Structure and stereochemistry of the monobromotigogenins

Bromination of desoxytigogenin has been found to yield two isomeric 23-bromo-5α-25R-spirostans. The corresponding 3β-acetoxy-23-bromo-5α-25R-spirostans were also prepared from tigogenin actetate. A combination of physical methods based on mass spectrometry and NMR measurements, at 60- and 100-mc/s, have been used to confirm attack by bromine at C-23 of the spiroketal system. The configuration of each epimeric pair (at C-23, cf. III and IV) of isomers was also determined.