Regioselective saponification

Figure 21 Esterase-mediated regioselective saponification of a dicaiboxylic acid ester.

Figure 24 Regioselective saponification of acylated methyl xylopyranosides, catalyzed by esterase.

Carbonylat ion of 13 4-nitrophenyloxycarbonyl chloride (J 6) in pyridine] gave a good yield of the trityl ether-carbonate 14 which was de-tritylated by hydrogenolysis (2 4) to afford the 7,8-diol-4,5-carbonate 15 in practically quantitative yield. Compound 15.. by way of its 7,8-0 -dibutylstannylidene derivative (25> 26) was regioselectively 8-0-benzylated (dibutyltin oxide, benzene, reflux then benzyl bromide, dimethyl formamide, 95 ), in fair yield (J ). Finally, the aglycon U3 was obtained in high yield by sequential 7-Q-tert.-butyldimethylsilylation (27) (compound 17) and Zemplen saponification. 

In order to introduce the carboxylic function, azide 30 was regioselectively debenzylated at the primary hydroxyl group by controlled treatment with acetic anhydride and trifluoroacetic acid, (14) followed by saponification of the acetate at C-5. The product 31 was finally converted into the carboxylic acid 32 by Jones oxidation. 

Saponification of steroid esters can also be steered with Candida cylindracea lipase (CCL),272, 273]. This process occurs in the presence of octanol in organic solvents and is characterized by a pronounced stereospecificity and regioselectivity. Thus, the 3a-... 

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