Enantioselective Total Syntheses of (+)-Arborescidine A, (−)-Arborescidine B, and (−)-Arborescidine C

Described are the first enantioselective total syntheses of (+)-arborescidine A ((+)-1), (−)-arborescidine B ((−)-2), and (−)-arborescidine C ((−)-3), via routes that proceeded in five steps and 50% overall yield, eight steps and 61% overall yield, and nine steps and 51% overall yield, respectively, from 6-bromotryptamine (7). The syntheses feature the use of the Noyori catalytic asymmetric hydrogen-transfer reaction to introduce chirality in dihydro-β-carbolines 6 and 8. On the basis of an ample precedent from Noyori's work, the reduction produces dihydro-β-carbolines, and ultimately the natural products, possessing the R absolute configuration. The synthetic arborescidines displayed optical rotations that were opposite in sign those of the natural products, thereby supporting the S configuration for natural arborescidines A (1) and B (2) and the (3S,17S) configuration for natural arborescidine C (3). Our results are in agreement with the initial stereochemical assignment by Païs and co-workers, and are counter to their recently revised assignment.