Abstract
Abstract : (2S,5S,8R)-Strobamine (+)-1a was synthesized by adding cinnamoyl cyanide 6a to tropinone 3 lithium enolate generated by treatment of (S,S')-alpha, alpha-dimethyldibenzylamide with butyl lithium in the presence of lithium chloride to give (-)-chalcostrobamine (-)-7a, which yielded a one-to-one mixture of (+)-1a and its C-2 epimer (-)-2a on treatment with 2N sulfuric acid. Compounds (+)-1a and (-)-2a could be separated by column chromatography. (-)-Strobamine (-)-1a and (+)-2a were synthesized by a similar set of reactions using the tropinone 3 lithium enolate generated with butyl lithium in the presence of (R,R')-alpha, alpha'-dimethyldibenzylamide and lithium chloride. (+)- and (-)-p-Methylstrobamine (+)- and (-)-1b and (+)- and (-)-epi-p-methylstrobamine (+)- and (-)-2b were synthesized by a similar procedure. The absolute configuration of (+)-epi-p-methylstrobamine (+)-2b was determined by X-ray analysis to have the (2S,5R,8S) configuration.
Highlights
Strobamine (+)-1a was first isolated from the leaves of the endemic New Caledonian plant Knightia strobilina Labill in 1980.1,2 the synthesis of (+)-1a has not been reported, and its absolute stereochemistry is unknown
Abstract (2S,5S,8R)-Strobamine (+)-1a was synthesized by adding cinnamoyl cyanide 6a to tropinone 3 lithium enolate generated by treatment of (S,S')-α,α-dimethyldibenzylamide with butyl lithium in the presence of lithium chloride to give (–)-chalcostrobamine (–)-7a, which yielded a one-to-one mixture of (+)-1a and its C-2 epimer (–)-2a on treatment with 2N sulfuric acid
Since Majewski and Lazny[4] reported that (–)-chalcostrobamine (–)-7a and (+)-entchalcostrobamine (+)-7a could be synthesized in >95% ee by enantioselective deprotonation of tropanone 3 using chiral lithium (S,S')-α,α'-dimethylbenzylamide 4 and (R,R')-α,α'dimethyldibenzylamide 5, we envisioned that (+)- and (–)-1a, (+)- and (–)-2a as well as (+)-1b and (–)-1b and (–)-2b and (+)-2b could be prepared by the route shown in Scheme 1
Summary
Strobamine (+)-1a was first isolated from the leaves of the endemic New Caledonian plant Knightia strobilina Labill in 1980.1,2 the synthesis of (+)-1a has not been reported, and its absolute stereochemistry is unknown. Abstract (2S,5S,8R)-Strobamine (+)-1a was synthesized by adding cinnamoyl cyanide 6a to tropinone 3 lithium enolate generated by treatment of (S,S')-α,α-dimethyldibenzylamide with butyl lithium in the presence of lithium chloride to give (–)-chalcostrobamine (–)-7a, which yielded a one-to-one mixture of (+)-1a and its C-2 epimer (–)-2a on treatment with 2N sulfuric acid. The synthesis of (±)-strobamine (±)-1a and (±)-epistrobamine (±)-2a was reported by Lounasmaa et al in 1983.3 In their preparation, tropinone was deprotonated with NaH, and the resulting enolate was treated with cinnamoyl cyanide to form chalcostrobamine (±)-7a.
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