Iminium Ions of Indolo(2,3-alpha)Quinolizidine Derivatives: The RegioselectiveFormation of delta (sup 5(6)) Iminium Ion Equivalents

摘要

The modified Polonovski reaction was used in combination with cyano-trapping tostudy the regioselective formation of delta 5(6) iminium ion of Na-Boc-protected indolo(2,3-a)quinolizidine derivatives. The formation of the delta 5(6) iminium ion was straightforward, but the easy isomerization of the iminium ions had to be taken into consideration through special attention to the reaction conditions. Increase in the reaction temperature caused delta 5(4) iminium ion to be strongly favored at the expense of delta 5(6) iminium ion and provided an easy access to heteroyohimbine structure from geissochizine skeleton without the use of a natural product as starting material. Isomerization of the C-3 ethylidene side-chain of indolo(2,3-a)quinolizindine derivatives was achieved by the use of E/Z side-chain equilibrium of conjugated delta 5(4) iminium ion. The amount and the direction of the isomerization were greatly influenced by the conformation of the indoloquinolizidine compound. Through isomerization of the ethylidene side-chain of deformyl-z-geissoschizine, a short route was found to the synthesis of (plus or minus)-geissoschizine, an important intermediate of many indole alkaloids.