Highly Diastereoselective Addition of Alkynylmagnesium Chlorides to <i>N</i>-<i>tert</i>-Butanesulfinyl Aldimines: A Practical and General Access to Chiral α-Branched Amines

The addition of alkynylmagnesium chlorides to N-tert-butanesulfinyl imines proceeded with a remarkably high diastereoselectivity (dr > 13:1, mostly diastereopure), and with a general scope of both reaction partners. The high dr translated into simplified purification and higher optical purity for the synthesis of a wide array of chiral α-branched amines. The alkyne functionality also provides a multitude of opportunities for further synthetic transformations. The short asymmetric synthesis of (+)-angustureine 7 and (−)-cuspareine 10 was realized with use of this approach.