Improved chemoenzymatic asymmetric synthesis of (S)-Rivastigmine

(S)-Rivastigmine [(S)-1] was obtained via a four-step synthesis using an asymmetric enzymatic transamination protocol as the key step. An early introduction of the carbamate pharmacophore side chain avoided the use of protective group strategies and hence led to a considerable shortcut. This strategy required a novel ω-transaminase from Paracoccus denitrificans, which could transform the highly polar key substrate 3-acetylphenyl ethyl(methyl)carbamate (4) to the corresponding amine (S)-5 in 99% ee and >80% conversion.     

Modular synthesis of the ClickFerrophos ligand family and their use in rhodium- and ruthenium-catalyzed asymmetric hydrogenation

A series of diphosphine ClickFerrophos ligands (CF), based on a triazoleferrocene backbone, was synthesized in a four-step sequence via click chemistry methodology. In addition to the four previously synthesized ligands CF1, CF4, CF7 and CF10, six novel CF ligands CF2–3 and CF5–8 were prepared. Hydrogenation reactions of alkenes and ketones were significantly improved upon by using CF ligands as rhodium- or ruthenium-complexes in which the % ee values can be optimized by choosing the appropriate CF ligand depending on the substrate.     

Pillar[5]arene based conjugated macrocycle polymers with unique photocatalytic selectivity

The development of heterogeneous catalysts with substrate shape, size or electronic constitution selectivity is a huge challenge in photocatalysis. Reported herein is a host-guest interaction strategy to endow photocatalysts with special selectivity. By adjusting the precursors, conjugated macrocycle polymers (CMPs) with pillar[5]arene struts (CMP-1 and CMP-2) and a corresponding non-pillar[5]arene-contained conjugated organic polymer (COP-1) were prepared and the photocatalytic activities toward sulfide derivatives were investigated. The sulfides showed similar conversions when COP-1 was used as a photocatalyst, but exhibited significant differences when it turned to the CMPs. Remarkably, the conversion yield of S-1 achieved near 18 folds over the one of S-2 when CMP-2 was used as a catalyst. Mechanism studies confirmed that the “host-guest” effect of pillar[5]arene struts in CMPs was the main cause of the difference. The present work establishes CMPs as novel heterogeneous photocatalysts with substrate selectivity, and such a method will inspire the researchers concerning preparation of heterogeneous catalysts with excellent selectivity.     

Simple amphiphilic isosteviol–proline conjugates as chiral catalysts for the direct asymmetric aldol reaction in the presence of water

Two novel amphiphilic catalysts 3 and 4 were synthesized by the condensation of isosteviol with l-proline in a one-pot process. With only 1 mol % loading, the catalyst 3 showed excellent activity (up to >99% yield) and stereoselectivity (up to 99:1 dr, >99% ee) for the direct aldol reaction of cyclohexanone and substituted benzaldehydes at room temperature in the presence of water. In addition, solvent effects, catalyst loading, substrate scope, temperature, and the influence of water on the reactions were investigated. These results demonstrate that the catalysts with a chiral concave and hydrophobic substituent in the 4-position of l-proline furnished high activity and stereoselectivity for the reaction.     

Enantioselective addition of organolithium reagents to imines mediated by C2-symmetric bis(aziridine) ligands

The C₂-symmetric bis(aziridine) ligands 1–5 have been screened in the enantioselective addition of organolithium reagents to imines. Ligand 1 (used in stoichiometric amounts) was found to be superior in terms of chemical yield and enantioselectivity, the best result being 90% yield and 89% e.e. in the addition of vinyllithium to imine 6a. Use of ligand 1 in substoichiometric amounts gave poorer yield and lower enantioselectivity. The enantioselectivity of the reaction was investigated as a function of substrate, reagent, stoichiometry and temperature, but no firm mechanistic conclusions could be drawn. Preliminary results with deuterium-labelled methyllithium indicate complexation/exchange processes involving ligand, reagent and substrate.     

Asymmetric acyloin condensation catalysed by phenylpyruvate decarboxylase. Part 2: Substrate specificity and purification of the enzyme

Phenylpyruvate decarboxylase from Achromobacter eurydice was used to catalyse the asymmetric acyloin condensation of phenylpyruvate 1 with various aldehydes 2 to produce optically active acyloins PhCH₂COCH(OH)R 3. The specific activity of the phenylpyruvate decarboxylase enzyme was increased by a factor of 332 after its purification. The molecular weight of the purified enzyme was shown to be 150 kDa by gel filtration chromatography, while SDS gel electrophoresis showed two sub-units with molecular weights of 90 and 40 kDa. The acyloin condensation yield decreased with increasing chain length for straight chain aliphatic aldehydes from 76% for acetaldehyde to 24% for valeraldehyde. The e.e.s of the acyloin products were 87–98%. Low yields of acyloin products were obtained with chloroacetaldehyde (13%) and glycoaldehyde (16%). Indole-3-pyruvate was a substrate of the enzyme and provided acyloin condensation product 3-hydroxy-1-(3-indolyl)-2-butanone 5 with acetaldehyde in 19% yield, while benzoylformate was not a substrate for the enzyme.     

Evaluating precursor-directed biosynthesis towards novel erythromycins through in vitro studies on a bimodular polyketide synthase

Background: Modular polyketide synthases (PKSs) catalyse the biosynthesis of complex polyketides using a different set of enzymes for each successive cycle of chain extension. Directed biosynthesis starting from synthetic diketides is a potentially valuable route to novel polyketides. We have used a purified bimodular derivative of the erythromycin-producing polyketide synthase (DEBS 1-TE) to study chain extension starting from a variety of diketide analogues and, in some cases, from the alternative acyl-CoA thioester substrates.Results: Chain initiation in vitro by DEBS 1-TE module 2 using a synthetic diketide analogue as a substrate was tolerant of significant structural variation in the starter unit of the synthetic diketide, but other changes completely abolished activity. Interestingly, a racemic β-keto diketide was found to be reduced in situ on the PKS and utilised in place of its more complex hydroxy analogue as a substrate for chain extension. The presence of a diketide analogue strongly inhibited chain initiation via the loading module. Significantly higher concentrations of diketide N-acetylcysteamine analogues than their corresponding acyl-CoA thioesters are required to achieve comparable yields of triketide lactones.Conclusions: Although a broad range of variation in the starter residue is acceptable, the substrate specificity of module 2 of a typical modular PKS in vitro is relatively intolerant of changes at C-2 and C-3. This will restrict the usefulness of approaches to synthesise novel erythromycins using synthetic diketides in vivo. The use of synthetic β-keto diketides in vivo deserves to be explored.     

Enantioselective reduction of propiophenone formed from 3-chloropropiophenone and stereoinversion of the resulting alcohols in selected yeast cultures

Biotransformation of 3-chloro-1-phenylpropan-1-one 1 in sixteen selected cultures of yeast strains has been carried out. For most of the biocatalysts studied the substrate was fully consumed after 1–9 h of transformation, with the exception of the culture of Saccharomyces brasiliensis KCh 905, in which after 24 h trace amounts of the substrate were still visible (2%). However, apart from the expected enantiospecific reduction of the substrate to 3-chloro-1-phenylpropan-1-ol 3, the main biotransformation products comprised of a dehalogenation product—propiophenone 2 and the product of its reduction—1-phenylpropan-1-ol 4. It was only in the cultures of five strains: Saccharomyces brasiliensis KCh 905, Rhodotorula marina KCh 77, Candida parapsilosis KCh 909, Candida viswanathii KCh 120, and Saccharomyces cerevisiae KCh 464 that 3-chloro-1-phenylpropan-1-ol 3 was observed in amounts of more than 10% of the product mixture. (S)-3-Chloro-1-phenylpropan-1-ol with ee = 91% was identified after 9 h of biotransformation in the culture of Candida viswanathii KCh 120, whereas (R)-3-chloro-1-phenylpropan-1-ol with ee = 28% was found in the culture of Aphanocladium album KCh 417. 1-Day biotransformation of propiophenone 2 in the cultures of Rhodotorula strains gave (S)-1-phenylpropan-1-ol 4 with a very high ee (95–99%) and 85–99% of substrate conversion, whereas transformation of this substrate in the cultures of Candida viswanathii KCh 120 and Candida parapsilosis KCh 909 led to (R)-1-phenylpropan-1-ol with ee = 98% and 97%, respectively. During biotransformation of propiophenone the percent composition of the reaction mixtures changed with time. Employment of the racemic mixture of 1-phenylpropan-1-ol 4 as a substrate for biotransformations allowed us to observe that the biocatalysts tested were capable of enantioselective oxidation of (S)-1-phenylpropan-1-ol. An exception was the culture of Rhodotorula glutinis KCh 242, in which after one day of biotransformation (S)-1-phenylpropan-1-ol was obtained with ee = 96%.     

Control of enantioselectivity in the baker's yeast asymmetric reduction of γ-chloro β-diketones to γ-chloro (S)-β-hydroxy ketones

1-Chloro-2,4-alkanediones 1a–f prepared in one step were reduced using baker's yeast to afford 1-chloro-2-hydroxy-4-alkanones 2a–f regioselectively in S 29% to R 58% ee. Use of a small amount of organic solvents to dissolve the substrate enhanced the enantiomeric excess in favor of the S configuration. The function of the organic solvents was studied with reducing enzymes isolated from baker's yeast. Some polar solvents selectively inhibited the enzymes, while nonpolar solvents enhanced the concentration of substrate in water. Application of inhibitors with heat-treatment and organic solvents as additives enhanced the enantiomeric excesses of the products to S 66–96% ee.     

Chemoenzymatic synthesis of β-carboline derivatives using McbA,a new ATP-dependent amide synthetase

McbA was characterized in vitro as a novel amide synthetase in the marinacarbolines A–D biosynthetic pathway, catalyzing amide bond formation between 1-acetyl-3-carboxy-β-carboline (1a) and substituted-β-phenethylamines (1b, 2b, 3b) and tryptamine (4b) in an ATP-dependent manner. Enzyme kinetic analyses highlight β-phenethylamine as the most suitable amine donor. McbA showed broad substrate compatibility with substituted amines; 10 new β-carboline analogues were chemoenzymatically generated.     

Molecular recognition of diketide substrates by a β-ketoacyl-acyl carrier protein synthase domain within a bimodular polyketide synthase

Background: Modular polyketide synthases (PKSs) are large multifunctional proteins that catalyze the biosynthesis of structurally complex bioactive products. The modular organization of PKSs has allowed the application of a combinatorial approach to the synthesis of novel polyketides via the manipulation of these biocatalysts at the genetic level. The inherent specificity of PKSs for their natural substrates, however, may place limits on the spectrum of molecular diversity that can be achieved in polyketide products. With the aim of further understanding PKS specificity, as a route to exploiting PKSs in combinatorial synthesis, we chose to examine the substrate specificity of a single intact domain within a bimodular PKS to investigate its capacity to utilize unnatural substrates.Results: We used a blocked mutant of a bimodular PKS in which formation of the triketide product could occur only via uptake and processing of a synthetic diketide intermediate. By introducing systematic changes in the native diketide structure, by means of the synthesis of unnatural diketide analogs, we have shown that the ketosynthase domain of module 2 (KS2 domain) in 6-deoxyerythronolide B synthase (DEBS) tolerates a broad range of variations in substrate structure, but it strongly discriminates against some others.Conclusions: Defining the boundaries of substrate recognition within PKS domains is crucial to the rationally engineered biosynthesis of novel polyketide products, many of which could be prepared only with great difficulty, if at all, by direct chemical synthesis or semi-synthesis. Our results suggest that the KS2 domain of DEBS1 has a relatively relaxed specificity that can be exploited for the design and synthesis of medicinally important polyketide products.     

Asymmetric synthesis of chiral Roche ester and its derivatives via Rh-catalyzed enantioselective hydrogenation with chiral phosphine-phosphoramidite ligands

Methyl 3-hydroxy-2-methylpropionate, known as the Roche ester, was prepared with high enantioselectivity (up to 96.7% ee) via the Rh-catalyzed asymmetric hydrogenation of methyl 2-hydroxymethylacrylate with a chiral 1,2,3,4-tetrahydro-1-naphthylamine-derived phosphine-phosphoramidite ligand (THNAPhos 5a) even at a low catalyst loading (0.1 mol %). An investigation on the substrate scope revealed that the ester group present in the substrate has a significant effect on the enantioselectivity, and the substrate with the bulkier ester group tended to give lower enantioselectivity.     

Biocatalytic oxidation of thiophosphoryl compounds: a new chemo-enzymatic approach to enantiomeric insecticidal thionophosphates and their oxons

The biocatalytic oxidation of racemic O-S-dimethyl O-p-nitrophenyl phosphorodithioate 5 catalyzed by chloroperoxidase from Caldariomyces fumago resulted in the formation of the (?)-(S)-enantiomer of the corresponding oxon 4 and unoxidized substrate 5 with a (+)-(R)-configuration. Both compounds were obtained with very high enantiomeric excesses, 99.6% and 97%, respectively. The thionation reaction of the resulting (?)-(S)-oxon 4 with Lawesson’s reagent gave (?)-(S)-phosphorodithioate 5 with full stereoselectivity, while the oxidation of unreacted substrate (+)-(R)-5 with iodoxybenzene afforded oxon (+)-(R)-4 with 94.9% ee.     

A novel atom-transfer cyclisation catalysed by indium metal in halogenated solvents

Treatment of tethered alkyne-allyl halides 1a-d with indium metal in halogenated solvents affords carbocyclic vinyl halides (3a-d) via a novel atom-transfer reaction. The reactions are operationally facile and proceed smoothly at room temperature even with sub-stoichiometric quantities of the metal. Use of a halogenated solvent containing a different halide than that contained in the substrate affords a mixture of products arising from intramolecular halide transfer and abstraction of a halide atom from solvent.     

Preparation of tricyclic imidazopyridines by asymmetric ketone hydrogenation in the presence of ruthenium phosphino-oxazoline catalyst

The asymmetric hydrogenation of the prochiral heterocyclic ketone 2 and its O-protected analogues 9 to 11 in the presence of the ruthenium POX catalyst RuCl₂(PPh₃)(Ph₂P-Fc-oxaⁱPr) was studied. The reactivity of the substrate and the enantioselectivity of the reduction depended on the nature of the protecting group. The best results were achieved with the thexyldimethylsilyl protecting group: the corresponding alcohol 13 was obtained in 86% yield and 88% ee and constitutes a valuable intermediate for the synthesis of the potassium-competitive acid blocker BYK 311319 1.     

Furanoside phosphite–phosphoroamidite and diphosphoroamidite ligands for Cu-catalyzed asymmetric 1,4-addition reactions

A sugar-based phosphite–phosphoroamidite and diphosphoroamidite ligand library L1–L5a–g was tested in the asymmetric Cu-catalyzed 1,4-conjugate addition reactions of β-substituted and β,β′-disubstituted enones. Our results indicated that the selectivity was strongly dependent on the ligand parameters and on the substrate structure. Moderate-to-good enantioselectivities (ees up to 84%) were obtained in the 1,4-addition of several types of β-substituted cyclic and linear substrates. Of particular note is the high enantioselectivity (ees up to 90%) obtained for the more challenging β,β′-disubstituted 3-methyl-cyclohexenone.     

Asymmetric hydrolysis of a meso-diester using pig liver esterase immobilised in hollow fibre ultrafiltration membrane

Pig liver esterase (PLE) was physically immobilised in a polysulphone ultrafiltration hollow fibre membrane reactor and used for the repetitive batch two-phase hydrolysis and separation, on a multigram scale, of the meso-diester dimethyl cis-cycloxex-4-ene-1,2-dicarboxylate 1 to enantiomerically pure (1S,2R)-cyclohex-4-ene-1,2-dicarboxylic acid monomethyl ester 2. After 25 days, the enzyme still retained its initial activity, which corresponds to 62% of its activity in the free form, and the enantiomeric purity of monoester 2 was still higher than 97%. Simple experimental conditions were established for the large laboratory scale preparation of substrate 1 and isolation of product 2 from the aqueous phase.     

Fine-tunable monodentate phosphoroamidite and aminophosphine ligands for Rh-catalyzed asymmetric hydroformylation

A biaryl-based monophosphoroamidite L1–L4a–f and aminophosphine L5–L7a–f ligand library was screened in the Rh-catalyzed asymmetric hydroformylation of several vinylarenes and heterocyclic olefins. Our results indicate that the selectivity is strongly dependent on the ligand parameters and on the substrate type. Enantioselectivities (up to 46%) were moderate in the hydroformylation of several vinylarenes S1–S5 and promising (up to 58%) for the more challenging heterocyclic olefins S6–S9.     

Synthesis,molecular docking with COX 1& II enzyme,ADMET screening and in vivo anti-inflammatory activity of oxadiazole,thiadiazole and triazole analogs of felbinac

Based on the core structure of Felbinac drug, three series (4a–d, 5a–d and 6a–n) of five membered heterocyclic derivatives containing three heteroatoms were designed and synthesized starting from Felbinac. In the rational design of the target molecules, the biphenyl ring along with the methylene bridge of felbinac was retained while the carboxyl group was substituted with biologically active substituents like 1,2,4-triazole, 1,3,4-thiadiazole and 1,3,4-oxadiazole, with an intent to obtain novel, better and safer anti-inflammatory agents with improved efficacy. The prepared molecules were then investigated for their anti-inflammatory, ulcerogenicity and analgesic activity in experimental animals. The tested compounds exhibited varying degrees of inflammatory activity (25.21–72.87%), analgesic activity (27.50–65.24%) and severity index on gastric mucosa in the range of 0.20–0.80 in comparison to positive control felbinac (62.44%, 68.70% and 1.5, respectively). Among all the prepared compounds, 2-(biphenyl-4-ylmethyl)-5-(4-chlorophenyl)-1,3,4-oxadiazole (6c) emerged as the most potent NSAID compound exhibiting the highest anti-inflammatory activity (72.87% inhibition) and analgesic activity (65.24%) along with the least severity index on gastric mucosa (0.20). Further, molecular docking on cyclooxygenase and in silico ADME-Toxicity prediction studies also supported the experimental biological results and indicated that 6c has a potential to serve as a drug candidate or lead compound for developing novel anti-inflammatory and analgesic therapeutic agent(s) with minimum toxicity on gastric mucosa.