Enantioselective synthesis of some tetrahydroisoquinoline and tetrahydro-β-carboline alkaloids

Four alkaloids: (R)-(+)-cryspine A 5, (R)-(+)-octahydroindolo2,3-a]quinolizidine 8, (R)-(+)-harmicine 19 and (R)-(+)-desbromoarborescidine 22 were prepared via the asymmetric transfer hydrogenation reaction of a prochiral enamine (iminium salt). The enantiomeric excesses of the isolated alkaloids were determined from their ¹H NMR spectra measured in the presence of (+)-(R)-tert-butylphenylphosphinothio(seleno)ic acids as chiral solvating agents. The absolute configurations at the newly created stereogenic carbon atoms were confirmed, in all cases, by X-ray crystallography.