Diastereocontrolled Monoprotodeboronation of β-Sulfinimido gem-Bis(boronates): A General and Stereoselective Route to α,β-Disubstituted β-Aminoalkylboronates.

β-Aminoalkylboronic acids are bioisosteres of the pharmaceutically important class of β-amino acids but few stereoselective methods exist for their preparation. The 1,2-addition of lithiated 1,1-diborylalkanes onto chiral N-tert-butanesulfinyl aldimines produces β-sulfinimido gem-bis(boronates) in good to excellent yields with high diastereoselectivity. The optimized conditions involve the use of rubidium fluoride and water, and are compatible with functionalized alkyl, aryl, alkenyl, and alkynyl substituents. Under these conditions, the geminal quaternary alkyl bis(pinacolatoboryl) intermediates undergo a highly diastereoselective monoprotodeboronation to afford a wide range of syn-α,β-disubstituted β-aminoalkylboronates. This novel application of protodeboronation chemistry was shown to result from a kinetically controlled, diastereotopic-group-selective B-C bond protolysis dictated by the configuration of the adjacent stereogenic C-N center. Facile acidic cleavage of the sulfinimide auxiliary produces the free aminoboronates with high enantiomeric purity.