article priority journal unclassified drug chemical structure drug synthesis nuclear magnetic resonance thiolactone 13C-1H coupling constants 2D NMR heteronucleus-coupled H-H correlation (HETLOC) 4 thio l lyxono 1,4 lactone 4 thio l rhamnofuranose Acetolysis Carbohydrates Potassium Compounds Potassium Thiocyanate Sugar Thiocyano Group aldopentono-1,4-thiolactone conformation antibiotic therapy conformational transition crystallography deacetylation drug structure enantiomer galactose gentamicin C glycosylation heteronuclear hydrogenation kanamycin derivative