Protected syn-Aldol Compounds from Direct, Catalytic, and Enantioselective Reactions of N-Acyl-1,3-oxazinane-2-thiones with Aromatic Acetals.

A direct and asymmetric syn-aldol reaction of N-acyl-1,3-oxazinane-2-thiones with dialkyl acetals from aromatic acetals in the presence of 2-5 mol % [DTBM-SEGPHOS]NiCl2, TMSOTf, and lutidine has been developed. It has been established that the oxazinanethione heterocycle, used for the first time as a scaffold in asymmetric carbon-carbon bond-forming reactions, can be smoothly removed to give access to a variety of enantiomerically pure compounds with high synthetic value.

Direct and Asymmetric Aldol Reactions of N-Azidoacetyl-1,3-thiazolidine-2-thione Catalyzed by Chiral Nickel(II) Complexes. A New Approach to the Synthesis of ß-Hydroxy-α-Amino Acids.

A direct and asymmetric triisopropylsilyltrifluoromethanesulfonate (TIPSOTf) mediated aldol reaction of N-azidoacetyl-1,3-thiazolidine-2-thione with aromatic aldehydes catalyzed by a chiral nickel(II)-Tol-BINAP complex has been developed (BINAP=2,2'-bis(diphenylphosphino)-1,1'-binaphthyl). The catalytic protocol gives the corresponding anti α-azido-ß-silyloxy adducts with outstanding stereocontrol and in high yields. Theoretical calculations account for the stereochemical outcome of the reaction and lay the foundations for a mechanistic model. In turn, the easy removal of the thiazolidinethione yields a wide array of enantiomerically pure derivatives in a straightforward and efficient manner. Such a noteworthy character of the heterocyclic scaffold together with the appropriate manipulation of the azido group open a new route to the synthesis of di- and tripeptide blocks containing a ß-aryl-ß-hydroxy-α-amino acid.

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.