Structures ofN-acetyl-DL-isoleucine,N-acetyl-DL-alloisoleucine and their ammonium salts; role of ammonium ions in crystal structure formation

Abstract

The crystal structures ofN-acetyl-DL-isoleucine,N-acetyl-DL-alloisoleucine and their ammonium salts show that these four compounds exist as racemic compounds around room temperature. The two ammonium salts are arranged around a 21screw axis, forming a helical column which consists of ammonium ions and single enantiomeric anions similar to the crystals of the ammonium salts of optically activeN-acetyl-L-isoleucine andN-acetyl-D-alloisoleucine. The ammonium ion and the carboxylate ion in the helix are connected by three hydrogen bonds, the fourth hydrogen bond being formed between the ammonium ion and an external acetyl amino group of the neighboring helical column. The fourth hydrogen bond is formed between the ammonium ion and an external acetyl amino group of the neighboring 21column. AmmoniumN-acetyl-DL-alloisoleucinate was revealed to exist as an unstable racemic compound due to conformational similarity between the racemic and optically active compounds in the solid state and was optically resolved by fractional crystallization at 293 K.