Search

Molecular hybrids obtained by connecting two or more bioactive molecules through a metabolizable linker are used as multi-target drugs for the therapy of multifactorial diseases. Ascorbic acid, as well as the ketone bodies acetoacetate and (R)-3-hydroxybutyrate, are bioactive molecules that have common fields of application in the treatment and prevention of neurodegenerative diseases and cardiac injuries as well. In spite of this, the preparation of ascorbic acid ketone body hybrids is uncovered by the literature. Herein, we report the lipase-catalyzed condensation of methyl acetoacetate with ascorbic acid, which affords the 6-O-acetoacetyl ascorbic acid in quantitative yield. The same approach, employing the methyl (R)-3-hydroxybutyrate in place of the methyl acetoacetate, allows the preparation of the 6-O-(R)-3-hydroxybutyryl ascorbic acid in 57% yield. A better result (90% overall yield) is achieved through the lipase-catalyzed coupling of ascorbic acid with methyl (R)-3-O-methoxymethyl-3-hydroxybutyrate followed by the cleavage of the MOM protecting group. The two novel products are fully characterized and additional information on the antioxidant activity of the new products is also given.

2,3-dihydroxy-2-methylbutyric acid, also known as 2,3-dimethylglyceric acid, constitutes the acyl and/or the alcoholic moiety of many bioactive natural esters. Herein, we describe a chemoenzymatic methodology which gives access to all the four possible stereoisomers of the 2,3-dimethylglyceric acid ethyl ester. The racemic ethyl α-acetolactate, produced by the N-heterocycle carbene (NHC)-catalyzed coupling of ethyl pyruvate and methylacetoin was employed as the starting material. The racemic mixture was resolved through (S)-selective reductions, promoted by the acetylacetoin reductase (AAR) affording the resulting ethyl (2R,3S)-2,3-dimethylglycerate; the isolated remaining (S)-ethyl α-acetolactate was successively treated with baker’s yeast to obtain the corresponding (2S,3S) stereoisomer. syn-2,3-Dimethylgliceric acid ethyl ester afforded by reducing the rac-α-acetolactate with NaBH4 in the presence of ZnCl2 was kinetically resolved through selective acetylation with lipase B from Candida antarctica (CAL-B) and vinyl acetate to access to (2S,3R) stereoisomer. Finally, the (2R,3R) stereoisomer, was prepared by C3 epimerization of the (2R,3S) stereoisomer recovered from the above kinetic resolution, achieved through the TEMPO-mediated oxidation, followed by the reduction of the produced ketone with NaBH4. The resulting 2,3-dimethylglycertate enriched in the (2R,3R) stereoisomer was submitted to stereospecicific acetylation with vinyl acetate and CAL-B in order to separate the major stereoisomer. The entire procedure enabled conversion of the racemic α-acetolactate into the four enantiopure stereoisomers of the ethyl 2,3-dihydroxy-2-methylbutyrate with the following overall yields: 42% for the (2R,3S), 40% for the (2S,3S), 42% for the (2S,3R) and 20% for the (2R,3R).

The enzymatic synthesis of biologically active compounds is receiving increasing interest from academic and industrial researchers. Herein, we report on the continuous‐flow enzymatic production of eugenol esters with packed‐bed microreactors prepared with the Novozym 435 and Lipozyme RM IM lipases. After optimization of the independent process variables, namely temperature (68 °C), flow rate (8 μL min⁻¹), and eugenol/isobutyric anhydride molar ratio (1 : 5) by means of experimental design methodologies, the unprecedented continuous‐flow production of eugenyl isobutyrate has been achieved with steady‐state conversions of 77 and 70%, using packed‐bed microreactors prepared with Novozym 435 and Lipozyme RM IM, respectively. With acetic anhydride as the acylating agent, the same reactors, under the same conditions, afforded eugenyl acetate with steady‐state conversions of 82% (Novozym 435) and 90% (Lipozyme RM IM). The two ester products have been tested against phytopathogenic and dermatophytic fungi, demonstrating interesting action against dermatophytic fungi. The reported results demonstrated the suitability of the enzymatic packed‐bed microreactors as a sustainable and scalable technology for the solvent‐free continuous‐flow production of biologically active esters of eugenol.

The application of N-heterocyclic carbene (NHC) catalysis to the polycondensation of diols and dialdehydes under oxidative conditions is herein presented for the synthesis of polyesters using fossil-based (ethylene glycol, phthalaldehydes) and bio-based (furan derivatives, glycerol, isosorbide) monomers. The catalytic dimethyl triazolium/1,8-diazabicyclo[5.4.0]undec-7-ene couple and stoichiometric quinone oxidant afforded polyester oligomers with a number-average molecular weight (Mn ) in the range of 1.5-7.8 kg mol-1 as determined by NMR analysis. The synthesis of a higher molecular weight polyester (polyethylene terephthalate, PET) by an NHC-promoted two-step procedure via oligoester intermediates is also illustrated together with the catalyst-controlled preparation of cross-linked or linear polyesters derived from the trifunctional glycerol. The thermal properties (TGA and DSC analyses) of the synthesized oligoesters are also reported.

The oral administration of (R)-3-hydroxybutyl-(R)-3-hydroxybutyrate, allows inducing a beneficial level of blood ketone bodies without the adverse effects due to the adhesion to a ketogenic diet. Several studies documented the therapeutic effectiveness of the (R)-3-hydroxybutyl (R)-3-hydroxybutyrate in treating neurodegenerative diseases as well as its boosting activity of athletic and cognitive performances during prolonged physical exercises. Further studies considering this ketone body ester for therapy of other pathologies are also underway. In the present work, we describe the synthesis of (R)-3-hydroxybutyl-(R)-3-hydroxybutyrate through the enantioselective transesterification of racemic ethyl 3-hydroxybutyrate with (R)-1,3-butanediol catalyzed by immobilized Candida antarctica lipase B (CAL-B). The enantiopure (R)-1,3-butanediol was in turn obtained from the kinetic resolution of the racemate by CAL-B catalyzed acetylation with vinyl acetate. The economy of the synthetic procedure has been improved by recycling the unreacted (S) enantiomers of the ethyl 3-hydroxybutyrate and 1,3-buatnediol after stereochemical inversion achieved by tosylation and SN2 with ammonium acetate. The overall procedure allows to incorporate up to 70% of the starting racemic reagents into the final product.

This work reports the enzymatic synthesis of butyl stearate and ethyl stearate, two esters with important industrial properties, ranging from cosmetic applications to emollient characteristics (butyl stearate) to food industries, imparting flavor and aroma to beverages and foods (ethyl stearate). Here, reactions were carried out using Novozym 435 as biocatalyst, considering different stirring levels and substrates molar ratios, to determine optimal conditions to produce both esters. The kinetics under optimized conditions, varying the substrates molar ratio was performed and the influence of the fractional addition of the alcohol in the reaction was also verified. It was observed for butyl and ethyl stearate that satisfactory results (92%) occurred when batch kinetics was carried out for 24 h at 60 °C, 1% (by weight of substrates) of the enzyme, agitation of 250 rpm and molar ratio of 1:2 and 1:5, respectively. Enzyme reuse up to five and three cycles led to satisfactory results for butyl stearate and ethyl stearate, respectively. This work represents, therefore, a novelty in the production of these esters by enzymatic route, leading to a high conversion of products, using a low amount of biocatalyst, proving the relevance of the production of these esters by enzymatic means, which makes the process ecologically correct and still add value to the final product.

A chiral NHC-catalyzed dearomatizing reaction of activated N-alkylpyridinium salts with aliphatic aldehydes is described. The resulting acylated 1,4-dihydropyridines have been obtained with complete C4 regioselectivity and enantioselectivities in the range 52-78% ee. The (4R)-absolute configuration of the synthesized compounds has been determined by the TD-DFT simulation of the electronic circular dichroism spectra.

Benzoin-type reactions allow the generation of α-hydroxy ketones through the (formal) carboligation of two aldehyde reactants. The synthetic relevance of the products and the wide distribution of the α-hydroxy ketone functionality in bioactive natural compounds have provided the motivation for intensive research efforts directed towards the development of ever more efficient and selective catalysts for reactions of this class. As in many other areas of study, the solution developed in nature – that is, the utilization of thiamine-diphosphate-dependent (ThDP-dependent) enzymes – also in this case allows levels of chemo- and stereoselectivity currently unattainable by biomimetic organocatalysts to be achieved. Here we present an overview of the structural diversity of the α-hydroxy ketone motif achievable through ThDP-dependent enzyme catalysis. Details relating to structure–activity relationships and to rational mutagenesis approaches for improving the catalytic performances of wild-type enzymes are also illustrated.

The application of the oxidative system composed of a heterogeneous triazolium pre-catalyst, iron(ii) phthalocyanine and air is described for the selective conversion of 5-hydroxymethylfurfural (HMF) into the added-value 5-hydroxymethyl-2-furancarboxylic acid (HMFCA). The disclosed one-pot two-step procedure involved sequential oxidative esterifications of HMF to afford a polyester oligomer having hydroxyl and carboxyl terminal groups (Mw = 389-1258), which in turn was hydrolyzed by a supported base (Ambersep 900 OH) to yield HMFCA in 87% overall yield. The same strategy was adopted for the effective synthesis of ester and amide derivatives of HMFCA by nucleophilic depolymerization of the oligomeric intermediate with methanol and butylamine, respectively. The utilization of the disclosed oxidative system for the direct conversion of HMF and furfural into their corresponding ester, amide, and thioester derivatives is also reported.

The retention mechanism of four major carotenoids, two xanthophylls (i.e., lutein and zeaxanthin) and two carotenes (i.e., lycopene and β-carotene), was investigated in reversed-phase liquid chromatography with the aim of thermodynamic analysis. The experimental variables considered in this study were the composition of mobile phase (MP) and the temperature. Chromatographic elutions were undertaken under linear, isocratic conditions by using a C18 stationary phase, four different MP compositions (by varying the ratio methanol/acetonitrile from 66.5/28.5 to 47.5/47.5 v/v), and column temperatures in the range 283–313 K. Traditional Van’t Hoff analysis has been used to estimate changes of standard enthalpy (ΔH∘) and Gibbs free energy (ΔG∘) associated with the solute transfer from the mobile to the stationary phase at each mobile phase composition. The thermodynamic quantities have been correlated to the structure of investigated carotenoids and their interaction with the octadecyl silica stationary phase.

The thiamine diphosphate (ThDP) dependent enzyme acetoin:dichlorophenolindophenol oxidoreductase (Ao:DCPIP OR) from Bacillus licheniformis was cloned and overexpressed in Escherichia coli. The recombinant enzyme shared close similarities with the acetylacetoin synthase (AAS) partially purified from Bacillus licheniformis suggesting that they could be the same enzyme. The product scope of the recombinant Ao:DCPIP OR was expanded to chiral tertiary α-hydroxy ketones through the rare aldehyde-ketone cross-carboligation reaction. Unprecedented is the use of methylacetoin as the acetyl anion donor in combination with a range of strongly to weakly activated ketones. In some cases, Ao:DCPIP OR produced the desired tertiary alcohols with stereochemistry opposite to that obtained with other ThDP-dependent enzymes. The combination of methylacetoin as acyl anion synthon and novel ThDP-dependent enzymes considerably expands the available range of C-C bond formations in asymmetric synthesis.

The design and synthesis of a set of supported azolium salt pre-catalysts is presented along with their utilization in the production of monoesters of glycerol and solketal by oxidative N-heterocyclic carbene (NHC)-catalysis through batch and continuous-flow approaches. After a propaedeutic study with soluble NHCs, the heterogeneous analogues (silica and polystyrene supports) were tested in a model monoesterification of glycerol using either the Kharasch oxidant or air (in the presence of electron transfer mediators) as the terminal oxidants. The best performing polystyrene-supported triazolium salt pre-catalyst afforded monoacylglycerols (MAGs) in high yields (up to 95%) and almost complete selectivity (monoester/diester >95 : 5) using air and the green solvent Me-THF. The synthesis of fully bio-based MAGs from furfural, 5-hydroxymethylfurfural (HMF), citronellal, and vanillin is also reported. Continuous-flow experiments have been finally performed by fabricating the corresponding packed-bed microreactor, which could be operated for ca. 120 hours with maintenance of conversion efficiency and selectivity.

A green scalable flow-synthesis process for the production of eugenyl acetate, an eugenol derivative with potential applications in food and medicinal chemistry, was developed. Through batch experiments, the anion-exchange resin Amberlyst A-21 was recognized as a suitable catalyst for the esterification of eugenol with acetic anhydride. Next, the process was switched from batch- to flow-mode by using a packed-bed microreactor integrated in an instrumental platform that permitted at the same time continuous control of the main process parameters (flow rate, feed mixture composition, temperature) and the on-line HPLC analysis of the reactor effluent. Thanks to this apparatus, a number of experiments with different reaction conditions have been easily performed to evaluate the effects of temperature and reagent molar ratios on eugenyl acetate production. The results have been used to carry out a central composite rotatable experimental design (CCRD) whose derived response surface model (RSM) suggested an optimal temperature and acetic anhydride to eugenol molar ratio of 95 °C and 3 : 1, respectively. The goodness of these theoretically deduced parameters has been experimentally confirmed obtaining, with a flow rate of 40 μL min−1, a 95% conversion. The Amberlyst A-21 packed-bed microreactor also demonstrated good long-term stability ensuring, under the above optimized conditions, a high and stable conversion (over 93%) for prolonged reaction times.

An operationally simple one-pot, two-step procedure for the desymmetrization of benzils is herein described. This consists in the chemoselective cross-benzoin reaction of symmetrical benzils with aromatic aldehydes catalyzed by the methyl sulfinyl (dimsyl) anion, followed by microwave-assisted oxidation of the resulting benzoylated benzoins with nitrate, avoiding the costly isolation procedure. Both electron-withdrawing and electron-donating substituents may be accommodated on the aromatic rings of the final unsymmetrical benzil.

Acetylacetoin synthase (AAS) from Bacillus licheniformis has been partially purified and immobilized on a silica support and its activity was tested under batch conditions in the homo-coupling of a set of α-diketones leading to valuable α-hydroxy ketone derivatives displaying a chiral tertiary alcohol functionality at the α-position. Next, the effectiveness of AAS heterogeneous catalysis has been evaluated under continuous-flow conditions by fabricating the corresponding packed-bed microreactors (pressure-resistant stainless-steel columns). It has been demonstrated that the covalent immobilization on a silica support and the flow regime synergistically contribute to preserve the enzyme activity over time, thus permitting the long-term operation (up to 15 days) of the prepared bioreactors for the production of the chiral targets via the umpolung strategy.

Ofrom C(11) C(12) of secosteroids 4a and 4b affords the compounds 5a (5%) and 5b (20%), respectively.Onthe otherhand,in thebiotransformation of hyocholicacid with R.ruber the9,10-secosteroid4c is notdetected,but,rearrangingtoanintramolecularhemiacetalform,itevolvestothefinalfuranderivative6c(35%) by easy elimination of two molecules of H

A strategy for the synthesis of biologically relevant 5-hydroxy-imidazolidine-2-thione derivatives is presented. A novel class of α-sulfonylamines have been suitably prepared (46–81% yield) as precursors of formal benzylidenethiourea acceptors; these are generated in situ and intercepted by N-heterocyclic carbene (NHC)-activated aldehydes affording open-chain aza-benzoin-type adducts, which in turn undergo an intramolecular aza-acetalization reaction in a one-pot fashion. A thiazolium salt/triethylamine couple proved to be the more effective system to trigger the domino sequence giving the target heterocycles in good yields (45–97%) and diastereoselectivities (up to 99 : 1 dr). The multigram scale synthesis and elaboration of a selected 5-hydroxy-imidazolidine-2-thione compound is also described.

A strategy for the immobilization of the valuable triazolium carbene Rovis catalyst onto polystyrene and silica supports is presented. Initially, the catalyst activity and recyclability were tested under batch conditions in the model stereoselective intramolecular Stetter reaction leading to optically active chromanones. Good results in terms of yield (95%) and enantioselectivity (ee 81–95%) were detected for the polystyrene-supported catalyst (10 mol %), while poorer results were collected for the silica-supported analogue. Also, continuous-flow experiments were performed by fabricating the corresponding polystyrene monolithic microreactors (pressure-resistant stainless-steel columns) to prove the benefits of the heterogeneous catalysis and the flow regime observing a high stability of the catalytic bed (48 h) with unaltered conversion efficiency and stereoselectivity.

Benzoin-type reactions allow the generation of α-hydroxy ketones through the (formal) carboligation of two aldehyde reactants. The synthetic relevance of the products and the wide distribution of the α-hydroxy ketone functionality in bioactive natural compounds have provided the motivation for intensive research efforts directed towards the development of ever more efficient and selective catalysts for reactions of this class. As in many other areas of study, the solution developed in nature – that is, the utilization of thiamine-diphosphate-dependent (ThDP-dependent) enzymes – also in this case allows levels of chemo- and stereoselectivity currently unattainable by biomimetic organocatalysts to be achieved. Here we present an overview of the structural diversity of the α-hydroxy ketone motif achievable through ThDP-dependent enzyme catalysis. Details relating to structure–activity relationships and to rational mutagenesis approaches for improving the catalytic performances of wild-type enzymes are also illustrated.

A convenient heterogeneous continuous-flow procedure for the polarity reversal of aromatic α-diketones is presented. Propaedeutic batch experiments have been initially performed to select the optimal supported base capable to initiate the two electron-transfer process from the carbamoyl anion of the N,N-dimethylformamide (DMF) solvent to the α-diketone and generate the corresponding enediolate active species. After having identified the 2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine on polystyrene (PS-BEMP) as the suitable base, packed-bed microreactors (pressure-resistant stainless-steel columns) have been fabricated and operated to accomplish the chemoselective synthesis of aroylated α-hydroxy ketones and 2-benzoyl-1,4-diones (benzoin- and Stetter-like products, respectively) with a good level of efficiency and with a long-term stability of the packing material (up to five days).

Teucrium chamaedrys, one of the most common and investigated species of the genus Teucrium, has been used for centuries in traditional medicine for many purposes. Its phytochemical components comprise, among others, phenylethanoid glycosides (PGs) and neo-clerodane diterpenoids. Several reports have demonstrated a wide range of beneficial biological and pharmacological activities of the phenylethanoid components, while the diterpenes were shown to be strongly hepatotoxic. In this work, in vitro cultures were established from leaf explants of T. chamaedrys. Both solid (callus) and liquid (cell suspension) cultures maintained the capacity to produce PGs, with teucrioside (TS) representing the most abundant one. Cell suspensions had a lower TS content than that found in leaf extracts, but higher than that of calli. An NMR-based metabolomics approach was used to compare the product profile of intact plants vs. cell suspension cultures, and results showed that neo-clerodane diterpenes, present in the intact plant, were not detected in cell cultures. Several elicitors were supplied to cell cultures with the aim of increasing TS production, and elicitation was tested at different growth phases and by exposing cells for different periods. Methyl jasmonate and fungal mycelia from Trichoderma viridae and Fusarium moniliforme were able to significantly increase TS production if supplied at the early-exponential growth phase for 24 h. Based on the proposed link between proline and the phenylpropanoid pathways, proline accumulation in cell cultures was followed throughout a 14-day culture period, showing that it strictly reflected that of TS production. Moreover, exogenously supplied proline, and its analogue hydroxyproline, turned out to be very effective in increasing teucrioside production. © 2012 Elsevier Ltd. All rights reserved.

Twelve strains of endophytic fungi, isolated from various plants (i.e. Eugenia hallii, Schinus molle, Crataegus monogyna, Juniperus communis and Sambucus nigra) sampled in Amazonian forest and in Italy, were screened for their reduction activity with a cocktail of ketones 1–4. The four most active strains [i.e. Phomopsis (FE86 and FE290), Pestalotia and Epicoccum] were chosen for the reduction of 5-hexen-2-one 1, acetophenone 2, cis-bicyclo[3.2.0]hept-2-en-6-one 3, 2-methylcyclohexanone 4, 6-methyl-5-hepten-2-one 5, 2-furyl methyl ketone 6, 1-indanone 7, and 2,4,4-trimethyl-2-cyclohexen-1-one 8 and in all cases the S-alcohols were obtained with variable yields and enantiomeric excesses depending on the strains.

The condensation of aromatic alpha-diketones (benzils) with aromatic aldehydes ( benzoin-type reaction) and chalcones ( Stetter-type reaction) in DMF in the presence of catalytic ( 25 mol%) KOtBu is reported. Both types of umpolung processes proceed with good efficiency and complete chemoselectivity. On the basis of spectroscopic evidence ( MS analysis) of plausible intermediates and literature reports, the occurrence of different ionic pathways have been evaluated to elucidate the mechanism of a model cross-benzoinlike reaction along with a radical route initiated by an electron-transfer process to benzil from the carbamoyl anion derived from DMF. This mechanistic investigation has culminated in a different proposal, supported by calculations and a trapping experiment, based on double electron-transfer to benzil with formation of the corresponding enediolate anion as the key reactive intermediate. A mechanistic comparison between the activation modes of benzils in KOtBu-DMF and KOtBu-DMSO systems is also described.

In this investigation, two novel multifunctional initiators for ring-opening polymerization were synthesized in three steps starting from cholic acid. Thus, cholic acid (1a) and its methyl ester (1b) were quantitatively transformed, via solvent-free reaction with succinic anhydride, to the corresponding 3, 7, 12-tri-hemisuccinate derivatives (3a-b). The polyacidic compounds (3a-b) were treated with thionyl chloride affording the corresponding acyl chlorides 4a-b which, in turn, were reacted with ethylene glycol to give the derivatives 2a-b having three and four primary alcohol end groups. These compounds, fully characterized by H, C NMR and mass spectrometry, have been assessed as initiators in the ring-opening polymerization of L-lactide using stannous octanoate as catalyst. The resulting three- and four-armed star-shaped poly(L-lactide)s, which were characterized by H NMR, SEC, DSC and TGA analysis, were amorphous, and their glass transition temperatures ranged from 13.7 to 36.5 °C. Additionally, some cholic acid-based star-shaped polylactic structures recently published have been critically reconsidered showing that these molecules, described as star polymers, were actually linear polymers.

Depending on the substitution pattern some products are obtained as mixtures of diastereomers.

A general access to a novel class of sugar α-amino acids composed of iminofuranose and iminopyranose residues anomerically linked to the glycinyl group through an alkyl chain is described. A set of eight compounds was prepared by the same reaction sequence involving as an initial step the Grubbs Ru−carbene-catalyzed cross-metathesis (CM) of various N-Cbz-protected allyl C-iminoglycosides with N-Boc-vinyl- and N-Boc-allyloxazolidine. The isolated yields of the CM products (mixtures of E- and Z-alkenes) varied in the range 40−70%. Each mixture was elaborated by first reducing the carbon−carbon double bond using in situ generated diimide and then unveiling the N-Boc glycinyl group [CH(BocNH)CO2H] by oxidative cleavage of the oxazolidine ring by the Jones reagent. All amino acids were characterized as their methyl esters. The insertion of a model C-iminoglycosyl-2-aminopentanoic acid into a tripeptide via sequential carboxylic and amino group coupling with l-phenylalanine derivatives was carried out as a demo...

The introduction of a formyl group at the anomeric center of 2,3,5-tri-O-benzyl furanoses and substitution of the ring oxygen with a basic nitrogen atom (aminohomologation) was carried out via stereoselective addition of 2-lithiothiazole to N-benzyl, N-furanosylhydroxylamines (masked N-benzyl sugar nitrones), followed by reductive dehydroxylation of the resulting open-chain adducts, and then ring closure via intramolecular displacement of the free hydroxy group by the amino group and unmasking of the formyl group from the thiazole ring. The resulting formyl aza-C-glycosides were transformed into 2,5-dideoxy-2,5-imino-hexitols (pyrrolidine homoazasugars) by reduction of the formyl to the hydroxymethyl group and removal of the O- and N-benzyl groups by hydrogenolysis. This reaction sequence was applied to four furanoses (D-arabino, D-ribo, D-lyxo, L-xylo) to give the hydroxy- and amino-free homoazasugars, including the natural product 2,5-dideoxy-2,5-imino-D-mannitol, in 17% overall yields (six steps). The formyl aza-C-glycosides proved to be valuable intermediates for the synthesis of more complex derivatives. In fact, these sugar aldehydes were employed in Wittig-type coupling reactions with galactose and ribose phosphoranes to give bis-glycosylated alkenes, which upon reduction of the double bond were transformed into methylene isosteres of (1--6)- and (1--5)-linked disaccharides in which one of the two sugar moieties was an azasugar (aza-(1--x)-C-disaccharides).

Formyl C-glycosides, prepared via the thiazole-based formylation of sugar lactones in three steps, are readily condensed with N-benzylhydroxylamine and hydroxylamine to give the corresponding glycosyl nitrones and oximes respectively. The latter imino derivatives are the precursors to nitrile oxides via the N-bromosuccinimide method.

ThDP-dependent cyclohexane-1,2-dione hydrolase (CDH) catalyzes the CC bond cleavage of cyclohexane-1,2-dione to 6-oxohexanoate, and the asymmetric benzoin condensation between benzaldehyde and pyruvate. One of the two reactivities of CDH was selectively knocked down by mutation experiments. CDH-H28A is much less able to catalyze the CC bond formation, while the ability for CC bond cleavage is still intact. The double variant CDH-H28A/N484A shows the opposite behavior and catalyzes the addition of pyruvate to cyclohexane-1,2-dione, resulting in the formation of a tertiary alcohol. Several acyloins of tertiary alcohols are formed with 54-94 % enantiomeric excess. In addition to pyruvate, methyl pyruvate and butane-2,3-dione are alternative donor substrates for CC bond formation. Thus, the very rare aldehyde-ketone cross-benzoin reaction has been solved by design of an enzyme variant.

Deprotonation of DMSO with catalytic amounts of KOtBu generates methylsulfinyl carbanions in situ.

Ofrom C(11) C(12) of secosteroids 4a and 4b affords the compounds 5a (5%) and 5b (20%), respectively.Onthe otherhand,in thebiotransformation of hyocholicacid with R.ruber the9,10-secosteroid4c is notdetected,but,rearrangingtoanintramolecularhemiacetalform,itevolvestothefinalfuranderivative6c(35%) by easy elimination of two molecules of H

A new entry to (1→6)- and (1→5)-aza- C -disaccharides featuring the pyrrolidine ring as the imino sugar component is described via Wittig condensation of polyhydroxylated 2-formylpyrrolidines with galactopyranose 6-phosphorane and ribofuranose 5-phosphorane, respectively.

The preparative-scale regio- and stereo-specific oxidation of hydroxy groups and reduction of keto functions at C(3) of several C21 bile acids, catalyzed by a new 3α-hydroxysteroid dehydrogenase (3α-HSDH), is reported. The crude enzyme, isolated from the cells of Pseudomonas paucimobilis, revealed the presence of a further enzymatic fraction containing a secondary alcohol dehydrogenase (SADH), that has been used to recycle the cofactor.

Preparation of various heteroaryl propanols 2a‐g and of the corresponding propanones 3a‐g as starting materials for microbial redox is described. The kinetic resolution of the racemic propanols 2a‐g is obtained via oxidation with Pseudomonas paucimobilisand Bacillus stearothermophilus[(R)-alcohols, ee 74‐100%]. Similar results are achieved with 3-(2-hydroxypropyl)trifluoromethylbenzene 7 (44%, ee 100% of the (R)-alcohol 6). The reduction of the propanones 3a‐d and 3g with baker’s yeast and other fungi gives the ( S)-alcohols (ee 100%). The pure (S)-alcohols are also obtained by reduction of 1-[3-(trifluoromethyl)phenyl]-2-propanone 7. 1-[(4,4-Dimethyl)2-( 2 )oxazolinyl]-2-propanone 3e and 1[2-( 2 )-thiazolinyl)-2-propanone 3f are not reduced. The heterocyclic rings of (S)-5-(2-hydroxypropyl)-3-methylisoxazole 2d and of (S)-2-(2-hydroxypropyl)-4-methylthiazole 2g are deblocked to the homochiral enamino ketone 8 (78%) and to the protected -hydroxy aldehyde 9 (73%), respectively. The (R)-3-(2-hydroxypropyl)trifluoromethylbenzene 6 is transfomed into the homochiral precursor of (S)-fenfluramine 10 (overall yield 65%). © 1998 Elsevier Science Ltd. All rights reserved.

o-phthalaldehyde inactivates homodimeric, NADP+ dependent, 6-phosphogluconate dehydrogenase from sheep liver, upon formation of a single isoindole derivative per enzyme subunit. This indicates that the thiol group of a cysteine residue or the epsilon-amino group of a lysine residue located within 3 A and crosslinked by the reagent is essential for catalysis. Fluorescence analyses of the modified enzyme suggest that the isoindole derivative forms at the binding site of the nicotinamide moiety of NADP+. The enzymes from Trypanosoma brucei and Lactococcus lactis are also inactivated suggesting a similar three-dimensional structure in this domain. The isoindole derivative does not form with two mutants of the T. brucei enzyme (Lys185His and Lys185Leu), this allowing to identify not only the lysine but also the cysteine involved in the cross-linking. The formation of the isoindole derivative inactivates not only the oxidative decarboxylation, but also two partial reactions catalysed by the enzyme.

The biotransformations of (RS)-linalool (1), (S)-citronellal (2), and sabinene (3) with fungi isolated from the epicarp of fruits of Citrus genus of the Amazonian forest (i.e., C. limon, C. aurantifolia, C. aurantium, and C. paradisiaca) are reported. The more active strains have been characterized, and they belong to the genus Penicillium and Fusarium. Different biotransformation products have been obtained depending on fungi and substrates. (RS)-Linalool (1) afforded the (E)- and (Z)-furanlinalool oxides (7 and 8, resp.; 39 and 37% yield, resp.) with Fusarium sp. (1D2), 6-methylhept-5-en-2-one (4; 49%) with F. fujikuroi, and 1-methyl-1-(4-methypentyl)oxiranemethanol (6; 42%) with F. concentricum. (S)-Citronellal (2) gave (S)-citronellol (12; 36-76%) and (S)-citronellic acid (11; 5-43%) with Fusarium species, while diastereoisomeric p-menthane-3,8-diols 13 and 14 (20 and 50% yield, resp.) were obtained as main products with Penicillium paxilli. Finally, both Fusarium species and P. paxilli biotransformed sabinene (3) to give mainly 4-terpineol (19; 23-56%), and (Z)- and (E)-sabinene hydrates (17 (3-21%) and 18 (11-17%), resp.).

Two methods with either elaborated temperature or microwave irradiation are also presented giving lower yields.

The heterogeneous proline-catalyzed aldol reaction was investigated under continuous-flow conditions by means of a packed-bed microreactor. Reaction-progress kinetic analysis (RPKA) was used in combination with nonlinear chromatography for the interpretation, under synthetically relevant conditions, of important mechanistic aspects of the heterogeneous catalytic process at a molecular level. The information gathered by RPKA and nonlinear chromatography proved to be highly complementary and allowed for the assessment of optimal operating variables. In particular, the determination of the rate-determining step was pivotal for optimizing the feed composition. On the other hand, the competitive product inhibition was responsible for the unexpected decrease in the reaction yield following an apparently obvious variation in the feed composition. The study was facilitated by a suitable 2D instrumental arrangement for simultaneous flow reaction and online flow-injection analysis.

The hitherto unreported ability of the methylsulfinyl carbanion (dimsyl anion) to generate acyl anion equivalents is described. The dimsyl anion, in fact, efficiently catalyzes chemoselective in- termolecular cross-benzoin condensations of diaryl a-diketones (benzils) with various aromatic and ali- phatic aldehydes to give the corresponding aryl-aryl and aryl-alkyl benzoin benzoates in an atom-eco- nomic fashion. The dimsyl anion acts as an environ- mentally friendly alternative to the toxic cyanide anion and it is obtained by in situ deprotonation of dimethyl sulfoxide (DMSO) solvent with a catalytic amount of a strong base, potassium tert-butoxide (t- BuOK) the optimal promoter. The assumption that the methylsulfinyl carbanion is the active catalyst in the title transformation is supported by electrospray ionization mass spectrometry (ESI-MS) experiments.

The biotransformations of hyodeoxycholic acid with various Rhodococcus spp. are reported. Some strains (i.e., Rhodococcus zopfii, Rhodococcus ruber, and Rhodococcus aetherivorans) are able to partially degrade the side chain at C(17) to afford 6α-hydroxy-3-oxo-23,24-dinor-5β-cholan-22-oic acid (2; 23%) and 6α-hydroxy-3-oxo-23,24-dinorchol-1,4-dien-22-oic acid (3; 23–30%), together with two new 9,10-secosteroids 4 and 5 (10–45%), still bearing the partial side chain at C(17) and adopting an intramolecular hemiacetal form. In addition, the 9,10-secosteroid 5 showed an unprecedented C(4)-hydroxylation. The new secosteroids were fully characterized by MS, IR, NMR, and 2D-NMR analyses.

The homocoupling reaction of α-diketones, e.g. (I), is smoothly mediated by thiazolium salt HETP or, more conveniently, by catalytic amounts of commercially available thiazolium salt HEBT.

The activity and stability under flow conditions of covalently and non-covalently silica supported proline and proline-like organocatalysts is herein described. The slow aldol reaction of cyclohexanone with p-nitro benzaldehyde and the fast α-amination of isovaleraldehyde with dibenzyl azodicarboxylate have been selected as model reactions for this study. Prospects and limitations of the disclosed continuous-flow organocatalytic approach are widely discussed.