Sparteine, mercuric acetate oxidation

Sparteine (43) is oxidized to a mixture of isomers /J -didehydro-sparteine (44) and id -dehydrosparteine (45) (57). The other two stereoisomers of sparteine, a-isosparteine (46) (3S,j9) and S-isosparteine (sparta-lupine) (47) (58,60) have been subjected to mercuric acetate oxidation, each giving Zl -didehydrosparteine (44). 

The idea was that sparteine (46) might arise from the double intramolecular Mannich reaction suggested by structure 48 (followed by reduction of the 8-keto group). A precursor of 48 might be 49. In the laboratory, 49 was assembled from piperidine, acetone and formaldehyde via a double intermol-ecular Mannich reaction. Mercuric acetate oxidation of 49 gave 8-ketosparteine (47), presumably via intermediates resembling 48 (stepwise oxidation-Mannich events seem most likely). A Wolff-Kishner reduction of 47 completed the synthesis of 46. Yields were not given for this synthesis and thus, the efficiency of this route (in terms of yield) cannot be evaluated. Efficiency in terms of the number of steps cannot be denied. 

For example, yohimbine containing an a-oriented axial hydrogen at C3 can be dehydrogenated by mercuric acetate, whereas reserpine or pseudoyohimbine with /1-oriented hydrogen does not react305,309-312. Sparteine (83) is oxidized to a mixture of isomers A5,11-didehydrosparteine (84) and A5-dehydrosparteine (85). The other two stereoisomers of sparteine, i.e. a-isosparteine (86) and jS-isosparteine (87) dehydrogenate to 84290,313-315. 

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