N-Acetyl-3,5-dibromo-l-tyrosine hemihydrate

The title compound, C(11)H(11)Br(2)NO(4)·0.5H(2)O, was prepared by an electrophilic bromination of N-acetyl-l-tyrosine in acetonitrile at room temperature. The two independent mol-ecules do not differ substanti-ally and a mol-ecule of water completes the asymmetric unit. The synthesis of the title compound does not modify the stereochemical center, as shown by the absolute configuration found in this crystal structure. Comparison with the non-bromo starting material differs mainly by rotation features. For instance the H(methine)-C(chiral center)-C(methyl-ene)-C(ipso) is 173.0 (2)° torsion angle in one mol-ecule and 177.3 (2)° in the other, indicating a trans arrangement. This is in contrast with approximately 50° in the starting material. A short inter-molecular Br?Br separation is observed [3.2938 (3) Å]. The molecules in the crystal are connected via a network of hydrogen bonds through an N-H?O hydrogen bond between the hydroxy group of the phenol of the tyrosine and the N-H of the amide of the other molecule and an O-H?O hydrogen bond between the hydroxy group of the carboxylic acid and the oxygen of the carbonyl of the amide.