Stereoselective Synthesis of Lower and Upper Rim Functionalized Tetra-α Isomers of Calix[4]pyrroles

Hydroxyaryl alkyl ketones with functionalized alkyl chains often fail to produce the corresponding tetra-α calix[4]pyrroles in Brönsted acid mediated condensations with pyrrole. A remarkable effect exerted by the addition of methyltrialkylammonium chloride during the acid-mediated syntheses of a series of meso-(tetrahydroxyaryl)-meso-tetraalkylcalix[4]pyrroles featuring alkyl terminal chloro or ester groups is reported. The ammonium salt enhances the cyclocondensation reaction and induces the almost exclusive formation of the tetra-α isomers.