Synthesis of Functionalized, Sterically Congested Calix[4]phyrin Macrocycles Using Donor-Acceptor-Substituted Cyclopropanes - First Example of a Mono-meso-spirolactone Incorporated into a Calix[4]phyrin

Calix[4]phyrins are an important class of hybrid macrocyclic systems at the interface between porphyrins and calixpyrroles (porphyrinogens). A new stepwise synthesis of oxidation-resistant meso-hydrogenated calix[4]phyrins is reported, which allows variable substitution in the residues of their sp(2)-meso-centers without the need for a porphyrin intermediate. It relies on the acid-catalyzed condensation of a sterically hindered donor-acceptor-substituted cyclopropane precursor with pyrrole to form a sterically congested dipyrromethane. This was subsequently condensed with a wide range of alkyl and aryl aldehydes bearing electron-donating or electron-withdrawing substituents followed by an oxidation step to form stable calix[4]phyrin(1.1.1.1)s with bridging meso-CH hydrogen atoms through an acid-promoted dehydrative condensation. The methodology avoids any need for premetallation of the macrocycle and/or the use of organometallic catalysts or reagents and allows the incorporation of the bulky cyclopropane-derived substituents specifically into the 5,15-meso-like positions. The resulting regioisomerically pure products display conformational features that reflect their mixed nature between porphyrins and calixpyrroles and they were assessed by X-ray diffraction analysis, NMR techniques and UV/Vis spectroscopy. The possibility to introduce key functional groups enables subsequent modification of these calix[4]phyrins and allows their connection to other groups such as biologically active moieties. Of special interest is an unprecedented example of an acid-driven lactonization that results in the incorporation of a mono-meso-spirolactone into a calix[4]phyrin(1.1.1.1). Moreover, it is demonstrated that this approach to calix[4]phyrins is also applicable to other sterically congested dipyrromethanes.