Corynanthe-Strychnos skeleton

The mechanism by which the Corynanthe skeleton is converted into the Strychnos system (42) is under current study and there are several chemically reasonable possibilities. The point is left open in Scheme 10 but clearly oxidation is necessary to reach preakuammicine (42) and this might occur before or after rearrangement. 

It is now considered [188 b, 188 c] that stemmadenine (XLIIA, Fig. 3) is likely to be a key intermediate between alkaloids of the Corynanthe type and the Ihoga and Aspidosperma types although whether it is a precursor [188b, 188 c] or further derivative [187a] of the Strychnos type skeleton is still unsettled. 

Simple rearrangement of the Corynanthe skeleton [as [6.243)] affords the Strychnos type, exemplified by akuammicine [6.242), and strychnine [6.263). Like [6.242), strychnine has been found to have the expected origins in tryptophan and in mevalonate via geraniol the extra C2 unit (C-22 and C-23) arises from acetate. Late intermediates have been identified and the pathway deduced is shown in Scheme 6.47 [182]. 

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