A Versatile Method, for the Resolution and Absolute Configuration Assignment of Substituted 1,1′-Bi-2-naphthols

摘要

Optically active 1,1′-bi-2-naphthols are extremely useful C_2-symmetrical compounds employed as chiral auxiliaries or chiral catalysts in asymmetric synthesis. A number of methodologies have been developed to prepare optically pure 1,1′-bi-2-naphthols ranging from classical resolution to enzymatic hydrolysis of their derivatives. Recently attempts have also been made to synthesize them via asymmetric oxidative coupling of 2-naphthols in the presence of chiral amines. While these methods produce enantiomerically pure 1,1′-bi-2-naphthols in good to excellent purities, they are usually not applicable to a wider range of substituted 1,1′-bi-2-naphthol derivatives. For example, we were unable to resolve 4,4′-dibromo-1,1′-bi-2-naphthol by Toda's procedure, although this method was employed to resolve the parent 1,1′-bi-2-naphthol in good optical purity. The unsubstituted 1,1′-bi-2-naphthol could also be resolved in 100 g quantities by an enzymatic method, but this procedure was again unsuccessful in our hands because of the poor solubility of the 4,4′-dibromo derivative in the reaction medium. Our interest in the preparation of conjugated polymers with main chain chirality called for a preparative route to optically active 4,4?-dibromo-1,1′-bi-2-naphthol to use as the monomeric unit. In the end, we discovered that racemic 4,4′-dibromo-1,1′-bi-2-naphthol, 1c, could be chromatographi-cally resolved via its diastereomeric bis((S)-camphorsul-fonates) 2c and 3c. Here we disclose that this resolution procedure could be applied to a wider range of substituted 1,1′-bi-2-naphthol derivatives. Moreover, on the basis of the spectral evidence, the absolute configurations of the chiral axis of the diastereomeric pairs 2 and 3 can be inferred from their relative chromatographic mobilities.