Synthesis of vinca alakaloids and realated compounds98. Oxidation with dimethyldioxirane of compounds containing the aspidospermane and quebrachamine ring system. A simple synthesis of (7S,20S)-(+)-rhazidigenine and (2R,7S,20S)-(+)-rhazidine

The oxidation of (-)-tabersonine (1) with dimethyldioxirane (DMD) in neutral and acidic medium gave 16-hydroxytabersonine-N-oxide (3) and the didehydrovincamine isomers 4 and 5, respectively. (+)-14,15-Didehydro-quebrachamine (7) furnished the hydroxyindolenine 9, and the pentacyclic derivative 11. (+)-Quebrachamine (8) and DMD in neutral medium gave (7S,20S)-(+)-rhazidigenine (12) which was converted to (2R,7S,20S)-(+)-rhazidine (13b) with hydrochloric acid.