Demonstration of endo-cis-(2S,3R)-Bicyclo[2.2.1]hept-5-en-2,3 dicarbonyl unit as a reverse-turn scaffold and nucleator of two-stranded parallel β- sheets: Design, synthesis, crystal structure, and self-assembling properties of norborneno peptide analogues

Article abstract of DOI:10.1021/ja980143+

endo-cis-(2S,3R)-Bicyclo[2.2.1]hept-5-en (norbornene) dicarbonyl unit with a built-in U-architecture has been demonstrated to be an excellent reverse-turn molecular scaffold. A large variety of endo-cis-(2S,3R)- norborneno bispeptides containing almost all of the coded amino acids were synthesized and examined for conformational preferences by ¹H NMR, FT-IR, CD, and X-ray crystallographic studies. While FT-IR and ¹H NMR variable- temperature studies ruled out the presence of any significant amount of intramolecular hydrogen bonding in simple bispeptides (3a-h) (except in Aib bispeptide), the CD studies were clearly in favor of a β-tum type structure. Single-crystal X-ray studies on Aib, Val and Leu containing norborneno bispeptides (3b-d) provided convincing proof for the presence of reverse- turn conformation. While the interstrand C(α)-C(α') distances (5.2-5.7 A?) were well within the range of those for β-turn structures, no interstrand intramolecular hydrogen bonding was seen in Val and Leu bispeptides; the Aib bispeptide forms a seven-membered hydrogen-bonded ring, thus, showing that the norbornene (2S, 3R)-dicarbonyl template assembles peptide chains in reverse-turn conformation by virtue of its built-in U-shaped architecture at these positions, and hydrogen bonding may not be necessary to stabilize the turn structure. The endo-cis-(2S,3R) orientation of bispeptide chains is essential for turn structure as shown by the crystal structure of trans- (2R,3R) and trans-(2S,3S) derivative of Val bispeptide wherein the two peptide chains move away from each other with the C(α)-C(α') distance increasing to 7.1-8.2 A?. The norbornene 5,6-double bond was hydrogenated to 5,6-dihydro derivative which showed almost the same CD spectrum as its olefinic analogue. Oxidative cleavage [Ru (VIII)] of the 5,6-double bond in norborneno bispeptides, as demonstrated with Leu bispeptide, afforded novel cyclopentanoid peptide analogues. The promise of norbornene unit as a template for nucleating the formation of two-stranded parallel β-sheets with minimum structural complexity is shown by the preparation of higher members of norbomeno bispeptides with the general structure NBE(Pep)₂ [NBE = endo- cis-(2S,3R)-bicyclo[2.2.1]hept-5-en (norbornene) dicarbonyl unit; Pep = peptide strand with two, three, or four (same or different) amino acid residues]. In ¹H NMR, the high ³J(HNα) values (7.0-9.3 Hz) observed for the amide protons (Table 3) coupled with the presence of medium to strong intrastrand sequential ROE connectivities d(αN(i,i+1)) spanning the entire three- or four-residue sequence in the peptide strands of 9a-e and 10 and the exhibition of relatively low-temperature coefficients (dδ/dT = - 0.2 to - 3.4 ppb/K) for amide protons in DMSO-d₆ solvent (Table 4) clearly suggested that hydrogen-bonded β-sheet conformers dominate the population. FT-IR and CD studies provided further support for parallel β-sheet structures. A particularly unique feature of the norborneno bispeptides is their strong tendency to self-assemble in the solid state. Thus, while endocis-(2S,3R)- Aib bispeptide (3b) forms 16-membered hydrogen bonded centrosymmetric dimers, the half-ester half-acid and the dicarboxylic acid derivatives of 3b self- assemble to form highly ordered hydrogen-bonded molecular ribbons. The Val and Leu cis-(2S,3R)-bispeptides organize into hydrogen-bonded chains and the trans isomer of Val bispeptide self-assembles into hydrogen-bonded β-sheet ribbon.

Full text of DOI:10.1021/ja980143+

8448
J. Am. Chem. Soc. 1998, 120, 8448-8460
Demonstration of endo-cis-(2S,3R)-Bicyclo[2.2.1]hept-5-en-2,3-
dicarbonyl Unit as a Reverse-Turn Scaffold and Nucleator of
Two-Stranded Parallel â-Sheets: Design, Synthesis, Crystal Structure,
and Self-Assembling Properties of Norborneno Peptide Analogues
Darshan Ranganathan,*,†,∇ V. Haridas,† Sunita Kurur,† Achamma Thomas,†
K. P. Madhusudanan,‡ R. Nagaraj,§ A. C. Kunwar,| A. V. S. Sarma,| and
Isabella L. Karle*,
Contribution from Biomolecular Research Unit, Regional Research Laboratory (CSIR), TriVandrum -
695019, India, Central Drug Research Institute, Lucknow 226 001, India, Centre for Cellular and
Molecular Biology, Hyderabad 500 007, India, Indian Institute of Chemical Technology, Hyderabad 500
007, India, and Laboratory for the Structure of Matter, NaVal Research Laboratory,
Washington D.C. 20375
ReceiVed January 13, 1998
Abstract: endo-cis-(2S,3R)-Bicyclo[2.2.1]hept-5-en (norbornene) dicarbonyl unit with a built-in U-architecture
has been demonstrated to be an excellent reverse-turn molecular scaffold. A large variety of endo-cis-(2S,3R)-
norborneno bispeptides containing almost all of the coded amino acids were synthesized and examined for
1
conformational preferences by H NMR, FT-IR, CD, and X-ray crystallographic studies. While FT-IR and
1H NMR variable-temperature studies ruled out the presence of any significant amount of intramolecular
hydrogen bonding in simple bispeptides (3a-h) (except in Aib bispeptide), the CD studies were clearly in
favor of a â-turn type structure. Single-crystal X-ray studies on Aib, Val and Leu containing norborneno
bispeptides (3b-d) provided convincing proof for the presence of reverse-turn conformation. While the
interstrand CR-CR′ distances (5.2-5.7 Å) were well within the range of those for â-turn structures, no interstrand
intramolecular hydrogen bonding was seen in Val and Leu bispeptides; the Aib bispeptide forms a seven-
membered hydrogen-bonded ring, thus, showing that the norbornene (2S,3R)-dicarbonyl template assembles
peptide chains in reverse-turn conformation by virtue of its built-in U-shaped architecture at these positions,
and hydrogen bonding may not be necessary to stabilize the turn structure. The endo-cis-(2S,3R) orientation
of bispeptide chains is essential for turn structure as shown by the crystal structure of trans-(2R, 3R) and
trans-(2S,3S) derivative of Val bispeptide wherein the two peptide chains move away from each other with
the CR-CR′ distance increasing to 7.1-8.2 Å. The norbornene 5,6-double bond was hydrogenated to 5,6-
dihydro derivative which showed almost the same CD spectrum as its olefinic analogue. Oxidative cleavage
[Ru (VIII)] of the 5,6-double bond in norborneno bispeptides, as demonstrated with Leu bispeptide, afforded
novel cyclopentanoid peptide analogues. The promise of norbornene unit as a template for nucleating the
formation of two-stranded parallel â-sheets with minimum structural complexity is shown by the preparation
of higher members of norborneno bispeptides with the general structure NBE(Pep)2 [NBE ) endo-cis-(2S,3R)-
bicyclo[2.2.1]hept-5-en (norbornene) dicarbonyl unit; Pep ) peptide strand with two, three, or four (same or
1
3
different) amino acid residues]. In H NMR, the high JHNR values (7.0-9.3 Hz) observed for the amide
protons (Table 3) coupled with the presence of medium to strong intrastrand sequential ROE connectivities
dRN(i,i+1) spanning the entire three- or four-residue sequence in the peptide strands of 9a-e and 10 and the
exhibition of relatively low-temperature coefficients (dδ/dT ) - 0.2 to -3.4 ppb/K) for amide protons in
DMSO-d6 solvent (Table 4) clearly suggested that hydrogen-bonded â-sheet conformers dominate the population.
FT-IR and CD studies provided further support for parallel â-sheet structures. A particularly unique feature
of the norborneno bispeptides is their strong tendency to self-assemble in the solid state. Thus, while endo-
cis-(2S,3R)-Aib bispeptide (3b) forms 16-membered hydrogen bonded centrosymmetric dimers, the half-ester
half-acid and the dicarboxylic acid derivatives of 3b self-assemble to form highly ordered hydrogen-bonded
molecular ribbons. The Val and Leu cis-(2S,3R)-bispeptides organize into hydrogen-bonded chains and the
trans isomer of Val bispeptide self-assembles into hydrogen-bonded â-sheet ribbon.
Introduction
of protein structure or function.1 Among the regular structures
found in proteins, reverse-turns are known to account for nearly
one-third of the residues in proteins of known structure.2 The
convenient location of turns on protein surface and their
desirable structural compactness with the side chains projecting
outward argue for the possibility that turns may participate
actively in protein folding and may also serve as ideal sites for
receptor binding, antibody recognition, and post-translational
modification in proteins.3 It is therefore not surprising that the
One of the important goals in peptide chemistry is to be able
to design short peptides that can mimic some important aspect
* Authors to whom correspondence should be addressed.
† Regional Research Laboratory; fax 91+0471-490186.
‡ Central Drug Research Institute.
§ Centre for Cellular and Molecular Biology.
| Indian Institute of Chemical Technology.
Naval Research Laborarory; fax (202) 767-6874.
∇ Present address: Discovery Laboratory, Indian Institute of Chemical
Technology, Hyerabad 500 007, India.
S0002-7863(98)00143-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/07/1998

Norborneno Peptide Analogues
J. Am. Chem. Soc., Vol. 120, No. 33, 1998 8449
reverse-turn peptidomimetics are the most sought-after second-
ary structure mimetics4 for study of molecular recognition and
as probes to explore structure-function relationship of receptors
and ligands.
ethene bridge (ii). In that sense, use of this template would
With an ever increasing number of reports describing
preformed peptides, pseudo-peptides, and non-peptide structures
that can be included in polypeptide chains to mimic, induce, or
stabilize various types of turn structures, a new dimension has
been added in the area of constrained peptides and their use in
peptide drug design.5 However, a major breakthrough in the
design of secondary structure mimetics, in particular, reverse-
turn mimetics, has been the use of appropriately crafted,
conformationally rigid, non-peptidic structural frameworks4,5
which serve as ideal scaffolds for supporting the side chain
functional elements in the desired reverse-turn geometry.
In this paper, we demonstrate the use of endo-cis-(2S,3R)-
bicyclo[2.2.1]hept-5-en (norbornene) dicarbonyl unit6 as a low
molecular weight, non-peptidic molecular scaffolding for the
design of reverse-turn peptide analogues and as a promising
template for the nucleation of two-stranded parallel â-sheets.
The endo-cis-(2S,3R)-norbornene scaffolding has a built-in
U-architecture and appeared particularly attractive as a turn
template since it has the structural elements of proline residue
embedded in its framework (i) and can be considered as a carbon
analogue of an N-acyl proline locked at 3,5-positions with an
represent the delineation of chemical simulation of protein
reverse-turns in nature.7 Additionally, the conformationally rigid
and nonlinear architecture of the norbornene skeleton, apart from
providing attractive models for protein folding studies,8 may
also prevent hydrophobic collapse of the peptide units in the
bioactive conformation9 and thus may serve as an ideal scaffold
for the design of peptide-based drugs.
Results and Discussion
The commercially available precursor template cis-5-nor-
bornene-endo-2,3 -dicarboxylic anhydride (1) was converted in
an overall yield of ∼80% into a variety of endo-cis-(2S,3R)-
norbornenobispeptides (3a-h) via a two-step sequence involv-
ing first the opening of the anhydride 1 with an appropriate
amino acid ester (freshly generated in situ at 0 °C from the
respective hydrochloride and triethylamine) at room temperature
in dry CH2Cl2 or THF, followed by coupling of the product
hemiamides 2a-i with their respective identical amino acid
partners using DCC/N-OH succinimide procedure (Scheme 1,
where DCC is dicyclohexylcarbodiimide). The symmetrical
endo-cis-(2S,3R)-norborneno bispeptides (3a-h) thus obtained
were fully characterized by spectral and analytical data.
A noteworthy feature of DCC coupling of amino acid esters
with hemiamides (2a-i) was that while small amounts (∼10%)
of cyclic imides (4a-h) were invariably obtained as side
products, the exclusive formation of imide 4b was noticed in
the case of serine condensation. The Trp imide 4e was the
exclusive product in the first step itself during the opening of
precursor anhydride with Trp-OMe. Parenthetically, cyclic
imides of type 4a-h may provide attractive models as transition-
state analogues for antibody catalysis of specific Diels-Alder
reactions.10
(1) (a) Degrado, W. F. AdV. Protein Chem. 1989, 39, 59. (b) Degrado,
W. F.; Wasserman, Z.; Lear, J. Science 1989, 243, 622. (c) Mutter, M.
Angew. Chem., Int. Ed. Engl. 1985, 24, 639. (d) Mutter, M. Trends Biochem.
Sci. 1988, 13, 26. (e) Mutter, M.; Altmann, K.-H. The Construction of New
Proteins, Die Angewandte Makromolekulare Chemie 1986, 145/146, 211.
(f) Mutter, M.; Vuilleumier, S. Angew. Chem., Int. Ed. Engl. 1989, 28,
535. (g) Richardson, J. S.; Richardson, D. C. Trends Biochem. Sci. 1989,
14, 304. (h) Altmann, K.-H.; Mutter, M. Int. J. Biochem. 1990, 22, 947. (i)
Moser, R.; Thomas, R. M.; Gutte, B. FEBS Lett. 1983, 157, 247. (j) Langen,
H,; Epprecht, T.; Linden, M.; Hehlgans, T.; Gutte, B.; Buser, H.-R. Eur. J.
Biochem. 1989, 182, 727. (k) Kullmann, W. J. Med. Chem. 1984, 27, 106.
(l) Lear, J. D.; Wasserman, Z. R.; Degrado, W. F. Science 1988, 240, 1177.
(m) Muir, T. W.; Kent, S. B. H. Curr. Opin. Biotechnol. 1993, 4, 420. (n)
Schnolzer, M.; Kent, S. B. H. Science 1992, 256, 221. (o) Dawson, P. E.;
Kent, S. B. H. J. Am. Chem. Soc. 1993, 115, 7263. (p) Dawson, P. E.;
Muir, T. W.; Clark-Lewis, I.; Kent, S. B. H. Science 1994, 266, 776. (q)
Canne, L. E.; Ferre´-D’Amare´, A. R.; Burley, S. K.; Kent, S. B. H. J. Am.
Chem. Soc. 1995, 117, 2998.
(2) (a) Chou, P. Y.; Fasman, G. D. J. Mol. Biol. 1977, 115, 135. (b)
Wilmot, C. M.; Thornton, J. M. J. Mol. Biol. 1988, 203, 221.
(3) (a) Smith, J. A.; Pease, L. G. CRC Crit. ReV. Biochem. 1980, 8,
315. (b) Kabasch, W.; Sander, C. Biopolymers 1983, 22, 2577. (c) Rose,
G. D.; Gierasch, L. M.; Smith, J. A. AdV. Protein Chem. 1985, 37, 1. (d)
Milner-White, E. J. J. Mol. Biol. 1990, 216, 385. (e) Rizo, J.; Dhingra, M.
M.; Gierasch, L. M. In Molecular Conformation and Biological Interactions;
Balaram, P., Ramaseshan, S., Eds.; Indian Academy Sciences: Bangalore,
1991; p 469. (f) Gierasch, L. M. Annu. ReV. Biochem. 1992, 61, 387.
(4) For reviews, see: (a) Kemp, D. S. Trends Biotech. 1990, 8, 249. (b)
Ball, J. B.; Alewood, P. F. J. Mol. Recog. 1990, 3, 55. (c) Giannis, A.;
Kolter, T. Angew. Chem., Int. Ed. Engl. 1993, 32, 1244. (d) Liskamp, R.
M. J. Recl. TraV. Chim. Pays-Bas 1994, 113, 1. (e) Saragovi, H. V.; Greene,
M. I.; Chrusciel, R. A.; Kahn, M. Biotechnology 1992, 10, 773. (f) Gante,
J. Angew. Chem., Int. Ed. Engl. 1994, 33, 1699. Also see ref 24c.
(5) (a) Hirschmann, R. Angew. Chem., Int. Ed. Engl. 1991, 30, 1278.
(b) Hirschmann, R.; Nicolaou, K. C.; Pietranico, S.; Leahy, E. M.; Salvino,
J.; Arison, B.; Cichy, M. A.; Spoors, P. G.; Shakespeare, W. C.; Sprengeler,
P. A.; Hamley, P.; Smith, A. B., III; Reisine, T.; Raynor, K.; Maechler, L.;
Donaldson, C.; Vale, W.; Freidinger, R. M.; Cascieri, M. A.; Strader, C.
D. J. Am. Chem. Soc. 1993, 115, 12550. (c) Hirschmann, R.; Sprengeler,
P. A.; Kawasaki, T.; Leahy, J. W.; Shakespeare, W. C.; Smith, A. B., III J.
Am. Chem. Soc. 1992, 114, 9699; Tetrahedron 1993, 49, 3665. (d) Beeley,
N. Tib. Tech. 1994, 12, 213. (e) Kazmierski, W. M. Tib. Tech. 1994, 12,
216. (f) Goodman, M.; Ro, S. Burger’s Medicinal Chemistry and Drug
DiscoVery; Wollf, M. E., Ed.; Wiley: New York, 1995; Vol. 1, pp 803-
861.
In ROESY NMR spectra, strong to medium cross-peaks were
seen for NH(I)-C2H and NH(II)-C3H in bispeptides 3a-h (I
and II denote the peptide strands at positions 2 and 3,
respectively, in norbornene skeleton 3a-h). The NH protons
(7) Stereochemically constrained proline is a well-known turn-inducer
in natural proteins (see ref 3). Proline may promote a â-turn or a γ-turn
depending upon the flanking moieties at the i-position or at i + 2 position
(for example, L, D, or ∆ amino acid residue. See: (a) Aubry, A.; Marraud,
M. Biopolymers 1989, 28, 109. (b) Pavone, V.; Lombardi, A.; D′Aurtia,
G.; Saviano, M.; Nastri, F.; Paolillo, L.; DiBlastio, B.; Pedone, C.
Biopolymers 1992, 32, 173. (c) Imperiali, B.; Fisher, S. L.; Moats, R. A.;
Prins, T. J. J. Am. Chem. Soc. 1992, 114, 3182.
(8) For recent reviews on protein folding, see: (a) Creighton, T. E. Proc.
Natl. Acad. Sci. U.S.A. 1988, 85, 5082. (b) Montelione, G. T.; Scheraga,
H. A. Acc. Chem. Res. 1989, 22, 70. (c) King, J. Chem. Eng. News 1989,
67, 32. (d) Baldwin, R. L. Trends Biochem. Sci. 1989, 14, 291. (e) Protein
Folding; Deciphering the Second Half of the Genetic Code; Gierasch, L.
M., King, J., Eds.; AAAS Pub.: Washington, DC, 1990. (f) Creighton, T.
E. Biochem. J. 1990, 270, 1. (g) Jaenicke, R. Biochemistry 1991, 30, 3147.
(9) A considerable amount of effort has recently been shown in the design
of scaffold peptidomimetics which can reduce or eliminate the hydrophobic
collapse of peptidic chains, thus avoiding the formation of inactive
conformation which appears to be the major problem during the binding of
inhibitors to their receptors in aqueous solution. For a review, see ref 4f.
(10) Hilvert, D.; Hill, K. W.; Nared, K. D.; Auditor, M.-T. M. J. Am.
Chem. Soc. 1989, 111, 9261.
(6) We thank a referee for bringing to our notice the work of North et
al. [(a) Hibbs, D. E.; Hursthouse, M. B.; Jones, I. G.; Jones, W.; Abdul
Malik, K. M.; North, M. J. Org. Chem. 1998, 63, 1496. (b) Jones, I. G.;
Jones, W.; North, M. J. Org. Chem. 1998, 63, 1505] wherein the endo-
(2S,3R)-norborn-5-ene unit in conjunction with a proline residue has been
used as a turn-inducer in peptides and pseudopeptides.

8450 J. Am. Chem. Soc., Vol. 120, No. 33, 1998
Ranganathan et al.
Figure 1. Crystal structure of norborneno bispeptides: (a) 3b; (b) 3c; (c) 3d. All peptides adopt endo-cis-(2S,3R)-configuration (torsion angle
C8-C1-C2-C9 ) +1° (3b), C8-C1-C2-C9 ) -10° (3c), and C8-C1-C2-C9 ) -8° (3d). While the Aib bispeptide (a) exhibits a seven-
membered, interstrand hydrogen-bonded ring (corresponding to γ-turn in proteins), the Val and Leu analogues show a C5 intramolecularly hydrogen-
bonded ring (d) within the same strand (II). The interstrand CR-CR′ distances in 3b-d range between ∼5.2 and 5.7 Å.
Scheme 1
also had medium intensity ROE with CR methyls in 3b and with
CâH2 protons in analogues 3c-3e (Supporting Information).
Although 1H NMR variable-temperature (VT) studies (con-
ducted with 3a-h in CDCl3 between 298 and 328 K) showed
dδ/dT values in the range of -0.8 to -3.6 ppb/K (except in
the case of 3b which exhibited dδ/dT value of -8.5 ppb/K)
indicating only small population of intramolecular hydrogen-
bonded structures (also corroborated by FT-IR studies in
chloroform), the presence of a strong positive band at ∼215
nm in 3c (Figure 3) and at ∼216 nm in 3d (Figure 4) in their
circular dichroism (CD) spectra clearly suggested the type II
â-turn conformation for these bispeptides.11 Final proof for the
endo-cis-(2S,3R)-orientation and the â-turn type conformation
for norborneno bispeptides was provided by single-crystal X-ray
studies.
Suitable crystals were obtained for bispeptides 3b-d. The
crystal structure showed that in Aib (Figure 1a), Val (Figure
1b), and Leu (Figure 1c) bispeptides, the two peptide chains
are oriented in endo-cis-(2S,3R)-configuration and run parallel
to each other (the torsion angle C8-C1-C2-C9 ) +1° (3b),
C8-C1-C2-C9 ) -10° (3c), and C8-C1-C2-C9 ) -8°
(11) Woody, W. W. In The Peptides: Analysis, Synthesis, Biology
Udenfriend, S., Meienhofer, J., Eds.; Academic Press: New York, 1985;
Vol. 17, pp 16-115.

Norborneno Peptide Analogues
J. Am. Chem. Soc., Vol. 120, No. 33, 1998 8451
Table 1. Torsion Angles (deg)
The CD spectrum of diastereomeric mixture of trans isomer
(6a) showed the absence of any turn features (Figure 3) and
thus was in agreement with the solid-state structure. The
presence of the 5,6-double bond in norbornene template did not
contribute much to the turn conformation as was shown by the
CD spectrum of the 5,6-dihydro derivative (obtained from 3c
by hydrogenation; Pd/C, 5%/H2/EtOAc) which still exhibited a
broad positive band although of reduced ellipticity at ∼211 nm
(Figure 3).
However, the ring double bond in norbornene scaffoldings
offered several advantages. Apart from acting as a marker in
1H NMR, the 5,6-double bond could easily be cleaved (Scheme
3), as demonstrated with 3d, under Ru(VIII)-catalyzed oxidative
conditions13,14 to afford a cyclopentanoid peptide analogue (7)
with four contiguous (all cis) asymmetric centers. Interestingly,
the cyclopentane scaffolding 7 retains its â-turn character as
shown by the presence of a broad positive band at ∼211 nm
(similar to that of 3c and 3d) in its CD spectrum (Figure 4).
The newly generated -COOH groups offer two additional
handles for attaching peptide chains thus providing attractive
scaffoldings for crafting peptide dendrimers or as templates for
designing artificial proteins.
From the spectral and the X-ray data, taken together, it can
be concluded that endo-cis-(2S,3R)-bispeptides supported on a
norbornene template would adopt a â-turn type conformation.
The parallel orientation of the chains suggested the use of these
scaffoldings as precursors for the construction of artificial, two-
stranded parallel â-sheetssa secondary structural motif much
sought-after in view of its potential use in the design of protease
inhibitors.
Norbornene Unit as a Template for the Nucleation of
Parallel â-Sheets. Recently, there has been a resurgence15,16
of activity in the design of simple models for â-sheet structures
that can provide deeper insights into the protein folding
mechanism and enzyme-substrate interaction and also serve
as building blocks for molecular receptors and catalysts. A
strategy that has emerged as the most popular approach17 for
creating artificial â-sheets is the use of appropriately crafted
a
angle
3b
3c
3d
6a(A) 6b(B)
C(1)-C(2)-C(3)-C(4)
C(2)-C(3)-C(4)-C(5)
C(3)-C(4)-C(5)-C(6)
C(4)-C(5)-C(6)-C(1)
C(5)-C(6)-C(1)-C(2)
C(6)-C(1)-C(2)-C(3)
C(7)-C(6)-C(1)-C(2)
C(7)-C(3)-C(2)-C(1)
C(8)-C(1)-C(6)-C(7)
C(9)-C(2)-C(3)-C(7)
-67 -63 -63
+70 +67 +69
+58
-70
-1
+71
-63
+3
+41
-50
+71
-62
-14
+83
-55
-9
0
0
-1
-71 -73 -70
+67 +71 +69
0
-3
-4
-38 -34 -35
+37 +40 +40
-169 -158 -160
+43
-25
-73 +173
+172 +169 +168 -176
+87
O(10)-C(1′)-C(1a)-N(1) Ψ1 -152 -40 +153
-18 +138
C(1b)-C(1a)-N(1)-C(8)
C(1′)-C(1a)-N(1)-C(8) Φ1
+173 +172 +176 +125 +163
+57 -63 -63
-98
-69
C(1a)-N(1)-C(8)-C(1)
N(1)-C(8)-C(1)-C(2)
C(8)-C(1)-C(2)-C(9)
C(1)-C(2)-C(9)-N(2)
C(2)-C(9)-N(2)-C(2a)
C(9)-N(2)-C(2a)-C(2b)
ωo1
180 -176 +170 +161 +169
+120 +155 +155 +145 -162
+1b -10b
+29 -66 -68 +161 -145
-8b -123c +111c
ωo2 +177 +168 +169 +173 +179
+178 +111 +105 +136 +125
C(9)-N(2)-C(2a)-C(2′) Φ2
N(2)-C(2a)-C(2′)-O(20) Ψ2 +162 +164 +165
-63 -127 -133 -101
-22
-85
-19
a Symbol used for torsional angles in peptides, IUPAC-IUB
Commission on Biochemical Nomenclature (Biochemistry 1970, 9,
3471-3479). b Cis. c Trans.
(3d)) (Table 1). The interstrand CR-CR′ distances of 5.67 Å
in 3b, 5.20 Å in 3c, and 5.52 Å in 3d were also consistent with
the â-turn conformation. While the crystal structures of Val
(3c) and Leu (3d) bispeptides were characterized by the presence
of an intrastrand C5 type hydrogen bond (N2‚‚‚O2, 2.687 Å;
H2B‚‚‚O2, 2.52 Å) in peptide strand II (Figure 1d), the Aib
analogue (3b) exhibits (Figure 1a) an interstrand seven-
membered hydrogen-bonded ring, (N2‚‚‚O8, 2.772 Å; H2‚‚‚
O8, 1.92 Å) (corresponding to γ-turn in proteins).
The presence of intramolecularly hydrogen-bonded NH in
3b was also suggested by the H NMR VT and FT-IR studies
1
(Supporting Information). Thus, a dδ/dT value of -8.5 ppb/K
for amide NHs in the VT spectrum of 3b (CDCl3, 298-358 K)
and the presence of a broad intense band at 3309 cm-1 in the
FT-IR (CHCl3, 298 K) clearly supported the seven-membered
hydrogen-bonded (NH‚‚‚OdC) ring in 3b.12
(13) Courtney, J. L. In Organic Synthesis by Oxidation with Metal
Compounds; Mijs, W. J., De Jonge, C. R. H. I., Eds.; Plenum Press: New
York, 1986; p 445.
The notion that the reverse-turn conformation in norbornene
scaffolding is controlled by endo-cis-(2S,3R)-orientation of the
two peptide chains was firmly established by the single-crystal
X-ray examination of 6a, the trans isomer of 3c, prepared
(Scheme 2a) by neat heating of the cis peptide 3c at ∼220 °C
for 0.5 h. Interestingly, the crystal structure showed (Figure
2) the presence of two diastereomers. Molecule A (Figure 2a)
with 2R,3R and molecule B (Figure 2b) with 2S,3S configura-
tions have trans orientations of the two Val chains (torsion angle
C8-C1-C2-C9 for molecule A ) +111° and for molecule B
) -123°). The interstrand C1aa-C2aa and C1ax-C2ax
distances in molecule A and B are 8.2 and 7.4 Å, respectively
(Figure 2), indicating that in trans isomers the peptide chains
move away from each other. However, in both cis and trans
isomers (3c and 6a, respectively), the side chains on C1aa and
C2aa are quite flexible and assume different conformations.
Thermolysis of the Leu analogue (3d) gave similar results. A
small amount of the olefinic product (5) obtained in the
thermolysis reaction was identified, by comparison with au-
thentic sample (Scheme 2b) as the trans olefin MeO-Aaa-CO-
CHdCH-CO-Aaa-OMe (E).
(14) (a) Ranganathan, S.; Ranganathan, D.; Bamezai, S.; Singh, W. P.;
Bhattacharyya, D.; Singh, G. P.; Rathi, R. K.; Saini, S.; Jayaraman, N.;
Patel, B. K. Pure Appl. Chem. 1990, 62, 1437. (b) Ranganathan, D.; Saini,
S. J. Am. Chem. Soc. 1991, 113, 1042. (c) Ranganathan, D.; Vaish, N. K.;
Shah, K. J. Am. Chem. Soc. 1994, 116, 6545.
(15) (a) Schneider, J. P.; Kelly, J. W. J. Am. Chem. Soc. 1995, 117,
2533. (b) Gardner, R. R.; Liang, G.-B.; Gellman, S. H. J. Am. Chem. Soc.
1995, 117, 3280. (c) Kobayashi, K.; Yonezawa, N.; Katakai, R. Macro-
molecules 1995, 28, 8242. (d) Yang, Y.; Erickson, B. W. Protein Sci. 1994,
3, 1069. (e) Minor, D. L. Jr.; Kim, P. S. Nature 1994, 371, 264. (f)
Brandmeier, V.; Sauer, W. H. B.; Feigel, M. HelV. Chim. Acta 1994, 77,
70. (g) Feigel, M. Liebigs Ann. Chem. 1989, 459. (h) Kemp, D. S.; Stites,
N. E. Tertahedron Lett. 1988, 29, 5057, 5081. (i) Moser, R.; Klauser, S.;
Leist, T.; Langen, H.; Epprecht, T.; Gutte, B. Angew. Chem., Int. Ed. Engl.
1985, 24, 719.
(16) (a) Winningham, M. J.; Sogah, D. Y. Macromolecules 1997, 30,
862. (b) Winningham, M. J.; Sogah, D. Y. J. Am. Chem. Soc. 1994, 116,
11173. (c) Nowick, J. S.; Pairish, M.; Lee, I. Q.; Holmes, D. L.; Ziller, J.
W. J. Am. Chem. Soc. 1997, 119, 5413. (d) Nowick, J. S.; Abdi, M.;
Bellamo, K. A.; Love, J. A.; Martinez, E. J.; Norohna, G.; Smith, E. M.;
Ziller, J. W. J. Am. Chem. Soc. 1995, 117, 89. (e) Wagner, G.; Feigel, M.
Tetrahedron 1993, 49, 10831.
(17) (a) Michne, W. F.; Schroeder, J. D. Int. J. Pept. Protein Res. 1996,
47, 2. (b) Boumendjel, A.; Roberts, J. C.; Hu, E.; Pallai, P. V.; Rebek, J.,
Jr. J. Org. Chem. 1996, 61, 4434. (c) Schrader, T.; Kirsten, C. Chem.
Commun. 1996, 2089. (d) Gante, J. Synthesis 1989, 405. (e) Hagihara, M.;
Anthony, N. J.; Stout, T. J.; Clardy, J.; Schreiber, S. L. J. Am. Chem. Soc.
1992, 114, 6568. (f) Han, H.; Janda, K. D. J. Am. Chem. Soc. 1996, 118,
2539. (g) Liskamp, R. M. J. Angew. Chem., Int. Ed. Engl. 1994, 33, 633.
(h) Moran, E. J.; Wilson, T. E.; Cho, C. Y.; Cherry, S. R.; Schultz, P. G.
Biopolymers 1995, 37, 213.
(12) Interestingly, peptides containing N-R-benzoyl-cis-4-amino-L-
proline are recently shown to adopt γ-turn conformation in chloroform
solution (Curran, T. P.; Chandler, N. M.; Kennedy, R. J.; Keaney, M. T.
Tetrahedron Lett. 1996, 37, 1933).

8452 J. Am. Chem. Soc., Vol. 120, No. 33, 1998
Ranganathan et al.
Scheme 2
Figure 3. A comparison of the CD spectra (in TFE) of 2,3-cis-Val
peptide 3c (a), 2,3-trans-Val peptide 6a (b) and 5,6-dihydro derivative
of 3c (c). While the cis peptide 3c exhibits a strong band at 215 nm
attributed to type-II â-turn conformation, its trans isomer 6a shows a
diffused broad band indicating loss of â-turn features in the structure.
The 5,6-dihydro derivative of 3c still shows a medium intensity positive
band at ∼211 nm.
Figure 2. Crystal structure of trans-2,3-bis-Val peptide (6a). Two
diastereomers are seen in the solid state. Molecule A (a) with 2R,3R
and molecule B (b) with 2S,3S (the numbering of carbon atoms same
as in 1) have the trans configuration (torsional angle C8-C1-C2-C9
for molecule A ) +111 Å and for molecule B ) -123 Å) for the
orientation of the two Val chains. The interstrand CR-CR′ (C9-C1aa-
C2aa and C1ax-C2ax) distances in molecule A and B are 8.2 and 7.4
Å, respectively.
Scheme 3
peptidic or nonpeptidic molecular scaffolds in combination with
peptide strands and â-strand mimics. The function of the
molecular scaffold being to bring the two peptide strands in
close proximity either by virtue of its molecular geometry or
by inducing intramolecular hydrogen bonding for â-sheet
formation. Interestingly, although several models for antiparallel
â-sheets are available,15 the reports on the design of artificial
parallel sheet structures are still scarce.16 The above results
with norbornene scaffold encouraged us to explore the potential
of this unit as a possible hairpin template for the construction
of simple, minimal models of two-stranded parallel â-sheets.
A large variety of higher norborneno bis-di-, -tri-, and
-tetrapeptides (8-10, Chart 1) containing â-branched amino acid
residues, with high â-sheet propensity, were prepared in a simple
manner by bi-directional elongation of the bis-dicarboxylic acids
of 3c and 3d precursor bispeptides using DCC/N-OH succin-

Norborneno Peptide Analogues
J. Am. Chem. Soc., Vol. 120, No. 33, 1998 8453
with tripeptide Ser-Val-Val-OMe. The bispeptides 8-10
obtained in good yields were purified by column chromatog-
raphy on silica gel using a gradient (CHCl3/MeOH) elution and
1
were fully characterized by H NMR, FT-IR, and FAB MS.
Interestingly, all of the norborneno bispeptides showed high
solubility18 in a wide range of organic solvents, including the
1
apolar chloroform. The 400 MHz H NMR spectra of 8-10
(Supporting Information) in CDCl3 and in DMSO-d6 were
extremely well-resolved with sharp resonances, indicating
presence of well-defined conformational species in solution.
Sequence-specific assignment of all backbone proton resonances
was readily achieved using a combination of TOCSY and
ROESY experiments. The consistently high values (7.0-9.3
3
Hz) observed for the JHNR coupling constants (Table 3)
throughout the strand segment clearly indicated19 extended (â-
strand) conformation in 8-10. The presence of medium to
strong intrastrand sequential ROE connectivities dRN(i,i+1) span-
ning the entire three- or four-residue sequence in peptide strands
of 9a-e and 10 (Supporting Information) further suggested that
â-sheet conformers dominate the population.20 The temperature
dependence of amide protons in 9a, 9c, and 10 (Supporting
Information) was studied in DMSO-d6 over the temperature
range of 293-363 K. The relatively low values for temperature
coefficients (dδ/dT varies between -0.2 to -3.4 ppb/K) of
amide NHs (Table 4) indicate the presence of intramolecular
interstrand hydrogen bonding.21 Although weak, the presence
of long-range ROEs between the NH (i+1) of strand I and the
R(i) proton of strand II in the ROESY spectra of bis-tripeptides
(9a and 9c) provided further evidence for parallel â-sheet
conformation.
Figure 4. A comparison of the CD spectra (TFE) of 2,3-cis-Leu peptide
3d (a) and cyclopentane derivatives 7 (b). The broad band at ∼211
nm in (b) shows that â-turn features are retained in 7.
Chart 1 The Family of Norborneno Bis-di (8a-c), -tri
(9a-e), and -tetra (10) Peptide Analogs (The Peptide Strands
at Positions 2 and 3 Are Denoted by I and II, Respectively)
Further support for intramolecular hydrogen bonding came
from FT-IR studies (298 K, CDCl3). Thus, in homologous Val
series of bispeptides, steady increase in the intensity of
concentration independent NH-stretch band at ∼3300 cm-1
(Supporting Information) with each successive addition of a pair
of Val residues (3c f 8a f 9a) was clearly supportive of
increasing interstrand hydrogen bonding.
CD studies of several members of norborneno bispeptides
(8-10) were carried out in TFE (trifluoroethanol) as well as in
MeOH solvents. The presence of a sharp -ve band at ∼218
nm in 8c and broad -ve band at ∼220 nm in 9a-c and 10
(Supporting Information) strongly supported high population of
(18) Although several non-peptidic, cyclic molecular scaffolds have been
used in the recent past15,16 for creating artificial â-sheet structures, almost
all of them possessed polycyclic aromatic frameworks and had posed serious
solubility problems. For example, Kemp’s 2,8-diaminoepindolidione was
soluble only in concentrated sulfuric acid.15h The parallel â-sheet models
reported here on norbornene molecular scaffold have much improved
solubility in organic solvents such as chloroform, ethyl acetate, and methanol
indicating low tendency of these mimics to self-associate.
3
(19) The JHNR coupling constants greater than 7 Hz are generally
considered to be consistent with â-sheet structure, while values of <6 Hz
are consistent with R-helical structure [(a) Kessler, H. Angew. Chem., Int.
Ed. Engl. 1982, 21, 512. (b) Dyson, H. J.; Wright, P. E. Annu. ReV. Biophys.
Chem. 1991, 20, 519. (c) Wuthrich, K. NMR of Proteins and Nucleic Acids;
Wiley: New York, 1986; pp 166-168)].
(20) Strong NOEs between the R-proton of one residue and the NH group
of the next residue are characteristic of the â-strand (extended) conformation.
(Wuthrich, K. NMR of Proteins and Nucleic Acids; Wiley: New York, 1986;
pp 124-129).
(21) In polar aprotic solvents such as DMSO-d6, the -dδ/dT values >5
ppb/K are interpreted for a solvent-exposed NH while values <3ppb/K
suggest an amide NH that is shielded from solvent through either
intramolecular hydrogen bonding or steric shielding [(a) Zerkout, S.; DuPont,
V.; Aubry, A.; Vidal, J.; Collet, A.; Vicherat, A.; Marraud, M. Int. J. Pept.
Protein Res. 1994, 44, 378. (b) Prasad, S.; Rao, R. B.; Balaram, P.
Biopolymers 1995, 35, 11. (c) Toniolo, C.; Bonora, G. M.; Stavropoulous,
G.; Cordopatis, P.; Theodoropoulos, D. Biopolymers 1986, 25, 281. (d)
Aubry, A.; Chung, M. T.; Marraud, M. J. Am. Chem. Soc. 1985, 107, 1825.
(e) Sakakibara, S. Biopolymers 1995, 37, 17)].
imide coupling procedure. Thus, for the preparation of 8a and
8b, the bisacid of Val analogue 3c was coupled with Val-OMe
and Phe-OMe, respectively, and 8c was obtained from the
bisacid of 3d and Ser-OMe. The bis-tripeptides 9a-e could
either be obtained by two stepwise elongations of 3c and 3d
via the intermediate bis-dipeptides 8a-c as for 9a and 9b or
by a single-step elongation of the precursor peptides 3c and 3d
with the corresponding dipeptides, for example, Val-Val-OMe
for 9a and 9b, Leu-Leu-OMe for 9c, Leu-Ser-OMe for 9d, and
Ser-Leu-OMe for 9e. For the preparation of norborneno bis-
tetrapeptide 10, the precursor Leu bisacid from 3d was coupled

8454 J. Am. Chem. Soc., Vol. 120, No. 33, 1998
Ranganathan et al.
Table 2. Hydrogen Bonds in Norborneno Compounds (NBE) (Å)
3b NBE(Aib)2 cis 3c NBE(Val)2 cis 3d NBE(Leu)2 cis NBE(Aib)2 cis half-ester; half-acid NBE(Aib)2 cis diacid 6a NBE(Val)2 trans
Intramolecular
N2‚‚‚O8 2.772
H‚‚‚O8 1.92
3 f 1
N2‚‚‚O2 2.687
H‚‚‚O2 2.52
C5
N2‚‚‚O2 2.701
H‚‚‚O2 2.50
C5
O2O‚‚‚O8 2.623
4 f 1
Intermolecular
N1‚‚‚O9 2.848
H‚‚‚O9 1.99
dimer
N1‚‚‚O9 2.971
H‚‚‚O9 2.14
infinite chain
N1‚‚‚O9 2.915
H‚‚‚O9 2.04
infinite chain
N1‚‚‚O9 2.886
H‚‚‚O9 2.02
N2‚‚‚O2 2.991
H‚‚‚O2 2.13
stack in infinite
ribbon
N1‚‚‚O1a 3.044
H‚‚‚O1a 2.18
N1‚‚‚O8A 2.942
H‚‚‚O8A 2.14
N2‚‚‚O9A 2.840
H‚‚‚O9A 2.02
N1A‚‚‚O8 2.911
H‚‚‚O8 2.05
N2‚‚‚O2b 2.969
H‚‚‚O2b 2.08
O10a‚‚‚O9 2.738
O20a‚‚‚O8 2.710
stack in â-ribbon
N2A‚‚‚O9 2.862
H‚‚‚O9 2.05
â- sheet
1
3
Table 3. The H NMR JHNR Coupling Constants (Hz) of the NH
Protonsa of 8a, 8b, 9a-c, and 9e (20 °C, 1 mm in CDCl3)b and 10
(in DMSO-d6)
unit shows the intrinsic property of promoting intermolecular
hydrogen bonding with the neighbors and creates highly
organized self-assemblies in the solid state as demonstrated with
the crystal structure of several norborneno bispeptides.
The crystal structure of norborneno bis Aib peptide 3b
showed, in addition to an internal seven-membered hydrogen-
bonded ring, the presence of dimeric structures formed through
intermolecular hydrogen bonding. The internally hydrogen-
bonded structure of 3b (Figure 1a) contains an unutilized NH
function on one Aib chain and a CdO group on the opposite
Aib chain. The bispeptide in this conformation is self-
complementary and dimerizes to form cyclic dimers (Figure 5a)
through a symmetrical pair of N-H‚‚‚O (N‚‚‚O 2.848 Å; H‚‚
‚O 1.99 Å) hydrogen bonds. The 16-membered centrosym-
metric dimer has a cavity with a size of 4.0 × 5.2 Å (N1‚‚‚O9
and C2‚‚‚C2 symmetric distances). The packing diagram of
dimers is shown in Figure 5b.
peptide
Hi
Hi+1 Hi+2 Hi+3 Hi′
Hi′+1 Hi′+2 Hi′+3
8a
8b
9a
9b
9c
9e
10
8.9
9.3
7.0
8.2
8.3
7.5
4.8
8.3
8.6
7.9
8.0
7.4
8.2
7.2
9.4
8.7
9.2
8.4
8.1
8.0
7.4
8.3
8.3
7.9
8.4
7.9
7.7
7.5
8.7
8.9
7.9
8.0
9.3
8.7
8.2
7.9
7.8
8.8
7.9
7.7
a i and i′ represent the first amino acid residue in strand I and II,
respectively. b The JHNR values were found to have the same profile
3
in DMSO-d6 also.
1
Table 4. Temperature Dependence of the H NMR Chemical
Shifts-(ppb/K) of the NH Protons of Bispeptides 9a, 9c, and 10 (1
mM in DMSO-d6)a
peptide
Hi
Hi+1 Hi+2 Hi+3 Hi′
Hi′+1 Hi′+2 Hi′+3
Interestingly, the hydrogen-bonded dimers of 3b are stable
in solution (as shown by the molecular weight value of 805
obtained in chloroform by vapor pressure osmometry method)
9a
9c
10
2.3
2.3
3.4
2.2
0.9
2.2
4.3
3.1
0.2
2.3
2.6
2.3
3.1
1.7
1.6
2.9
1.0
2.5
1.8
2.8
(23) (a) Smith, A. B., III.; Keenan, T. P.; Holcomb, R. C.; Sprengeler,
P. A.; Guzman, M. C.; Wood, J. L.; Carroll, P. J.; Hirschmann, R. J. Am.
Chem. Soc. 1992, 114, 10672. (b) Smith A. B., III.; Holcomb, R. C.;
Guzman, M. C.; Keenan, T. P.; Sprengeler, P. A.; Hirschmann, R.
Tetrahedron Lett. 1993, 34, 63. (c) Smith, A. B., III.; Hirschmann, R.;
Pasternak, A.; Akaishi, R.; Guzman, M. C.; Jones, D. R.; Keenan, T. P.;
Sprengeler, P. A.; Darke. P. L.; Emini, E. A.; Holloway, M. K.; Schleif,
W. A. J. Med. Chem. 1994, 37, 215. (d) Smith, A. B., III.; Guzman, M. C.;
Sprengeler, P. A.; Keenan, T. P.; Holcomb, R. C.; Wood, J. L.; Carroll, P.
J.; Hirschmann, R. J. Am. Chem. Soc. 1994, 116, 9947. (e) Smith, A. B.;
Akaishi, R.; Jones, D. R.; Keenan, T. P.; Guzman, M. C.; Holcomb, R. C.;
Sprengeler, P. A.; Wood, J. L.; Hirschmann, R.; Holloway, M. K.
Biopolymers 1995, 37, 29. (f) Smith, A. B. III; Hirschmann, R.; Pasternak,
A.; Guzman, M. C.; Yokoyama, A.; Sprengeler, P. A.; Darke, P. L.; Emini,
E. A.; Schlief. W. A. J. Am. Chem. Soc. 1995, 117, 11113.
(24) For recent reviews in self-assembly, see: (a) Lehn, J.-M. Supramo-
lecular chemistry: Concepts and perspectiVes; VCH: Weinheim, 1995. (b)
Whitesides, G. M.; Simanek, E. E.; Mathias, J. P.; Seto. C. T.; Chin, D.
N.; Mammen, M.; Gordon, D. M. Acc. Chem. Res. 1995, 28, 37. (c)
Lawrence, D. S.; Jiang, T.; Levett, M. Chem. ReV. 1995, 95, 2229. (d)
McDonald, J. C.; Whitesides, G. M. Chem. ReV. 1994, 94, 2383. (e)
Aakeroy, C. B.; Seddon, K. R. Chem. Soc. ReV. 1993, 22, 397.
(25) (a) Ranganathan, D. Pure Appl. Chem. 1996, 68, 671. (b) Karle, I.
L.; Ranganathan, D. Int. J. Pept. Protein Res. 1995, 46, 65. (c) Karle, I. L.;
Ranganathan, D. Int. J. Pept. Protein Res. 1995, 46, 18. (d) Karle, I. L.;
Ranganathan, D. Int. J. Pept. Protein Res. 1995, 46, 24. (e) Karle, I. L.
Ranganathan, D. Biopolymers 1995, 36, 323. (f) Ranganathan, D.; Kurur,
S.; Madhusudanan, K. P.; Karle, I. L. Tetrahedron Lett. 1997, 38, 4659.
(g) Ranganathan, D.; Kurur, S.; Madhusudanan, K. P.; Roy, R.; Karle, I.
L. J. Pept. Res. 1998, 51, 297. (h) Coe, S.; Kane, J. J.; Nguyen, T. L.;
Toledo, L. M.; Wininger, E.; Fowler, F. W.; Lauher, J. W. J. Am. Chem.
Soc. 1997, 119, 86.
(26) (a) Nowick, J. S.; Pairish, M.; Lee, I. Q.; Holmes, D. L.; Ziller, J.
W. J. Am. Chem. Soc. 1997, 119, 5413. (b) Schrader, T.; Kirsten, C. Chem.
Commun. 1996, 2089. (c) Hagihara, M.; Anthony, N. J.; Stout, T. J.; Clardy,
J.; Schreiber, S. L. J. Am. Chem. Soc. 1992, 114, 6568.
a VT spectra recorded at 10 °C intervals from 20 to 70 °C.
â-sheet conformers. In bis-tripeptides 9a and 9b a new
minimum appeared at ∼205 nm which was not seen in the CD
of other bispeptides.22
Thus, collectively, the spectroscopic (1H NMR, FT-IR and
CD) studies establish the parallel â-stranded nature of the two
peptide chains in higher norborneno bispeptides 8-10 as was
originally seen in the precursor bispeptides 3c and 3d. Fur-
thermore, the increasing population of interstrand hydrogen-
bonded structures in higher bispeptides clearly shows that
norbornene unit is an effective template for nucleating parallel
â-sheet structures.
Self-Assembling Properties of Norborneno Peptide Ana-
logues. Although, in the past decade there has been virtually
a near explosion in the reports describing the design of protein
secondary structure mimetics,23 an important aspect of pepti-
domimetics which had hitherto received scant attention is their
tendency to self-associate24 either in solid or in solution state
through intermolecular hydrogen bonding. This particular aspect
gains more importance in view of their preferred conformations3
during receptor binding. Only very recently has this area been
mentioned in a few reports on the self-assembling properties
of core-modified small linear peptides25 and secondary structure
mimetics.26 In the present design, the presence of norbornene
(22) A shoulder at ∼210 nm in the CD of longer norborneno bispeptides
was attributed to the constrained norbornene unit (ref 6).

Norborneno Peptide Analogues
J. Am. Chem. Soc., Vol. 120, No. 33, 1998 8455
Figure 5. Hydrogen-bonded cyclic dimers of 3b in the solid state (a)
X-ray structure and (b) crystal packing.
as well as in vapor phase (FAB MS shows 6% of dimer peak).
In contrast, the dicarboxylic acid of 3b did not exhibit any
intramolecular hydrogen bonds in its crystal structure (Figure
6a). There was no inter- or intramolecular hydrogen bonding
between the -COOH groups as is normally found in dicar-
boxylic acids. As shown in Figure 6b, the U-shaped dicar-
boxylic acid molecules stack in an antiparallel fashion, con-
nected to each other, through a network of N-H‚‚‚O and O-H‚
‚‚O hydrogen bonds. Each molecule participates in a total of
eight hydrogen bonds (four to each neighbor), thus utilizing all
of its hydrogen-bond donors and acceptors in intermolecular
hydrogen bonding which extends into an infinite double-stranded
â-ribbon (Table 2). The four independent hydrogen bonds are
locked in a contiguous alternating array of 11-membered and
14-membered (a characteristic feature of â-sheet) hydrogen-
bonded rings in the double-stranded â-ribbon. In the dicar-
boxylic acid ribbon assembly, the molecules are repeated by a
vertical 2-fold screw axis which directs the norbornene units
toward the edges of the molecular ribbon giving the edges a
very hydrophobic character. A schematic representation of the
self-assembly of dicarboxylic acid is shown in Figure 6c.
Figure 6. (a) Crystal structure of the dicarboxylic acid of 3b (b). X-ray
picture of the infinite double-stranded ribbon assembly formed by
antiparallel stacking of dicarboxylic acid molecules through a network
of N-H‚‚‚O and O-H‚‚‚O hydrogen bonds. The molecules are repeated
by a vertical 2-fold screw axis which directs the norbornene units toward
the edges of the ribbon (c) a schematic representation of the hydrogen-
bonded molecular ribbon.
structure and the schematic representation of the self-assembly
of hemiester of 3b.
The self-assembly pattern of endo-cis-(2S,3R)-bis-Val peptide
3c and its trans isomer 6a differ from each other and from the
Aib bis peptide 3b. Thus, in endo-cis-peptide 3c while one
amide NH is locked in an intrachain C5 hydrogen-bonded ring
with the ester carbonyl (Figure 1d), the NH of the second Val
residue participates in intermolecular hydrogen bonding with
the norbornene carbonyl creating a hydrogen-bonded infinite
chain wherein each molecule bonds with two neighbors through
a pair of N-H‚‚‚O bonds [N‚‚‚O 2.971 Å, H‚‚‚O 2.14 Å (Figure
8a)]. The molecules in the chain organize in an antiparallel
manner with a 2-fold screw axis (schematic representation
shown in Figure 8b).
The trans-bis-Val peptide 6a on the other hand, by virtue of
its nearly extended backbone, shows a different self-assembly.
The two diastereomers 2R,3R and 2S,3S alternate in the stack
to form a highly ordered â-sheet ribbon wherein each molecule
is bonded to two neighbors through two pairs of symmetrical
intermolecular NH‚‚‚OdC hydrogen bonds forming the char-
The half-ester half-acid derivative of 3b showed in its crystal
structure (Figure 7a) the presence of a 10-membered intramo-
lecularly hydrogen-bonded ring involving norbornene carbonyl
and acid -OH group (O20‚‚‚O8, 2.71 Å; H20‚‚‚O8, 1.91 Å)
as compared to a seven-membered ring involving N2‚‚‚O8 in
3b. Like the dicarboxylic acid, the cis half-ester half-acid
molecules self-assemble in infinite ribbons by a pair of
intermolecular N-H‚‚‚O hydrogen bonds with norbornene
carbonyl and carboxylic carbonyl groups acting as hydrogen-
bond acceptors. The molecules alternate in orientation along
the ribbon. Figures 7b and c show, respectively, the crystal

8456 J. Am. Chem. Soc., Vol. 120, No. 33, 1998
Ranganathan et al.
Figure 7. (a) Crystal structure of half-ester half-acid of 3b. The
molecule is locked in a 10-membered intramolecularly hydrogen-bonded
ring (b), the infinite ribbon assembly formed by the molecules which
alternate in orientation along the ribbon, and (c) a schematic representa-
tion of the hydrogen-bonded ribbon stabilized by a network of
intramolecular (O-H‚‚‚O) and intermolecular (N-H‚‚‚O) hydrogen
bonds.
Figure 8. (a) A hydrogen-bonded chain assembly of 3c in solid state.
The molecules in the chain organize in an antiparallel manner with a
2-fold screw axis. The intramolecular C5 hydrogen-bonded ring is not
shown. (b) A schematic representation of the hydrogen-bonded mo-
lecular chains in 3c and (c) X-ray picture of the chain assembly in
structurally similar 3d. The molecules are connected by continuous
N-H‚‚‚O hydrogen bonds.
acteristic 14-membered rings found in â-sheets (Figure 9a). The
N-H‚‚‚O hydrogen bonds are formed between NH of valine
and CdO of norbornene units (N1‚‚‚O1A, 2.976 Å; H1A‚‚‚
O1A, 2.20 Å; N2‚‚‚O4A, 2.832 Å; H2B‚‚‚O4A, 2.01 Å; N1A‚
‚‚O1, 2.901 Å; H1A‚‚‚O1, 2.05 Å; N2A‚‚‚O4, 2.952 Å, H2A‚
‚‚O4, 2.13 Å) in the adjacent neighbors. A noteworthy feature
of the self-assembly of trans-bis-Val peptide is the lining-up
of the Val and carbomethoxy methyls at the edges and
norbornene units in the middle of the â-sheet ribbon giving these
regions a very hydrophobic character. Figure 9b shows the
schematic representation of the sheet assembly.
Conclusion
In summary, the (2S,3R)-norbornene dicarbonyl unit with a
built-in U-architecture has been demonstrated as a reliable
molecular scaffold for the construction of reverse-turn peptide
analogues and as a nucleator of two-stranded parallel â-sheets.
A large variety of (2S,3R)-substituted norborneno bispeptides
have been synthesized and examined for conformational prefer-
ences by 1H NMR, FT-IR, CD, and X-ray studies. While FT-
IR and VT studies are not in favor of hydrogen-bonded
structures in simple bispeptides (except in Aib case), CD spectra
clearly suggested the presence of â-turn type features in endo-
cis-(2S,3R)-bispeptides 3c and 3d, also, supported by ROESY
NMR spectra. The 5,6-double bond although not necessary for
turn structure can be exploited to generate cyclopentanoid
peptide analogues by Ru(VIII)-catalyzed oxidative cleavage. The
reverse-turn structure has been firmly established by single-
crystal X-ray studies in Aib, Val, and Leu containing norborneno
As anticipated, the endo-cis-(2S,3R)-bis-Leu peptide 3d shows
strikingly similar self-assembly (Figure 8c) with a characteristic
chain motif as observed in norborneno bis-Val peptide 3c. The
N1H‚‚‚O9 hydrogen bonds link the molecules into a continuous
string around a 2-fold screw axis (N1‚‚‚O9, 2.915 Å; H‚‚‚O9,
2.04 Å). The strong tendency of norborneno peptide analogues
to self-assemble into contiguous ribbons or â-sheet-like struc-
tures through intermolecular hydrogen bonding is noteworthy
and may have relevance in the design of protease inhibitors.26

Norborneno Peptide Analogues
J. Am. Chem. Soc., Vol. 120, No. 33, 1998 8457
demonstration of a conformationally rigid, small, low molecular
weight, non-peptidic (2S,3R)-norbornene dicarbonyl unit as a
molecular scaffold for the construction of minimal models of
parallel two-stranded â-sheets and reverse-turn peptide ana-
logues is particularly important in the context of designing
peptide-based drugs.
Experimental Section
All amino acids used were of L-configuration. Melting points were
recorded on a Fischer-Johns melting point apparatus and are uncor-
rected. Optical rotations were measured with an automatic JASCO
polarimeter; concentrations are given in grams/100 mL. Infrared spectra
were recorded on a Perkin-Elmer/1600FT spectrometer in chloroform
solutions, as neat liquids, or as KBr pellets, and prominent peaks are
expressed in cm-1 1H NMR spectra were recorded on Varian UNITY-
.
400, Bruker WM 300, Hitachi R 600 and JEOL 90 MHz instruments.
The chemical shifts are reported in δ (ppm) with TMS at 0.00 as an
internal reference. ROESY experiments were performed using 0.2 s
mixing time with pulsed spin locking with 30° pulses and 2 kHz spin
locking field. FAB MS were obtained on a JEOL SX-120/DA-6000
instrument using m-nitrobenzyl alcohol as the matrix. The circular
dichroism (CD) spectra were recorded on JASCO J 20 spectropola-
rimeter in quartz cells of 1 mm path length at 25 °C. Reactions were
monitored wherever possible by TLC. Silica gel G (Merck) was used
for TLC, and column chromatography was done on silica gel (100-
200 mesh) columns, which were generally made from a slurry in hexane
or a mixture of hexane and ethyl acetate. Products were eluted
(gradient) with either a mixture of ethyl acetate/hexane or chloroform/
methanol.
Reaction of cis-5-Norbornene-endo-2,3-dicarboxylic Anhydride
(1) with Amino Acid Esters. Preparation of N-[3-Carboxybicyclo-
[2.2.1]hept-5-en-2-ylcarbonyl] Amino Acid Methyl Esters (2a-i).
General Procedure. A well-stirred and ice-cooled solution of cis-5-
norbornene-endo-2,3-dicarboxylic anhydride (1, 10 mmol) in dry CH2-
Cl2 (∼20 mL) was admixed with amino acid methyl ester free base
(freshly generated in situ at 0 °C from corresponding methyl ester
hydrochloride and triethylamine, 10 mmol each, in dry CH2Cl2 (∼50
mL)) and stirred at room temperature for 12 h. The reaction mixture
was worked up by washing, sequentially, with ice-cold 2 N H2SO4
and water (20 mL each), drying (anhyd. MgSO4), and evaporating in
vacuo. The residue in most cases was directly crystallized from a
mixture of ethyl acetate and hexane.
Figure 9. (a) A â-sheet ribbon assembly of 2,3-trans-Val bispeptides
(6a). The two mirror-image isomers (2R,3R and 2S, 3S) alternate in
the stack, each molecule bonded to the neighbor through a symmetrical
pair of N-H‚‚‚O hydrogen bonds forming the characteristic 14-
membered rings found in â-sheets and (b) a schematic representation
of the hydrogen-bonded â-sheet ribbon. A noteworthy feature of the
assembly is the lining-up of the carbomethoxy methyls at the edges
and norbornene units in the middle of the ribbon, giving these regions
a very hydrophobic character.
bispeptides 3b, 3c, and 3d, respectively. Crystal structure
analyses showed that except in Aib bispeptide which has an
intramolecular interstrand seven-membered ring (corresponding
to a γ-turn in peptides), all bispeptides adopt open-turn structures
with interstrand CR-CR′ distances in the range of 5.2-5.7 Å.
That the endo-cis-(2S,3R)-orientation of bispeptide chains is
essential for turn conformation was convincingly shown by the
crystal structure of trans-(2R,3R)- and trans-(2S,3S)-Val deriva-
tives wherein the two chains moved away from each other (CR-
CR′, 7.2-8.2 Å). Spectroscopic studies have clearly established
the parallel â-stranded nature of the peptide chains in higher
norborneno bispeptides (8-10) and thus have shown that
norbornene unit can act as an efficient template for the
nucleation of hydrogen-bonded parallel â-sheets.
A particularly noteworthy feature of the norborneno bispep-
tides is their strong tendency to self-assemble in solid state.
Thus, while endo-cis-(2S,3R)-Val and -Leu bispeptides self-
assemble in infinite hydrogen-bonded chains, the trans isomer
of Val bispeptide organizes to form an extensively hydrogen-
bonded â-sheet ribbon. Interestingly, while the internally
hydrogen-bonded dimethyl ester of Aib bispeptide (3b) forms
16-membered hydrogen-bonded centrosymmetric dimers, the
half-ester half-acid and the dicarboxylic acid derivatives of 3b
form highly ordered molecular ribbons through participation of
all of the available hydrogen-bonding donors and acceptors. An
attractive feature of these assemblies is the lining up of
norbornene units to provide strong hydrophobic regions. The
N-(3-Carboxybicyclo[2.2.1]hept-5-en-2-ylcarbonyl) alanine meth-
yl ester (2a): yield 86%; mp 116-118 °C; IR (KBr) 3510, 3005, 2950,
1745, 1703, 1558, 1514, 1461, 1394 cm-1; 1H NMR (60 MHz, CDCl3)
δ 1.33 (3H, d, J ) 7.5 Hz), 1.58 (2H, dd), 3.24 (4H, m), 3.65 (3H, s),
4.50 (1H, m), 6.02 (2H, brs), 6.43(2H, br, exchangeable with D2O).
N-(3-Carboxybicyclo[2.2.1]hept-5-en-2-ylcarbonyl) Aib methyl
ester (2b): yield 94%; mp 124-128 °C; IR (KBr) 3366, 3080, 2977,
2743, 2680, 2497, 1727, 1705, 1674, 1541, 1472, 1439, 1392 cm-1
;
1H NMR (60 MHz, CDCl3) δ 1.04-1.64 (8H, s+m), 3.16 (4H, m),
3.65 (3H, s), 6.19 (2H, m), 7.28 (1H, s, exchangeable).
N-(3-Carboxybicyclo[2.2.1]hept-5-en-2-ylcarbonyl) serine methyl
ester (2c): yield 66%; syrup; IR (KBr) 3451 (br), 2990 (br), 1750,
1
1712, 1574, 1544, 1524, 1435, 1395, 1340 cm-1; H NMR (60 MHz,
CDCl3) δ 1.64 (2H, br dd), 3.33 (4H, brs), 3.68 (3H, s), 3.88 (2H, d,
J ) 6.0 Hz), 4.66 (1H, br t), 6.17 (3H, m).
N-(3-Carboxybicyclo[2.2.1]hept-5-en-2-ylcarbonyl) valine methyl
ester (2d): yield 80%; mp 114-116 °C; IR (KBr) 3549, 3375, 3279,
3088, 2978, 2672, 1727, 1657, 1571, 1531, 1469, 1440, 1388 cm-1
;
1H NMR (60 MHz, CDCl3) δ 0.89 (6H, d, J ) 6.0 Hz), 1.41 (2H, brs),
2.02 (1H, m), 3.20 (4H, brd), 3.65 (3H, s), 4.39 (1H, m), 5.98-6.41
(3H, m), 6.49 (1H, brd, exchangeable).
N-(3-Carboxybicyclo[2.2.1]hept-5-en-2-ylcarbonyl) leucine meth-
yl ester (2e): yield 90%; mp 94-96 °C; IR (KBr) 3532, 3418, 3274,
1
3096, 2971, 2620, 1730, 1657, 1569, 1393 cm-1; H NMR (60 MHz,
CDCl3) δ 0.96 (6H, d, J ) 5.0 Hz), 1.51 (5H, m), 3.14 (4H, m), 3.70
(3H, s), 4.53 (1H, m), 6.03-6.79 (4H, m); FAB MS m/z 310 (MH)+.
N-(3-Carboxybicyclo[2.2.1]hept-5-en-2-ylcarbonyl) phenylalanine
methyl ester (2f): yield 90%; mp 98-100 °C; IR (KBr) 3531, 3417,

8458 J. Am. Chem. Soc., Vol. 120, No. 33, 1998
Ranganathan et al.
3283, 3092, 2978, 2647, 1721, 1666, 1565, 1504, 1452, 1387 cm-1
;
CDCl3) δ 1.60 (2H, br), 2.27 (4H, m), 3.32 (4H, brd), 3.61, 3.63 (3H,
s; 3H, s), 4.52 (1H, m), 6.10 (2H, s); FAB MS m/z 322 (MH)+.
1H NMR (60 MHz, CDCl3) δ 1.32 (2H, brs), 2.77-3.33 (6H, m), 3.62
(3H, s), 4.57 (1H, m), 5.72 (2H, brs), 6.27 (1H, brd), 7.04 (5H, brs);
FAB MS m/z 344 (MH)+.
Preparation of endo-cis-(2S,3R)-diyl-norborneno Bispeptides
(3a-h). General Procedure. Solid N-hydroxy succinimide (1 mmol)
and dicyclohexylcarbodiimide (DCC, 1 mmol) were added sequentially
at 0 °C to a stirred solution of N-(3-carboxy bicyclo[2.2.1]hept-5-en-
2-ylcarbonyl) amino acid methyl ester (2a-i, 1 mmol) in dry CH2Cl2
(∼20 mL) or in a mixture of dry DMF and CH2Cl2 in cases where
solubility was poor in CH2Cl2. After a period of ∼0.25 h, the reaction
mixture was admixed with the amino acid methyl ester, prepared in
situ at 0 °C from the corresponding methyl ester hydrochloride and
triethylamine (1.2 mmol each) in dry CH2Cl2 or in a mixture of dry
DMF and CH2Cl2. The combined mixture was stirred at room
temperature for 2 days, the precipitated dicyclohexyl urea filtered, and
the residue washed with CH2Cl2 (2 × 10 mL), and the combined filtrates
were washed sequentially with cold 2 N H2SO4 (∼20 mL), water (∼20
mL), and saturated bicarbonate solution (∼20 mL). The organic extract
was dried (MgSO4) and evaporated in vacuo. The residue in most cases
was directly crystallized from EtOAc/hexane or purified on a short
column of silica gel using EtOAc/hexane as eluents.
N-(3-Carboxybicyclo[2.2.1]hept-5-en-2-ylcarbonyl) methionine
methyl ester (2 g): yield 54%; mp 102-104 °C; IR (KBr) 3525, 3390,
3287, 3087, 2978, 2950, 2660, 1726, 1656, 1561, 1436, 1387 cm-1
;
1H NMR (60 MHz, CDCl3) δ 1.39 (2H, brs), 2.03 (5H, s+m), 2.46
(2H, m), 3.19 (4H, brd), 3.66 (3H, s), 4.51 (1H, m), 6.21 (3H, m),
7.31 (1H, brd).
N-(3-Carboxybicyclo[2.2.1]hept-5-en-2-ylcarbonyl) aspartic acid
dimethyl ester (2h): yield 73%; syrup; IR (KBr) 3403, 3072, 3025,
3003, 2959, 2702, 2487, 1733, 1654, 1523, 1446, 1400, 1380 cm-1
;
1H NMR (60 MHz, CDCl3) δ 1.30 (2H, brs), 2.75 (2H, brd), 3.15 (4H,
brd), 3.61 (6H, s), 4.65 (1H, m), 6.20 (2H, br), 7.85 (1H, brd).
N-(3-Carboxybicyclo[2.2.1]hept-5-en-2-ylcarbonyl) glutamic acid
1
dimethyl ester (2i): yield 71%; syrup; H NMR (60 MHz, CDCl3) δ
1.35 (2H, brs), 2.25 (4H, m), 3.30 (4H, m), 3.65 (6H, s), 4.50 (1H, m),
6.10 (2H, br), 7.85 (1H, br).
Norbornenyl Cyclic Imides (4a-h) of Amino Acid Methyl Esters
Isolated during the DCC/N-OH Succinimide Coupling of Mono-
carboxylic Acid Derivatives 2a-i. N-(endo-Bicyclo[2.2.1]hept-5-
en-2,3-diyldicarbonyl) Aib methyl ester (4a): yield 10%; mp 140-
142 °C; IR (KBr) 3077, 2996 (br), 2878, 1778 (sh), 1745, 1713, 1551
(sh), 1458, 1439 (sh), 1365, 1287 cm-1; 1H NMR (90 MHz,CDCl3) δ
1.35-1.65 (8H, s+m), 3.20 (2H, brs), 3.30 (2H,brs), 3.65 (3H, s), 6.09
(2H, brs); 13C NMR (22.40 MHz, CDCl3) δ 177.3, 172.8, 134.3, 60.1,
52.2, 51.7, 45.3, 45.0, 23.3; FAB MS m/z 264 (MH)+.
N,N′-(endo-Bicyclo[2.2.1]hept-5-en-2,3-diyldicarbonyl) bis-alanine
methyl ester (3a): yield 88%; mp 125-127 °C; [R]2D6 -27.56 (c 1.70,
CHCl3); IR (KBr) 3350, 2950, 1743 (br), 1637, 1574, 1551, 1453, 1389
1
cm-1; H NMR (60 MHz, CDCl3) δ 1.34 (3H, d, J ) 6.5 Hz), 1.38
(3H, d, J ) 6.5 Hz), 1.53 (2H, dd), 3.35 (4H, m), 3.67 (6H, brs), 4.48
(2H, m), 6.05 (2H, m), 6.34 (1H, m), 6.83 (1H, m); FAB MS m/z 353
(MH)+. Anal. Calcd. for C17H24N2O6 (mol wt 352): C, 57.95; H,
6.82; N, 7.95. Found: C, 57.80; H, 7.12; N, 7.58.
N-(endo-Bicyclo[2.2.1]hept-5-en-2,3-diyldicarbonyl) serine methyl
ester (4b): yield 90%; mp 127-129 °C; IR (KBr) 3336, 2933, 2856,
N,N′-(endo-Bicyclo[2.2.1]hept-5-en-2,3-diyldicarbonyl) bis-Aib
methyl ester (3b): yield 88%; mp 154-155 °C; IR (KBr) 3320, 3290
(sh), 3067, 2997, 2962 (sh), 2875 (sh), 1741, 1689, 1650, 1633 (sh),
1777 (sh), 1750, 1704, 1629, 1575, 1551, 1473 (sh), 1436, 1396 cm-1
;
1H NMR (60 MHz, CDCl3) δ 1.60 (2H, dd), 3.33 (4H, brs), 3.68 (3H,
s), 3.87 (2H, d, J ) 5.0 Hz), 4.60 (1H, t), 6.03 (2H, s); FAB MS m/z
266 (MH)+. Anal. Calcd. for C13H15NO5 (mol wt 265): C, 58.86; H,
5.66; N, 5.28. Found: C, 58.42; H, 5.32; N, 4.88.
1
1554, 1531, 1465, 1441 cm-1; H NMR (400 MHz, CDCl3) δ 1.33
(1H, d, J ) 8.4 Hz), 1.43 (1H, d, J ) 8.4 Hz), 1.47 (12H, s), 3.12 (4H,
s), 3.71 (6H, s), 6.36 (2H, s), 7.18 (2H, s); FAB MS m/z 381 (M +
H)+ (100%), 761 (2 × M + H)+ (6%). Anal. Calcd. for C19H28N2O6
(mol wt 380): C, 60.00; H, 7.36; N, 7.36. Found: C, 60.14; H, 7.48;
N, 7.67.
N-(endo-Bicyclo[2.2.1]hept-5-en-2,3-diyldicarbonyl) valine methyl
ester (4c): yield 10%; mp 58-59 °C; IR (KBr) 3068, 2989, 2955, 2881,
1775 (sh), 1756, 1705, 1628, 1579, 1547, 1458, 1434, 1388, 1353, 1286
Hydrolysis (2 N aqueous NaOH) of bis methyl ester afforded a
1
cm-1; H NMR (60 MHz, CDCl3) δ 0.80 (3H, d, J ) 6.0 Hz), 1.03
mixture of half-ester half-acid [mp 208-210 °C; IR (KBr) 3338, 3286,
(3H, d, J ) 6.0 Hz), 1.65 (2H, dd,), 2.55 (1H, m), 3.37 (4H, brd), 3.68
(3H, s), 4.24 (1H, d, J ) 9 Hz), 6.15 (2H, s); FAB MS m/z 278 (MH)+.
Anal. Calcd. for C15H19NO4 (mol wt 277): C, 64.98; H, 6.85.
Found: C, 65.28; H, 6.84.
3087, 2999, 2880, 1745 (br), 1642, 1556, 1532, 1475, 1458, 1393 cm-1
;
FAB MS m/z 367, (MH)+, 389 (M + Na+)] and diacid [mp 238-240
°C; IR (KBr) 3357, 3077, 2997, 1725, 1646, 1556, 1467, 1405, 1367
cm-1; FAB MS m/z 353 (MH)+].
N-(endo-Bicyclo[2.2.1]hept-5-en-2,3-diyldicarbonyl) leucine meth-
yl ester (4d): yield 9%; mp 75-77 °C; IR (KBr) 3070, 2969, 2876,
N,N′-(endo-Bicyclo[2.2.1]hept-5-en-2,3-diyldicarbonyl) bis-valine
methyl ester (3c): yield 88%; mp 158-160 °C; [R]2D6 -5.39 (c 4.92,
CHCl3); IR (KBr) 3405, 3375, 3333, 3067, 2978, 2882, 1749, 1663,
1
1779 (sh), 1746, 1708, 1550, 1509, 1439, 1386 cm-1; H NMR (90
MHz, CDCl3) δ 0.90 (6H, d, J ) 5.0 Hz), 1.45-2.05 (5H, m), 3.36
(4H, brd), 3.70 (3H, s), 4.60 (1H, m), 6.13 (2H, s); FAB MS m/z 292
(MH)+. Anal. Calcd. for C16H21NO4 (mol wt 291: C, 65.97; H, 7.21,
N, 4.81; Found: C,65.88; H, 7.26; N, 5.07.
1
1539, 1505, 1463, 1441 cm-1; H NMR (400 MHz, CDCl3) δ 0.88-
(12H, m), 1.33 (1H, d, J ) 8.4 Hz), 1.47 (1H, d, J ) 8.4 Hz), 2.06
(2H, m), 3.16 (2H, brs), 3.27 (2H, m), 3.68 (6H, s), 4.34 (2H, m), 6.27
(1H, d, J ) 8.1 Hz), 6.38 (1H, m), 6.43 (1H, m), 6.47 (1H, d, J ) 7.7
Hz); 13C NMR (22.40 MHz, CDCl3) δ 172.53, 172.41, 172.05, 135.41,
57.448, 51.70, 51.46, 51.34, 49.40, 47.82, 47.55, 30.87, 30.63, 18.73,
18.10, 17.89; FAB MS m/z 409 (MH)+. Anal. Calcd. for C21H32N2O6
(mol wt 408): C, 61.76, H, 7.84, N, 6.86. Found: C, 61.97; H, 7.63;
N, 6.88.
N-(endo-Bicyclo[2.2.1]hept-5-en-2,3-diyldicarbonyl) tryptophan
methyl ester (4e): yield 88%; mp 144-146 °C; [R]2D6 -87.21 (c 3.13,
CHCl3); IR (KBr) 3422, 3376, 3133, 3066, 2991, 2939, 2870, 1767
1
(sh), 1748, 1693, 1575, 1557, 1488, 1455, 1392 cm-1; H NMR (60
MHz, CDCl3) δ 1.43 (2H, dd), 3.00 (2H, m), 3.37 (2H, brs), 3.52 (2H,
brs), 3.66 (3H, s), 4.91 (1H, t), 5.40 (2H, s), 6.65-7.50 (5H, m), 8.15
(1H, s exchangeable); FAB MS m/z 365 (MH)+.
N,N′-(endo-Bicyclo[2.2.1]heptane-2,3-diyl dicarbonyl) bis-valine
methyl ester, 5,6-dihydro derivative of 3c: prepared by hydrogenation
N-(endo-Bicyclo[2.2.1]hept-5-en-2,3-diyldicarbonyl) methionine
methyl ester (4f): yield 12%; syrup; [R]2D6 -35.61 (c 4.45, CHCl3); IR
(KBr) 3070, 2993, 2957, 2925, 2877, 1779 (sh), 1750, 1710, 1638,
1
of 3c in EtOAc with Pd/C, 5%, H2; yield 98%; mp172-173 °C; H
NMR (400 MHz, CDCl3) δ 0.93 (12H, m), 1.47 (4H, m), 1.90 (2H, d,
J ) 7.3 Hz), 2.15 (2H, m), 2.54 (2H, brs), 2.88 (2H, s), 3.72 (6H, s),
4.55 (2H, m), 6.49 (1H, d, J ) 8.33 Hz), 6.72 (1H, d, J ) 8.34 Hz);
FAB MS m/z 411 (MH)+.
1
1440, 1387 cm-1; H NMR (60 MHz, CDCl3) δ 1.60 (2H, dd), 1.95
(3H, s), 2.07-2.60 (4H, m), 3.22 (4H, brs), 3.60 (3H, s), 4.60 (1H, t),
5.99 (2H, s); FAB MS m/z 310 (MH)+.
N-(endo-Bicyclo[2.2.1]hept-5-en-2,3-diyldicarbonyl) aspartic acid
methyl ester (4 g): yield 12%; syrup; IR (KBr) 2985 (br), 1770 (sh),
1745, 1710, 1440, 1391 cm-1; 1H NMR (90 MHz, CDCl3) δ 1.70 (2H,
br), 2.80 (2H, brd), 3.40 (4H, brs), 3.70 (6H, s), 5.10 (1H, m), 6.11
(2H, brs); FAB MS m/z 308 (MH)+.
N,N′-(bicyclo[2.2.1]hept-5-ene-trans-2,3-diyldicarbonyl) bis-valine
methyl ester (6a): prepared by neat heating of endo isomer (3c) at
∼220 °C for 30 min, followed by purification on a small column of
silica gel using a mixture of ethyl acetate/hexane (60:40) as eluents;
yield 35%; mp 175-177 °C; IR (KBr) 3316, 3071, 2976, 2883, 1748,
1
N-(endo-Bicyclo[2.2.1]hept-5-en-2,3-diyldicarbonyl) glutamic acid
dimethyl ester (4h): yield 9.5%; syrup; IR (KBr) 3003, 2961, 2883,
1648, 1544 cm-1; H NMR (90 MHz, CDCl3) δ 0.90 (12H, m), 1.61
(2H, m), 1.90-2.60 (4H, m), 3.03 (2H, m), 3.67 (6H, s), 4.46 (2H, m),
6.20 (2H, m), 6.68 (1H, brd), 7.03 (1H, brd).
1
1771 (sh), 1745, 1710, 1543, 1442, 1387 cm-1; H NMR (90 MHz,

Norborneno Peptide Analogues
J. Am. Chem. Soc., Vol. 120, No. 33, 1998 8459
Further elution with the same solvent afforded core fumaroyl bis-
Val peptide (5a) arising from the retro-Diels-Alder of 3c: yield 15%;
mp 225-227 °C; [R]2D6 -50.89 (c 1.29, MeOH); IR (KBr) 3309, 3078,
FAB MS m/z 473 (MH)+. Anal. Calcd. for C21H32N2O6S2: C, 53.38;
H, 6.77; N, 5.93. Found: C, 53.80; H, 7.18; N, 6.13.
N,N′-(endo-Bicyclo[2.2.1]hept-5-en-2,3-diyldicarbonyl) bis-aspar-
tic acid methyl ester (3 g): yield 58%; syrup; IR (KBr) 3391, 2960,
1
2973, 2938, 2858, 1748, 1643, 1552, 1442 cm-1; H NMR (90 MHz,
1
1743 (br), 1689, 1521, 1441, 1373 cm-1; H NMR (90 MHz, CDCl3)
CDCl3) δ 0.95 (12H, dd, J ) 9.0 Hz, 2.0 Hz), 2.20 (2H, m), 3.75 (6H,
s), 4.70 (2H, q), 7.13 (2H, s), 7.35 (2H, d, J ) 11.25 Hz); FAB MS
m/z 343 (MH)+.
δ 1.40 (2H, brs), 2.81 (4H, m), 3.29 (4H, m), 3.62, 3.64 (6H, s; 6H, s),
4.60 (2H, m), 6.20 (2H, brs), 7.10 (2H, brd).
N,N′-(endo-Bicyclo[2.2.1]hept-5-en-2,3-diyldicarbonyl) bis-glutam-
ic acid methyl ester (3h): yield 75%; syrup; IR (KBr) 3382, 2965,
Authentic MeO-Val-CO-CHdCH-CO-Val-OMe (E). To a
well-stirred and ice-cooled mixture of MeO-Val-CO-CHdCH-
COOH (1 mmol, prepared from maleic anhydride and Val-OMe), DCC,
and N-OH succinimide (1 mmol each) in dry CH2Cl2 was added a
solution of freshly generated Val-OMe (from 1 mmol each of Val-
OMe hydrochloride and triethylamine in CH2Cl2) and stirred for 24 h.
The reaction mixture was worked up by washing, sequentially with
ice-cold 2 N H2SO4, water, and saturated bicarbonate solution (20 mL
each), drying (anhydrous MgSO4), and evaporating in Vacuo. Purifica-
tion on a small column of silica gel and elution with a mixture of ethyl
acetete and hexane (80:20) afforded the trans isomer in 70% yield;
found identical in all respects with the olefinic product (5a) obtained
by the thermolysis of 3c. A small amount (∼ 20%) of cis isomer
MeO-Val-CO-CHdCH- CO-Val-OMe (Z) was also obtained in
the above reaction: syrup; IR (KBr) 3274, 3033, 2974, 1748, 1677,
1645, 1618, 1541 cm-1; 1H NMR (90 MHz, CDCl3) δ 1.00 (12H, d, J
) 7.5 Hz), 2.20 (2H, m), 3.74 (6H, s), 4.55 (2H, m), 6.32 (2H, s), 9.43
(2H, d, J ) 8.0 Hz); FAB MS m/z 343 (MH)+.
1
1743, 1677, 1542, 1444, 1379 cm-1; H NMR (90 MHz, CDCl3) δ
1.40 (2H, brd), 2.05-2.81 (8H, m), 3.28 (4H, m), 3.70, 3.73 (6H, s;
6H, s), 4.55 (2H, m), 6.40 (2H, brs), 6.80 (2H, brd); FAB MS m/z 497
(MH)+.
N,N′-(1S,3R)-dicarboxy-(4S,5R)-dicarbonyl)cyclopentane bis-leu-
cine methyl ester (7): Oxidative cleavage of 3d. A solution of 3d (4
mmol) in MeCN (16 mL) was admixed with CCl4/H2O (1:2, 48 mL),
NaIO4 (15.4 g, 72 mmol), and RuCl3‚3H2O (0.030 g, 2.2 mol %), sealed,
and left shaken for 6 h at room temperature, cautiously opened and
filtered, and the residue washed with CCl4 (3 × 10 mL). The filtrates
were combined and evaporated without heating in vacuo, and the residue
was digested with saturated aqueous NaHCO3 (∼40 mL) for 3 h. The
bicarbonate extract was adjusted to pH ≈3 with 2 N H2SO4, saturated
with NaCl, extracted with EtOAc (3 × 25 mL), dried, and evaporated
to yield the dicarboxylic acid 7: yield 50%; mp 168-170 °C; [R]D26
-44.76 (c 3.60, CHCl3); IR (KBr) 3386, 3324, 2968, 2499, 1756, 1724,
1
1690, 1605, 1563, 1445 cm-1; H NMR (90 MHz, CDCl3 + DMSO-
N,N′-(endo-Bicyclo[2.2.1]hept-5-en-2,3-diyldicarbonyl) bis-leucine
methyl ester (3d): yield 89%; mp 138-140 °C; [R]2D6 -4.50 (c 4.30,
CHCl3); IR (KBr) 3405, 3351, 2961, 2876, 1749, 1665, 1574, 1532,
d6) δ 0.90 (12H, m), 1.57 (6H, m), 2.48 (2H, m), 3.44 (4H, m), 3.68
(6H, s), 4.42 (2H, m), 7.63 (1H, d, J ) 7.9 Hz), 7.92 (1H, d, J ) 7.9
Hz); FAB MS m/z 523 (M + Na+), 501 (MH)+. Anal. Calcd. for
C23H36N2O10 (mol wt 500): C, 55.20; H, 7.20; N, 5.60. Found: 54.98;
H, 7.37; N, 5.08.
Preparation of Higher Norborneno Bispeptides 8-10 Listed in
Chart 1. General Procedure. (a) Preparation of precursor bis-
carboxylic acids from bismethyl esters 3c and 3d: An ice-cooled
solution of bisester 3c/3d (1 mmol, in 2 mL of MeOH) was admixed
with 2 N aqueous NaOH (4 mmol) and stirred at room temperature for
4 h (TLC). The reaction mixture was concentrated to half the volume
in vacuo (without heating), acidified to pH ≈3 with ice-cold 2 N H2SO4,
saturated with solid NaCl, extracted with ethyl acetate (3 × 20 mL),
dried (anhydrous MgSO4), and evaporated in vacuo to give near
quantitative yield of the biscarboxylic acid which was pure enough for
direct use in the next reaction.
(b) Bidirectional coupling of biscarboxylic acids of 3c/3d with
amino acids/dipeptides or tripeptides: To a well-stirred and ice-cooled
solution of biscarboxylic acid (1 mmol) in dry dichloromethane (∼10
mL) or a mixture of dry DMF (∼0.5 mL) and dichloromethane (∼10
mL) was added, sequentially, N-OH succinimide and DCC (2 mmol
each). After 15 min of stirring, the reaction mixture was treated with
freshly prepared free base of an amino acid ester (generated at 0 °C
from 2.5 mmol each of the corresponding ester hydrochloride and
triethylamine in dry CH2Cl2) or an NR-deprotected dipeptide or
tripeptide ester and left stirred at room temperature for 24 h. The
precipitated DC-urea was filtered, the residue was washed with CH2Cl2
(3 × 20 mL), and the combined filtrates were washed sequentially with
ice-cold 2 N H2SO4, H2O and saturated aqueous bicarbonate (20 mL
each), dried (anhydrous MgSO4) and solvent removed in vacuo. The
residue was purified by column chromatography on silica gel and eluted
with either a mixture of ethyl acetate/hexane or chloroform/methanol
(gradient) to afford the higher bispeptides 8-10.
Compound 8a: prepared from dicarboxylic acid of 3c and Val-OMe;
yield 67%; hygroscopic solid; [R]2D6 -49.52 (c 4.08, CHCl3); 1H NMR
(400 MHz, CDCl3) δ 0.87 (24 H, m), 1.40 (1H, d, J ) 8.4 Hz), 1.57
(1H, d, J ) 8.4 Hz), 2.11 (1H, m), 2.16 (1H, m), 2.25 (1H, m), 2.34
(1H, m), 3.12 (2H, brs), 3.29 (2H, m), 3.69 (3H, s), 3.71 (3H, s), 4.34
(1H, m), 4.44 (3H, m), 6.12 (1H, d, J ) 9.4 Hz), 6.32 (1H, m), 6.47
(1H, m), 6.50 (1H, d, J ) 8.8 Hz), 7.40 (1H, d, J ) 8.3 Hz), 7.57 (1H,
d, J ) 8.3 Hz); FAB MS m/z 607 (MH)+. Anal. Calcd. for C31H50O8N4
(mol wt 606): C, 61.38; H, 8.25; N, 9.24. Found: C, 61.74; H, 7.88;
N, 9.06.
1
1513, 1468, 1440 cm-1; H NMR (400 MHz, CDCl3) δ 0.90 (12H, d,
J ) 4.5 Hz), 1.34 (1H, d, J ) 8.7 Hz), 1.49 (1H, d, J ) 8.7 Hz), 1.62
(6H, m), 3.22 (4H, m), 3.73 (6H, s), 4.45 (2H, m), 6.27 (1H, d, J )
7.6 Hz), 6.40 (2H, m), 6.55 (1H, d, J ) 7.4 Hz); 3C NMR (22.40 MHz,
CDCl3) δ 173.09, 172.50, 172.23, 135.47, 135.17, 51.87, 51.81, 51.58,
51.13, 50.98, 49.28, 47.88, 47.64, 41.16, 41.01, 24.67, 22.46, 22.10,
21.98; FAB MS m/z 437 (MH)+.
N,N′-(Bicyclo[2.2.1]hept-5-ene-trans-2,3-diyldicarbonyl) bis-leu-
cine methyl ester (6b): prepared by neat heating of 3d at ∼220 °C
using identical conditions as with 3c. Chromatography yielded a
mixture of two products identified as trans isomer 6b; [yield 33%; mp
184-186 °C; IR (KBr) 3302, 3076, 2965, 1751, 1649, 1557, 1474,
1
1443 cm-1; H NMR (CDCl3, 90 MHz) δ 0.96 (12H, m), 1.67 (8H,
m), 3.11 (4H, m), 3.74 (6H, s), 4.62 (2H, m), 6.10-6.60 (3H, m), 7.30
(1H, br); FAB MS m/z 437 (MH)+] and core fumaryl retro bis peptide
MeO-Leu-CO-CHdCH-CO-Leu-OMe (5b); [yield; 10%; mp
196-198 °C; IR (KBr) 3321, 2957, 1738, 1633, 1536, 1437 cm-1; 1H
NMR (80 MHz, CDCl3) δ 0.93 (12H, d, J ) 5.0 Hz), 1.71 (6H, br),
3.81 (6H, s), 4.75 (2H, m), 7.03 (2H, s), 7.28 (2H, d, J ) 7.5 Hz);
FAB MS (m/z) 371 (MH)+].
The authentic sample prepared from maleic anhydride and Leu-OMe
by procedure described as under Val case was found to be identical
with the thermolysis product.
N,N′-(endo-Bicyclo[2.2.1]hept-5-en-2,3-diyldicarbonyl) bis-phen-
ylalanine methyl ester (3e): yield 91%; mp 154-156 °C; [R]D26
+56.40 (c 3.95, CHCl3); IR (KBr) 3437, 3326, 3030, 3001, 2955, 1756,
1
1664, 1536, 1501, 1440, 1366 cm-1; H NMR (400 MHz, CDCl3) δ
1.28 (1H, d, J ) 8.5 Hz); 1.42 (1H, d, J ) 8.5 Hz); 3.05 (6H, m), 3.18
(2H, s), 3.66 (3H, s), 3.69 (3H, s), 4.66 (1H, m), 4.73 (1H, m), 5.93
(1H, m), 6.17 (1H, m), 6.24 (1H, d, J ) 7.3 Hz), 6.28 (1H, d, J ) 7.2
Hz), 7.13 (4H, m), 7.30 (6H, m); 13C NMR (22.40 MHz, CDCl3) δ
172.11, 171.99, 171.90, 171.81, 136.16, 135.98, 135.44, 135.17, 129.27,
129.12, 128.40, 128.31, 126.91, 53.63, 53.46, 51.87, 51.22, 49.37, 47.16,
37.67, 37.49; FAB MS m/z 505 (MH)+. Anal. Calcd. for C29H32N2O6
(mol wt 504): C, 69.04; H, 6.34; N, 5.55; Found: C, 69.22; H, 6.43;
N, 5.27.
N,N′-(endo-Bicyclo[2.2.1]hept-5-en-2,3-diyldicarbonyl) bis-meth-
ionine methyl ester (3f): yield 70%; mp 90-92 °C; -5.30 (c 4.53,
CHCl3); IR (KBr) 3412, 3363, 2978, 2923, 1743, 1659, 1574, 1552,
1
1531, 1439 cm-1; H NMR (60 MHz, CDCl3) δ 1.32 (2H, m), 1.69-
Compound 8b: prepared from dicarboxylic acid of 3c and Phe-
2.19 (10H, s+m), 2.39 (4H, m), 3.09 (2H, brs), 3.24 (2H, brs); 3.58
(6H, brs), 4.64 (2H, m), 6.26 (2H, brs), 6.84 (2H, m, exchangeable);
OMe; yield 62% sticky solid; [R]2D6 +6.02 (c 5.5, CHCl3); H NMR
1

8460 J. Am. Chem. Soc., Vol. 120, No. 33, 1998
Ranganathan et al.
(400 MHz, CDCl3) δ 0.87 (12 H, m), 1.47 (1H, d, J ) 8.7 Hz), 1.63
(1H, d, J ) 8.7 Hz), 2.10 (1H, m), 2.35 (1H, m), 3.05-3.22 (6H, m),
3.33 (2H, m), 3.49 (3H, s), 3.60 (3H, s), 4.47 (1H, m), 4.56 (1H, m),
4.86 (2H, m), 5.84 (1H, d, J ) 9.3 Hz), 6.33 (1H, m), 6.42 (1H, d, J
) 8.7 Hz), 6.53 (1H, m), 7.30 (10H, m), 7.62 (1H, d, J ) 8.3 Hz),
7.86 (1H, d, J ) 8.6 Hz); FAB MS m/z 703 (MH)+.
(400 MHz, DMSO-d6) δ 0.68-0.98 (36 H, m), 1.24 (2H, m), 1.44
(4H, m), 1.68 (2H, m), 2.02 (4H, m), 2.93 (1H, brs), 3.12 (1H, m),
3.16 (1H, m), 3.38 (1H, m), 3.48 (1H, m), 3.56 (1H, m), 3.60 (6H, s),
3.69 (2H, m), 3.90 (1H, m), 4.14 (5H, m), 4.34 (2H, m), 4.68 (1H, m),
4.90 (1H, m), 5.92 (1H, m), 6.19 (1H, m), 7.25 (1H, d, J ) 9.3 Hz),
7.67 (1H, d, J ) 7.5 Hz), 7.75 (1H, d, J ) 8.8 Hz), 7.80 (1H, d, J )
7.9 Hz), 7.91 (1H, d, J ) 4.8 Hz), 8.12 (1H, d, J ) 7.7 Hz), 8.17 (1H,
d, J ) 7.2 Hz), 8.26 (1H, d, J ) 7.4 Hz); FAB MS m/z 1007 (MH)+.
X-ray Structure Analyses of Norborneno Bispeptides. (1) cis-
bis-Aib methyl ester 3b: C19H28N2O6, space group P1h, a ) 8.891(1)
Å, b ) 10.526(0) Å, c ) 11.751(0) Å, R ) 71.25(0)o, â ) 79.09(0)o,
γ ) 79.03(0)o, V ) 1012.6 Å3, Z ) 2, dcalcd ) 1.248 g/cm3, R ) 5.18%.
(2) cis-bis-Val methyl ester 3c: C21H32N2O6, space group P212121, a
) 10.076(1) Å, b ) 11.147(1) Å, c ) 20.118(1) Å, V ) 2259.6 Å3, Z
) 4, dcalcd ) 1.201 g/cm3, R ) 8.75%. (3) cis-bis-Leu methyl ester 3d:
C23H36N2O6, space group P21, a ) 10.66(2) Å, b ) 9.951(8) Å, c )
12.39(2) Å, â ) 106.31(12)o, V ) 1261.3 Å3, Z ) 2, dcalcd ) 1.150
g/cm3, R ) 11.1%. (4) trans-bis-Val methyl ester 6a: C21H32N2O6,
space group P212121, a ) 9.626(1) Å, b ) 16.659(4) Å, c ) 29.847(7)
Compound 8c: prepared from dicarboxylic acid of 3d and Ser-OMe;
yield 77% syrup; [R]2D6 -65.05 (c 4.35, CHCl3); H NMR (400 MHz,
1
CDCl3) δ 0.94 (12 H, m), 1.43-1.84 (8H, m), 3.09 (2H, m), 3.30 (2H,
m), 3.79 (6H, s), 3.97 (4H, m), 4.42 (2H, m), 4.68 (2H, m), 6.07 (1H,
d, J ) 8.3 Hz), 6.27 (2H, m), 6.82 (1H, d, J ) 7.5 Hz), 7.67 (1H, d,
J ) 7.5 Hz), 7.74 (1H, d, J ) 8.3 Hz). Anal. Calcd. for C24H46N4O10
(mol wt 550): C, 57.04; H, 7.54; N, 9.18. Found: C, 57.02; H, 7.29;
N, 9.00.
Compound 9a: prepared from dicarboxylic acid of 3c and Val-Val-
OMe; yield 52%; mp 254-256 °C; [R]2D6 -80.03 (c 1.80, CHCl3); H
1
NMR (400 MHz, CDCl3) δ 0.88 (36 H, m), 1.38 (1H, d, J ) 8.4 Hz),
1.54 (1H, d, J ) 8.4 Hz), 2.13 (4H, m), 2.33 (2H, m), 3.14 (2H, brs),
3.24 (2H, brs), 3.69 (6H, s), 3.98 (1H, m), 4.11 (1H, m), 4.40 (1H, m),
4.44 (1H, m), 4.49 (1H, m), 4.50 (1H, m), 6.24 (1H, d, J ) 9.2 Hz),
6.30 (1H, m), 6.39 (1H, m), 6.65 (1H, d, J ) 7.0 Hz), 6.87 (2H, d, J
) 8.7 Hz); 7.69 (1H, d, J ) 7.9 Hz), 7.94 (1H, d, J ) 7.9 Hz); FAB
MS m/z 805 (MH)+. Anal. Calcd. for C41H68O10N6 (mol wt 804): C,
61.19; H, 8.45; N, 10.45. Found: C, 60.97 H, 8.28; N, 10.95.
Compound 9b: prepared from dicarboxylic acid of 3d and Val-
Val-OMe; yield 55%; mp 155-157 °C; [R]2D6 -73.66 (c 3.80, CHCl3);
1H NMR (400 MHz, CDCl3) δ 0.89 (36 H, m), 1.36 (1H, d, J ) 8.3
Hz), 1.44-1.80 (7H, m), 2.13 (3H, m), 2.29 (1H, m), 3.14 (2H, brs),
3.25 (2H, m), 3.72 (6H, s), 4.09 (2H, m), 4.36 (1H, m), 4.51 (2H, m),
4.60 (1H, m), 6.29 (2H, m), 6.33 (1H, d, J ) 8.2 Hz), 6.51 (1H, d, J
) 8.2 Hz), 7.01 (1H, d, J ) 8.9 Hz), 7.10 (1H, d, J ) 8.4 Hz), 7.63
(1H, d, J ) 8.4 Hz), 7.75 (1H, d, J ) 8.0 Hz); FAB MS m/z 833
(MH)+.
Å, V ) 4786.1 Å3, Z ) 8 (contains a pair of diastereomers), dcalcd
)
1.132 g/cm3, R ) 14.0%. (5) Diacid of 3b: C17H25N2O6, space group
Cc, a ) 16.257(2) Å, b ) 9.192(1) Å, c ) 11.875(1) Å, â ) 97.72(1)o,
V ) 1758.4 Å3, Z ) 4, dcalcd ) 1.327 g/cm3, R ) 5.43%. (6) Half-acid
half-ester of 3b: C36H52N4O12, space group Cc, a ) 12.047(1) Å, b )
16.556(2) Å, c ) 11.427(2) Å, â ) 120.09(1)o, V ) 1971.8(5) Å3, Z
) 2, dcalcd ) 1.234 g/cm3, R ) 4.47%.
X-ray data were collected with Cu KR radiation (λ ) 1.54178 Å)
on a Siemens automated diffractometer in the θ/2θ mode, constant scan
speed of 10 deg/min, 2° scan width, and 2θmax ) 115° (0.9 Å
resolution). Crystal structures were determined by direct phase
determination. Full-matix, anisotropic least-squares refinement was
performed on the parameters for all atoms except the H atoms.
Hydrogen atoms involved in hydrogen bonding were located in electron
density maps. The remainder of the H atoms were placed in idealized
positions and allowed to ride with the C atoms to which each was
bonded.
Compound 9c: prepared from dicarboxylic acid of 3d and Leu-
1
Leu-OMe; yield 45%; mp 143-144 °C; H NMR (400 MHz, CDCl3)
δ 0.88 (36 H, m), 1.38 (1H, m), 1.43-1.87 (19H, m), 3.12 (2H, brs),
3.21 (1H, dd, J ) 3.1, 10.6 Hz), 3.27 (1H, dd, J ) 2.9, 10.6 Hz), 3.70
(6H, s), 4.36 (3H, m), 4.49 (3H, m), 6.04 (1H, d, J ) 8.1 Hz), 6.23
(1H, d, J ) 8.3 Hz), 6.27 (1H, m), 6.35 (1H, m), 6.68 (1H, d, J ) 7.9
Hz), 6.78 (1H, d, J ) 7.9 Hz), 7.65 (1H, d, J ) 7.9 Hz); 7.78 (1H, d,
J ) 7.4 Hz); FAB MS m/z 889 (MH)+.
Acknowledgment. We are most grateful to Professor S.
Ranganathan (RRL, Trivandrum) for inspiration, encouragement
and valuable advice and Professor D. Balasubramanian (CCMB,
Hyderabad) and Professor P. Balaram (IISc. Bangalore) for
helpful comments. Financial support from DST, New Delhi,
and from the Office of Naval Research and the National Institute
of Health (GM-30902) is acknowledged.
Compound 9d: prepared from dicarboxylic acid of 3d and Leu-
Ser-OMe; yield 59%; syrup; [R]2D6 -59.53 (c 0.55, CHCl3); H NMR
1
(300 MHz, CDCl3) δ 0.93 (24 H, m), 1.35-2.1 (14H, m), 3.12 (2H,
brs), 3.26 (2H, brs), 3.75 (6H, s), 3.80 (4H, m), 4.37 (4H, m), 4.60
(2H, m), 6.05 (2H, brd), 6.27 (2H, brs), 6.50 (1H, brd), 7.26 (1H, brd),
7.41 (1H, brd), 8.02 (2H, m); FAB MS m/z 837 (MH)+.
Supporting Information Available: 1H NMR spectra of
1
3b-e, 8a, 8b, 9a, 9b, 9c, 9e, and 10, H NMR VT spectra of
Compound 9e: prepared from dicarboxylic acid of 3d and Ser-Leu-
3b-e, 8a, 8b, 9a-c, 9e, and 10, ROESY NMR spectra of 3b-
e, 8a, 8b, 9a-c, 9e, and 10, FT-IR spectra (NH stretch region
in CHCl3) of 3b, 9b, and 9c, comparison of FT-IR for 3c, 8a,
and 9a, FAB MS of 8a, 9a-c, and 10, CD spectra of 8c, 9c,
and 10, X-ray diffraction structure determination summaries and
tables of atomic coordinates, bond lengths, bond angles,
anisotropic thermal coefficients and hydrogen atom coordinates
for compounds 3b-d, 6a, 3b (diacid), and 3b (half-acid half-
ester) (90 pages, print/PDF). See any current masthead page
for ordering information and Web access instructions.
OMe; yield 52%; mp 97-99 °C; [R]2D6 -48.24 (c 0.65, CHCl3); H
1
NMR (400 MHz, CDCl3) δ 0.89 (24 H, m), 1.39 (1H, d, J ) 8.6 Hz),
1.47-1.80 (13H, m), 3.15 (2H, brs), 3.29 (2H, m), 3.72 (6H, s), 3.85
(4H, m), 4.27 (1H, m), 4.36 (1H, m), 4.44 (1H, m), 4.55 (2H, m), 4.64
(1H, m), 6.28 (2H, m), 6.54 (1H, d, J ) 7.5 Hz), 6.77 (1H, d, J ) 7.8
Hz), 7.32 (1H, d, J ) 8.0 Hz), 7.36 (1H, d, J ) 8.0 Hz), 7.80 (1H, d,
J ) 8.2 Hz), 7.85 (1H, d, J ) 7.7 Hz); FAB MS m/z 837 (MH)+.
Compound 10: prepared from dicarboxylic acid of 3d and Ser-Val-
Val-OMe which in turn was obtained by RN-Z deprotection (Pd/C,
5%, H2) of Z-Ser-Val-Val-OMe [prepared by coupling Z-Ser with Val-
Val-OMe in 69% yield; mp 168-169 °C, [R]2D6 -17.59 (c 2.9,
CHCl3)]. Selected data for 10: yield 35%; hygroscopic solid; 1H NMR
JA980143+