Semisynthesis of Kopsidine a and B via a Polonovski-Potier Reaction

Abstract
Kopsidine A 4a and B 4b were synthesised from kopsingine N-oxide via a biomimetic Polonovski-Potier reaction. 1,4-Addition of CH3OH or C2H5OH on the intermediate iminium salts, followed by 1,2-addition of the C-17 hydroxyl function produced the expected alkaloids.