CHIROPTICAL PROPERTIES OF 1-DEOXY-1-THIOGALACTOPYRANOSIDES

Abstract

Continuing our studies of optically active sulfides we have investigated flexible systems incorporating the 1.3-O,S-chromophore, e.q., alkyl α-and B-D-thiogalactopyranosides and their tetraacetates. Although most substances were known from the literature, constitutional, configurational, and conformational assignments had hardly ever been proved rigorously; we therefore had to provide a firm basis by a systematic analysis of the ¹H and ¹³C NMR spectra of thioglycosides, the complexity of the carbohydrate signals necessitating high field studies. The compounds were thus, in the end, correlated with the natural thiogalactoside lincomycin, whose structure including its absolute configuration is known from X-ray investigations. CD curves have been measured at various temperatures in the range of 185–300 nm on a commercial spectropolarimeter equipped with a data processor and connected to a table calculator and plotter, directly furnishing data suitable for, e.g., Kronig-Kramers type transformations or curve analysis and displaying the results. These CD data were correlated with the UV spectra and the ORD curves of the substrates. Typically, the UV curves are devoid of detail except in the case of the acetates, where a shoulder in the region of the n→π? transition is discernible. In the ORD, the Cotton effects (CE) are largely suppressed by the background contribution of the “invisible giant” in the far UV. In the thiogalactopyranosides all accessible Cotton effects are associated with electron transitions within the sulfur or possibly the coupled O-C-S chromophore, whereas a relatively small negative, composite CE is contributed by the acetate carbonyl groups in the acetates.