Linear oligopeptides. Part 147. A chemical and crystallographic study of the reaction between benzyloxycarbonyl chloride and alpha-aminoisobutyric acid

From the reaction mixture of benzyloxycarbonyl chloride and alpha-aminoisobutyric acid three crystalline compounds were isolated and characterized by chromatographic and spectroscopic techniques and X-ray diffraction. Two of them are polymorphic forms of the N-protected amino acid, which differ in the orientation of the phenyl ring relative to the urethane moiety and in the packing modes of the molecules, including the intermolecular hydrogen-bonding arrangements. The third compound is the N-protected dipeptide, the formation of which is not unexpected in the synthesis of the N-benzyloxycarbonyl amino acid derivative. The crystal structure of the symmetric carboxylic anhydride from the N-protected amino acid, a reasonable reactive intermediate in the formation of the N-protected dipeptide, is also described.