Can a consecutive double turn conformation be considered as a peptide based molecular scaffold for supramolecular helix in the solid state?

Article abstract of DOI:10.1016/j.tet.2005.03.057

Helices and sheets are ubiquitous in nature. However, there are also some examples of self-assembling molecules forming supramolecular helices and sheets in unnatural systems. Unlike supramolecular sheets there are a very few examples of peptide sub-units

Full text of DOI:10.1016/j.tet.2005.03.057

Tetrahedron 61 (2005) 5027–5036
Can a consecutive double turn conformation be considered
as a peptide based molecular scaffold for supramolecular helix
in the solid state?
Apurba Kumar Das,a Arijit Banerjee,a Michael G. B. Drew,b Sudipta Ray,a Debasish Haldara
and Arindam Banerjeea,
*
aDepartment of Biological Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
bSchool of Chemistry, The University of Reading, Whiteknights, Reading RG6 6AD, UK
Received 18 November 2004; revised 21 February 2005; accepted 10 March 2005
Available online 12 April 2005
Abstract—Helices and sheets are ubiquitous in nature. However, there are also some examples of self-assembling molecules forming
supramolecular helices and sheets in unnatural systems. Unlike supramolecular sheets there are a very few examples of peptide sub-units that
can be used to construct supramolecular helical architectures using the backbone hydrogen bonding functionalities of peptides. In this report
we describe the design and synthesis of two single turn/bend forming peptides (Boc-Phe-Aib-Ile-OMe 1 and Boc-Ala-Leu-Aib-OMe 2) (Aib:
a-aminoisobutyric acid) and a series of double-turn forming peptides (Boc-Phe-Aib-Ile-Aib-OMe 3, Boc-Leu-Aib-Gly-Aib-OMe 4
and Boc-g-Abu-Aib-Leu-Aib-OMe 5) (g-Abu: g-aminobutyric acid). It has been found that, in crystals, on self-assembly, single turn/bend
forming peptides form either a supramolecular sheet (peptide 1) or a supramolecular helix (peptide 2), unlike self-associating double turn
forming peptides, which have only the option of forming supramolecular helical assemblages.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
self-assembled, oligonuclear coordination compounds,7 the
helicates.8 Different approaches to construct supramolecular
helices without metal ions and stabilized only by inter-
molecular and intramolecular hydrogen bonding have also
been pursued.6,9 Recently pyrene-4, 5-dione derivatives
have been used to design supramolecular helical struc-
tures.10 Peptide derivatives11 and even chiral amino acids
like the chiral 2,6-pyridinedicarboxamide containing the
podand L-histidyl moieties and the corresponding D-
derivative12a and ferrocene bearing the podand chiral
dipeptide moieties (-L-Ala-L-Pro-OEt) and the correspond-
ing D-derivative12b have been used to form supramolecular
helical assemblages. Parthasarathi and his colleagues have
synthesized and characterized a series of tripeptides which
form extended helical structures with intervening water
molecules between two consecutive peptide molecules and
they demonstrated the hydrated helix pattern in crystals.13
Our group is involved in constructing supramolecular
peptide helices utilizing the backbone hydrogen bonding
functionalities of the peptide molecules.14 From our
previous report, it has been observed that the short synthetic
terminally blocked peptides with single turn/bend confor-
mations can form either supramolecular sheets15 or
supramolecular helices.14a However, the peptide molecules
with a double turn structure, self-assemble to form
The challenge in molecular self-assembly is to design
molecular building blocks that are predisposed to give
definite supramolecular structures using non-covalent
interactions. Supramolecular helices and sheets are the
most common and indispensable form of structures in
biological systems. Helicity exists in numerous biological
and chemical systems. In proteins, the a-helical structure is
a very common motif. In DNA-double helices,1 collagen
triple helix2 and even in the coat protein complex of
Tobacco Mosaic Virus (TMV),3 helicity is a common
observable feature. Surprisingly, a considerable amount of
helicity is also present in some misfolded, neurodegenera-
tive disease-causing protein aggregates popularly known as
amyloid plaques.4 Unnatural supramolecular helical struc-
tures can be constructed using conformational restriction of
macromolecules,5 intra or intermolecular hydrogen bonds6
or by metal ion chelation.7 The most common and well-
studied example of supramolecular single-, double- and
triple-stranded helical conformations are metal chelated,
Keywords: Aib; Double turn conformation; ‘S’ shaped molecular structure;
Supramolecular helix.
Corresponding author. Tel.: C91 33 2473 4971; fax: C91 33 2473 2805;
e-mail: bcab@mahendra.iacs.res.in
*
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.03.057

5028
A. K. Das et al. / Tetrahedron 61 (2005) 5027–5036
double-turn forming peptides can only form supramolecular
helical structures or whether other supramolecular archi-
tectures are possible.
2. Results and discussion
Peptides (peptides 1, 2, 3, 4 and 5) reported in this study
have been synthesized with conformationally constrained,
helicogenic Aib (a-aminoisobutyric acid) residue(s) in order
to induce the helical nature of the individual peptide
backbone.16 The terminally protected tripeptides Boc-Phe-
Aib-Ile-OMe 1 and Boc-Ala-Leu-Aib-OMe 2 (Fig. 1) have
been designed and synthesized to obtain single b-turn/bend
forming molecular conformations. The tetrapeptides Boc-
Phe-Aib-Ile-Aib-OMe 3 and Boc-Leu-Aib-Gly-Aib-OMe 4
and Boc-g-Abu-Aib-Leu-Aib-OMe 5, each containing two
Aib residues have been synthesized to obtain double bend
conformations with two consecutive b-turns or other
unusual turn structures. Previous studies from our group
have established that the helix-nucleating Aib (a-amino-
isobutyric acid) residue needs to be placed adjacent to the
flexible N-terminally located g-Abu (g-aminobutyric acid
residue) to obtain a turn structure.17 All the reported
peptides were studied using X-ray crystallography, NMR
and mass spectrometry.
2.1. Single crystal X-ray diffraction study
Tripeptide 1 contains a helicogenic Aib residue at the
central region whereas tripeptide 2 possesses the Aib
residue at the C-terminus. The molecular conformations of
tripeptides 1 and 2 in the crystal state are illustrated in
Figure 2. Most of the f and j values (except j1 and j3) of
the constituent amino acid residues of tripeptide 1 fall
within the helical region of the Ramachandran plot
(Table 2). Hence the tripeptide 1 backbone, though it fails
to form any intramolecular hydrogen bonded b turn
conformation, adopts a bend (turn-like) structure (Fig. 2a),
which self-assembles through two intermolecular hydrogen
bonds (N6–H6/O8, 2.36 A, 3.02 A, 135.008 with sym-
metry element Kx, K0.5Cy, 1Kz) along the crystallo-
graphic b axis to form a monolayer b-sheet structure. These
monolayer structures of tripeptide 1 are further self-
assembled into higher order supramolecular b-sheet struc-
ture along the crystallographic a and c directions via van der
Figure 1. Schematic presentation of peptides 1–5.
supramolecular helices in crystals.14b–d Therefore, it is
necessary to define the role of a consecutive b-turn structure
in the formation of supramolecular helical architecture. In
this context we have designed and synthesized a series of
single and double turn/ bend forming peptides (Fig. 1). All
our previously reported peptides14,15 and the peptides
described in the present work are listed in Table 1. In this
paper, we address the question of whether self-assembling
˚
˚
Table 1. List of single and double turn/bend forming peptides from our group
Sequence
No. of bends/turns in
molecular structure
Supramolecular
structure
Reference
1. Boc-Leu-Aib-Phe-OMe
One
Two
Two
Two
Two
One
One
One
One
One
One
Two
Two
Two
Helix
Helix
Helix
Helix
Helix
Sheet
Sheet
Sheet
Sheet
Sheet
Helix
Helix
Helix
Helix
14a
14b
14c
14d
14e
15a
15b
15b
2. Boc-Leu-Aib-Phe-Aib-OMe
3. Boc-Ala-Aib-Leu-Aib-OMe
4. Boc-b-Ala-Aib-Leu-Aib-OMe
5. Boc-Aib-Val-Aib-b-Ala-OMe
6. Boc-Leu-Aib-b-Ala-OMe
7. Boc-Ala-Aib-Val-OMe
8. Boc-Ala-Aib-Ile-OMe
9. Boc-Ala-Gly-Val-OMe
10. Boc-Phe-Aib-Ile-OMe
11. Boc-Ala-Leu-Aib-OMe
12. Boc-Phe-Aib-Ile-Aib-OMe
13. Boc-Leu-Aib-Gly-Aib-OMe
14. Boc-g-Abu-Aib-Leu-Aib-OMe
15b
In current study
In current study
In current study
In current study
In current study

A. K. Das et al. / Tetrahedron 61 (2005) 5027–5036
5029
Figure 2. (a) The structure of peptide 1 showing the atomic numbering scheme. Ellipsoids at 30% probability. (b) The ORTEP diagram of the peptide 2
showing the atomic numbering scheme. Ellipsoids at 20% probability. Intramolecular hydrogen bonds are shown as dotted line.
Table 2. Selected backbone torsional angles of peptides 1–5
Compound
f1
K59.2
K66.6
62.0
K62.6
j1
154.2
K15.0
23.6
116.9
f2
62.5
K53.5
53.4
54.8
j2
30.9
K42.8
26.6
30.0
f3
j3
146.8
46.6
25.5
2.6
f4
j4
q1
q2
Peptide 1
Peptide 2
Peptide 3
Peptide 4
Peptide 5
Molecule A
Molecule B
K75.0
48.2
64.4
—
—
K46.4
K47.1
—
—
—
—
—
—
—
—
—
—
134.6
K50.1
83.2
K115.6
K121.7
K140.8
K134.8
K53.5
K55.5
K37.3
K37.3
K86.5
K80.2
K6.0
K14.0
K55.5
42.5
K47.8
K128.4
57.6
60.2
64.2
56.6
Waals interactions (Fig. 3). By comparison Figure 2b shows
that there is a 4/1 hydrogen bond between Boc CO and
Aib(3) N–H (N9–H/O2, 2.37, 3.04 A, 135.008) forming a
distorted type III b-turn (10-membered hydrogen bonded
ring) conformation in tripeptide 2. The backbone torsion
angles of tripeptide 2 fall within the helical region of the
Ramachandran plot (Table 2). For the first two residues, that
is Ala(1) and Leu(2), the f, j values lie within the right
handed helical region of the Ramachandran plot while f, j
values of the last residue (Aib(3)) fall in the left-handed
helical region. Each individual sub-unit of tripeptide 2 then
self-assembles through intermolecular hydrogen bonds
(N3–H/O8, 2.11, 2.93 A, 159.008 and N6–H/O5, 2.37,
˚
3.2 A, 164.008 with symmetry element K0.5Cx, K0.5Ky,
Kz) atop one another to form a supramolecular helix along
the crystallographic a axis (Fig. 4). In the former example
and other previous studies15 it has been demonstrated that
tripeptides with single b-turn conformations can form
supramolecular b-sheet architectures upon self-assembly.
The structure of peptide 2 reported here and other earlier
work in the literature14a has indicated that tripeptides
containing a turn/bend conformation can also form
supramolecular helical structure upon self-association in
the solid state. So, molecular self-assembly of single-turn
forming peptides does not only lead to one particular type
of supramolecular structure, but can form either supra-
molecular helices or supramolecular b-sheets depending
upon the nature of crystal packing.
˚
˚
We are trying to find a peptide-based molecular scaffold,
which can only form a definite type of supramolecular
structure: namely the supramolecular helix. In this regard
we have designed and synthesized tetrapeptides 3, 4 and 5.
From X-ray crystallography, it is evident that the tetrapep-
tide 3 adopts a consecutive b-turn structure in the crystal
(Fig. 5a). Incorporation of helicogenic Aib residues has
driven the tetrapeptide 3 backbone to form two overlapping
type III0 b-turn conformations through intramolecular
Figure 3. Packing of peptide 1 along crystallographic b axis showing
intermolecularly hydrogen bonded sheet-like structure.

5030
A. K. Das et al. / Tetrahedron 61 (2005) 5027–5036
Figure 6. Cross-eye stereo representation of the packing of peptide 3 shows
the intermolecular hydrogen-bonded supramolecular helix along the b axis.
Hydrogen bonds are shown as dotted lines.
Figure 4. A cross-eye stereo representation of the higher order packing of
peptide 2 along crystallographic a axis showing the formation of
supramolecular helix via intermolecular hydrogen bonds.
(Table 2). However, the dihedral angles of tetrapeptide 4
indicate that the first turn is a type II b-turn whereas the
second turn is a type I0 b-turn. In both tetrapeptides 3 and 4,
the Aib(2) residue occupies simultaneously the iC2th
position of first turn and the iC1th position of the second
turn. Here the remarkable feature is that these two
tetrapeptides form an ‘S’ shaped molecular scaffold as a
result of the double turn structure (Fig. 5). The individual
tetrapeptide 3 subunits are regularly stacked atop one
another through multiple intermolecular hydrogen bonds
˚
hydrogen bonds (N9–H/O2, 2.31, 3.17 A, 175.008 and
˚
˚
N12–H/O5, 2.17 A, 2.95 A, 151.008). Most of the back-
bone torsion angles (except j4) of tetrapeptide 3 are in the
left-handed helical region of the Ramachandran diagram
(Table 2). The tetrapeptide 4 also forms two successive b-
turns in the crystal (Fig. 5b). Ten-membered intramolecular
hydrogen bonds are formed between the CO of the Boc
˚
group and Gly(3) NH (N13–H/O4, 2.21, 3.02 A, 157.008)
˚
˚
and the CO of Leu(1) and Aib(4) NH (N17–H/O7, 2.05 A,
(N6–H/O11, 2.23, 2.95 A, 141.008 and N4–H/O8,
˚
2.88 A, 162.008), respectively, in tetrapeptide 4. The
˚
˚
2.10 A, 2.90 A, 154.008 with symmetry element 1Kx,
0.5Cy, 0.5Kz) to form a supramolecular helix along the
axis parallel to crystallographic b direction (Fig. 6). The
hydrogen bonds between the Phe(1) and Aib(2) NH groups
and the Aib(2) and Ile(3) C]O groups are responsible for
connecting individual molecules to form and stabilize the
supramolecular one-dimensional helical column of peptide
3. The supramolecular building blocks of tetrapeptide 4 are
further self-assembled through selective intermolecular
backbone torsion angles (except j1) of tetrapeptide 4 are
mostly in the helical region of the Ramachandran diagram
˚
hydrogen bonds (N5–H/O16, 2.01, 2.86 A, 174.008 with
symmetry element 2Kx, 0.5Cy, 0.5Kz) to generate a
supramolecular helical architecture along the crystallo-
graphic b axis (Fig. 7). One interesting feature of the crystal
structure of the tetrapeptide 4 is that, the individual helical
columns of 4 are linked to the adjacent column exploiting
the hydrogen bond functionalities of Aib(2) NH and Aib(4)
˚
CO (N8–H/O20, 2.09, 2.93 A, 165.008 with symmetry
element 1Cx, y, z).
The molecular conformation of tetrapeptide 5 (Fig. 8) was
also established by single crystal X-ray diffraction studies.
Figure 8 illustrates that there are two molecules (namely A
and B) in the asymmetric unit, which have equivalent
conformations. Both molecules A and B in the asymmetric
unit form a hydrogen bond to a water molecule (N3A–H/
˚
O200, 2.18, 3.00 A, 160.008 for molecule A and N3B–H/
Figure 5. (a) Formation of ‘S’ shaped molecular scaffold by peptide 3 and
(b) Formation of ‘S’ shaped molecular scaffold by peptide 4.
˚
O100, 2.22, 3.04 A, 160.008 for molecule B with symmetry

A. K. Das et al. / Tetrahedron 61 (2005) 5027–5036
5031
membered hydrogen bonded ring (Fig. 8). An additional
10-atom hydrogen bonded b-turn (N18A–H/O7A, 2.18,
˚
3.02 A, 165.008 for molecule A and N18B–H/O7B, 2.26,
˚
3.05 A, 152.008 for molecule B) between the CO of g-Abu
and the NH of Aib(4) is also formed adjacent to the
12-membered hydrogen bonded ring. Hence, tetrapeptide 5
also has a double bend ‘S’ shaped molecular conformation.
Most of the f and j values (except f1 and j1 of molecule A
and f1, j1 and j4 of molecule B) of the constituent amino
acid residues of tetrapeptide 5 fall within the helical region
of Ramachandran plot (Table 2). The torsions about the
polymethylene groups of the N-terminal g-Abu residue
(namely q1 and q2) in peptide 5 are in gauche conformation.
The peptide self-assembles through intermolecular hydro-
˚
gen bonds (N8A–H/O10A, 2.15, 2.92 A, 149.008 with
symmetry equivalent 0.5Kx, K0.5Cy, 1Kz for molecule A
˚
and N8B–H/O10B, 2.15, 2.92 A, 149.008 with symmetry
equivalent 0.5Kx, 0.5Cy, Kz for molecule B) maintaining
the proper registry of molecules A and B along the
crystallographic b axis to form a supramolecular helical
structure (Fig. 9). Crystal data for all the reported peptides
are listed in Table 3.
3. Conclusion
All the reported peptides are composed of one or more
conformationally constrained Aib residues and their back-
bone dihedral angles demonstrate that they mostly adopt
helical conformations. Crystallographic studies of tripep-
tides 1 and 2 demonstrate that the self-association of pep-
tides containing only one turn have two options to form a
supramolecular architecture: either a supramolecular sheet
or a supramolecular helix. Tetrapeptides 3 and 4 share a
common structural motif, consecutive double bend molecu-
lar conformations and self-association of each peptide
Figure 7. The packing of the peptide 4 illustrating the intermolecular
hydrogen-bonded supramolecular helix along the b axis. Hydrogen bonds
are shown as dotted lines.
equivalent x, K1Cy, z and x, 1Cy, z, respectively). The
insertion of two CH2 groups into the backbone (g-Abu)
causes an unusual 4/1 type hydrogen bond (N11A–H/
˚
O2A, 2.24, 3.07 A, 163.008 for molecule A and N11B–H/
˚
O2B, 2.19, 3.03 A, 165.008 for molecule B) between the CO
of the Boc group and the NH of Leu(3) forming a 12-
Figure 8. Molecular conformation of peptide 5 with atom numbering scheme. The asymmetric unit contains two molecules (namely A and B) each of which
has a water molecule. Each molecule forms a 12 membered unusual turn adjacent with a 10 membered distorted type III b-turn. The hydrogen bonds are shown
as dotted line. Thermal ellipsoids are shown at the level of 20% probability.

5032
A. K. Das et al. / Tetrahedron 61 (2005) 5027–5036
conformation, which in turn leads to the formation of a
common S shaped molecular structure. This S shaped
structure may increase the inherent propensity of individual
tetrapeptide subunits to form a supramolecular helical
architecture through intermolecular hydrogen bonds in the
solid state.
Our work offers an elegant approach to controlling the
three-dimensional structure using the stereochemical nature
of a conformationally constrained amino acid (viz. Aib).
This study should be useful for designing and calibrating
supramolecular architectures having importance both in
supramolecular chemistry and crystal engineering.
4. Experimental
4.1. Synthesis of peptide 1
4.1.1. Boc-Phe(1)-OH 6. A solution of phenylalanine
(4.95 g, 30 mmol) in a mixture of dioxane (60 mL), water
(30 mL) and 1 M NaOH (30 mL) was stirred and cooled in
an ice-water bath. Di-tert-butyldicarbonate (7.2 g,
33 mmol) was added and stirring was continued at room
temperature for 6 h. Then the solution was concentrated
under vacuum to about 40–60 mL, cooled in an ice water
bath, covered with a layer of ethyl-acetate (about 50 mL)
and acidified with a dilute solution of KHSO4 to pH 2–3
(Congo red). The aqueous phase was extracted with ethyl
acetate and this operation was done repeatedly. The ethyl
acetate extracts were pooled, washed with water and dried
over anhydrous Na2SO4 and evaporated under vacuum. The
pure material as white solid was obtained.
Figure 9. Packing diagram of peptide 5 along the crystallographic b axis
showing intermolecular hydrogen-bonded supramolecular helices. The A
and B molecules packed through proper registry, individual helix is formed
by only a particular molecule that is either molecule A or B. Intermolecular
hydrogen bonds are shown as dotted lines. The hydrogen bonded water
molecule with each peptide is shown as *.
YieldZ96% (7.63 g, 28.8 mmol). Mp 85–87 8C. Elemental
Analysis Calcd for C14H19NO4 (265): C, 63.50; N, 5.24; H,
7.20. Found: C, 63.39; N, 5.28; H, 7.17.
create supramolecular helices in crystals. Tetrapeptide 5,
instead of having a conformationally flexible g-amino-
butyric acid residue at the N terminus, also forms a double
bend molecular conformation with two consecutive turns;
one of which is an unusual turn with a 12-atom hydrogen
bonded ring structure followed by a classical b-turn
conformation. Thus, all these tetrapeptides, despite sub-
stantial differences in sequences adopt a double turn
4.1.2. Boc-Phe(1)-Aib(2)-OMe 7. Boc-Phe-OH (7.42 g
28 mmol) was dissolved in a mixture of 30 mL dichloro-
methane (DCM) in an ice-water bath. H-Aib-OMe was
isolated from the corresponding methyl ester hydrochloride
(8.6 g, 56 mmol) by neutralization and subsequent extrac-
tion with ethyl acetate and the ethyl acetate extract was
Table 3. Crystal and data collection parameters of peptides 1–5
Peptide 1
Peptide 2
Peptide 3
Peptide 4
Peptide 5
Empirical formula
Crystallizing solvent
Crystal system
C25H39N3O6
Methanol–water
Monoclinic
P21
10.224(11)
11.378(12)
12.578(14)
105.28(10)
1411.5
2
477.59
1.124
5066
C19H35N3O6
Methanol–water
Orthorhombic
P212121
10.613(12)
10.476(12)
21.132(2)
(90)
2349.50
4
401.50
1.135
3249
C29H46N4O7
Methanol–water
Orthorhombic
P212121
10.179(12)
15.606(17)
20.727(23)
(90)
3292.55
4
562.7
1.135
5268
C22H40N4O7
Methanol–water
Orthorhombic
P212121
10.421(17)
13.361(2)
20.171(3)
(90)
2808(7)
4
472.58
1.117
4296
C24H44N4O7, H2O
Methanol–water
Monoclinic
C2
29.557(3)
12.237(14)
20.703(2)
121.31(1)
6397.21
8
516.63
1.073
9986
4607
0.1178
0.2280
0.370,K0.251
Space group
˚
a (A)
˚
b (A)
˚
c (A)
b (8)
3
˚
U (A )
Z
Mol. Wt.
Density (Calcd, mg/mm3)
Unique data
Observed reflns. (IO2s(I))
R
wR2
3291
0.0881
0.1331
0.259, K0.180
1757
0.1157
0.2915
0.376, K0.321
1994
0.1015
0.2014
0.269–0.260
2701
0.0839
0.2394
0.204, K0.205
3
˚
Largest diff. peak and hole (e/A )

A. K. Das et al. / Tetrahedron 61 (2005) 5027–5036
5033
concentrated to 10 mL. This was added to the reaction
mixture, followed immediately by di-cyclohexylcarbodi-
imide (DCC) (5.77 g, 28 mmol). The reaction mixture was
allowed to come to room temperature and stirred for 24 h.
DCM was evaporated, and the residue was taken up in ethyl
acetate (60 mL), and dicyclohexylurea (DCU) was filtered
off. The organic layer was washed with 2 M HCl (3!
50 mL), brine, then 1 M sodium carbonate (3!50 mL) and
brine (2!50 mL) and dried over anhydrous sodium sulfate,
and evaporated under vacuum to yield 8.74 g (24 mmol,
86%) of 7 as white solid.
(s, 6H); 1.25–1.13 (m, 2H); 0.89–0.94 (m, 6H). 13C NMR
(CDCl3, 300 MHz, d ppm): 173.58, 172.17, 170.65, 136.56,
129.15, 129.06, 128.58, 126.87, 80.24, 57.16, 56.56, 51.83,
37.49, 28.05, 25.40, 24.90, 24.80, 24.38, 15.33, 11.37.
[a]2D5.7ZC5.57 (c 2.07, CHCl3). Mass Spectral data (MC
Na)CZ500.7, MCalcdZ477. Elemental Analysis Calcd for
C25H39N3O6 (477): C, 66.89; N, 8.80; H, 8.47. Found: C,
66.81; N, 8.85; H, 8.53.
4.1.5. Boc-Ala(1)-OH 9. See Ref. 15b.
4.1.6. Boc-Ala(1)-Leu(2)-OMe 10. Boc-Ala-OH (5.67 g,
30 mmol) was dissolved in a mixture of dichloromethane
(DCM) (30 mL) in an ice-water bath. H-Leu-OMe was
isolated from the corresponding methyl ester hydrochloride
(10.89 g, 60 mmol) by neutralization and subsequent
extraction with ethyl acetate and the ethyl acetate extract
was concentrated to 10 mL. This was added to the reaction
mixture, followed immediately by di-cyclohexylcarbodi-
imide (DCC) (6.18 g, 30 mmol). The reaction mixture was
allowed to come to room temperature and stirred for 24 h.
DCM was evaporated, and the residue was taken up in ethyl
acetate (60 mL), and dicyclohexylurea (DCU) was filtered
off. The organic layer was washed with 2 M HCl (3!
50 mL), brine, then 1 M sodium carbonate (3!50 mL) and
brine (2!50 mL) and dried over anhydrous sodium sulfate,
and evaporated under vacuum to yield 10 as waxy solid.
1
YieldZ86% (8.74 g, 24 mmol). Mp 119–121 8C. H NMR
(CDCl3, 300 MHz, d ppm): 7.22–7.35 (m, 5H); 6.3 (s, 1H);
5.1 (d, JZ9 Hz, 1H); 4.28 (m, 1H); 3.71 (s, 3H); 2.96–3.07
(m, 2H); 1.45 (s, 6H); 1.42 (s, 9H). Elemental Analysis
Calcd for C19H28N2O5 (364): C, 62.45; N, 7.72; H, 7.71.
Found: C, 62.63; N, 7.69; H, 7.69.
4.1.3. Boc-Phe(1)-Aib(2)-OH 8. To 7 (8.5 g 23.35 mmol),
MeOH (50 mL) and 2 M NaOH (20 mL) were added and the
progress of saponification was monitored by thin layer
chromatography (TLC). The reaction mixture was stirred at
room temperature. After 10 h methanol was removed under
vacuum, the residue was taken up in 50 mL of water,
washed with diethyl ether (2!50 mL). Then the pH of the
aqueous layer was adjusted to 2 using 1 M HCl and it was
extracted with ethyl acetate (3!50 mL). The extracts were
pooled, dried over anhydrous sodium sulfate, and evapor-
ated in vacuum to yield a waxy solid.
YieldZ83.3% (7.9 g, 25 mmol). 1H NMR (CDCl3,
300 MHz, d ppm): 6.71 (d, JZ6 Hz, 1H); 5.15 (d, JZ
7.5 Hz, 1H); 4.64–4.57 (m, 1H); 4.15–4.11 (m, 1H); 3.74 (s,
3H); 1.61–1.69 (m, 3H); 1.45 (s, 9H); 1.35 (m, 3H); 0.93 (m,
6H). Elemental Analysis Calcd for C15H28N2O5 (316): C,
56.85; N, 8.72; H, 8.90. Found: C, 56.96; N, 8.86; H, 8.86.
1
YieldZ92% (7.525 g, 21.5 mmol). H NMR ((CD3)2SO,
300 MHz, d ppm): 12.27 (br, 1H); 8.05 (s, 1H); 7.17–7.26
(m, 5H); 6.77 (d, JZ9 Hz, 1H); 4.13–4.21 (m, 1H); 2.85–
3.02 (m, 2H); 1.34 (s, 6H); 1.29 (s, 9H). Elemental Analysis
Calcd for C18H26N2O5 (350): C, 61.60; N, 7.96; H, 7.56.
Found: C, 61.71; N, 8.00; H, 7.43.
4.1.7. Boc-Ala(1)-Leu(2)-OH 11. To 10 (6.32 g, 20 mmol),
MeOH (50 mL) and 2 M NaOH (20 mL) were added and the
progress of saponification was monitored by thin layer
chromatography (TLC). The reaction mixture was stirred at
room temperature. After 10 h methanol was removed under
vacuum, the residue was taken up in 50 mL of water,
washed with diethyl ether (2!50 mL). Then the pH of the
aqueous layer was adjusted to 2 using 1 M HCl and it was
extracted with ethyl acetate (3!50 mL). The extracts were
pooled, dried over anhydrous sodium sulfate, and evapor-
ated under vacuum to yield 4.23 g of 11 as white solid.
4.1.4. Boc-Phe(1)-Aib(2)-Ile(3)-OMe 1. Boc-Phe(1)-
Aib(2)-OH (7 g, 20 mmol) in DMF (15 mL) was cooled in
an ice-water bath and H-Ile-OMe was isolated the
corresponding methyl ester hydrochloride from (7.26 g,
40 mmol) by neutralization and subsequent extraction with
ethyl acetate and the ethyl acetate extract was concentrated
to 8 mL. It was then added to the reaction mixture, followed
immediately by 4.12 g (20 mmol) DCC and HOBt (2.7 g,
20 mmol). The reaction mixture was stirred for 3 days. The
residue was taken up in ethyl acetate (40 mL) and the DCU
was filtered off. The organic layer was washed with 2 M
HCl (3!40 mL), brine, 1 M sodium carbonate (3!40 mL),
brine (2!40 mL), dried over anhydrous sodium sulfate and
evaporated in vacuum to yield 8.1 g (17 mmol) of white
solid. Purification was done by silica gel column (100–200
mesh) using ethyl acetate as eluent. Colorless single crystals
were grown from methanol/water solvent system (methanol/
waterZ80:20) by slow evaporation.
1
YieldZ70% (4.23 g, 14 mmol). Mp 124–126 8C. H NMR
((CD3)2SO, 300 MHz, d ppm): 12.41 (br, 1H); 7.88 (d, JZ
9 Hz, 1H); 6.85 (d, JZ6 Hz, 1H); 4.24–4.17 (m, 1H); 3.99–
3.94 (m, 1H); 1.49–1.66 (m, 3H); 1.35 (s, 9H); 1.14 (m, 3H);
0.81–0.88 (m, 6H). Elemental Analysis Calcd for
C14H26N2O5 (302): C, 55.60; N, 9.21; H, 8.56. Found: C,
55.63; N, 9.27; H, 8.60.
4.1.8. Boc-Ala(1)-Leu(2)-Aib(3)-OMe 2. Boc-Ala(1)-
Leu(2)-OH (2.74 g, 10 mmol) in DMF (15 mL) was cooled
in an ice-water bath and H-Aib-OMe was isolated from the
corresponding methyl ester hydrochloride (3.07 g,
20 mmol) by neutralization and subsequent extraction with
ethyl acetate and the ethyl acetate extract was concentrated
to 8 mL. It was then added to the reaction mixture, followed
immediately by DCC (2.06 g, 10 mmol) and HOBt (1.35 g,
YieldZ85%, (8.1 g, 17 mmol). Rf (ethyl acetate) 0.78. Mp
104–106 8C. nmax (KBr)/cmK1 1655, 1697, 3321, 3395,
3427. H NMR (CDCl3, 300 MHz, d ppm): 7.22–7.35 (m,
5H); 7.01 (d, JZ9 Hz, 1H); 6.17 (s, 1H); 5.07 (d, JZ9 Hz,
1H); 4.54–4.50 (m, 1H); 4.28–4.20 (m, 1H); 3.72 (s, 3H);
3.08–3.03 (m, 2H); 1.93–1.88 (m, 1H); 1.42 (s, 9H); 1.39
1

5034
A. K. Das et al. / Tetrahedron 61 (2005) 5027–5036
10 mmol). The reaction mixture was stirred for 3 days. The
residue was taken up in ethyl acetate (40 mL) and the DCU
was filtered off. The organic layer was washed with 2 M
HCl (3!40 mL), brine, 1 M sodium carbonate (3!40 mL),
brine (2!40 mL), dried over anhydrous sodium sulfate and
evaporated under vacuum to yield 3.21 g (8 mmol) of white
solid. Purification was done by silica gel column (100–200
mesh) using ethyl acetate–toluene (3:1) as eluent. Colorless
single crystals were grown from methanol/water solvent
system (methanol/waterZ90:10) by slow evaporation.
were obtained from methanol–water (90:10) by slow
evaporation.
YieldZ81% (4.14 g, 7.37 mmol). Rf (25% toluene/ethyl
acetate) 0.65. Mp 168–170 8C. nmax (KBr)/cmK1 1647,
1730, 1747, 3308, 3551. 1H NMR (CDCl3, 300 MHz d
ppm): 7.20–7.34 (m, 5H); 7.13 (s, 1H); 6.64 (d, JZ9 Hz,
1H); 6.31 (s, 1H); 5.01 (d, JZ7 Hz, 1H); 4.44 (m, 1H);
4.11–4.14 (m, 1H); 3.70 (s, 3H); 3.07–3.11 (m, 2H); 2.22
(m, 1H); 1.56, 1.53 (s, 6H); 1.43 (s, 9H); 1.26 (s, 6H); 0.89–
0.91 (m, 6H). 13C NMR (CDCl3, 300 MHz, d ppm): 174.89,
173.64, 171.19, 170.37, 155.82, 129.13, 129.02, 128.87,
128.78, 128.61, 127.19, 80.91, 57.93, 56.91, 55.80, 52.03,
27.92, 25.20, 25.03, 24.42, 24.35, 23.99, 15.53, 15.47,
11.57, 11.51, 11.49. [a]2D5.7ZK15.35 (c 2.10, CHCl3). Mass
Spectral data (MCNaC2H)CZ587.8, MCalcdZ562.7.
Elemental Analysis Calcd for C29H46N4O7 (562): C,
62.02; N, 9.91; H, 8.12. Found: C, 61.90; N, 9.96; H, 8.19.
YieldZ80% (3.21 g, 8 mmol). Rf (25% toluene/ethyl
acetate) 0.68. Mp 117–119 8C. nmax (KBr)/cmK1 1659,
1684, 1753, 3244, 3321, 3350. 1H NMR (CDCl3, 300 MHz,
d ppm): 6.84 (s, 1H); 6.60 (d, JZ9 Hz, 1H); 5.00 (d, JZ
6 Hz, 1H); 4.44–4.36 (m, 1H); 4.16–4.12 (m, 1H); 3.71 (s,
3H); 1.59–1.73 (m, 3H); 1.50 (s, 3H); 1.53 (s, 3H); 1.45 (s,
9H); 1.37 (d, JZ9 Hz, 3H); 0.90–0.94 (m, 6H). 13C NMR
(CDCl3, 300 MHz, d ppm): 174.52, 172.73, 170.85, 155.44,
80.08, 56.07, 52.28, 51.44, 40.27, 28.09, 24.60, 24.56, 24.48,
24.39, 22.76, 21.76, 15.00. [a]2D5.7ZK57.27 (c 2.10, CHCl3).
Mass Spectral data (MCNaCH)CZ425.1, MCalcdZ401.
Elemental Analysis Calcd for C19H35N3O6 (401): C, 56.85; N,
10.47; H, 8.72. Found: C, 56.81; N, 10.85; H, 8.53.
4.1.11. Boc-Leu(1)-OH 13. See Ref. 18.
4.1.12. Boc-Leu(1)-Aib(2)-OMe 14. See Ref. 18.
4.1.13. Boc-Leu(1)-Aib(2)-OH 15. See Ref. 18.
4.1.9. Boc-Phe(1)-Aib(2)-Ile(3)-OH 12. To Boc-Phe(1)-
Aib(2)-Ile(3)-OMe 1 (5 g, 10.48 mmol), methanol (50 mL)
and of 2 M NaOH (20 mL) were added and the progress of
saponification was monitored by thin layer chromatography
(TLC). The reaction mixture was stirred at room tempera-
ture. After 10 h methanol was removed under vacuum, the
residue was taken in 50 mL of water, washed with diethyl
ether (2!50 mL). Then pH of the aqueous layer was
adjusted to 2 by adding 1 M HCl and it was extracted with
ethyl acetate (3!40 mL). The extracts were pooled, dried
over anhydrous sodium sulfate and evaporated under
vacuum to yield a waxy solid.
4.1.14. Boc-Leu(1)-Aib(2)-Gly(3)-OBz 16. Boc-Leu(1)-
Aib(2)-OH (6.32 g, 20 mmol) in DMF (20 mL) was cooled
in an ice-water bath and H-Gly-OBz was isolated from
the corresponding benzyl ester p-toluenesulfonate (8.62 g,
40 mmol) by neutralization, subsequent extraction with
ethyl acetate and concentration (10 mL) and it was added to
the reaction mixture, followed immediately by DCC
(4.12 g, 20 mmol) and HOBt (2.7 g, 20 mmol). The reaction
mixture was stirred for 3 days. The residue was taken in
ethyl acetate (60 mL) and the DCU was filtered off. The
organic layer was washed with 2 M HCl (3!50 mL), brine,
1 M sodium carbonate (3!50 mL), brine (2!50 mL), dried
over anhydrous sodium sulfate and evaporated under
vacuum to yield 8.11 g (17 mmol) of 16 as waxy solid.
YieldZ89% (4.32 g, 9.33 mmol). 1H NMR ((CD3)2SO,
300 MHz, d ppm): 12.52 (br, 1H); 7.95 (s, 1H); 7.28–7.35
(m, 5H); 7.21 (d, JZ9 Hz, H); 6.9 (d, JZ9 Hz, 1H); 4.36–
4.44 (m, 1H); 4.16–4.12 (m, 1H); 2.87–2.96 (m, 2H); 1.77–
1.89 (m, 1H); 1.30 (s, 6H); 1.28 (s, 9H); 1.07–1.18 (m, 2H);
0.81–0.82 (m, 6H). Elemental Analysis Calcd for
C24H37N3O6 (463): C, 62.26; N, 9.02; H, 8.02. Found: C,
62.20; N, 9.07; H, 7.99.
YieldZ81% (7.87 g, 17 mmol). 1H NMR (CDCl3, 300 MHz,
d ppm): 7.26–7.30 (m, 5H); 7.35(t, JZ9 Hz, 1H); 6.52 (s,
1H); 4.99 (d, JZ9 Hz, 1H); 3.78–3.90 (m, 3H); 1.71 (s, 6H);
1.59–1.68 (m, 3H); 1.52 (s, 9H); 0.94–0.99 (m, 6H).
Elemental Analysis Calcd for C24H37N3O6 (463): C, 62.20;
H, 7.99; N, 9.07. Found: C, 62.28; H, 7.95; N, 10.17.
4.1.10. Boc-Phe(1)-Aib(2)-Ile(3)-Aib(4)-OMe 3. To 12
(4.2 g, 9.1 mmol) in DMF (10 mL) was cooled in an ice
water bath. H-Aib-OMe was isolated from the correspond-
ing methyl ester hydrochloride (2.8 g, 18.2 mmol) by
neutralization, subsequent extraction with ethyl acetate
and concentration (7 mL) and this was added to the reaction
mixture, followed immediately by DCC (1.87 g, 9.1 mmol)
and HOBt (1.23 g, 9.1 mmol). The reaction mixture was
stirred for 3 days. The residue was taken in ethyl acetate
(50 mL) and DCU was filtered off. The organic layer was
washed with 2 M HCl (3!50 mL), brine, 1 M sodium
carbonate (3!50 mL), brine (2!50 mL), dried over
anhydrous sodium sulfate and evaporated under vacuum
to yield 4.14 g (7.37 mmol, 81%) of white solid. Purification
was done by silica gel column (100–200 mesh) using ethyl
acetate–toluene (3:1) as eluent. Colorless single crystals
4.1.15. Boc-Leu(1)-Aib(2)-Gly(3)-OH 17. To Boc-Leu(1)-
Aib(2)-Gly(3)-OBz 16 (4.63 g, 10 mmol), methanol
(50 mL) and of 2 M NaOH (20 mL) were added and the
progress of saponification was monitored by thin layer
chromatography (TLC). The reaction mixture was stirred at
room temperature. After 10 h methanol was removed under
vacuum, the residue was taken in 50 mL of water, washed
with diethyl ether (2!50 mL). Then pH of the aqueous
layer was adjusted to 2 by adding 1 M HCl and it was
extracted with ethyl acetate (3!40 mL). The extracts were
pooled, dried over anhydrous sodium sulfate and evaporated
under vacuum to yield a white solid.
1
YieldZ70% (2.60 g, 7 mmol). Mp 140–142 8C. H NMR
((CD3)2SO, 300 MHz, d ppm): 12.38 (br, 1H); 8.02 (s, 1H);
7.72 (t, JZ5 Hz, 1H); 7.01 (d, JZ6 Hz, 1H); 3.83–3.91 (m,

A. K. Das et al. / Tetrahedron 61 (2005) 5027–5036
5035
2H); 3.53–3.61 (m, 1H); 1.52–1.60 (m, 3H); 1.44 (s, 6H);
1.37 (s, 9H); 0.84–0.89 (m, 6H). Elemental Analysis Calcd
for C17H31N3O6 (373): C, 54.69; H, 8.31; N, 11.26. Found:
C, 54.67; H, 8.16; N, 11.12.
0.99–0.92 (m, 6H). Elemental Analysis Calcd for
C20H37N3O6 (415): C, 57.83; H, 8.91; N, 10.12. Found: C,
57.50; H, 8.40; N, 10.00.
4.1.21. Boc-g-Abu(1)-Aib(2)-Leu(3)-OH 22. To Boc-g-
Abu(1)-Aib(2)-Leu(3)-OMe 21 (1.23 g, 2.9 mmol), MeOH
(5 mL) and 2 M NaOH (3 mL) were added and the progress
of saponification was monitored by thin layer chromato-
graphy (TLC). The reaction mixture was stirred at room
temperature. After 10 h methanol was removed under
vacuum, the residue was taken in 50 mL of water, washed
with diethyl ether (2!50 mL). Then the pH of the aqueous
layer was adjusted to 2 using 1 M HCl and it was extracted
with ethyl acetate (3!50 mL). The extracts were pooled,
dried over anhydrous sodium sulfate, and evaporated under
vacuum to yield 0.9 g of Boc-g-Abu(1)-Aib(2)-Leu(3)-OH
as white solid.
4.1.16. Boc-Leu(1)-Aib(2)-Gly(3)-Aib(4)-OMe 4. To 17
(1.30 g, 5 mmol) in DMF (10 mL) was cooled in an ice
water bath. H-Aib-OMe was isolated from the correspond-
ing methyl ester hydrochloride (1.53 g, 10 mmol) by
neutralization, subsequent extraction with ethyl acetate
and concentration (7 mL) and this was added to the reaction
mixture, followed immediately by DCC (1 g, 5 mmol) and
HOBt (0.7 g, 5 mmol). The reaction mixture was stirred for
3 days. The residue was taken in ethyl acetate (50 mL) and
DCU was filtered off. The organic layer was washed with
2 M HCl (3!50 mL), brine, 1 M sodium carbonate (3!
50 mL), brine (2!50 mL), dried over anhydrous sodium
sulfate and evaporated in vacuum to yield 2.25 g (4 mmol,
80%) of white solid. Purification was done by silica gel
column (100–200 mesh) using ethyl acetate as eluent. Single
crystals were obtained from methanol–water (methanol/
waterZ90:10) by slow evaporation.
YieldZ77.58% (0.9 g, 2.25 mmol). Mp 203–205 8C. 1H
NMR ((CD3)2SO, 300 MHz, d ppm): 12.34 (br, 1H); 7.75 (s,
1H); 7.58 (d, JZ6 Hz, 1H); 6.81 (t, JZ5.4 Hz, 1H); 4.23–
4.15 (m, 1H); 2.95–2.89 (m, 2H); 2.01–2.15 (m, 2H); 1.61–
1.54 (m, 2H); 1.54–1.50 (m, 3H); 1.31 (s, 9H); 1.37 (s, 6H);
0.87–0.79 (m, 6H). Elemental Analysis Calcd for
C19H35N3O6 (401): C, 54.85; H, 8.72; N, 10.47. Found: C,
54.30; H, 8.40; N, 10.20.
YieldZ80% (1.90 g, 4 mmol). Rf (ethyl acetate) 0.68. Mp
1
145–147 8C. nmax (KBr)/cmK1 1657, 1728, 3273, 3368. H
NMR (10% (CD3)2SO in CDCl3, 300 MHz, d ppm): 8.05 (s,
1H); 7.73 (t, JZ11 Hz, 1H); 7.56 (s, 1H); 6.23 (d, JZ6 Hz,
1H); 3.99–4.04 (m, 2H); 3.83–3.9 (m, 1H); 3.65 (s, 3H);
1.50–1.55 (m, 3H); 1.48 (6H, s); 1.47 (s, 6H); 1.44 (s, 9H);
0.92–0.97 (m, 6H). 13C NMR (CDCl3, 300 MHz, d ppm):
174.98, 174.18, 173.25, 169.00, 156.61, 80.89, 56.95, 55.83,
54.47, 52.19, 43.32, 28.19, 25.63, 25.11, 24.96, 24.92,
24.66, 22.78, 21.89. [a]2D5.7ZK19.93 (c 2.06, CHCl3). Mass
Spectral data (MCNa)CZ495.5. Elemental Analysis Calcd
for C22H40N4O7 (472): C, 55.93; H, 8.74; N, 11.86. Found:
C, 54.88; H, 8.77; N, 11.89.
4.1.22. Boc-g-Abu(1)-Aib(2)-Leu(3)-Aib(4)-OMe 5. Boc-
g-Abu(1)-Aib(2)-Leu(3)-OH (0.8 g, 2 mmol) in DMF
(5 mL) was cooled in an ice-water bath and Aib-OMe was
isolated from the corresponding methyl ester hydrochloride
(0.614 g, 4 mmol) by neutralization, subsequent extraction
with ethyl acetate and concentration (10 mL) and it was
added to the reaction mixture, followed immediately by
DCC (0.412 g, 2 mmol) and HOBt (0.270 g, 2 mmol). The
reaction mixture was stirred for 3 days. The residue was
taken in ethyl acetate (60 mL) and the DCU was filtered off.
The organic layer was washed with 2 M HCl (3!50 mL),
brine, 1 M sodium carbonate (3!50 mL), brine (2!
50 mL), dried over anhydrous sodium sulfate and evapor-
ated under vacuum to yield 0.75 g (1.5 mmol) of white
solid. Purification was done by silica gel column (100–200
mesh) using ethyl acetate as eluent. Single crystals were
obtained from methanol–water (methanol/waterZ90:10) by
slow evaporation.
4.1.17. Boc-g-Abu(1)-OH 18. See Ref. 17.
4.1.18. Boc-g-Abu(1)-Aib(2)-OMe 19. See Ref. 17.
4.1.19. Boc-g-Abu(1)-Aib(2)-OH 20. See Ref. 17.
4.1.20. Boc-g-Abu(1)-Aib(2)-Leu(3)-OMe 21. Boc-g-
Abu(1)-Aib(2)-OH (1.0 g, 3.5 mmol) in DMF (5 mL) was
cooled in an ice-water bath and H-Leu-OMe was isolated
from the corresponding methyl ester hydrochloride (1.27 g,
7 mmol) by neutralization, subsequent extraction with ethyl
acetate and concentration (10 mL) and it was added to the
reaction mixture, followed immediately by DCC (0.721 g,
3.5 mmol) and HOBt (0.472 g, 3.5 mmol). The reaction
mixture was stirred for 3 days. The residue was taken in
ethyl acetate (60 mL) and the DCU was filtered off.The
organic layer was washed with 2 M HCl (3!50 mL), brine,
1 M sodium carbonate (3!50 mL), brine (2!50 mL), dried
over anhydrous sodium sulfate and evaporated under
vacuum to yield 1.23 g (2.9 mmol) of white solid.
YieldZ75% (0.75 g, 1.5 mmol). Rf (ethyl acetate) 0.72. Mp
1
104–106 8C. nmax (KBr)/cmK1 1651, 1692, 1746, 3296. H
NMR ((CD3)2SO, 300 MHz, d ppm): 7.54 (d, JZ9 Hz, 1H);
7.41 (s, 1H); 6.08 (s, 1H); 4.70 (t, JZ9.5 Hz, 1H); 4.5.–4.42
(m, 1H); 3.70 (s, 3H); 3.28–3.23 (m, 2H); 2.32–3.28 (m,
2H); 2.17–2.14 (m, 1H); 1.94–1.88 (m, 1H); 1.57 (s, 3H);
1.54 (s, 3H); 1.48 (s, 6H); 1.46 (s, 9H); 0.91–0.86 (m, 6H).
13C NMR (CDCl3, 300 MHz, d ppm): 175.16, 174.27,
173.00, 172.00, 157.04, 141.39, 104.48, 79.62, 56.90, 55.92,
52.20, 51.70, 38.94, 38.39, 32.37, 28.36, 27.00, 26.42,
25.35, 24.94, 24.49, 23.72, 23.29, 20.84. [a]2D5.7ZK21.61
(C 2.04, CHCl3). Mass Spectral data (MCH)CZ501.4,
McalcdZ500. Elemental Analysis Calcd for C22H44N4O7
(500): C, 57.60; H, 8.80; N, 11.20. Found: C, 57.12; H, 8.40;
N, 10.94.
YieldZ85% (1.23 g, 2.9 mmol). Mp 134–136 8C. 1H NMR
(CDCl3, 300 MHz, d ppm): 7.35 (d, JZ9 Hz, 1H); 6.49 (s,
1H); 4.76 (t, JZ9 Hz, 1H); 4.63–4.56 (m, 1H); 3.71 (s, 3H);
3.20–3.18 (m, 2H); 2.21–2.19 (m, 2H); 1.79–1.82 (m, 2H);
1.61–1.64 (m, 3H); 1.53 (s, 3H); 1.55 (s, 3H); 1.45 (s, 9H);

5036
A. K. Das et al. / Tetrahedron 61 (2005) 5027–5036
Simard, M.; Roelens, S. J. Am. Chem. Soc. 1995, 117,
7630–7645.
4.2. Single crystal X-ray diffraction study
7. (a) Rowan, I. E.; Notle, R. J. M. Angew. Chem., Int. Ed. 1998,
37, 62–68. (b) Feiters, M. C.; Notle, R. J. M. Adv. Supramol.
Chem. 2000, 6, 41–156. (c) Lehn, J. M. Chem. Eur. J. 2000, 6,
2097–2102. (d) Lightfoot, M. P.; Mair, F. S.; Pritchard, R. G.;
Warren, J. E. Chem. Commun. 1999, 1945–1946. (e) Tobellion,
F. M.; Seidel, S. R.; Arif, A. M.; Stang, P. J. J. Am. Chem. Soc.
2001, 123, 7740–7741.
For peptides 1–5, intensity data were collected with MoKa
radiation using the MARresearch Image Plate System. For
all peptides, the crystals were positioned at 70 mm from the
Image Plate. Selected details of the structure solutions and
refinements are given in Table 3. 100 frames were measured
at 28 intervals with a counting time of 2 min. Data analyses
were carried out with the XDS program.19 The structures
were solved using direct methods with the Shelx8620
program. For peptide 4, the t-butyl group was disordered,
each methyl group taking up two different sites each refined
with 50% occupancy. For peptide 5 some atoms had high
thermal parameters but no satisfactory disordered model
could be obtained. Apart from disordered atoms, all non-H
atoms were refined with anisotropic thermal parameters.
The hydrogen atoms bonded to nitrogen and carbon were
included in geometric positions and given thermal para-
meters equivalent to 1.2 times those of the atom to which
they were attached. Those bonded to the water molecules in
5 could not be located. The structures were refined on F2
using Shelxl.21 Crystallographic data have been deposited at
the Cambridge Crystallographic Data Centre reference
CCDC 254598-254602 for peptides 1–5.
8. Lehn, J. M.; Rigault, A.; Siegel, J.; Harrowfield, J.; Chevrier,
B.; Moras, D. Proc. Natl. Acad. Sci. U.S.A. 1987, 84,
2565–2569.
9. (a) Allen, W. E.; Fowler, C. J.; Lynch, V. M.; Sessler, J. L.
Chem. Eur. J. 2001, 7, 721–729. (b) Geib, S. J.; Vicent, C.;
Fan, E.; Hamilton, A. D. Angew. Chem., Int. Ed. Engl. 1993,
32, 119–121.
10. Wang, Z.; Enkelmann, V.; Negri, F.; Mullen, K. Angew.
Chem., Int. Ed. 2004, 43, 1972–1975.
11. (a) Moriuchi, T.; Nomoto, A.; Yoshida, K.; Ogawa, A.; Hirao,
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We thank EPSRC and the University of Reading, U.K. for
funds for the Image Plate System. S. Ray thanks the Council
for Scientific and Industrial Research (C.S.I.R), New Delhi,
India for financial assistance. This research is also supported
by a grant from Department of Science and Technology
(DST), India [Project No. SR/S5/OC-29/2003].
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