NITRILE OXIDES IN MEDICINAL CHEMISTRY .4. CHEMOENZYMATIC SYNTHESIS OF CHIRAL HETEROCYCLIC-DERIVATIVES

The two enantiomers of 3-bromo-5-(hydroxymethyl)-DELTA-2-isoxazoline (1) and 2-phenyl-5-(hydroxymethyl)-isoxazolidin-3-one (9) have been prepared in enantiomeric excess higher than 90% by hydrolysis of the corresponding butyrates under the catalysis of lipase PS, which was the most selective catalyst of the enzymes tested. The pairs of enantiomers of 1 and 9 were transformed into the chiral forms of the potent muscarinic ligands 3 and 5. The results obtained with the homogeneous set of esters 6, 7, 10a-d evidence a strong dependence of reaction rate and enantioselectivity of the lipase PS-catalyzed transformations upon both the size of the acyl moiety and the shape of the group carrying the alcoholic part of the ester. In the series of esters 10a-d, the best results were obtained with butyrate 10b. Quite interestingly, on passing from the butyrate of 1 to that of 9, the value of the enantiomeric ratio remained remarkably high but the enantiopreference switched from R to S. In between lies the butyrate of 2-methyl-5-(hydroxymethyl)isoxazolidin-3-one [(+/-)-6] which was barely recognized by lipase PS and yielded alcohol (R)-(-)-2 in a modest enantiomeric excess.