13C-1H coupling constants 2D NMR heteronucleus-coupled H-H correlation (HETLOC) 2 4 thio l lyxono 1,4 lactone 2 aldopentono-1,4-thiolactone conformation 2 article 2 crystallography 2 drug structure 2 heteronuclear 2 nuclear magnetic resonance 2 priority journal 2 thiolactone 2 unclassified drug 2