A simple preparation of d-alloisoleucine from l-isoleucine via acetylation and separation of the ammonium salts

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

“…In the crystals of metal complexes containing Ile moieties, no B-type conformation has yet been observed, but A-type and C-type conformations are seen with similar probabilities (Cai et al, 2000;Rao et al, 2008;Li et al, 2009;Jing et al, 2011;Li et al, 2012;Wang et al, 2012;Shabari et al, 2012;Tašner et al, 2013). In organic compounds and salts, the A-type conformation is more common than the other two types (Trommel & Bijvoet, 1954;Benedetti et al, 1973;Cotrait et al, 1979;Dalhus & Gö rbitz, 2000;Gö rbitz & Hartviksen, 2006;Yamada et al, 2008;Yajima et al, 2008;Akazome et al, 2010). Such variations of conformation arise from the conformation around the C -C bond, which may be readily influenced by interactions between the N atom and the carboxy group connected to the C atom.…”

“…The separation of enantiomeric pairs dl-Ile and dl-alloisoleucine [(2RS,3SR)-(1); dl-aIle] may appear to be easy, but recrystallizing a mixture of four stereoisomers did not give a simple racemate, dl-Ile or dl-aIle, because these stereoisomers form molecular compounds consisting of various combinations of stereoisomers (Dalhus & Gö rbitz, 2000). Previously we revealed that separation of N-acetyl-l-isoleucine (Ac-l-Ile) and N-acetyl-d-alloisoleucine (Ac-d-aIle) by recrystallization is also difficult due to the formation of a molecular compound, whereas their ammonium salts are easily separated by recrystallization from 95% ethanol because they do not form molecular compounds (Yajima et al, 2008(Yajima et al, , 2009. Additionally, some ammonium salts of N-acylated amino acids have been found to exist as conglomerates, which were resolved optically by preferential crystallizations and replacing crystallizations (Shiraiwa et al, 1986(Shiraiwa et al, , 1987(Shiraiwa et al, , 1991ISSN 2052-5206 # 2016 International Union of Crystallography Yajima et al, 2010).…”

Structures and Properties of a Diastereoisomeric Molecular Compound of (2S,3S)- and (2R,3S)-N-Acetyl-2-amino-3-methylpentanoic Acids

“…The solubility in water of mixtures of 1 and 2 at various ratios is shown in the ternary phase solubility diagram (Fig. 6) which indicates that a mixture with a molar ratio around 0.5 may give molecular compound 3 11) this giving crystals of 3 and an Ac-L-Ile-excess solution by acidification in water (vide supra).…”

“…On the other hand, a diastereoisomeric mixture can be easily separated by managing the chiral derivatives of amines 9) or tartaric acid, 10) but the diastereoisomers can be separated without using chiral resolving reagents, as we have shown in the previous report. 11) In general, the solubility and crystal properties of a single enantiomer and its racemates have strong influence on the separation and optical resolution into a single enantiomer. A racemate existing as a conglomerate can be separated into a single enantiomer by preferential crystallization or replacing crystallization.…”

Preparation of Optically Active Allothreonine by Separating from a Diastereoisomeric Mixture with Threonine

“…N-Acetylated amino acids tend to form good crystalline ammonium salts; for example, the ammonium salts of N-acetyl-D-alloisoleucine and N-acetyl-L-isoleucine can be separated from their mixture by recrystallization. 14,15) The networks of hydrogen bonds with the ammonium ions in the crystals of these ammonium salts determine the rigidity of the crystal packing, so that a difference in the side chain conformation of the amino acids would greatly affect their solubility. An ammoniacal aqueous solution of a mixture of Ac-L-Thr and Ac-D-aThr was evaporated to dryness in vacuo to give a mixture of the ammonium salts of Ac-L-Thr and Ac-D-aThr (Ac-L-Thr .…”