Hydroxyaryl alkyl ketones with functionalized alkyl chains often fail to produce the corresponding tetra-α calixpyrroles 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-tetraalkylcalixpyrroles 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.
Stereoselective Synthesis of Lower and Upper Rim Functionalized Tetra-α Isomers of Calixpyrroles
Org. Lett. 2017, 19 (1), 226-229, DOI: 10.1021/acs.orglett.6b03505.