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 o verlap with that of the substrate, except for cases in which the coupl ing between the existing and the introduced chromophores are deliberat ely sought for. Thus, the availability of red-shifted chromophores tha t do not overlap with the substrate absorption would greatly expand th e applicability of this versatile CD method. Four such red-shifted chr omophores, 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 derivatiz e hydroxyl groups of the substrates on a microscale. The bischromophor ic esters of 1(R),2(R)-cyclohexanediol (14-18) exhibited intense excit on-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 str ong absorptions at 260-275 nm, were esterified with the new chromophor es; this gave rise to strong couplets isolated from the CD Cotton effe cts of starting materials, the signs of which were in agreement with t he absolute configurations of these two natural products. These O-acyl ating chromophores should be useful for determinations of absolute con figurations and conformations of chiral substrates, including biopolym ers; they could also be conveniently used in conjunction with the red- shifted chromophores developed recently for primary amino groups.