A key chiral intermediate [(3R-cis)-3-(acetyloxy)-4-phenyl-2-
azetidinone (2)] for the semi-synthesis of
paclitaxel (
taxol; 5), an anti-
cancer compound, was prepared by an enzymic process. The stereoselective enzymic hydrolysis of cis-3-(acetyloxy)-4-phenyl-2-azetidinone (1) to the corresponding (S)-(-)-alcohol (3) was carried out using various lipases.
Lipase PS-30 (Pseudomonas cepacia) and BMS (Bristol-Myers Squibb)
lipase (Pseudomonas sp. SC13856) catalysed hydrolysis of the undesired enantiomer of racemic compound 1, producing the (S)-(-)-alcohol (3) and the desired (R)-(+)-
acetate (2). Reaction yields of > 96% and optical purities of > 99.5% were obtained. For a very efficient
enzyme source (BMS
lipase), a
lipase fermentation using Pseudomonas sp. SC13856 was developed. In a fed-batch process using
soybean oil, the fermentation resulted in 1500 units of extracellular
lipase activity/ml. Crude BMS
lipase (1.7 kg, containing 140,000 units/g) was recovered from the filtrate by
ethanol precipitation. BMS
lipase and commercially available
lipase PS-30 were independently immobilized on Accurel
polypropylene. These immobilized lipases were re-used (ten cycles) without loss of
enzyme activity, productivity or optical purity of the product. The enzymic reaction process was scaled up to 75 and 150 litres using immobilized BMS
lipase and
lipase PS-30 respectively. From the reaction mixture, compound 2 was isolated in 88-90 mol% yield and 99.5% optical purity. A purity of 99.9 (area %) was demonstrated by g.c. for isolated compound 2.