A Chiral Pyrrolic Tripodal Receptor Enantioselectively Recognizes beta-Mannose and beta-Mannosides

The prototype of a new family of chiral receptors based on a tripodal scaffold and featuring pyrrolic binding arms containing the trans-1,2- diaminocyclohexane motif, was reported. A cage endowed with a somewhat enlarged cavity was designed, in the belief that fine-tuning of the cage size would lead to a significantly improved affinity even in more polar media. Reaction of the trialdehyde 4 with the mono-BOC-protected diamine 2, followed by reduction of the resulting Schiff base and subsequent deprotection of the amino groups, yielded the tripodal hexaamine 5, which was condensed with pyrrole-2,5- dicarbaldehyde under the conditions used for preparing the bicyclic receptor 1. A preliminary screening indicated that, while Glc, Gal, and GlcNAc were moderately bound, strong recognition occurred with mannosides. βMannose was also extracted in benzene, though to a lesser extent (10%), and αMannose could not be detected.