Short synthesis of noscapine, bicuculline, egenine, capnoidine, and corytensine alkaloids through the addition of 1-siloxy-isobenzofurans to imines

A concise diastereotioselective strategy for the synthesis of noscapine, bicuculline, and egenine (1a–c), as well as capnoidine and corytensine (2a,b), was developed using diastereoselective addition of 1-siloxy-isobenzofurans 4a and 4b to iminium ion 5 in a one-pot approach. The synthesis features the use of imine 13 obtained through Bischler–Napieralsky reaction from amine 11. The addition of ionic liquids as addictives in the reactions afforded erythro configuration in major adduct compounds. The synthetic route can also be applied in the total synthesis of promising tubulin binding agent EM105 (3).