One result did however give us hope. Using platinum oxide as
the catalyst, some reduction of the phenyl ring of the benzoyl
group occurred, and the resulting cyclohexyl derivative clearly
showed the presence of the singlet (ca. d 2.6) characteristic of
the ajmaline series.17 We reasoned that the bulkier protecting
group was partially blocking the si face of the imine, allowing
reduction to the ajmaline series. We therefore tried to optimize
this with bulkier protecting groups and reducing agents, eventually
leading to a 72:28 preference for the ajmaline series—the best
selectivity ever reported for this cyclization.