α-Alkylation of amino acid derivatives: Synthesis and chiral resolution of [C]β-aminoisobutyric acid

A synthesis of ¹¹C-labeled β-aminoisobutyric acid (¹¹C]β-AIB) and its enantiomeric resolution by high performance liquid chromatography (HPLC) are reported. β-Alanine ethyl ester 2 was converted to benzaldimine-β-alanine ethyl ester 3 in 87% yield. Treatment of the imine derivative 3 with lithium diisopropylamide (1.1 eq) in tetrahydrofuran at −78 °C, followed by addition of cold iodomethane (1.1 eq) produced the α-methylated benzaldimine-β-alanine ethyl ester 4 in 73% chemical yield. Deprotection of the amino group by acidic hydrolysis followed by basic hydrolysis of the ester group produced the desired product 1 in 37% chemical yield. Labeling was accomplished using ¹¹C]methyl iodide. The radiola-beled product was purified by HPLC using a semipreparative reversed-phase C-18 column and 2 mM phosphate buffer (pH 5.9) as the mobile phase. The synthesis time was 35–40 min including HPLC purification, with 20–60% radiochemical yield (decay corrected). Radiochemical purity was >99%, with average specific activity being 450 mCi/μmol. Enantiomers of β-AIB were well separated by analytical HPLC using a chiral column and aqueous perchloric acid as the mobile phase. (S)-β-AIB was eluted at 17.4 min and the (R)-enantiomer was eluted at 20.0 min when the jacketed column was maintained at low temperature by circulation of ice-cold water, and the pH of the mobile phase was 1.05.