Convenient and highly efficient chromatographic resolution of BINOL and of 6,6′-dibromo-BINOL via N(α)-Boc-tryptophan esters

Racemic [1,1′]binaphthalenyl-2,2′-diol (BINOL, (±)-1) has been esterified with various commercially available N-protected-l-amino acids, giving the corresponding diastereomeric esters. Their TLC separation factors were highly dependent on the amino acid pattern. Diesters of (±)-1 and N(α)-Boc-tryptophan (3a) showed unusually large separation factors, which allowed their efficient separation by simple column chromatography. Removal of the tryptophan moieties under very mild conditions furnished each enantiomer of 1 in high overall yield and 100% ee. This procedure was also successful for the resolution of racemic 6,6′-dibromo-[1,1′]binaphthalenyl-2,2′-diol (6,6′-dibromo-BINOL, (±)-2).