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 doe s not occur because the two electric transition moments which should i nteract are coplanar. This is seen with cyclohexane-1,4-diols (both ee or ea) and a wide variety of 3-hydroxy carotenoids, 3-hydroxyretinoid s, etc. A general approach to deal with such cases is to acylate one o f the hydroxyl groups with a chiral allenic acid substituted with a su itable chromophore, e.g., CHROM-CH=C=CH-COOH. The allenic bond introdu ces a 90 degrees twist at the italicized central carbon so that the al lenic CHROM now couples with the second chromophore. This concept of i ntroducing an auxiliary allenic center should be of general applicabil ity in other similar cases. (C) 1995 Wiley-Liss, Inc.