STEREOSELECTIVE RADICAL-ADDITION OF CARBON-CENTERED RADICALS TO THE DEHYDROALANINE MOIETY OF THE CHIRAL NICKEL(II) COMPLEX OF THE SCHIFFS BASE DERIVED FROM (S)-2-[N-(N'-BENZYLPROLYL)AMINO]BENZOPHENONE AND DEHYDROALANINE

A new approach to the asymmetric synthesis of beta-substituted alpha-a minopropanoic acids by 2.2'-azoisobutyronitrile (AIBN) and Bu(3)SnH-in itiated radical addition of Etl, (PrBr)-Br-i, Bu(t)Br, and PhCH(2)Br t o the dehydroalanine moiety of the Ni-II complex 1 of a Schiff's base derived from (S)-o-[N-(N'-benzylprolyl)amino] benzophenone (BPB) and d ehydroalanine is described. The radical addition produced a mixture of diastereoisomeric complexes 4a-d with a 40-90% excess of (S,S)-diaste reoisomers over the (S,R)-ones, giving the reaction products in almost quantitative yields. The diastereoselectivity of the reaction depende d on the size of the entering radicals, the most effective asymmetric induction being achieved for the Bu(t) radical addition. Enantiomerica lly pure '(S)-2-amino-3-(tert-butyl)propanoic acid' [(S)-gamma-methyll eucine] and the chiral auxiliary BPB were recovered from compound 4d a fter its decomposition with HCl. The reactivities of the carbon-centre d radicals towards the carbon-carbon double bond in the amino acid moi ety of the complex 1 was quantitatively established by using ESR spect roscopy in the spin-trap technique.