Design and synthesis of chiral macrocycles as receptors for enantioselective molecular recognition and as catalysts for asymmetric reactions

The design and the synthesis of chiral macrocycles acting as enantioselective receptors is of great interest in the field of enzyme mimicking enantioselective catalysis as well as in that of racemic resolution and of chiral sensing. Recently particular attention has been devoted to neutral ditopic receptors able to simultaneously bind chiral ammonium cations and their counteranions. In this work we report the synthesis of new chiral macrocycles which can be employed as ligands to obtain neutral chiral ditopic receptors for ion pairs as well as catalysts for asymmetric epoxidation reactions. The two different abilities are modulated both by varying the position of different calix[4]arene scaffolds with respect to the salen-type moiety and varying the transition metal inside the salen unit. The candidates we chose for the first case are calix[4]arene based receptors in 1,3-alternate conformations, linked by different polyether spacers to UO2-salen framework to get ditopic receptor of chiral alkylammonium salts ion pairs. In the second case a salicylaldehyde functionality was incorporated in the calix[4]arene moiety to yield two couples of enantiomeric Mn-salen complexes that have the requisites to act as catalysts in selective oxygen transfer reactions to prochiral olefins.