Crown Ethers in Enantioselective Synthesis

A number of new chiral crown compounds and lariat ethers have been synthesized starting from phenyl-D-glucopyranoside, methyl-D-glucopyranoside and galactopyranoside, methyl-D-mannopyranoside and 1,2:5,6-di-O-isopropylidene-D-mannitol, but the main emphasis was laid on their application. Some of the sugar-based macrocycles showed significant asymmetric induction as phase transfer catalysts in liquid-liquid and solid-liquid phase reactions such as in two Michael addition reactions (84–95 % ee), in a Darzens condensation (74 % ee) and in the epoxidation of chalcones (92 % ee). The proposed mechanism of the selective reactions was supported by molecular mechanics calculations. The asymmetric self-condensation of phenacyl chloride took place in 64 % ee value. A novel deracemization of CH-acids has also been discovered and the reason of the enantioselective protonation is discussed.