Enantioselective formation of a dynamic hydrogen-bonded assembly based on the chiral memory concept

In this paper, we report the enantioselective formation of a dynamic noncovalent double rosette assembly 1a₃·(CYA)₆ composed of three 2-pyridylcalix[4]arene dimelamines (1a) and six butylcyanuric acid molecules (BuCYA). The six 2-pyridyl functionalities of the assembly interact stereoselectively with chiral dicarboxylic acids 3a-e via two-point hydrogen-bonding interactions. One of the two enantiomeric assemblies (P- or M-) 1a3·(CYA)₆ is formed in excess as the result of the complexation of the chiral diacids, resulting in formation of optically active assemblies. The complexations with dibenzoly tartaric acids D-3a and L-3a (3 equivalent), respectively, leading to the formation of diastereomeric assemblies (P)-1a₃· (BuCYA)₆(D-3a)₃ and (M)-1a₃·(BuCYA)₆·(L-3a)₃ with 90% diastereomeric excess. The diastereomeric excess in (M)-1a₃·(BuCYA)₆·(L-3a)₃ is "memorized" when L-3a is removed by precipitation with ethlylenediamine (EDA). The assembly (M)-1a₃·(BuCYA)₆ is still optically active (90% enantiomeric excess), although none of its individual components are chiral. (M)-1a₃·(BuCYA)₆ has a high kinetic stability toward racemization (E_a = 119 kJ mol^-1, half-life of (M)-1a₃·(BuCYA)₆ is ca. 1 week at 20 °C).