Design, synthesis, and structural analysis of D, L -mixed polypyrrolinones. 2. Macrocyclic hexapyrrolinones

Article abstract of DOI:10.1021/ol101008y

The design, synthesis, and structural analysis of two macrocyclic d,l-alternating hexapyrrolinones have been achieved. These cyclic peptide mimics adopt a flat, hexagonal conformation, stabilized by intramolecular hydrogen bonding between adjacent pyrrolinone rings. Extensive NMR studies and X-ray analysis reveal, respectively, that the macrocyclic hexapyrrolinones aggregate in solution and in the solid state form staggered stacked nanotube-like assemblies.

Full text of DOI:10.1021/ol101008y

ORGANIC
LETTERS
2010
Vol. 12, No. 13
2994-2997
Design, Synthesis, and Structural
Analysis of D,L-Mixed Polypyrrolinones.
2. Macrocyclic Hexapyrrolinones
Amos B. Smith, III,* Hui Xiong, Adam K. Charnley, Meinrad Brenner, Eugen
F. Mesaros, Craig S. Kenesky, Luigi Di Costanzo, David W. Christianson,* and
Ralph Hirschmann*,‡
Department of Chemistry, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104
smithab@sas.upenn.edu
Received May 2, 2010
ABSTRACT
The design, synthesis, and structural analysis of two macrocyclic D,L-alternating hexapyrrolinones have been achieved. These cyclic peptide
mimics adopt a flat, hexagonal conformation, stabilized by intramolecular hydrogen bonding between adjacent pyrrolinone rings. Extensive
NMR studies and X-ray analysis reveal, respectively, that the macrocyclic hexapyrrolinones aggregate in solution and in the solid state form
staggered stacked nanotube-like assemblies.
The evolution of the pyrrolinone scaffold toward a universal
peptidomimetic possessing both conformational control and
diversity was a significant theme of the Hirschmann-Smith
collaboration for over 15 years. Through these efforts, we
learned that the combined effects of R-stereogenicity of the
pyrrolinone ring, intramolecular hydrogen bonding, and
choice of side-chains determined the global minimum energy
conformation of the polypyrrolinone chain. Homochiral
polypyrrolinones (eg., all D, Figure 1)1 that preferentially
adopt an extended conformation proved to be excellent
ꢀ-strand/ꢀ-sheet mimics2 and, as such, led to potent, orally
bioavailable pyrrolinone-based enzyme inhibitors of aspartic
acid proteases,3 as well as modest metalloprotease inhibitors4
and peptide-pyrrolinone hybrid ligands for the class II MHC
protein HLA-DR1.5 Alternatively, heterochiral polypyrroli-
nones (e.g., alternating D,L pyrrolinones; Figure 1), much
like heterochiral polypeptides, adopt a turn structure6 and
as such have been employed to generate functional ꢀ-turn
mimetics.7 Subsequent investigations of the extended het-
(2) (a) Smith, A. B., III; Keenan, T. P.; Holcomb, R. C.; Sprengler,
P. A.; Guzman, M. C.; Wood, J. L.; Carrol, 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; Guzman, M. C.;
Sprengler, P. A.; Keenan, T. P.; Holcomb, R. C.; Wood, J. L.; Carrol, P. J.;
Hirschmann, R. J. Am. Chem. Soc. 1994, 116, 9947.
(3) (a) Smith, A. B., III; Akaishi, R.; Jones, D. R.; Keenan, T. P.;
Guzman, M. C.; Holcomb, R. C.; Sprengeler, P. A.; Wood, J.; Hirschmann,
R.; Holloway, M. K. Biopolymers (Pept. Sci.) 1995, 37, 29. (b) Smith, A. B.,
III; Cantin, L.-D.; Pasternak, A.; Guise-Zawacki, L.; Yao, W. Q.; Charnley,
A. K.; Barbosa, J.; Sprengeler, P. A.; Hirschmann, R.; Munshi, S.; Olsen,
D. B.; Schleif, W. A.; Kuo, L. C. J. Med. Chem. 2003, 46, 1831, and
references cited therein. (c) Smith, A. B., III; Charnley, A. K.; Harada, H.;
Beiger, J. J.; Cantin, L.-D.; Kenesky, C. S.; Hirschmann, R.; Munshi, S.;
Olsen, D. B.; Stahlhut, M. W.; Schleif, W. A.; Kuo, L. C. Bioorg. Med.
Chem. Lett. 2006, 16, 859.
‡ May 6, 1922-June 20, 2009.
(1) While D, L descriptors are not directly applicable to the pyrrolinone
units, their use simplifies comparison with peptidal structures.
(4) Smith, A. B., III; Nittoli, T.; Sprengeler, P. A.; Duan, J. W.; Liu,
R.-Q.; Hirschmann, R. Org. Lett. 2000, 2, 3809.
10.1021/ol101008y  2010 American Chemical Society
Published on Web 06/02/2010

erochiral pyrrolinone motif led to the discovery that hexapy-
rrolinone (-)-1 adopts a flat G-shaped conformation that
aggregates in solution and in the solid state self-assembles
into a nanotube-like stucture.8
Figure 2. (a) Prospective macrocyclic hexapyrrolinones 2 and 3.
Figure 1. (a) Homochiral (DDD) and heterochiral pyrrolinones
(LDL). (b) Structure of D,L-hexapyrrolinone (-)-1.
(b) Stereoview of the lowest energy conformation of 2 derived via
Monte Carlo conformational analysis.
The nanotube-like architecture of (-)-1 in the solid-state,
possessing termini in close proximity, readily suggested the
design of macrocyclic hexapyrrolinones 2 and 3 (Figure 2a).
Unencumbered with terminal substituents, we reasoned that
such cyclic polypyrrolinones might self-assemble into nano-
tubes.9 Pleasingly, Monte Carlo conformational searches10
for 2 predicted that the lowest energy conformations would
possess a flat, hexagonal conformation (Figure 2b), in
agreement with previous structural analysis of the acyclic
heterochiral pyrrolinones such as (-)-1.
Importantly, the predicted conformation presents hydrogen
bonding acceptors and donors (cf. CdO and N-H, respec-
tively) in an alternating pattern directed above and below
the plane of the molecule, thus providing the potential for
intermolecular hydrogen bonding in a nanotube-like array.
To access 2, we initially employed our iterative polypyr-
rolinone synthetic tactic in a linear fashion,2,6 beginning with
the C terminus to generate the open-chain pentamer (-)-10.
Although this approach to (+)-2 eventually proved successful
(Supporting Information), we subsequently designed a more
effective, convergent synthesis, beginning with (+)-411 and
(-)-5 (Scheme 1).12 Condensation to afford an intermediate
imine, followed by treatment with KHMDS, generated
monopyrrolinone (+)-6, a common precursor for both (+)-7
and (-)-8. Hydrogenolysis furnished amine (+)-7, while
treatment with LiBF4 led to aldehyde (-)-8. Union of these
two pyrrolinone building blocks was achieved in 82% yield
by imine formation, followed by treatment with KHMDS.
Acetal hydrolysis furnished trispyrrolinone (-)-9; a two-step
sequence with pyrrolinone amine (+)-7 then delivered the
pentapyrrolinone (-)-10. The critical final pyrrolinone ring
construction, leading to macrocycle (+)-2, was achieved in
a similar fashion, albeit in this case the yield was at best
modest (ca. 12-13%). Notwithstanding the efficiency of the
final cyclization, a sample (ca. 100 mg) of (+)-2 was
prepared for structural analysis.
Assignment of structure (+)-2 was based principally on
1
simplification of both the H and 13C NMR spectra, in
conjunction with HRMS identification of the parent ion.
Pentapyrrolinone (-)-10 (an unsymmetrical molecule, Scheme
1) displays a distinct set of signals for the five chemically
(and magnetically) different pyrrolinone units (e.g., vinyl and
benzyl hydrogens, etc.). Conversion to the cyclic C3-
symmetrical hexamer (+)-2 (Figure 3, Scheme 1) renders
each benzyl and isobutyl pyrrolinone chemically and mag-
netically identical, resulting in isochronous NMR signals for
the three monomeric units. Indeed, only two sets of signals
(5) Lee, K. H.; Olson, G. L.; Bolin, D. R.; Benowitz, A. B.; Sprengeler,
P. A.; Smith, A. B., III; Hirschmann, R. F.; Wiley, D. C. J. Am. Chem.
Soc. 2000, 122, 8370, and references cited therin.
1
are observed in both H and 13C NMR spectra of (+)-2,
(6) Smith, A. B., III; Wang, W.; Sprengler, P. A.; Hirschmann, R. J. Am.
Chem. Soc. 2000, 122, 11037.
corresponding to the two types of pyrrolinone rings.
(7) Smith, A. B., III; Charnley, A. K.; Mesaros, E. F.; Kikuchi, O.; Wang,
W.; Benowitz, A.; Chu, C.-L.; Feng, J.-J.; Chen, K.-H.; Lin, A.; Cheng,
F.-C.; Taylor, L.; Hirschmann, R. Org. Lett. 2005, 7, 399.
(8) See preceeding article in this journal (DOI: 10.1021/ol101007n).
(9) (a) Scanlon, S.; Aggeli, A. Nano Today 2008, 3, 22. (b) Fischer, L.;
Decossas, M.; Briand, J.-P.; Didierjean, C.; Guichard, G. Angew. Chem.,
Int. Ed. 2009, 48, 1625. (c) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri,
M. R. Angew. Chem., Int. Ed. 2001, 40, 988.
The propensity of macrocycle (+)-2 to self-assemble in
1
solution was demonstrated via a series of H NMR studies
in CDCl3 similarly employed for in the study of (-)-1.8,13
The cyclic structure of (+)-2 permits each N-H of the
individual macrocycles to be involved in intramolecular
hydrogen bonding, thereby lessening their solvent exposure,
(10) Chang, G.; Guida, W. C.; Still, W. C. J. Am. Chem. Soc. 1989,
111, 4379.
(11) We employed the bis(2-TMS-ethyl) acetal (+)-4 in the synthesis
of (+)-2, as opposed to a dimethyl acetal, due to incompatibility of acid
cleavage of the latter with the Cbz-protected amines. See: (a) Paquette,
L. A.; Backhaus, D.; Braun, R. J. Am. Chem. Soc. 1996, 118, 11990. (b)
Walkup, R. D.; Obeyesekere, N. U. Synthesis 1987, 607. For hydrolysis of
acetals with LiBF4, see: Lipshutz, B. H.; Harvey, D. F. Synth. Commun.
1982, 12, 267.
(12) The amino ester building blocks are readily available via the
methods of: (a) Karady, S.; Amato, J. S.; Weinstock, L. M. Tetrahedron
Lett. 1984, 25, 4337. (b) Seebach, D.; Fadel, A. HelV. Chim. Acta 1985,
68, 1243.
(13) Haque, T. S.; Little, J. C.; Gellman, S. H. J. Am. Chem. Soc. 1996,
118, 6975
.
Org. Lett., Vol. 12, No. 13, 2010
2995

Scheme 1. Convergent Synthesis of Macrocyclic Hexapyrrolinone (+)-2
and thus a relatively small effect of concentration was
observed on the chemical shifts.6 Nonetheless, a concentra-
tion dependence of the N-H protons was observed (Figure
3) despite the intramolecular interactions. The measurable
downfield shift for the N-H signals with increasing con-
centration is consistent with aggregation mediated by inter-
molecular hydrogen bonding.
dependent NMR analysis of (+)-3 revealed nearly identical
solution-state aggregation as observed for (+)-2 (see Sup-
porting Information). Gratifyingly, compared to (+)-2,
superior crystals of (+)-3 could be obtained by slow
evaporation in 1:1 CHCl3:trifluoroethanol, and the solid-state
structure analysis using synchrotron X-ray diffraction was
achieved.14 Interestingly, unlike the open chain hexapyrroli-
none (-)-1, (+)-3 assembles into a staggered nanotube-like
array (Figure 4). Comparison of this structure, with that of
(-)-1,8 provides both interesting similarities and differences.
The monomers of (+)-3 assemble in an antiparallel fashion,
as observed for (-)-1. Alternatively, the staggered array
adopted by (+)-3 possesses only four intermolecular
pyrrolinone-pyrrolinone hydrogen bonds between each pair
of monomers, compared to six for (-)-1. Additionally, the
staggered nanotube structure of (+)-3 forms an infinite array
in the crystal lattice (four molecules/unit cell as illustrated
in Figure 4b-d). Given the pyrrolinone scaffold comprises
a designed peptidomimetic, our work has obvious parallels
to the cyclic peptides pioneered by Karle,15 Lorenzi,16 and
Ghadiri.17,9 Of particular interest, the 1987 report by G. P.
Lorenzi et al. discloses a series of D,L-alternating cyclic
hexapeptides.16 In contrast to the hexapeptides, which were
reported to be completely insoluble in all common organic
solvents, (+)-2 and (+)-3 are moderately soluble in most
polar organic solvents (i.e., EtOAc, EtOH) and display
excellent solubility in CHCl3. Importantly, the structure of
cyclic hexapyrrolinone (+)-3 overlays remarkably well with
the Lorenzi cyclic hexapeptides (Figure 5), illustrating the
close correspondence between the pyrrolinone and peptide
1
Figure 3
.
Concentration dependence of H NMR chemical shifts
in CDCl3 of N-H(1) (0) and NH(2) ([) protons of (+)-2.
Single crystal X-ray analysis of (+)-2 would provide the
strongest evidence possible for nanotube assembly; unfor-
tunately, crystals of (+)-2 suitable for X-ray analysis have
not, as yet, been forthcoming. The failure to obtain suitable
crystals of (+)-2 prompted the synthesis of hexapyrrolinone
3, wherein the isobutyl groups were substituted for isopropyl
groups, with the expectation that the reduced flexibility of
the isopropyl side-chains might facilitate crystal growth.
The synthesis of (+)-3 was completed in a convergent
fashion, similar to that employed for (+)-2 (see Supporting
Information). Not surprisingly, the macrocyclization step
proceeded in even lower yield given the steric encumberance
of the isopropyl side-chains (ca. 2% yield). Concentration-
(14) For crystallographic data for hexapyrrolinone (+)-3, see Supporting
Information.
(15) Karle, I. L.; Handa, B. K.; Hassall, C. H. Acta Crystallogr. 1975,
B31, 555.
(16) (a) Tomasic, L.; Lorenzi, G. P. HelV. Chim. Acta 1987, 70, 1012.
(b) Sun, X.; Lorenzi, G. P. HelV. Chim. Acta 1994, 77, 1520, and references
cited therein.
(17) (a) Ghadiri, M. R.; Granja, J. R.; Milligan, R. A.; McRee, D. E.;
Khazanovich, N. Nature 1993, 366, 324. (b) Ghadiri, M. R.; Kobayashi,
K.; Granja, J. R.; Chadha, R. K.; McRee, D. E. Angew. Chem., Int. Ed.
Engl. 1995, 34, 93. (c) Clark, T. D.; Buriak, J. M.; Kobayashi, K.; Isler,
M. P.; McRee, D. E.; Ghadiri, M. R. J. Am. Chem. Soc. 1998, 120, 8949,
and references cited therein
.
2996
Org. Lett., Vol. 12, No. 13, 2010

Figure 5. Overlay of the X-ray structure of cyclic hexapyrrolinone
(+)-3 (blue) with the X-ray structure of cyclo(-D-Leu-L-MeLeu-
D-Leu-L-MeLeu-D-Leu-L-MeLeu-) (red).16
units and suggesting the possibility that the two macrocycles
would form heterogeneous aggregates.
In summary, the design, syntheses, and structural analysis
of macrocyclic D,L-hexapyrrolinones (+)-2 and (+)-3 have
been achieved. Studies by NMR suggest that the macrocyclic
hexapyrrolinones aggregate in solution, while the solid-state
structure of (+)-3, determined via X-ray crystallography,
revealed an extended, staggered nanotube-like array, stabi-
lized by four intermolecular hydrogen bonds between pairs
of pyrrolinone rings. Importantly, the solid state structure
of (+)-3 demonstrates the ability of macrocyclic pyrrolinones
to assemble into nanotube-like structures. Studies to both
improve the final macrocyclization as well as to exploit the
structural properties of such cyclic heterochiral polypyrroli-
nones continue in our laboratory.
Acknowledgment. Support was provided by the NIH
through Grants AI-42010 and GM-49758. Additional support
was provided by Postdoctoral Fellowships from the Department
of Defense (PC-030136) to E.F.M., from the Swiss National
Science Foundation to M.B., and Graduate Fellowships from
Bristol-Myers Squibb and the University of Pennsylvania to
A.K.C. We thank the Advanced Light Source (Berkeley, CA)
for access to synchrotron beamline 5.0.2. We also thank Samuel
H. Gellman, the Ralph Hirschmann Professor of Chemistry at
the University of Wisconsin, for insightful discussion.
Supporting Information Available: Experimental pro-
cedures and spectroscopic data for all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 4. X-ray structure of (+)-3: (a) a single molecule; (b)
representative stereoview of the infinite staggered nanotube array,
viewed from the side. The staggered nanotube structure is more
clearly visible with the side-chains removed, from the top (c) and
from the side (d).
OL101008Y
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