PREFERRED CONFORMATION OF C-GLYCOSIDES .12. SYNTHESIS AND CONFORMATIONAL-ANALYSIS OF ALPHA,ALPHA-, ALPHA,BETA-, AND BETA,BETA-C-TREHALOSES

A single, unified strategy for the stereocontrolled synthesis of alpha,alpha-, alpha,beta-, and beta,beta-C-trehaloses (1-3) was developed. The solution conformations of C-trehaloses 1-3, as well as their permethyl derivatives, were determined on the basis of vicinal coupling constants observed in the H-1 NMR spectra. The preferred conformations for alpha,alpha- and beta,beta-C-trehaloses (1 and 3), shown in Figure 1, were predicted and experimentally proven. A diamond-lattice analysis of alpha,beta-C-trehalose (2), shown in Figure 2, revealed the relative stability of the three staggered conformers to be 2A > 2B > 2C, and the experimental data were found to be consistent with this trend. It was demonstrated that the inversion of the C.2 or C.2' hydroxyl group of 2 affected its conformational preference in a predictable manner; The H-1 NMR spectra of alpha,beta-C-trehalose 2 provided direct experimental evidence to illustrate that the alpha-C-glycosidic bond is conformationally more rigid than the beta-C-glycosidic bond,