Asymmetric Michael addition reactions of chiral Ni(II)-complex of glycine with (N-trans-enoyl)oxazolidines: improved reactivity and stereochemical outcome

Application of the (N-trans-enoyl)oxazolidines as Michael acceptors in the kinetically controlled additions with a Ni(II)-complex of the chiral Schiff base of glycine with (S)-o-[N-(N-benzylprolyl)amino]benzophenone 1 was sho wn to be synthetically advantageous over the alkyl enoylates, allowing for remarkable improvement in reactivity and, in most cases, diastereoselectivi ty of the reactions. While the stereochemical outcome of the Michael additi ons of the aliphatic (N-trans-enoyl)oxazolidines with complex 1 depended on the steric bulk of the alkyl group on the starting oxazolidines, the diast ereoselectivity of the aromatic (N-trans-enoyl)oxazolidines reactions was f ound to be controlled by the electronic properties of the aryl ring. In par ticular, the additions of complex 1 with (N-cinnamoyl)oxazolidines, bearing electron-withdrawing substituents on the phenyl ring, afforded the (2S,3R) configured products with synthetically useful selectivity and in quantitati ve chemical yield, thus allowing an efficient access to sterically constrai ned beta-substituted pyroglutamic acids and related compounds. (C) 1999 Els evier Science Ltd. All rights reserved.