Efficient asymmetric synthesis of 2,3-diamino-3-phenylpropanoic acid derivatives

An efficient, stereoselective synthesis of selectively-protected anti and syn, methyl 2-amino-3-(Boc-amino)-3-phenylpropanoate is described. Preparation of syn β-acetylamino-α-hydroxy ester was from isopropyl cinnamate via an acetamide-based Sharpless aminohydroxylation (AA), and its anti isomer was obtained via C-α epimerization of the syn isomer. For the installation of a second amino group, two different approaches, involving substitution of the β-hydroxyl group with azide, were investigated. The first was a ring-opening reaction of trans-oxazoline-5-carboxylate with trimethylsilyl azide, which produced anti β-(acetylamino)-α-azido esters, which then transformed into the anti isomer; whereas the cis-oxazoline-5-carboxylate was found to be unreactive under this reaction condition. The second approach used the Mitsunobu reaction of syn and anti β-(Boc-amino)-α-hydroxy esters with hydrazoic acid, followed by catalytic hydrogenation, which gave both anti and syn isomers, respectively.