Catalysis enantiomerically pure nucleophiles

Catalysis by acids, which is only rarely effective for aliphatic amines but better suited to the less basic aromatic amines [334], can promote nucleophilic attack at the most strongly polarized C-0 bond of the epoxide (Scheme 4.75) [333, 334, 339]. Vinyl epoxides react with amines in the presence of Pd(0) under mild conditions to yield allylamines [340], If such reactions are performed in the presence of an enantiomerically pure ligand, racemic vinyl epoxides can be converted into enantiomerically enriched products of nucleophilic ring opening (last example, Scheme 4.75). 

Catalytic Michael additions of a-nitroesters 38 catalyzed by a BINOL (2,2 -dihydroxy-l,r-bi-naphthyl) complex were found to yield the addition products 39 as precursors for a-alkylated amino acids in good yields and with respectable enantioselectivities (8-80%) as shown in Scheme 9 [45]. Asymmetric PTC (phase transfer catalysis) mediated by TADDOL (40) as a chiral catalyst has been used to synthesize enantiomeri-cally enriched a-alkylated amino acids 41 (up to 82 % ee) [46], A similar strategy has been used to access a-amino acids in a stereoselective fashion [47], Using azlactones 42 as nucleophiles in the palladium catalyzed stereoselective allyla-tion addition, compounds 43 were obtained in high yields and almost enantiomerically pure (Scheme 9) [48]. The azlactones 43 can then be converted into the a-alkylated amino acids as shown in Scheme 4. 

Enantioselective catalysis with nucleophiles that attack the metal of the intermediate rr-allylpal-ladium complexes prior to allylic C-C bond formation has met with only limited success so far30-31. Phenylation of the allyl acetates 1 and 4 via re-allyl complexes with a me,sY -7r-allyl ligand using phenylzinc chloride and enantiomerically pure monophosphines 231 or 53a yield 3 and 6 of low enantiomeric excess, respectively. 

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