Self-assembly of noncyclic bis-D- and L-tripeptides into higher order tubular constructs: Design, synthesis, and X-ray crystal superstructure

Article abstract of DOI:10.1021/jo701621c

(Figure Presented) Trans (1R,2R)-diaminocyclohexane was used as a semirigid vicinal diamine to anchor two N-protected tripeptides consisting of L-D-L amino acids as carboxy terminal amides. The bis-tripeptide consisting of L-Ser (OBn)-D-Ser (OBn)-L-Ser (O-p-bromobenzyl) Boc afforded X-ray quality crystals containing benzene and chloroform solvent molecules. Analysis of the solid-state structure revealed a highly H-bonded helical open-ended tubular superstructure. The tripeptide strands intertwine like a pair of self-embracing arms, held together by a γ-turn and a 14-membered antiparallel H-bonded macroring spanning the first and third amino acid residues within each strand. Whereas the tripeptide from the R,R anchor gave beautiful crystals from benzene and chloroform, the analogous construct from the S,S-anchored diamine gave a gel. Related bis-tripeptides with different amino acids showed extensive intramolecular H-bonding based on NMR titration and dilution experiments.

Full text of DOI:10.1021/jo701621c

VOLUME 73, NUMBER 4
FEBRUARY 15, 2008
© Copyright 2008 by the American Chemical Society
Self-Assembly of Noncyclic Bis-D- and L-tripeptides into Higher
Order Tubular Constructs: Design, Synthesis, and X-ray Crystal
Superstructure
Stephen Hanessian,* Valerio Vinci, Kamal Fettis,† Thierry Maris, and Minh Tan Phan Viet
Department of Chemistry, UniVersite´ de Montre´al, P.O. Box 6128, Station Centre-Ville, Montre´al, QC,
H3C 3J7, Canada
stephen.hanessian@umontreal.ca
ReceiVed August 10, 2007
Trans (1R,2R)-diaminocyclohexane was used as a semirigid vicinal diamine to anchor two N-protected
tripeptides consisting of L-D-L amino acids as carboxy terminal amides. The bis-tripeptide consisting of
L-Ser (OBn)-D-Ser (OBn)-L-Ser (O-p-bromobenzyl) Boc afforded X-ray quality crystals containing benzene
and chloroform solvent molecules. Analysis of the solid-state structure revealed a highly H-bonded helical
open-ended tubular superstructure. The tripeptide strands intertwine like a pair of self-embracing arms,
held together by a γ-turn and a 14-membered antiparallel H-bonded macroring spanning the first and
third amino acid residues within each strand. Whereas the tripeptide from the R,R anchor gave beautiful
crystals from benzene and chloroform, the analogous construct from the S,S-anchored diamine gave a
gel. Related bis-tripeptides with different amino acids showed extensive intramolecular H-bonding based
on NMR titration and dilution experiments.
Introduction
in general.1 Helical and sheet arrangements are common
supramolecular motifs encountered in practically all functional
proteins.2 A better understanding of the underlying principles
that govern the architecture of such motifs is a prerequisite for
The three-dimensional architecture of proteins and polypep-
tides is a fundamental topological property that is exploited by
nature for molecular recognition, function, and life processes
(1) Petsko, G. A.; Ringe, D. In Protein Structure and Function; Sinauer
Associates Inc.: Sunderland, MA, 2004; pp 49-83.
(2) Lesk, A. M. In Introduction to Protein Architecture, The Structural
Biology of Proteins; Oxford University Press: Oxford, 2000.
* To whom correspondence should be addressed. Fax: (514) 343-5728.
† Current address: Altana Pharma, Moltkestrasse 4, 78467 Konstanz,
Switzerland.
10.1021/jo701621c CCC: $40.75 © 2008 American Chemical Society
Published on Web 12/07/2007
J. Org. Chem. 2008, 73, 1181-1191
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Hanessian et al.
probing the origin of many physical, biophysical, and biomedical
phenomena related to specific diseases.3
The simulation of ordered supramolecular assemblies using
short oligopeptides has been a challenging area of intensivere-
search in recent years.4,5 Moreover, the advent of nanotechnol-
ogy has instigated creative ways to construct well-defined three-
dimensional oligopeptide architectures.6 For example, the
insightful and scholarly contributions of Ghadiri and co-workers7
have laid a strong foundation for the construction of tubular
assemblies consisting of cyclic D- and L-oligopeptides with wide-
ranging applications.8 Although synthetic helical structures
arising from short peptidic units are of common occurrence,9
the design and synthesis of â-sheets as models for â-stranded
peptide sequences presents a different challenge.10 To this
end, stabilization of â-sheets by a network of intermolecular
H-bonds has been achieved with the incorporation of rigid
templates.11 Inclusion of noncoded amino acids within a
tripeptide such as Boc-Leu-Aib-â-AlaOMe has led to a self-
assembled â-sheet type or helical structures.12 Further modifica-
tions have produced helical assemblies that were reported to
form polydispersed nanorods.13 However, such self-assembled
motifs have invariably incorporated the helicogenic noncoded
amino acid Aib, known for its helix-promoting character.14 The
higher order autoassembly of uncharged, metal free, noncyclic
short peptides consisting of natural amino acids (and their
FIGURE 1. Top: Head-to-head parallel â-sheet array. Bottom: Head-
to-tail antiparallel â-sheet array. Partial structure with side chains
removed for clarity.
D-counterparts) into tubular superstructures is a relatively
unexplored area.15
We report herein on the design and synthesis of a tubular
superconstruct that is formed from the self-assembly of non-
cyclic bis-tripeptides anchored on a semirigid trans-1,2-diami-
nocyclohexane spacer unit. The sequential attachment of
alternating L- and D-amino acids to trans-(1R,2R)-diaminocy-
clohexane (DACH)16 can potentially lead to two types of
idealized H-bonded arrays. A head-to-head orientation can
produce a â-sheet resulting from a parallel H-bonding alignment
(Figure 1, top). A head-to-tail deployment of the anchoring
DACH units on the other hand leads to an antiparallel alignment
encompassing interstrand 10-membered H-bonded rings (Figure
1, bottom). The bis-amide of (1R,2R)-DACH with palmitic acid
is reported to produce a helical gel structure in which the
anchoring DACH units are slightly staggered in a head-to-head
arrangement.17
(3) See for example, Alzheimers disease: (a) Rochet, J. C.; Lansbury,
P. T. Curr. Opin. Struct. Biol. 2000, 10, 60-68. (b) Baumeister, R.; Eimer,
S. Angew. Chem., Int. Ed. 1998, 37, 2978-2982. Huntington’s disease:
(c) Perutz, M. F. Curr. Opin. Struct. Biol. 1996, 6, 848-858. (d) MacDonald,
M. E.; Gusella, J. F. Curr. Opin. Neurobiol. 1996, 6, 638-643. Prion protein
disease: (e) Prusiner, S. B. Science 1997, 278, 245-251. (f) Ng, S. B. L.;
Doig, A. J. Chem. Soc. ReV. 1997, 26, 425-432.
(4) See for example: (a) Betz, S. F.; Liebman, P. A.; DeGrado, W. F.
Biochemistry 1997, 36, 2450-2458. (b) Nautiyal, S.; Woolfson, D. N.; King,
D. S.; Alber, T. Biochemistry 1995, 34, 11645-11651. (c) Olofsson, S.;
Johansson, G.; Baltzer, L. J. Chem. Soc., Perkin Trans. 2 1995, 11, 2047-
2056. (d) Ghadiri, R.; Soares, C.; Choi, C. J. Am. Chem. Soc. 1992, 114,
825-831.
(5) For selected reviews, see: (a) Sanford, A. R.; Gong, B. Curr. Org.
Chem. 2003, 7, 1649-1659. (b) Cheng, R. P.; Gellman, S. H.; De Grado,
W. F. Chem. ReV. 2001, 101, 3219-3232. (c) Hill, D. J.; Mio, M. J.;
Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem. ReV. 2001, 101, 3893-
4012. (d) Cubberley, M.; Iverson, B. L. Curr. Opin. Chem. Biol. 2001, 5,
650-653.
Results and Discussion
(6) (a) Hartgerink, J. D.; Clark, T. D.; Ghadiri, M. R. Chem.sEur. J.
1998, 4, 1367-1372. (b) Ghadiri, M. R. AdV. Mater 1995, 7, 675-677.
(7) (a) Ghadiri, M. R.; Granja, J. R.; Milligan, R. A.; Mckee, D. E.;
Khazanovich, N. Nature 1993, 366, 324-327. (b) Hartgerink, J. D.; Granja,
J. R.; Milligan, R. A.; Ghadiri, M. R. J. Am. Chem. Soc. 1996, 118, 43-
50. (c) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M. R. Angew.
Chem., Int. Ed. 2001, 46, 988-1011. (d) Clark, T. D.; Buehler, L. K.;
Ghadiri, M. R. J. Am. Chem. Soc. 1998, 120, 651-656.
(8) (a) Rosenthal-Aizman, K.; Svensson, G.; Unden, A. J. Am. Chem.
Soc. 2004, 126, 3372-3373. (b) Amorin, M.; Castedo, L.; Granja, J. R. J.
Am. Chem. Soc. 2003, 125, 2844-2845. (c) Seebach, D.; Mathews, J. L.;
Meden, A.; Wessels, T.; Baerlocher, C.; McCusker, L. B. HelV. Chim. Acta
1997, 80, 173-182.
(9) Hodgson, D. R.; Sanderson, J. Chem. Soc. ReV. 2004, 33, 422-430.
(10) For a recent review, see: Longhlin, W. A.; Tyndall, J. D. A.; Glenn,
M. P.; Fairlie, V Chem. ReV. 2004, 104, 6085-6118.
(11) For selected references, see: (a) Schrader, T.; Kirsten, C. Chem.
Commun. 1996, 2089-2090. (b) Kelly, J. W.; Lashuel, H. L.; La Brenz, S.
R.; Woo, L.; Serpell, L. C. J. Am. Chem. Soc. 2000, 122, 5262-5277. (c)
Nowick, J. S.; Chung, D. M.; Maitra, K.; Maitra, S.; Stigen, K.; Sun, Y. J.
Am. Chem. Soc. 2000, 122, 7654-7661.
(12) (a) Das, A. K.; Haldar, D.; Hegde, R. P.; Shamala, N.; Banerjee,
A. Chem. Commun. 2005, 1836-1838. (b) Banerjee, A.; Maji, S. K.; Drew,
M. G. B.; Haldar, D.; Banerjee, A. Tetrahedron Lett. 2003, 44, 335, 699-
702. (c) Haldar, D.; Maji, S. K.; Drew, M. G. B.; Banerjee, A.; Banerjee,
A. Tetrahedron Lett. 2002, 43, 5465-5468.
(13) (a) Haldar, D.; Banerjee, A.; Drew, M. G. B.; Das, A. K.; Banerjee,
A. Chem. Commun 2003, 1406-1407. (b) Ray, S.; Haldar, D.; Drew, G.
D. B.; Banerjee, A. Org. Lett. 2004, 6, 4463-4465.
Initial studies involving the successive coupling of L- and
D-CbzAlaOH to (R,R)-DACH 1 gave a series of bis-peptides
2-5 (Scheme 1). Although the bis-dipeptide 3 and the bis-
tripeptide and 4 were soluble in CHCl3, the tetrapeptide 5 formed
a gel. Removal of the N-Cbz group by hydrogenolysis afforded
the free amine 6, which was converted to the corresponding
1
N-Boc and N-Ac derivatives 7 and 8, respectively. H NMR
dilution experiments in CDCl3 with peptides 2-5, 7, and 8
showed little if any NH signal change over the concentration
range of 10 to 0.6 mM. Titration with increasing amounts of
(14) (a) Balaram, P. Curr. Opin. Struct. Biol. 1992, 2, 845-851. (b)
Karle, I. I.; Gurunath, R. B.; Prasad, S.; Kaul, R.; Rao, R. B.; Balaram, P.
J. Am. Chem. Soc. 1995, 117, 9632-9637.
(15) See for example: (a) Srinivasulu, G.; Kumar, S. K.; Sharma, G. V.
M.; Kunwar, A. C. J. Org. Chem. 2006, 71, 8395-8400. (b) Barsby, T.;
Warabi, K.; Sørensen, D.; Zimmerman, W. T.; Kelly, M. T.; Andersen, R.
J. J. Org. Chem. 2006, 71, 6031-6037. (c) Go¨rbitz, C. H. Chem. Commun.
2006, 2332-2334. (d) Ray, S.; Haldar, D.; Drew, M. G. B.; Banerjee, A.
Org. Lett. 2004, 6, 4463-4465. (e) For a recent review, see: Moriuchi, T.;
Hirao, T. Chem. Soc. ReV. 2004, 33, 294-301. (f) Go¨rbitz, C. H. Chem.s
Eur. J. 2001, 7, 2332-2334.
(16) Bennani, Y. L.; Hanessian, S. Chem. ReV. 1997, 97, 3161-3195.
(17) Hanabusa, K.; Yamada, M.; Kimura, M.; Shirai, H. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 1949-1950.
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Self-Assembly of Noncyclic Bis-D- and L-tripeptides
SCHEME 1. Synthetic Scheme for the Alanine Derivatives
SCHEME 2. Synthetic Scheme for the Serine Derivatives
DMSO-d6 also suggested a relatively stable, H-bonded structure
as judged from the minimal NH shifts.18
In an effort to obtain solids suitable for X-ray crystallographic
analysis, we varied the nature of the amino acids (Scheme 2).
Thus, N-Boc mono-, di-, tri-, and tetrapeptides 9-12, consisting
of L- and D-O-benzyl serines, afforded microcrystalline products
with fully engaged H-bonded structures as indicated by 1H NMR
titration experiments.18 Ultimately, the bis-tripeptide analogue
L-Ser (OBn)-D-Ser (OBn)-L-Ser (OPBB) Boc 13, (OPBB )
p-bromobenzyl ether) afforded X-ray quality crystals from a
mixture of benzene and CHCl3.19 Much to our surprise (and
delight), the crystallographic data revealed that neither of the
hypothetical linear â-sheet motifs shown in Figure 1 were
actually formed. Instead, the crystal structure, consisting of two
symmetry independent conformers A and B differing slightly
in the positions of some side chains, revealed an array of tubular
constructs arranged in a parallel spatial alignment separated by
benzene and CHCl3 molecules (Figure 2, left).
Individual DACH-bis-tripeptide units had in fact self-
assembled by engaging in intra- and intermolecular H-bonds,
thus forming a supramolecular helical continuum along the
vertical axis of the tubular motif (Figure 2, right). The manner
in which a carboxy-linked anchored bis-peptide 13 accom-
modates both intra- and intermolecular H-bonded interactions
to achieve the observed helical superstructure is intriguing and,
to the best of our knowledge, unprecedented.
Thus, each tripeptide strand assumes a 31° turn at the i + 1/i
+ 2 juncture to expose partially extended strands that cross “over
and under” each other in left- and right-handed directions akin
to a pair of self-embracing arms reminiscent of Foyatier’s
Spartacus (Figure 3). Three intramolecular H-bonds consisting
of a γ-turn and a 14-membered antiparallel H-bonded ring,
spanning the first and third amino acid residues within each
strand, can be observed (Figure 3, red dotted line).
a way that brings its first residue at a hydrogen-bonded distance
to the first residue of the “lower” strand (N10‚‚‚O20, 2.903(4)
Å). A further turn (C10-C11-N11-C12, -104.6(4)°) is
induced by an intramolecular hydrogen bond between the sec-
ond residue (N11) and the third residue from the lower strand
(N11‚‚‚O24, 2.935(4) Å). The conformation of the upper strand
(N14-C14-C15-N16,157.7(3)°,C14-C15-N16-C16,-158.0(3)°)
is then aligned so that the carbonyl group of the third resi-
due accepts a hydrogen bond from the second residue of the
lower strand (N21‚‚‚O14, 2.800(4) Å). A similar conformation
with the same intramolecular hydrogen bond pattern between
the two tripeptide strands is observed for the second con-
former B.18
In the conformer A (Figure 3), the “upper” strand (N10 f
N16, Figure 3) is rotated (N10-C10-C11-N11, 32.8(5)°) in
(18) For details, see Supporting Information.
(19) For selected examples of benzene inclusion in supramolecular
complexes, see: (a) Hanessian, S.; Saladino, R.; Margarita, R.; Simard,
M.; Chem.sEur. J. 1999, 5, 2169-2183. (b) Endo, K.; Sawaki, T.;
Koyanagi, M.; Kobayashi, K.; Matuda, H.; Ayama, Y. J. Am. Chem. Soc.
1995, 117, 8341-8352. (c) Collet, A. Tetrahedron 1987, 43, 5725-5729.
(d) Cerrini, S.; Giglio, E.; Mazza, F.; Pavel, N. Y. Acta Crystallogr., Sect.
B 1979, 35, 2605-2609.
This type of folding pattern allows further interstrand
H-bonding with a classical antiparallel arrangement between
J. Org. Chem, Vol. 73, No. 4, 2008 1183

Hanessian et al.
FIGURE 2. Left: View along the c-axis of a 2 × 2 × 2 array of unit cells of the packing in structure of DACH-bis-tripeptide 13 displayed in
red with the included CHCl3 and benzene molecules shown as sphere of van der Waals radii. Right: View of one column along the a-axis showing
the stacking of alternating DACH-bis-tripeptide 13 conformers A (green) and B (purple) with the included CHCl3 and benzene molecules shown
as sphere of van der Waals radii.
residue in the upper strand in conformer A (O12) is engaged in
a bifurcated intermolecular hydrogen bond involving the NH
group in the terminal N-Boc (N46‚‚‚O12, 2.845(5) Å) and the
amide group nitrogen of the third residue (N44‚‚‚O12, 2.990-
(4) Å) of the DACH tripeptide unit B. This last intermolecular
hydrogen bond, together with the intramolecular H-bond
N31‚‚‚O44, results in the turn of the terminal N-Boc in B
(C42-C43-N43-C44, 83.1(4)°). The link between the two
consecutive conformers A and B is completed by two additional
hydrogen bonds involving the first and second residues of A
with the third residue and the carbonyl group of the terminal
N-Boc group, respectively, in conformer B (N14‚‚‚O42, 2.869-
(4) Å, N40‚‚‚O16, 3.094(5) Å). A similar hydrogen bond pattern
is observed between the lower strand of conformer A (N20 f
N26) and that in conformer B (N30 f N36).18
The combination of intra- and intermolecular H-bond net-
works results in a helical head-to-tail arrangement of alternating
DACH-bis-tripeptide units A and B assembled in a tubular
shape with an average outer diameter of 18 Å (Figure 4, right).
The hydrophobic aromatic side chains that are oriented toward
FIGURE 3. X-ray structure of an individual (1R,2R)-DACH-bis-
tripeptide 13 showing intramolecularly H-bonded strands that cross over
each other like a pair of self-embracing arms. Side chains and hydrogen
atoms (except those attached to nitrogen) have been omitted for clarity.
Two distinct but similar conformations A and B are observed in the
crystal; only one is shown.
the outer periphery do not appear to be within van der Waals
contact.
The superstructure contains concave regions surrounded by
hydrophobic benzyl and Boc groups (Figure 4, right, arrows).
These large cavities are occupied by benzene and CHCl3 solvent
molecules, while the internal hydrophilic regions are solvent-
free.19 The benzene molecules occupying the hydrophobic
cavities are deployed almost in parallel and perpendicular
opposing peptide strands from the different conformers (Figure
4, left, blue hashed lines). The amide carbonyl of the second
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Self-Assembly of Noncyclic Bis-D- and L-tripeptides
FIGURE 4. Left: H-bonded arrangement of three DACH-bis-tripeptide units in 13 with intra- (red) and inter- (blue) H-bonds (c-axis vertical).
Side chains are not shown for clarity. Right: Four consecutive DACH-bis-tripeptide units 13 with side chains along the b-axis, showing two
different conformers A (green) and B (purple). Arrows indicate concave regions occupied by benzene molecules as in Figure 2. Hydrogen atoms
not involved in H-bonds have been omitted for clarity.
orientations relative to the crystallographic c-axis (Figure 2, left).
Together with the remaining CHCl3 molecules, they line up the
sides of the densely packed tube-shaped motifs in a symmetry-
related pattern coinciding with the locations of hydrophobic side
chains.20 The inner diameter of the central core is 2.5 Å (Figure
5). Although structure 13 is one of several ensembles in the
crystals, it represents the overall three-dimensional topology of
the system as a whole.
1H NMR dilution experiments with 13 revealed the existence
of relatively stable H-bonded structures within the concentration
range of 0.4 to 4.0 mM. Titrations with increasing amounts of
DMSO-d6 in CDCl3 showed minimal if any chemical shift
differences of the NH resonances.18
Finally, hydrogenolysis of the benzyl ether groups in 11 led
1
to 14 as an amorphous solid which gave a nicely resolved H
NMR spectrum in pyridine-d5. Addition of incremental amounts
of DMSO-d6 only slightly shifted the NH resonances.18 A 20-
fold equivalent of CD3OD per NH group in pyridine-d5 resulted
in only a 20% exchange over a period of 24 h at rt. This suggests
FIGURE 5. Top view along the c-axis of one column of DACH-
bis-tripeptide 13 showing the internal hydrophilic cavity with an inner
diameter of 2.5 Å.
the existence of a H-bonded structure of 14 in pyridine-d5.
(20) Crystal data for bis-tripeptide 13‚3(C6H6)‚CHCl3: C171H207BrH-
C13N16O32, FW ) 3424.52, orthorhombic, space group P212121, a ) 18.146-
(3) Å, b ) 26.468 Å, c ) 35.413(3) Å, V ) 17009 Å3, Z ) 4, dcalc ) 1.377
g cm-3. For details, see Supporting Information.
We also prepared the diastereomeric bis-tripeptide 18 with
an (S,S)-DACH anchor (Scheme 2). Although titration experi-
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Hanessian et al.
FIGURE 6. Left: crystals of 13 exhibiting birefringence under polarized light. Right: gel form of 18 from a benzene solution.
SCHEME 3. Synthetic Scheme for the Unsymmetrical Ala-Leu Derivatives
ments revealed a behavior almost identical to that of 13,18 its
solubility properties were markedly different. Whereas beauti-
fully crystalline 13 was freely soluble in benzene, 18 afforded
a gel upon heating and cooling from the same solvent (Figure
6).
We next studied the synthesis of the analogous bis-tripeptides
comprising D-Ala and L-Ala on one strand in combination with
D-Leu and L-Leu on the other strand (Scheme 3). We hoped
that these unsymmetrical constructs would provide information
regarding possible conformationally distinct secondary structures
in solution as revealed by 2D NMR.
Deprotection of the N-Boc group and coupling with N-Boc Leu-
OH afforded the bis-monopeptide 21, which was sequentially
deprotected and coupled as above to give the bis-peptides 23-
25. Finally, cleavage of the N-Cbz and N-Boc groups from 25
gave the tripeptide 26, isolated as the bis-hydrochloride salt.
Alternatively, hydrogenolysis of the N-Cbz group in 25 followed
by protection of the resulting free amine as the Boc derivative
afforded the intended bis-N-Boc tripeptide 27 as an amorphous
solid. Numerous attempts to grow crystals from the Ala-Leu
tripeptide series were not successful. Titration with DMSO-d6
in CDCl3 showed minimal if any chemical shift differences of
Starting with the mono-N-Boc derivative of trans-(1R,2R)-
diaminocyclohexane 1921 and following the standard coupling
with N-Cbz Ala-OH, 20 was obtained in excellent yields.
(21) Uppadine, L. H.; Keene, F. R.; Beer, P. D. J. Chem. Soc., Dalton
Trans. 2001, 2188-2198.
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Self-Assembly of Noncyclic Bis-D- and L-tripeptides
energetically favorable conformations in the solid state when
anchored to a C2-symmetrical turn motif such as DACH. As a
result of a unique H-bonding network between the strands of
the tripeptides, tube-shaped supramolecular assemblies are
formed reminiscent of cyclic peptide nanotubes7 and other non-
peptidic columnar aggregates.28 Thus, the need to further exploit
the chemistry of short open-ended D- and L-oligopeptides in
relation to structure and function is warranted.29
Experimental Section
(1R,2R)-N,N′-Bis(L-Ala-NHCbz)diaminocyclohexane (2). To
an ice-chilled solution of 1 (200 mg, 1.75 mmol) in 49 mL of DMF
were added (L) N-Cbz-alanine (937 mg, 4.20 mmol), HOBt (708
mg, 5.25 mmol), EDC (1.0 g, 5.25 mmol), and DIEA (1.8 mL,
10.5 mmol). The reaction mixture was stirred for 1 h at 0 °C and
36 h at room temperature. DMF was then removed under reduced
pressure, and the residue was dissolved in 28 mL of dichlo-
romethane. Water (28 mL) was then added to the solution, and the
phases were separated. The organic layer was washed successively
with a saturated aqueous solution of NaHCO3, aq 1 N HCl, and
brine, then dried (Na2SO4) and evaporated to dryness. The resulting
crude was purified by flash column chromatography on silica gel
(elution with AcOEt), and the desired product was obtained as a
FIGURE 7. Representations of the interstrand cross-talk between D-Ala
R-proton and L-LeuNH (red arrow) in 27.
the NH resonances. Several NOESY and ROESY spectra of
27 at different temperatures (268 and 318 K) and mixing times
(0.5, 0.7, 0.8, and 1.2 s) were recorded.18 In spite of the
observation of more than 30 NOESY cross-peaks, most were
not significant and indicative of an ordered H-bonded structure.
Nevertheless, an nOe was observed between the D-Ala R-proton
in one strand and the first L-Leu NH of the other, indicating
relative proximity in space (Figure 7).
1
white solid (750 mg, 82%): [R]D +2 (c 1.0, MeOH); H NMR
(CDCl3, 400 MHz) δ (ppm) 7.28-7.20 (m, 10H), 6.94 (br s, 2H),
6.13 (d, J ) 7.7 Hz, 2H), 5.12-5.09 (d, J ) 12.3 Hz, 2H), 4.91-
4.88 (d, J ) 12.3 Hz, 2H), 4.17-4.13 (m, 2H), 3.68 (br s, 2H),
1.94 (br s, 2H), 1.68 (br s, 2H), 1.28-1.23 (m, 10H); 13C NMR
(CDCl3, 100 MHz) δ (ppm) 173.6, 156.8, 136.7, 128.8 (2), 128.4,
67.3, 53.8, 51.0, 32.5, 25.1, 18.5; FT-IR (neat) ν (cm-1) 3421, 3325,
1708, 1678, 1524, 1509; MS (FAB) m/e 525 [M + H]+; HRMS
(FAB) for C28H37N4O6 calcd 525.27131; found 525.27366.
(1R,2R)-N,N′-Bis(L-Ala-D-Ala-NHCbz)diaminocyclohexane (3).
Compound 2 (500 mg, 0.95 mmol) was dissolved in methanol (9.5
mL) and hydrogenated (1 atm) in the presence of 10% Pd/C for 16
h at room temperature. After removal of the catalyst by filtration
through Celite, the solvent was evaporated under vacuum, affording
the corresponding free diamine as a white solid. This material was
then coupled with (D) N-Cbz-alanine under the same conditions
described for the synthesis of 2 (purification by flash chromatog-
raphy on silica gel elution with AcOEt/MeOH 9/1), affording the
desired product as a white solid (344 mg, 66% over two steps):
Unfortunately, other across strand interactions could not be
observed, thus precluding the unambiguous assignment of a
definitive secondary structure for the Ala-Leu derivative 27
in solution as in the case of 13 in the solid state.
The helical superstructure of 13 in the solid state is
reminiscent of the structure of the antibacterial peptide grami-
cidin A,22 which consists of a sequence of seven alternating D-
and L-amino acids. Dimerization of gramicidin A affords a head-
to-head, so-called â-helical, motif.23,24 Valinomycin is a cyclic
dodecadepsipeptide that contains D- and L-valine residues among
others which, like gramicidin A, is involved in cation transport
across membranes.25 The structure of polytheonamide A, a 48
amino acid residue cytotoxic polypeptide consisting of alternat-
ing D- and L-tert-butyl leucines among others, has been recently
elucidated by NMR techniques.26 It is only the second naturally
occurring noncyclic polypeptide to harbor D- and L-amino acid
sequences after gramicidin A. The structure-activity relation-
ship among helix-forming â-amino acid oligomers as mimics
of host-defense peptides has been studied by Gellman and co-
workers.27
1
[R]D -14 (c 0.5, MeOH); H NMR (CDCl3, 400 MHz) δ (ppm)
7.36-7.27 (m, 12H), 6.95 (br s, 2H), 6.09-6.06 (d, J ) 7.5 Hz,
2H), 5.14-5.10 (d, J ) 12.2 Hz, 2H), 5.05-5.01 (d, J ) 12.2 Hz,
2H), 4.41-4.30 (m, 4H), 3.70 (br s, 2H), 1.94 (br s, 2H), 1.70 (br
s, 2H), 1.40-1.24 (m, 16H); 13C NMR (CDCl3, 100 MHz) δ (ppm)
173.5, 172.9, 156.8, 136.5, 128.9, 128.6, 128.4, 67.4, 54.2, 50.9,
49.0, 32.2, 25.0, 18.9, 17.9; FT-IR (neat) ν (cm-1) 3425, 3334,
1713, 1669, 1524, 1508; MS (FAB) m/e 667 [M + H]+; HRMS
(FAB) for C34H47N6O8 calcd 667.34553; found 667.34590.
(1R,2R)-N,N′-Bis(L-Ala-D-Ala-L-Ala-NHCbz)diaminocyclo-
hexane (4). The title compound was obtained starting from 3 (250
mg, 0.37 mmol) and following the procedures described for the
Conclusions
In conclusion, we have shown that even a minimal noncyclic
tripeptide consisting of L- and D-amino acids as 13 can adopt
(22) (a) Wallace, B. A.; Ravikumar, K. Science 1988, 241, 182-187.
(b) Langs, D. A. Science 1988, 241, 188-191. (c) Ketchen, R. R.; Hu, W.;
Cross, T. A. Science 1993, 261, 1457-1460. (d) Burkhart, B. M.; Li, N.;
Langs, D. A.; Pangborn, W. A.; Duax, W. L. Proc. Natl. Acad. Sci. U.S.A.
1998, 95, 12950-12955.
(23) For relevant articles, see: (a) Åkerfelt, K.; Lear, L. D.; Wasserman,
Z. R.; Chung, L. A.; De Grado, W. F. Acc. Chem. Res. 1993, 26, 191-
197. (b) Gokel, G. W.; Mukhopadhyay, A. Chem. Soc. ReV. 2001, 30, 274-
286.
(24) For a study of tartaric acid-gramicidin hybrids, see for example:
(a) Stankovic, C. J.; Heinemann, S. H.; Schreiber, S. L. J. Am. Chem. Soc.
1990, 112, 3702-3704. (b) Xiao, J.; Weisblum, B.; Wipf, P. J. Am. Chem.
Soc. 2005, 127, 5742-5743.
(27) (a) Sadowsky, J. D.; Murray, J. K.; Tomita, Y.; Gellman, S. H.
ChemBioChem 2007, 8, 903-916. (b) Price, J. L.; Horne, W. S.; Gellman,
S. H. J. Am. Chem. Soc. 2007, 129, 6376-6377. (c) Horne, W. S.; Price,
J. L.; Keck, J. L.; Gellman, S. H. J. Am. Chem. Soc. 2007, 129, 4178-
4180. (d) Sadowsky, J. D.; Fairlie, W. D.; Hadley, E. B.; Lee, H.-S.;
Umezawa, N.; Nikolovska-Coleska, Z.; Wang, S.; Huang, D. C. S.; Tomita,
Y.; Gellman, S. H. J. Am. Chem. Soc. 2007, 129, 139-154. (e) Porter, F.
A.; Weisblum, B.; Gellman, S. H. J. Am. Chem. Soc. 2005, 127, 11516-
11529.
(28) For helical cation-promoted oligomeric, open-ended heterocycles,
see: Petitjean, A.; Cuccia, A, L. A.; Lehn, J.-M.; Nierengarten, H.; Schmutz,
M. Angew. Chem., Int. Ed. 2002, 41, 1195-1198.
(29) Keizer, H. M.; Sijbesma, R. P. Chem. Soc. ReV. 2005, 34, 226-
234.
(25) Streinrauf, L. K.; Hamilton, J. A.; Sabesan, M. N. J. Am. Chem.
Soc. 1982, 104, 4085-4091.
(26) Hamada,T.; Matsunaga, S.; Yano, G.; Fusetani, N. J. Am. Chem.
Soc. 2005, 127, 110-118.
J. Org. Chem, Vol. 73, No. 4, 2008 1187

Hanessian et al.
synthesis of 3 from 2 (i.e., coupling with (L) N-Cbz-alanine and
purification by flash chromatography on silica gel). Elution with
AcOEt/MeOH 9/1 gave 4 as a white solid (72 mg, 24%, over two
3.71 (m, 2H), 1.99 (s, 6H), 1.91 (br s, 2H), 1.74 (br s, 2H), 1.40-
1.35 (m, 28H); 13C NMR (CD3OD, 100 MHz) δ (ppm) 174.5, 173.9,
173.8, 173.6, 172.3, 52.9, 50.1, 49.9, 49.8, 49.7, 31.9, 24.7, 21.4,
17.1, 16.7, 16.5, 16.4; FT-IR (neat) ν (cm-1) 3302, 1656, 1541,
1523; MS (FAB) m/e 790 [M + Na]+, 768 [M + H]+.
1
steps): [R]D -3 (c 0.5, MeOH); H NMR (CDCl3, 300 MHz) δ
(ppm) 7.70 (br s, 2H), 7.36-7.28 (m, 12H), 6.65 (br s, 2H), 5.24-
5.20 (d, J ) 12.1 Hz, 2H), 5.15-5.12 (d, J ) 8.7 Hz, 2H), 5.08-
5.04 (d, J ) 12.1 Hz, 2H), 4.67-4.63 (m, 2H), 4.43-4.40 (m,
2H), 4.32-4.28 (m, 2H), 3.51 (br s, 2H), 1.81-1.78 (m, 2H), 1.55
(m, 2H), 1.35 (d, J ) 6.7 Hz, 6H), 1.24-1.22 (m, 16H); 13C NMR
(CDCl3, 100 MHz) δ (ppm) 173.7, 173.4, 172.7, 156.9, 136.5,
128.9, 128.8, 128.7, 67.6, 53.4, 50.7, 49.0 (2), 32.5, 25.0, 18.4 (2),
17.9; FT-IR (neat) ν (cm-1) 3414, 3314, 1714, 1670, 1539, 1507;
MS (FAB) m/e 810 [M + H]+; HRMS (FAB) for C40H57N8O10
calcd 809.41976; found 809.42151.
(1R,2R)-N,N′-Bis(L-Ser(OBn)-NHBoc)diaminocyclohexane (9).
To an ice-chilled solution of 1 (300 mg, 2.63 mmol) in 13 mL of
DMF were added (L) N-Boc-serine (OBn) (1.86 g, 6.31 mmol),
HATU (2.4 g, 6.31 mmol), and DIEA (2.75 mL, 15.78 mmol). The
reaction mixture was stirred for 1 h at 0 °C and 48 h at room
temperature. Water (50 mL) and EtOAc (50 mL) were added and
the layers separated. The organic phase was washed successively
with water, a saturated solution of NaHCO3, brine, then dried on
Na2SO4 and evaporated to dryness. The solid obtained was purified
by flash column chromatography on silica gel (elution with AcOEt/
hexanes 1/1), affording the desired product as a white solid (1.12
g, 96%): [R]D +6 (c 1.0, CHCl3); 1H NMR (CDCl3, 400 MHz) δ
(ppm) 7.24-7.19 (m, 10H), 6.75 (d, J ) 6.3 Hz, 2H), 5.45 (br s,
2H), 4.49-4.46 (d, J ) 12.1 Hz, 2H), 4.41-4.38 (d, J ) 12.1 Hz,
2H), 4.20 (br s, 2H), 3.75 (br s, 2H), 3.59 (br s, 2H), 3.52-3.48
(dd, J ) 9.3, 5.4 Hz, 2H) 1.92 (br s, 2H), 1.65 (d, J ) 7.6 Hz, 2H),
1.37 (s, 18H), 1.25-1.22 (m, 4H); 13C NMR (CDCl3, 100 MHz) δ
(ppm) 170.9, 156.0, 138.0, 128.5, 128.1, 127.9, 80.3, 73.5, 69.8,
54.4, 54.1, 32.3, 28.6, 24.9; FT-IR (neat) ν (cm-1) 3416, 1710,
1668, 1528, 1496; MS (FAB) m/e 669 [M + H]+, 569 [M - Boc]+,
469 [M - 2Boc]+; HRMS (FAB) for C36H53N4O8 calcd 669.38634;
found 669.38370.
(1R,2R)-N,N′-Bis(L-Ala-D-Ala-L-Ala-D-Ala-NHCbz)diaminocy-
clohexane (5). The title compound was obtained starting from 4
(680 mg, 0.84 mmol) and following the procedures described for
the synthesis of 3. Coupling with (D) N-Cbz-alanine and purification
by flash chromatography on silica gel (elution with AcOEt/MeOH
4/1) gave 5 as a white solid (380 mg, 48%, over two steps): [R]D
-24 (c 0.125, MeOH); 1H NMR (CDCl3, 400 MHz) δ (ppm) 7.64
(br s, 2H), 7.34-7.31 (m, 12H), 7.22 (br s, 2H), 6.84 (br s, 2H),
6.23 (d, J ) 6.0 Hz, 2H), 5.16-5.13 (d, J ) 12.2 Hz, 2H), 5.08-
5.05 (d, J ) 12.2 Hz, 2H), 4.55-4.40 (m, 8H), 3.66 (m, 2H), 1.77-
1.75 (m, 2H), 1.69-1.66 (m, 2H), 1.36-1.15 (m, 28H); 13C NMR
(CD3OD, 100 MHz) δ (ppm) 174.8, 173.9, 173.8, 173.6, 157.4,
137.2, 128.5, 128.0, 127.7, 66.6, 52.9, 51.3, 50.0, 49.8, 49.7, 31.9,
24.7, 17.1, 16.7, 16.4, 16.3; FT-IR (neat) ν (cm-1) 3306, 1662,
1530; MS (FAB) m/e 951 [M]+; HRMS (FAB) for C46H67N10O12
calcd 951.49399; found 951.49100.
(1R,2R)-N,N′-Bis(L-Ala-D-Ala-L-Ala-D-Ala-NH2)diaminocy-
clohexane (6). The title compound was obtained starting from 5
(300 mg, 0.31 mmol), following the procedures for the deprotection
of 2. Hydrogenolysis and filtration through Celite afforded 6 as a
white solid (225 mg, 94%). The crude product was used in the
next step without any further purification: [R]D -9 (c 0.55, MeOH);
1H NMR (CD3OD, 300 MHz) δ (ppm) 4.32-4.18 (m, 6H), 3.67
(br s, 2H), 3.47-3.40 (t, J ) 6.8 Hz, 2H), 1.92 (br s, 2H), 1.74 (br
s, 2H), 1.37-1.25 (m, 28H); 13C NMR (CD3OD, 75 MHz) δ (ppm)
177.4, 174.1, 173.7 (2), 53.1, 50.2, 49.8, 49.7, 49.5, 32.0, 24.8,
20.2, 17.2, 16.9, 16.4; MS (FAB) m/e 705 [M + Na]+, 683 [M +
H]+; HRMS (FAB) for C30H55N10O8 calcd 683.42043; found
683.42077.
(1R,2R)-N,N′-Bis(L-Ala-D-Ala-L-Ala-D-Ala-NHBoc)diaminocy-
clohexane (7). Boc2O (19 mg, 0.088 mmol) was added to a stirred
solution of diamine 6 (15 mg, 0.022 mmol) in methanol (1 mL).
The reaction mixture was stirred at room temperature for 60 h.
The solvent was removed under reduced pressure, and the residue
was purified by flash column chromatography on silica gel (AcOEt/
MeOH 9/1) to give the desired product as a white solid (11 mg,
58%, over two steps): [R]D -5 (c 0.55, MeOH); 1H NMR (CDCl3,
300 MHz) δ (ppm) 7.63 (br s, 2H), 7.49 (br s, 2H), 7.23 (br s,
2H), 7.11 (br s, 2H), 5.79 (br s, 2H), 4.53-4.43 (m, 6H), 4.24-
4.22 (m, 2H), 3.67 (m, 2H), 1.92 (br s, 2H), 1.75 (br s, 2H), 1.46
(s, 18H), 1.49-1.25 (m, 28H); 13C NMR (CD3OD, 100 MHz) δ
(ppm) 175.0, 173.9, 173.8, 173.7, 156.7, 79.5, 53.0, 50.9, 50.1,
49.9, 49.5, 31.9, 27.7, 24.7, 17.1, 17.0, 16.6, 16.4; FT-IR (neat) ν
(cm-1) 3309, 1664, 1534, 1509; MS (FAB) m/e 905 [M + Na]+,
882 [M]+, 782 [M - Boc]+, 683 [M - 2Boc]+; HRMS (FAB) for
C40H71N10O12 calcd 883.52529; found 883.52250.
(1R,2R)-N,N′-Bis(L-Ser(OBn)-D-Ser(OBn)-NHBoc)diaminocy-
clohexane (10). Compound 9 (1.6 g, 2.39 mmol) was dissolved in
10 mL of dichloromethane, trifluoroacetic acid (10 mL) was then
added, and the resulting solution was stirred at room temperature
for 2 h. The solvents were removed under reduced pressure, and
the residue was coevaporated with toluene, then dried overnight
under vacuum. The solid obtained was used without any further
purification in the ensuing coupling to (D) N-Boc-serine (OBn)
under the same conditions described for the synthesis of 9
(purification by flash column chromatography on silica gel, eluting
with AcOEt/hexanes 3/2), affording the desired product as a white
1
solid (1.88 g, 77% over two steps): [R]D -46 (c 1.0, CHCl3); H
NMR (CDCl3, 400 MHz) δ (ppm) 7.32-7.20 (m, 22H), 6.73 (br s,
2H), 6.18 (br s, 2H), 4.71 (br s, 2H), 4.55-4.39 (m, 10H), 4.06-
3.96 (m, 2H), 3.96-3.92 (m, 2H), 3.69-3.56 (m, 4H), 3.56-3.46
(br s, 2H), 1.86 (br s, 2H), 1.59 (br s, 2H), 1.45 (s, 18H), 1.07 (br
s, 4H); 13C NMR (CDCl3, 100 MHz) δ (ppm) 171.0, 170.3, 156.3,
138.1, 137.9, 128.9, 128.8, 128.7, 128.5, 128.1, 127.9, 80.5, 73.6,
72.9, 70.7, 68.3, 55.3, 54.2, 52.6, 32.0, 28.8, 24.8; FT-IR (neat) ν
(cm-1) 3334, 1668, 1538, 1497; MS (FAB) m/e 1023 [M]+, 923
[M - Boc]+, 823 [M - 2Boc]+; HRMS (ESI) for C56H75N6O12
(MH+) calcd 1023.54375; found 1023.54210.
(1R,2R)-N,N′-Bis(L-Ser(OBn)-D-Ser(OBn)-L-Ser(OBn)-NHB-
oc)diaminocyclohexane (11). The title compound was obtained
starting from 10 (1.5 g, 1.47 mmol) following the general
procedures for the synthesis of 10 from 9 (i.e., coupling with (L)
N-Boc-serine (OBn) and purification by flash column chromatog-
raphy on silica gel). Elution with AcOEt/hexanes 2/1 gave 11 as a
white solid (1.56 g, 77% over two steps: [R]D -7.5 (c 1.0, CHCl3);
1H NMR (CDCl3, 400 MHz) δ (ppm) 7.57 (br s, 2H), 7.41 (d, J )
6.9 Hz, 2H), 7.28-7.22 (m, 30H), 6.85 (br s, 2H), 5.72 (d, J ) 6.9
Hz, 2H), 4.76-4.74 (m, 2H), 4.63-4.60 (m, 2H), 4.51-4.38 (m,
14H), 3.83-3.79 (m, 6H), 3.62-3.52 (m, 8H), 1.89 (d, J ) 9.6
Hz, 2H), 1.61-1.58 (m, 2H), 1.43 (s, 18H), 1.16-1.10 (m, 4H);
13C NMR (CDCl3, 100 MHz) δ (ppm) 171.3, 170.3, 170.2, 155.9,
138.0 (2), 137.9, 128.9, 128.8, 128.6, 128.5, 128.2, 128.1 (2), 128.0,
127.9, 80.3, 73.5, 73.3, 70.4, 69.9 (2), 69.2, 54.4, 54.2, 53.9, 53.0,
32.0, 28.7, 24.9; FT-IR (neat) ν (cm-1) 3332, 1666, 1496; MS
(FAB) m/e 1377 [M]+, 1277 [M - Boc]+, 1177 [M - 2Boc]+;
HRMS (ESI) for C76H97N8O16 (MH+) calcd 1377.70171; found
1377.69973.
(1R,2R)-N,N′-Bis(L-Ala-D-Ala-L-Ala-D-Ala-NHAc)diaminocy-
clohexane (8). Acetic anhydride (10 µL, 0.076 mmol) was added
to a stirred solution of diamine 6 (10 mg, 0.015 mmol) in methanol
(500 µL). The reaction mixture was stirred at room temperature
for 48 h. The solvents were removed under reduced pressure, and
the residue was triturated with ether, the white solid filtered and
dried under vacuum. The desired product was obtained as a white
1
solid (10 mg, 91% over two steps): [R]D -8 (c 0.1, MeOH); H
NMR (CD3OD, 400 MHz) δ (ppm) 4.31-4.26 (m, 8H), 3.74-
1188 J. Org. Chem., Vol. 73, No. 4, 2008

Self-Assembly of Noncyclic Bis-D- and L-tripeptides
(1R,2R)-N,N′-Bis(L-Ser(OBn)-D-Ser(OBn)-L-Ser(OBn)-D-Ser-
(OBn)-NHBoc)diaminocyclohexane (12). The title compound was
obtained starting from 11 (500 mg, 0.36 mmol) following the
general procedures for the synthesis of 10 (i.e., coupling with (D)
N-Boc-serine (OBn) and purification by flash column chromatog-
raphy on silica gel). Elution with AcOEt/hexanes 3/1 gave 12 as a
white solid (560 mg, 89% over two steps): [R]D -2 (c 0.9, CHCl3);
1H NMR (CDCl3, 400 MHz) δ (ppm) 7.69 (br s, 2H), 7.57 (br s,
2H), 7.42-7.18 (m, 40H), 7.15 (br s, 2H), 6.93 (br s, 2H), 5.70 (br
s, 2H), 4.77 (br s, 2H), 4.71-4.55 (m, 4H), 4.53-4.31 (m, 18H),
3.89-3.70 (m, 8H), 3.68-3.49 (m, 10H), 1.86 (br s, 2H), 1.59 (br
s, 2H), 1.49-1.32 (m, 20H), 1.20-1.04 (m, 4H); 13C NMR (CDCl3,
100 MHz) δ (ppm) 171.0, 170.7, 170.3, 170.2, 156.1, 138.1, 138.0
(2), 137.9, 128.8 (2), 128.7 (2), 128.5 (2), 128.2 (2), 128.1 (2),
128.0, 127.9, 80.6, 73.5 (2), 73.2, 70.2, 69.7, 69.5 (2), 69.1, 54.8,
53.9 (2), 53.5, 53.1, 32.1, 28.7, 24.9; FT-IR (neat) ν (cm-1) 3327,
1667, 1497; MS (FAB) m/e 1754 [M + Na]+, 1732 [M]+, 1632
[M - Boc]+, 1532 [M - 2Boc]+; HRMS (ESI) for C96H119N10O20
(MH+) calcd 1731.85966; found 1731.85850.
2.00-1.85 (m, 2H), 1.82-1.69 (m, 2H), 1.50 (s, 18H), 1.32-1.10
(m, 4H); MS (ESI) m/e 669 [M + H]+, 569 [M - Boc + H]+.
(1S,2S)-N,N′-Bis(L-Ser(OBn)-D-Ser(OBn)-NHBoc)diaminocy-
clohexane (17). The title compound was obtained starting from 16
(668 mg, 1.0 mmol) following the procedures for the preparation
of 10 from 9 (i.e., deprotection with trifluoroacetic acid in
dichloromethane, then coupling with (D) N-Boc-serine (OBn) and
purification by flash column chromatography on silica gel). Elution
with hexanes/AcOEt 20/80 gave 17 as a white fluffy solid (880
mg, 86% on two steps): 1H NMR (CDCl3, 400 MHz) δ (ppm)
7.42 (d, J ) 6.8 Hz, 2H), 7.37-7.27 (m, 20H), 6.81 (br s, 2H),
5.69 (br s, 2H), 4.62 (d, J ) 11.9 Hz, 2H), 4.57-4.45 (m, 8H),
4.41 (br s, 2H), 3.98-3.84 (m, 4H), 3.73-3.65 (m, 2H), 3.65-
3.52 (m, 4H), 1.88-1.81 (m, 2H), 1.65 (d, J ) 6.8 Hz, 2H), 1.43
(s, 18H), 1.25-1.15 (m, 2H); MS (ESI) m/e 1023 [M + H]+, 923
[M - Boc + H]+.
(1S,2S)-N,N′-Bis(L-Ser(OBn)-D-Ser(OBn)-L-Ser(OPBB)-NH-
Boc)diaminocyclohexane (18). The title compound was obtained
starting from 17 (322 mg, 0.31 mmol) following the procedures
for the preparation of 13 from 10 (i.e., deprotection with trifluo-
roacetic acid in dichloromethane, then coupling with (L) N-Boc-
serine (OPBB) and purification by flash column chromatography
on silica gel). Elution with CHCl3/MeOH 97/3 gave 18 as a glassy
(1R,2R)-N,N′-Bis(L-Ser(OBn)-D-Ser(OBn)-L-Ser(OPBB)-NH-
Boc)diaminocyclohexane (13). The title compound was obtained
starting from 10 (100 mg, 0.098 mmol) and following the general
procedures for the synthesis of 10 (i.e., coupling with (L) N-Boc-
serine (OPBB) and purification by flash column chromatography
on silica gel). Elution with AcOEt/hexanes 2/1 gave 13 as a white
crystalline solid (76 mg, 51% over two steps): crystallization from
1:1 benzene/CHCl3 (hexanes atmosphere); [R]D -5 (c 0.5, CHCl3);
1H NMR (CDCl3, 300 MHz) δ (ppm) 7.58 (d, J ) 6.2 Hz, 2H),
7.48-7.38 (m, 6H), 7.38-7.23 (m, 20H), 7.21-7.13 (m, 4H), 6.82
(d, J ) 6.3 Hz, 2H), 5.74 (d, J ) 6.8 Hz, 2H), 4.85-4.74 (m, 2H),
4.70-4.58 (m, 2H), 4.54-4.40 (m, 14H), 3.93-3.74 (m, 6H),
3.70-3.46 (m, 8H), 1.99-1.85 (m, 2H), 1.75-1.62 (m, 2H), 1.46
(s, 18H), 1.22-1.09 (m, 4H); 13C NMR (CDCl3, 100 MHz) δ (ppm)
171.3, 170.3 (2), 155.9, 137.9 (2), 137.1, 131.9, 129.7, 128.8, 128.2,
128.1, 127.9 (2), 127.6, 122.0, 80.5, 73.5, 73.3, 72.7, 70.5, 69.9,
1
solid (317 mg, 66% on two steps): [R]D -6 (c 1.0, CHCl3); H
NMR (CDCl3, 500 MHz) δ (ppm) 7.51 (d, J ) 6.8 Hz, 2H), 7.44-
7.39 (m, 3H), 7.35-7.23 (m, 28H), 7.17-7.13 (m, 4H), 6.85 (d, J
) 7.3 Hz, 2H), 5.74 (d, J ) 7.3 Hz, 2H), 4.67 (dd, J1 ) 5.4 Hz, J2
) 8.3 Hz 2H), 4.53-4.39 (m, 14H), 3.91-3.78 (m, 6H), 3.72-
3.63 (m, 6H), 3.53 (dd, J1 ) 5.4 Hz, J2 ) 8.3 Hz, 2H), 1.91-1.84
(m, 2H), 1.44 (s, 18H), 1.30-1.22 (m, 4H); 13C NMR (CDCl3,
100 MHz) δ (ppm) 170.5 (2), 169.4, 155.4, 137.3, 137.1, 136.3,
131.1, 128.9, 128.1, 128.0, 127.5, 127.4 (2), 127.2, 121.2, 79.7,
72.9, 72.8, 72.0, 69.5, 69.1, 68.8, 53.9, 53.2, 52.7, 52.4, 31.7, 27.9,
24.3; FT-IR (neat) ν (cm-1) 3270, 1635, 1553; MS (ESI) m/e 1536
[M + H]+, 1535 [M]+, 1435 [M - Boc]+; HRMS (ESI) for C79H95-
Br2N8O16 (MH+) calcd 1533.52273; found 1533.52101.
69.1, 54.4, 54.2, 53.9, 53.0, 32.1, 28.7, 24.9; FT-IR (neat) ν (cm-1
)
(1R,2R)-N-(L-Ala-NHCbz),N′-Boc-diaminocyclohexane (20).
Compound 19 (65 mg, 0.3 mmol) was dissolved in 8.4 mL of DMF,
the resulting solution ice-chilled, and (L) N-Cbz-alanine (160 mg,
0.72 mmol), HOBt (121 mg, 0.9 mmol), EDC (172 mg, 0.9 mmol),
and DIEA (313 µL, 1.8 mmol) were added in sequence. The reaction
mixture was stirred for 1 h at 0 °C and 36 h at room temperature.
DMF was then removed under reduced pressure, and the residue
was dissolved in 8.4 mL of dichloromethane. Water (8.4 mL) was
then added to the solution, and the two phases were separated. The
organic layer was washed successively with a saturated solution
of NaHCO3, aq 1 N HCl, and brine, then dried (Na2SO4) and
evaporated to dryness. The resulting crude material was purified
by flash column chromatography on silica gel (eluting with CHCl3/
MeOH 96/4), affording the desired product as a pale yellow solid
3334, 1669, 1537; MS (FAB) m/e 1536 [M + H]+, 1535 [M]+,
1435 [M - Boc]+; HRMS (ESI) for C79H95Br2N8O16 (MH+) calcd
1533.52273; found 1533.52085.
(1R,2R)-N,N′-Bis(L-Ser-D-Ser-L-Ser-NHBoc)diaminocyclohex-
ane (14). Compound 11 (100 mg, 0.098 mmol) was dissolved in
methanol (10 mL) and hydrogenated (60 psi) in the presence of
10% Pd/C (40 mg) for 16 h at room temperature. After removal of
the catalyst by filtration through Celite, the solvent was evaporated
under vacuum, affording the title compound as a white solid (64
mg, quantitative yield): [R]D -3 (c 1.4, MeOH); 1H NMR
(pyridine-d5, 400 MHz) δ (ppm) 9.18 (d, J ) 7.0 Hz, 2H), 8.76 (d,
J ) 7.4 Hz, 2H), 8.38 (br s, 2H), 8.02 (d, J ) 7.4 Hz, 2H), 6.25
(br s, 6H), 5.20 (d, J ) 5.3 Hz, 2H), 5.16-5.09 (m, 2H), 5.01 (d,
J ) 6.6 Hz, 2H), 4.54-4.51 (m, 2H), 4.50-4.48 (m, 2H), 4.47 (d,
J ) 4.58 Hz, 2H), 4.45 (d, J ) 5.0 Hz, 2H), 4.31-4.24 (m, 4H),
4.18 (d, J1 ) 4.7 Hz, 1H), 4.16 (d, J ) 4.7 Hz, 1H), 4.02-3.95
(m, 2H), 1.99-1.89 (m, 2H), 1.47 (s, 18H), 1.41-1.30 (m, 4H),
0.98-0.88 (m, 2H); 13C NMR (pyridine-d5, 100 MHz) δ (ppm)
172.5, 171.5, 171.0, 156.4, 78.9, 63.3, 62.6, 62.2., 57.8, 56.8, 56.3,
53.7, 31.9, 28.2, 24.6; FT-IR (neat) ν (cm-1) 3308, 1655, 1529;
MS (ESI) m/e 859 [M + Na]+, 837 [M + H]+, 737 [M - Boc +
H]+; HRMS (ESI) for C34H61N8O16 (MH+) calcd 837.42000; found
837.41958.
1
(102 mg, 81%): [R]D +20 (c 0.9, CHCl3); H NMR (CDCl3, 400
MHz) δ (ppm) 7.42-7.29 (m, 5H), 6.78 (d, J ) 6.8 Hz, 1H), 5.55
(d, J ) 6.4 Hz, 1H), 5.14 (d, J ) 12.2 Hz, 1H), 5.09 (d, J ) 12.2
Hz, 1H), 4.81 (d, J ) 7.3 Hz, 1H), 4.21 (t, J ) 6.8 Hz, 1H), 3.64-
3.49 (m, 1H), 3.43-3.27 (m, 1H), 2.10-1.93 (m, 2H), 1.82-1.64
(m, 2H), 1.49-1.35 (m, 12H), 1.34-1.12 (m, 3H); 13C NMR
(CDCl3, 100 MHz) δ (ppm) 171.9, 156.5, 155.3, 136.0, 128.1, 127.7
(2), 79.3, 66.4, 54.6, 53.5, 50.4, 32.1, 31.8, 28.0, 24.6, 24.1, 19.1;
FT-IR (neat) ν (cm-1) 3306, 1660, 1530; MS (ESI) m/e 442 [M +
Na]+, 420 [M + H]+, 320 [M - Boc + H]+.
(1S,2S)-N,N′-Bis(L-Ser(OBn)-NHBoc)diaminocyclohexane (16).
The title compound was obtained starting from 15 (207 mg, 1.81
mmol) following the procedures for the preparation of 9 from 1
(i.e., coupling with (L) N-Boc-serine (OBn) and purification by flash
column chromatography on silica gel). Elution with hexanes/AcOEt
60/40 gave 16 as a white solid (1.04 g, 87%): 1H NMR (CDCl3,
400 MHz) δ (ppm) 7.43-7.30 (m, 10H), 6.92 (d, J ) 6.4 Hz, 2H),
5.67 (d, J ) 6.4 Hz, 2H), 4.55 (app s, 4H), 4.38-4.25 (m, 2H),
3.94 (d, J ) 6.8 Hz, 2H), 3.80-3.68 (m, 2H), 3.60-3.49 (m, 2H),
(1R,2R)-N-(L-Ala-NHCbz),N′-(L-Leu-NHBoc)diaminocyclo-
hexane (21). Compound 20 (92 mg, 0.219 mmol) was dissolved
in a 4 N HCl solution in dioxane (1.1 mL), and the resulting solution
was stirred at room temperature until complete consumption of the
starting material. The solvent was removed under reduced pressure,
and the residue was used in the next coupling step without any
further purification. The monoamine chlorohydrate was then
dissolved in 6 mL of DMF, the resulting solution was ice-chilled,
and (L) N-Boc-leucine monohydrate (131 mg, 0.52 mmol), HOBt
J. Org. Chem, Vol. 73, No. 4, 2008 1189

Hanessian et al.
(89 mg, 0.66 mmol), EDC (126 mg, 0.66 mmol), and DIEA (230
µL, 1.3 mmol) were added in sequence. The reaction mixture was
stirred for 1 h at 0 °C and 36 h at room temperature. DMF was
then removed under reduced pressure, and the residue was dissolved
in 6 mL of dichloromethane. Water (6 mL) was then added to the
solution, and the phases were separated. The organic layer was
washed successively with a saturated solution of NaHCO3, aq 1 N
HCl, and brine, then dried (Na2SO4) and evaporated to dryness.
The resulting crude material was then purified by flash column
chromatography on silica gel (eluting with CHCl3/MeOH 98/2),
affording the desired product as a pale yellow oil (82 mg, 71%,
617.2 [M - Boc + H]+; HRMS (ESI) for C37H61N6O8 calcd
717.45454; found 717.45364.
(1R,2R)-N-(L-Ala-D-Ala-L-Ala-NHCbz),N′-(L-Leu-D-Leu-NH-
Boc)diaminocyclohexane (24). The title compound was obtained
starting from 23 (89 mg, 0.124 mmol) following the general
procedures described for the preparation of 23 from 22 (i.e.,
coupling with (L) N-Cbz-alanine and purification by flash column
chromatography on silica gel). Elution with CHCl3/MeOH 95/5 gave
24 as a white solid (85 mg, 87%, over two steps): [R]D -7 (c 0.6,
1
MeOH); H NMR (CDCl3, 400 MHz) δ (ppm) 7.52 (d, J ) 6.6
Hz, 1H), 7.41-7.30 (m, 5H), 7.27-7.16 (m, 2H), 7.04 (d, J ) 8
Hz, 1H), 6.69 (d, J ) 6.6 Hz, 1H), 6.26 (d, J ) 7.0 Hz, 1H), 5.47
(d, J ) 6.6 Hz, 1H), 5.19 (d, J ) 11.8 Hz, 1H), 5.11 (d, J ) 11.8
Hz, 1H), 4.66-4.51 (m, 2H), 4.47-4.26 (m, 3H), 4.47-4.28 (m,
3H), 3.65-3.40 (m, 2H), 2.00-1.86 (m, 2H), 1.76-1.52 (m, 6H),
1.52-1.38 (m, 12H), 1.37-1.27 (m, 6H), 1.01-0.78 (m, 12H);
13C NMR (CDCl3, 100 MHz) δ (ppm) 174.0, 173.6, 172.9 (2),
156.7, 156.4, 136.6, 128.9, 128.5, 128.4, 80.3, 67.3, 54.4, 54.2,
53.6, 51.9, 51.0, 48.9, 41.9, 41.0, 32.3, 32.1, 28.8, 25.1 (2), 25.0
(2), 23.5, 22.1, 22.6, 19.2, 17.9; FT-IR (neat) ν (cm-1) 3292, 2933,
1654, 1529; MS (ESI) m/e 810.6 [M + Na]+, 788.5 [M + H]+,
688.5 [M - Boc + H]+.
1
over two steps): [R]D -24 (c 0.6, CHCl3); H NMR (CDCl3, 500
MHz, -5 °C) δ (ppm) 7.40-7.34 (m, 5H), 6.55 (d, J ) 7.0 Hz,
1H), 6.48 (d, J ) 7.3 Hz, 1H), 5.80 (d, J ) 8.3 Hz, 1H), 5.39 (d,
J ) 8.3 Hz, 1H), 5.18 (d, J ) 12.2 Hz, 1H), 4.98 (d, J ) 12.2 Hz,
1H), 4.14 (q, J1 ) 7.3 Hz, J2 ) 14.7 Hz, 1H), 4.02-3.95 (m, 1H),
3.69-3.59 (m, 2H), 1.97 (d, J ) 11.5 Hz, 2H), 1.92-1.81 (m,
2H), 1.74 (d, J ) 6.7 Hz, 2H), 1.65-1.55 (m, 2H), 1.48-1.39 (m,
2H), 1.37-1.31 (m, 12H), 0.90 (dd, J1 ) 3.5 Hz, J2 ) 5.4 Hz,
6H); 13C NMR (CDCl3, 100 MHz) δ (ppm) 173.2, 172.8, 156.0,
155.6, 135.9, 128.1, 127.8 (2), 79.6, 66.6, 53.4, 53.1, 52.6, 50.2,
40.6, 31.8, 31.7, 27.9, 24.3, 24.2, 22.7, 21.4, 17.6; FT-IR (neat) ν
(cm-1) 3298, 1712, 1646, 1532; MS (ESI) m/e 555.4 [M + Na]+,
533.3 [M + H]+, 433.3 [M - Boc + H]+; HRMS (ESI) for
C28H45N4O6 calcd 533.33336; found 533.33243.
(1R,2R)-N-(L-Ala-D-Ala-L-Ala-NHCbz),N′-(L-Leu-D-Leu-L-
Leu-NHBoc)diaminocyclohexane (25). The title compound was
obtained starting from 24 (65 mg, 0.08 mmol) following the
procedures described for the preparation of 21 from 20 (i.e.,
coupling with (L) N-Boc-leucine monohydrate and purification by
flash column chromatography on silica gel). Elution with AcOEt/
hexanes gradient elution, from 90/10 to 100% gave 25 as a white
solid (42 mg, 57%): [R]D -6.5 (c 0.5, CDCl3); 1H NMR (CDCl3,
500 MHz, 0 °C) δ (ppm) 7.5 (d, J ) 7.0 Hz, 1H), 7.65 (d, J ) 7.0
Hz, 1H), 7.43 (d, J ) 8.1 Hz, 1H), 7.41-7.36 (m, 6H), 7.09 (d, J
) 7.7 Hz, 1H), 6.84 (d, J ) 7.7 Hz, 1H), 6.23 (d, J ) 7.4 Hz, 1H),
5.25-5.16 (m, 2H), 5.12 (d, J ) 12.0 Hz, 1H), 4.68-4.57 (m,
2H), 4.45-4.37 (m, 2H), 4.36-4.30 (m, 1H), 4.26-4.17 (m, 1H),
3.69-3.58 (m, 1H), 3.57-3.49 (m, 1H), 1.96-1.87 (m, 2H), 1.66-
1.58 (m, 6H), 1.57-1.52 (m, 3H), 1.47 (s, 9H), 1.41 (d, J ) 6.7
Hz, 3H), 1.35 (d, J ) 6.7 Hz, 3H), 1.31 (d, J ) 6.7 Hz, 3H), 1.28-
1.23 (m, 6H), 0.99-0.94 (m, 7H), 0.93-0.89 (m, 7H), 0.89-0.83
(m, 4H); 13C NMR (CDCl3, 100 MHz) δ (ppm) 173.2, 172.7, 172.6,
172.5, 172.0, 171.7, 156.0, 155.6., 135.9, 128.1, 127.7 (2), 79.4,
66.6, 53.2 (2), 52.6, 52.4, 51.2, 51.0, 50.0, 48.2 (2), 40.8 (2), 40.6,
32.0, 31.6, 29.3 (2), 28.0, 24.4, 24.2, 22.8, 22.7, 22.5, 21.6, 21.4,
21.3, 17.9, 17.6, 17.4; FT-IR (neat) ν (cm-1) 3285, 2927, 1642,
1538; MS (ESI) m/e 923.7 [M + Na]+, 901.5 [M + H]+, 801.5 [M
- Boc + H]+; HRMS (ESI) for C46H77N8O10 (MH)+ calcd
901.57572; found 901.57245.
(1R,2R)-N-(L-Ala-D-Ala-L-Ala-NH2),N′-(L-Leu-D-Leu-L-Leu-
NH2)diaminocyclohexane (26). Compound 25 (28 mg, 0.031
mmol) was dissolved in methanol (2 mL) and hydrogenated (40
psi) in the presence of 10% Pd/C (3 mg) for 16 h at room
temperature. After removal of the catalyst by filtration through
Celite, the solvent was evaporated under vacuum, affording the
corresponding free amine as a white solid. This was then dissolved
in 3 mL of a 4 N HCl solution in dioxane and 200 µL of water and
stirred at room temperature for 2 h. Evaporation of the solvent under
reduced pressure afforded the title compound as a bis-hydrochloride
salt in quantitative yield: [R]D -14 (c 0.3, MeOH); 1H NMR
(pyridine-d5, 500 MHz) δ (ppm) 10.16 (d, J ) 7.51 Hz, 1H), 9.95
(d, J ) 7.3 Hz, 1H), 8.96-8.87 (m, 2H), 8.78 (d, J ) 7.2 Hz, 1H),
8.47 (d, J ) 7.7 Hz, 1H), 5.96 (br s, 2H), 5.25-5.18 (m, 1H),
5.17-5.10 (m, 2H), 5.10-5.04 (m, 2H), 4.99 (q, J1 ) 7.2 Hz, J2
) 7.5 Hz, 1H), 4.16-4.00 (m, 2H), 2.28-2.22 (m, 2H), 2.22-
2.12 (m, 5H), 2.09-1.98 (m, 5H), 1.96 (d, J ) 6.9 Hz, 3H), 1.77
(d, J ) 7.1 Hz, 3H), 1.74 (d, J ) 7.0 Hz, 3H), 1.49-1.37 (m, 4H),
1.06 (d, J ) 6.2 Hz, 3H), 1.03 (d, J ) 6.2 Hz, 3H), 1.00 (d, J )
6.2 Hz, 3H), 0.99 (d, J ) 6.2 Hz, 3H), 0.95 (d, J ) 6.2 Hz, 3H),
0.93 (d, J ) 6.2 Hz, 3H); 13C NMR (pyridine-d5, 100 MHz) δ
(ppm) 173.3, 172.7, 172.5 (2), 170.7, 170.4, 53.0, 52.5(2), 52.4,
(1R,2R)-N-(L-Ala-NHCbz),N′-(L-Leu-D-Leu-NHBoc)diami-
nocyclohexane (22). The title compound was obtained starting from
21 (79 mg, 0.15 mmol) following the procedures for the preparation
of 21 from 20 (i.e., coupling with (D) N-Boc-leucine and purification
by flash column chromatography on silica gel). Elution with CHCl3/
MeOH 98/2 gave 22 as a white solid (82 mg, 85%, over two
1
steps): [R]D -8.3 (c 0.6, CDCl3); H NMR (CDCl3, 400 MHz) δ
(ppm) 7.45-7.28 (m, 5H), 7.02-6.85 (m, 2H), 6.75 (d, J ) 6.35
Hz, 1H), 6.14 (d, J ) 6.8 Hz, 1H), 5.60-5.49 (m, 1H), 5.20 (d, J
) 12.2 Hz, 1H), 5.03 (d, J ) 12.2 Hz, 1H), 4.44-4.32 (m, 1H),
4.20 (q, J1 ) 6.13 Hz, J2 ) 13.2 Hz, 1H), 4.15-4.06 (m, 1H),
3.70-3.54 (m, 2H), 2.06-1.89 (m, 2H), 1.77-1.66 (m, 3H), 1.64-
1.51 (m, 3H), 1.42 (s, 9H), 1.38-1.22 (m, 7H), 0.99-0.78 (m,
12H); 13C NMR (CDCl3, 100 MHz) δ (ppm) 172.8, 172.2, 162.3,
155.9, 155.5, 135.8, 128.2, 127.8 (2), 79.7, 66.7, 53.7, 53.3, 53.0,
51.2, 50.2, 40.2 (2), 31.5, 27.9, 24.3, 24.2 (2), 22.7, 22.6, 21.3 (2),
17.8; FT-IR (neat) ν (cm-1) 3389, 2934, 1694, 1640, 1531, 1455,
1256; MS (ESI) m/e 667.2 [M + Na]+, 646.2 [M + H]+, 546.3 [M
- Boc + H]+.
(1R,2R)-N-(L-Ala-D-Ala-NHCbz),N′-(L-Leu-D-Leu-NHBoc)di-
aminocyclohexane (23). Compound 22 (79 mg, 0.122 mmol) was
dissolved in ethanol (1.2 mL) and hydrogenated (40 psi) in the
presence of 10% Pd/C (8 mg) for 16 h at room temperature. After
removal of the catalyst by filtration through Celite, the solvent was
evaporated under vacuum, affording the corresponding free monoam-
ine. The resulting crude material was used without any further
purification for the coupling to (D) N-Cbz-alanine, as described for
the preparation of 20. Purification by flash column chromatography
on silica gel (AcOEt/hexanes gradient elution, from 50/50 to 75/
25) afforded the title compound as a white solid (57 mg, 68%,
1
over two steps): [R]D -30 (c 0.6, CDCl3); H NMR (CDCl3, 500
MHz, 0 °C) δ (ppm) 7.43-7.32 (m, 6H), 7.26 (d, J ) 6.8 Hz, 1H),
6.85 (d, J ) 6.4 Hz, 1H), 6.74 (d, J ) 6.35 Hz, 1H), 6.00 (d, J )
6.35 Hz, 1H), 5.38 (d, J ) 6.4 Hz, 1H), 5.20 (d, J ) 12.2 Hz, 1H),
5.04 (d, J ) 12.2 Hz, 1H), 4.55-4.43 (m, 2H), 4.4.0-4.32 (m,
1H), 4.31-4.24 (m, 1H), 3.65-3.54 (m, 2H), 2.07-1.91 (m, 2H),
1.85-1.58 (m, 6H), 1.56-1.50 (m, 2H), 1.45 (s, 9H), 1.38 (d, J )
7 Hz, 3H), 1.33 (d, J ) 7 Hz, 3H), 1.30-1.22 (m, 4H), 0.99-0.84
(m, 12H); 13C NMR (CDCl3, 100 MHz) δ (ppm) 174.0, 173.6, 172.9
(2), 156.7, 156.4, 136.6, 128.9, 128.5, 128.4, 80.3, 67.3, 54.4, 54.2,
53.6, 51.9, 51.0, 48.9, 41.9, 41.0, 32.3, 32.1, 28.8, 25.1 (2), 25.0
(2), 23.5, 22.1, 22.6, 19.2, 17.9; FT-IR (neat) ν (cm-1) 3294, 2933,
1644, 1532; MS (ESI) m/e 738.2 [M + Na]+, 717.2 [M + H]+,
1190 J. Org. Chem., Vol. 73, No. 4, 2008

Self-Assembly of Noncyclic Bis-D- and L-tripeptides
50.0, 49.7, 49.5, 40.7, 40.6, 40.5, 31.7 (2), 31.6, 24.6 (2), 24.4,
22.9, 22.8, 22.2, 22.1, 21.0, 21.3, 18.0, 17.4, 17.2; FT-IR (neat) ν
(cm-1) 3235, 2955, 2929, 1655, 1548; MS (ESI) m/e 689.4 [M +
Na]+, 667.4 [M + H]+; HRMS (ESI) for C33H62N8O6 (MH)+ calcd
667.48651; found 667.48705.
0.93-0.91 (m, 7H), 0.90-0.88 (m, 3H); 13C NMR (CDCl3, 125
MHz, 45 °C) δ (ppm) 173.4, 173.0, 172.9, 172.7, 172.3, 172.0,
155.7, 155.6, 79.9 (2), 53.6, 53.3, 52.9, 51.7, 51.5, 50.7, 48.7 48.4,
41.3, 41.0, 40.8, 32.3, 32.1, 31.9, 29.6 (2), 29.3, 28.4, 28.3, 24.8
(2), 24.7 (2), 23.0 (2), 22.8, 22.6, 22.0, 21.9, 21.7,18.1 (2), 17.3;
FT-IR (neat) ν (cm-1) 3288, 2930, 1995, 1644, 1548, 1452, 1366,
1250, 1169, 1050; MS (ESI) m/e 889.3 [M + Na]+, 867.4 [M +
H]+; HRMS (ESI) for C43H78N8O10 (MH)+ calcd 867.59137; found
867.59252.
(1R,2R)-N-(L-Ala-D-Ala-L-Ala-NHBoc),N′-(L-Leu-D-Leu-L-
Leu-NHCbz)diaminocyclohexane (27). Compound 25 (10 mg,
0.011 mmol) was dissolved in ethanol (0.5 mL). Boc2O (22 mg),
10% Pd/C (10 mg), DMAP (a spatula tip), and 1,4-cyclohexadiene
(20 µL) were added in sequence, and the resulting solution was
stirred under argon at room temperature overnight. After removal
of the catalyst by filtration through a 45 µm syringe filter, the
solvent was evaporated under vacuum, affording the corresponding
crude product. This was purified by flash column chromatography
on silica gel (AcOEt/hexanes gradient elution, from 65/35 to 100%),
affording the title compound as a white solid (8.6 mg, 86%): [R]D
-15 (c 0.55, CHCl3); 1H NMR (CDCl3, 500 MHz, 45 °C) δ (ppm)
7.49 (d, J ) 7.6 Hz, 1H), 7.40-7.32 (m, 1H), 7.20-7.07 (br m,
2H), 6.78 (d, J ) 6.1 Hz, 1H), 6.51 (br s, 1H), 5.46 (br s, 1H),
5.26 (br s, 1H), 4.64 (dd, J1 ) 6.4 Hz, J2 ) 13.7 Hz, 1H), 4.58
(dd, J1 ) 10.7 Hz, J2 ) 7.3 Hz, 1H), 4.38 (dd, J1 ) 5.8 Hz, J2 )
12.2 Hz, 1H), 4.33-4.29 (m, 1H), 4.26 (br s, 1H), 4.23-4.15 (m,
1H), 3.66-3.53 (m, 2H), 2.01 (d, J ) 11.9 Hz, 1H), 1.96 (d, J )
10.7 Hz, 1H), 1.76-1.60 (m, 9H), 1.48-1.46 (m, 18H), 1.38 (d, J
) 7.0 Hz, 3H), 1.38 (d, J ) 7.0 Hz, 3H), 1.36 (d, J ) 7.0 Hz, 3H),
1.31 (d, J ) 7.0 Hz, 3H), 1.29-1.24 (m, 6H), 0.97-0.94 (m, 8H),
Acknowledgment. We thank NSERC and FQRNT (Que´bec)
of Canada for financial support. TOC/Abstract graphic and cover
art copyright 2004 Urban, accessed at http://en.wikipedia.org/
wiki/Image:Spartacusl.jpg. Permission is granted to copy,
distribute, and/or modify this image under the terms of the GNU
Free Documentation License, Version 1.2 or any later version
published by the Free Software Foundation.
Supporting Information Available: Complete crystallographic
data for compound 13, 1H DMSO-d6 titration experiments, copies
1
of H and 13C spectra for all the new compounds, and 2D spectra
for 27. This material is available free of charge via the Internet at
http://pubs.acs.org.
JO701621C
J. Org. Chem, Vol. 73, No. 4, 2008 1191