RELATIVE AND ABSOLUTE CONFIGURATIONAL ASSIGNMENTS OF ACYCLIC POLYOLS BY CIRCULAR-DICHROISM .2. DETERMINATION OF NONDEGENERATE EXCITON COUPLING INTERACTIONS BY ASSIGNMENT OF PROCHIRAL ARYLOXYMETHYLENE PROTONS FOR H-1-NMR CONFORMATIONAL-ANALYSIS

A general procedure for assigning relative and absolute stereochemistry in 1,2,3-triols, 1,2,3,4-tetrols, and 1,2,3,4,5-pentols by circular dichroism was recently reported. Selective 9-anthroylation of primary hydroxyls followed by per-p-methoxy cinnamoylation of secondary hydroxyls affords “bichromophoric” derivatives, the CD spectra of which are characteristic and predictable for each stereochemical pattern. The distinctive CD spectra result from two types of exciton coupling interactions: degenerate cinnamate/cinnamate interactions that give rise to Cotton effects at 287 and 322 nm, and nondegenerate anth roate/cinnamate interactions that result in a strong sharp Cotton effect at 253 nm and smaller Cotton effects in the 300-380-cm region. The 287- and 322-nm Cotton effects were previously accounted for by consideration of the additive contributions from pairwise cinnamate/cinnamate interactions in 1H NMR determined conformations. Here the 253-nm Cotton effects are accounted for by analysis of the anthroate/cinnamate exciton coupling modes. These have been determined by NMR, for which stereospecifically deuterated compounds 1 and 2 were prepared from (6S)-(6-2H1)-1-deoxy-D-galactitol and (6S)-(6-2H1)-1-deoxy-D-glucitol, respectively, in order to assign pro-R and pro-S protons in anthroyloxymethylene groups. Distributions of the three staggered anthroyloxymethyl rotamers were calculated from vicinal coupling constants. CD spectral contributions of anthroate/cinnamate interactions in the dominant rotamers were determined via model compounds and shown to account for the predominance of positive 253-nm Cotton effects in these derivatives. The large number of acyclic derivatives allowed for graphical analysis of the interdependence between C1-C2 and C2-C3 rotamers for conformational analysis. Exciton coupling interactions between 1-O-(9-anthroate) and 3-O-p-methoxycinnamate groups have been elucidated for both 2,3-erythro and threo configurations, and in certain cases minor conformations were found to be responsible for distinctive features of the CD spectra. © 1990, American Chemical Society. All rights reserved.