Abstract
Dynamic atroposelective resolution of chiral salts derived from oily racemic nicotinamides and enantiopure dibenzoyltartaric acid (DBTA) was achieved by crystallization. The absolute structures of the axial chiral nicotinamides were determined by X-ray structural analysis. The chirality could be controlled by the selection of enantiopure DBTA as a chiral auxiliary. The axial chirality generated by dynamic salt formation was retained for a long period after dissolving the chiral salt in solution even after removal of the chiral acid. The rate of racemization of nicotinamides could be controlled based on the temperature and solvent properties, and that of the salts was prolonged compared to free nicotinamides, as the molecular structure of the pyridinium ion in the salts was different from that of acid-free nicotinamides.
Highlights
Axial chirality in aromatic amides is a stereogenic element that arises from the hindered rotation of an aryl-C(=O) single bond [1]
Deracemization of oily racemic nicotinamides with a basic group was examined by dynamic salt formation with enantiopure acids such as dibenzoyltartaric acid (DBTA), based on the fact that chiral materials tend to crystallize in chiral fashion [34,47,48]
If an enantiopure (R)-acid is added to the racemic base under conditions of efficient racemization, and the crystallinity of the (S)-base/(R)-acid salt is superior to that of the other diastereomeric salt [(R)-base/(R)-acid], effective dynamic optical resolution can be achieved and the (S)-base/(R)-acid form will be obtained in enantiopure form with good chemical yield (Scheme 2)
Summary
Axial chirality in aromatic amides is a stereogenic element that arises from the hindered rotation of an aryl-C(=O) single bond [1]. Molecules 2013, 18 amides is important, and this has received increasing attention over the past decades The importance of these chiral elements has been exemplified in attractive asymmetric reactions leading to optically active materials, such as anilides [2,3,4,5,6], N-arylimides [7,8,9,10], benzamides [11,12,13,14,15] and naphthamides [16,17,18,19,20,21]. Racemic mixtures may be converted to diastereomers through the formation of chiral salts with enantiopure materials followed by optical resolution by preferential crystallization [43,44]. There have been no previous examples of control of axially chiral nicotinamides by salt formation
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