Lipase-catalyzed regioselective hydrolysis of 3(5)-methylpyrazole-N- carboxylates in water-saturated organic solvents

Pure N-substituted-azole regioisomers containing substituents on the C-atom of azole ring may exert different biological activities, just like pure enantiomers do in comparison with their antipodes. It is therefore highly desirable that they can be effectively synthesized from their precursors or separated from regioisomeric mixtures. With lipase-catalyzed hydrolysis of (R)-1-phenylethyl 3(5)-methylpyrazole-N-carboxylate mixture (1a and 1b) in water-saturated organic solvents as the model system, excellent regioselectivity (V_1a/V_1b > 100) for 3-methylpyrazole-N-carboxylate (1a) is obtainable when selecting Novozym 435 from Candida antartica lipase B, but not Lipase MY-30 from Candida rugosa and Lipase PS-D from Pseudomonas cepacia, as the biocatalyst. Increasing of solvent hydrophobicity and temperature, or decreasing of water content, may enhance the enzyme activity but not regioselectivity. A change of the 1-phenylethyl moiety to an isopropyl or straight alkyl chain is longer than propyl for other substrates also results in excellent regioselectivity. The biocatalysis is successfully extended to the hydrolysis of (R)-1-phenylethyl 3(5)-methyl-4-bromopyrazole-N-carboxylate mixture (8a and 8b) with V_8a/V_8b > 100 for preparing pure 5-methyl-4-bromopyrazole-N-carboxylate.