Thionoglycine as a multifunctional spectroscopic reporter of screw-sense preference in helical foldamers

Article abstract of DOI:10.1039/c3ob42167h

A single thionoglycine (glycine thioamide, -HNCH₂C(S)-) residue inserted into a peptide foldamer provides both a pair of germinal protons for use as a ¹H NMR stereochemical probe and a chromophore giving rise to a well defined Cotton

Full text of DOI:10.1039/c3ob42167h

Organic &
Biomolecular Chemistry
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PAPER
Thionoglycine as a multifunctional spectroscopic
reporter of screw-sense preference in helical
foldamers†
Cite this: Org. Biomol. Chem., 2014,
12, 836
Matteo De Poli and Jonathan Clayden*
A single thionoglycine (glycine thioamide, –HNCH2C(vS)–) residue inserted into a peptide foldamer pro-
vides both a pair of germinal protons for use as a 1H NMR stereochemical probe and a chromophore
giving rise to a well defined Cotton effect in CD. Comparison of the response of these two features to a
local helically chiral environment validates them as independent methods for quantifying the confor-
mational screw-sense preference of a helical oligomer, in this case a peptide made of repeated Aib units.
The sign of the Cotton effect provides a measure of the sign of the screw-sense preference, while both
the chemical shift separation of the anisochronous signals of the glycine CH2 group and the magnitude
of the Cotton effect give an estimate of the helicity excess of the oligomer. The thionoglycine unit is
readily introduced synthetically by a thionation of a BocGlyAibOMe dipeptide.
Received 1st November 2013,
Accepted 25th November 2013
DOI: 10.1039/c3ob42167h
www.rsc.org/obc
method for assigning screw sense as left or right handed.12
CD may also be quantitative provided a maximum value of
Introduction
Foldamers are extended molecular structures with well defined molar ellipticity characteristic of a compound with a com-
global conformations.1–4 Many foldamers are helical,4,5 and plete preference for a single screw sense may be deter-
such structures fall into two conceptual classes. Oligomeric mined.13,14 However, in many cases this value is difficult to
helical foldamers built from chiral monomers typically prefer a estimate accurately, and CD spectra may be difficult to inter-
single screw sense because the two screw-sense conformers are pret quantitatively if other absorption bands, for example
diastereoisomers, one being of higher energy than the other. from chiral residues, overlap with those characteristic of the
In contrast the oligomers of achiral monomers, though they helix itself.15
may adopt a helical conformation, must necessarily populate
We have reported an alternative, practically simple method
equally left (M) and right (P) handed screw-sense conformers. for quantifying screw-sense preference which relies on the be-
In some cases the screw sense isomers may interconvert haviour of a pair of diastereotopic groups located within the
slowly, even to the point of being separable,6–8 while in others foldamer chain under conditions where the rate of interconver-
the racemisation rate is fast,9 even on the NMR timescale.10
sion of the screw-sense conformers is either slow or fast on the
The equilibrium between these screw-sense conformers of NMR timescale.16–19 At slow exchange, the diastereotopic
achiral helical foldamers may be perturbed in a number of groups appear as a pair of anisochronous signals by virtue of
ways – by incorporation of a chiral residue in the achiral chain, their location in the chiral environment of the helix. At fast
for example, or by covalent or non-covalent interactions of exchange, the diastereotopic groups may still appear aniso-
chiral ligands with the body or terminus of the chain. We have chronous, but only if the local chiral environment is main-
used such strategies as a means of communicating infor- tained by the existence of an imbalance between the helical
mation through foldamers by a mechanism analogous to that screw-sense conformers. We proposed that the ratio of the aniso-
exploited by the G-protein coupled receptor, or for achieving chronicity of the two signals at slow and fast exchange is
remote stereocontrol over up to 60 bonds.11
directly proportional to this screw-sense preference, expressed
Classically, screw-sense preference has been analysed by as a ‘helical excess’. Using the NMR signals arising from 13C-
circular dichroism (CD), which is a particularly powerful labelled Me groups, we explored this hypothesis by using line
shape analysis at a range of temperatures, and extracted from
these data thermodynamic and kinetic parameters for screw-
sense inversion.17 Importantly, the value of the equilibrium
School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK.
E-mail: clayden@man.ac.uk
†Electronic supplementary information (ESI) available. CCDC 969434 and
constant K extracted by this method matched closely the value
obtained simply from the quotient of the anisochronicities at
fast and slow exchange.
969435. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3ob42167h
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More convenient than relying on a specifically synthesised
and incorporated 13C- (or 19F-)19 labelled residue is the use of a
pair of diastereotopic protons present in a methylene group –
for example the protons of a glycine residue.18 On the assump-
tion that these diastereotopic signals behave in a comparable
manner to those of the 13C-labelled Aib residue, we have used
the AB system arising from the coupled anisochronous signals
of the CH2 group of a glycine residue to quantify the different
levels of screw-sense control from the N- and C-terminals of a
peptide helix,16 and to compare different controllers of helicity
with one another.18 We have also used the CH2 groups
of related C-terminal amino alcohol residues to assess the
decay of conformational preference over extended foldamer
lengths.20
Fig. 1 The endothiopeptides used in this study. A right-handed helix is
shown for convenience only; see text for further explanation.
However, while these studies gave values that were self-con-
sistent,19 we have thus far never demonstrated that the degree
of screw-sense control measured by NMR may be correlated
directly with the degree of screw-sense control observed by cir-
cular dichroism. In this paper we do this by making use of a
thionated glycine residue (–NHCH2(CvS)–) as a reporter of
screw-sense preference. Among the possible peptide bond
modifications, thioamides are one of the closest mimics of a
peptide linkage. These ‘endothiopeptides’ have received much
attention because of their interesting physical and chemical
properties. For example: (1) the thioamide C–N bond under-
goes reversible cis/trans photoisomerization upon irradiation
at 260 nm;21,22 (2) the S atom of a thioamide is a weaker hydro-
gen bond acceptor than the O atom of an amide, while the
more acidic adjacent NH is a stronger hydrogen bond donor;23 (3)
endothiopeptides have been used in structure–activity relation-
ship studies to alter biological activities;24,25 (4) thioamides
can act as isosteric peptide bond fluorescence quenchers.26
Thionoglycine contains both a chromophore suitable as a
CD reporter in the form of a thioamide group and a pair of
1
Scheme 1 Reagents and conditions: (a) Lawesson’s reagent, THF,
60 °C; (b) HCl–Et2O 2 M, HSCH2CH2OH, anisole; (c) TFA, CH2Cl2; (d)
EDC, MeCN; (e) 9, iPr2NEt, MeCN; (f) PMe3, THF, H2O; (g) Cbz-L-AA-OH,
HOAt, EDC, iPr2NEt.
potentially diastereotopic H nuclei for use as an NMR repor-
ter, directly comparable with the 1H NMR spectra of the
related glycinamides.18 By using a thioamide, we ensure that
the Cotton effect arising from this chromophore is well separ-
ated in the CD spectrum from the bands associated with the
amide carbonyls themselves. Typical applications of CD
measure helicity in the foldamer structure as a whole, in con-
trast with our NMR methods, which measure conformational
preference at a single site.17 Here we use the thioamide func-
tion to achieve a localised CD response at a particular site in
the foldamer chain.27 The thionoglycine was located one
residue from the C-terminus of the peptide chain, sufficiently
far from any chiral residue for its chiral environment to be
affected only by the local helicity of the chain, but still incor-
porated into the hydrogen-bonded network forming the helix.
made by the route shown in Scheme 1. BocGlyAibOMe 7 was
thionated regioselectively at the amide carbonyl group using
Lawesson’s reagent31–33 to give the thioamide 8. The Boc
group of 8 was removed under acidic conditions31,33 to provide
a common precursor to 1–6, HCl·H-Gly-ψ[CSNH]AibOMe 9
(Scheme 1),34 which was coupled with a range of Aib-contain-
ing oligomers by opening of their C-terminal azlactones. Thus
11, 12 and N3Aib3OH gave, respectively, 1, 2 and 13. Cbz-pro-
tected endothiopeptides 3–6 were obtained by Staudinger
reduction of azide 13 followed by N-terminal capping with a
range of protected L-amino acids (L-Xxx).
Design and synthesis of the
endothiopeptides
NMR and CD analysis
Aib-rich, helical28–30 endothiopeptides 1–6 bearing either an 1H NMR spectra of the endothiopeptides 1–6 in CD3OD35 at
N-terminal Cbz group or an N-terminal acetamide (Fig. 1) were 23 °C showed characteristic AB signals for the methylene
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Table 1 Measured anisochronicities (Δδ) in the CHAHBC(vS)NH
methylene groups of oligomers 1–6
a See Fig. 2.
Fig. 3 CD spectra of the endothiopeptides 1–6 in methanol. Inset: the
region of the spectrum arising from the π–π* transition of the thioamide
chromophore.
band is assigned to the π–π* transition of the thioamide
chromophore;22,37–45 it is interesting that the n–π* band
(usually lying at around 340 nm32) is hardly visible. The posi-
tions of these two bands match those observed in the UV
spectra in the same solvent (see ESI†). Analysis of the far-UV
portion of the CD spectrum, which usually reveals valuable
information on helical conformations, is complicated by the
contribution of the thioamide chromophore transitions at
lower wavelengths.12,46
The ability to identify the contribution of bonding at a
single residue to an overall circular dichroism spectrum is a
valuable result of the thionation of the glycine peptide bond.
Given the spatial separation of this thioamide chromophore
from the nearest stereogenic centre, the sign of the thioamide
Fig. 2 Comparing the performance of the glycine thioamide as a 1H
NMR probe with other NMR reporters of helical screw-sense; a for 13C
Aib probe (blue), Δδ/10; b measured anisochronicity of the signals from
highlighted nuclei in the 1H, 13C and 19F NMR spectrum.
protons, centred around 4.25 ppm. Table
1 shows the π–π* absorption band centred at 268 nm (Fig. 3, inset) must
measured anisochronicity Δδ36 for each Aib-containing oligo- result solely from the screw-sense preference experienced in
mer 1–6 XxxAib4Gly-ψ[CSNH]AibOMe carrying an N-terminal the vicinity of the thionoglycine probe. Despite the common
controller of screw-sense. Plotting these values against the an- L-configuration of all the endothiopeptides at their N-terminal
isochronicities previously measured in non-thionated peptides stereocontrolling residue, 2 and 6, which are capped with a Cα-
XxxAib4GlyNH2,18 XxxAib4Aib**Aib4Ot-Bu,17 and XxxAib4- tetrasubstituted amino acid (Ac- or Cbz-(αMe)Val), show a posi-
FibOMe19 bearing identical N-terminal protected chiral tive Cotton effect, while 1 and 3–5, which are capped with Cα-
residues Xxx (Fig. 2) shows that the response of the thiono- tertiary, proteinogenic amino acids (Ac- and Cbz-protected Val,
glycine probe is directly proportional to that of other NMR repor- Phe and Ala), show a negative Cotton effect. This observation
ters of helical screw sense, validating its applicability in this is in agreement with the reported behaviour of peptides of the
context. This proportionality also strongly suggests that the sequence L-XxxAibn-, which adopt a left-handed (M) helical
thioamide-containing foldamers adopt, at least in their Aib4 screw sense if Xxx is a tertiary L-amino acid and right-handed
portion, the same 310-helical conformation as that adopted by (P) helical screw sense if Xxx is a quaternary L-amino acid.18,47
related all-amide oligomers.28–30
Previous studies of this behaviour by circular dichroism,
CD spectra of the thionated peptides 1–6 were recorded in initially identified by using an enantiomerically enriched 13C
MeOH solution, in order to keep the experimental conditions isotopic label, had been frustrated by the lack of separation
consistent with those used in the NMR experiments. The thio- between the characteristic chromophores of the helical chain
nated peptides display a distinct Cotton effect at about and those of the chiral N-terminal residue.
268 nm, clearly separated from the rest of the peptide bond
Circular dichroism is a powerful technique for identifying
absorptions, which fall below 240 nm (Fig. 3). This strong the sign of screw-sense preference (M or P), but NMR has been
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Fig. 5 X-ray crystal structures of endothiopeptides
3 (top) and 5
(bottom) in their M (left) and P (right) conformations as observed in the
unit cell.‡ The M conformations display an N-terminal Type II β-turn
while the P conformations display a Type III β-turn.47 Cbz phenyl rings
have been removed for clarity.
Fig. 4 Relationship between the measured absolute molar ellipticities
at 268 nm and the h.e. values for peptides 1–6.
Conclusions
of more use for quantifying the magnitude of the preference
because (at least for 13C NMR) maximum peak separation
(corresponding to 100% helical excess) may be evaluated
from low temperature NMR spectra, in which screw-sense
conformers are in slow exchange.48 However, the reported
values of screw-sense preference induced by each controller
determined by this method can now be correlated with the
absolute molar ellipticities ([θ]M) measured at 268 nm (the
CSNH π–π* absorption) (Fig. 4). In confirmation that the sign
and value of [θ]2M68 is a reporter of both sign and magnitude of
screw-sense preference, these two values are proportional, with
[θ]2M68 = 10.6 × Δδ (in ppb).17
A thionoglycine residue inserted into a peptide provides a
probe of the magnitude and sign of local helical screw sense,
providing a qualitative and quantitative reporter of confor-
mational preference. The versatile probe is readily introduced
by selective thionation, and is responsive towards both mag-
netic and optical spectroscopies, with the helical excess values
derived by NMR correlating closely those obtained by CD. The
absolute measurement of a single thioamide chromophore
contribution to the CD spectrum is made possible by virtue of
its well-separated absorption wavelength, which may also
permit its use in solvents that absorb in the far-UV region of
the CD spectrum. Thioamides may prove useful in the future
as tools for the conformational analysis of foldamers and
peptide mimics embedded into membranes.
As seen in earlier work on related compounds,18 bulkier
and Cβ-branched amino acids induce a greater level of screw-
sense control, while the role played by protecting groups
(amide vs. carbamate) depends on whether the stereocontrol-
ling residue is Cα-tri- or tetrasubstituted, probably because of the
different type of β-turn47 adopted at the N-terminus of the helix.
Experimental
BocGlyAibOMe49
7
Isobutyl chloroformate (0.74 mL, 5.7 mmol) was added
dropwise to a cooled (−15 °C) solution of BocGlyOH (1 g,
5.7 mmol) and N-methylmorpholine (0.66 mL, 6 mmol) in dry
THF (15 mL). The suspension was stirred at −15 °C for 15 min,
and a mixture of H-AibOMe·HCl50 (1.75 g, 11.4 mmol) and
N-methylmorpholine (1.3 mL, 11.4 mmol) in dry THF (20 mL)
was added. The mixture was warmed to room temperature and
left stirring overnight. After solvent evaporation, the residue
was taken up in EtOAc (100 mL) and washed with KHSO4 5%
(3 × 10 mL), sat. NaHCO3 (3 × 10 mL) and brine (10 mL). The
organic phase was dried (MgSO4) and the solvent removed
under reduced pressure. The crude product was purified by
column chromatography (CHCl3–MeOH, 97 : 3) to give pure
BocGlyAibOMe 7 (1.44 g, 92%) as a white solid. Rf 0.35
(CHCl3–MeOH, 97 : 3); mp = 94–96 °C; IR (ATR, cm−1): 3340,
3275, 1716, 1667, 1534, 1303, 1279, 1244, 1157; 1H NMR
(300 MHz, CDCl3) δH 6.72 (s, 1H), 5.21 (s, 1H), 3.74 (d, J =
6.0 Hz, 1H), 3.73 (s, 3H), 1.54 (s, 6H), 1.45 (s, 9H); 13C NMR
(75 MHz, CDCl3) δC 175.0, 168.9, 156.3, 80.5, 56.8, 52.9, 44.9,
X-ray crystal structure analysis
Endothiopeptides 3 and 5, having respectively Cbz-Val and
Cbz-Ala at their N-terminus, formed crystals suitable for X-ray
analysis (Fig. 5). Interestingly, the unit cell contains equal
numbers of peptides in M and P screw-sense conformations,
indicating that both helical conformers are of similar energy
in the solid state as well as in solution. Furthermore, each
helix is initiated by either one of the two types of β-turn (a dis-
torted Type II or Type III) proposed47 to account for the induc-
tion of screw-sense preference from the helix N-terminus. It is
likely that the asymmetric environment experienced by the
thioamide chromophore in solution results from the popu-
lation of an unequal mixture of these two diastereoisomeric
conformers.
‡X-ray data for 3 and 5 have been deposited with the Cambridge crystallographic
data centre. Deposition numbers 3: 969434; 5: 969435.
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44.8, 44.8, 28.5, 24.9; MS (ES+, MeOH): 275 ([M + H]+, 55%), 24.8, 24.7, 24.4, 24.4, 24.2, 24.1, 22.5, 19.7; MS (ES+, MeOH):
297 ([M + Na]+, 100%); HRMS (ES+, MeOH): Calcd for 500 ([M + H]+, 40%), 522 ([M + Na]+, 100%); HRMS (ES+,
C12H22N2O5Na [M + Na]+ 297.1426, found 297.1437.
MeOH): Calcd for C23H41N5O7Na [M + Na]+ 522.2904, found
522.2906.
BocGly-ψ[CSNH]AibOMe 8
A stirred solution of BocGlyAibOMe 7 (1.410 g, 5.1 mmol)
and Lawesson’s reagent (2,4-bis(4-methoxyphenyl)-1,3,2,4-
dithiadiphosphetane-2,4-dithione, 1.041 g, 2.55 mmol) in dry
THF (15 mL) was heated to 60 °C for 4 hours. The mixture was
cooled to room temperature and the volatiles evaporated
under reduced pressure. After purification by column chromato-
graphy (EtOAc–PE 2 : 3) endothiopeptide BocGly-ψ[CSNH]-
AibOMe 8 (0.87 g, 57%) was obtained as an off-white waxy
solid. Rf 0.45 (EtOAc–PE, 2 : 3); mp = 96–98 °C; IR (ATR, cm−1):
AcValAib4Gly-ψ[CSNH]AibOMe 1
To a mixture of AcValAib4OH 11 (60 mg, 0.12 mmol) and HOAt
(1-hydroxy-7-azabenzotriazole, 18 mg, 0.13 mmol) in a mixture
of dry CH2Cl2 (3 mL) and dry DMF (0.4 mL), was added
EDC·HCl (25 mg, 0.11 mmol). After stirring for 20 min, a solu-
tion of HCl·H-Gly-ψ[CSNH]AibOMe 9 (28 mg, 0.12 mmol) and
N,N-diisopropylethylamine (0.061 mL, 0.35 mmol) in dry
CH2Cl2 (2 mL) was added. The mixture was left stirring at
room temperature for 6 days, after which the solvent was evap-
orated. The residue was taken up in EtOAc (10 mL) and
washed with KHSO4 5% (3 × 2 mL), sat. NaHCO3 (3 × 2 mL)
and brine (2 mL). The organic phase was dried (MgSO4) and
the solvent removed in vacuo. The crude product was purified
by column chromatography (CHCl3–MeOH, 95 : 5) to give pure
AcValAib4Gly-ψ[CSNH]AibOMe (45 mg, 52%) as a white solid.
Rf 0.35 (CH2Cl2–MeOH, 95 : 5); mp = 116–117 °C; [α]2D5 = −29.6;
IR (ATR, cm−1): 3273, 1650, 1530, 1416, 1362, 1226, 1156;
1H NMR (500 MHz, MeOD) δH 7.82 (s, 1H), 4.31 (d, A of AB,
J = 17.7 Hz, 1H), 4.11 (d, B of AB, J = 17.7 Hz, 1H), 3.87 (d,
J = 8.1 Hz, 1H), 3.63 (s, 3H), 2.06–1.98 (m, 1H), 2.00 (s, 3H),
1.68 (s, 3H), 1.67 (s, 3H), 1.53–1.34 (m, 24H), 1.04 (d, J = 6.7
Hz, 3H), 1.01 (d, J = 6.8 Hz, 3H); 13C NMR (101 MHz, MeOD)
δC 199.9, 177.9, 177.9, 177.9, 176.8, 175.5, 174.6, 174.1, 61.9,
61.4, 58.3, 58.1, 57.9, 57.9, 52.9, 52.3, 31.0, 30.9, 26.7, 26.6,
26.5, 26.4, 25.0–24.8 (overlapping signals), 24.3, 24.2, 22.5,
19.7, 19.6; MS (ES+, MeOH): 694 ([M + Na]+, 100%); HRMS
(ES+, MeOH) m/z calcd for C30H53N7O8NaS [M + Na]+ 694.3569,
found 694.3555.
1
3324, 1741, 1667, 1520, 1427, 1409, 1286, 1256, 1153; H NMR
(300 MHz, CDCl3) δH 8.63 (s, 1H), 5.39 (s, 1H), 4.08 (d, J =
6.4 Hz, 2H), 3.71 (s, 3H), 1.67 (s, 6H), 1.45 (s, 9H); 13C NMR
(101 MHz, CDCl3) δC 199.0, 173.4, 156.8, 80.9, 60.0, 53.2, 52.7,
28.2, 23.9; MS (ES+, MeOH): (292 [M + H]+, 100%); HRMS (ES+,
MeOH): Calcd for C12H23N2O4S [M + H]+ 291.1373, found
291.1372.
HCl·H-Gly-ψ[CSNH]AibOMe 9
The method of Jensen et al. was used.33 Anisole (1.1 mL,
1 mmol) and 2-mercaptoethanol (0.4 mL, 0.6 mmol) were
added to a stirred solution of BocGly-ψ[CSNH]AibOMe 8
(565 mg, 1.95 mmol) in dry dioxane (4 mL), followed by HCl
(2 M solution in diethyl ether, 10 mL, 20 mmol) at room temp-
erature. The reaction was monitored by TLC. A precipitate was
observed after 45 min. Stirring was continued for 2 h and the
product was collected by filtration to give HCl·H-Gly-ψ[CSNH]-
AibOMe as a white solid (241 mg, 65%). Rf 0.05 (EtOAc–PE,
2 : 3); mp = 206–207 °C; IR (ATR, cm−1): 3223, 3045, 2928,
1
2864, 1741, 1542, 1438, 1428, 1352, 1276, 1181, 1153; H NMR
(500 MHz, MeOD) δH 3.83 (s, 2H), 3.66 (s, 3H), 1.63 (s, 6H);
13C NMR (101 MHz, MeOD) δC 194.9, 174.6, 61.3, 52.9, 46.9, 24.4;
Ac(αMe)ValAib4Gly-ψ[CSNH]AibOMe 2
MS (ES+, MeOH): 191 ([M + H]+, 100%); HRMS (ES+, MeOH): To a mixture of Ac(αMe)ValAib4OH47 (74 mg, 0.14 mmol) and
Calcd for C7H15N2O2S [M + Na]+ 191.0849, found 191.0848.
HOAt (21 mg, 0.15 mmol) in a mixture of dry CH2Cl2 (3 mL)
and dry DMF (0.4 mL), EDC·HCl (N-(3-dimethylaminopropyl)-
N′-ethylcarbodiimide hydrochloride, 30 mg, 0.16 mmol)
AcValAib4OH 11
To a solution of AcValAib4OtBu47 (100 mg, 0.18 mmol) in dry was added. After stirring for 20 min,
a solution of
CH2Cl2 (2 mL) TFA was added (2 mL) dropwise. The progress HCl·H-Gly-ψ[CSNH]AibOMe 9 (33 mg, 0.14 mmol) and N,N-di-
of the reaction was followed by TLC. After reaction completion isopropylethylamine (0.080 mL, 0.46 mmol) in dry CH2Cl2
(4 h) the mixture was concentrated in vacuo. Et2O was added to (2 mL) was added. The mixture was left stirring at room temp-
the residue (5 mL) and the solvent evaporated. This process erature for 7 days, after which the solvent was evaporated. The
was repeated 5 times. No further purification was required. residue was taken up in EtOAc (10 mL) and washed with
AcValAib4OH was obtained as a white solid. Yield: 86 mg, KHSO4 5% (3 × 2 mL), sat. NaHCO3 (3 × 2 mL) and brine
96%. Rf 0.10 (CH2Cl2–MeOH, 95 : 5); mp = 242–245 °C (dec.). (2 mL). The organic phase was dried (MgSO4) and the solvent
[α]2D5 = +19.2; IR (ATR, cm−1): 3296, 1648, 1528, 1386, 1364, removed in vacuo. The crude product was purified by column
1290, 1169; 1H NMR (300 MHz, MeOD) δH 8.57 (s, 1H), 8.27 chromatography (CHCl3–MeOH, 97 : 3 → 95 : 5) to give pure
(d, J = 5.7 Hz, 1H), 7.70 (s, 1H), 7.65 (s, 1H), 7.51 (s, 1H), Ac(αMe)ValAib4Gly-ψ[CSNH]AibOMe (54 mg, 55%) as a white
3.90–3.81 (m, 1H), 2.08–1.98 (m, 1H), 2.00 (s, 3H), 1.53–1.39 solid. Rf 0.7 (CH2Cl2–MeOH, 9 : 1); mp = 191–193 °C; [α]D25
=
(m, 24H), 1.04 (d, J = 6.7 Hz, 3H), 1.00 (d, J = 6.8 Hz, 3H); +31.2; IR (ATR, cm−1): 3283, 1654, 1530, 1418, 1286; H NMR
13C NMR (126 MHz, MeOD) δC 178.4, 177.0, 176.8, 176.8, (400 MHz, MeOD) δH 7.76 (s, 1H), 4.22 (d, A of AB, J = 17.7 Hz,
174.6, 174.5, 174.1, 62.0, 58.1, 58.0, 58.0, 57.9, 57.3, 31.0, 1H), 4.02 (d, B of AB, J = 17.7 Hz, 1H), 3.53 (s, 3H), 1.98–1.89
27.0–26.8 (overlapping signals), 26.5, 26.0, 25.9, 25.0, 24.9, (m, 1H), 1.93 (s, 3H), 1.58 (s, 3H), 1.57 (s, 3H), 1.45–1.26 (m,
1
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24H), 0.92 (d, J = 6.8 Hz, 3H), 0.88 (d, J = 6.8 Hz, 3H); 13C NMR 18.7, 17.7; MS (ES+, MeOH): 553 ([M + Na]+, 100%); HRMS
(126 MHz, MeOD) δC 199.8, 178.2, 177.8, 177.7, 177.1, 175.3, (ES+, MeOH) m/z calcd for C23H42N6O6NaS [M + Na]+ 553.2784,
175.1, 173.7, 63.9, 61.3, 58.2, 58.0, 57.9, 57.8, 52.7, 52.2, 36.2, found 553.2775.
26.5, 26.4, 26.4, 24.9, 24.7, 24.6–24.2 (overlapping signals),
23.3, 18.6, 17.9, 17.7; MS (ES+, MeOH): 708 ([M + Na]+, 100%);
CbzValAib4Gly-ψ[CSNH]AibOMe 3
HRMS (ES+, MeOH) m/z calcd for C31H55N7O8NaS [M + Na]+
To a mixture of CbzValOH (31 mg, 0.12 mmol), HOAt (17 mg,
708.3725, found 708.3712.
0.12 mmol) and H-Aib4Gly-ψ[CSNH]AibOMe 14 (33 mg,
0.06 mmol) in dry CH2Cl2 (3 mL) was added EDC (0.022 mL,
0.12 mmol). The mixture was left stirring at room temperature
for 3 days, and then the solvent was evaporated. The residue
was taken up in EtOAc (5 mL) and washed with KHSO4 5%
(3 × 1 mL), sat. NaHCO3 (3 × 1 mL) and brine (1 mL). The
organic phase was dried (MgSO4) and the solvent removed
in vacuo. The crude product was purified by column chromato-
graphy (EtOAc–PE, 4 : 1) to give pure CbzValAib4Gly-ψ[CSNH]-
AibOMe (35 mg, 74%) as a white solid. Rf 0.65 (EtOAc–PE,
4 : 1); mp = 198–200 °C; [α]2D5 = −22.4; IR (ATR, cm−1): 3279,
2983, 1651, 1529, 1284, 1226, 1157; 1H NMR (500 MHz, MeOD)
δH 7.47–7.23 (m, 5H), 5.11 (s, 2H), 4.28 (d, J = 17.7 Hz, 1H),
4.14 (d, J = 17.6 Hz, 1H), 3.75 (d, J = 7.4 Hz, 1H), 3.63 (s, 3H),
2.08–1.97 (m, 1H), 1.67 (s, 6H), 1.52–1.32 (m, 24H), 1.02 (d, J =
7.0 Hz, 3H), 1.00 (d, J = 6.9 Hz, 3H; 13C NMR (126 MHz,
MeOD) δC 199.8, 177.7, 177.7, 177.7, 176.6, 175.3, 174.6, 159.0,
138.2, 129.6, 129.1, 128.7, 67.7, 62.8, 61.3, 58.1, 58.0, 57.7,
57.7, 52.7, 52.2, 31.2, 26.2, 26.2, 26.0, 25.9, 25.1–24.7 (over-
lapping signals), 24.5, 24.3, 19.6, 19.4, 19.3 ppm; MS (ES+,
MeOH): 786 ([M + Na]+, 100%); HRMS (ES+, MeOH) m/z calcd
for C36H57N7O9NaS [M + Na]+ 786.3831, found 786.3817.
N3Aib4Gly-ψ[CSNH]AibOMe 13
(1) Azlactone formation. N3Aib4OH (500 mg, 1.3 mmol) was
dissolved in dry CH2Cl2 (5 mL) and EDC (0.252 mL,
1.43 mmol) was added. The resulting solution was left stirring
for 3 h at room temperature. The solvent was evaporated in
vacuo and the residue was taken up in EtOAc (20 mL) and
washed with KHSO4 5% (3 × 5 mL). The organic phase was
dried (MgSO4) and concentrated to give the crude azlactone
product, which was used directly in the next step.
(2) Azlactone coupling. The crude azlactone was dissolved in
dry CH2Cl2 (5 mL) and HCl·H-Gly-ψ[CSNH]AibOMe 9 (230 mg,
1.01 mmol) was added, together with N,N-diisopropylethyl-
amine (0.232 mL, 1.33 mmol). The mixture was left stirring at
room temperature for 7 days, after which the solvent was
evaporated under reduced pressure. The residue was taken up
in EtOAc (10 mL) and washed with KHSO4 5% (3 × 2 mL), sat.
NaHCO3 (3 × 2 mL) and brine (2 mL). The organic phase was
dried (MgSO4) and the solvent removed in vacuo. The crude
product was purified by column chromatography (CH2Cl2–
MeOH, 97 : 3 → 95 : 5) to give pure N3Aib4Gly-ψ[CSNH]AibOMe
(250 mg, 44%) as a colourless solid. Rf 0.75 (CH2Cl2–MeOH,
95 : 5); mp = 190–192 °C (dec.); IR (ATR, cm−1): 3277, 2112,
1745, 1657, 1530, 1382, 1275, 1152; 1H NMR (500 MHz, CDCl3)
CbzPheAib4Gly-ψ[CSNH]AibOMe 4
δH 8.68 (s, 1H), 7.76 (t, J = 6.2 Hz, 1H), 7.36 (s, 1H), 6.93 To a mixture of CbzPheOH (37 mg, 0.12 mmol), HOAt (17 mg,
(s, 1H), 6.28 (s, 1H), 4.31 (d, J = 6.2 Hz, 2H), 3.64 (s, 3H), 1.67 0.12 mmol) and H-Aib4Gly-ψ[CSNH]AibOMe 14 (33 mg,
(s, 6H), 1.49 (s, 12H), 1.44 (s, 6H), 1.40 (s, 6H); 13C NMR 0.06 mmol) in dry CH2Cl2 (3 mL) was added EDC (0.022 mL,
(126 MHz, CDCl3) δC 198.6, 175.0, 174.2, 173.9, 173.7, 173.3, 0.12 mmol), followed by N,N-diisopropylethylamine (0.05 mL,
64.0, 60.3, 57.3, 57.0, 56.9, 52.4, 51.7, 25.5–25.4 (overlapping 0.03 mmol). The mixture was left stirring at room temperature
signals), 24.9, 24.4; MS (ES+, MeOH): 579 ([M + Na]+, 100%); for 3 days, and then the solvent was evaporated. The residue
HRMS (ES+, MeOH) m/z calcd for C23H40N8O6NaS [M + Na]+ was taken up in EtOAc (5 mL) and washed with KHSO4 5%
579.2689, found 579.2697.
(3 × 1 mL), sat. NaHCO3 (3 × 1 mL) and brine (1 mL). The
organic phase was dried (MgSO4) and the solvent removed
in vacuo. The crude product was purified by column chromato-
H-Aib4Gly-ψ[CSNH]AibOMe 14
N3Aib4Gly-ψ[CSNH]Aib-OMe 13 (112 mg, 0.2 mmol) was dis- graphy (CH2Cl2–MeOH, 95 : 5) to give pure CbzPheAib4-
solved in distilled THF (2 mL). Water (0.018 mL, 1 mmol) was Gly-ψ[CSNH]AibOMe (39 mg, 78%) as a white solid. Rf 0.5
added, followed by PMe3 (1 M solution in THF, 2 mL, (CH2Cl2–MeOH, 95 : 5); mp = 110–113 °C; [α]2D5 = −19.2; IR
2 mmol). The reaction mixture was stirred overnight at room (ATR, cm−1): 3291, 2984, 1657, 1529, 1383, 1226, 1155; 1H
temperature. The volatiles were removed under vacuum and NMR (500 MHz, MeOD) δH 7.90 (s, 1H), 7.81 (s, 2H), 7.39–7.16
the residue purified by column chromatography (CH2Cl2– (m, 1H), 5.10 (d, A of AB, J = 12.6 Hz, 1H), 5.06 (d, B of AB,
MeOH, 9 : 1 → 8 : 2) to give H-Aib4Gly-ψ[CSNH]AibOMe (85 mg, 12.6 Hz, 1H), 4.30 (d, A of AB, J = 17.7 Hz, 1H), 4.23 (t, J = 7.8
81%). Rf 0.10 (CH2Cl2–MeOH, 95 : 5); mp = 212–213 °C; Hz, 1H), 4.11 (d, B of AB, J = 17.7 Hz, 1H), 3.63 (s, 3H), 3.00
IR (ATR, cm−1): 3278, 1730, 1651, 1530, 1383, 1362, 1282, 1221, (ddd, J = 31.6, 13.6, 7.8 Hz, 2H), 1.67 (d, J = 2.5 Hz, 6H), 1.50
1
1155; H NMR (500 MHz, CDCl3) δH 8.73 (s, 1H), 8.22 (s, 1H), (s, 3H), 1.49 (s, 6H), 1.42 (s, 3H), 1.38 (s, 3H), 1.34 (s, 3H), 1.28
7.81 (t, J = 6.2 Hz, 1H), 7.69 (s, 1H), 6.36 (s, 1H), 4.31 (d, J = (s, 3H), 1.24 (s, 3H). 13C NMR (101 MHz, MeOD) δC 199.9,
6.2 Hz, 2H), 3.64 (s, 3H), 1.75 (brs, 2H), 1.68 (s, 6H), 1.50 177.9, 177.9, 177.8, 176.7, 175.5, 174.5, 158.7, 138.3, 138.2,
(s, 6H), 1.42 (s, 6H), 1.39 (s, 6H), 1.32 (s, 6H); 13C NMR 130.7, 129.7, 129.7, 129.2, 128.7, 128.1, 67.8, 61.4, 58.4, 58.3,
(75 MHz, CDCl3) δC 198.7, 175.4, 174.8, 174.4, 174.1, 174.1, 58.1, 57.9, 57.7, 52.8, 52.4, 38.4, 30.9, 26.6–26.3 (overlapping
60.4, 57.3, 56.7, 56.6, 55.5, 52.5, 51.8, 28.0, 25.5, 25.0, 24.4, signals), 25.1, 24.9, 24.8, 24.3, 24.1; MS (ES+, MeOH): 834
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Organic & Biomolecular Chemistry
([M
+
Na]+, 100%); HRMS (ES+, MeOH) m/z calcd for J = 12.7 Hz, 1H), 5.07 (d, B of AB, J = 12.7 Hz, 1H), 4.34 (d, A of
C40H57N7O9NaS [M + Na]+ 834.3831, found 834.3818.
AB, J = 17.7 Hz, 1H), 4.09 (d, B of AB, J = 17.7 Hz, 1H), 3.63
(s, 3H), 2.02 (m, 1H), 1.68 (s, 3H), 1.67 (s, 3H), 1.53–1.33
(m, 24H), 1.27 (s, 3H), 1.00 (d, J = 6.8 Hz, 3H), 0.94 (d, J =
CbzAlaAib4Gly-ψ[CSNH]AibOMe 5
To a mixture of Cbz-Ala-OH (28 mg, 0.12 mmol), HOAt (17 mg, 6.8 Hz, 3H); 13C NMR (101 MHz, MeOD) δC 199.8, 178.2, 177.8,
0.12 mmol) and H-Aib4Gly-ψ[CSNH]AibOMe 14 (33 mg, 177.7, 176.9, 175.9, 175.4, 158.2, 138.7, 129.6, 129.1, 128.7,
0.06 mmol) in dry CH2Cl2 (3 mL) was added EDC (0.023 mL, 79.5, 73.4, 67.7, 63.9, 61.3, 58.2, 58.1, 58.0, 57.8, 57.7, 52.7,
0.12 mmol), followed by N,N-diisopropylethylamine (0.05 mL, 52.2, 36.0, 26.7, 26.6, 24.8, 24.7–24.4 (overlapping signals),
0.03 mmol). The mixture was left stirring at room temperature 23.9, 18.0, 17.7; MS (ES+, MeOH): 800 ([M + Na]+, 100%);
for 3 days, and then the solvent was evaporated. The residue HRMS (ES+, MeOH) m/z calcd for C37H63N8O9NaS [M + NH4]+
was taken up in EtOAc (5 mL) and washed with KHSO4 5% 795.4439, found 795.4433.
(3 × 1 mL), sat. NaHCO3 (3 × 1 mL) and brine (1 mL). The
organic phase was dried (MgSO4) and the solvent removed
in vacuo. The crude product was purified by column chromato-
graphy (CH2Cl2–MeOH, 97 : 3) to give pure CbzAlaAib4-
Acknowledgements
Gly-ψ[CSNH]AibOMe (35 mg, 77%) as a white solid. Rf 0.4
This work was funded by the European Research Council (ERC
(CH2Cl2–MeOH, 97 : 3); mp = 204–205 °C; [α]2D5 = −13.2 IR
Advanced Grant ROCOCO) and was presented at the 23rd Amer-
(ATR, cm−1) 3287, 2984, 1703, 1643, 1529, 1420, 1361, 1208,
ican Peptide Society Symposium in Hawaii, 20–27 June 2013.
1
1171; H NMR (500 MHz, MeOD) δH 7.90 (s, 1H), 7.81 (s, 1H),
Jonathan Clayden is a recipient of a Royal Society Wolfson
7.38–7.27 (m, 5H), 5.12 (d, A of AB, J = 12.7 Hz, 1H), 5.09 (d, B
Research Merit Award.
of AB, J = 12.7 Hz, 1H), 4.27 (d, A of AB, J = 17.7 Hz, 1H), 4.16
(d, B of AB, J = 17.7 Hz, 1H), 4.00 (q, J = 7.1 Hz, 1H), 3.63 (s,
3H), 1.67 (s, 6H), 1.50 (s, 6H), 1.47–1.36 (m, 18H), 1.34 (d, J =
7.1 Hz, 3H); 13C NMR (126 MHz, MeOD) δC 199.9, 177.9, 177.9,
177.8, 176.8, 176.1, 175.5, 158.8, 138.4, 129.7, 129.3, 128.8,
Notes and references
79.6, 67.8, 61.4, 58.2, 58.1, 57.9, 57.6, 52.8, 52.7, 52.4, 26.2,
25.9, 25.8, 25.4, 25.2–24.8 (overlapping signals), 24.7, 17.3; MS
(ES+, MeOH): 758 ([M + Na]+, 100%); HRMS (ES+, MeOH) m/z
calcd for C34H53N7O9NaS [M + Na]+ 758.3518, found 758.3505.
1 S. H. Gellman, Acc. Chem. Res., 1998, 31, 173.
2 G. Guichard and I. Huc, Chem. Commun., 2011, 5933.
3 S. Hecht and I. Huc, Foldamers, Wiley-VCH, Weinheim,
2007.
4 D. J. Hill, M. J. Mio, R. B. Prince, T. S. Hughes and
J. S. Moore, Chem. Rev., 2001, 101, 3893.
5 E. Yashima, K. Maeda, H. Iida, Y. Furusho and K. Nagai,
Chem. Rev., 2009, 109, 6102.
6 P. N. Shah, J. Min, H.-J. Kim, S.-Y. Park and J.-S. Lee, Macro-
molecules, 2011, 44, 7917.
7 P. N. Shah, J. Min, C.-G. Chae, N. Nishikawa, D. Suemasa,
T. Kakuchi, T. Satoh and J.-S. Lee, Macromolecules, 2012,
45, 8961.
Cbz(αMe)ValAib4Gly-ψ[CSNH]AibOMe 6
Acyl fluoride formation.51 A solution of Cbz-(αMe)Val-OH
(58 mg, 0.22 mmol) in dry CH2Cl2 (2 mL) was treated with pyri-
dine (0.018 mL, 0.22 mmol) and TFFH (tetramethyl-fluoroform-
amidinium hexafluorophosphate, 87 mg, 0.33 mmol). The
mixture was left stirring at room temperature for 4 h, after
which ice-cold water was added (2 mL). The organic phase was
separated and washed again with ice-cold water (2 mL), dried
(MgSO4) and concentrated to give crude Cbz-(αMe)Val-F as a
colorless solid.
8 N. Ousaka, Y. Inai and R. Kuroda, J. Am. Chem. Soc., 2008,
130, 12266.
Acyl fluoride coupling. The freshly prepared amino acyl
fluoride was dissolved in dry CH2Cl2 (2 mL) and the resulting
9 N. Delsuc, T. Kawanami, J. Lefeuvre, A. Shundo, H. Ihara,
M. Takafuji and I. Huc, ChemPhysChem, 2008, 9, 1882.
solution was added to H-Aib4Gly-ψ[CSNH]AibOMe 14 (39 mg, 10 R. P. Hummel, C. Toniolo and G. Jung, Angew. Chem., Int.
0.073 mmol) in dry CH2Cl2 (3 mL). N,N-Diisopropylethylamine Ed., 1987, 99, 1180.
(0.134 mL, 0.77 mmol) was added and the mixture was left stir- 11 L. Byrne, J. Solà, T. Boddaert, T. Marcelli, R. W. Adams,
ring at room temperature for 5 days, after which the solvent
was evaporated. The residue was taken up in EtOAc (5 mL) and
G. A. Morris and J. Clayden, Angew. Chem., Int. Ed., 2013,
DOI: 10.1002/anie.201308264.
washed with KHSO4 5% (3 × 1 mL), sat. NaHCO3 (3 × 1 mL) 12 N. Sreerama and R. W. Woody, Circular Dichroism of
and brine (1 mL). The organic phase was dried (MgSO4) and
the solvent removed in vacuo. The crude product was purified
by column chromatography (CH2Cl2–MeOH, 95 : 5) to give pure
Peptides and Proteins in “Circular Dichroism: Principles and
Applications”, ed. N. Berova, K. Nakanishi and R. W. Woody,
Wiley, New York, NY, 2000.
Cbz(αMe)ValAib4Gly-ψ[CSNH]Aib-OMe (51 mg, 70%) as
a
13 M. Suginome, S. Collet and Y. Ito, Org. Lett., 2002, 4, 351.
white solid. Rf 0.25 (CH2Cl2–MeOH, 95 : 5); mp = 203–205 °C; 14 Y. Ito, T. Miyake, S. Hatano, R. Shima, T. Ohara and
[α]2D5 = +34.7; IR (ATR, cm−1): 3281, 2983, 1707, 1651, 1530,
M. Suginome, J. Am. Chem. Soc., 1998, 120, 11880.
1454, 1382, 1361, 1287, 1163; 1H NMR (400 MHz, MeOD) δH 15 Such a difficulty has led to misassignment of screw sense:
7.91 (s, 1H), 7.83 (s, 1H), 7.43–7.27 (m, 5H), 5.20 (d, A of AB,
see ref. 18.
842 | Org. Biomol. Chem., 2014, 12, 836–843
This journal is © The Royal Society of Chemistry 2014

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Organic & Biomolecular Chemistry
Paper
16 J. Solà, M. Helliwell and J. Clayden, J. Am. Chem. Soc., 2010,
132, 4548.
17 J. Solà, G. A. Morris and J. Clayden, J. Am. Chem. Soc., 2011,
133, 3712.
over a four-fold range of concentration. Spectra obtained in
less polar solvents were broad and concentration-
dependent, suggesting aggregation. See: J. Clayden,
A. Castellanos, J. Solà and G. A. Morris, Angew. Chem.,
Int. Ed., 2009, 48, 5962; S. J. Pike, V. Diemer, J. Raftery,
18 R. A. Brown, T. Marcelli, M. De Poli, J. Solà and J. Clayden,
Angew. Chem., Int. Ed., 2012, 51, 1395.
S. J. Webb and J. Clayden, manuscript in preparation.
2 1/2
19 S. J. Pike, M. De Poli, W. Zawodny, J. Raftery, S. J. Webb 36 Δδ for an AB system is given by ν0Δδ = [( f1 − f3)2 − JAB
]
=
2 1/2
and J. Clayden, Org. Biomol. Chem., 2013, 11, 3168.
20 J. Solà, S. P. Fletcher, A. Castellanos and J. Clayden, Angew.
Chem., Int. Ed., 2010, 49, 6836.
[( f2 − f4)2 − JAB
]
= [( f1 − f4) ( f2 − f3)]1/2 where f1,2,3,4 are
the observed resonance frequencies in order of the four
lines comprising the AB multiplet, JAB is the coupling
constant and ν0 is the spectrometer frequency.
21 J. Helbing, H. Bregy, J. Bredenbeck, R. Pfister, P. Hamm,
R. Huber, J. Wachtveitl, L. De Vico and M. Olivucci, J. Am. 37 J. H. Miwa, L. Pallivathucal, S. Gowda and K. E. Lee, Org.
Chem. Soc., 2004, 126, 8823. Lett., 2002, 4, 4655.
22 D. Wildemann, C. Schiene-Fischer, T. Aumüller, 38 J. Zhao, J.-C. Micheau, C. Vargas and C. Schiene-Fischer,
A. Bachmann, T. Kiefhaber, C. Lücke and G. Fischer, J. Am.
Chem. Soc., 2007, 129, 4910.
23 E. P. Dudek and G. Dudek, J. Org. Chem., 1967, 32, 823.
Chem.–Eur. J., 2004, 10, 6093.
39 J. H. Miwa, A. K. Patel, N. Vivatrat, S. M. Popek and
A. M. Meyer, Org. Lett., 2001, 3, 3373.
24 S. Yao, R. Zutshi and J. Chmielewski, Bioorg. Med. Chem. 40 M. Kajtár, M. Hollósi, J. Kajtár, Z. Majer and K. E. Kövér,
Lett., 1998, 8, 699. Tetrahedron, 1986, 42, 3931.
25 J. Lehmann, A. Linden and H. Heimgartner, Tetrahedron, 41 M. Hollósi, E. Kollát, J. Kajtár, M. Kajtár and G. D. Fasman,
1998, 98, 8721. Biopolymers, 1990, 30, 1061.
26 J. M. Goldberg and R. L. Baldwin, Biochemistry, 1998, 37, 42 T. T. Sifferlen, M. Rueping, K. Gademann, B. Jaun and
2546. D. Seebach, Helv. Chim. Acta, 1999, 82, 2067.
27 Inai (N. Ousaka and Y. Inai, J. Org. Chem., 2009, 74, 1429) 43 A. Bachmann, D. Wildemann, F. Praetorius, G. Fischer and
has used dehydrophenylalanine derivatives containing T. Kiefhaber, Proc. Natl. Acad. Sci. U. S. A., 2011, 108, 3952.
different chromophores to distinguish different locations 44 D. F. Mierke, A. Geyer and H. Kessler, Int. J. Pept. Protein
in an extended helix.
Res., 1994, 44, 325.
28 C. Toniolo, CRC Crit. Rev. Biochem., 1980, 9, 1.
29 C. Toniolo and E. Benedetti, Trends Biochem. Sci., 1991, 16,
350.
45 Y. Otani, T. Hori, M. Kawahata, K. Yamagouchi and
T. Ohwada, Tetrahedron, 2012, 68, 4418.
46 M. Goodman and C. Toniolo, Biopolymers, 1968, 6, 1673.
30 C. Toniolo, M. Crisma, F. Formaggio and C. Peggion, Bio- 47 M. De Poli, M. De Zotti, J. Raftery, J. A. Aguilar,
polymers, 2001, 60, 396. G. A. Morris and J. Clayden, J. Org. Chem., 2013, 78, 2248.
31 K. Clausen, M. Thorsen and S.-O. Lawesson, Tetrahedron, 48 13C NMR may be used to determine absolute screw sense
1981, 37, 3635.
preferences by employing an enantiomerically enriched
probe incorporating an isotopic label. See: J. Solà,
S. P. Fletcher, A. Castellanos and J. Clayden, Angew. Chem.,
Int. Ed., 2010, 49, 6836; S. P. Fletcher, J. Solà, D. Holt,
R. A. Brown and J. Clayden, Beilstein J. Org. Chem., 2011, 7,
1304.
32 M. De Zotti, B. Biondi, C. Peggion, M. De Poli, H. Fathi,
S. Oancea, C. Toniolo and F. Formaggio, Beilstein J. Org.
Chem., 2012, 8, 1161.
33 O. E. Jensen, S.-O. Lawesson, R. Bardi, A. M. Piazzesi and
C. Toniolo, Tetrahedron, 1985, 41, 5595.
34 ψ[X] indicates replacement of a peptide CONH bond by X: 49 P. Jana, S. Maity, S. K. Maity and D. Haldar, Chem.
see: A. F. Spatola, in Chemistry and Biochemistry of Amino Commun., 2011, 47, 2092.
Acids, Peptides and Proteins, ed. B. Dekker, Weinstein, 50 C. Kolano, K. Gomann and W. Sander, Eur. J. Org. Chem.,
New York, 1983. 2004, 4167.
35 Studies of similar Aib-containing oligomers have shown 51 L. A. Carpino and A. El-Faham, J. Am. Chem. Soc., 1995,
that NMR spectra obtained in CD3OD remained identical
117, 5401.
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