Preparation of chiral building blocks for the enantioselective total synthesis of ent-kauranoids by the pig liver esterase-catalyzed asymmetric hydrolysis of a dialkyl malonate-type prochiral diester

Syntheses of polynorbornadienes by ring-opening metathesis polymerizations of symmetric and non-symmetric 2,3-bis(alkoxycarbonyl)norbornadienes and their conversion to half-ester derivatives

Libraries of polynorbornadienes were synthesized with good yields with a ruthenium-containing 2nd generation Grubbs catalyst by ring-opening metathesis polymerization (ROMP) of a variety of symmetric and non-symmetric 2,3-bis(alkoxycarbonyl)norbornadiene monomer units prepared from the half-esters obtained efficiently by the selective monohydrolysis reactions of symmetric diesters we reported earlier. Among these polymers, the polynorbornadienes with t-butoxycarbonyl groups derived from non-symmetric monomer units were converted to the half-ester derivatives by deprotection with trifluoroacetic acid, yielding amphiphilic polymers. The hydrogenation reactions of the obtained polymers were carried out to yield polymers having saturated structures in the main chains for improvement of the thermal stabilities. All these polymers were characterized by their molecular weights and thermal properties along with the spectroscopic data. Our selective monohydrolysis reactions have been proven to be a versatile tool for production of relatively homogeneous polymer libraries.

Preparation of Sterically Demanding 2,2-Disubstituted-2-Hydroxy Acids by Enzymatic Hydrolysis

Preparation of optically-pure derivatives of 2-hydroxy-2-(3-hydroxyphenyl)-2-phenylacetic acid of general structure 2 was accomplished by enzymatic hydrolysis of the correspondent esters. A screening with commercial hydrolases using the methyl ester of 2-hydroxy-2-(3-hydroxyphenyl)-2-phenylacetic acid (1a) showed that crude pig liver esterase (PLE) was the only preparation with catalytic activity. Low enantioselectivity was observed with substrates 1a–d, whereas PLE-catalysed hydrolysis of 1e proceeded with good enantioselectivity (E = 28), after optimization. Enhancement of the enantioselectivity was obtained by chemical re-esterification of enantiomerically enriched 2e, followed by sequential enzymatic hydrolysis with PLE. The preparation of optically-pure (S)-2e was validated on multi-milligram scale.

Regioselective Hydrolysis and Transesterification of Dimethyl 3-Benzamidophthalates Assisted by a Neighboring Amide Group

An efficient, highly regioselective hydrolysis and transesterification of dimethyl 3-benzamidophthalates into the corresponding carboxylic acid monoesters and mixed esters (including tert-butyl esters) under basic conditions is presented. The selectivity is governed by the neighboring 3-benzamido moiety's participation and by the nature of the solvent. In alcohols the reaction occurred exclusively at the ortho-position to the benzamido functionality, in pyridine or acetonitrile at both ester groups. An insight into the mechanistic pathway was obtained from a (1)H NMR study in perdeuteromethanol.

Esterases as stereoselective biocatalysts

Non-lypolitic esterases are carboxylester hydrolases with preference for the hydrolysis of water-soluble esters bearing short-chain acyl residues. The potential of esterases as enantioselective biocatalysts has enlarged in the last few years due to the progresses achieved in different areas, such as screening methodologies, overproduction of recombinant esterases, structural information useful for understanding the rational behind enantioselectivity, and efficient methods in protein engineering. Contributions of these complementary know-hows to the development of new robust enantioselective esterases are critically discussed in this review.