A new approach in exciton-coupled circular dichroism (ECCD)--insertion of an auxiliary stereogenic center

There are cases in which exciton coupling between two chromophores does not occur because the two electric transition moments which should interact are coplanar. This is seen with cyclohexane-1,4-diols (both ee or ea) and a wide variety of 3-hydroxy carotenoids, 3-hydroxyretinoids, etc. A general approach to deal with such cases is to acylate one of the hydroxyl groups with a chiral allenic acid substituted with a suitable chromophore, e.g., CHROM-CH = C = CH-COOH. The allenic bond introduces a 90 degrees twist at the italicized central carbon so that the allenic CHROM now couples with the second chromophore. This concept of introducing an auxiliary allenic center should be of general applicability in other similar cases.