INFLUENCE OF INTRAMOLECULAR HYDROGEN-BONDING ON THE CONFORMATIONAL PROPERTIES OF SUGAR THIOUREAS

Reaction of deoxyisothiocyanato derivatives of 2:3,4-di-O-isopropylide ne-alpha-D-galactopyranose, 1,2:3,5-di-O-isopropylidene-alpha-D-glucof uranose and 2,3:4,5-di-O-isopropylidene-beta-D-fructopyranose with amm onia afforded the corresponding sugar thioureas. Both the Z and E ster eoisomers around the NH-C(=S) bond were observed in the H-1 and C-13 N MR spectra of the later in the low temperature range, their relative p roportions being a function of the sugar configuration. Experimental e vidence for the existence of seven-membered NH...O intramolecular hydr ogen bonds in the E isomers of thioureas has been obtained from DNMR e xperiments, rotational barrier height calculations, measurements of te mperature coefficients for the H-1 chemical shifts of the NH signals, and study of the influence of the solvent polarity in the rotameric ra tio.