Eduardo Busto

Hydrolases: catalytically promiscuous enzymes for non-conventional reactions in organic synthesis.

During the last three decades the use of hydrolases for the catalysis of environmentally friendly organic processes under mild reaction conditions has been well documented. Hydrolases have shown themselves to be ideal tools for the acceleration of synthetic transformations because of their high stability, catalytic efficiency, commercial availability and broad substrate specificity in a wide spectrum of biocatalyzed processes. In recent years, novel examples have appeared related to non-conventional reactions catalyzed by hydrolytic enzymes. Amongst these, lipases and acylases have gained much attention as promiscuous biocatalysts showing good levels of reactivity in C-C bond formation, C-heteroatom bond formation, oxidative processes, and novel hydrolytic reactions. This critical review covers recent investigation in the field of catalytic promiscuity, and highlights the most surprising and uncommon activities that this class of enzymes shows in organic synthetic transformations (111 references).

Chemoenzymatic synthesis of rivastigmine based on lipase-catalyzed processes.

A straightforward chemoenzymatic synthesis of enantiomerically pure rivastigmine has been efficiently carried out under mild reaction conditions, with Candida antarctica lipase B responsible for the stereoselective acetylation of the corresponding (R)-alcohol or amine. An exhaustive enzymatic study has been developed exploring the possibilities of carry out enzyme recycling, scaling up the enzymatic process and development of a dynamic kinetic resolution procedure for the production of adequate enantiomerically pure precursors of rivastigmine. Total chemoenzymatic synthesis of this pharmaceutical has been performed in good overall yield from commercially available 3-methoxyacetophenone.

From salts to ionic liquids by systematic structural modifications: a rational approach towards the efficient modular synthesis of enantiopure imidazolium salts.

This paper reports a simple and robust modular synthetic strategy that leads to a large variety of configurationally and structurally diverse imidazole-based chiral ionic liquids (CILs) by lipase-catalyzed resolution. The intimate microscopic interactions of the supramolecular ionic network of these imidazolium chiral salts at the molecular level are investigated both spectroscopically (NMR, FT-IR-ATR) and theoretically, and a topological analysis of the experimental electron densities obtained by X-ray diffraction of single crystals is performed. Our results support the key role played by the relative configuration of the -OR group on the hydrogen-bonding pattern and its strong influence on the final physical properties of the imidazolium salt. We also obtained a reasonable correlation between the observed melting point and the non-covalent interactions. The spectroscopic data and the topological analysis reflect the key role played by hydrogen bonds between the OH and imidazolium C2H groups in both cation-anion and cation-cation interactions, with the presence of an OH group leading to an additional inter-cation interaction. This interaction significantly affects the properties of stereoisomeric salts. Even more interestingly, we also studied the effect of the chirality by comparing enantiopure CILs with their racemic mixtures and found that, with the exception of trans-Cy6-OH-Im-Bn-Br, the melting points of the racemic mixtures are higher than those of the corresponding enantiomerically pure forms. For stereoisomeric examples, we have successfully explained the differences in melting temperatures in light of the corresponding structural data. Chirality should therefore be taken into account as a highly attractive design vector in the preparation of ILs with specifically desired properties.

Polymeric imidazolium ionic liquids as valuable stationary phases in gas chromatography: chemical synthesis and full characterization.

Seven new functionalized polymerizable ionic liquids were chemically prepared, and later applied for the preparation of polymeric stationary phases in gas chromatography. These coated GC columns, which exhibited good thermal stabilities (240-300°C) and very high efficiencies (3120-4200 plates/m), have been characterized using the Abraham solvation parameter model. The chromatographic behavior of these polymeric IL columns has been deeply studied observing excellent selectivities in the separation of many organic substances such as alkanes, ketones, alcohols, amines or esters in mixtures of polar and non polar solvents or fragrances. Remarkably, the challenging separation of xylene isomers has been possible using a bis(trifluoromethylsulfonyl)amide based imidazolium IL coated column as a gas chromatography stationary phase.

Asymmetric chemoenzymatic synthesis of miconazole and econazole enantiomers. The importance of chirality in their biological evaluation.

A simple and novel chemoenzymatic route has been applied for the first time in the synthesis of miconazole and econazole single enantiomers. Lipases and oxidoreductases have been tested in stereoselective processes; the best results were attained with oxidoreductases for the introduction of chirality in an adequate intermediate. The behaviors of a series of ketones and racemic alcohols in bioreductions and acetylation procedures, respectively, have been investigated; the best results were found with alcohol dehydrogenases A and T, which allowed the production of (R)-2-chloro-1-(2,4-dichlorophenyl)ethanol in enantiopure form under very mild reaction conditions. Final chemical modifications have been performed in order to isolate the target fungicides miconazole and econazole both as racemates and as single enantiomers. Biological evaluation of the racemates and single enantiomers has shown remarkable differences against the growth of several microorganisms; while (R)-miconazole seemed to account for most of the biological activity of racemic miconazole on all the strains tested, both enantiomers of econazole showed considerable biological activities. In this manner, (R)-econazole showed higher values against Candida krusei , while higher values were observed for (S)-econazole against Cryptococcus neoformans, Penicillium chrysogenum, and Aspergillus niger.

Straightforward Synthesis of Enantiopure 2,3-Dihydrobenzofurans by a Sequential Stereoselective Biotransformation and Chemical Intramolecular Cyclization

A new family of optically active 2,3-dihydrobenzofurans has been prepared by a simple chemoenzymatic asymmetric strategy. This synthetic approach is based on the combination of a lipase-mediated kinetic resolution of 1-aryl-2-propanols or bioreduction of the corresponding ketones followed by an intramolecular cyclization reaction. These novel compounds have been prepared in enantiopure form and in good overall yield through a straightforward route.

Enzymatic Desymmetrization of Prochiral 2-Substituted-1,3-Diamines : Preparation of Valuable Nitrogenated Compounds

A wide range of prochiral 1,3-diamines were first efficiently synthesized and subsequently desymmetrized by using lipase from Pseudomonas cepacia as catalyst and diallyl carbonate as alkoxycarbonylating agent. In all cases, the amino carbamates of R-configuration were recovered. Final selective cleavage of the N-allyloxycarbonyl moiety was carried out under mild reaction conditions, which demonstrates the high versatility and potential of this chemoenzymatic route as a source of intermediates in the synthesis of related optically active nitrogenated derivatives.

One-Pot Synthesis of Enantiopure 3,4-Dihydroisocoumarins through Dynamic Reductive Kinetic Resolution Processes

A straightforward chemoenzymatic synthesis of enantiopure 4-alkyl-3-methyl-3,4-dihydroisocoumarins through a ketoreductase-catalyzed one-pot dynamic reductive kinetic resolution is reported. E. coli/ADH-A cells have shown outstanding diastereo- and enantioselectivity toward the bioreduction of a series of racemic ketones, with the use of anion exchange resins or triethylamine being compatible in the same aqueous reaction medium. The so-obtained enantiopure alcohols were subsequently cyclized in acid media affording the corresponding lactones in good to excellent conversions (72-96%) and excellent selectivities (dr ≥99:1 and ee >99%).

Asymmetric chemoenzymatic synthesis of ramatroban using lipases and oxidoreductases.

A chemoenzymatic asymmetric route for the preparation of enantiopure (R)-ramatroban has been developed for the first time. The action of lipases and oxidoreductases has been independently studied, and both were found as excellent biocatalysts for the production of adequate chiral intermediates under very mild reaction conditions. CAL-B efficiently catalyzed the resolution of (±)-2,3,4,9-tetrahydro-1H-carbazol-3-ol that was acylated with high stereocontrol. On the other hand, ADH-A mediated bioreduction of 4,9-dihydro-1H-carbazol-3(2H)-one provided an alternative access to the same enantiopure alcohol previously obtained through lipase-catalyzed resolution, a useful synthetic building block in the synthesis of ramatroban. Inversion of the absolute configuration of (S)-2,3,4,9-tetrahydro-1H-carbazol-3-ol has been identified as a key point in the synthetic route, optimizing this process to avoid racemization of the azide intermediate, finally yielding (R)-ramatroban in enantiopure form by the formation of the corresponding amine and the convenient functionalization of both exocyclic and indole nitrogen atoms.

Biocontrolled Formal Inversion or Retention of L‐α‐Amino Acids to Enantiopure (R)‐ or (S)‐Hydroxyacids

Natural L-α-amino acids and L-norleucine were transformed to the corresponding α-hydroxy acids by formal biocatalytic inversion or retention of absolute configuration. The one-pot transformation was achieved by a concurrent oxidation reduction cascade in aqueous media. A representative panel of enantiopure (R)- and (S)-2-hydroxy acids possessing aliphatic, aromatic and heteroaromatic moieties were isolated in high yield (67-85 %) and enantiopure form (>99 % ee) without requiring chromatographic purification.