Solid-phase synthesis of chiral stationary phases based on 2,4,5,6-tetrachloro-1,3-dicyanobenzene derivatives spaced from N-3,5-dinitrobenzoyl alpha-amino acids: comparative study of their resolution efficacy

Two new chiral stationary phases, 3-5-chloro-1,3-dicyano-2,4-2'-(N'-1,3-dinitrobenzoyl-D-phenylglycinyl) aminoethyl]aminophen-1-yl] aminopropyl silica (CSP-1) and 3-5-chloro-1,3-dicyano-2,4-2'-(N'-1,3-dinitrobenzoyl-L-leucinyl) aminoethyl] aminophen-1-yl] aminopropyl silica (CSP-2), were prepared by solid-phase synthesis. They comprise chiral unit, 3,5-dinitrobenzoyl derivative of the amino acid, D-PhGly or L-Leu, bound via spacer 1,2-diaminoethane to 2,4-positions of the persubstituted benzene ring, derived from compound 1, and possess pseudo-C2 symmetry. Preparation of model compounds 6 and 7 confirmed the structure of chiral selectors, which comprise pi-donor persubsituted aromatic ring and two strong pi-acceptor 3,5-dinitrobenzoyl amido units. CD spectra of model selectors 6 and 7, run in DMSO above 250 nm, exhibit negative exciton coupling (EC) between pi-acceptor and pi-donor chromophores, C(1) symmetric model compound 8 exhibited much weaker EC and 9, devoid of pi-donor unit, does not exhibit any significant CD. Combined pi-donor and pi-acceptor properties enable the new CSPs to separate a broad range of racemates. The columns with CSP-1 and CSP-2 were tested for the separation of 22 racemates by HPLC with two different mobile phase systems and the results are compared with those obtained by using a structurally related commercial column.