General access to tacamine and vinca-eburna alkaloids through tandem non-biomimetic oxidation of dihydropyridines/Zn-mediated radical addition processes - Unexpected facial selectivity of flattened cyclohexyl-type radicals

A general route to the eburnamine-vincamine and tacamine indole alkaloid gr oups has been developed. The key features of the synthetic strategy are the halocyclization of a dihydropyridine (a non-biomimetic oxidation of these compounds) and the radical addition of the resulting halo derivative to act ivated alkenes. The latter process shows an unusual stereos electivity, fav ouring the generation of a cis stereochemistry in a 1,2-disubstituted 6-mem bered ring. Theoretical calculations suggest conformational control at the intermediate radical level, and a preferential face selectivity producing t he observed products.