Batch and in-flow kinetic resolution of racemic 1-(N-acylamino)alkylphosphonic and 1-(N-acylamino)alkylphosphinic acids and their esters using immobilized penicillin G acylase

The kinetic resolution of racemic 1-(N-acylamino)alkylphosphonic acids 3 (R³ = OH) and their dimethyl esters 1, as well as 1-(N-acylamino)alkylphosphinic acids 4 (R³ = H or Ph) using penicillin G acylase (PGA) immobilized on three types of mesoporous silicas in both a batch slurry system and in a continuous-flow reactor was studied. The initial hydrolytic deacylation rates in the presence of those catalysts were measured and the relationships between the substrate structure and the enzyme efficiency are discussed. The stereospecific hydrolysis of the N-acyl group of both racemic N-acylated phosphorus analogues of amino acids and their esters catalyzed by the immobilized PGA proved to be a highly effective method for the kinetic resolution of all the investigated compounds, with the stereochemical preference of PGA for (R)-substrates.     

Lipase-mediated kinetic resolution of (RS)-1-bromo-3-[4-(2-methoxy-ethyl)-phenoxy]-propan-2-ol to (R)-1-bromo-3-(4-(2-methoxyethyl) phenoxy) propan-2-yl acetate

A novel biocatalytic method for the enantioselective synthesis of (R)-bromo-3-[4-(2-methoxy-ethyl) phenoxy]-2-propanol [(R)-BMEPP], a precursor for the synthesis of (S)-metoprolol, an anti hypertensive drug is described. We have developed kinetic resolution of rac-BMEPP by transesterification using Candida rugosa lipase and vinyl acetate as the acyl donor affording the product with excellent conversion (49%) and ee (>99%). Various reaction parameters (source of enzyme, reaction media, and concentration of substrate and acylating agent) for the enzymatic kinetic resolution have been reported.     

Enzymatic resolution of trans-2-hydroxycyclohexanecarbonitrile in supercritical carbon dioxide

A novel, highly enantioselective (E ? 100) and environmentally benign method is presented for the kinetic resolution of trans-2-hydroxycyclohexanecarbonitrile in supercritical carbon dioxide. Using Candida antarctica Lipase B as a biocatalyst and vinyl acetate as an acyl donor, enantiomerically pure (1S,2R)-acetoxycyclohexanecarbonitrile and (1R,2S)-hydroxycyclohexanecarbonitrile were obtained in quantitative yields and excellent ee (98%) values.     

Novel thiophosphonodiamides as efficient hydrogen bond donor catalysts in tandem Michael addition–cyclization of malononitrile and 2-(E)-2-nitrovinylphenols

From l-amino acid, three chiral thiophosphonodiamides were prepared as new hydrogen bond donor organocatalysts. Under the mediation of the catalyst derived from l-valine, a tandem Michael addition/cyclization reaction between (E)-2-(2-nitrovinyl)phenols and malononitrile proceeds smoothly, giving pharmaceutically valuable 2-amino-4H-chromene-3-carbonitriles in high yields with 66–95% ee within 2 h.     

Kinetic study on enantioselective resolution of (R,S)-2-phenylpropionic acid through Novozyme 435–catalyzed esterification

2-Phenylpropionic acid (2-PPA) is a very important chiral intermediate in the synthesis of aryl propionic acid drugs with anti-inflammatory and analgesic effects. Enzymatic kinetic resolution of (R,S)-2-PPA using n-hexanol as an acyl donor was carried out in n-hexane. Lipases from different sources were used to catalyze the esterification of 2-PPA, among which Novozyme 435 had the highest catalytic efficiency. The effects of reaction conditions on conversion (c) and enantiomeric excess (ee), involving temperature, substrate concentrations, enzyme loading, and reaction time were investigated. The kinetic model based on the Ping-Pong bi-bi mechanism was established to simulate the enzymatic esterification process. The experimental values of initial rates of various 2-PPA concentrations were consistent with the simulated values.     

Highly efficient dynamic kinetic resolution of secondary aromatic alcohols with low-cost and easily available acid resins as racemization catalysts

A new and efficient dynamic kinetic resolution (DKR) process of secondary aromatic alcohols was developed with acid resins as racemization catalysts. Acid resin CD8604 was shown to have excellent racemization activity and good biocompatibility. When employing CD8604 and complex acyl donors as racemization catalyst and acyl donor, respectively, enantiomerically pure aromatic acetate was obtained with excellent yield and ee values through the DKR process. It is noteworthy that the system could be reused more than 10 times with little loss of yield and ee value.     

Straightforward Access to Enantioenriched cis-3-Fluoro-dihydroquinolin-4-ols Derivatives via Ru(II)-Catalyzed-Asymmetric Transfer Hydrogenation/Dynamic Kinetic Resolution

Herein we report a practical method for the asymmetric transfer hydrogenation/dynamic kinetic resolution of N-Boc 3-fluoro-dihydrotetrahydroquinolin-4-ones into the corresponding cis-fluoro alcohols in 70–96% yields, up to 99:1 diastereomeric ratio (dr) and up to >99% ee (enantiomeric excess) by using the ruthenium complex Ts-DENEB and a formic acid/triethylamine (1:1) mixture as the hydrogen donor under mild conditions.     

Chiral N-heterocyclic carbenes as asymmetric acylation catalysts

Chiral N-heterocyclic carbenes are generated from C₂-symmetric 1,3-bis(1-arylethyl)imidazolium salts and potassium tert-butoxide. These C₂-symmetric imidazolidenyl carbenes catalyze enantioselective acylation of racemic secondary alcohols. The asymmetric acylation of 1-(1-naphthyl)ethanol was achieved in up to 68% ee of the acylated product, using (R,R)-1,3-bis[(1-naphthyl)ethyl]imidazolium tetrafluoroborate as a precursor of the chiral N-heterocyclic carbene and vinyl propionate as the acyl donor.     

α-Chymotrypsin-catalyzed peptide synthesis in frozen aqueous solution using N-protected amino acid carbamoylmethyl esters as acyl donors

A kinetically controlled peptide synthesis catalyzed by α-chymotrypsin was performed in frozen aqueous solution (ice, −24 °C). The yield of the peptide was significantly improved by the use of the carbamoylmethyl (Cam) ester as the acyl donor instead of the conventional ethyl ester. The peptide yield increased up to ca. 90% when N-benzyloxycarbonyl (CBZ)-Phe-OCam and H-Phe-NH₂ were used as the acyl donor and nucleophile, respectively. Such an improvement of the peptide yield in ice was also observed in the coupling of other CBZ-amino acid Cam esters as acyl donors. Furthermore, this approach was applied to the synthesis of peptides containing d-amino acids. The peptides such as CBZ-d-Phe-Phe-NH₂, CBZ-Phe-d-Phe-NH₂ and CBZ-d-Phe-d-Phe-NH₂ were also obtained in excellent to moderate yields in ice. A high diastereoselectivity towards the l–l peptide was observed when the racemic amino acid Cam ester was used as the acyl donor in ice.     

Synthesis of N-(2-Hydroxy-1-phenoxyacetyl)- prolylproline and Related Cation Binding and Molecular Mechanics Studies

The synthesis of N-(2-hydroxy-1-phenoxyacetyl)prolylproline 2 from 2-acetoxyphen-oxyacetic acid is described. A firstsynthesis led toN-(2-acetoxy-1-phenoxyacetyl)prolylproline methylester 8b that fragmented upon attempted esterhydrolysis with 1N NaOH. A second synthesis gave thecorresponding benzyl ester 13, which wasconverted to 2 by deacylation of the phenolicacetoxy group with pyrrolidine followed byhydrogenolysis of the ester. Cation binding by 2 in methanol and related molecular mechanics (MM)geometric optimizations are discussed.     

Enzymatic resolution of 4-N-phenylacetylamino-derivatives obtained from multicomponent reactions using PenG amidase and in silico studies

The three component coupling reaction of aldehydes, phenylacetamide and dienophiles gave the corresponding N-phenylacetamidocyclohexene derivatives in good yields (55-70%) and high regioselectivity. For the first time, enzymatic kinetic resolution of this class of compounds has been achieved. The enantioselective hydrolysis of 4-N-phenylacetylamino-cis-3a,4,7,7a-hexahydroisoindole-1,3-dione derivatives using Penicillin G amidase (PGA) from Escherichia coli gave the remaining enantiomer with ee-values from 30 to 70% at 50% conversion. On the basis of molecular modeling predictions, 1-N-phenylacetylamino-2-cyano-5-cyclohexene derivatives were synthesized. The kinetic resolution resulted in ees up to 99%. The differences of the observed selectivities confirmed the in silico predictions based on the simulation of enzyme-substrate interactions.     

Enzymatic kinetic resolution of tropic acid

A new strategy has been developed for the CAL-B catalysed kinetic resolution of tropic acid by which both enantiomers of tropic acid can be obtained in good enantiomeric excess. (R)-Tropic acid was synthesised with 90% ee and (S)-tropic acid butyl ester in 99% ee by the hydrolysis of tropic acid butyl ester. The other enantiomers were available through the enzymatically catalysed reaction of tropic acid lactone with butanol to give (S)-tropic acid lactone and (R)-tropic acid ester in >98% ee.     

Penicillin acylase-catalyzed peptide synthesis: a chemo-enzymatic route to stereoisomers of 3,6-diphenylpiperazine-2,5-dione

Chiral dipeptides of phenylglycine were synthesized using immobilized Escherichia coli penicillin acylase. The high selectivity of penicillin acylase for l-amino acids as the nucleophile resulted in the efficient acylation of l-phenylglycine by d-phenylglycine amide at pH 9.7 to give d-phenylglycyl-l-phenylglycine in 69% yield. No isomers or tripeptides were formed. The low enantiospecificity of the enzyme for the acyl donor provided the possibility of preparing the corresponding l,l-dipeptides, starting from l-phenylglycine methyl ester as both donor and acceptor at pH 7.5, resulting in a 63% yield of l-phenylglycyl-l-phenylglycine methyl ester. The product precipitated under the reaction conditions; this effectively prevented the formation of oligomers as well as chemical transformation of the product.The dipeptide esters of phenylglycine easily cyclized to diketopiperazines in aqueous methanol. l-Phenylglycyl-l-phenylglycine methyl ester formed l,l-3,6-diphenylpiperazine-2,5-dione (cis); the achiral trans isomer was obtained from d-phenylglycyl-l-phenylglycine methyl ester.     

Synthesis of N-phenyl β-amino acids via iridium-catalyzed asymmetric hydrogenation using mixed monodentate ligands

The iridium-catalyzed asymmetric hydrogenation of N-phenyl-β-dehydroamino acid derivatives was examined using monodentate phosphoramidite ligands. The highest yields and enantioselectivities were obtained using a mixed ligand approach with PipPhos L1 and achiral triphenylphosphine (full conversion, 70% ee).     

Thermolysin catalyses the synthesis of cyclodextrin esters in DMSO

Fatty acid esters of cyclodextrins (CDs) were synthesised in a one-step reaction with native CDs as acyl acceptors and vinyl-activated fatty acid esters as acyl donors. Immobilised preparations of thermolysin, subtilisin, the alkaline protease AL-89 and Candida antarctica lipase B were investigated for their catalytic properties regarding transesterification in solvents of increasing hydrophilicity. The synthesis of cyclodextrin fatty acid esters was proved to be catalysed enzymatically by thermolysin in DMSO. The obtained products were analysed by TLC and their structures characterised by NMR, MS and FTIR spectroscopy. With vinyl decanoate as acyl donor β-CD was esterified at all seven glucose C-2 positions resulting in heptakis(2-O-decanoyl)-β-cyclodextrin as the major product. With vinyl butyrate, substitution occurred at all the C-2 and partially at the C-3 or C-6 positions resulting in an average degree of substitution of nine. Between 20% and 25% (w/w) of the acyl donor was converted to esters in 20 h corresponding to an estimated total conversion of the acyl acceptor in the case of maltosyl-β-CD. In the subtilisin and AL-89 catalysed reactions, product formation was simultaneously catalysed non-enzymatically by inorganic buffer salts in aprotic, hydrophilic solvents and with the lipase no products were formed in any of the solvents investigated.     

Enzyme-catalyzed preparation of methyl (R)-N-(2,6-dimethylphenyl)alaninate: a key intermediate for (R)-metalaxyl

A biocatalytic approach for the production of (R)-metalaxyl, mefenoxam, has been developed. A practical synthesis of methyl (R)-N-(2,6-dimethylphenyl)alaninate, a key intermediate for (R)-metalaxyl, has been developed by the use of lipase-catalyzed hydrolytic kinetic resolution and chemical racemization of the remaining ester. At high concentrations in aqueous media (300 g/L) lipases were stable and gave moderate to good conversions and excellent enantioselectivities (>98% ee). A simple extraction procedure was used to separate the acid product from the remaining ester and the acid was esterified with methanol to give methyl (R)-N-(2,6-dimethylphenyl)alaninate without any reduction in enantiomeric excess (>98% ee). Subsequent chemical coupling with methoxyacetyl chloride provided enantiomerically pure (R)-metalaxyl (>98% ee) without racemization.     

Lipase catalyzed chemo-enzymatic synthesis of propranolol: A newer enzymatic approach

The biocatalytic processes are greener and safer alternatives for synthesis of drug and drug intermediates. This study reports one such approach of chemo-enzymatic synthesis of propranolol, a β-adrenergic receptor blocker. A green synthetic route was employed for synthesis of propranolol using enzymatic kinetic resolution. Racemic mixture of secondary alcohol [1-chloro-3-(naphthalen-1-yloxy)propan-2-ol] (RS)-5 was synthesized chemically in two step reaction and further its enzymatic kinetic resolution was carried out by using lipase to synthesize enantiomeric forms (R)-5 and S-(6) of propranolol. The kinetic resolution of (RS)-5 involves the transesterification of secondary racemic alcohol in which vinyl acetate is used as acyl donor. Initially, we screened commercially available lipases for kinetic resolution and of all the screened lipases Addzyme 001 showed the best results. The several reaction parameters such as organic solvent, acyl donor, temperature, reaction time and enzyme concentration were optimized to improve rate of reaction and to achieve maximum enantioselectivity. Addzyme 001 at 40 mg shows the maximum conversion rate of 49% using cyclohexane as the organic solvent, vinyl acetate as the acyl donor and it was found that the reaction yield was higher for (R)-5 along with the (S)-6 (eep = 98%, ees = 97%) at 40 °C in 48 h. Further, the treatment of (R)-5 with isopropyl amine resulted into formation of (S)-propranolol eep = 98%, and overall yield 29% independently. To synthesize (R)-propranolol, S-(6) acetate was produced enzymatically and was further deacylated by chemical hydrolysis for the production of (R)-propranolol. This study reports newer approach for synthesis of (R) and (S) propranolol using chemoenzymatic route.     

Metal/linked-BINOL complexes: Applications in direct catalytic asymmetric Mannich-type reactions

Development of metal/linked-BINOL complexes and their applications in direct catalytic asymmetric Mannich-type reactions of hydroxyketones are reviewed. A Et₂Zn/linked-BINOL complex was effective for diastereo- and enantioselective synthesis of β-amino alcohols. By choosing the proper protective groups on the imine nitrogen, either anti- or syn-β-amino alcohol was obtained in excellent enantioselectivity (up to >99.5% ee) using the same zinc catalysis. Y{N(SiMe₃)₂}₃/linked-BINOL complex was effective for various hydroxyketones, affording syn-β-amino alcohols with high enantioselectivity (up to 98% ee). To broaden the nucleophile scope to carboxylic acid derivatives, N-acylpyrrole was utilized as an ester equivalent donor. In(O-iPr)₃/linked-BINOL complex was effective for generating an In-enolate from N-acylpyrrole in situ, giving Mannich adducts with high enantioselectivity (up to 98% ee).     

A comparative study on the acylative kinetic resolution of racemic fluorinated and non-fluorinated 2-methyl-1,2,3,4-tetrahydroquinolines and 3,4-dihydro-3-methyl-2H-[1,4]benzoxazines

The acylative kinetic resolution of racemic 6-fluoro-2-methyl-1,2,3,4-tetrahydroquinoline, 7,8-difluoro-3,4-dihydro-3-methyl-2H-[1,4]benzoxazine, and their non-fluorinated analogues with (S)-naproxen and N-phthaloyl-(S)-amino acyl chlorides has been carried out. It has been shown that the presence of fluorine atoms in the aromatic fragment of a heterocyclic amine results in the increasing stereoselectivity of acylation with (S)-naproxen acyl chloride and in a decrease in the efficiency of acylative kinetic resolution using N-phthaloyl-(S)-amino acyl chlorides. A method for the preparation of enantiopure (S)-6-fluoro-2-methyl-1,2,3,4-tetrahydroquinoline (ee >99%) was developed.     

Acylative kinetic resolution of racemic heterocyclic amines using N-phthaloyl-(S)-amino acyl chlorides with alkyl side chains

A comparative study of the acylative kinetic resolution of racemic 2-methyl-1,2,3,4 tetrahydroquinoline and 3,4-dihydro-3-methyl-2H-[1,4]benzoxazine using N-phthaloyl-(S)-amino acyl chlorides with alkyl side chains has been carried out. The influence of steric factors on the stereoselectivity of the acylation was demonstrated. The (S)-enantiomers of the heterocyclic amines (ee >99%) were obtained in good yields via a kinetic resolution protocol using N-phthaloyl-(S)-leucyl chloride.