Enantioselective borane reduction of ketones catalyzed by tricyclic 1, 3, 2-oxazaborolidines. Issue 23 (8th June 2016) - Kaldun, Johannes
Graphical abstract: Highlights: Two new tricyclic oxazaborolidines were synthesized from methyl Boc-l -pyroglutamate. Their potential in the asymmetric borane reduction of ketones was evaluated. One derivative permitted excellent enantioselectivities of up to 98%. The relative reactivities of the catalysts were judged from competition experiments. Abstract: Two novel tricyclic 1, 3, 2-oxazaborolidines were synthesized in seven steps from methyl Boc-l -pyroglutamate. They are characterized by an ortho- and peri- fused 5/5/6-ring system with the B–N bond forming one ring junction. In the asymmetric borane reduction of ketones, the B -alkoxy bridged derivative permits excellent enantioselectivities of up to 98% ee and its activity is comparable to that of the standard CBS catalyst. The closely related, B -alkyl bridged derivative is less enantioselective and less active, as determined by competition experiments.