A Chiral Axis due to an Acyclic Imide–Ar Bond: a Study of Steric Effects of Acyl Groups on Racemization

Studies on the racemization in a series of optically active compounds 3a–e, including the steric effect of their acyl groups, are described. The first example of optically active compounds 3c and 3d, which possess axial chirality based on an acyclic imide–Ar bond, has been reported. A quite interesting result has been revealed, namely, that 3a bearing a bulky acyl group rather than a relatively small one racemized more easily. To explain this observed phenomenon, 13C NMR experiments and the reaction with benzylamine of 3a–e were undertaken. These results suggested that the t-BuCO–N bond in 3a which racemized easily, is more twisted, compared with the RCO–N bonds in 3b–e which are relatively stable to racemization. Furthermore, the absolute configuration of 3b and 3c has been determined to be R by the CD spectrum and the X-ray crystallographic analysis of racemic 3f has been accomplished.