CD EXCITON CHIRALITY METHOD - NEW RED-SHIFTED CHROMOPHORES FOR HYDROXYL-GROUPS

When hydroxyl groups are derivatized to apply the CD exciton chirality method, the absorption maxima of introduced chromophores should not overlap with that of the substrate, except for cases in which the coupling between the existing and the introduced chromophores are deliberately sought for. Thus, the availability of red-shifted chromophores that do not overlap with the substrate absorption would greatly expand the applicability of this versatile CD method. Four such red-shifted chromophores, chrom-II, -III, -IV, and -V, have been developed to convert hydroxyl groups into esters that absorb strongly in the range 360-410 nm. Using the chromophoric triazole amide, they can readily derivatize hydroxyl groups of the substrates on a microscale. The bischromophoric esters of 1(R),2(R)-cyclohexanediol (14-18) exhibited intense exciton-split CD curves with the signs correctly representing the absolute sense of twist between the two hydroxyl groups. The 1,2-diol moieties of taxinine (2) and chromomycin A3 (3) derivatives, already having strong absorptions at 260-275 nm, were esterified with the new chromophores; this gave rise to strong couplets isolated from the CD Cotton effects of starting materials, the signs of which were in agreement with the absolute configurations of these two natural products. These O-acylating chromophores should be useful for determinations of absolute configurations and conformations of chiral substrates, including biopolymers; they could also be conveniently used in conjunction with the red-shifted chromophores developed recently for primary amino groups.