Stereospecific iminium ion-vinylsilane cyclization

The stereocontrolled formation of exocyclic double bonds by stereospecific iminium ion-vinylsilane cyclizations has also been employed by Overman and coworkers as the key step in total syntheses of Co-rynanthe alkaloids. - The crucial cyclization step in the synthesis of ( )-(19Z)-isositsirikine is shown in equation (10). The (Z)-vinylsilane (103) cyclizes in 1 1 methanol-water to provide, in essentially quantitative yield, the (Z)-ethylideneindoloquinolizidine (104). 

An elegant application of vinylsilane chemistry reported this year is the clean and stereospecific iminium ion-vinylsilane cyclization to dendrobatid toxin 25 ID shown in Scheme and a spiroannulation synthesis has been described in which the final stage is Lewis-acid-catalysed cyclization of (111) to (112)/ °... 

Overman, L. E., and K. L. Bell Enantiospecific total synthesis of dendrobatid toxin 25ID. A short chiral entry to the cardiac-active pumiliotoxin A alkaloids via stereospecific iminium ion — vinylsilane cyclizations. J. Amer. Chem. Soc. 103, 1851— 1852 (1981). 

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See also in sourсe #XX -- [ , ]

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