Preparation of enantiopure pregabalin intermediate using cross linked enzyme aggregates (CLEAs) in basket reactor

Route to the synthesis of enantiomerically pure ethyl (S)-3-cyano-5-methylhexanoate, (S)-5, a key chiral intermediate for Pregabalin has been improved. The racemic β-cyano diester, 3 was prepared in 98% purity via gelatine catalysed Knoevenagel condensation of diethylmalonate with isovaleraldehyde followed by hydrocyantion of α,β-unsaturated diester 14 using acetone cyanohydrin and K2CO3. Racemic diethyl 2-(1-cyano-3-methylbutyl)malonate, rac-3, has been resolved using lipase from Thermomyces lanuginosus immobilised in form of crosslinked enzyme aggregates, CLEAs. The CLEAs were made by employing commercial soymilk as an additional protein source and a reaction was carried out in a moving basket reactor. The immobilised enzyme was found to be stable in many organic solvents and temperature up to 50 °C. The resolution reaction was studied in a basket reactor at 50% substrate loading in calcium acetate buffer, pH 7.5 at 30 °C by using 20% w/w enzyme loading. The apparent kinetic parameters were Vmax,app = (8.74 ± 0.43) mM/h/g and Km,app = (1.5 ± 0.07) M (correlation coeff. r = 0.98). The desired ethyl (S)-3-cyano-5-methylhexanoate, (S)-5 is obtained in 90-92% theoretical yield and e.e > 99%. The advantages of this improved process are mild reaction conditions; an alternate method for hydrocyanation step avoiding the use of highly toxic potassium cyanide at large scale operation and an immobilised enzyme that can be reused for at least 11 recycles.