Preparation of 2-acyl- and 2-alkoxycarbonyl-octahydroindolo[2,3-a]quinolizines

A method for the synthesis of 2-acyl- and 2-alkoxycarbonyl-octahydroindolo[2,3-a]quinolizines is described which involves as a key step the isomerisation of a 4-acyl- or 4-alkoxycarbonyl-1-[2-(indol-3-yl)ethyl]-1,2,3,6-tetra-hydropyridine to a 4-acyl- or 4-alkoxycarbonyl-1-[2-(indol-3-yl)ethyl]-1,2,3,4-tetrahydropyridine (not isolated). This, in turn, in reacting as a cyclic enamine, provides by protonation an electrophilic centre for closure on to the indole α-position. The 4-acyl-1-[2-(indol-3-yl)ethyl]-1,2,3,6-tetrahydropyridines can be cyclised in an alternative manner to give compounds comprising four of the five skeletal rings of the Iboga alkaloids.