RESUMEN
The paper describes the first synthesis of the enantiomerically pure cis-alpha,beta-propanoleucines 6c and 6d by means of asymmetric Strecker synthesis. Furthermore, an improved procedure for the preparation of the stereomeric trans compounds 6a and 6b is proposed. Finally, the four feasible stereomeric alpha,alpha-quaternary-alpha-amino acids are resolved on a penicillamine based chiral stationary phase allowing the determination of ee values ranging from 92.9% to > 98%.
Asunto(s)
Concentración de Iones de Hidrógeno , Leucina/análogos & derivados , Leucina/síntesis química , Cromatografía Líquida de Alta Presión , Indicadores y Reactivos , Espectroscopía de Resonancia Magnética , EstereoisomerismoRESUMEN
High-performance liquid chromatographic (HPLC) separation of stereoisomeric cyclic beta-substituted alpha-quaternary alpha-amino acids was performed by ligand-exchange on a copper(II)-D-penicillamine chiral stationary phase. The investigated amino acids are the 1-amino-2-methylcyclohexanecarboxylic acids, the 1-amino-2-hydroxycyclohexanecarboxylic acids, the 1-amino-2-methylcyclopentanecarboxylic acids and the trans-configured 1,2-diaminocyclohexanecarboxylic acids. The effects of the mobile phase composition (copper(II) concentration, type and content of organic modifier, pH) and the temperature on the enantio- and diastereoselectivity were studied and the conditions were optimised to resolve the four stereoisomers of each of the said amino acids in single chromatographic runs. A reversal of the elution order occurred for enantiomers of some of the amino acids in dependence on the acetonitrile content of the eluent. This phenomenon is explained by at least two different copper(II) complexes of the tridentate ligand penicillamine.
Asunto(s)
Aminoácidos Cíclicos/aislamiento & purificación , Cromatografía Líquida de Alta Presión/métodos , Penicilamina/química , Aminoácidos Cíclicos/química , EstereoisomerismoRESUMEN
Three of ten sesquiterpene lactones isolated from the herb of Arctotis fastuosa (Asteraceae) belong to the heliangolide series. The first was identified as hirsutolide, the second is new and named dihydrohirsutolide, and the third, dichloromethyldihydrohirsutolide, is an artifact. The known degraded carotenoid loliolide was also isolated. Structures were established by spectroscopic methods including IR, 1D-/2D-1H-NMR, 13C-NMR, GHSQC, GHMBC, HREIMS and CD.