The spatial structure of 13α-hydroxy-2-thionosparteine – a potential hypoglycemic agent – and some related compounds

13α-Hydroxy-2-thionosparteine (7) was synthesized from 13α-hydroxylupanine (4) via 13α-t-butyldimethylsilyloxylupanine (8) and 13α-t-butyldimethyl-2-thionosparteine (9). The similarity of the stereochemistry of compounds 4, 8 and 9 as well as 13α-trimethylsilyloxylupanine (10) was established on the basis of their 13C and 1H NMR spectra. In solution, the conformation with ring C a boat predominates in all compounds studied. However, they differ in the fraction of conformers: the thiolactams have a slightly greater population of the chair conformer than lactams and the hydroxy derivatives have a bit greater population of the same conformer than the parent lactam lupanine (2) or thiolactam 2-thionosparteine (5). The fraction of the chair conformer in 7 amounts to ca. 22%, almost as much as in multiflorine (6) known to be a good hypoglycemic agent. The trialkylsilyloxy derivates have a similar composition at the conformational equilibrium as that of the appropriate hydroxy compounds. The substituent does not influence the geometry of ring D.