Optimization of the direct chiral separation of potential cytotoxic α-methylene-γ-butyrolactones and α-methylene-γ-butyrolactams by liquid chromatography

Abstract

The direct enantiomeric resolution of a series of racemic α-methylene-γ-butyrolactones and α-methylene-γ-butyrolactams was carried out on various commercially available chiral stationary phases (CSPs). Particular interest was paid to compounds which exhibit physiological activity as cytotoxic agents. On a Pirkle-type column packed with ( R)-N-(3,5-dinitrobenzoyl)phenylglycine (DNBPG) covalently bonded to γ-aminopropylsilanized silica gel, only α-methylene-γ-lactams containing two aromatic groups were resolved; a chiral recognition mechanism is proposed. (+)-Poly-(triphenylmethyl methacrylate) (PTrMA) coated on silica gel [Chiralpak OT(+)] with methanol as eluent afforded fairly good selectivities (up to 1.8) especially for α-methylene γ-lactones. Temperature had a great influence on the resolution. Cellulose tribenzoate and triphenylcarbamate (Chiralcel OB and OC, respectively) coated on macroporous silica gel displayed selectivities from 1.1 to 1.3, but, because of the very poor efficiency of these CSPs, no baseline resolution was achieved. Investigations on two protein CSPs are reported, α 1-acid glycoprotein ( α 1-AGP) and bovine serum albumin (BSA) immobilized on microparticulate silica gel (Enantiopac and Resolvosil-BSA, respectively). The separations were performed using an aqueous sodium phosphate buffer as eluent and 2-propanol as organic modifier to regulate retention and selectivity. Both CSPs exhibited a good chiral recognition ability, especially towards cytotoxic solutes with resolution factors between 1 and 2.