A systematic study of the solid state and solution phase conformational preferences of β-peptides derived from transpentacin

Article abstract of DOI:10.1016/j.tetasy.2010.06.001

The solid state and solution phase conformational preferences of a homologous series of β-peptides derived from (S,S)-2- aminocyclopentanecarboxylic acid (transpentacin) have been investigated using a variety of spectroscopic and crystallographic techniqu

Full text of DOI:10.1016/j.tetasy.2010.06.001

Tetrahedron: Asymmetry 21 (2010) 1797–1815
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
A systematic study of the solid state and solution phase conformational
preferences of b-peptides derived from transpentacin
a
Elin Abraham a, Callum W. Bailey a, Timothy D. W. Claridge a, Stephen G. Davies a, , Kenneth B. Ling ,
*
Barbara Odell a, Thomas L. Rees a, Paul M. Roberts a, Angela J. Russell a, Andrew D. Smith a, Lorna J. Smith b,
Helen R. Storr a, Miles J. Sweet a, Amber L. Thompson a, James E. Thomson a, George E. Tranter a,c
David J. Watkin a
,
a Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford OX1 3TA, UK
b Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, UK
c Chiralabs Ltd, BCIE, Oxford University Begbroke Science Park, Sandy Lane, Yarnton, Oxfordshire OX5 1PF, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The solid state and solution phase conformational preferences of a homologous series of b-peptides
derived from (S,S)-2-aminocyclopentanecarboxylic acid (transpentacin) have been investigated using a
variety of spectroscopic and crystallographic techniques. These studies indicate that the hexamer and
pentamer persist as a 12-helix in both the solid state and solution phase. Although the conformational
traits of a 12-helix are exhibited by oligomers with as few as three residues in the solid state, in solution
the trimer exists as an equilibrium of many alternative conformers whilst the tetramer has been shown to
predominantly exist in either a 12-helix or a turn-type conformation.
Received 20 May 2010
Accepted 27 May 2010
Available online 13 July 2010
Dedicated to Professor Henri Kagan on the
occasion of his 80th birthday
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
These previous studies have not, however, systematically inves-
tigated the effect of increasing transpentacin oligomer length on
b-Amino acids are an attractive moiety for the formation of pep-
tidomimetics, whereby their replacement for specific -amino acids
the predilection to adopt a 12-helix. Upon inspection of the 12-he-
lix adopted by hexamer 5 we envisaged that this secondary struc-
ture motif could, in principle, be adopted by the corresponding
transpentacin pentamer 7, tetramer 8 and trimer 9 through the
formation of three, two and one inter-residue 12-membered
hydrogen bonds, respectively (Fig. 2).11 In order to facilitate subse-
quent analysis of alkyl-substituted derivatives, our initial investi-
a
in existing peptide compounds offers remarkable opportunities to
mimic the action of these peptides whilst offering improved bio-
availability and therapeutic lifetimes.1 b-Peptides (i.e., those
composed solely of b-amino acid residues) themselves display
biological activities such as inhibition of fat and cholesterol absorp-
tion2 and antibiotic properties,3 and as such have been the subject of
several investigations.4 For instance, Seebach and co-workers re-
ported their investigations into the conformational characterisation
of the b3-substituted hexapeptide 1 which adopts a 14-helix5 con-
formation in organic solutions1a,6 (Fig. 1). Gellman investigated
the introduction of covalent constraints to pre-organise the mole-
cule for specific secondary structure motifs without blocking the
H-bonding sites along the peptide backbone. Hexamer 4, assembled
from residues of the b-amino acid (R,R)-2-aminocyclohexanecar-
boxylic acid (transhexacin) 2, populates a 14-helix in both the solu-
tion and solid state,7 whilst hexamer 5, assembled from residues of
the b-amino acid (R,R)-2-aminocyclopentanecarboxylic acid (ACPC
or transpentacin) 3, adopts a 12-helical conformation7b,8,9 (Fig. 1).
A variety of analogues have since been reported and analysed for
the presence of secondary structural preferences.10
gations set out to determine when
a 12-helical motif is
substantially populated within the progressively increasing tran-
spentacin oligomeric series, or whether alternative conformations
are accessible, in both the solid state and solution phase. It was
envisaged that investigation of a range of protecting group strate-
gies for the transpentacin oligomers may facilitate crystallisation
and thus enable solid state investigations to be performed. The
results of these investigations are disseminated herein.
2. Results and discussion
2.1. Asymmetric synthesis of transpentacin-derived oligomers
Initial studies into either an N-Boc/O-Me- or an N-Cbz/O-tBu-pro-
tecting group strategy revealed the latter to be more efficacious, and
therefore multigram quantities of the requisite monomers 13 and 15
were prepared. b-Amino ester 12 was prepared on a >10 g scale
following the previously described procedure.12 Conjugate addition
* Corresponding author.
E-mail address: steve.davies@chem.ox.ac.uk (S.G. Davies).
0957-4166/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2010.06.001

1798
E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
of lithium (S)-N-benzyl-N-(a-methylbenzyl)amide to tert-butyl
cyclopent-1-enecarboxylate 10 was followed by quenching with
satd aq NH4Cl, and treatment of the initially formed cis-diastereoiso-
O
O
O
•TFA
t
mer 1113 (87% yield, 96:4 dr) with KOtBu in dry BuOH14 gave the
N
H
N
H
N
H
OH
2
trans-diastereoisomer 12 in >99:1 dr, and as a white solid in 81% iso-
lated yield after chromatographic purification.15 Subsequent pro-
tecting group manipulation gave NH/O-tBu monomer (‘monomer
amine’) 13 and N-Cbz/OH monomer (‘monomer acid’) 1516 in 92%
and 62% yields over one and three steps, respectively, from 12
(Scheme 1).
1
H-[β3Val-β3Ala-β3Leu]2-OH•TFA
14-helix
H2N
H
CO2
H2N
CO2H
Under optimised conditions, a fragment condensation approach
to peptide synthesis was employed, utilising an HOBt/EDCꢀHCl-
mediated coupling protocol in CHCl3. Coupling of monomer amine
13 and monomer acid 15 afforded dimer 16 in 77% yield and >99:1
dr. Orthogonal N- and O-deprotection of dimer 16 was achieved
using Pd(OH)2/C under H2 (1 atm) and TFA, respectively, giving
the corresponding dimer amine 17 and dimer acid 18 in 97% and
86% yields, respectively, and in >99:1 dr in each case, as crystalline
solids. Trimer 19 was synthesised from monomer acid 15 and di-
mer amine 17 in 72% isolated yield and >99:1 dr, and was subse-
quently N-deprotected using Pd(OH)2/C under H2 (1 atm) to
afford the corresponding trimer amine 21 in 89% yield. Coupling
of either dimer amine 17 or trimer amine 21 with dimer acid 18
([2+2] and [2+3] fragment condensations, respectively) occurred
with quantitative conversion in both cases giving tetramer 20
and pentamer 23 as single diastereoisomers >99:1 dr), in 73%
and 56% yields, respectively, after column chromatography. The
synthesis of hexamer 24 was achieved through hydrogenolysis of
tetramer 20 to give the corresponding tetramer amine 22 in 89%
yield, which was subsequently coupled with dimer acid 18 ([2+4]
fragment condensation) to afford a mixture of products (85%
conversion) that was purified to give hexamer 24 in 66% isolated
yield (Scheme 2).
(R,R)-2
(R,R)-3
transpentacin
transhexacin
O
O
Boc NH
NH
NH
CO2Bn
4
4
14-helix
O
O
NH
Boc NH
NH
CO2Bn
4
5
12- helix
Figure 1. Structure of b3-substituted hexamer 1, transhexacin 2, transpentacin 3
and their respective hexamers 4 and 5.
O
Hβ
O
Hβ
O
Hβ
O
Hβ
O
Hβ
O
Hβ
O
RO
N
H
A
N
H
B
N
H
C
N
H
D
N
H
E
N
H
OR'
F
Hα
Hα
Hα
Hα
Hα
Hα
6
4 x 12-membered N-H---O=C
Hexamer:
7 Pentamer: 3 x 12-membered N-H---O=C
8 Tetramer: 2 x 12-membered N-H---O=C
9
Trimer:
1 x 12-membered N-H---O=C
Figure 2. Potential 12-membered inter-residue hydrogen bonds within generic transpentacin hexamer 6, pentamer 7, tetramer 8 and trimer 9.
Ph
N
Ph
N
t
CO2Bu
(i)
(ii)
t
t
CO2 Bu
CO2Bu
Ph
Ph
10
11, 87%
96:4 dr
12, 81%
>99:1 dr
(iii)
CO2tBu
H2N
CO2tBu
Cbz NH
CO2H
Cbz NH
(v)
(iv)
15, 87%
14, 77%
>99:1 dr
13, 92%
>99:1 dr
>99:1 dr
Scheme 1. Reagents and conditions: (i) lithium (S)-N-benzyl-N-(
Pd(OH)2/C, H2 (5 atm), MeOH, rt, 16 h; (iv) CbzCl, Et3N, THF, rt, 16 h; (v) TFA, CH2Cl2, 0 °C to rt, 3 h.
a
-methylbenzyl)amide, THF, ꢁ78 °C, then NH4Cl (satd aq), ꢁ78 °C to rt; (ii) KOtBu, tBuOH, reflux, 8 h; (iii)

E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
1799
O
O
OtBu
(vi)
OtBu
Cbz
NH
H
NH
CO2tBu
Cbz NH
CO2H
H2N
5
3
Transpentacin pentamer 23
+
21, 89%, >99:1 dr
(ii)
13
15
(i)
O
O
OtBu
OtBu
Cbz
NH
H
NH
(ii)
(iv)
O
Cbz
NH
OtBu
3
2
Transpentacin trimer 19
17, 97%, >99:1 dr
2
O
O
Transpentacin dimer 16
(v)
(iii)
OtBu
OH
Cbz
NH
Cbz
NH
2
4
Transpentacin tetramer 20
18, 86%, >99:1 dr
(ii)
O
O
H
NH
OtBu
(vi)
OtBu
Cbz
NH
4
6
Transpentacin hexamer 24
22, 89%, >99:1 dr
Scheme 2. Reagents and conditions: (i) EDCꢀHCl, HOBt, Et3N, CHCl3, rt, 16 h; (ii) Pd(OH)2/C, H2 (1 atm), MeOH, rt 16 h; (iii) TFA, CH2Cl2, 0 °C to rt, 12 h; (iv) monomer acid 15,
EDCꢀHCl, HOBt, Et3N, CHCl3, rt, 16 h; (v) dimer amine 17, EDCꢀHCl, HOBt, Et3N, CHCl3, rt, 16 h; (vi) dimer acid 18, EDCꢀHCl, HOBt, Et3N, CHCl3, rt, 16 h.
[H-(ACPC)n-OtBu] and O-deprotected oligomers [Cbz-(ACPC)n-
O
O
OH]). Single crystals of N-Boc/O-tBu hexamer 29, N-Boc/O-tBu
pentamer 28 and N-Boc/O-tBu tetramer 27 were produced and sub-
jected to single crystal X-ray analyses which revealed the existence
of a 12-helix motif in the solid state conformation of hexamer 29,
with four intramolecular hydrogen bonds observed, in accordance
with the results of Gellman and co-workers.7 Analysis of the solid
state structures of pentamer 28 and tetramer 27 realised the pre-
diction that these oligomers may also adopt a 12-helix motif
through the presence of three and two intramolecular 12-mem-
bered H-bonds in the X-ray crystal structures of these two com-
pounds, respectively (Figs. 3–5). All attempts to crystallise the
fully protected trimer derivatives Cbz-(ACPC)3-OtBu 19 and Boc-
(ACPC)3-OtBu 26 proved unsuccessful. However, a single crystal
of trimer acid Cbz-(ACPC)3-OH 3017 was produced and single
crystal X-ray analysis revealed that the predicted 12-membered
H-bonded ring was present in addition to an eight-membered
C(O)O–Hꢀ ꢀ ꢀO@C(B) interaction, involving the carboxylic acid
group. It is postulated that this additional intramolecular
H-bonded ring confers extra stability upon this motif not possible
in the O-protected derivatives (Fig. 6).
Boc NH
OtBu
Cbz NH
OtBu (i) or (ii)
n
n
n
Conditions Product Yield (dr) %
2 (dimer)
3 (trimer)
16
19
20
23
24
(i)
(i)
(i)
(i)
(ii)
25
26
27
28
29
81 (>99:1)
80 (>99:1)
65 (>99:1)
61 (>99:1)
23 (>99:1)
4 (tetramer)
5 (pentamer)
6 (hexamer)
Scheme 3. Reagents and conditions: (i) Pd(OH)2/C, Boc2O, H2 (1 atm), MeOH, rt,
12 h; (ii) Pd(OH)2/C, H2 (1 atm), MeOH, rt, 12 h, then NaHCO3, Boc2O, MeOH,))), rt,
16 h.
Finally, to facilitate subsequent analysis by CD spectroscopy, the
N-Cbz functionality within oligomers 16, 19, 20 and 23 was ex-
changed for an N-Boc-protecting group via hydrogenolysis medi-
ated by Pd(OH)2/C in the presence of Boc2O, giving the
corresponding N-Boc/O-tBu-protected oligomers 25–28 in good
yields. Transformation of hexamer 24 required hydrogenolysis
and subsequent sonication in the presence of NaHCO3 in MeOH
to install the N-Boc group within 29 (Scheme 3).
The X-ray crystal structures of tetramer 27, pentamer 28 and
hexamer 29 were overlaid in order to compare the solid state con-
formations within these molecules. These results highlighted good
parity between the three structures (Fig. 7).18
2.2. Conformational analysis of transpentacin-derived
oligomers
To date, the smallest fully protected transpentacin b-peptide to
display a 12-helical conformation in the crystal lattice is N-Boc tet-
ramer 27, which is stabilised by only two intramolecular hydrogen
bonds. Furthermore, the results of these solid state investigations
suggest that oligomers derived from transpentacin have a strong
predisposition to adopt the 12-helical motif, conformational traits
2.2.1. Solid state conformational preferences: single crystal
X-ray analysis
A variety of crystallisation techniques were applied to the range
of transpentacin b-peptides (fully protected oligomers [Cbz-
(ACPC)n-OtBu and Boc-(ACPC)n-OtBu], N-deprotected oligomers

1800
E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
Figure 3. Chem-3D representation of the X-ray crystal structure of hexamer Boc-(ACPC)6-OtBu 29: (A) perpendicular to and (B) along the helical axis (some H atoms omitted
for clarity).
of which are exhibited by molecules containing as few as three res-
idues, as demonstrated through analysis of trimer acid 30. With
these promising results in hand, attention focused upon whether
these observed folding preferences in the smaller b-peptides are
manifested in solution, or if alternative conformers or irregular
structures are populated.
and i ꢁ 1). In order to verify that the 12-helical conformation per-
sisted in solution for Cbz-(ACPC)6-OtBu hexamer 24 (which
showed better resonance dispersion in CDCl3 than Boc-(ACPC)6-
OtBu hexamer 29) a full tabulation of NOE correlations was sought.
Tr-ROESY data for hexamer 24 were acquired, giving rise to a num-
ber of long range inter-residue NOEs. The NH(i)?C H(i) and
a
NH(i)?C H(iꢁ1) pattern of NOEs, characteristic of the 12-helix,
a
2.2.2. Solution phase conformational preferences
was observed within hexamer 24. Two sets of long range correla-
Solution phase NMR studies were carried out on a range of
transpentacin oligomers harbouring varying protecting groups:
N-Cbz/O-tBu (trimer to hexamer 19, 20, 23 and 24), N-Boc/O-tBu
(trimer to hexamer 26–29) and N-Cbz/OH (trimer 30). Initial inves-
tigations sought to fully assign the primary structure of each oligo-
mer, utilising a combination of 2D COSY, edited HSQC and TOCSY
experiments. Additionally, 1D TOCSY experiments were also an as-
set when chemical shift overlap was problematic, allowing isola-
tion of an individual ring system by transfer from the NH proton,
and sequential assignments were performed using NOESY and
ROESY spectra.19 Boc-(ACPC)6-OBn hexamer 5 has previously been
shown by Gellman to adopt the 12-helix secondary structural
motif in solution phase and solid state.7 In the 12-helix, each NH
proton [except NH(A)] has been shown to give rise to two strong,
tions were also recorded: CbH(i)?C H(i + 2) and CbH(i)?NH(i + 2),
a
which are also indicative of a 12-helix motif7b (Fig. 8).
With these data in hand, structure calculations were performed
for the hexamer 24 with XPLOR, using distance restraints from the
experimental 1H NMR NOE data.20 A simulated annealing proto-
col21 was employed and an ensemble of 20 structures was calcu-
lated. All these structures contained the 12-membered hydrogen
bonds CO(A)?NH(D), CO(B)?NH(E) and CO(C)?NH(F) and were
further refined with distance restraints for these hydrogen bonds
included. An overlay of the 10 lowest energy structures thus gener-
ated showed remarkable parity; this suggests that the produced
structures offer an accurate representation of the solution phase
conformation of hexamer 24, and thus substantiates the existence
of a 12-helix motif for this compound in solution (Fig. 9). When
the lowest energy calculated NMR structure of hexamer 24 was
through space NOE correlations to two C H protons (of residues i
a

E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
1801
Figure 4. Chem-3D representation of the X-ray crystal structure of pentamer Boc-(ACPC)5-OtBu 28: (A) perpendicular to and (B) along the helical axis (some H atoms omitted
for clarity).
overlaid with the solid-state X-ray crystal structure of hexamer 29,
excellent agreement was noted, which suggests that the 12-helix is
favoured for hexamers 24 and 29 in both the solid state and solution
phase (Fig. 9). Further affirmation of this assertation was obtained
from the IR spectrum of hexamer 29 at high dilution (2 mM), which
revealed two peaks associated with N–H bonds. A sharp peak at
3439 cmꢁ1 was indicative of a non-hydrogen bonding situation:
in a 12-helix NH(A) and NH(B) would be able only to participate
in intermolecular hydrogen bonds but at such high dilution this is
unlikely and it is therefore possible that these two protons are
responsible for this sharp peak. A second, broader peak centred
on 3326 cmꢁ1 is indicative of intramolecular hydrogen bonding,
which may be ascribed to the environments of NH protons (C)–(F)
within a 12-helix.
The 12-helix solid-state X-ray crystal structure of Boc-(ACPC)5-
OtBu pentamer 28 exhibited three CO(i)?NH(i + 3) intramolecular
hydrogen bonds involving NH protons (C)–(E) and it was therefore
envisaged that if this motif persisted in solution, the equivalent key
NOE correlations reported by Gellman for hexamer 57b would also
be present within pentamer 28. Unlike Boc-(ACPC)6-OtBu hexamer
29, Boc-(ACPC)5-OtBu pentamer 28 displayed good resonance dis-
persion in CDCl3 and therefore this oligomer was investigated to
facilitate direct comparison of the solid state and solution phase
structures. A set of NOEs were produced for pentamer 28 that con-
tained all the characteristic correlations for a 12-helix, although
some could not be unambiguously assigned a relative intensity
due to resonance overlap: two of the three Hb(i)–NH(i + 2) correla-
tions were unambiguously assigned, as were two of the three
Hb(i)–H (i + 2) correlations, with the remainder being present
a
although non-quantifiable (Fig. 10).
Structure calculations using the NOEs for pentamer 28 were
performed in an analogous fashion to those previously described
for hexamer 24. All 20 of the NMR structures contained the 12-
membered hydrogen bonds CO(A)?NH(D) and CO(B)?NH(E),
and so the structures were further refined with distance restraints
for these hydrogen bonds included. The 10 lowest energy struc-
tures were superimposed, affording a conformation that closely
resembled a 12-helix (Fig. 11). Good agreement with the solid-
state X-ray crystal structure was again observed. The IR spectrum
of pentamer 28 showed a sharp non-hydrogen bonding stretch at
3436 cmꢁ1 and a broader peak centred on 3336 cmꢁ1 with a shoul-
der at 3271 cmꢁ1, suggesting both inter- and intramolecular
hydrogen bonding N–H stretches. Taken together, these results
suggest that the 12-helix represents a dominant conformation for
pentamer 28 in solution, although an alternative conformation
may be partially adopted.
Analysis of the Tr-ROESY spectrum for tetramer 27 revealed the
presence of a large number of inter-residue NOEs, some of which

1802
E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
Figure 5. Chem-3D representation of the X-ray crystal structure of tetramer Boc-(ACPC)4-OtBu 27: (A) perpendicular to and (B) along the helical axis (some H atoms omitted
for clarity).
were characteristic of a 12-helical motif being present in solution
(Fig. 12). Structure calculations were performed for tetramer 27
using the experimental NOE data and the same simulated anneal-
ing protocol, but without the inclusion of any hydrogen bond re-
straints. The resultant NMR structures revealed that
conformation of tetramer 29 is less well defined than that of the
hexamer 24 or pentamer 28. Some of the generated structures con-
tained the 12-membered hydrogen bond CO(A)?NH(D) as ob-
served in the crystal structure, but in other structures this was
replaced with an eight-membered hydrogen bond CO(A)?NH(C),
indicating an alternative turn-type conformation. The IR spectrum
of tetramer 29 was indicative of two possible types of N–H stretch.
In the N–H hydrogen bonding region, there are two distinct peaks
(at 3273 and 3340 cmꢁ1), suggesting that there is more than one
type of hydrogen bonding occurring. A sharper, non-hydrogen
bonding peak at 3438 cmꢁ1 is also noted. On diluting the sample
10-fold, much of the character from the original tetramer IR still re-
mains, suggesting most of the hydrogen bonding in this oligomer is
intramolecular. These studies suggest that this oligomer may pop-
ulate multiple conformations in solution, of which the 12-helix
may be only one possibility (Fig. 13).
2.2.3. Comparisons across the oligomer series
Analysis of the N–H 1H NMR chemical shifts within the series of
Cbz-(ACPC)n-OtBu oligomers 19, 20, 23 and 24 and Boc-(ACPC)n-
OtBu oligomers 26–29 showed similar trends, which provided
information about hydrogen bonding chemical environments
(Fig. 14). In the Boc-(ACPC)n-OtBu oligomers 27–29 (tetramer to
hexamer), the chemical shift values of NH(C), NH(D), NH(E) and
NH(F) (where relevant) are all very deshielded, appearing within
the narrow range of 7.30–8.26 ppm in both homologous oligomer
series, suggesting significantly similar hydrogen bonding chemical
environments, likely to be intramolecular: it is intramolecular
hydrogen bonding involving these protons that stabilises the 12-
helical motif of 27–29 in both the X-ray crystal structures and solu-
tion phase energy-minimised NMR models. Conversely, the 1H
NMR chemical shift (and hence chemical environment) of the car-
bamate NH(A) protons was noted to be largely independent of oli-
gomer length, appearing in the range 4.67–4.94 ppm as the trimer
26 to hexamer 29 series is traversed, suggesting that the hydrogen
bonding environment of all NH(A) protons is similar. The chemical
shift of NH(B) showed the greatest change upon increasing oligo-
mer length, suggesting a considerable change in hydrogen bonding

E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
1803
the coefficients for this residue and for NH(A), whereas those for
NH(C)–NH(F) remain constant. Pentamer 23 and tetramer 20
exhibited a trend akin to that observed within hexamer 24: with
increasing concentration, the values recorded for protons NH(A)
and NH(B) displayed the greatest increase. However, the data pro-
duced for tetramer 20 are not typical of a 12-helix conformation
with NH(B) displaying the lowest values within the data set, and
the only variation in concentration occurring with NH(A). This sug-
gests that intramolecular hydrogen bonding behaviour is observed
with NH(B) in shorter oligomers (Fig. 15).
Gellman et al. have produced experimentally derived CD traces
of their transpentacin hexamer 5 (in the opposite enantiomeric
series to 24 and 29). These investigations demonstrated that exper-
imentally the 12-helix secondary structure motif displays a CD sig-
nature in MeOH-d3 with a reported positive-maximum at 204 nm,
zero crossing at 214 nm and negative-maximum at 221 nm.8 CD
spectroscopy was performed on the transpentacin N-Boc series (di-
mer 25?hexamer 29) as solutions in TFE. The slight discrepancy
between the data obtained from these experiments and with Gell-
man’s reported data is likely due to the effects of different concen-
trations and solvents between the two systems. Comparison of
these data indicated that N-Boc pentamer 28 and hexamer 29 show
good correlation with the values reported by Gellman
(k+max = 219 nm for both compounds, k0 = 210 and 211 nm,
kꢁmax = 200 and 202 nm, respectively), thus supporting the pres-
ence of a 12-helix conformation for these oligomers. The values re-
corded for N-Boc tetramer 27, however, do not fit well
(k+max = 216 nm, k0 = 206 nm and kꢁmax = 198 nm), indicating that
the 12-helix may not be the most highly populated conformation
of this molecule in solution and other competing conformations
(regular or irregular) may be present. The traces obtained for N-
Boc dimer 25 and N-Boc trimer 26 display negative maxima at
ca. 190 nm, which is characteristic of an irregular conformation
for many peptide systems. These results suggest that a noticeable
preference for a 12-helical conformation in solution occurs be-
tween the tetramer?pentamer transition, whilst the conforma-
tional ensemble of the lower oligomers could potentially include
partially 12-helical and alternative conformers and irregular struc-
tures that inter-convert rapidly (Fig. 16).
Figure 6. Chem-3D representation of the X-ray crystal structure of trimer Cbz-
(ACPC)3-OH 30 (some H atoms omitted for clarity).
environment, with the largest difference noted between the tetra-
mer 27 and pentamer 28. When DMSO-d6 was added to a solution
of pentamer 23 and hexamer 24 in CDCl3 (26 mM), the volume of
DMSO-d6 added showed relatively little perturbation upon the
shifts of amide protons NH(C)–NH(F) (where relevant), indicating
that these protons are effectively shielded from the strong H-bond-
ing solvent, consistent with their involvement in strong, intramo-
lecular hydrogen bonds. Amide protons NH(A) and NH(B)
displayed high sensitivity towards the presence of DMSO-d6, sug-
gesting that they are predominantly solvent exposed. An analogous
study conducted on tetramer 20 did not display similar trends,
with only NH(A) displaying a significant change in chemical shift
with the addition of DMSO-d6. These data suggest that the shorter
oligomers (trimer, tetramer) have NH(C) and NH(D) involved in
intramolecular hydrogen bonds, but the N-terminus of the mole-
cule can adopt a conformation in which NH(B) can also form an
intramolecular hydrogen bond. The population of such alternative
conformers is depleted as the length of the oligomer increases and
the 12-helix becomes the dominant conformation in which NH(A)
and NH(B) remain solvent exposed.
3. Conclusion
In conclusion, b-peptides derived from (S,S)-2-aminocyclopen-
tanecarboxylic acid (transpentacin) show a strong predisposition
to adopt a 12-helix secondary structure, with the conformational
traits of the 12-helix being exhibited by oligomers with as few as
three residues in the solid state. The hexamer and pentamer persist
as a 12-helix in solution phase, although in solution the tetramer
appears to exist as an equilibrium of several different conformers,
of which the 12-helix is one. Investigations into the ability of the
12-helix to tolerate alkyl substituents on the cyclopentane back-
bone are currently in progress in our laboratory.
These observations are consistent with pentamer 28 (but not
necessarily tetramer 27) showing a strong preference to adopt
the 12-helix conformation in solution, which does not involve an
intramolecular hydrogen bond to NH(B). To investigate this postu-
late further, the dependence of the NH chemical shift values on
concentration was probed. NH(A) and NH(B) in both pentamers
23 and 28 and both hexamers 24 and 29 exhibited the greatest
concentration dependence, whilst NH(C), NH(D), NH(E) and NH(F)
exhibited negligible concentration dependence upon their chemi-
cal shift values, suggesting that the former partake in intermolec-
ular hydrogen bonding between molecules. Determination of the
4. Experimental
4.1. General experimental
All reactions involving organometallic or other moisture-sensi-
tive reagents were carried out under a nitrogen or argon atmo-
sphere using standard vacuum line techniques and glassware that
was flame dried and cooled under nitrogen before use. Solvents
were dried according to the procedure outlined by Grubbs and
co-workers.22 Other solvents and reagents were used as supplied
(analytical or HPLC grade) without prior purification. Organic layers
were dried over MgSO4. Thin layer chromatography was performed
temperature coefficients (DdNY/D
T in ppb Kꢁ1) of the amide pro-
tons of hexamer 24 revealed that at low concentrations (7 mM),
NH(B) produced the lowest value, indicating that the environment
of this proton remains constant throughout the temperature range.
Increasing the concentration produces a corresponding increase in

1804
E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
Figure 7. Chem-3D overlays of the X-ray crystal structures of tetramer 27 (red), pentamer 28 (green) and hexamer 29 (purple), perpendicular to and along the helical axis:
(A) tetramer 27 versus pentamer 28; (B) pentamer 28 versus hexamer 29; (C) tetramer 27 versus hexamer 29.
O
Hβ
O
O
Hβ
O
Hβ
O
O
Hβ
O
O
Hβ
O
Hβ
O
O
BnO
BnO
N
H
A
N
H
B
N
H
OtBu
C
N
H
D
N
H
E
N
H
F
Hα
Hα
Hα
Hα
Hα
Hα
strong
medium
weak
O
Hβ
Hβ
O
Hβ
Hβ
Hβ
O
Hβ
N
H
A
N
H
B
N
H
OtBu
C
N
H
D
N
H
E
N
H
F
Hα
Hα
Hα
Hα
Hα
Hα
Figure 8. Summary of the resolved intra- and inter-residue NOEs observed in hexamer 24.
on aluminium plates coated with 60 F254 silica. Plates were visual-
ised using UV light (254 nm), iodine, 1% aq KMnO4 or 10% ethanolic
phosphomolybdic acid. Flash column chromatography was per-
formed on Kieselgel 60 silica on a glass column, or on a Biotage
SP4 automated flash column chromatography platform.
Elemental analyses were recorded by the microanalysis service
of the Inorganic Chemistry Laboratory, University of Oxford, UK.
Melting points were recorded on a Gallenkamp Hot Stage appara-
tus and are uncorrected. Optical rotations were recorded on a Per-
kin–Elmer 241 polarimeter with a water-jacketed 10 cm cell.
Specific rotations are reported in 10ꢁ1 deg cm2 gꢁ1 and concentra-
tions in g/100 mL. IR spectra were recorded on a Bruker Tensor 27
FT-IR spectrometer as either a thin film on NaCl plates (film) or a
KBr disc (KBr), as stated. Selected characteristic peaks are reported
in cmꢁ1. NMR spectra were recorded on Bruker Avance spectrom-
eters in the deuterated solvent stated. Spectra were recorded at rt

E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
1805
Figure 9. (A) Overlay of the 10 lowest energy NMR structures of hexamer 24 viewed perpendicular to the helical axis; (B) overlay of the lowest energy NMR structure of
hexamer 24 (purple) and the X-ray crystal structure of hexamer 29 (grey) viewed perpendicular to the helical axis.
O
Hβ
O
O
Hβ
O
Hβ
O
O
Hβ
O
O
Hβ
O
tBuO
N
H
A
N
H
OtBu
B
N
H
C
N
H
D
N
H
E
Hα
Hα
Hα
Hα
Hα
strong
medium
weak
O
Hβ
Hβ
O
Hβ
Hβ
Hβ
O
tBuO
N
H
A
N
H
B
N
H
OtBu
C
N
H
D
N
H
E
Hα
Hα
Hα
Hα
Hα
Figure 10. Summary of the resolved intra- and inter-residue NOEs observed in the transpentacin pentamer 28.
unless otherwise stated. The field was locked by external referenc-
H2 for 16 h, after which time the reaction mixture was filtered
through CeliteÒ (eluent MeOH) and concentrated in vacuo.
ing to the relevant deuteron resonance. Low-resolution mass spec-
tra were recorded on either a VG MassLab 20–250 or a Micromass
Platform 1 spectrometer. Accurate mass measurements were run
on either a Bruker MicroTOF internally calibrated with polyalanine,
or a Micromass GCT instrument fitted with a Scientific Glass
Instruments BPX5 column (15 m ꢂ 0.25 mm) using amyl acetate
as a lock mass.
4.2.2. General procedure 2 for O-deprotection
TFA was added to a stirred solution of the requisite tert-butyl
ester in CH2Cl2 at 0 °C. The reaction mixture was allowed to warm
to rt and stirred for 3 h before being concentrated in vacuo and
dried under high vacuum.
4.2. General experimental procedures
4.2.3. General procedure 3 for HOBt/EDCꢀHCl-mediated peptide
coupling23
4.2.1. General procedure 1 for N-deprotection
Pd(OH2)/C was added to a stirred, degassed solution of the req-
uisite amine in MeOH. The resulting suspension was stirred under
A stirred solution of the requisite amine in CHCl3 was succes-
sively treated with Et3N, HOBt, the requisite carboxylic acid and
EDCꢀHCl. After 16 h, the reaction mixture was washed sequentially

1806
E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
Figure 11. (A) Overlay of the 10 lowest energy NMR structures of 28 viewed perpendicular to the helical axis; (B) overlay of the lowest energy NMR structure (green) and the
X-ray crystal structure (grey) of 28 viewed perpendicular to the helical axis.
O
Hβ
O
with 1 M aq HCl, satd aq NaHCO3 and brine and then dried and
concentrated in vacuo.
Hβ
O
Hβ
O
Hβ
O
tBuO
N
H
A
N
H
B
N
H
D
OtBu
C
N
H
4.2.4. General procedure 4 for N-protecting group swap
(N-Cbz?N-Boc)
Hα
Hα
Hα
Hα
strong
Pd(OH2)/C was added to a stirred, degassed solution of the req-
uisite N-Cbz protected amine and Boc2O in MeOH. The resulting
suspension was stirred under H2 (1 atm) for 16 h, after which time
the reaction mixture was filtered through CeliteÒ (eluent MeOH)
and concentrated in vacuo.
medium
weak
O
Hβ
O
Hβ
O
Hβ
O
Hβ
O
tBuO
N
H
A
N
H
B
N
H
C
N
H
D
OtBu
Hα
Hα
Hα
Hα
4.3. tert-Butyl cyclopent-1-enecarboxylate 10
CO2tBu
Figure 12. Summary of the resolved intra- and inter-residue NOEs observed in the
transpentacin tetramer 27.
NaH (60% suspension in mineral oil, 72.5 g, 1.80 mol) was sus-
pended in PhMe (1.6 L) and the resultant mixture was heated to
60 °C. A solution of di-tert-butyladipate (2.60 g, 0.01 mol) in tBuOH
(10 mL) was added via syringe. After 30 min, more di-tert-butylad-
ipate (232 g, 0.90 mol) as a solution in PhMe (300 mL) was added
dropwise. The resultant mixture was heated at 100 °C for 3 h, then
allowed to cool to rt before being cooled to 0 °C prior to the
sequential addition of MeOH (25 mL), H2O (25 mL) and satd aq
NH4Cl (250 mL). The organic layer was separated, dried and con-
centrated in vacuo. Purification via vacuum distillation gave tert-
butyl 2-oxocyclopentanecarboxylate as a colourless oil (122 g,
73%);25 bp 110–112 °C (7.5 mmHg); dH (400 MHz, CDCl3) 1.45
(9H, s, CMe3), 1.79–1.90 (1H, m, C(4)HA), 2.07–2.31 (5H, m,
C(3)H2, C(4)HB, C(5)H2), 3.05 (1H, app t, J 8.8, C(1)H).
A solution of adipoyl chloride (600 g, 3.28 mol) in Et2O (1.2 L)
was added dropwise to a solution of tBuOH (941 mL, 9.83 mol)
and N,N-dimethylaniline (1.25 L, 9.83 mol) in Et2O (2.4 L) at 0 °C.
The resultant mixture was allowed to warm slowly to rt and stirred
for 24 h. The solution was diluted with H2O (1.5 L) and the aqueous
phase was separated. The organic portion was washed sequentially
with 1 M aq HCl (5 ꢂ 500 mL), satd aq NaHCO3 (1 L) and brine (1 L),
then dried and concentrated in vacuo to give di-tert-butyl adipate
as a colourless oil that crystallised on standing (694 g, 82%);24 mp
29–30 °C; {lit.24 mp 29–31 °C}; dH (400 MHz, CDCl3) 1.43 (18H, s,
2 ꢂ CMe3), 1.58–1.68 (4H, m, C(3)H2, C(4)H2), 2.20–2.23 (4H, m,
C(2)H2, C(5)H2).

E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
1807
Figure 13. (A) Overlay of the 10 lowest energy NMR structures of tetramer 27, containing a 12-membered ring hydrogen bond N(D)–Hꢀ ꢀ ꢀO@C(A), viewed perpendicular to
the helical axis; (B) Overlay of the 10 lowest energy NMR structures of tetramer 27, containing an eight-membered ring hydrogen bond N(C)–Hꢀ ꢀ ꢀO@C(A), viewed
perpendicular to the helical axis.
NaBH4 (15.6 g, 0.41 mol) was added portionwise to a solution of
tert-butyl 2-oxocyclopentanecarboxylate (76.0 g, 0.41 mol) in
EtOH (1 L) at 0 °C. The reaction mixture was allowed to warm to
rt and the progress of the reaction was monitored by TLC. Once
all the starting material had been consumed the reaction was
cooled to 0 °C before the dropwise addition of H2O until the resul-
tant solid aggregated, followed by the addition of excess satd aq
NH4Cl (1 L). The mixture was diluted with Et2O (1 L), the layers
were separated and the aqueous layer was extracted with Et2O
(3 ꢂ 1 L). The combined organic layers were dried and concen-
trated in vacuo. The residue was dissolved in CH2Cl2 (1 L), cooled
to 0 °C, and Et3N (387 mL, 2.77 mol) and DMAP (4.84 g, 39.6 mmol)

1808
E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
Figure 14. 1H NMR spectra of Boc-(ACPC)n-OtBu oligomers 26–29 [trimer (n = 3) to hexamer (n = 6)] at 298 K in CDCl3. NH protons are labelled from the N-terminus. Peaks
between 4.00 and 4.50 ppm are due to Hb protons, and those between 2.00 and 3.00 ppm are due to H protons.
a
were added, followed by the dropwise addition of a solution of
MsCl (123 mL, 1.58 mol) in CH2Cl2 (500 mL). The resultant mixture
was stirred at 0 °C for 1 h before being allowed to warm slowly to
rt. The reaction mixture was stirred at rt for a further 8 h before
the addition of H2O (1.5 L). The layers were separated and the or-
ganic layer was washed sequentially with 1 M aq HCl (1 L), H2O
(1 L), satd aq NaHCO3 (1 L) and brine (1 L), then dried and concen-
trated in vacuo. The residue was dissolved in CH2Cl2 (1 L), cooled
to 0 °C, and a solution of DBU (79.1 mL, 0.53 mol) in CH2Cl2
(79.1 mL) was added dropwise. After 4 h, the reaction mixture
was washed sequentially with 1 M aq HCl (1 L) and brine (1 L),
then dried and concentrated in vacuo. Purification via flash col-
umn chromatography (eluent 30–40 °C petrol/Et2O, 50:1) gave
10 as a colourless oil (45.1 g, 65% over three steps); Rf 0.64
(30–40 °C petrol/Et2O, 10:1); dH (400 MHz, CDCl3) 1.49 (9H, s,
CMe3), 1.90–1.98 (2H, m, C(4)H2), 2.45–2.55 (4H, m, C(3)H2,
C(5)H2), 6.67–6.68 (1H, m, C(2)H).
allowed to warm to rt before being concentrated in vacuo. The res-
idue was dissolved in CH2Cl2 (100 mL), washed sequentially with
10% aq citric acid (100 mL), satd aq NaHCO3 (100 mL) and brine
(100 mL), then dried and concentrated in vacuo to give a 96:4 mix-
ture of 11:12 as a colourless oil that solidified upon standing to a
white crystalline solid (24.6 g, 87%).12a Recrystallisation of an ali-
quot from MeOH gave 11 as colourless crystals (>99:1 dr); mp
53–55 °C; ½a 2D2
ꢃ
¼ ꢁ69:9 (c 0.6 in CHCl3); {lit.12a
½
a 2D5
ꢃ
¼ ꢁ69:1 (c
1.2 in CHCl3)}; dH (400 MHz, CDCl3) 1.38–1.85 (6H, m, C(3)H2,
C(4)H2, C(5)H2) overlapping 1.38 (3H, d, J 6.9, C(a)Me) and 1.51
(9H, s, CMe3), 2.89–2.93 (1H, m, C(1)H), 3.09–3.15 (1H, m, C(2)H),
3.54 (1H, d, J 15.5, NCHA), 4.03 (1H, d, J 15.5, NCHB), 4.31 (1H, q, J
6.9, C(a)H), 7.17–7.43 (10H, m, Ph).
4.4.1. X-ray crystal structure determination for 11
Data were collected using an Enraf-Nonius -CCD diffractome-
ter with graphite monochromated Mo K radiation using standard
j
a
procedures at 190 K. The structure was solved by direct methods
(SIR92); all non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms were added at idealised
positions. The structure was refined using CRYSTALS.26
4.4. tert-Butyl (1R,2S,S)-2-[N-benzyl-N-(-methylbenzyl)amino]-
cyclopentanecarboxylate 11
Ph
X-ray crystal structure data for 11 [C25H33NO2]: M = 379.54,
monoclinic, space group P21, a = 10.0500(5) Å, b = 9.2191(4) Å,
c = 12.1262(7) Å, b = 90.9075(17)°, V = 1112.36(10) Å3, Z = 2,
l =
N
CO2tBu
0.071 mmꢁ1, colourless plate, crystal dimensions = 0.20 ꢂ 0.30 ꢂ
0.40 mm3. A total of 2694 unique reflections were measured for
5 < h < 27 and 1589 reflectionswere used in therefinement. Thefinal
Ph
parameters were wR2 = 0.072 and R1 = 0.074 [I >2.5r(I)].
BuLi (2.5 M in hexanes, 28.4 mL, 71.0 mmol) was added drop-
wise to solution of (S)-N-benzyl-N-( -methylbenzyl)amine
Crystallographic data (excluding structure factors) have been
deposited with the Cambridge Crystallographic Data Centre as sup-
plementary publication number CCDC 772467. Copies of the data
can be obtained, free of charge, on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [fax: +44(0) 1223 336033 or e-mail:
deposit@ccdc.cam.ac.uk].
a
a
(15.5 g, 73.0 mmol) in THF (400 mL) at ꢁ78 °C. The resultant solu-
tion was allowed to stir at ꢁ78 °C for 30 min before the dropwise
addition of a solution of 10 (7.70 g, 46.0 mmol) in THF (230 mL)
at ꢁ78 °C. The reaction mixture was stirred at ꢁ78 °C for 3 h before
the addition of satd aq NH4Cl (50 mL). The reaction mixture was

E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
1809
4.5. tert-Butyl (S,S,S)-2-[N-benzyl-N-(-
methylbenzyl)amino]cyclopentanecarboxylate 12
O
OtBu
Cbz
NH
Ph
6
Transpentacin hexamer 24
N
CO2tBu
14
12
10
8
Ph
KOtBu (40% w/w substrate, 3.52 g) was added to a stirred solution
of 11 (8.80 g, 23.2 mmol, 96:4 dr) in tBuOH (200 mL) and the resul-
tant mixture was heated at reflux for 8 h. The reaction mixture
was then allowed to cool to rt prior to the addition of satd aq NH4Cl
(200 mL). The resultant mixture was diluted with Et2O (500 mL), the
layers were separated and the organic layer was washed with brine
(500 mL), dried and concentrated in vacuo to give a 96:4 mixture of
6
4
2
12 and (S,S,S)-2-[N-benzyl-N-(a-methylbenzyl)amino]cyclopentan-
ecarboxylic acid. Purification via flash column chromatography(elu-
ent 30–40 °C petrol/Et2O, 20:1) gave 12 as a colourless oil that
solidified upon standing to a white crystalline solid (7.1 g, 81%,
>99:1 dr);12a Rf 0.48 (30–40 °C petrol/Et2O, 10:1); mp 51–52 °C
0
0
5
10
15
20
25
30
35
Concentration (mM)
O
OtBu
(MeOH); ½a 2D0
ꢃ
¼ þ62:4 (c 1.0 in CHCl3); {lit.12a
½ ꢃ
a 2D5
¼ þ64:5 (c 1.2
Cbz
NH
in CHCl3)}; dH (400 MHz, CDCl3) 1.33 (3H, d, J 6.8, C(
a
)Me), 1.39
(9H, s, CMe3), 1.57–1.70 (4H, m, C(3)HA, C(4)H2, C(5)HA), 1.74–1.81
(2H, m, C(3)HB, C(5)HB), 2.61–2.66 (1H, m, C(1)H), 3.55–3.63 (1H,
m, C(2)H), 3.69 (1H, d, J 14.8, NCHA), 3.78 (1H, d, J 14.8, NCHB), 3.90
5
Transpentacin pentamer 23
14
12
10
8
(1H, q, J 6.8, C(
a
)H), 7.14–7.46 (10H, m, Ph). Further elution (eluent
-methyl-
white solid
¼ þ58:9 (c 1.2 in CHCl3);
30–40 °C petrol/EtOAc, 2:1) gave (S,S,S)-2-[N-benzyl-N-(
a
benzyl)amino]cyclopentanecarboxylic acid as
a
(94 mg, 1%, >99:1 dr); mp 130–134 °C; ½a D21
ꢃ
mmax (KBr) 1696 (C@O); dH (400 MHz, CDCl3); 1.54 (3H, d, J 6.8,
C(
m, C(1)H), 3.67 (1H, app q, J 9.1, C(2)H), 4.01 (1H, d, J 14.1, NCHA),
4.23 (1H, d, J 14.1, NCHB), 4.29 (1H, q, J 6.8, C( )H), 7.13–7.59 (10H,
m, Ph); dC (100 MHz, CDCl3) 14.1 (C( )Me), 22.5, 27.0, 27.0 (C(3),
C(4), C(5)), 46.8 (C(1)), 50.3 (NCH2), 60.1 (C( )), 64.5 (C(2)), 128.3,
a)Me), 1.58–2.05 (6H, m, C(3)H2, C(4)H2, C(5)H2), 2.79–2.90 (1H,
6
a
4
a
a
2
128.4, 128.6, 128.7, 128.9, 129.4 (o,m,p-Ph), 139.5, 136.6 (i-Ph),
178.3 (CO2H); m/z (ESI+) 382 ([M+MeCN+NH4]+, 28%), 346
0
0
5
10
15
20
25
30
35
([M+Na]+, 11%), 324 ([M+H]+, 100%); HRMS (ESI+) C21H26NO2
þ
Concentration (mM)
([M+H]+) requires 324.1964; found 324.1966.
O
OtBu
Cbz
NH
4.5.1. X-ray crystal structure determination for 12
Data were collected using an Enraf-Nonius
j-CCD diffractome-
4
ter with graphite monochromated Mo K radiation using standard
a
Transpentacin tetramer 20
procedures at 190 K. The structure was solved by direct methods
(SIR92); all non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms were added at idealised
positions. The structure was refined using CRYSTALS.26
14
12
10
8
X-ray crystal structure data for 12 [C25H33NO2]: M = 379.54,
orthorhombic, space group P212121, a = 6.09210(10) Å, b =
13.0837(2) Å, c = 27.7214(6) Å, V = 2209.60(7) Å3, Z = 4,
l = 0.71
mmꢁ1, colourless plate, crystal dimensions = 0.1 ꢂ 0.2 ꢂ 0.3 mm3.
A total of 2855 unique reflections were measured for 5 < h < 27
and 2232 reflections were used in the refinement. The final param-
6
4
eters were wR2 = 0.039 and R1 = 0.037 [I >3r (I)].
2
Crystallographic data (excluding structure factors) have been
deposited with the Cambridge Crystallographic Data Centre as sup-
plementary publication number CCDC 772468. Copies of the data
can be obtained, free of charge, on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [fax: +44(0) 1223 336033 or e-mail:
deposit@ccdc.cam.ac.uk].
0
0
5
10
15
20
25
30
35
Concentration (mM)
Figure 15. Temperature coefficients of amide protons within hexamer 24, penta-
mer 23 and tetramer 20 over a range of concentrations.

1810
E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
30
25
20
15
10
5
Hexamer
Pentamer
Tetramer
Trimer
Dimer
0
190
200
210
220
230
240
-5
-10
-15
-20
O
O
5
27
28
29
NH
NH
CO2tBu
Boc NH
n = 2 n = 3 n = 4 n = 4
λ+max 216
219
210
200
219
211
202
204
214
221
0
max
206
198
λ−
n
Figure 16. CD spectra of N-Boc dimer 25, trimer 26, tetramer 27, pentamer 28 and hexamer 29 (1 mg/mL in TFE).
4.6. tert-Butyl (S,S)-2-aminocyclopentanecarboxylate 13
C(3)HA) overlapping 1.43 (9H, s, CMe3), 1.65–1.76 (2H, m, C(4)H2),
1.83–1.89 (1H, m, C(5)HA), 1.92–1.97 (1H, m, C(5)HB), 2.08–2.20
(1H, m, C(3)HB), 2.46–2.54 (1H, m, C(1)H), 4.12–4.21 (1H, m,
C(2)H), 4.81 (1H, br s, NH), 5.10 (2H, app s, CH2Ph), 7.29–7.39 (5H,
m, Ph); dC (125 MHz, CDCl3) 22.7 (C(4)), 27.9 (C(5), CMe3), 33.1
(C(3)), 51.6 (C(1)), 56.1 (C(2)), 66.5 (CH2Ph), 80.4 (CMe3), 127.9,
128.4, 128.5 (o,m,p-Ph), 136.4 (i-Ph), 155.5 (NCO), 173.5
t
H2N
CO2Bu
Following General Procedure 1, 12 (6.8 g, 17.8 mmol) and
Pd(OH)2/C (25% w/w substrate, 1.70 g) in MeOH (80 mL) under
H2 (5 atm) gave 13 as a colourless oil that solidified upon standing
to an off white solid (3.05 g, 92%, >99:1 dr); mp 84–86 °C;
(CO2 Bu); m/z (ESI+) 378 ([M+MeCN+NH4]+, 100%); HRMS (ESI+)
t
+
þ
C18H25NNaO4 ([M+Na] ) requires 342.1681; found 342.1679.
4.8. (S,S)-2-[N-
½
a 2D3
ꢃ
¼ þ59:4 (c 1.1 in CHCl3); mmax (KBr) 3433 (N–H), 1725
(Benzyloxycarbonyl)amino]cyclopentanecarboxylic acid 15
(C@O); dH (400 MHz, CDCl3, 26 mM) 1.30–1.42 (1H, m, C(3)HA),
1.46 (9H, s, CMe3), 1.55 (2H, s, NH2), 1.66–1.70 (2H, m, C(4)H2),
1.79–1.83 (1H, m, C(5)HA), 1.91–2.05 (2H, m, C(3)HB, C(5)HB),
2.27–2.35 (1H, m, C(1)H), 3.34–3.42 (1H, m, C(2)H); dC (100 MHz,
CDCl3) 22.4 (C(4)), 27.9 (C(5)), 28.0 (CMe3), 35.1 (C(3)), 54.5
Cbz NH
CO2H
(C(1)), 57.0 (C(2)), 79.9 (CMe3), 174.5 (CO2 Bu); m/z (ESI+) 186
t
([M+H]+ 100%); HRMS (ESI+) C10H20NO2 ([M+H]+) requires
Following General Procedure 2, 14 (5.90 g, 0.02 mol) and TFA
(12 mL) in CH2Cl2 (48 mL) gave, after purification via recrystallisa-
tion (CHCl3/heptane), 15 as a white crystalline solid (4.23 g, 87%,
>99:1 dr); C14H17NO4 requires C, 63.9; H, 6.5; N, 5.3; found C,
þ
186.1494; found 186.1489.
4.7. tert-Butyl (S,S)-2-[N-
(benzyloxycarbonyl)amino]cyclopentanecarboxylate 14
64.1; H, 6.5; N, 5.2; mp 100–101 °C (CHCl3/heptane); ½a D18
¼
ꢃ
þ23:5 (c 1.3 in CHCl3); mmax (film) 3323 (O–H), 1706, 1704
(C@O); dH (400 MHz, CDCl3, 26 mM) 1.42–1.55 (1H, m, C(3)HA),
1.66–1.79 (2H, m, C(4)H2), 1.86–2.00 (1H, m, C(5)HA), 2.07–2.21
(2H, m, C(3)HB, C(5)HB), 2.73–2.85 (1H, m, C(1)H), 4.06–4.20 (1H,
m, C(2)H), 5.08 (1H, s, NH), 5.14 (2H, s, CH2Ph), 7.31–7.44 (5H, m,
Ph); dC (125 MHz, CD3OD) 23.2 (C(4)), 28.9 (C(5)), 32.8 (C(3)),
50.6 (C(1)), 56.3 (C(2)), 66.3 (CH2Ph), 127.8, 127.9, 128.4 (o,m,p-
Ph), 137.4 (i-Ph), 157.4 (NCO), 176.2 (CO2H); m/z (ESI+) 322
Cbz NH
CO2tBu
Et3N (8.31 mL, 0.06 mol) and CbzCl (8.51 mL, 0.06 mol) were
added successively to a stirred solution of 13 (9.20 g, 0.05 mol) in
THF (100 mL) at 0 °C. The resultant mixture was then allowed to
warm to rt and stirred for 16 h before the addition of brine
(100 mL). The layers were separated and the aqueous layer was ex-
tracted withCH2Cl2 (2 ꢂ 100 mL). The combined organic layers were
then dried and concentrated in vacuo. Purification via flash column
chromatography (eluent 30–40 °C petrol/Et2O, 7:1) gave 14 as a col-
ourless oil (12.2 g, 77%, >99:1 dr); C18H25NO4 requires C, 67.7; H, 7.9;
N, 4.4; found C, 67.5; H, 7.9; N, 4.3; Rf 0.64 (30–40 °C petrol/Et2O,
([M+MeCN+NH4]+, 100%), 286 ([M+Na]+, 95%); HRMS (ESI+)
þ
C
14H17NNaO4 ([M+Na]+) requires 286.1055; found 286.1056.
4.8.1. X-ray crystal structure determination for 15
Data were collected using an Enraf-Nonius -CCD diffractome-
ter with graphite monochromated Mo K radiation using standard
j
a
procedures at 190 K. The structure was solved by direct methods
(SIR92); all non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms were added at idealised
positions. The structure was refined using CRYSTALS.26
1:1); ½a 2D3
¼ þ34:9 (c 1.0 in CHCl3); mmax (film) 3366 (N–H), 1725,
ꢃ
1704 (C@O); dH (400 MHz, CDCl3, 26 mM) 1.36–1.54 (1H, m,

E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
1811
X-ray crystal structure data for 15 [C14H17NO4]: M = 263.29,
monoclinic, space group P21, a = 15.9879(8) Å, b = 4.9448(3) Å,
c = 18.8621(13) Å, b = 112.521(2), V = 1377.46(15) Å3, Z = 4,
4.11. Cbz-[(S,S)-ACPC]2-OH 18
l
=
O
0.093 mmꢁ1
,
colourless plate, crystal dimensions = 0.1 ꢂ 0.1 ꢂ
OH
Cbz
NH
0.3 mm3. A total of 3027 unique reflections were measured for
5 < h < 27 and 2261 reflections were used in the refinement. The
final parameters were wR2 = 0.110 and R1 = 0.058 [I >2.0r(I)]. Crys-
2
tallographic data (excluding structure factors) have been deposited
with the Cambridge Crystallographic Data Centre as supplemen-
tary publication number CCDC 772469. Copies of the data can be
obtained, free of charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK [fax: +44(0) 1223 336033 or e-mail:
deposit@ccdc.cam.ac.uk].
Following General Procedure 2, 16 (2.50 g, 5.80 mmol) and TFA
(7 mL) in CH2Cl2 (21 mL) gave a colourless oil. Addition of Et2O
(40 mL) resulted in the formation of a white powder that was col-
lected by suction filtration to give 18 as a white solid (1.87 mg,
86%, >99:1 dr); C20H26N2O5 requires C, 64.15; H, 7.0; N, 7.5; found
C, 64.0; H, 7.0; N, 7.4; mp 144–146 °C; ½a D18
¼ þ28:6 (c 1.6 in
ꢃ
4.9. Cbz-[(S,S)-ACPC]2-OtBu 16
CHCl3); mmax (film) 3302 (O–H), 1694, 1554 (C@O); dH (400 MHz,
CDCl3, 26 mM) 1.42–1.55 (1H, m, C(3)HA), 1.60–2.17 (10H, m,
C(3)HA, 2 ꢂ C(3)HB, 2 ꢂ C(4)H2, 2 ꢂ C(5)HA, C(5)HB), 2.18–2.30
(1H, m, C(5)HB), 2.68–2.72 (1H, m, C(1)H), 2.75–2.79 (1H, m,
C(1)H), 3.97–4.03 (1H, m, C(2)H), 4.04–4.09 (1H, m, C(2)H), 5.03
(1H, d, J 6.1, NH), 5.11 (2H, app s, CH2Ph), 7.31–7.43 (5H, m, Ph),
8.25 (1H, br s, NH); dC (100 MHz, CD3OD) 22.8, 23.4 (2 ꢂ C(4)),
28.6, 29.4 (2 ꢂ C(5)), 32.0, 38.4 (2 ꢂ C(3)), 48.4, 50.0 (2 ꢂ C(1)),
54.4, 57.4 (2 ꢂ C(2)), 66.1 (CH2Ph), 127.2, 127.4, 127.6, 128.1,
129.1 (o,m,p-Ph), 137.7 (i-Ph), 157.0, 175.7 (C@O); m/z (ESI+) 433
O
Cbz
NH
OtBu
2
Following General Procedure 3 15 (4.20 g, 16.0 mmol), 13
(2.95 g, 16.0 mmol), HOBt (2.59 g, 19.0 mmol), EDCꢀHCl (3.67 g,
19.0 mmol) and Et3N (11.1 mL, 80.0 mol) in CHCl3 (10 mL) gave,
after purification via recrystallisation (CHCl3/heptane), 16 as a
white solid (5.30 g, 77%, >99:1 dr); C24H34N2O5 requires C, 66.95;
H, 8.0; N, 6.5; found C, 66.8; H, 8.0; N, 6.5; mp 142–144 °C
([M+MeCN+NH4]+, 100%), 375 ([M+H]+, 95%); HRMS (ESI+)
þ
C
20H27N2O5 ([M+H]+) requires 375.1920; found 375.1913.
4.11.1. X-ray crystal structure determination for 18
Data were collected using an Enraf-Nonius -CCD diffractome-
ter with graphite monochromated Mo K radiation using standard
j
(CHCl3/heptane); ½a D23
¼ þ39:0 (c 1.1 in CHCl3); mmax (film) 3324
ꢃ
a
(N–H), 1720, 1689, 1645, 1539 (C@O); dH (400 MHz, CDCl3,
26 mM) 1.37–1.55 (2H, m, 2 ꢂ C(3)HA) overlapping 1.44 (9H, s,
CMe3), 1.60–1.77 (4H, m, 2 ꢂ C(4)H2), 1.81–1.88 (2H, m,
2 ꢂ C(5)HA), 2.01–2.06 (4H, m, 2 ꢂ C(3)HB, 2 ꢂ C(5)HB), 2.52–2.62
(1H, m, C(1)H), 2.63–2.67 (1H, m, C(1)H), 3.97–4.07 (1H, m,
C(2)H), 4.33–4.45 (1H, m, C(2)H), 4.92 (1H, d, J 4.6, NH), 5.12 (2H,
s, CH2Ph), 7.32–7.41 (6H, m, NH, Ph); dC (100 MHz, CDCl3) 23.1,
24.3 (2 ꢂ C(4)), 27.3, 28.2 (2 ꢂ C(5)), 28.0 (CMe3), 32.9, 33.3
(2 ꢂ C(3)), 51.7, 52.8 (2 ꢂ C(1)), 54.5, 57.1 (2 ꢂ C(2)), 66.9 (CH2Ph),
80.2 (CMe3), 128.1, 128.3, 128.6 (o,m,p-Ph), 136.2 (i-Ph), 156.7,
174.1, 175.5 (C@O); m/z (ESI+) 489 ([M+MeCN+NH4]+, 100%);
procedures at 190 K. The structure was solved by direct methods
(SIR92); all non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms were added at idealised
positions. The structure was refined using CRYSTALS.26
X-ray crystal structure data for 18 [C20H26N2O5]: M = 374.44,
triclinic, space group P1, a = 5.9632(2) Å, b = 8.8669(3) Å,
c = 10.0968(4) Å,
a
= 76.807(2)°, b = 82.639(2)°,
c
= 75.7607(14)°,
V = 502.34(3) Å3, Z = 1,
l
= 0.089 mmꢁ1, colourless plate, crystal
dimensions = 0.1 ꢂ 0.1 ꢂ 0.2 mm3. A total of 2226 unique reflec-
tions were measured for 5 < h < 27 and 1526 reflections were used
in the refinement. The final parameters were wR2 = 0.052 and
HRMS (ESI+) C24H34N2NaO5 ([M+Na]+) requires 453.2365; found
þ
R1 = 0.046 [I >1.5r(I)].
453.2360.
Crystallographic data (excluding structure factors) have been
deposited with the Cambridge Crystallographic Data Centre as sup-
plementary publication number CCDC 772470. Copies of the data
can be obtained, free of charge, on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [fax: +44(0) 1223 336033 or e-mail:
deposit@ccdc.cam.ac.uk].
4.10. H-[(S,S)-ACPC]2-OtBu 17
O
H
NH
OtBu
4.12. Cbz-[(S,S)-ACPC]3-OtBu 19
2
O
Following General Procedure 1, 16 (1.96 g, 4.60 mmol) and
Pd(OH)2/C (25% w/w substrate, 490 mg) in MeOH (20 mL) under
H2 (1 atm) gave 17 as a white solid (1.31 g, 97%, >99:1 dr); mp
OtBu
Cbz
NH
101–103 °C; ½a 2D3
¼ þ50:1 (c 0.8 in CHCl3); mmax (film) 3290 (N–
ꢃ
3
H), 1723, 1646 (C@O); dH (400 MHz, CDCl3, 26 mM) 1.22–1.52
(2H, m, 2 ꢂ C(3)HA) overlapping 1.45 (9H, s, CMe3), 1.57–1.81
(6H, m, 2 ꢂ C(4)H2, NH2), 1.82–2.21 (7H, m, C(1)H, 2 ꢂ C(3)HB,
2 ꢂ C(5)H2), 2.45–2.53 (1H, m, C(1)H), 3.16–3.25 (1H, m, C(2)H),
4.34–4.43 (1H, m, C(2)H), 7.03 (1H, d, J 7.6 NH); dC (100 MHz,
CDCl3) 21.8, 22.8 (2 ꢂ C(4)), 26.6, 27.9 (2 ꢂ C(5)), 28.0 (CMe3),
32.9, 36.7 (2 ꢂ C(3)), 52.1, 52.9 (2 ꢂ C(1)), 54.4, 57.4 (2 ꢂ C(2)),
80.5 (CMe3), 173.8, 175.5 (C@O); m/z (ESI+) 355 ([M+MeCN+NH4]+,
Following General Procedure 3, 15 (874 mg, 3.32 mmol), 17
(984 mg, 3.32 mmol), HOBt (538 mg, 3.98 mmol), EDCꢀHCl
(764 mg, 3.98 mmol) and Et3N (2.31 mL, 16.6 mmol) in CHCl3
(10 mL) gave, after purification via recrystallisation (CHCl3/hep-
tane), 19 as a white solid (1.29 mg, 72%, >99:1 dr); mp 193–195 °C
(CHCl3/heptane); ½a D23
¼ þ46:6 (c 1.0 in CHCl3); mmax (KBr) 3319
ꢃ
(N–H), 1719, 1684, 1644, 1539 (C@O); dH (400 MHz, CDCl3,
26 mM) 1.38–1.61 (3H, m, 3 ꢂ C(3)HA) overlapping 1.43 (9H, s,
CMe3), 1.65–1.77 (6H, m, 3 ꢂ C(4)H2), 1.90–2.12 (9H, m, 3 ꢂ
100%), 297 ([M+H]+, 95%); HRMS (ESI+) C16H29N2O3 ([M+H]+) re-
þ
quires 297.2178; found 297.2180.

1812
E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
C(3)HAHB, 3 ꢂ C(5)H2), 2.43–2.64 (1H, C(1)H), 2.65–2.68 (2H, m,
2 ꢂ C(1)H), 3.96–4.05 (1H, m, C(2)H), 4.15 (1H, app t, J 6.1, C(2)H),
4.35–4.46 (1H, m, C(2)H), 5.00 (1H, d, J 6.5, NH), 5.10 (2H, s, CH2Ph),
7.30–7.41 (5H, m, Ph), 7.45 (1H, d, J 6.1, NH), 7.93 (1H, d, J 7.9, NH); dC
(100 MHz, CDCl3) 23.2, 24.0, 24.4, 27.3, 27.6, 28.4, 32.9, 33.0, 33.6
(3 ꢂ C(3), 3 ꢂ C(4), 3 ꢂ C(5)), 28.0 (CMe3), 51.7, 53.0, 53.2 (3 ꢂ
C(1)), 54.5, 55.8, 56.9 (3 ꢂ C(2)), 67.1 (CH2Ph), 80.1 (CMe3), 128.1,
128.3, 128.6 (o,m,p-Ph), 136.1 (i-Ph), 156.8 (NCO [carbamate]),
4.15. H-[(S,S)-ACPC]4-OtBu 22
O
H
NH
OtBu
4
173.1, 174.2, 174.3 (2 ꢂ NCO [amide], CO2 Bu); m/z (ESI+) 564
t
Following General Procedure 1, 20 (158 mg, 0.24 mmol) and
Pd(OH)2/C (50% w/w substrate, 79.2 mg) in MeOH (5 mL) under H2
(1 atm) gave 22 as a white solid (112 mg, 89%, >99:1 dr); mp 201–
([M+Na]+, 41%), 542 ([M+H]+, 32%), 486 ([MꢁC4H9]+, 100%); HRMS
(ESI+) C30H43N3NaO6þ ([M+Na]+) requires 564.3044; found 564.3045.
203 °C (dec); ½a 2D3
ꢃ
¼ þ59:7 (c 0.6 in CHCl3);
mmax (KBr) 3307 (N–H),
4.13. Cbz-[(S,S)-ACPC]4-OtBu 20
1722, 1653 (C@O); dH (400 MHz, CDCl3, 26 mM) 1.29–2.27 (27H,
m, C(1)H, 4 ꢂ C(3)H2, 4 ꢂ C(4)H2, 4 ꢂ C(5)H2, NH2) overlapping
1.42 (9H, s, CMe3), 2.51–2.65 (3H, m, 3 ꢂ C(1)H), 3.08–3.18 (1H, m,
C(2)H), 4.08–4.21 (2H, m, 2 ꢂ C(2)H), 4.36–4.46 (1H, m, C(2)H),
7.80 (1H, d, J 6.8, NH), 8.05 (1H, d, J 7.9, NH), 8.35 (1H, d, J 6.5, NH);
dC (100 MHz, CDCl3) 21.5, 23.3, 24.2, 24.2, 26.2, 27.5, 27.6, 28.4,
32.9, 32.9, 33.3, 37.5 (4 ꢂ C(3), 4 ꢂ C(4), 4 ꢂ C(5)), 28.0 (CMe3),
51.7, 51.8, 53.1, 53.6 (4 ꢂ C(1)), 54.4, 55.1, 55.5, 57.4 (4 ꢂ C(2)),
O
Cbz
NH
OtBu
4
Following General Procedure 3, 18 (500 mg, 1.34 mmol), 17
(396 mg, 1.34 mmol), HOBt (217 mg, 1.60 mmol), EDCꢀHCl
(307 mg, 1.60 mmol) and Et3N (0.93 mL, 6.68 mmol) in CHCl3
(7 mL) gave, after purification via recrystallisation (CHCl3/hep-
tane), 20 as a white solid (632 mg, 73%, >99:1 dr); C36H52N4O7 re-
quires C, 66.2; H, 8.0; N, 8.6; found C, 65.9; H, 7.9; N 8.5; mp 224–
80.1 (CMe3), 173.2, 174.4, 175.6 (3 ꢂ NCO, CO2 Bu); m/z (ESI+) 541
t
([M+Na]+, 42%), 519 ([M+H]+, 23%), 463 ([MꢁC4H8]+, 100%); HRMS
+
(ESI+) C28H47N4O5 ([M+H] ) requires 519.3541; found 519.3545.
þ
4.16. Cbz-[(S,S)-ACPC]5-OtBu 23
226 °C (CHCl3/heptane); ½a D23
ꢃ
¼ þ45:2 (c 0.6 in CHCl3);
m
max/cmꢁ1
(film) 3330 (br, NH), 1720, 1703, 1647, 1553 (C@O); dH
(400 MHz, CDCl3, 26 mM) 1.35–1.53 (4H, m, 4 ꢂ C(3)HA) overlap-
ping 1.43 (9H, s, CMe3), 1.65–1.98 (20H, m, 4 ꢂ C(3)AHB,
4 ꢂ C(4)H2, 4 ꢂ C(5)H2), 2.31–2.40 (1H, m, C(1)H), 2.45–2.52 (1H,
m, C(1)H), 2.55–2.61 (1H, m, C(1)H), 2.64–2.70 (1H, m, C(1)H),
4.01 (1H, app qt, J 6.8, C(2)H), 4.14–4.26 (2H, m, 2 ꢂ C(2)H),
4.36–4.46 (1H, m, C(2)H), 5.09 (2H, s, CH2Ph), 5.18 (1H, d, J 6.5,
NH), 7.08 (1H, d, J 6.8, NH), 7.30–7.41 (5H, m, Ph), 7.86 (1H, d, J
6.5, NH), 8.10 (1H, d, J 7.5, NH); dC (125 MHz, CDCl3) 22.2, 23.0,
23.3, 23.5 (4 ꢂ C(4)), 26.2, 26.5, 26.9, 27.0, 27.5 (4 ꢂ C(5), CMe3),
31.8, 31.9, 32.2, 32.3 (4 ꢂ C(3)), 49.0, 51.5, 51.8, 52.0 (4 ꢂ C(1)),
52.3, 53.4, 54.5, 56.1 (4 ꢂ C(2)), 66.1 (CH2Ph), 79.0 (CMe3), 127.1,
127.2, 127.3, 127.4, 127.6 (o,m,p-Ph), 135.0 (i-Ph), 155.7, 172.3,
173.3, 173.4, 173.5 (C@O); m/z (ESI+) 711 ([M+MeCN+NH4]+,
O
OtBu
Cbz
NH
5
Following General Procedure 3, 18 (66.0 mg, 0.18 mmol), 21
(71.8 mg, 0.18 mmol), HOBt (28.6 mg, 0.21 mmol), EDCꢀHCl
(40.5 mg, 0.21 mmol) and Et3N (0.12 mL, 0.88 mmol) in CHCl3
(3 mL) gave, after purification via recrystallisation (CHCl3/heptane),
23 as a white solid (74.9 mg, 56%, >99:1 dr); mp 232–234 °C (CHCl3/
heptane); ½a 2D3
¼ þ53:8 (c 0.3 in CHCl3); mmax (KBr) 3279 (N–H),
ꢃ
1716, 1698, 1640, 1563 (C@O); dH (400 MHz, CDCl3, 14 mM) 1.20–
2.23 (16H, m, C(1)H, 5 ꢂ C(3)H2, 5 ꢂ C(4)H2, 5 ꢂ C(5)H2) overlapping
1.43 (1H, s, CMe3), 2.31–2.52 (1H, m, C(1)H), 2.54–2.68 (2H, m,
2 ꢂ C(1)H), 2.70–2.83 (1H, m, C(1)H), 4.01–4.07 (1H, m, C(2)H),
4.20–4.26 (3H, m, 3 ꢂ C(2)H), 4.36–4.49 (1H, m, C(2)H), 5.07 (1H,
d, J 12.3, CHAHBPh), 5.13 (1H, d, J 12.3, CHAHBPh), 5.28 (1H, d, J 7.5,
NH), 6.49 (1H, d, J 7.9, NH), 7.22–7.43 (5H, m, Ph), 7.47 (1H, d, J 8.2,
NH), 8.09(1H, d, J 7.2, NH), 8.19 (1H, d, J 7.5, NH); dC (125 MHz, CDCl3)
23.8, 24.4, 24.6, 24.8, 27.7, 28.2, 28.4, 28.5, 28.6, 32.7, 33.3, 33.5, 33.6,
33.7 (5 ꢂ C(3), 5 ꢂ C(4), 5 ꢂ C(5)), 28.0 (CMe3), 51.3, 52.4, 52.6, 53.6,
53.6, 54.4, 55.3, 55.4, 57.3 (5 ꢂ C(1), 5 ꢂ C(2)), 67.0 (CH2Ph), 79.9
(CMe3), 127.8, 127.9, 128.4, 128.6 (o,m,p-Ph), 136.2 (i-Ph), 156.5
(NCO [carbamate]), 173.8, 174.1, 174.5, 174.8 (4 ꢂ NCO [amide],
100%), 675 ([M+Na]+, 50%), 653 ([M+H]+, 20%); HRMS (ESI+)
þ
C
36H53N4O7 ([M+H]+) requires 653.3914; found 653.3915.
4.14. H-[(S,S)-ACPC]3-OtBu 21
O
H
NH
OtBu
3
CO2 Bu); m/z (ESI+) 786 ([M+Na]+, 52%), 764 ([M+H]+, 58%), 708
t
Following General Procedure 1, 19 (405 mg, 0.75 mmol) and
Pd(OH)2/C (25% w/w substrate, 101 mg) in MeOH (10 mL) under
H2 (1 atm) gave 21 as a white solid (272 mg, 89%, >99:1 dr); mp
[(MꢁC4H8]+, 100%); HRMS (ESI+) C42H62N5O8 ([M+H]+) requires
þ
764.4593; found 764.4597.
198–200 °C (dec); ½a D23
¼ þ46:4 (c 0.9 in CHCl3); mmax (KBr) 3264
ꢃ
4.17. Cbz-[(S,S)-ACPC]6-OtBu 24
(N–H), 1725, 1634 (C@O); dH (400 MHz, CDCl3, 26 mM) 1.24–1.78
(11H, m, 3 ꢂ C(3)HA, 3 ꢂ C(4)H2, NH2) overlapping 1.43 (9H, s,
CMe3), 1.79–2.26 (10H, m, C(1)H, 3 ꢂ C(3)HB, 3 ꢂ C(5)H2), 2.50–
2.67 (2H, m, 2 ꢂ C(1)H), 3.07–3.17 (1H, m, C(2)H), 4.12–4.21 (1H,
m, C(2)H), 4.35–4.45 (1H, m, C(2)H), 7.68 (1H, d, J 6.3, NH), 8.14
(1H, d, J 7.6, NH); dC (100 MHz, CDCl3) 21.5, 23.2, 24.3, 26.2, 27.3,
28.3, 33.0, 33.1, 37.5 (3 ꢂ C(3), 3 ꢂ C(4), 3 ꢂ C(5)), 28.0 (CMe3),
51.7, 52.0, 53.5 (3 ꢂ C(1)), 54.5, 55.3, 57.4 (3 ꢂ C(2)), 80.1 (CMe3),
O
Cbz
NH
OtBu
6
172.9, 174.3, 175.5 (2 ꢂ NCO, CO2 Bu); m/z (ESI+) 430 ([M+Na]+,
Following General Procedure 3, 18 (121 mg, 0.32 mmol), 22
(168 mg, 0.32 mmol), HOBt (52.5 mg, 0.39 mmol), EDCꢀHCl
(74.5 mg, 0.39 mmol) and Et3N (0.20 mL, 1.62 mmol) in CHCl3
t
95%), 352 ([MꢁC4H9]+, 100%); HRMS (ESI+) C22H38N3O4 ([M+H]+)
þ
requires 408.2857; found 408.2857.

E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
1813
(40 mL) gave, after purification via flash column chromatography
can be obtained, free of charge, on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [fax: +44(0) 1223 336033 or e-mail:
deposit@ccdc.cam.ac.uk].
(eluent CHCl3/MeOH, 20:1), 24 (187 mg, 66%, >99:1 dr) as a white
solid; Rf 0.48 (CHCl3/MeOH, 10:1); mp 185–187 °C; ½a D20
¼ þ49:2 (c
ꢃ
0.2 in CHCl3); mmax (KBr) 3298 (N–H), 1723, 1704, 1648, 1556
(C@O); dH (400 MHz, CDCl3, 7 mM) 1.31–2.20 (37H, m, C(1)H,
6 ꢂ C(3)H2, 6 ꢂ C(4)H2, 6 ꢂ C(5)H2) overlapping 1.44 (9H, s,
CMe3), 2.29–2.36 (2H, m, 2 ꢂ C(1)H), 2.41–2.46 (1H, m, C(1)H),
2.62–2.71 (1H, m, C(1)H), 2.79–2.89 (1H, m, C(1)H), 3.91–3.98
(1H, m, C(2)H), 4.22–4.34 (4H, m, 4 ꢂ C(2)H), 4.38–4.48 (1H, m,
C(2)H), 5.06 (1H, d, J 12.5, CHAHBPh), 5.50 (1H, d, J 12.5, CHAHBPh),
5.50 (1H, d, J 7.9, NH), 6.30 (1H, d, J 7.9, NH), 4.26–7.41 (6H, m, NH,
Ph), 8.10 (1H, d, J 8.5, NH), 8.22 (1H, d, J 7.9, NH), 8.31 (1H, d, J 7.5,
NH); dC (100 MHz, CDCl3) 22.6, 23.8, 24.4, 24.7, 24.8, 25.2
(6 ꢂ C(4)), 27.2, 28.0, 28.1, 28.2, 28.3, 28.5, 29.0 (6 ꢂ C(5), CMe3),
31.8, 32.3, 32.6, 33.0, 33.1, 33.3 (6 ꢂ C(3)), 50.5, 51.1, 51.2, 52.1,
52.2, 52.3 (6 ꢂ C(1)), 54.2, 54.3, 55.2, 55.3, 55.4, 56.0 (6 ꢂ C(2)),
66.6 (CH2Ph), 79.9 (CMe3), 127.7, 128.1, 128.5 (o,m,p-Ph), 136.6
(i-Ph), 156.7 (NCO [carbamate]), 173.3, 173.4, 173.5, 174.4, 174.6,
4.19. Boc-[(S,S)-ACPC]3-OtBu 26
O
Boc NH
OtBu
3
Following General Procedure 4, 19 (196 mg, 0.36 mmol), Boc2O
(86.7 mg, 0.40 mmol) and Pd(OH)2/C (25% w/w substrate, 49 mg)
in MeOH (10 mL) gave, after purification via recrystallisation
(CHCl3/heptane), 26 as a white solid (147 mg, 80%, >99:1 dr); mp
208–210 °C (CHCl3/heptane); ½a D23
¼ þ36:2 (c 0.5 in CHCl3); mmax
ꢃ
(2 mM solution in CHCl3) 3441, 3280 (N–H); mmax (KBr) 3424
(N–H), 1717, 1646, 1559, 1541 (C@O); dH (400 MHz, CDCl3,
26 mM) 1.36–1.62 (3H, m, 3 ꢂ C(3)HA) overlapping 1.43 (9H, s,
CMe3) and 1.45 (9H, s, CMe3), 1.65–1.80 (6H, m, 3 ꢂ C(4)H2),
1.80–2.19 (9H, m, 3 ꢂ C(3)HB, 3 ꢂ C(5)H2), 2.52–2.67 (3H, m,
3 ꢂ C(1)H), 3.91–4.00 (1H, m, C(2)H), 4.12–4.20 (1H, m, C(2)H),
4.37–4.46 (1H, m, C(2)H), 4.71 (1H, d, J 7.1, NH), 4.80 (1H, d, J
5.6, NH), 8.03 (1H, d, J 7.6, NH); dC (100 MHz, CDCl3) 23.2, 23.9,
24.4, 27.3, 27.8, 33.0, 33.7 (3 ꢂ C(3), 3 ꢂ C(4), 3 ꢂ C(5)), 28.0, 28.4
(2 ꢂ CMe3), 51.7, 53.3, 53.5 (3 ꢂ C(1)), 54.5, 55.8, 56.1 (3 ꢂ C(2)),
80.1, 80.3 (2 ꢂ CMe3), 156.5 (NCO [carbamate]), 173.1, 174.3
174.8 (5 ꢂ NCO [amide], CO2 Bu); m/z (ESI+) 933 ([M+MeCN+NH4]+,
t
100%), 897 ([M+Na]+, 50%); HRMS (ESI+) C48H71N6O9 ([M+H]+) re-
þ
quires 875.5283; found 875.5313.
4.18. Boc-[(S,S)-ACPC]2-OtBu 25
O
Boc NH
OtBu
(2 ꢂ NCO [amide], CO2 Bu); m/z (ESI+) 530 ([M+Na]+, 100%) 509
t
2
([M+H]+, 25%); HRMS (ESI+) C27H46N3O6 ([M+H]+) requires
þ
Following General Procedure 4, 16 (148 mg, 0.34 mmol), Boc2O
(82.4 mg, 0.38 mmol) and Pd(OH)2/C (25% w/w substrate, 37 mg)
in MeOH (10 mL) gave, after purification via trituration with
Et2O, 25 as a white crystalline solid (110 mg, 81%, >99:1 dr);
508.3381; found 508.3385.
4.20. Boc-[(S,S)-ACPC]4-OtBu 27
C
21H36N2O5 requires C, 63.6; H, 9.2; N, 7.1; found C, 63.7; H, 9.2;
N, 7.0; mp 200–202 °C (CHCl3/heptane); ½a D18
¼ þ33:1 (c 0.7 in
ꢃ
O
CHCl3); mmax (KBr) 3317 (N–H), 1718, 1684, 1652, 1558 (C@O); dH
(400 MHz, CDCl3, 26 mM) 1.35–1.57 (2H, m, 2 ꢂ C(3)HA) overlap-
ping 1.44 (9H, s, CMe3) and 1.45 (9H, s, CMe3), 1.63–1.78 (4H, m,
2 ꢂ C(4)H2), 1.81–1.92 (2H, m, 2 ꢂ C(5)HA), 1.94–2.14 (4H, m,
2 ꢂ C(3)HB, 2 ꢂ C(5)HB), 2.53–2.65 (2H, m, 2 ꢂ C(1)H), 3.91–4.00
(1H, m, C(2)H), 4.39 (1H, app qt, J 7.1 C(2)H), 4.67 (1H, d, J 6.1,
NH), 7.69 (1H, br s, NH); dC (100 MHz, CDCl3) 23.1, 24.1, 27.5,
28.3, 32.9, 33.5 (2 ꢂ C(3), 2 ꢂ C(4), 2 ꢂ C(5)), 28.0, 28.4 (2 ꢂ CMe3),
51.6, 55.3 (2 ꢂ C(1)), 54.5, 56.3 (2 ꢂ C(2)), 80.0, 80.1 (2 ꢂ CMe3),
Boc NH
OtBu
4
Following General Procedure 4, 20 (76.2 mg, 0.12 mmol), Boc2O
(28.0 mg, 0.13 mmol) and Pd(OH)2/C (50% w/w substrate, 38.1 mg)
in MeOH (10 mL) gave, after purification via recrystallisation
(CHCl3/heptane), 27 as a white solid (47.0 mg, 65%, >99:1 dr); mp
233–235 °C (dec); ½a D23
¼ þ48:7 (c 0.4 in CHCl3); mmax (2 mM solu-
ꢃ
t
156.4 (NCO [carbamate]), 172.9, 174.2 (2 ꢂ NCO [amide], CO2 Bu);
tion in CHCl3) 3438, 3340, 3273 (N–H); mmax (KBr) 3300 (N–H),
1720, 1690, 1647, 1553 (C@O); dH (400 MHz, CDCl3, 26 mM)
1.36–2.18 (24H, m, 4 ꢂ C(3)H2, 4 ꢂ C(4)H2, 4 ꢂ C(5)H2) overlapping
1.43 (9H, s, CMe3) and 1.45 (9H, s, CMe3), 2.42–2.49 (2H, m,
2 ꢂ C(1)H), 2.55–2.63 (1H, m, C(1)H), 2.62–2.69 (1H, m, C(1)H),
3.90–4.01 (1H, m, C(2)H), 4.14–4.25 (2H, m, 2 ꢂ C(2)H), 4.36–
4.45 (1H, m, C(2)H), 4.80 (1H, d, J 7.2, NH), 7.33 (1H, d, J 7.5, NH),
7.98 (1H, d, J 6.8, NH), 8.09 (1H, d, J 7.5, NH); dC (100 MHz, CDCl3)
23.4, 23.7, 24.5, 27.5, 28.1, 28.6, 30.9, 32.8, 33.3, 33.3, 33.6
(4 ꢂ C(3), 4 ꢂ C(4), 4 ꢂ C(5)), 28.0, 28.4 (2 ꢂ CMe3), 51.5, 52.7,
53.5, 53.6 (4 ꢂ C(1)), 54.4, 55.4, 55.5, 56.5 (4 ꢂ C(2)), 80.0, 80.1
(2 ꢂ CMe3), 156.3 (NCO [carbamate]), 173.4, 174.4, 174.6 (3 ꢂ NCO
m/z (ESI+) 419 ([M+Na]+, 100%), 397 ([M+H]+, 24%); HRMS (ESI+)
21H36N2NaO5 ([M+Na]+) requires 419.2516; found 419.2515.
þ
C
4.18.1. X-ray crystal structure determination for 25
Data were collected using an Enraf-Nonius -CCD diffractome-
ter with graphite monochromated Mo K radiation using standard
procedures at 190 K. The structure was solved by direct methods
(SIR92); all non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms were added at idealised
positions. The structure was refined using CRYSTALS.26
j
a
X-ray crystal structure data for 25 [C21H36N2O5]: M = 396.53,
orthorhombic, space group P212121, a = 5.25520(10) Å, b =
[amide], CO2 Bu); m/z (ESI+) 619 ([M+H]+, 92%), 563 ([MꢁC4H9]+,
t
100%); HRMS (ESI+) C33H54N4NaO7 ([M+Na]+) requires 641.3885;
þ
18.6766(3) Å, c = 22.3554(4) Å, V = 2194.17(7) Å3, Z = 4,
l = 0.085
mmꢁ1
,
colourless plate, crystal dimensions = 0.05 ꢂ 0.05 ꢂ 0.1
found 641.3881.
mm3. A total of 2876 unique reflections were measured for
5 < h < 27 and 1582 reflections were used in the refinement. The fi-
4.20.1. X-ray crystal structure determination for 27
nal parameters were wR2 = 0.035 and R1 = 0.030 [I >3.0r(I)].
Data were collected using an Enraf-Nonius
j-CCD diffractome-
Crystallographic data (excluding structure factors) have been
deposited with the Cambridge Crystallographic Data Centre as sup-
plementary publication number CCDC 772471. Copies of the data
ter with graphite monochromated Mo K radiation using standard
procedures at 190 K. The structure was solved by direct methods
(SIR92); all non-hydrogen atoms were refined with anisotropic
a

1814
E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
thermal parameters. Hydrogen atoms were added at idealised
positions. The structure was refined using CRYSTALS.26
4.22. Boc-[(S,S)-ACPC]6-OtBu 29
X-ray crystal structure data for 27 [C33H54N4O7]: M = 618.81, tri-
clinic, space group P1, a = 9.3085(3) Å, b = 9.2742(3) Å, c =
O
Boc NH
OtBu
11.2806(6) Å,
a = 93.1887(17)°, b = 93.0542(17)°, c = 113.577(2)°,
V = 888.13(6) Å3, Z = 1,
l
= 0.081 mmꢁ1, colourless plate, crystal
dimensions = 0.1 ꢂ 0.1 ꢂ 0.3 mm3. A total of 3520 unique reflec-
tions were measured for 5 < h < 27 and 2585 reflections were used
in the refinement. The final parameters were wR2 = 0.101 and
6
Following General Procedure 4, 24 (120 mg, 0.14 mmol), Boc2O
(36.0 mg, 0.17 mmol) and Pd(OH)2/C (25% w/w substrate, 60.2 mg)
in MeOH (10 mL) gave the crude reaction mixture. The residue was
dissolved in MeOH (60 mL), NaHCO3 (12.3 mg, 0.15 mmol) was
added and the mixture was allowed to stand in an ultrasonic bath
for 24 h at rt. The reaction mixture was then diluted with CHCl3
(100 mL) and washed sequentially with 1 M aq HCl (2 ꢂ 100 mL)
and brine (100 mL), then dried and concentrated in vacuo. Purifica-
tion via flash column chromatography (gradient elution, 0?18%
MeOH in CHCl3) gave 29 as a white crystalline solid (27.6 mg,
23%, >99:1 dr); Rf 0.04 (CHCl3/MeOH, 20:1); mp 250–251 °C;
R1 = 0.077 [I >2.0r(I)].
Crystallographic data (excluding structure factors) have been
deposited with the Cambridge Crystallographic Data Centre as sup-
plementary publication number CCDC 772472. Copies of the data
can be obtained, free of charge, on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [fax: +44(0) 1223 336033 or e-mail:
deposit@ccdc.cam.ac.uk].
4.21. Boc-[(S,S)-ACPC]5-OtBu 28
½
a 2D0
ꢃ
¼ þ48:7 (c 1.1 in CHCl3); mmax (2 mM solution in CHCl3)
O
3439, 3326 (N–H); dH (400 MHz, CDCl3, 26 mM) 1.31–2.53 (40H,
m, 4 ꢂ C(1)H, 6 ꢂ C(3)H2, 6 ꢂ C(4)H2, 6 ꢂ C(5)H2) overlapping
1.43 (9H, s, CMe3) and 1.44 (9H, s, CMe3), 2.60–2.71 (1H, m,
C(1)H), 2.80–2.91 (1H, m, C(1)H), 3.98–4.31 (1H, m, C(2)H), 4.20–
4.31 (4H, m, 4 ꢂ C(2)H), 4.37–4.43 (1H, m, C(2)H), 5.62 (1H, d, J
8.9, NH), 6.86 (1H, d, J 8.2, NH), 7.61 (1H, d, J 9.2, NH), 8.25–8.35
(2H, m, 2 ꢂ NH), 8.40 (1H, d, J 7.5, NH); dC (125 MHz, CDCl3) 23.3,
23.5, 24.5, 24.7, 25.0, 28.2, 28.3, 28.9, 29.3, 29.7, 32.6, 33.2, 33.4,
33.5 (6 ꢂ C(3), 6 ꢂ C(4), 6 ꢂ C(5)), 28.1, 28.5 (2 ꢂ CMe3), 51.2,
52.1, 52.3, 52.3, 53.0 (6 ꢂ C(1)), 54.3, 55.2, 55.3, 57.2 (6 ꢂ C(2)),
79.7, 79.9 (2 ꢂ CMe3), 156.2 (NCO [carbamate]), 174.3, 174.5,
Boc NH
OtBu
5
Following General Procedure 4, 23 (173 mg, 0.23 mmol), Boc2O
(54.5 mg, 0.25 mmol) and Pd(OH)2/C (25% by wt, 86.7 mg) in
MeOH (20 mL) gave, after purification via recrystallisation
(CHCl3/heptane), 28 (101 mg, 61%, >99:1 dr) as a white solid; mp
230–232 °C (CHCl3/heptane); ½a D23
¼ þ45:8 (c 0.6 in CHCl3); mmax
ꢃ
(2 mM solution in CHCl3) 3436, 3336 (N–H); dH (400 MHz, CDCl3,
26 mM) 1.30–2.18 (30H, m, 5 ꢂ C(3)H2, 5 ꢂ C(4)H2, 5 ꢂ C(5)H2)
overlapping 1.44 (18H, app s, 2 ꢂ CMe3), 2.21–2.68 (4H, m,
4 ꢂ C(1)H), 2.72–2.84 (1H, m, C(1)H), 3.92–4.07 (1H, m, C(2)H),
4.14–4.34 (3H, m, 3 ꢂ C(2)H), 4.36–4.50 (1H, m, C(2)H), 5.09
(1H, d, J 7.5, NH), 6.74 (1H, d, J 7.5, NH), 7.65 (1H, d, J 8.2, NH),
8.15 (1H, d, J 7.2, NH), 8.20 (1H, d, J 7.2, NH); dC (100 MHz, CDCl3)
23.4, 23.5, 24.6, 24.8, 27.9, 28.1, 28.8, 28.9, 32.7, 33.2, 33.4, 33.5
(5 ꢂ C(3), 5 ꢂ C(4), 5 ꢂ C(5)), 28.0, 28.4 (2 ꢂ CMe3), 51.3, 52.4,
52.5, 53.3, 54.1, 54.3, 55.3, 55.4, 56.9 (5 ꢂ C(1), 5 ꢂ C(2)), 79.9
(2 ꢂ CMe3), 156.2 (NCO [carbamate]), 174.1, 174.4, 174.6,
174.5, 174.8, 175.1 (5 ꢂ NCO [amide], CO2 Bu); m/z (ESI+) 864
t
([M+Na]+, 84%), 842 ([M+H]+, 100%); HRMS (ESI+) C45H73N6O9
þ
([M+H]+) requires 841.5434; found 841.5434.
4.22.1. X-ray crystal structure determination for 29
Data were collected using an Enraf-Nonius
ter with graphite monochromated Mo K radiation using standard
procedures at 190 K. The structure was solved by direct methods
(SIR92); all non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms were added at idealised
positions. The structure was refined using CRYSTALS.26
j-CCD diffractome-
a
174.9 (4 ꢂ NCO [amide], CO2 Bu); m/z (ESI+) 789 ([M+NH4+
t
MeCN]+, 52%), 752 ([M+Na]+, 89%), 730 ([M+H]+, 100%); HRMS
X-ray crystal structure data for 29 [C45H72N6O9]: M = 841.10,
tetragonal, space group P43, a = 10.3345(2) Å, b = 10.3358(2) Å,
(ESI+) C39H63N5NaO8
752.4557.
([M+Na]+) requires 752.4569; found
þ
c = 44.0581(9) Å, V = 4706.08(16) Å3, Z = 4,
l
= 0.083 mmꢁ1, colour-
less block, crystal dimensions = 0.2 ꢂ 0.2 ꢂ 0.2 mm3. A total of
4699 unique reflections were measured for 5 < h < 27 and 4120
reflections were used in the refinement. The final parameters were
4.21.1. X-ray crystal structure determination for 28
Data were collected using an Enraf-Nonius -CCD diffractome-
ter with graphite monochromated Mo K radiation using standard
j
wR2 = 0.178 and R1 = 0.106 [I >ꢁ3.0
r(I)].
a
Crystallographic data (excluding structure factors) have been
deposited with the Cambridge Crystallographic Data Centre as sup-
plementary publication number CCDC 772474. Copies of the data
can be obtained, free of charge, on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [fax: +44(0) 1223 336033 or e-mail:
deposit@ccdc.cam.ac.uk].
procedures at 190 K. The structure was solved by direct methods
(SIR92); all non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms were added at idealised
positions. The structure was refined using CRYSTALS.26
X-ray crystal structure data for 28 [C39H63N5O8]: M = 729.96,
monoclinic, space group P21, a = 9.1420(3) Å, b = 19.7069(5) Å,
c = 11.6211(4) Å, b = 92.5033(12)°, V = 2091.67(11) Å3, Z = 2,
l =
0.81 mmꢁ1
,
colourless plate, crystal dimensions = 0.1 ꢂ 0.2 ꢂ
4.23. Cbz-[(S,S)-ACPC]3-OH 30
0.3 mm3. A total of 4852 unique reflections were measured for
5 < h < 27 and 2848 reflections were used in the refinement. The fi-
O
nal parameters were wR2 = 0.085 and R1 = 0.071 [I >1.9r(I)].
Cbz NH
OH
Crystallographic data (excluding structure factors) have been
deposited with the Cambridge Crystallographic Data Centre as sup-
plementary publication number CCDC 772473. Copies of the data
can be obtained, free of charge, on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [fax: +44(0) 1223 336033 or e-mail:
deposit@ccdc.cam.ac.uk].
3
Following General Procedure 2, 19 (250 mg, 0.46 mmol) and
TFA (0.70 mL) in CH2Cl2 (2.10 mL) gave, after purification via tritur-
ation with Et2O, 30 as a white solid (171 mg, 76%, >99:1 dr); mp

E. Abraham et al. / Tetrahedron: Asymmetry 21 (2010) 1797–1815
1815
213–215 °C; ½a 2D3
¼ þ29:1 (c 0.3 in MeOH); mmax (KBr) 3313 (N–H,
ꢃ
8. Appella, D. H.; Christianson, L. A.; Klein, D. A.; Powell, D. R.; Huang, X.; Barchi, J.
J., Jr.; Gellman, S. H. Nature 1997, 387, 381; Applequist, J.; Bode, K. A.; Appella,
D. H.; Christianson, L. A.; Gellman, S. H. J. Am. Chem. Soc. 1998, 120, 4891.
O–H), 1706, 1649, 1560 (C@O); dH (400 MHz, CDCl3) 1.23–2.31
(18H, m, 3 ꢂ C(3)H2, 3 ꢂ C(4)H2, 3 ꢂ C(5)H2), 2.34–2.51 (2H, m,
2 ꢂ C(1)H), 2.89–2.99 (1H, m, C(1)H), 3.94–4.14 (2H, m,
2 ꢂ C(2)H), 4.15–4.27 (1H, m, C(2)H), 5.00 (1H, d, J 7.2, NH), 5.08
(1H, d, J 12.3, CHAHBPh), 5.13 (1H, d, J 12.3, CHAHBPh), 6.89 (1H,
d, J 7.9, NH), 7.31–7.45 (5H, m, Ph), 8.58 (1H, br s, NH); dC
(125 MHz, CDCl3) 23.9, 24.2, 25.4, 28.0, 28.0, 28.2, 33.0, 33.2,
33.7 (3 ꢂ C(3), 3 ꢂ C(4), 3 ꢂ C(5)), 53.0, 53.2, 53.5 (3 ꢂ C(1)), 55.4,
55.7, 57.2 (3 ꢂ C(2)), 67.2 (CH2Ph), 128.0, 128.4, 128.6 (o,m,p-Ph),
136.0 (i-Ph), 156.6 (NCO [carbamate]), 174.5, 175.3, 177.0 (2 ꢂ NCO
[amide], CO2H); m/z (ESI+) 545 ([M+NH4+MeCN]+, 15%), 508
9. The ability to adopt alternative conformations is
a consequence of the
relationship between alkyl ring size and torsional angle h. Transhexacin
favours a torsional angle h of around 60°, which specifically stabilises the 14-
helix, whereas the smaller ring size of transpentacin biases h towards larger
values, rendering the 12-helix the most favourable conformer, see: Banerjee,
A.; Balaram, P. Curr. Sci. 1997, 73, 1067.
10. For instance, see: Wang, X.; Espinosa, J. F.; Gellman, S. H. J. Am. Chem. Soc. 2000,
122, 4821; Lee, H.-S.; Syud, F. A.; Wang, X.; Gellman, S. H. J. Am. Chem. Soc.
2001, 123, 7721; Winkler, J. D.; Piatnitski, E. L.; Mehlmann, J.; Kasparec, J.;
Axelsen, P. H. Angew. Chem., Int. Ed. 2001, 40, 743; Woll, M. G.; Fisk, J. D.; LePlae,
P. R.; Gellman, S. H. J. Am. Chem. Soc. 2002, 124, 12447; Raguse, L.; Lai, J. R.;
Gellman, S. H. J. Am. Chem. Soc. 2003, 125, 5592; Park, J.-S.; Lee, H.-S.; Lai, J. R.;
Kim, B. M.; Gellman, S. H. J. Am. Chem. Soc. 2003, 125, 8539; Peelen, T. J.; Chi, Y.;
English, E. P.; Gellman, S. H. Org. Lett. 2004, 6, 4411; Simpson, G. L.; Gordon, A.
H.; Lindsay, D. M.; Promsawan, N.; Crump, M. P.; Mulholland, K.; Hayter, B. R.;
Gallagher, T. J. Am. Chem. Soc. 2006, 128, 10638; Martinek, T. A.; Mándity, I. M.;
Fülöp, L.; Tóth, G. K.; Vaas, E.; Hollósi, M.; Forró, E.; Fülöp, F. J. Am. Chem. Soc.
2006, 128, 13539.
([M+Na]+, 100%), 486 ([M+H]+, 60%); HRMS (ESI+) C26H35N3NaO6
þ
([M+Na]+) requires 508.2418; found 508.2417.
4.23.1. X-ray crystal structure determination for 30
11. Throughout this manuscript, the cyclopentane rings are represented as
puckered on C(2) for clarity; however, a conformational preference is not
implied.
12. (a) Davies, S. G.; Ichihara, O.; Walters, I. A. S. Synlett 1993, 461; (b) Davies, S. G.;
Ichihara, O.; Lenoir, I.; Walters, I. A. S. J. Chem. Soc., Perkin Trans. 1 1994, 1411;
Data were collected using an Enraf-Nonius
j-CCD diffractome-
ter with graphite monochromated Mo K radiation using standard
a
procedures at 190 K. The structure was solved by direct methods
(SIR92); all non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms were added at idealised
positions. The structure was refined using CRYSTALS.26
a
,b-Unsaturated ester 10 was synthesised by an analogous procedure to that
outlined by Davies, S. G.; Garner, A. C.; Long, M. J. C.; Smith, A. D.; Sweet, M. J.;
Withey, J. M. Org. Biomol. Chem. 2004, 2, 3355.
13. Recrystallisation of an aliquot gave a sample of 11 in >99:1 dr that was
subjected to single crystal X-ray analysis, thus unambiguously confirming the
X-ray crystal structure data for 30 [C26H35N3O6]: M = 485.58,
tetragonal, space group P41, a = 10.15760(10) Å, c = 24.3046(5) Å,
relative cis-configuration, with the absolute (1R,2S,aS)-configuration being
V = 2507.67(6) Å3, Z = 4,
l
= 0.092 mmꢁ1, colourless plate, crystal
assigned from the known (S)-configuration of the
a-methylbenzyl
dimensions = 0.2 ꢂ 0.2 ꢂ 0.3 mm3. A total of 2908 unique reflec-
tions were measured for 5 < h < 27 and 2360 reflections were used
in the refinement. The final parameters were wR2 = 0.056 and
stereocentre.
14. The presence of water resulted in partial hydrolysis of the tert-butyl ester,
thought to arise through the in situ production of potassium hydroxide. This
phenomenon was also observed by McErlean, C. S. P.; Moody, C. J. J. Org. Chem.
2007, 72, 10298.
15. Single crystal X-ray analysis of 12 unambiguously confirmed the trans-relative
configuration, with the absolute (S,S,S)-configuration being assigned from the
R1 = 0.046 [I >2.5r(I)].
Crystallographic data (excluding structure factors) have been
deposited with the Cambridge Crystallographic Data Centre as sup-
plementary publication number CCDC 772475. Copies of the data
can be obtained, free of charge, on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [fax: +44(0) 1223 336033 or e-mail:
deposit@ccdc.cam.ac.uk].
known (S)-configuration of the a-methylbenzyl stereocentre.
16. Single crystal X-ray analysis of 15 unambiguously confirmed the relative trans-
configuration, indicating that the stereochemical integrity of the molecule was
not compromised using this protecting group strategy. The absolute (S,S)-
configurations within 13, 14 and 15 could therefore also be confidently
assigned from the known absolute configurations of 11 and 12.
17. A sample of trimer acid 30 was prepared via treatment of trimer 19 with TFA in
CH2Cl2.
18. Structures were overlaid using the Chem-3D structure mapping function.
19. This procedure took advantage of the fact that the C-terminus C(1)H proton
forms just one short-range NOE correlation to one NH proton, when all
remaining C(1)H protons form short-range NOE correlations to two NH
protons. Similarly the N-terminus NH proton forms just one short-range NOE
correlation to one C(1)H proton when all remaining NH protons form short-
range NOE correlations to two C(1)H protons; see Ref. 7b.
20. NMR structure calculations for all the transpentacin oligomers was performed
using the XPLOR software package (Brünger, A.T. XPLOR Version 3.1: A system for
X-ray crystallography and NMR, Yale University Press, New Haven CT, 1992)
with the parameter and topology files modified so they are appropriate for b-
amino acids. For each peptide the NOE intensities were categorised as either
very strong, strong, medium, weak or very weak. The corresponding distance
restraints used in the structure calculations for the five categories were 1.8–
2.5 Å (very strong), 1.8–3.0 Å (strong), 1.8–3.5 Å (medium), 1.8–4.0 Å (weak)
and 1.8–5.0 Å (very weak). Pseudoatoms were used in the structure
calculations where no stereospecific assignments have been made. Where
hydrogen bond restraints were used in the structure refinement, the distance
restraints used were NH(i)-O(j), 1.3–2.3 Å and N(i)-O(j), 2.3–3.3 Å. A simulated
annealing protocol was used with ensembles of 20 structures being calculated
and the lowest energy 10 structures used for analysis.
References
1. (a) Seebach, D.; Overhand, M.; Kuhnle, F. N. M.; Martinoni, B.; Oberer, L.;
Hommel, U.; Widmer, H. Helv. Chim. Acta 1996, 79, 913; (b) Hintermann, T.;
Seebach, D. Chimia 1997, 50, 244; (c) Seebach, D.; Abele, S.; Schreiber, J. V.;
Martinoni, B.; Nussbaum, A. K.; Schild, H.; Schulz, H.; Hennecke, H.; Woessner,
R.; Bitsch, F. Chimia 1998, 52, 734; (d) Seebach, D.; Hook, D. F.; Glätti, A.
Biopolymers (Peptide Science) 2006, 84, 23.; (e) Aguilar, M.-I.; Purcell, A. W.;
Devi, R.; Lew, R.; Rossjohn, J.; Smith, I.; Perlmutter, P. Org. Biomol. Chem. 2007,
5, 2884.
2. Werder, M.; Hauser, H.; Abele, S.; Seebach, D. Helv. Chim. Acta 1999, 82, 1774.
3. Hamuro, Y.; Schneider, J. P.; DeGrado, W. F. J. Am. Chem. Soc. 1999, 121, 12200;
Liu, D.; DeGrado, W. F. J. Am. Chem. Soc. 2001, 123, 7553.
4. For instance, see: Hintermann, T.; Seebach, D. Synlett 1997, 437; Seebach, D.;
Abele, S.; Gademann, K.; Guichard, G.; Hintermann, T.; Jaun, B.; Matthews, J.
L.; Schreiber, J. V. Helv. Chim. Acta 1998, 81, 932; Gung, B. W.; Zou, D. J. Org.
Chem. 1999, 64, 2176; Raguse, L.; Lai, J. R.; Gellman, S. H. Helv. Chim. Acta
2002, 85, 4154; Glättli, A.; Seebach, D.; van Gunsteren, W. F. Helv. Chim. Acta
2004, 87, 24872; Luppi, G.; Galeazzi, R.; Garavelli, M.; Formaggio, F.;
Tomasini, C. Org. Biomol. Chem. 2004, 2, 2187; Izquierdo, S.; Kogan, M. J.;
Parella, T.; Moglioni, A. G.; Branchadell, V.; Giralt, E.; Ortuño, R. M. J. Org.
Chem. 2004, 69, 5093.
21. Nilges, M.; Gronenborn, A. M.; Brunger, A. T.; Clore, G. M. Protein Eng. 1988, 2,
27.
22. Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J.
Organometallics 1996, 15, 1518.
23. Gung, B. W.; Zou, D.; Stalcup, A. M.; Cottrell, C. E. J. Org. Chem. 1999, 64, 2176.
24. Babler, J. H.; Sarusi, S. J. J. Org. Chem. 1987, 52, 3462.
25. Henderson, D.; Richardson, K. A.; Taylor, R. J. K.; Saunders, J. Synthesis 1983, 12,
996.
5. The n-helix nomenclature is used to define the C@Oꢀ ꢀ ꢀH–N hydrogen bonded
ring sizes observed within the peptide backbone.
6. In contrast, it is often noted that a-peptides usually require at least 15 residues
to elicit any noticeable conformational preferences; see: Quinkirt, G.; Egert, E.;
Griesinger, C. In Peptides: Synthesis, Structure, and Applications; Gutte, B., Ed.;
Academic Press: San Diego, 1995.
7. (a) Appella, D. H.; Christianson, L. A.; Karle, I. L.; Powell, D. R.; Gellman, S. H. J.
Am. Chem. Soc. 1996, 118, 13071; (b) Barchi, J. J., Jr.; Huang, X.; Appella, D. H.;
Christianson, L. A.; Durell, S. R.; Gellman, S. H. J. Am. Chem. Soc. 2000, 122,
2711.
26. Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, C. K.; Watkin, D. J.
CRYSTALS, 2001, Issue 11, Chemical Crystallography Laboratory, University of
Oxford: UK.