Conformers of sparteine

Dioxo-3-isoparteine was isolated from Lupinus sericeus (143). The mass spectrum, with M+ at miz 262 and signals at miz 234 (M" — 28) and 206 (M+ - 56), is characteristic for 10- and 17-oxosparteines and successive splitting of two carbonyl groups. Oxidation of p-isosparteine (14) by potassium ferricyanide resulted in 10-oxosparteine (108) as well as 10,17-dioxo-p-isospar-teine (109) (Scheme 13). This confirmed the alkaloid structure. Although 109 was found as a natural compound it had already been synthesized by Bohlmann et al. (144). The problems of configuration and conformation of sparteine (6), a-isosparteine (7), and (3-isosparteine (14) were discussed (145). 

The conformation of sparteine in solution has been determined by means of NMR-spectra. Ring C is present in the boat form and hence rings C and D are joined trans (6). 

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