t, J =
8.30 Hz, 1 H), 7.11 (t, J = 7.93 Hz, 1 H), 3.92 (s, 3 H), 3.05 (s, 3
H) ppm. 13C NMR (CDCl3, 125 MHz): δ = 168.3, 140.7, 134.8,
131.5, 131.4, 122.8, 117.9, 115.3, 52.5, 39.9 ppm. MS (ESI): m/z =
230.0162 [M + H]+, 252.0270 [M + Na]+. C9H11NO4S (229.25):
calcd. C 47.15, H 4.84, N 6.11; found C 46.85, H 5.14, N 5.88.
tert-Butyl
[1-{2-[2-(4-Bromophenylcarbamoyl)pyrrolidine-1-carb-
onyl]phenylcarbamoyl}-2-methylpropyl]carbamate (17): To a solu-
Eur. J. Org. Chem. 2013, 3529–3542
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3539

P. R. Rajamohanan, G. J. Sanjayan et al.
FULL PAPER
Na]+, 597.3860 [M + K]+. C26H31BrN4O5 (559.46): calcd. C 55.82,
H 5.59, N 10.01; found C 55.88, H 5.47, N 10.22.
(0.11 mL, 0.8 mmol) were added and the reaction mixture was
stirred at room temperature for 4 h. The reaction mixture was di-
luted with CH2Cl2 (10 mL), the organic layer was washed sequen-
tially with satd. NaHCO3 solution followed by satd. KHSO4 solu-
tion, water and brine. The organic layer was dried with anhydrous
Na2SO4 and the crude product was subjected to column purifica-
tion (30% acetone/petroleum ether; Rf = 0.4) to furnish 21b as a
white solid (0.058 g, 50%), m.p. 117–118 °C. [α]2D5 = –152.15 (c =
tert-Butyl
(2S)-1-{2-[2-(4-Bromophenylcarbamoyl)pyrrolidine-1-
carbonyl]phenylamino}-4-methyl-1-oxopentan-2-ylcarbamate (20):
To a solution containing 12b (0.25 g, 0.92 mmol) and H-Ant-OMe
(0.19 g, 0.71 mmol) in CH2Cl2 (10 mL), DCC (0.176 g, 0.85 mmol)
and DMAP (cat. amount) were added and the reaction mixture
was stirred at room temperature for 12 h. The precipitated DCU
was filtered and the filtrate was washed sequentially with satd.
NaHCO3 solution followed by satd. KHSO4 solution, water and
brine. The organic layer was dried with anhydrous Na2SO4 and the
crude product was subjected to column purification (30% acetone/
petroleum ether; Rf = 0.5) to afford 20 (0.2 g, 47%) as a white solid,
1.01, MeOH). IR (CHCl ): ν = 3310, 3020, 2400, 1676, 1591, 1523,
˜
3
1
1421, 1046, 928, 755 cm–1. H NMR (400 MHz, CDCl3): δ = 9.75
(s, 1 H), 9.54 (s, 1 H), 9.28–9.26 (d, J = 8.48 Hz, 1 H), 7.44–7.41
(m, 3 H), 7.36–7.34 (m, 1 H), 7.28–7.25 (m, 2 H), 7.18–7.14 (t, J =
7.51 Hz, 1 H), 6.51–6.49 (d, J = 8.06 Hz, 1 H), 4.96–4.93 (m, 1 H),
4.84–4.79 (m, 1 H), 3.56–3.44 (m, 2 H), 2.55 (br. s, 1 H), 2.34–2.23
(m, 2 H), 2.05–1.99 (m, 1 H), 1.91–1.84 (m, 1 H), 1.71–1.67 (m, 2
H), 1.22 (s, 9 H), 0.94–0.93 (m, 6 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 178.5,171.8, 169.9, 169.2, 137.0, 135.1, 131.6, 130.7,
126.6, 125.9, 123.8, 122.1, 121.2, 116.6, 60.7, 52.7, 50.1, 41.5, 38.7,
28.9, 27.4, 24.9, 23.0, 22.0 ppm. MS (ESI): m/z = 585.2769 [M +
H]+, 609.2656 [M + Na]+, 625.2522 [M + K]+. C29H37BrN4O4
(585.54): calcd. C 59.49, H 6.37, N 9.57; found C 59.62, H 6.20, N
9.64.
m.p. 118–120 °C. [α]2D5 = –66.88 (c = 1.02, MeOH). IR (CHCl ): ν
˜
3
= 3306, 3271, 2958, 1693, 1537, 1454, 1301, 1163, 1045, 756 cm–1.
1H NMR (200 MHz, CDCl3): δ = 9.58 (s, 1 H), 9.48 (s, 1 H), 8.30–
8.26 (d, J = 7.83 Hz, 1 H), 7.47–7.31 (m, 4 H), 7.23–7.12 (m, 3 H),
5.31–5.27 (m, 1 H), 4.93–4.90 (m, 1 H), 4.52–4.29 (m, 1 H), 3.59–
3.98 (m, 2 H), 2.42–2.27 (m, 2 H), 2.10–1.83 (m, 2 H), 1.76–1.70
(m, 3 H), 1.38 (s, 9 H), 1.06–1.00 (m, 6 H) ppm. 13C NMR
(50 MHz, CDCl3): δ = 172.8, 170.1, 168.8, 155.4, 137.0, 134.8,
131.3, 130.7, 126.5, 124.0, 122.3, 121.2, 116.4, 79.5, 61.1, 53.8, 41.5,
29.5, 28.2, 24.8, 23.1, 21.8 ppm. MS (ESI): m/z = 623.4253 [M +
Na]+, 639.3826 [M + K]+. C29H37BrN4O5 (601.54): calcd. C 57.90,
H 6.20, N 9.31; found C 57.78, H 6.32, N 9.45.
tert-Butyl 2-{2-[2-(4-Bromophenylcarbamoyl)pyrrolidine-1-carb-
onyl]phenylamino}-2-oxoethylcarbamate (22): To an ice-cold solu-
tion of 14b (0.457 g, 1.55 mmol) and 3b (0.5 g, 1.86 mmol) in anhy-
drous MeCN (15 mL), HBTU (0.767 g, 2.02 mmol) and Et3N
(0.30 mL, 2.17 mmol) were added. The reaction mixture was stirred
at 0 °C for 10 min and then at room temperature for 12 h. The
organic layer was washed sequentially with satd. NaHCO3 solution
followed by satd. KHSO4 solution, water and brine. The organic
layer was dried with anhydrous Na2SO4 and the crude product was
subjected to column purification (30% acetone/petroleum ether; Rf
= 0.5) to yield 22 as a white solid (0.35 g, 42%), m.p. 228–230 °C.
N-(4-Bromophenyl)-1-{2-[(R)-4-methyl-2-l-pivalamido-pentanamido]-
benzoyl}pyrrolidine-2-carboxamide (21a): To an ice-cold solution of
13b (0.3 g, 0.85 mmol) and 3b (0.275 g, 1.02 mmol) in anhydrous
MeCN (10 mL), HBTU (0.422 g, 1.11 mmol) and Et3N (0.17 mL,
1.19 mmol) were added. The reaction mixture was stirred at 0 °C
for 10 min and then at room temperature for 12 h. The organic
layer was washed sequentially with satd. NaHCO3 solution fol-
lowed by satd. KHSO4 solution, water and brine. The organic layer
was dried with anhydrous Na2SO4 and the crude product was sub-
jected to column purification (30% acetone/petroleum ether; Rf =
0.5) to furnish 21a as a white solid (0.285 g, 55%), m.p. 115–116 °C.
[α]2D5 = –136.73 (c = 0.65, MeOH). IR (nujol): ν = 2964, 2852, 1712,
˜
1681, 1633, 1491, 1454, 1377, 1169, 1051 cm–1
.
1H NMR
(200 MHz, CDCl3): δ = 9.53 (s, 1 H), 9.41 (s, 1 H), 8.22–8.18 (d, J
= 8.15 Hz, 1 H), 7.46–7.28 (m, 5 H), 7.24–7.13 (m, 2 H), 5.57 (br.
s, 1 H), 4.90 (br. s, 1 H), 4.16–3.94 (m, 2 H), 3.58–3.44 (m, 2 H),
2.49–2.21 (m, 2 H), 2.07–1.78 (m, 2 H), 1.44 (s, 9 H) ppm. 13C
NMR (50 MHz, CDCl3): δ = 170.3, 168.8, 134.7, 131.6, 130.8,
126.4, 126.3, 124.1, 121.1, 116.7, 60.9, 49.9, 30.8, 29.4, 28.2,
25.0 ppm. MS (ESI): m/z = 567.4713 [M + Na]+, 583.4742 [M +
K]+. C25H29BrN4O5 (545.43): calcd. C 55.05, H 5.36, N 10.27;
found C 55.30, H 5.28, N 10.12.
[α]2D5 = –68.53 (c = 1, MeOH). IR (CHCl ): ν = 3311, 3020, 1675,
˜
3
1622, 1541, 1427, 1398, 1303, 1216, 1045, 758 cm–1 1H NMR
.
(400 MHz, CDCl3): δ = 9.83rotamer (1 H), 9.80rotamer (0.17 H), 9.57–
9.51rotamer (0.20 H), 9.20rotamer (0.1 H), 8.58rotamer (0.20 H), 8.46–
8.44rotamer (d, J = 8.10 Hz, 1 H), 8.31rotamer (0.10 H), 7.47–
7.43rotamer (t, J = 8.10 Hz, 1 H), 7.34–7.32 (m, 1 H), 7.27–7.25 (m,
1 H), 7.18–7.14 (m, 4 H), 5.99–5.97 (d, J = 8.99 Hz, 1 H), 4.99–
4.96 (m, 1 H), 4.53–4.50rotamer (1 H), 4.23rotamer (0.25 H), 3.57–3.46
(m, 2 H), 2.54–2.36 (m, 2 H), 2.27–2.22 (m, 1 H), 2.09–1.96 (m, 1
H), 1.77–1.64 (m, 2 H), 1.42rotamer (9 H), 1.38rotamer (1 H), 0.99–
0.95 (m, 6 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 173.2, 172.6,
170.7, 170.4, 168.8, 168.4, 155.3, 136.9, 136.0, 131.3, 131.0, 130.8,
126.5, 125.5, 125.4, 123.7, 121.4, 120.9, 116.6, 116.4, 79.6, 61.2,
61.0, 54.7, 49.6, 49.4, 41.3, 40.3, 29.9, 29.6, 28.3, 28.2, 25.3, 24.9,
23.1, 21.5, 21.0 ppm. MS (ESI): m/z = 601.3143 [M + H]+, 623.3112
[M + Na]+, 639.2879 [M + K]+. C29H37BrN4O5 (601.54): calcd. C
57.90, H 6.20, N 9.31; found C 57.98, H 6.32, N 9.40.
N-(4-Bromophenyl)-1-[2-(4-methylphenylsulfonamido)benzoyl]pyr-
rolidine-2-carboxamide (23a): To a solution of 15b in anhydrous
MeCN (10 mL) was added 3b (1.5 g, 5.77 mmol), HBTU (3.3 g,
8.55 mmol) and DIEA (2.2 mL, 12.69 mmol). The reaction mixture
was stirred at 0 °C for 15 min and then at room temperature for
12 h. The solvent was evaporated and the residue was dissolved in
CH2Cl2 (15 mL) and the organic layer was washed sequentially
with satd. NaHCO3 solution followed by satd. KHSO4 solution,
water and brine. The organic layer was dried with anhydrous
Na2SO4 and the crude product was subjected to column purifica-
tion (petroleum ether/ethyl acetate, 25:75; Rf = 0.5) to furnish 23a
as a white solid (3.24 g, 86.4%), m.p. 83–85 °C. [α]2D9 = –23.53 (c =
N-(4-Bromophenyl)-1-{2-[(R)-4-methyl-2-pivalamidopentan-
amido]benzoyl}pyrrolidine-2-carboxamide (21b): Compound 21a
(0.1 g, 0.2 mmol) was subjected to Boc deprotection using TFA/
CH2Cl2 (2 mL, 1:1). Upon completion of the reaction, the solvents
were removed and the residue was neutralized with satd. NaHCO3
and extracted with CH2Cl2. The organic layer was dried with anhy-
drous Na2SO4, evaporated, and the residue obtained was taken in
anhydrous CH2Cl2 (10 mL). The reaction mixture was cooled to
0 °C, then pivaloyl chloride (0.07 mL, 0.6 mmol) and Et3N
0.3, CHCl ). IR (CHCl ): ν = 3315, 3018.7, 1716, 1697, 1683,
˜
3
3
1622 cm–1. 1H NMR ([D6]DMSO, 500 MHz): δ = 10.37 (s, 1 H),
9.41 (s, 1 H), 7.67–7.64 (m, 4 H), 7.54–7.52 (m, 2 H), 7.38–7.35 (m,
5 H), 7.20–7.17 (m, 2 H), 4.58–4.55 (m, 1 H), 3.23–3.21 (m, 1 H),
3.13–3.12 (m, 1 H), 3.34 (s, 3 H), 2.30–2.29 (m, 1 H), 1.97–1.86 (m,
2 H), 1.76–1.71 (m, 1 H) ppm. 13C NMR ([D6]DMSO, 125 MHz):
δ = 170.8, 166.9, 164.8, 143.8, 138.4, 134.5, 131.7, 130.7, 129.8,
3540
www.eurjoc.org
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 3529–3542

Characterizing the Ant-Pro Reverse-Turn Motif
128.7, 127.8, 127.1, 124.8, 122.0, 121.4, 115.2, 60.6, 49.5, 39.7, 29.9,
24.8, 21.1 ppm. MS (ESI): m/z = 542.2380 [M + H]+, 564.1860 [M
+ Na]+. C25H24BrN3O4S (542.45): calcd. C 55.35, H 4.46, N 7.75;
found C 54.95, H 4.76, N 7.95.
for fellowships. This work was partly funded by the International
Foundation for Science, Sweden and NCL-IGIB (New Delhi).
[1]
[2]
[3]
C. M. Wilmot, J. M. Thornton, J. Mol. Biol. 1988, 203, 221–
232.
For a classification of reverse turns, see: K. C. Chou, Anal.
Biochem. 2000, 286, 1–16.
N-(4-Bromophenyl)-1-[5-iodo-2-(4-methylphenylsulfonamido)benz-
oyl]pyrrolidine-2-carboxamide (23b): A solution of 23a (0.2 g,
0.35 mmol) in anhydrous MeCN (2 mL) was cooled to 0 °C.
DMAP (0.298 g, 2.43 mmol) was added followed by the dropwise
addition of iodine monochloride (0.09 mL, 1.742 mmol). The reac-
tion mixture was stirred at room temperature for 2 h, then the reac-
tion was quenched by stirring with a saturated solution containing
NaCl, Na2S2O8 and KHSO4 in water (1 mL) for 15 min. The crude
product was extracted with CH2Cl2 (5 mL) and the organic layer
was dried with anhydrous Na2SO4 and evaporated under reduced
pressure. The residue was purified by column chromatography (pe-
troleum ether/ethyl acetate, 50:50; Rf = 0.5) to afford 23b (0.18 g,
72%) as a white solid, m.p. 215–217 °C; [α]2D9 = –13.82 (c = 0.5,
D. H. J. Mackay, A. J. Cross, A. T. Hagler, in: Prediction of Pro-
tein Structure and the Principles of Protein conformation (Ed.:
G. D. Fasman), Plenum, New York, 1989, pp. 317–358.
For selected reviews on reverse-turn mimetics, see: a) R. F.
Hirschmann, K. C. Nicolaou, A. R. Angeles, J. S. Chen, A. B.
Smith, Acc. Chem. Res. 2009, 42, 1511–1520; b) P. G. Vasudev,
S. Chatterjee, N. Shamala, P. Balaram, Acc. Chem. Res. 2009,
42, 1628–1639; c) N. T. Ross, W. P. Katt, A. D. Hamilton,
Philos. Trans. R. Soc. London Ser. A 2010, 368, 989–1008; d)
J. M. Holub, K. Kirshenbaum, Chem. Soc. Rev. 2010, 39, 1325–
1337; e) R. M. J. Liskamp, D. T. S. Rijkers, J. A. W. Kruijtzer,
J. Kemmink, ChemBioChem 2011, 12, 1626–1653; f) T. A. Mar-
tinek, F. Fulop, Chem. Soc. Rev. 2012, 41, 687–702.
For excellent reviews on this topic (GPCR), see: J. S. Blakeney,
R. C. Reid, G. T. Le, D. P. Fairlie, Chem. Rev. 2007, 107, 2960–
3041.
a) M. J. M. Niesen, S. Bhattacharya, N. Vaidehi, J. Am. Chem.
Soc. 2011, 133, 13197–13204; b) I. E. Andersson, T. Batsalova,
S. Haag, B. Dzhambazov, R. Holmdahl, J. Kihlberg, A. Linus-
son, J. Am. Chem. Soc. 2011, 133, 14368–14378.
a) S. Hanessian, N. G. Smith, H. G. Lombart, W. D. Lubell,
Tetrahedron 1997, 53, 12789–12854; b) J. M. Mason, Future
Med. Chem. 2010, 2, 1813–1822; c) J. Vagner, H. Qu, V. J.
Hruby, Curr. Opin. Chem. Biol. 2008, 12, 292–296; d) C.
Caumes, O. Roy, S. Faure, C. Taillefumier, J. Am. Chem. Soc.
2012, 134, 9553–9556; e) L. Berlicki, L. Pilsl, E. Weber, I. M.
Mandity, C. Cabrele, T. A. Martinek, F. Fulop, O. Reiser, An-
gew. Chem. 2012, 124, 2251; Angew. Chem. Int. Ed. 2012, 51,
2208–2212.
For selected secondary structure mimics defined by H-bond-
ings, see: a) R. Chapman, J. L. I. Kulp, A. Patgiri, N. R. Kal-
lenbach, C. Bracken, P. S. Arora, Biochemistry 2008, 47, 4189–
4195; b) J. A. Robinson, Acc. Chem. Res. 2008, 41, 1278–1288;
c) P. G. Vasudev, S. Chatterjee, N. Shamala, P. Balaram, Chem.
Rev. 2010, 111, 657–687; d) C. J. Craig, J. L. Goodman, A.
Schepartz, ChemBioChem 2011, 12, 1035–1038; e) V. V. Koren-
dovych, S. J. Shandler, G. L. Montalvo, W. F. DeGrado, Org.
Lett. 2011, 13, 3474–3477; f) A. Roy, P. Prabhakaran, P. K.
Baruah, G. J. Sanjayan, Chem. Commun. 2011, 47, 11593–
11611; g) J. Fremaux, L. Fischer, T. Arbogast, B. Kauffmann,
G. Guichard, Angew. Chem. 2011, 123, 11584–11587; Angew.
Chem. Int. Ed. 2011, 50, 11382–11385; h) R. A. Brown, T. Mar-
celli, M. D. Poli, J. Sol, J. Clayden, Angew. Chem. 2012, 124,
1–6; Angew. Chem. Int. Ed. 2012, 51, 1–6; i) P. Schramm, H.-
J. Hofmann, J. Pept. Sci. 2010, 16, 276–283; j) G. Guichard, I.
Huc, Chem. Commun. 2011, 47, 5933–5941; k) C. Tomasini, G.
Angelici, N. Castellucci, Eur. J. Org. Chem. 2011, 3648, and
references cited therein.
[4]
CHCl ). IR (CHCl ): ν = 3321, 3020, 1714, 1681, 1614, 1537, 1489,
˜
3
3
1435, 1398, 1338, 1215, 1166 cm–1. 1H NMR (CDCl3, 500 MHz):
δ = 9.09 (s, 1 H), 8.91 (s, 1 H), 7.68–7.69 (d, J = 7.98 Hz, 2 H),
7.63 (m, 1 H), 7.60 (br. s, 1 H), 7.37 (m, 3 H), 7.29 (m, 2 H), 7.21–
7.22 (d, J = 7.98 Hz, 2 H), 4.78 (m, 1 H), 3.38–3.35 (m, 1 H), 3.21–
3.19 (m, 1 H), 2.37 (s, 3 H), 2.30–2.29 (m, 1 H), 2.03–2.00 (m, 1
H), 1.79–1.77 (m, 1 H) ppm. 13C NMR (CDCl3, 125 MHz): δ =
169.4, 167.3, 144.2, 140.0, 136.9, 136.2, 135.8, 135.0, 131.7, 129.7,
128.2, 127.3, 123.2, 116.8 ppm. MS (ESI): m/z = 668.1056 [M +
H]+, 689.9918 [M + Na]+, 705.9593 [M + K]+. C25H23BrIN3O4S
(668.34): calcd. C 44.93, H 3.47, N 6.29; found C 45.26, H 3.72, N
6.59.
[5]
[6]
[7]
N-(4-Bromophenyl)-1-[2-(methylsulfonamido)benzoyl]pyrrolidine-2-
carboxamide (24): To a solution of 16b in anhydrous MeCN
(10 mL), 3b (1.0 g, 4.646 mmol), HBTU (2.12 g, 5.597 mmol) and
anhydrous DIEA (1.5 mL, 8.208 mmol) were added at 0 °C. The
reaction mixture was stirred at room temperature for 12 h, then the
solvent was evaporated and the residue was dissolved in CH2Cl2
(10 mL). The organic layer was washed sequentially with satd.
NaHCO3 solution followed by satd. KHSO4 solution, water and
brine. The organic layer was dried with anhydrous Na2SO4 and the
crude product was subjected to column purification (pet ether/ethyl
acetate, 30:70; Rf = 0.5) to afford 24 as a white solid (1.086, 86%),
[8]
m.p. 178–180 °C. [α]2D8 = –22.78 (c = 0.3, CHCl ). IR (CHCl ): ν =
˜
3
3
3020, 2399, 2360.95, 2343, 1716, 1697, 1683, 1653, 1647, 1635,
1616 cm–1. 1H NMR (CDCl3, 400 MHz): δ = 9.69 (s, 1 H), 8.80 (s,
1 H), 7.57–7.59 (d, J = 8.04 Hz, 2 H), 7.48–7.46 (m, 2 H), 7.36–
7.33 (m, 3 H), 7.29–7.27 (m, 3 H), 4.75–4.72 (m, 1 H), 3.59–3.29
(m, 2 H), 2.94 (s, 3 H), 2.28–2.27 (m, 1 H), 2.10–2.03 (m, 1 H),
1.98–1.92 (m, 1 H), 1.86–1.78 (m, 1 H) ppm. 13C NMR (CDCl3,
100 MHz): δ = 170.4, 168.0, 166.6, 137.3, 134.6, 133.6, 131.3, 130.7,
129.1, 128.1, 127.7, 126.7, 124.5, 122.2, 121.1, 116.1, 60.7, 60.1,
49.8, 42.3, 39.6, 39.3, 38.3, 29.3, 24.7, 20.8, 13.9 ppm. MS (ESI):
m/z = 466.0795 [M + H]+, 488.2980 [M + Na]+. C19H20BrN3O4S
(466.35): calcd. C 48.93, H 4.32, N 9.01; found C 49.26, H 4.63, N
8.73.
[9]
Structurally intriguing “peptoid reverse-turns” have been re-
ported recently, see: a) B. C. Gorske, J. R. Stringer, B. L. Bas-
tian, S. A. Fowler, H. E. Blackwell, J. Am. Chem. Soc. 2009,
131, 16555–16567; b) J. R. Stringer, J. A. Crapster, I. A. Guzei,
H. E. Blackwell, J. Org. Chem. 2010, 75, 6068–6078.
For interesting accounts of universal peptidomimetics, see: a)
E. Ko, J. Liu, K. Burgess, Chem. Soc. Rev. 2011, 40, 4411–4421;
b) E. Ko, J. Liu, L. M. Perez, G. Lu, A. Schaefer, K. Burgess,
J. Am. Chem. Soc. 2011, 133, 462–477.
a) J. S. Nowick, Acc. Chem. Res. 2008, 41, 1319–1330; b) O.
Khakshoor, A. J. Lin, T. P. Korman, M. R. Sawaya, S.-C. Tsai,
D. Eisenberg, J. S. Nowick, J. Am. Chem. Soc. 2010, 132,
11622–11628.
Supporting information (see footnote on the first page of this arti-
1
cle): H, 13C, DEPT-135, 2D NMR spectra, crystal data and ESI
[10]
[11]
mass spectra of all new compounds.
Acknowledgments
V. T., T. I., K. N. V., R. V., S. S. K., and E. V. V. are thankful to the
Council of Scientific and Industrial Research (CSIR), New Delhi
Eur. J. Org. Chem. 2013, 3529–3542
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3541

P. R. Rajamohanan, G. J. Sanjayan et al.
FULL PAPER
[12]
[23] Ideal β-turns and their mimics have Cαi to Cαi+3 distances of
less than 7.0 Å, see: J. S. Richardson, Adv. Protein Chem. 1981,
34, 167–339.
For pyrrolinone-based peptidomimetics, see: A. B. Smith,
A. K. Charnley, R. Hirschmann, Acc. Chem. Res. 2011, 44,
180–193.
[24] S. Chatterjee, R. S. Roy, P. Balaram, J. R. Soc. Interface 2007,
[13]
Triazole-based peptide mimics have been reported very re-
cently, see: a) Ahsanullah, P. Schmieder, R. Kuhne, J. Radem-
ann, Angew. Chem. 2009, 121, 5143; Angew. Chem. Int. Ed.
2009, 48, 5042–5045; b) K. Holland-Nell, M. Meldal, Angew.
Chem. 2011, 123, 5310–5312; Angew. Chem. Int. Ed. 2011, 50,
5204–5206; c) M. Empting, O. Avrutina, R. Meusinger, S. Fab-
ritz, M. Reinwarth, M. Biesalski, G. Buntkowsky, H. Kolmar,
Angew. Chem. 2011, 123, 5313–5317; Angew. Chem. Int. Ed.
2011, 50, 5207–5211.
a) A. J. Souers, J. A. Ellman, Tetrahedron 2001, 57, 7431–7448;
b) G. Jeanotte, W. D. Lubell, J. Am. Chem. Soc. 2004, 126,
14334–14335; c) E. Ko, K. Burgess, Org. Lett. 2011, 13, 980–
983.
For an excellent account of validation of a β-turn mimetic li-
brary targeting protein-protein and peptide-receptor interac-
tions, see: L. R. Whitby, Y. Ando, V. Setola, P. K. Vogt, B. L.
Roth, D. L. Boger, J. Am. Chem. Soc. 2011, 133, 10184–10194.
J. Chatterjee, C. Gilon, A. Hoffman, H. Kessler, Acc. Chem.
Res. 2008, 41, 1331–1342.
4, 587–606.
[25] a) T. S. Haque, J. C. Little, S. H. Gellman, J. Am. Chem. Soc.
1996, 118, 6975–6985; b) J. M. Langenhan, S. H. Gellman, Org.
Lett. 2004, 6, 937–940; c) S. Royo, W. M. D. Borggraeve, C.
Peggion, F. Formaggio, M. Crisma, A. I. Jimenez, C. Cativiela,
C. Toniolo, J. Am. Chem. Soc. 2005, 127, 2036–2037.
[26] a) T. S. Haque, J. C. Little, S. H. Gellman, J. Am. Chem. Soc.
1994, 116, 4105–4106; b) Y. Demizu, M. Doi, Y. Sato, M.
Tanaka, H. Okuda, M. Kurihara, J. Org. Chem. 2010, 75,
5234–5239.
[27] Sulfonamides have distinctly different H-bonding preferences,
when compared to carboxamides, see: a) J. M. Fisk, J. D. Lan-
genhan, S. H. Gellman, Org. Lett. 2001, 3, 2559–2562; b)
R. M. J. Liskamp, A. W. Kruijtzer, Mol. Diversity 2004, 8, 79–
87.
[28] a) M. J. Pérez de Vega, M. Martín-Martínez, R. González-
Muñiz, Curr. Top. Med. Chem. 2007, 7, 33–62; b) G. M. Grot-
enbreg, M. S. M. Timmer, A. L. Llamas-Saiz, M. Verdoes,
G. A. V. Marel, M. J. Van Raaij, H. S. Overkleeft, M. J. Over-
hand, J. Am. Chem. Soc. 2004, 126, 3444–3446.
[29] a) M. Chorev, M. Goodman, Trends Biotechnol. 1995, 13, 438–
445; b) M. Taylor, S. Moore, J. Mayes, E. Parkin, M. Beeg, M.
Canovi, M. Gobbi, D. M. A. Mann, D. Allsop, Biochemistry
2010, 49, 3261–3272; c) T. Weeden, J. Stefano, S. Duan, A.
Edling, L. Hou, W. L. Chuang, M. A. Perricone, C. Pan, J. L.
Dzuris, J. Pept. Sci. 2011, 17, 47–55.
[14]
[15]
[16]
[17] a) Y. Hamuro, S. J. Geib, A. D. Hamilton, J. Am. Chem. Soc.
1996, 118, 7529–7541; b) Y. Hamuro, S. J. Geib, A. D. Hamil-
ton, Bioorg. Med. Chem. 2001, 9, 2355–2363; c) J. M. Davis,
L. K. Tsou, A. D. Hamilton, Chem. Soc. Rev. 2007, 36, 326–
334; d) I. Saraogi, A. D. Hamilton, Chem. Soc. Rev. 2009, 38,
1726–1743; e) Y. Hamuro, S. J. Geib, A. D. Hamilton, J. Am.
Chem. Soc. 1997, 119, 10587–10593.
[18] H-bonding is a powerful force in the creation of synthetic
oligomers (foldamers) with potential applications in material
science. For a highlight on this topic, see: P. Prabhakaran, G.
Priya, G. J. Sanjayan, Angew. Chem. Int. Ed. 2012, 51, 4006–
4008.
[30] a) E. G. Hutchinson, J. M. Thornton, Protein Sci. 1994, 3,
2207–2216; b) M. P. Williamson, Biochem. J. 1994, 297, 249–
260.
[31] For an extensive study on the mechanism of hydrolysis of
benzoxazinones, see: a) M. C. Etter, Z. U. Lipkowska, C. Deco-
reta, J. Royer, F. Bayarda, F. Vicens, J. Mol. Cryst. Liq. Crsyt.
1988, 161, 43–53; b) M. C. Etter, Z. U. Lipkowska, G. Leroy,
F. Precigoux, C. Decoret, F. Bayard, J. Royer, J. Vicens, J. Mol.
Cryst. Liq. Cryst. 1988, 161, 67–76; c) J. Vicens, C. Decoret, J.
Royer, M. C. Etter, Isr. J. Chem. 1985, 25, 306–11.
[32] M. Kuemin, Y. A. Nagel, S. Schweizer, F. W. Monnard, C. Och-
senfeld, H. Wennemers, Angew. Chem. 2010, 122, 6468–6471;
Angew. Chem. Int. Ed. 2010, 49, 6324–6327.
[19] M. Kuemin, J. Engel, H. Wennemers, J. Pept. Sci. 2010, 16,
596–600.
[20] a) B. Chatterjee, I. Saha, S. Raghothama, S. Aravinda, R. Rai,
N. Shamala, P. Balaram, Chem. Eur. J. 2008, 14, 6192–6204; b)
S. R. Raghothama, S. K. Awasthi, P. Balaram, J. Chem. Soc.
Perkin Trans. 2 1998, 137–143; c) S. Chatterjee, P. G. Vasudev,
S. Raghothama, C. Ramakrishnan, N. Shamala, P. Balaram, J.
Am. Chem. Soc. 2009, 131, 5956–5965.
[21] a) Y. J. Chung, L. A. Christianson, H. E. Stanger, D. R. Powell, [33] M. L.-Kyriakides, R. E. Galardy, Biochemistry 1979, 18, 1952–
S. H. Gellman, J. Am. Chem. Soc. 1998, 120, 10555–10556; b)
J. F. Espinosa, S. H. Gellman, Angew. Chem. 2000, 112, 2420–
2423; Angew. Chem. Int. Ed. 2000, 39, 2330–2333; c) J. M. Lan-
genhan, I. A. Guzei, S. H. Gellman, Angew. Chem. 2003, 115,
2504–2507; Angew. Chem. Int. Ed. 2003, 42, 2402–2405; d) F.
Freire, J. D. Fisk, A. J. Peoples, M. Ivancic, I. A. Guzei, S. H.
Gellman, J. Am. Chem. Soc. 2008, 130, 7839–7841.
1957.
[34] K. Raatikainen, K. Rissanen, CrystEngComm 2009, 11, 750–
752.
[35] a) G. N. Ramachandran, C. Ramakrishnan, V. Sasisekharan, J.
Mol. Biol. 1963, 7, 95–99; b) G. N. Ramachandran, V. Sasisek-
haran, Adv. Protein Chem. 1968, 23, 283–437.
[36] M. M. Vasbinder, E. R. Jarvo, S. J. Miller, Angew. Chem. 2001,
113, 2806–2809; Angew. Chem. Int. Ed. 2001, 40, 2824–2827.
[37] a) J. L. Gustafson, D. Lim, S. J. Miller, Science 2010, 328,
1251–1255; b) M. Wiesner, H. Wennemers, Synthesis 2010,
1568–1571.
[38] S. J. Grabowski, Chem. Rev. 2011, 111, 2597–625.
[39] N. J. Greenfield, Nat. Protoc. 2006, 1, 2876–2890.
[40] CCDC-838713 (for 5a), -838714 (for 6a), -838715 (for 7a),
-838716 (for 8a), -838709 (for 17), -838710 (for 18),
-838711 (for 19), -838712 (for 22), -812837 (for 23b), and
-812836 (for 24) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[22] a) D. Srinivas, R. Gonnade, S. Ravindranathan, G. J. Sanjayan,
Tetrahedron 2006, 62, 10141–10146; b) P. K. Baruah, R. Gon-
nade, P. R. Rajamohanan, H.-J. Hofmann, G. J. Sanjayan, J.
Org. Chem. 2007, 72, 5077–5084; c) D. Srinivas, R. Gonnade,
S. Ravindranathan, G. J. Sanjayan, J. Org. Chem. 2007, 72,
7022–7025; d) P. K. Baruah, N. K. Sreedevi, R. Gonnade, S.
Ravindranathan, K. Damodaran, H.-J. Hofmann, G. J. San-
jayan, J. Org. Chem. 2007, 72, 636–639; e) A. M. Kendhale, R.
Gonnade, P. R. Rajamohanan, H.-J. Hofmann, G. J. Sanjayan,
Chem. Commun. 2008, 2541–2543; f) P. K. Baruah, N. K. Sree-
devi, B. Majumdar, R. Pasricha, P. Poddar, R. Gonnade, S.
Ravindranathan, G. J. Sanjayan, Chem. Commun. 2008, 712–
714; g) P. Prabhakaran, S. S. Kale, V. G. Puranik, P. R. Raja-
mohanan, O. Chetina, J. A. K. Howard, H.-J. Hofmann, G. J.
Sanjayan, J. Am. Chem. Soc. 2008, 130, 17743–17754.
Received: December 24, 2012
Published Online: April 25, 2013
3542
www.eurjoc.org
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 3529–3542