molecules are more rare,3 particularly in the realm of
conformationally defined (e.g., shape-persistent) macrocycles
that have been designed to contain from two- to six- but not
five-fold symmetries.4 The peripheral functionalization of
these distinct easily modifiable organic templates that possess
unusual pentagon-shaped topologies may lead to a good
expectation in generating some useful functional materials
such as quasicrystals,2g DNA nanostructures,5a virus capsid-
like nanocontainers,5b dendritic molecules,5c discotic liquid
crystals,5c,d and hosts for organic/inorganic species. Further-
more, these rigid pentagon-shaped organic entities may be
utilized to augment our ability in protein engineering and
drug discovery that target biological pentamers found
abundantly in Nature.6 For example, up to a ten million-
fold gain in inhibitory activity against Shiga-like toxin, when
compared to its univalent counterpart, has been achieved by
a bifurcated pentavalent inhibitor that cross-links two identi-
cal toxin pentamers to form an inhibitor-sandwiched decamer-
ic ensemble.6h
Scheme 1. Synthetic Route That Affords Pentamer 1
We describe here the synthesis and detailed characteriza-
tion of a highly rigid and structurally well-defined circular
aromatic pentamer 1 (Scheme 1). Excluding the five interior
methoxy groups, the five identical aromatic building blocks
meta-linked by secondary amide groups are nearly ideally
disposed around a rotational axis of C5 symmetry with
internal angles of close to 108° and a geometric shape of a
nearly planar pentagon. This five-fold folding pattern,
possessing a good planarity, on the repeating structural motifs
is quite unusual among all the hitherto reported conforma-
tionally defined macrocycles4 or synthetic foldamers7 with
biased conformations enforced by noncovalent forces.
Our bottom-up pentameric molecular design lies in the
preceding literature observations that suggest to us the
likelihood for the amide linkages to display a breathing-type
behavior in bond angles, resulting in the backbone being
curved toward the hydrogen bonded (H-bonded) side.7f,8 We
reasoned that an oligomeric backbone incorporating an
inward-pointing, continuous H-bonding network might re-
quire less than six benzene units per turn. As such, the end-
to-tail cyclization of a rigidified, crescent acyclic pentamer
into a circular form as sketched in pentamer 1 might not
impose too much angle strain on the molecular backbone
and so a planar conformation with a smaller cavity may
result. Following this speculation, a total of four conforma-
tions of circular pentamer 1, depending on the orientation
(3) For a recent review on a C5-symmetric, bowl-shaped corannulene
consisting of five benzene rings built around a five-membered ring, see:
(a) Wu, Y.-T.; Siegel, J. S. Chem. ReV. 2006, 106, 4843; the regioselective
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neethynylene-based covalent pentacycle obtained as a side product with its
point symmetry remaining unknown, see: (b) Zhang, W.; Moore, J. S. J. Am.
Chem. Soc. 2004, 126, 12796. For the metal-assisted templation of a roughly
C5-symmetric, expanded porphyrin pentacycle (in this case, the metal-free
organic pentacycle cannot be formed and isolated), see: (c) Day, V. W.;
Marks, T. J.; Wachter, W. A. J. Am. Chem. Soc. 1975, 97, 4519. For a
pyrrole-based macrocyclic pentaphyrin with its point symmetry remaining
unknown, see: (d) Rexhausen, H.; Gossauer, A. J. Chem. Soc., Chem.
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