A nuclear magnetic resonance study of sparteine delta metabolite structure

Originally, the enamines 2,3- and 5,6-didehydrosparteine (2a and 3a, respectively) had been characterized by GC/MS as metabolites after administration of (-)-sparteine sulfate (1a.H2SO4). Since the existence of free enamines in aqueous medium seemed rather doubtful, the metabolism of (-)-sparteine was reinvestigated by high-resolution 1H-, 2H-, and 13C-NMR spectroscopy. When synthetic 1,2-didehydrosparteinium monoperchlorate (4a.ClO4) is dissolved in aqueous medium at pH 4-9, sterically uniform (2S)-hydroxysparteine (6a) is formed, as is proven unequivocally by 1H- and 13C-NMR. However, at pH less than or equal to 2, this carbinolamine eliminates water under reconstitution of the iminium structure 4a. No carbinolamine is formed, in contrast, from synthetic 1,6-didehydrosparteinium monoperchlorate (5a.ClO4). Upon oral application of [2R-2H]sparteine (1b), as the sulfate, the respective carbinolamine and iminium structures 6b and 5b are identified in the urines by 2H-NMR spectroscopy. Reversed-phase TLC likewise confirms the different structural principle of the two main sparteine metabolites. The free enamine bases, characterized by GC/MS, thus must be regarded as artifacts, formed in the work-up requisite for GC analysis.