Comparative study on the use of ortho-phthalaldehyde, naphthalene-2,3-dicarboxaldehyde and anthracene-2,3-dicarboxaldehyde reagents for alpha-amino acids followed by the enantiomer separation of the formed isoindolin-1-one derivatives using quinine-type chiral stationary phases

Cinchona alkaloid based chiral stationary phases (CSPs) were evaluated and compared for the enantiomer separation of a set of alpha-amino acid derivatives as selectands (SA), using ortho-phthalaldehyde (OPA), naphthalene-2,3-dicarboxaldehyde (NDA) and anthracene-2,3-dicarboxaldehyde (ADA) as reagents in the presence of acetonitrile. Protocols have been developed for the derivatization of most common amino acids in the absence of the usual thiol components (2-mercaptoethanol, mercaptosulphonic acid, sodium sulfite) under acidic and neutral conditions providing the corresponding isoindolin-1-one (phthalimidine) derivatives. They are stable for hours at various reaction conditions compared to thiol or sulfide modified isoindoles resulted by the OPA-thiol reaction type. Among the derivatizing agents, ADA afforded the highest retention factors (k) and for the majority of the analytes also resolution (Rs) and enantioselectivity (alpha) values (i.e. for tryptophan k1 = 23, Rs = 4.93 and alpha = 1.43). Structure variation of the CSPs and selector (SO), respectively indicates that steric arrangement around the binding cleft plays a major role in the enantiodiscriminating events. To provide more detailed information about the derivatization reaction itself, the proposed mechanism for the formation of the OPA derivative (isoindolin-l-one) was further evaluated by deuterium labeling and LC-MS analysis.