Calix[4]phyrins. Effect of Peripheral Substituents on Conformational Mobility and Structure within a Series of Related Systems

A new, stepwise synthesis of calix[4]phyrins is described. It relies on the condensation of a
ketone with pyrrole to form a dipyrromethane containing a quaternary carbon center that is subsequently
condensed with an aromatic aldehyde. This methodology, in contrast to the previous rational approach
described by this group (involving formation of a trisubstituted dipyrromethane via the condensation of an
aldehyde with pyrrole, followed by condensation of this dipyrromethane with acetone), allows for a variety
of bulky, ketone-derived substituents to be incorporated into the meso-like positions. The resulting systems,
while all containing the same central macrocyclic core, display conformational properties that reflect the
nature of these meso-like substituents; these conformational features were independently assessed by
X-ray diffraction analysis, NMR spectroscopy, and quantum chemical calculations.