Improved Synthesis of β‑Aminoboronate Esters via Copper-Catalyzed Diastereo- and Enantioselective Addition of 1,1-Diborylalkanes to Acyclic Arylaldimines

We
describe herein an improved and scalable procedure to achieve
the copper-catalyzed diastereo- and enantioselective 1,2-addition
of 1,1-diborylalkanes to N-protected acyclic arylaldimines.
We find that the installation of an N,N-dimethylsulfamoyl-group as N-protecting
group in acyclic arylaldimines allows the 1,2-addition reaction to
proceed with good-to-excellent diastereoselectivity and enantioselectivity,
thus providing β-aminoboronate esters bearing vicinal chiral
secondary amines and secondary boronate esters in good yields. The
reaction can be easily scaled up to gram scale (3.25 g), highlighting
the usefulness of the developed process. Synthetic applications whereby
the obtained β-aminoboronate ester is converted to other enantioenriched
compounds are also demonstrated.