Geometry of some α-isosparteine derivatives

13C and 1H NMR spectra of five compounds with the α-isosparteine skeleton: α-isosparteine (2), 2-oxo-α-isosparteine (α-isolupanine, 4), 2,13-dioxo-α-isosparteine (5), 2-tiono-α-isosparteine (6), and 13-oxo-2-tiono-α-isosparteine (7) were compared. Precise determination of the effects of ketone, lactam and thiolactam groups allowed identification of differences in the geometry of the compounds compared. Ring A of the two lactams investigated, 4 and 5, has the same or very similar geometry but is different than that in 2-oxosparteine (lupanine) while in thiolactams 6 and 7 it has the same (or very close) geometry as in 2-thionosparteine. Ketone group in position 13 in 5 and 7 causes a flattening of ring D in a similar way, and different to that in 2,13-dioxosparteine (13-oxolupanine).