Concave Pyridines for Bifunctional Acid–Base Catalysis

Two bifunctional concave acid–base catalysts, 1 and 2 , have been synthesized starting from 2,6‐dibromopyridine ( 9 ) and 2,6‐bis(ω‐alkenyloxy)phenylboronic acids 8 and 10 which end up as bridgeheads in final bimacrocycles 1 and 2 . One bridgehead contained an additional substituent in the 4‐position. The respective protected 4‐hydroxymethyl‐substituted phenylboronic acids 8 were synthesized from 4‐bromo‐3,5‐dihydroxybenzoic acid ( 3 ) in five steps. 4‐Unsubstituted boronic acid 10 and 4‐substituted boronic acid 8 were then attached to 9 by subsequent Suzuki couplings to give tetra‐ω‐alkenes 12 . By ring‐closing metathesis of 12 , bimacrocyclic dienes 13 and 17 were formed. After deprotection of the 4‐hydroxymethyl group of one bridgehead, a 3‐hydroxybenzoate was coupled to 14 to give ester 15 which gave bifunctional acid–base catalyst 1 upon hydrolysis. Analogously, homologue 2 was synthesized, but before coupling the bimacrocycle to the benzoate, tetraene 14 was hydrogenated to 18 . Acidic and basic centers in 1 (49 % from 9 ) and 2 (19 % from 9 ) are at least 5 Å apart.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)