Ester vs. amide on folding: A case study with a 2-residue synthetic peptide

Article abstract of DOI:10.1039/c3ob41967c

Although known for their inferiority as hydrogen-bonding acceptors when compared to amides, esters are often found at the C-terminus of peptides and synthetic oligomers (foldamers), presumably due to the synthetic readiness with which they are obtained using protected peptide coupling, deploying amino acid esters at the C-terminus. When the H-bonding interactions deviate from regularity at the termini, peptide chains tend to "fray apart". However, the individual contributions of C-terminal esters in causing peptide chain end-fraying goes often unnoticed, particularly due to diverse competing effects emanating from large peptide chains. Herein, we describe a striking case of a comparison of the individual contributions of C-terminal ester vs. amide carbonyl as a H-bonding acceptor in the folding of a peptide. A simple two-residue peptide fold has been used as a testing case to demonstrate that amide carbonyl is far superior to ester carbonyl in promoting peptide folding, alienating end-fraying. This finding would have a bearing on the fundamental understanding of the individual contributions of stabilizing/destabilizing non-covalent interactions in peptide folding.

Full text of DOI:10.1039/c3ob41967c

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Ester vs. amide on folding: a case study with a 2-residue
synthetic peptide†
Cite this: Org. Biomol. Chem., 2013, 11,
8348
Kuruppanthara N. Vijayadas,a Roshna V. Nair,a Rupesh L. Gawade,b
Amol S. Kotmale,c Panchami Prabhakaran,a,d Rajesh G. Gonnade,b
Vedavadi G. Puranik,b Pattuparambil R. Rajamohananc and Gangadhar J. Sanjayan*a
Although known for their inferiority as hydrogen-bonding acceptors when compared to amides, esters
are often found at the C-terminus of peptides and synthetic oligomers (foldamers), presumably due to
the synthetic readiness with which they are obtained using protected peptide coupling, deploying amino
acid esters at the C-terminus. When the H-bonding interactions deviate from regularity at the termini,
peptide chains tend to “fray apart”. However, the individual contributions of C-terminal esters in causing
peptide chain end-fraying goes often unnoticed, particularly due to diverse competing effects emanating
from large peptide chains. Herein, we describe a striking case of a comparison of the individual contri-
butions of C-terminal ester vs. amide carbonyl as a H-bonding acceptor in the folding of a peptide. A
simple two-residue peptide fold has been used as a testing case to demonstrate that amide carbonyl is
far superior to ester carbonyl in promoting peptide folding, alienating end-fraying. This finding would
have a bearing on the fundamental understanding of the individual contributions of stabilizing/destabi-
lizing non-covalent interactions in peptide folding.
Received 28th September 2013,
Accepted 15th October 2013
DOI: 10.1039/c3ob41967c
www.rsc.org/obc
“frayed”, the exposed termini may interact with solvent mole-
cules for hydrogen-bonding and when the hydrogen bonding
Introduction
Hydrogen-bonding interactions play a central role in the stabil- solvents are not available, peptides may aggregate by inter-
ization of secondary structures – the local conformations that molecular hydrogen-bonding interactions.2 In large peptides,
impart a specific shape to the peptide backbone. There are including the synthetic oligomers called foldamers,5–7 it would
innumerable examples of peptide-based systems known in the often be a challenging task to pinpoint the individual contri-
literature wherein the C-terminal esters, as clearly evident butions of the C-terminal esters in causing end-fraying. This is
from their crystal structures, are seen not participating in particularly due to the competing diverse effects originating at
intramolecular hydrogen-bonding that would have further the peptide termini such as: π–π interactions,2a NH–π inter-
reinforced the stability of the secondary structure.1–4 In fact, in actions,2b,c a similar chirality array of amino acids in the
most of the peptide hairpin crystal structures, the terminal hairpin region,4f,g temperature effects2d and bulkiness of
hydrogen bond that connects the N- and the C-termini of the groups at the end terminals4h etc., which are all well known to
peptide is absent as a consequence of strand fraying.1,2 Once cause peptide chain end-fraying. Using a simple two-residue
reverse-turn mimic,8–19 herein we demonstrate the conse-
quences of swapping C-terminal amides with esters on the
aDivision of Organic Chemistry, National Chemical Laboratory, Dr Homi Bhabha
Road, Pune 411 008, India. E-mail: gj.sanjayan@ncl.res.in; Fax: +91-020-2590-2629;
Tel: +91-020-2590-2082
overall folding ability of a synthetic peptide. Crystallographic
studies of several C-terminal esters and amides unequivocally
reveal the high propensity of the amide carbonyls to promote a
closed hydrogen-bonded network adopting a folded confor-
mation, whilst the C-terminal esters suffer end-fraying leaving
the termini open-ended without intramolecular hydrogen-
bonding. Employing the ready-to-understand 2-residue reverse
turn motif with least interference from other effects, as noted
above,20 we also exemplify how tiny structural changes at the
peptide termini can have dramatic consequences for the
folding characteristics.20b,21,22
bCentre for Materials Characterisation, National Chemical Laboratory,
Dr Homi Bhabha Road, Pune 411 008, India
cCentral NMR Facility, National Chemical Laboratory, Dr Homi Bhabha Road,
Pune 411 008, India
dDepartment of Chemistry & Biology, University of Leeds, Leeds, LS2 9JT, UK
†Electronic supplementary information (ESI) available: 1H, 13C, DEPT-135 NMR,
2D study spectra, ESI mass spectra and theoretical study of new compounds are
included. CCDC 874039, 699500, 874040, 874041, 874042, 874043, 679454,
874044, 874045, 874046 and 874047. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c3ob41967c
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In order to unequivocally establish the individual contri-
butions of amide and ester carbonyls in modulating the
Results and discussion
While investigating the conformational characteristics of the folding propensity, we present herein two sets of compounds –
2-residue Ant-Pro peptide fold (Ant – anthranilic acid; Pro – five each from both categories of diversely substituted C-term-
proline),20 which forms
a
reverse-turn-like secondary inal amides 1–5 (Fig. 1, left panel) and esters 6–10 (Fig. 1,
structure,8–19 we stumbled upon the finding that the C-term- right panel). Substituents around the folding segment have
inal peptide ester 6 (Fig. 1) did not show, surprisingly, the been varied extensively at both termini of the amides and
folded C9-turn conformation with a closed hydrogen-bonded esters in order to unequivocally reveal the role of substituent
ring, although its corresponding C-terminal amide 1 (Fig. 1) effects in hydrogen-bonding interactions. Extensive crystal
did show the expected folding. Intrigued by this finding, we structure investigations and solution-state studies have been
synthesized diversely substituted, at both N- and C-termini, undertaken to correlate the structural modifications with their
dipeptide sets of Ant-Pro C-terminal esters and amides, in consequences for conformation.
order to compare the individual contributions of C-terminal
ester vs. amide carbonyl as a hydrogen-bonding acceptor in
Synthesis
the formation of a folded structure featuring a closed network
of 9-membered hydrogen-bonded rings.
The Ant-Pro reverse-turn analogues featuring C-terminal
amides 1–5 and C-terminal esters 6–11, required for the
Fig. 1 Molecular and crystal structures of C-terminal amides 1–5 (left panel) and C-terminal esters 6–10 (right panel). Note: the amides assume folded confor-
mation featuring 9-membered-ring H-bonding (9-H-bonding), whilst the esters display fraying without 9-H-bonding. In the crystal structures, all hydrogens, except
the polar ones, have been deleted for clarity.
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present study, have been synthesized in a multi-step synthetic
strategy, as depicted in Scheme 1 (see ESI,† page S3). The
reason for functionalizing the C-terminus amides 1, 3–5 with
4(Br)-anilide was owing to the relative ease of bromo
compounds to yield crystal formation.6c,f It should be empha-
sized that the 4(Br)-aryl group does not interfere in the folding
process, as it is positioned away from the hydrogen-bonding
turn region. This is also vindicated from the fact that the
C-terminus methyl amide 2, which is devoid of 4(Br)-aryl
group, participates in folding.
Fig. 2 Molecular and crystal structures of a C-terminal ester showing intra-
molecular C–H⋯O interactions.
Crystal structure investigations
structure revealed that the N-terminus benzoyl group restricts
the C-terminus ester group from fraying, apparently mediated
Comparison of all crystal structures of the C-terminal amides
1–5 unequivocally reveals that folding, aided by a robust
9-membered ring hydrogen-bonding involving the 1Ant NH
by
a
C–H⋯O hydrogen-bonding interaction.24,25 The
additional stabilization factor emanating from the C–H⋯O
bond facilitates the formation of the 9-membered ring hydro-
gen-bonding, although very weak, between the amide NH and
ester carbonyl with a large N–H⋯O distance] [d(N–H⋯O: 3.17),
d(N⋯O: 4.51)].
2
and Pro carbonyl hydrogen-bonding codes, is clearly persist-
ent throughout. The hydrogen-bonding parameters are: hydro-
gen-bonding distances [Δ(N⋯O) = 2.881–3.080 Å and Δ(H⋯O)
= 2.030–2.268 Å]. In stark contrast, all the C-terminal esters
6–10 show absence of folding involving 9-membered ring
hydrogen-bonding featuring the 1Ant NH and 2Pro carbonyl
hydrogen-bonding codes. As the ester carbonyls in 6–10 move
Conformational investigation of amides (1 and 5) and esters
(6 and 10) in solution
away causing end-fraying without involving any hydrogen-
1
bonding, intriguingly though, the Ant NH finds another car- Extensive solution-state NMR studies have been undertaken to
bonyl partner for hydrogen-bonding – which happens to be its verify the nature of the hydrogen-bonding interactions in the
own carbonyl forming a 6-membered-ring hydrogen-bonding. amide and esters. Conformational investigations by solution
The consequences of end-fraying of the termini resulting in state 2D NOESY studies (500 MHz, CDCl3) revealed that the
the lack of folding in the esters 6–10 are readily discernible solid-state conformational features of the C-terminal amides
from the high ψ torsion angles of the Ant residues (ψav = 145°) are clearly preserved in the solution-state as well (Fig. 3 and
when compared to that of the amides 1–5 (ψav = 68°, Table 1). ESI,† pages S49–S52).20a The significant nOes observed: C20
The disinclination of the C-terminal ester carbonyl in partici- vs. C19H and NH1 vs. C12H support their spatial proximity
pating in hydrogen-bonding which would have otherwise led thereby suggesting the folded conformation for the amide 1,
to folding is apparently due to its poorer hydrogen-bonding as observed in their solid-state by crystal structure studies.
acceptor potential.21e,22
Other characteristic nOes observed for 5 which supported
While investigating the effects of substituents on the struc- its folded conformation in the solution-state include: C24H vs.
tural features of the folded conformation, we noticed an ano- C19H, C24H vs. C18H and NH1 vs. C12H. However, similar
malous example of an ester 11 that purportedly forms a very characteristic nOes are weakened or absent in the case of
weak C9 H-bonding23 (Fig. 2). Closer investigation of its crystal esters, suggesting significant fraying, as observed in the solid-
Table 1 Torsion angles and intramolecular hydrogen bonding parameters
Torsion angles (°)
Ant
Hydrogen bonding parameters
Pro
Distances (Å)
Angles (°)
Compound no.
φ
ψ
φ
ψ
O⋯H
N⋯O
CvO⋯N
NH⋯O
Amides
1
2
3
4
5
173.40
170.23
−172.38
176.23
166.51
−65.37
−69.58
−70.08
−72.66
−71.89
−55.97
−60.64
−58.82
−55.94
−56.49
164.59
174.04
164.32
166.62
162.41
2.173
2.030
2.259
2.108
2.178
3.025
2.881
3.075
2.960
3.033
127.28
131.10
123.31
132.15
127.17
170.80
169.81
158.34
171.33
172.17
Esters
6
7
8
9
10
11
153.97
−177.80
−144.62
167.51
−138.63
−177.28
150.75
140.07
146.41
−144.35
146.28
−61.60
−69.70
−57.70
−56.68
−59.04
−59.62
−139.47
153.71
135.55
−38.27
134.8
5.263
5.452
5.237
4.737
5.366
3.177
5.842
5.930
5.790
5.552
5.874
4.510
65.58
72.70
64.08
48.65
62.70
111.12
128.93
120.03
125.28
156.00
122.65
148.44
−130.50
156.72
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Fig. 3 Selected NOE extracts from the 2D NOESY data of C-terminal amide 1 (left panel: A) and C-terminal ester 6 (right panel: B). See ESI,† pages S50 and S51, for
2D NOESY spectra for 5 and 10, respectively. Note: the amides assume folded conformation featuring 9-membered-ring H-bonding (9-H-bonding), whilst the esters
display fraying without 9-H-bonding.
state (crystal structures). It is also noteworthy that the charac- model systems are due to the differences in their hydrogen-
teristic NH1 vs. C12H nOe interaction that is suggestive of bonding patterns. The 9-membered-ring hydrogen-bondings
folding, as observed in the amide 1, is significantly weakened that led to folding in the case of the C-terminal amides
in the ester counterpart 6. The nOes due to the interaction of are retained in the solution-state, as evidenced from the
C24H vs. C19H and C24H vs. C18H in amide 1 are absent in solvent titration experiments (Fig. 4A).20a However, the poten-
the ester counterpart 6. On the other hand, esters (6 and 10) tial intervention of 6-membered hydrogen-bonding, in the
display strong nOes involving the interactions between absence of 9-membered hydrogen-bonding, has made it
δ-hydrogens of proline and aromatic hydrogens of Ant difficult to unequivocally distinguish the nature of the hydro-
(i.e. C12H vs. C5H), suggestive of the fraying. These gen-bonded rings of the C-terminal esters in the solution-state
differences in the conformational features of ester and amide (Fig. 4B).
Fig. 4 NMR DMSO-d6 titration studies of C-terminal amide 5 (A) and C-terminal ester 10 (B).
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orthorhombic, space group P212121, a = 8.9732(8), b = 10.4292
Conclusions
(9), c = 25.653(2) Å, V = 2400.7(4) Å3, Z = 4, Dc = 1.230 mg m−3
,
In conclusion, this article describes a striking case of a com- µ (Mo-Kα) = 0.088 mm−1, 12 181 reflections measured, 4239
parison of the individual contributions of C-terminal ester vs. unique [I > 2σ(I)], R value 0.0467, wR2 = 0.1223.
amide carbonyl as a hydrogen-bonding acceptor in the folding
Crystal data for 3. Single crystals of 3 were grown by slow
of a peptide secondary structure featuring a closed network of evaporation of the mixture of methanol and dichloromethane.
hydrogen-bonded rings. A simple two-residue peptide fold A colorless plate of approximate size 0.17 × 0.12 × 0.02 mm
served as a simple model, with the least interference from was used for data collection. Temperature = 296 K, wavelength
other competing interactions emanating from the backbone = 0.71073 Å. Total scans = 4, F(000) = 944, θ range = 1.62 to
and/or side chains, in evaluating the extent of individual con- 25.00°, completeness to θ of 24.99° is 100.0%. SADABS correc-
tributions of C-terminal ester vs. amide carbonyl as a hydro- tion applied, goodness-of-fit on F2 = 1.203, C21H22BrN3O4, M =
gen-bonding acceptor in the formation of a peptide fold. The 460.33. Crystals belong to orthorhombic, space group P212121,
results of the crystallographic studies26 of two large sets of a = 7.1599(7), b = 11.8460(11), c = 25.160(2) Å, V = 2134.0(4) Å3,
C-terminal esters and amides unequivocally reveal the high Z = 4, Dc = 1.433 mg m−3, µ (Mo-Kα) = 1.958 mm−1, 17 385
propensity of the C-terminal amides, when compared to their reflections measured, 3754 unique [I > 2σ(I)], R value 0.0749,
ester counterparts, to stabilize a folded conformation. The wR2 = 0.1330.
comprehensive demonstration of the individual contributions
Crystal data for 4. Single crystals of 4 were grown by slow
– ester vs. amide – in modulating the stability of the hydrogen- evaporation of methanol. A colorless thin plate of appro-
bonded network, free from intervention of other effects which ximate size 0.48 × 0.14 × 0.02 mm was used for data collection.
would have otherwise manifested in large oligomer systems, is Temperature = 297 K, wavelength = 0.71073 Å. Total scans = 4,
noteworthy.
F(000)
to
= 488, θ range = 1.89 to 25.00°, completeness
θ
of 25.00° is 99.7%. SADABS correction applied,
goodness-of-fit on F2 = 0.985, C20H17BrF3N3O3, M = 484.28.
Crystals belong to monoclinic, space group P21, a = 11.016(5),
b = 7.612(3), c = 12.920(5) Å, V = 1059.2(7) Å3, Z = 2, Dc
=
Experimental procedures
Crystal X-ray crystallographic studies
1.518 mg m−3, µ (Mo-Kα) = 1.992 mm−1, 5304 reflections
measured, 3754 unique [I > 2σ(I)], R value 0.0469, wR2
=
Crystallographic data for compounds 1–11 were collected at 0.0954.
T = 293 K on a SMART APEX CCD Single Crystal X-ray diffracto- Crystal data for 5. Single crystals of 5 were grown by slow
meter using Mo KR radiation (λ = 0.7107 Å). The structures evaporation of methanol. colorless needle of appro-
A
were solved by direct methods using SHELXTL. All the data ximate size 0.62 × 0.15 × 0.11 mm was used for data collection.
were corrected for Lorentz polarization and absorption effects. Temperature = 297 K, wavelength = 0.71073 Å. Total scans = 4,
SHELX-97 (ShelxTL) was used for structure solution and full F(000) = 504, θ range = 2.19 to 25.00°, completeness to θ of
matrix least-squares refinement on F2. Hydrogen atoms were 25.00° is 98.7%. SADABS correction applied, goodness-of-fit on
included in the refinement in the riding mode. The refine- F2 = 1.203, C23H26BrN3O4, M = 488.38. Crystals belong to
ments were carried out using SHELXL-97.27
monoclinic, space group P21, a = 12.8420(19), b = 7.0231(10),
Crystal data for 1. Single crystals of 1 were grown by slow c = 13.3863(19) Å, V = 1207.3(3) Å3, Z = 2, Dc = 1.343 mg m−3
,
evaporation of the mixture of methanol and dichloromethane. µ (Mo-Kα) = 1.734 mm−1, 9259 reflections measured, 4152
Colorless prism crystals of approximate size 0.31 × 0.21 × unique [I > 2σ(I)], R value 0.1045, wR2 = 0.1533.
0.12 mm were used for data collection. Temperature = 297 K,
Crystal data for 6. Single crystals of 6 were grown by slow
wavelength = 0.71073 Å, hemisphere acquisition. Total scans = evaporation of the mixture of methanol and dichloromethane.
4, F(000) = 440, θ range = 1.90 to 25.00, completeness to θ of A colorless needle of approximate size 1.37 × 0.54 × 0.47 mm
25.00° is 99.6%, SADABS correction applied, goodness-of-fit on was used for data collection. Temperature = 297 K, wavelength
F2 = 0.990, C20H20BrN3O3, M = 430.30. Crystals belong to = 0.71073 Å. Total scans = 4, F(000) = 744, θ range = 2.28 to
monoclinic, space group P21, a = 11.1418(8), b = 7.1748(5), c = 25.00°, completeness to θ of 25.00° is 99.8%. SADABS correc-
12.9148(10) Å, V = 994.55(13) Å3, Z = 2, Dc = 1.437 mg m−3
,
tion applied, goodness-of-fit on F2 = 1.042, C18H24N2O5, M =
µ (Mo-Kα) = 2.091 mm−1, 5062 reflections measured, 2888 348.39. Crystals belong to orthorhombic, space group P2(1)2(1)-
unique [I > 2σ(I)], R value 0.0297, wR2 = 0.0714.
2(1), a = 8.547(5), b = 9.694(5), c = 22.943(12) Å, V = 1900.9(18)
Crystal data for 2. Single crystals of 2 were grown by slow Å3, Z = 4, Dc = 1.217 mg m−3, µ (Mo-Kα) = 0.089 mm−1, 18 354
evaporation of the mixture of ethyl acetate and pet. ether. A reflections measured, 3348 unique [I > 2σ(I)], R value 0.0479,
colorless needle of approximate size 0.17 × 0.12 × 0.12 mm was wR2 = 0.1257.
used for data collection. Temperature = 295 K, wavelength =
Crystal data for 7. Single crystals of 7 were grown by slow
0.71073 Å, hemisphere acquisition. Total scans = 3, F(000) = evaporation of the mixture of ethyl acetate and pet. ether. Col-
952, θ range = 2.11 to 24.99°, completeness to θ of 24.99° is orless rectangular crystals of approximate size 0.43 × 0.31 ×
100.0%. SADABS correction applied, goodness-of-fit on F2
=
0.14 mm were used for data collection. Temperature = 296 K,
0.999, C23H32N4O5,
M
=
444.53. Crystals belong to wavelength = 0.71073 Å. Total scans = 4, F(000) = 904, θ range
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= 2.09 to 25.00°, completeness to θ of 25.00° is 100%. SADABS
correction applied, goodness-of-fit on F2 = 1.066, C24H28N2O5,
Acknowledgements
M = 424.48. Crystals belong to orthorhombic, space group KNV, RVN and ASK are thankful to CSIR, New Delhi, for
P2(1)2(1)2(1), a = 10.6231(7), b = 10.9569(7), c = 19.5280(12) Å, research fellowships. GJS thanks CSIR network project BSC
V = 2273.0(3) Å3, Z = 4, Dc = 1.240 mg m−3, µ (Mo-Kα) = 0120 for financial support. This article is dedicated to Prof.
0.087 mm−1
, 11 578 reflections measured, 4004 unique Goverdhan Mehta on the occasion of his 70th birth
[I > 2σ(I)], R value 0.0402, wR2 = 0.0946.
anniversary.
Crystal data for 8. Single crystals of 8 were grown by
slow evaporation of the mixture of ethyl acetate and pet. ether.
Colourless thick plate crystals of approximate size 0.50 ×
0.27 × 0.22 mm were used for data collection. Temperature =
173 K, wavelength = 0.71073 Å. Total scans = 4, F(000) = 680,
θ range = 2.318 to 28.349°, completeness to θ of 28.41°
is 99.1%. SADABS correction applied, goodness-of-fit on
F2 = 1.031, C16H20N2O5, M = 320.34. Crystals belong to
Notes and references
1 R. Mahalakshmi and P. Balaram, Protein Design: Methods
and Applications, in Methods in Molecular Biology, ed.
R. Guerois, M. L. De La Paz and J. M. Walker, Humana,
Totowa, NJ, 2006, vol. 340, pp. 71–94.
orthorhombic, space group P2(1)2(1)2(1),
b = 9.756(2), c = 20.126(6) Å, V = 1592.6(7) Å3, Z = 4, Dc
1.336 mg m−3, µ (Mo-Kα) = 0.100 mm−1, 8457 reflections
a = 8.111(2),
=
2 “End-fraying” is common in peptides featuring C-terminal
esters, in particular β-hairpins, due to the absence of
hydrogen-bonds that connect the N- and the C-termini of
the peptide chain. See: (a) R. Mahalakshmi, A. Sengupta,
S. Raghothama, N. Shamala and P. Balaram, Biopolymers,
2006, 88, 36; (b) V. V. Harini, S. Aravinda, R. Rai,
N. Shamala and P. Balaram, Chem.–Eur. J., 2005, 11, 3609;
(c) S. Aravinda, V. V. Harini, N. Shamala, C. Das and
P. Balaram, Biochemistry, 2004, 43, 1832; (d) A. W. Smith,
J. Lessing, Z. Ganim, C. S. Peng, A. Tokmakoff, S. Roy,
T. L. C. Jansen and J. Knoester, J. Phys. Chem. B, 2010, 114,
10913; (e) R. Rai, P. G. Vasudev, K. Ananda, S. Raghothama,
N. Shamala, I. L. Karle and P. Balaram, Chem.–Eur. J., 2007,
13, 5917; (f) C. L. Nesloney and J. W. Kelly, J. Am. Chem.
Soc., 1996, 118, 5836.
3 For examples of helix fraying – as unambiguously proved by
crystal structures, see: (a) L. Guo, Y. Chi, A. M. Almeida,
I. A. Guzei, B. K. Parker and S. H. Gellman, J. Am. Chem.
Soc., 2009, 131, 16018; (b) L. Guo, A. M. Almeida,
W. Zhang, A. G. Reidenbach, S. H. Choi, I. A. Guzei and
S. H. Gellman, J. Am. Chem. Soc., 2010, 132, 7868;
(c) P. Balaram, Biopolymers, 2010, 94, 733; (d) S. Chatterjee,
P. G. Vasudev, S. Raghothama, C. Ramakrishnan,
N. Shamala and P. Balaram, J. Am. Chem. Soc., 2009, 131,
5956.
4 (a) G. Yoder, A. Polese, R. A. G. D. Silva, F. Formaggio,
M. Crisma, Q. B. Broxterman, J. Kamphuis, C. Toniolo and
T. A. Keiderling, J. Am. Chem. Soc., 1997, 119, 10278;
(b) S. H. Choi, I. A. Guzei, L. C. Spencer and S. H. Gellman,
J. Am. Chem. Soc., 2009, 131, 2917; (c) P. G. Vasudev,
K. Ananda, S. Chatterjee, S. Aravinda, N. Shamala and
P. Balaram, J. Am. Chem. Soc., 2007, 129, 4039; (d) R. Rai,
S. Raghothama and P. Balaram, J. Am. Chem. Soc., 2006,
128, 2675; (e) S. H. Choi, I. A. Guzei, L. C. Spencer and
S. H. Gellman, J. Am. Chem. Soc., 2008, 130, 6544;
(f) B. Chatterjee, I. Saha, S. Raghothama, S. Aravinda,
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measured, 3901 unique [I > 2σ(I)], R value 0.0353, wR2
=
0.0823.
Crystal data for 9. Single crystals of 9 were grown by slow
evaporation of the mixture of ethyl acetate and pet. ether. Col-
orless block crystals of approximate size 0.62 × 0.59 × 0.27 mm
were used for data collection. Temperature = 100 K, wavelength
= 0.71073 Å. Total scans = 4, F(000) = 952, θ range = 2.10
to 25.00°, completeness to θ of 25.00° is 99.8%. SADABS cor-
rection applied, goodness-of-fit on F2 = 1.090, C25H34N2O5, M =
442.54. Crystals belong to orthorhombic, space group P2(1)2(1)-
2(1), a = 7.4115(4), b = 16.5480(9), c = 19.3577(11) Å, V = 2374.1(2)
Å3, Z = 4, Dc = 1.238 mg m−3, µ (Mo-Kα) = 0.086 mm−1, 17 494
reflections measured, 4176 unique [I > 2σ(I)], R value 0.0442,
wR2 = 0.1054.
Crystal data for 10. Single crystals of 10 were grown by slow
evaporation of the mixture of ethyl acetate and pet. ether. Col-
orless plate crystals of approximate size 0.44 × 0.34 × 0.14 mm
were used for data collection. Temperature = 297 K, wavelength
= 0.71073 Å. Total scans = 4, F(000) = 744, θ range = 1.77
to 25.00°, completeness to θ of 25.00° is 100%. SADABS correc-
tion applied, goodness-of-fit on F2 = 1.062, C18H24N2O5, M =
348.39. Crystals belong to orthorhombic, space group P2(1)2(1)-
2(1), a = 8.3515(8), b = 9.6398(9), c = 22.979(2) Å, V = 1850.0(3)
Å3, Z = 4, Dc = 1.251 mg m−3, µ (Mo-Kα) = 0.092 mm−1, 9400
reflections measured, 3251 unique [I > 2σ(I)], R value 0.0430,
wR2 = 0.1150.
Crystal data for 11. Single crystals of 11 were grown by slow
evaporation of the mixture of diethyl ether and pet. ether. Col-
orless plate crystals of approximate size 0.24 × 0.12 × 0.03 mm
were used for data collection. Temperature = 297 K, wavelength
= 0.71073 Å. Total scans = 4, F(000) = 744, θ range = 1.96
to 25.99°, completeness to θ of 25.99° is 99.9%. SADABS cor-
rection applied, goodness-of-fit on F2 = 1.117, C40H40N4O8, M =
704.76. Crystals belong to orthorhombic, space group P2(1)2(1)-
2(1), a = 8.173(5), b = 13.001(8), c = 17.350(11) Å, V = 1843.5(19)
Å3, Z = 2, Dc = 1.270 mg m−3, µ (Mo-Kα) = 0.089 mm−1, 10 075
reflections measured, 3616 unique [I > 2σ(I)], R value 0.0488,
wR2 = 0.1084.
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19 Kesslers’ group has been highly successful devising an
N-methylation strategy to conformationally restrict
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N. R. Kallenbach, J. Am. Chem. Soc., 2001, 123, 6451;
Organic & Biomolecular Chemistry
isotropically. Other hydrogen atoms were refined in the
riding mode. Crystallographic data of 1–11 have been de-
posited with the Cambridge Crystallographic Data Centre:
CCDC 874039, 699500, 874040, 874041, 874042, 874043,
679454, 874044, 874045, 874046, and 874047 for com-
pounds 1–11, respectively.
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26 Crystal structure was solved by direct methods and refined
by full matrix least squares on F2 for all data using
SHELXTL software. The hydrogen atom of the hydroxy 27 G. M. Sheldrick, SHELX-97 program for crystal structure solu-
group was located on the difference map and refined
tion and refinement, University of Gottingen, Germany, 1997.
8356 | Org. Biomol. Chem., 2013, 11, 8348–8356
This journal is © The Royal Society of Chemistry 2013