The absolute configuration of an intermediate in the asymmetric synthesis of unusual amino acids.

(4R)-3-[(2'R,3'R)-2'-Bromo-3'-(phenylbutanoyl)]-4-(phenylmethyl)-2 - oxazolidinone, C20H20Br-NO3, M(r) = 402.30, monoclinic, P2(1), a = 11.542 (2), b = 7.625 (1), c = 11.667 (1) A, beta = 113.97 (1) degrees, V = 938.2 (2) A3, Z = 2, Dx = 1.42 g cm-3, lambda(Mo K alpha) = 0.71073 A, mu = 21.8 cm-1, F(000) = 412, T = 296 +/- 1 K, final R = 0.028 for 2369 observed reflections. Since a D-chiral auxiliary was used the configuration at the alpha-carbon was R as expected. The two carbonyls are aligned in opposite directions to each other to overcome van der Waals repulsions.