article carbon nuclear magnetic resonance drug structure lactone derivative nonhuman unclassified drug 13C NMR spectra 20,16 gamma carbolactone derivative 3beta,16alpha dihydroxy 5alpha androstan 17 one diacetate 3β, 16β‐Dihydroxy‐androst‐5‐en‐17‐one 3β, 17β‐Dihydroxy‐androst‐5‐en‐16‐one Acid‐catalysed rearrangement Androstanols Biosynthesis Lactones Liliaceae Magnetic Resonance Spectroscopy Models, Chemical Models, Molecular NMR Natural products Nothoscordum gramineum var. philippianum Spironolactone Stereospecificity Steroids and sterols Tristagma uniflorum androstane derivative carbonyl derivative drug stability drug synthesis