Sweets for Catalysis - Facile Optimisation of Carbohydrate-Based Bis(oxazoline) Ligands

Abstract

A new type of carbohydrate-based bis(oxazoline) ligands was prepared from inexpensive D-glucosamine and tested in asymmetric cyclopropanation reactions. For optimisation, modified ligands with 3-O substituents of varying size and electronic properties were prepared as well as a 3-OH unprotected and a perpivaloylated derivative. All new ligands were tested in asymmetric cyclopropanation, revealing a strong dependence of enantioselectivity on steric demand and electronic properties of the 3-O residue. Also, a significant influence of the pyranose confomation, which is determined by the presence or absence of the cyclic acetal group, was observed. Thus, it was easily possible to tune the new carbohydrate bis(oxazoline) ligands to a given reaction. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)