Novel Rearrangements of Vinylaziridines: Aza-[2,3]-Wittig Rearrangements and [1,5]-Hydrogen Shifts

Abstract: A route from epoxy alcohols to cis- and trans-trisubstituted vinylaziridines has been developed. The vinylaziridines have been used as substrates in the base induced aza-[2,3]-Wittig rearrangement to substituted 1,2,3,6-tetrahydropyridines. The influence of the anion-stabilising group on the reaction outcome has been investigated and it was found that tert-butyl ester enolate promotes a clean conversion to the cis-2,6-disubstituted tetrahydropyridine whereas the use of a TMS-alkyne as anion-stabilising group gives a poor trans-2,6-selectivity. A study on the rearrangement outcome when using disubstituted alkenes as substrates was perused and it was found that the stereochemical information in the substrate alkene moiety is transmitted to C3 in the 2,3,6-trisubstituted tetrahydropyridine product. The scope and limitations of this transformation has been investigated. The mechanism of this reaction has been probed to some extent and it reveals a complicated picture. Enantioselective total synthesis of (-)-indolizidine alkaloids 209B and 209D with the aza-[2,3]-Wittig rearrangement as key transformation is described. Vinylaziridines undergoes a thermal homodienyl-[1,5]-hydrogen shift to the corresponding allylic imine. The scope and limitations of this rearrangement has been investigated. The rate of the rearrangement is influenced by the N-substituent and the substitution on the alkene moiety of the substrate. The result of these rearrangements could be explained by invoking a chair-like transition structure for this reaction.