Novel chemoenzymatic oxidation of amines into oximes based on hydrolase-catalysed peracid formation.

The efficient transformation of benzylamines into the corresponding oximes has been described by means of a chemoenzymatic process. This strategy is based on a two-step sequence developed in one-pot at 30 °C and atmospheric pressure. First, the formation of a reactive peracid intermediate occurs by means of a lipase-catalysed perhydrolysis reaction, and then this peracid acts as a chemical oxidising agent of the amines. A total of nine ketoximes were isolated in high purity after a simple extraction protocol (90-98% isolated yield), while for the eleven synthesised aldoximes a further column chromatography purification was required (71-82% isolated yield). In all cases excellent selectivities were attained, offering a practical method for amine oxidation in short reaction times (1 hour). The environmental impact of the process was analysed and compared with a recently published alternative chemical synthesis, finding for this metric a good E-factor value.

Stereoselective Access to 1-[2-Bromo(het)aryloxy]propan-2-amines Using Transaminases and Lipases; Development of a Chemoenzymatic Strategy Toward a Levofloxacin Precursor.

Two independent enzymatic strategies have been developed toward the synthesis of enantioenriched 1-[2-bromo(het)aryloxy]propan-2-amines. With that purpose a series of racemic amines and prochiral ketones have been synthesized from commercially available 2-bromophenols or brominated pyridine derivatives bearing different pattern substitutions in the aromatic ring. Biotransamination experiments have been studied using ketones as starting materials, yielding both the (R)- and (S)-amine enantiomers with high selectivity (91-99% ee) depending on the transaminase source. In a complementary approach, the classical kinetic resolutions of the racemic amines have been investigated using Candida antarctica lipase type B as biocatalyst. Ethyl methoxyacetate was found as a suitable acyl donor leading to the corresponding (S)-amines (90-99% ee) and (R)-amides (88-99% ee) with high selectivity in most of the cases. A preparative biotransamination process has been developed for the synthesis of (2S)-1-(6-bromo-2,3-difluorophenoxy)propan-2-amine in 61% isolated yield after 24 h, a valuable precursor of the antimicrobial agent Levofloxacin.

Catalytic Promiscuity of Transaminases: Preparation of Enantioenriched ß-Fluoroamines by Formal Tandem Hydrodefluorination/Deamination.

Transaminases are valuable enzymes for industrial biocatalysis and enable the preparation of optically pure amines. For these transformations they require either an amine donor (amination of ketones) or an amine acceptor (deamination of racemic amines). Herein transaminases are shown to react with aromatic ß-fluoroamines, thus leading to simultaneous enantioselective dehalogenation and deamination to form the corresponding acetophenone derivatives in the absence of an amine acceptor. A series of racemic ß-fluoroamines was resolved in a kinetic resolution by tandem hydrodefluorination/deamination, thus giving the corresponding amines with up to greater than 99 % ee. This protocol is the first example of exploiting the catalytic promiscuity of transaminases as a tool for novel transformations.

Chemoenzymatic asymmetric synthesis of 1,4-benzoxazine derivatives: application in the synthesis of a levofloxacin precursor.

A versatile and general route has been developed for the asymmetric synthesis of a wide family of 3-methyl-3,4-dihydro-2H-benzo[b][1,4]oxazines bearing different pattern substitutions in the aromatic ring. Whereas hydrolases were not suitable for resolution of these racemic cyclic nitrogenated amines, alternative chemoenzymatic strategies were designed through independent pathways leading to both amine antipodes. On one hand, bioreduction of 1-(2-nitrophenoxy)propan-2-ones allowed the recovery of the enantiopure (S)-alcohols in high yields using the alcohol dehydrogenase from Rhodococcus ruber (ADH-A), whereas evo-1.1.200 ADH led to their counterpart (R)-enantiomers also with complete selectivity and quantitative conversion. Alternatively, lipase-catalyzed acetylation of these racemic alcohols, and the complementary hydrolysis of the acetate analogues, gave access to the corresponding optically enriched products with high stereodiscrimination. Particularly attractive was the design of a chemoenzymatic strategy in six steps for the production of (S)-(-)-7,8-difluoro-3-methyl-3,4-dihydro-2H-benzo-[b][1,4]oxazine, which is a key precursor of the antimicrobial agent Levofloxacin.

Enzymatic preparation of cis and trans-3-amino-4-hydroxytetrahydrofurans and cis-3-amino-4-hydroxypyrrolidines.

The lipase catalyzed resolution of cis and trans-3-amino-4-hydroxytetrahydrofurans and cis-3-amino-4-hydroxypyrrolidines have been studied. For all the heterocycles, the best enantioselectivity was obtained using Candida antarctica lipases A and B as catalysts in hydrolytic processes. The absolute configuration of the optically pure obtained heterocycles has been assigned.

Steric vs. electronic effects in the Lactobacillus brevis ADH-catalyzed bioreduction of ketones.

Lactobacillus brevis ADH (LBADH) is an alcohol dehydrogenase that is commonly employed to reduce alkyl or aryl ketones usually bearing a methyl, an ethyl or a chloromethyl as a small ketone substituent to the corresponding (R)-alcohols. Herein we have tested a series of 24 acetophenone derivatives differing in their size and electronic properties for their reduction employing LBADH. After plotting the relative activity against the measured substrate volumes we observed that apart from the substrate size other effects must be responsible for the activity obtained. Compared to acetophenone (100% relative activity), other small substrates such as propiophenone, α,α,α-trifluoroacetophenone, α-hydroxyacetophenone, and benzoylacetonitrile had relative activities lower than 30%, while medium-sized ketones such as α-bromo-, α,α-dichloro-, and α,α-dibromoacetophenone presented relative activities between 70% and 550%. Moreover, the comparison between the enzymatic activity and the obtained final conversions using an excess or just 2.5 equiv. of the hydrogen donor 2-propanol, denoted again deviations between them. These data supported that these hydrogen transfer (HT) transformations are mainly thermodynamically controlled. For instance, bulky α-halogenated derivatives could be quantitatively reduced by LBADH even employing 2.5 equiv. of 2-propanol independently of their kinetic values. Finally, we found good correlations between the IR absorption band of the carbonyl groups and the degrees of conversion obtained in these HT processes, making this simple method a convenient tool to predict the success of these transformations.

Synthesis of vitamin D3 analogues with A-ring modifications to directly measure vitamin D levels in biological samples.

C-3-substituted 25-hydroxyvitamin D3 analogues were synthesized as tools to directly measure levels of vitamin D in biological samples. The strategy involves vinyloxycarbonylation of the 3ß-hydroxy group and formation of a carbamate bond with a hydroxyl or amino group at the end of the alkyl chain. Biotinylated conjugates of synthesized derivatives were generated to be linked with vitamin D binding protein (DBP). The spacer group present in the alkyl chain is important in the binding of antibodies to the analogue-DBP complex. When compared to 25-hydroxyvitamin D3-DBP, the binding of some antibodies to the analogue-DBP complex of the 25-hydroxyvitamin D3 derivative 10 that posses an 8-aminoctyl alkyl chain is significantly reduced, but this analogue displaced [26,27-(3)H]-25-hydroxyvitamin D3 from DBP. In contrast, the 8-hydroxyoctyl alkyl chain analogue 9 showed less displacement.

Expanding dynamic kinetic protocols: transaminase-catalyzed synthesis of α-substituted ß-amino ester derivatives.

Several α-alkylated ß-amino esters have been obtained via DKR processes employing a kit of transaminases and isopropylamine as an amino donor in aqueous medium under mild conditions. Thus, while acyclic α-alkyl-ß-keto esters afforded excellent conversions and enantioselectivities, although usually low diastereoselectivities, using more constrained cyclic ß-keto esters high to excellent inductions were obtained.

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%).

Synthesis of enantiopure fluorohydrins using alcohol dehydrogenases at high substrate concentrations.

The use of purified and overexpressed alcohol dehydrogenases to synthesize enantiopure fluorinated alcohols is shown. When the bioreductions were performed with ADH-A from Rhodococcus ruber overexpressed in E. coli, no external cofactor was necessary to obtain the enantiopure (R)-derivatives. Employing Lactobacillus brevis ADH, it was possible to achieve the synthesis of enantiopure (S)-fluorohydrins at a 0.5 M substrate concentration. Furthermore, due to the activated character of these substrates, a huge excess of the hydrogen donor was not necessary.

Coupling biocatalysis and click chemistry: one-pot two-step convergent synthesis of enantioenriched 1,2,3-triazole-derived diols.

A fully convergent one-pot two-step synthesis of different chiral 1,2,3-triazole-derived diols in high yields and excellent enantio- and diastereoselectivities has been achieved under very mild conditions in aqueous medium by combining a single alcohol dehydrogenase (ADH) with a Cu-catalysed 'click' reaction.

Mimicking nature: synthetic nicotinamide cofactors for C═C bioreduction using enoate reductases.

A series of synthetic nicotinamide cofactors were synthesized to replace natural nicotinamide cofactors and promote enoate reductase (ER) catalyzed reactions without compromising the activity or stereoselectivity of the bioreduction process. Conversions and enantioselectivities of >99% were obtained for C═C bioreductions, and the process was successfully upscaled. Furthermore, high chemoselectivity was observed when employing these nicotinamide cofactor mimics (mNADs) with crude extracts in ER-catalyzed reactions.

Chiral triazolium salts and ionic liquids: from the molecular design vectors to their physical properties through specific supramolecular interactions.

An exhaustive experimental study based on X-ray diffraction analysis, NMR, FTIR-ATR (attenuated total reflection), and Raman spectroscopy as well as theoretical calculations is reported in order to understand how the non-covalent intermolecular contacts are fundamental to explain structure-property relationships and allowing us to correlate a basic macroscopic property (i.e., the melting point, T(m)) with the structural variables of a family of enantiopure 1,4-dialkyl-1,2,4-triazolium salts. The effect of different structural vectors such as the ring size, the spatial disposition of the substituent, the substitution on the oxygen atom, the nature of the anion, or the N4 alkylation of the triazole on the intermolecular interactions of these chiral salts of a well-defined 3D structure is reported. The non-covalent intermolecular contacts mainly implicating the triazolium H3 proton are fundamental to explain structure-property relationships and, therefore, the physical properties of these new chiral salts, rather than simple anion-cation interactions. Overall, our findings highlight the importance of the specific supramolecular interactions for the understanding of the physical properties of triazolium salts and ionic liquids.

Synthesis, evaluation of anti-HIV-1 and anti-HCV activity of novel 2',3'-dideoxy-2',2'-difluoro-4'-azanucleosides.

A series of 2',3'-dideoxy-2',2'-difluoro-4'-azanucleosides of both pyrimidine and purine nucleobases were synthesized in an efficient manner starting from commercially available L-pyroglutamic acid via glycosylation of difluorinated pyrrolidine derivative 15. Several 4'-azanucleosides were prepared as a separable mixture of α- and ß-anomers. The 6-chloropurine analogue was obtained as a mixture of N(7) and N(9) regioisomers and their structures were identified based on NOESY and HMBC spectral data. Among the 4'-azanucleosides tested as HIV-1 inhibitors in primary human lymphocytes, four compounds showed modest activity and the 5-fluorouracil analogue (18d) was found to be the most active compound (EC(50)=36.9µM) in this series. None of the compounds synthesized in this study demonstrated anti-HCV activity.

Stereoselective synthesis of 2,3-disubstituted indoline diastereoisomers by chemoenzymatic processes.

Racemic indolines including a variety of structural motifs such as C-2 and C-3 substitutions (alkyl or aryl), cis/trans relative stereochemistry and functionalization of the aromatic ring (fluoro, methyl or methoxy groups) have been efficiently prepared through Fischer indolization and subsequent diastereoselective reduction of the unprotected indoles. Combination of Candida antarctica lipase type A and allyl 3-methoxyphenyl carbonate has been identified as the best tandem for their kinetic resolutions, observing excellent stereodiscriminations for most of the tested indolines.

Lipase-catalyzed preparation of chromomycin A3 analogues and biological evaluation for anticancer activity.

Several acyl derivatives of the aureolic acid chromomycin A(3) were obtained via lipase-catalyzed acylation. Lipase B from Candida antarctica (CAL-B) was found to be the only active biocatalyst, directing the acylation regioselectively towards the terminal secondary hydroxyl group of the aglycone side chain. All new chromomycin A(3) derivatives showed antitumor activity at the micromolar or lower level concentration. Particularly, chromomycin A(3) 4'-vinyladipate showed 3-5 times higher activity against the four tumor cell lines assayed as compared to chromomycin A(3).

Design and divergent synthesis of aza nucleosides from a chiral imino sugar.

Several novel nucleoside analogues as potential inhibitors of glycosidases and purine nucleoside phosphorylase (PNP) have been synthesized via selective coupling of an appropriate nucleobase at different positions of an orthogonally protected imino sugar as a common precursor. This synthetic strategy offers a straightforward protocol for the assembly of imino sugar containing nucleosides, establishing a new repertoire of molecules as potential therapeutics.

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.

Dynamic kinetic resolution of 1,3-dihydro-2H-isoindole-1-carboxylic acid methyl ester: asymmetric transformations toward isoindoline carbamates.

Asymmetric syntheses of isoindoline carbamates have been successfully achieved through enzyme-mediated dynamic kinetic resolution processes and without requirement of metal or acid-base catalyst for the substrate racemization. Optically active carbamates were obtained in good yields and an excellent degree of stereoselectivity when Pseudomonas cepacia lipase (PSL) was used as biocatalyst, with diallyl or dibenzyl carbonates being both adequate reagents in alkoxycarbonylation reactions.

Highly stereoselective chemoenzymatic synthesis of the 3H-isobenzofuran skeleton. Access to enantiopure 3-methylphthalides.

A straightforward synthesis of (S)-3-methylphthalides has been developed, with the key asymmetric step being the bioreduction of 2-acetylbenzonitriles. Enzymatic processes have been found to be highly dependent on the pH value, with acidic conditions being required to avoid undesired side reactions. Baker's yeast was found to be the best biocatalyst acting in a highly stereoselective fashion. The simple treatment of the reaction crudes with aqueous HCl has provided access to enantiopure (S)-3-methylphthalides in moderate to excellent yields.