135981-02-1

Both diastereoisomers of 2-amino-3-hydroxybutanoic acid and 2-amino-3-hydroxy-3-phenylpropanoic acid have been prepared from enantiopure α-hydroxy-β-amino esters via the intermediacy of the corresponding cis- and trans-aziridines. Aminohydroxylation of two α,β-unsaturated esters produced enantiopure 2,3-anti-α-hydroxy-β-amino esters in >99:1 dr. Subsequent epimerisation at the C(2)-position via a sequential oxidation/diastereoselective reduction protocol gave the corresponding enantiopure 2,3-syn-α-hydroxy-β-amino esters in >99:1 dr. These syn- and anti-substrates were then converted into the corresponding N-Boc protected cis- and trans-aziridines, respectively, via a three step reaction sequence: (i) hydrogenolysis and in situ N-Boc protection; (ii) OH-activation; and (iii) aziridine formation. Subsequent regioselective ring-opening of the C(3)-methyl-aziridines with Cl3CCO2H proceeded with inversion of configuration to give the corresponding 2-amino-3-trichloroacetate esters, whereas the analogous reaction with the C(3)-phenyl-aziridines resulted in rearrangement to the corresponding oxazolidin-2-ones with retention of configuration. In each case, hydrolysis of the products from these ring-opening reactions produced the corresponding enantiopure β-hydroxy-α-amino acids as single diastereoisomers.

The highly diastereoselective conjugate addition of lithium N-benzyl-N-α-methylbenzylamide with enoate acceptors, and the electrophilic hydroxylation of the resultant β-amino enolates with (camphorsulfonyl)oxaziridine, is identified as a direct and general strategy for the asymmetric synthesis of homochiral β-amino-α-hydroxy acids and their derivatives.A structurally diverse array of β-amino enolate substrates can be hydroxylated with generally excellent anti diastereoselectivity (>90percent d.e.) using this protocol; an alternative stepwise hydroxylation procedure, where the β-amino enolate is prepared by enolisation of the preformed conjugate adduct is also found to lead to formation of the anti diastereomer.The diastereofacial selectivity of enolate hydroxylation appears to be under predominantly substrate-controlled asymmetric induction, although a measurable degree of chirality predominantly substrate-controlled asymmetric induction, although a measurable degree of chirality recognition with the oxaziridine reagent can be observed.Homochiral β-amino-α-keto esters are also prepared and their stereoselective reductions examined.

New methodology is described for the enantioselective coupling of t-butyl bromoacetate with aldehydes to give anti-α-bromo β-hydroxy esters (1), useful precursors of chiral glycidic esters (2), acetate aldols (3), β-amino acid esters (4) and α-amino acid