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.