Direct, Stereodivergent, and Catalytic Michael Additions of Thioimides to α,ß-Unsaturated Aldehydes - Total Synthesis of Tapentadol.

Direct and stereodivergent Michael additions of N-acyl 1,3-thiazinane-2-thiones to α,ß-unsaturated aldehydes catalyzed by chiral nickel(II) complexes are reported. The reactions proceed with a remarkable regio-, diastereo-, and enantioselectivity, so access to any of the four potential Michael stereoisomers is granted through the appropriate choice of the chiral ligand of the nickel(II) complex. Simple removal of the heterocyclic scaffold furnishes a wide array of either syn or anti enantiomerically pure derivatives, which can be exploited for the asymmetric synthesis of biologically active compounds, as demonstrated in a new approach to tapentadol. In turn, a mechanism, based on theoretical calculations, is proposed to account for the stereochemical outcome of these transformations.

Direct and Enantioselective Aldol Reactions Catalyzed by Chiral Nickel(II) Complexes.

A direct and asymmetric aldol reaction of N-acyl thiazinanethiones with aromatic aldehydes catalyzed by chiral nickel(II) complexes is reported. The reaction gives the corresponding O-TIPS-protected anti-aldol adducts in high yields and with remarkable stereocontrol and atom economy. Furthermore, the straightforward removal of the achiral scaffold provides enantiomerically pure intermediates of synthetic interest, which involve precursors for anti-α-amino-ß-hydroxy and α,ß-dihydroxy carboxylic derivatives. Theoretical calculations explain the observed high stereocontrol.