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Continuous flow synthesis of 3-hydroxy-2-pyrone from galactaric acid.

The review will discuss the methods that have been optimized so far for the enzymatic hydrolysis of soapstock into enriched mixtures of free fatty acids, in order to offer a sustainable alternative to the procedure which is currently employed at the industrial level for converting soapstock into the by-product known as acid oil (or olein, i.e., free fatty acids removed from raw vegetable oil, dissolved in residual triglycerides). The further biocatalyzed manipulation of soapstock or of the corresponding acid oil for the production of biodiesel and fine chemicals (surfactants, plasticizers, and additives) will be described, with specific attention given to processes performed in continuous flow mode. The valorization of soapstock as carbon source in industrial lipase production will be also considered.

Piperonal is a key ingredient in the flavour and fragrance industry and a useful intermediate for the synthesis of fine chemicals. It is currently prepared by either ozonolysis or chromic acid oxidation of isosafrole, obtained upon isomerization of naturally abundant safrole. A chemo-enzymatic three-step procedure for the conversion of isosafrole into piperonal is herein described. Lipase-mediated perhydrolysis of EtOAc in the presence of H2O2 is employed to generate peracetic acid in situ and promote the epoxidation of isosafrole. The reaction mixture is submitted to methanolic KOH treatment to recover the corresponding mixture of vicinal diols. The latter is efficiently oxidized to piperonal using MnO2, periodically regenerated at the expense of tert-butylhydroperoxide (TBHP), which represents the terminal oxidant of this transformation. The use of continuous-flow conditions using a continuously-stirred tank reactor for the epoxidation, and a packed bed reactor for the final oxidation improves the productivity and stability of the whole method, with space–time yields of 75 mmol g−1 h−1 and 120 mmol g−1 h−1, calculated as amount of generated product per amount of catalyst per unit time.

The asymmetric hydrogenation of conjugated tetrasubstituted alkenes with transition-metal catalysts is a challenging reaction, especially for substrates bearing a halide substituent. We describe a two-step multienzymatic reduction of a series of α-halo β-alkyl tetrasubstituted cyclic enones, affording halohydrins with three contiguous stereogenic centers, in good yield and with a high stereoselectivity. The reduction is catalyzed by a stereospecific ene-reductase (OYE2-3 or NemA) and a highly enantioselective alcohol dehydrogenase (ADH). The use of two enantiodivergent ADHs allows the control of the diastereoselectivity. The absolute stereochemical configurations of the products have been determined from the analysis of single-crystal structures (Flack's parameter). The enantiomeric excess (ee) has been determined by derivatization of the products with (R) Mosher's acid. Lastly, we extended our methodology also to a nonhalogenated substrate: the α-methyl ketoisophorone was reduced by two distinct enantiodivergent ene-reductases (flavin mononucleotide- and F 420-dependent), affording each enantiomer of the saturated ketone with ee > 98%.

We describe the asymmetric synthesis of the most pleasant enantiomer of Jessemal fragrance. The key steps are (i) the one-pot reduction of an α-chloro-tetrasubstituted cyclohexenone to give the chlorohydrin, catalyzed by two stereoselective redox enzymes (an ene-reductase and an alcohol dehydrogenase); (ii) the regioselective epoxide ring-opening with organocuprate or organolithium nucleophiles. Density functional theory calculations together with the Curtin-Hammett principle allowed the rationalization of the regioselectivity.

Ene-reductases from the Old Yellow Enzyme (OYE) superfamily are a well-known and efficient biocatalytic alternative for the asymmetric reduction of C=C bonds. Considering the broad variety of substituents that can be tolerated, and the excellent stereoselectivities achieved, it is apparent why these enzymes are so appealing for preparative and industrial applications. Different classes of C=C bonds activated by at least one electron-withdrawing group have been shown to be accepted by these versatile biocatalysts in the last decades, affording a vast range of chiral intermediates employed in the synthesis of pharmaceuticals, agrochemicals, flavours, fragrances and fine chemicals. In order to access both enantiomers of reduced products, stereodivergent pairs of OYEs are desirable, but their natural occurrence is limited. The detailed knowledge of the stereochemical course of the reaction can uncover alternative strategies to orient the selectivity via mutagenesis, evolution, and substrate engineering. An overview of the ongoing studies on OYE-mediated bioreductions will be provided, with particular focus on stereochemical investigations by deuterium labelling.

The combination of a biocatalytic asymmetric C=C reduction with a simple sequence of chemical transformations was implemented in a new chemoenzymatic synthesis of various substituted aryloxyalkanoic acids, used as weed-killing agrochemicals or chiral precursors. By careful selection of the biocatalyst, either enantiomer of the product could be obtained in good yield and moderate to good ee. The method relies on the use of simple and commercially available starting materials, and requires neither purified enzymes nor chromatographic separations.

Production of azelaic and pelargonic acids from renewable high-oleic soapstock, a by-product of vegetable oil refining industry.

Trifluoroacetic acid catalysed lactonization of vicinal disubstituted γ-hydroxyesters was investigated in different solvents. The reaction kinetics, monitored by NMR spectroscopy, showed that: (i) the vic-disubstituent effect is stereoselective since the anti diastereoisomer ring closes substantially more rapidly than the syn isomer ring; (ii) the anti-vic effect is much stronger than the classical Thorpe-Ingold effect (known also as the gem-disubstituent effect), instead the syn diastereoisomers have rate constants comparable to that of the gem-disubstituted ester; (iii) the vic-effect can be enhanced by increasing the steric hindrance of one of the two substituents or carrying out the reaction in non-polar solvents. DFT computations of energy barriers (ΔG‡) were in good agreement with the experimental data. The distortion/interaction-activation strain model together with the Winstein-Holness kinetic scheme gave more insights into the origin of the vic-effect. An application of this effect consists of the diastereomeric resolution of disubstituted γ and δ lactones, among which are the naturally occurring Nicotiana t. lactone, the whisky and cognac oak lactones, and the Aerangis lactone. Both cis and trans diastereoisomers of these lactones were isolated in good yield and with high diastereomeric excess (de >92%). The selectivities of the diastereomeric resolution process, determined by NMR spectroscopy, are reported as well.

The text describes the main techniques used in the pharmaceutical field to mask the bitter taste of active pharmaceutical ingredients and make the product more pleasant for oral intake by the patient.

The reduction of (Z)-beta-acylaminonitroalkenes catalysed by ene-reductases is described for the first time. The reaction occurs with high conversion and excellent enantioselectivity, and shows a wide substrate scope. The reduced products are valuable chiral synthons characterized by two vicinal nitrogen-containing functional groups, and they can be further modified by functional group interconversion thanks to the synthetic versatility of the nitro moiety. The chemoenzymatic synthesis of (R)-N,N'-(1-phenylethane-1,2-diyl)diacetamide from easily accessible (Z)-N-(2-nitro-1-phenylvinyl)acetamide is herein reported as a representative application of this synthetic procedure.

Ene-reductases (ERs) belonging to the old yellow enzyme (OYE) family have been thoroughly investigated for the stereospecific reduction of activated prochiral C=C double bonds. In this work, OYE3 was immobilized both by covalent binding on glyoxyl-agarose (OYE3-GA), and by affinity-based adsorption on EziGTM particles (OYE3-EziG). The immobilized OYE3-GA was demonstrated to be active (activity recovery = 52%) and to retain almost 100% of its activity under the enzymatic assay conditions (50 mM phosphate buffer pH 7, 28 °C) for six days, whereas the activity of the non-immobilized enzyme dropped to 50% after two days. In the case of EziGTM, the highest activity recovery (54%) was achieved by using the most hydrophilic carrier (EziGTM Opal) that was selected for the full characterization of this type of enzyme preparation (stability, recycling, re-use, enzyme leakage). OYE3-EziG was slightly less stable than OYE3-GA under the same experimental conditions. OYE3-GA could be recycled and re-used for up to 12 reaction cycles in the bioreduction of α-methyl-trans-cinnamaldehyde; after 12 runs, the highest conversion achieved was 40%. In the case of the co-immobilized OYE3/GDH-EziG, the conversion dropped to 56% after two reaction cycles. No enzyme leakage was detected over 48 h for both OYE3-GA and OYE3/GDH-EziG (50 mM phosphate buffer pH 7, 28 °C). These seed results pave the way for a true optimization of the immobilization of OYE3, as well as for the use of immobilized OYE3 for preparative applications both in batch and continuous flow conditions.

Leather cyclohexanol (4-(isopropyl)cyclohexanol) and woody acetate (4-(tert-butyl)cyclohexyl acetate) are commercialized for functional perfumery applications as mixtures of cis- and trans-isomers. The cis-isomers are more potent odorants than the corresponding trans counterparts, but they are the less favoured products in most of the classical synthetic routes. Known stereoselective routes to cis-4-alkylcyclohexanols are characterized by a high environmental burden and/or troublesome reaction work-up. In this work, we examine the use of commercial alcohol dehydrogenases (ADHs) to produce cis-4-alkylcyclohexanols, including the two derivatives with isopropyl and tert-butyl substituents, by the stereoselective reduction of the corresponding ketones. High conversions and diastereoisomeric excess values were achieved with five of the eighteen tested ADHs. To complete the synthetic approach to woody acetate, Candida antarctica A (CALA) was employed as a catalyst for the enzymatic acetylation of cis-4-(tert-butyl)cyclohexanol. In order to provide a technological upgrade to the production of the most odorous isomers of the two commercial fragrances, we designed a continuous-flow process based on the combination of in-line enzymatic steps with in-line work-up, effectively providing samples of cis-leather cyclohexanol and cis-woody acetate with high diastereoisomeric purity.

The four isomeric forms of the flavour linalool oxide (pyranoid) were synthesized starting from the inexpensive monoterpene terpinolene. The key steps of the process include mCPBA epoxidation of the starting diene, alumina-catalysed rearrangement of the obtained diepoxide and the diastereoselective reduction of the ketone 2,2,6-trimethyl-6-vinyldihydro-2H-pyran-3(4H)-one. The resulting racemic cis- and trans-linalool oxides were resolved through an enzyme-mediated acetylation procedure. More specifically, we found that Candida rugosa lipase and lipase PS are the catalysts of choice for the resolution of cis- and trans-linalool oxide, respectively.

The chemoselective hydrogenation of alkenes in the presence of alkynes is a very challenging transformation to achieve with traditional chemical methods. The development of an effective procedure to perform this transformation would enrich the tool-kit available to organic chemists for the development of useful synthetic routes, and the creation of novel structural motifs. The reduction of activated alkene bonds by ene-reductases (ERs) is completely chemoselective, because of the mechanism of the reaction. Thus, we investigated the use of ERs belonging to the Old Yellow Enzyme family for the reduction of ?,?-unsaturated aldehydes with a conjugated C?C triple bond at the ? position. This reaction was exploited as the key step for the development of an effective homologation route to convert aryl and alkyl substituted propynals and butynones into 4-alkynals and 4-alkynols, avoiding some troublesome or hazardous steps of known synthetic routes. (Figure presented.).

The integration of biocatalytic steps in retrosynthetic analysis of a target molecule offers multiple advantages, such as reduction of the environmental footprint of the process, viability of milder and safer reaction conditions, and accessibility of transformations that are challenging with traditional chemical synthesis. Herein, six chemo-enzymatic routes are described for the synthesis of a fluorinated D-phenylalanine derivative, precursor of the blockbuster antidiabetic drug sitagliptin. All routes start from the same aldehyde precursor and involve at least one biocatalytic step, including reductive amination, transamination, deracemisation, hydroamination, and alkene reduction. The target molecule was obtained in 2–5 steps from the aldehyde, with ee up to >99% and in 36–62% isolated yield. Furthermore, as part of one of the routes, the first example of a fully biocatalytic conversion of a cinnamic acid derivative to the corresponding D-phenylalanine (formal D-selective hydroamination) is reported.

Chiral &beta, nitroalcohols are important building blocks in organic chemistry. The synthetic approach that is based on the enzyme-mediated reduction of &alpha, nitroketones has been scarcely considered. In this work, the use of commercial alcohol dehydrogenases (ADHs) for the reduction of aromatic and aliphatic nitroketones is investigated. High conversions and enantioselectivities can be achieved with two specific ADHs, affording either the (S) or (R)-enantiomer of the corresponding nitroalcohols. The reaction conditions are carefully tuned to preserve the stability of the reduced product, and to avoid the hydrolytic degradation of the starting substrate. The further manipulation of the enantioenriched nitroalcohols into Boc-protected amminoalcohols is also described.

The zygomycete fungus Syncephalastrum racemosum shows the remarkable capability of reducing carboxylic acids to primary alcohols in water medium, at ambient temperature and pressure. The reaction does not require molecular hydrogen, and in most cases affords quantitative transformations. The results herein reported highlight the basic molecular scaffold that can be accepted by the fungus, the effects due to substituents, and also the possibility that carboxylic acids can be generated in the reaction medium by enzymatic hydrolysis of the corresponding methyl esters. This biocatalysed reduction implements the scarcely supplied enzymatic toolbox for the conversion of carboxylic groups into primary alcohols, which can be exploited for the optimisation of sustainable synthetic procedures. (C) 2015 Elsevier B.V. All rights reserved.

The Trp 116 mutants of Old Yellow Enzyme 1 that catalyse the reduction of (Z)-?-aryl-?-cyanoacrylates give the opposite enantioselectivity according to the nature of the amino acid in position 116. Small amino acids (e.g., alanine) make the substrate bind to the enzyme?s active site in a "classical" orientation, affording the (S)-enantiomer of the reduced product. When the size of the amino acid increases (e.g., leucine), a "flipped" binding mode is adopted by the substrate, which is converted into the corresponding (R)-derivative. With bulky amino acids (e.g., tryptophan in the wild-type) the reduction does not occur. The enantiomerically enriched cyanopropanoates thus prepared can be converted into the corresponding (S)- and (R)-?-aryl-?-lactams, precursors of inhibitory neurotransmitters belonging to the class of ?-aminobutyric acids, by a simple functional group interconversion in a sequential one-pot procedure.

The most odorous stereoisomers of the chiral commercial fragrance Muguesia® are prepared by a very effective linear biocatalysed cascade reaction, in which a suitable unsaturated ketone is submitted to the sequential action of two enzymes, an ene-reductase and an alcohol dehydrogenase, which are added together to the same reaction vessel with the cofactor regeneration system. Two stereogenic centres in 1,2 relative position are thus created under high stereochemical control by a two-step one-pot enzymatic procedure.

A chemo-enzymatic catalytic system, comprised of Bobbitt's salt and laccase from Trametes versicolor, allowed the [1,3]-oxidative rearrangement of endocyclic allylic tertiary alcohols into the corresponding enones under an Oxygen atmosphere in aqueous media. The yields were in most cases quantitative, especially for the cyclopent-2-en-1-ol or the cyclohex-2-en-1-ol substrates without an electron withdrawing group (EWG) on the side chain. Transpositions of macrocyclic alkenols or tertiary alcohols bearing an EWG on the side chain were instead carried out in acetonitrile by using an immobilized laccase preparation. Dehydro-Jasmone (R), dehydro-Hedione (R), dehydro-Muscone and other fragrance precursors were directly prepared with this procedure, while a synthetic route was developed to easily transform a cyclopentenone derivative into trans-Magnolione (R) and dehydro-Magnolione (R). The rearrangement of exocyclic allylic alcohols was tested as well, and a dynamic kinetic resolution was observed: alpha,beta-unsaturated ketones with (E)-configuration and a high diastereomeric excess were synthesized. Finally, the 2,2,6,6-tetramethyl-1-piperidinium tetrafluoroborate (TEMPO+BF4-)/laccase catalysed oxidative rearrangement was combined with the ene-reductase/ alcohol dehydrogenase cascade process in a one-pot three-step synthesis of cis or trans 3-methylcyclohexan-1-ol, in both cases with a high optical purity.

A fast and sensitive colorimetric assay (FRED, fast and reliable ene-reductases detection) that allows the estimation of levels of conversion of ene-reductase (ER)-catalysed reactions has been developed. The activated olefin is reduced by ER at the expense of NAD(P)H cofactor, whose regeneration is carried out in situ by the glucose/glucose dehydrogenase system. Subsequently, the consumption of the co-substrate glucose is determined colorimetrically by a multienzymatic system. The FRED assay offers a wide range of possible applications, from enzyme fingerprinting and kinetic analysis, to primary screening of enzyme libraries and optimisation of ERs' performances under different reaction conditions.

A three-step biocatalytic procedure is described for the conversion of methyl and ethyl cyclopentene- and cyclohexenecarboxylates into both the enantiomers of the corresponding chiral 3-oxoesters, which are useful building blocks for the synthesis of active pharmaceutical ingredients. The allylic hydroxylation of the starting cycloalkenecarboxylates is carried out by using R. oryzae resting cells entrapped in alginate beads, in acetate buffer solution at 25 °C. The oxidation of the intermediate allylic alcohols to unsaturated ketones, performed by the laccase/TEMPO system, and the ene-reductase mediated hydrogenation of the alkene bond were carried out in the same reaction vessel in a sequential mode at 30 °C. Being entirely biocatalytic, our multistep procedure provides considerable advantages in terms of selectivity and environmental impact over reported chemical methods.

Enantiopure 2-methyl-3-substituted tetrahydrofurans are key precursors of several biologically active products (drugs, flavors, and agrochemicals). Thus, a stereocontrolled and efficient methodology for the obtainment of these synthons is highly desirable. We exploited a two-step multienzymatic stereoselective cascade reduction of alpha-bromo-alpha,beta-unsaturated ketones to give the corresponding bromohydrins in good yields, with high ee and de values. The cascade process is catalyzed by an ene-reductase and an alcohol dehydrogenase. Further manipulations of these bromohydrins, by two diastereodivergent routes, allowed the preparation of the tetrahydrofuran synthons. One route is based on a lipase catalyzed cleavage of the protecting group. The second route is characterized by a camphor sulfonic acid mediated isomerization of a beta-hydroxyepoxide to give the tetrahydrofuran-2-ol. Finally, the synthesis of the most odorous and pleasant stereoisomer of the roasted meat aroma, i.e., (2S,3R)-2-methyl-3-thioacetate tetrahydrofuran, is reported as well.

The reduction of C=C double bond, a key reaction in organic synthesis, is mostly achieved by traditional chemical methods. Therefore, the search for enzymes capable of performing this reaction is rapidly increasing. Old Yellow Enzymes (OYEs) are flavin-dependent oxidoreductases, initially isolated from Saccharomyces pastorianus. In this study, the presence and activation of putative OYE enzymes was investigated in the filamentous fungus Mucor circinelloides, which was previously found to mediate C=C reduction. Following an in silico approach, using S. pastorianus OYE1 amminoacidic sequence as template, ten putative genes were identified in the genome of M. circinelloides. A phylogenetic analysis revealed a high homology of McOYE1-9 with OYE1-like proteins while McOYE10 showed similarity with thermophilic-like OYEs. The activation of mcoyes was evaluated during the transformation of three different model substrates. Cyclohexenone, α-methylcinnamaldehyde and methyl cinnamate were completely reduced in few hours and the induction of gene expression, assessed by qRT-PCR, was generally fast, suggesting a substrate-dependent activation. Eight genes were activated in the tested conditions suggesting that they may encode for active OYEs. Their expression over time correlated with C=C double bond reduction.

The Baker's Yeast (BY) reduction of (Z)-2-chloromethyl-3-arylacrylaldehydes was found to afford (R)-2-methyl-3-aryl-propanols showing high enantiomeric excess values. Deuterium incorporation experiments were performed, in order to investigate the mechanism of the bioreduction: the formation of the corresponding substituted 2-benzylacrylaldehydes. as intermediates to be effectively reduced by Baker's Yeast, was suggested. These intermediates were synthesized and submitted to BY reduction to afford the corresponding saturated (R)-alcohols, thus confirming the conclusions drawn from labelling experiments. The enantioselectivity of their bioreduction was found to be opposite with respect to that observed for the corresponding regioisomeric 2-methylcinnamaldehydes. The preparation of the two enantiomers of 2-methyl-3-aryl-propanols by fermentation of two regioisomers represents an interesting example of substrate-controlled enantioselective reaction. (C) 2012 Elsevier B.V. All rights reserved.

The coupling of the enantioselective reduction catalyzed by Old Yellow Enzymes (OYEs), together with the in situ substrate feeding product removal (SFPR) concept, significantly improved the productivity of the g-scale preparation of ethyl (S)-2-ethoxy-3-(p-methoxyphenyl)propanoate (EEHP), an important precursor of several PPAR-α/γ agonists, such as Tesaglitazar. The OYEs and the glucose dehydrogenase for cofactor regeneration were cloned, overexpressed in Escherichia coli, and purified. The synthetic sequence was completed by a NaClO2 oxidation employing cheap and environmentally friendly conditions. The product was obtained in 94% yield and with an ee of 98% over the two steps.

The enantioselective biocatalyzed reduction of the C=C bond of some (Z)-methyl alpha-halo-beta-arylacrylates was investigated. The reaction was performed by baker's yeast fermentation and Old Yellow Enzymes 1-3 mediated biotransformations. The final products were, respectively, enantiomerically enriched (S)-alpha-halo-beta-arylpropionic acids and their methyl esters, and ester hydrolysis was promoted in the whole cell system. High conversions and enantioselectivity values were observed when the aromatic ring was substituted by an electron-withdrawing group. Further manipulation of two of these enantiomerically enriched (S)-haloacids afforded p-substituted D-phenylalanines.

A new synthetic route of to pharmaceutical intermediates (S)-1a–b and (S)-14 is reported. The reaction pathway is based on the baker’s yeast-mediated reduction of the α-alkoxy cinnamaldehydes 9a–c to give the corresponding (S)-alcohols in good yields and excellent ee.

A new scaleable synthetic route to the title compounds has been developed. The reaction pathway is based on the α-chymotrypsin-catalysed hydrolysis of the racemic ethyl 2-ethoxy-3-( p -methoxyphenyl)propanoate or of the racemic ethyl 2-methoxy-3-( p -methoxyphenyl)propanoate to give the corresponding resolved ( S )-esters with excellent ee. The acids were easily separated from the ( S )-esters by a simple acid–base work-up. The overall yields of 1 and 2 were 16% and 17%, respectively.

We determined the D/H isotope ratios of some ibuprofen and naproxen samples by 2 H NMR spectroscopy. Some of these values were found to be useful for collecting hints on the synthetic procedures employed to prepare these drugs. Site-specific isotope ratio analysis shows great potentials in the fight against patent infringement.