Diethylanilineborane:  A Practical, Safe, and Consistent-Quality Borane Source for the Large-Scale Enantioselective Reduction of a Ketone Intermediate in the Synthesis of (R,R)-Formoterol

The development of a process for the use of N,N-diethylaniline−borane (DEANB) as a borane source for the enantioselective preparation of a key intermediate in the synthesis of (R,R)-formoterol l-tartrate, bromohydrin 2, from ketone 3 on kilogram scale is described. DEANB was found to be a more practical, safer, and higher-quality reagent when compared to other more conventional borane sources: borane−THF and borane−DMS.