Total Synthesis of (?-a-Isosparteine, (?-b-Isosparteine, and (?-Sparteine from a Common Tetraoxobispidine Intermediate.

[摘要]：The three title alkaloids were sep. prepd. in stereocontrolled fashion from a common tetraoxobispidine precursor, 3,7-diallyl-2,4,6,8-tetraoxo-3,7-diazabicyclo[3.3.1]nonane (I). Bisimide I was generated from malonate via acid promoted cyclization of the Knoevenagel condensation adduct 1,1,3,3-propanetetracarboxamide. (?-a-Isosparteine was elaborated from I in 28% overall yield by a two-directional synthetic sequence composed of four reactions: double addn. of allylmagnesium bromide, ring-closing olefin metathesis (RCM), hydrogenation, and borane mediated redn. (?-b-Isosparteine was targeted along similar lines by a strategic reversal in allylation and redn. operations on the core synthon. Thus, I was advanced to (?-b-isosparteine in five steps and 12% overall yield by a reaction sequence commencing with sodium borohydride mediated redn. and followed by double Sakurai-type allylation of the resulting bishemiaminal. The synthesis of (?-b-isosparteine was concluded by RCM and then global redn. (H2, Pd/C; LiAlH4). The final target, (?-sparteine, was secured in six steps and 11% overall yield from I by monoredn. and Sakurai allylation, followed by allyl Grignard addn. and then RCM and global redn. as before. Reasons for the inherent C2-type regioselectivity of net double nucleophilic addns. to tetraoxobispidines are discussed and enantioselective oxazaborolidine mediated redn. of the N,N'-dibenzyl congener of I is reported.

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