Lipase-Catalyzed Strategies for the Preparation of Enantiomeric THIQ and THßC Derivatives: Green Aspects.

This report reviews the most important lipase-catalyzed strategies for the preparation of pharmaceutically and chemically important tetrahydroisoquinoline and tetrahydro-ß-carboline enantiomers through O-acylation of the primary hydroxy group, N-acylation of the secondary amino group, and COOEt hydrolysis of the corresponding racemic compounds with simple molecular structure, which have been reported during the last decade. A brief introduction describes the importance and synthesis of tetrahydroisoquinoline and tetrahydro-ß-carboline derivatives, and it formulates the objectives of this compilation. The strategies are presented in chronological order, classified according to function of the reaction type, as kinetic and dynamic kinetic resolutions, in the main text. These reactions result in the desired products with excellent ee values. The pharmacological importance of the products together with their synthesis is given in the main text. The enzymatic hydrolysis of the hydrochloride salts as racemates of the starting amino carboxylic esters furnished the desired enantiomeric amino carboxylic acids quantitatively. The enzymatic reactions, performed in tBuOMe or H2O as usable solvents, and the transformations carried out in a continuous-flow system, indicate clear advantages, including atom economy, reproducibility, safer solvents, short reaction time, rapid heating and compression vs. shaker reactions. These features are highlighted in the main text.

A Sustainable Green Enzymatic Method for Amide Bond Formation.

A sustainable enzymatic strategy for the preparation of amides by using Candida antarctica lipase B as the biocatalyst and cyclopentyl methyl ether as a green and safe solvent was devised. The method is simple and efficient and it produces amides with excellent conversions and yields without the need for intensive purification steps. The scope of the reaction was extended to the preparation of 28 diverse amides using four different free carboxylic acids and seven primary and secondary amines, including cyclic amines. This enzymatic methodology has the potential to become a green and industrially reliable process for direct amide synthesis.

Macrocyclic glycopeptides- and derivatized cyclofructan-based chiral stationary phases for the enantioseparation of fluorinated ß-phenylalanine analogs.

The enantioseparation of five fluorinated ß-phenylalanine analogs together with the nonfluorinated α- and ß-phenylalanines has been investigated utilizing chiral stationary phases. The employed chiral selectors include macrocyclic antibiotics, such as vancomycin, teicoplanin, and teicoplanin aglycone, isopropyl carbamate functionalized cyclofructan-6, and Cinchona alkaloid-based tert.-butyl carbamate quinine, all covalently bonded to 2.7 µm superficially porous silica particles. The applied conditions included reversed-phase and polar-ionic modes where the vancomycin-, and the cyclofructan-6-based core-shell particles proved to offer suitable efficiency. Under reversed-phase conditions typical hydrophobic chromatographic behavior was observed, especially in the H2O/MeOH system. The improved selectivity with increasing MeOH content observed in polar ionic mode suggests that H-bonding may not play a major role in the chiral recognition. The stoichiometric displacement model was probed to gather information on the ionic interactions. The ion-exchange process was found to affect retention, but it has no essential contribution to chiral recognition. Without paying special attention to the optimization of the system volume of the UHPLC instrument plate heights varying in the range of 10-50 µm were obtained. In all cases, retention and selectivity decreased with increasing temperature, and enthalpy-driven enantiorecognition was observed. Elution sequences were determined in all cases.

Enzymatic Strategies for the Preparation of Pharmaceutically Important Amino Acids through Hydrolysis of Amino Carboxylic Esters and Lactams.

The most relevant lipase-catalyzed strategies for the synthesis of pharmaceutically important cyclic and acyclic α-, ß- and γ-amino carboxylic acid enantiomers through hydrolysis of the corresponding amino carboxylic esters and lactams, over the last decade are overviewed. A brief Introduction part deals with the importance and synthesis of enantiomeric amino acids, and formulates the objectives of the actual work. The strategies are presented in the Main Text, in chronological order, classified as kinetic, dynamic kinetic and sequential kinetic resolution. Mechanistic information of the enzymatic transformations is also available at the end of this overview. The pharmacological importance of the enantiomeric amino acids is given next to their synthesis, in the Main Text, and it is also illustrated in the Conclusions and Outlook sections.

Green Strategies for the Preparation of Enantiomeric 5-8-Membered Carbocyclic ß-Amino Acid Derivatives through CALB-Catalyzed Hydrolysis.

Candida antarctica lipase B-catalyzed hydrolysis of carbocyclic 5−8-membered cis ß-amino esters was carried out in green organic media, under solvent-free and ball-milling conditions. In accordance with the high enantioselectivity factor (E > 200) observed in organic media, the preparative-scale resolutions of ß-amino esters were performed in tBuOMe at 65 °C. The unreacted ß-amino ester enantiomers (1R,2S) and product ß-amino acid enantiomers (1S,2R) were obtained with modest to excellent enantiomeric excess (ee) values (ees > 62% and eep > 96%) and in good chemical yields (>25%) in one or two steps. The enantiomers were easily separated by organic solvent/H2O extraction.

Enantioselective high-performance liquid chromatographic separation of fluorinated ß- phenylalanine derivatives utilizing Cinchona alkaloid-based ion-exchanger chiral stationary phases: Enantioselective separation of fluorinated ß-phenylalanine derivatives.

The enantioselective separation of newly synthesized fluorine-substituted ß-phenylalanines has been performed utilizing Cinchona alkaloid-based ion-exchanger chiral stationary phases. Experiments were designed to study the effect of eluent composition, counterion content, and temperature on the chromatographic properties in a systematic manner. Mobile phase systems containing methanol or mixtures of methanol and acetonitrile together with acid and base additives ensured highly efficient enantioseparations. Zwitterionic phases [Chiralpak ZWIX (+) and ZWIX(-)] were found to provide superior performance compared to that by the anion-exchangers (Chiralpak QN-AX and QD-AX). A detailed thermodynamic characterization was also performed by employing van't Hoff analysis. Using typical liquid chromatographic experimental conditions, no marked effect of the flow rate could be observed on the calculated thermodynamic parameters. In contrast, a clear tendency has been revealed about the effect of the eluent composition on the thermodynamics for the zwitterionic phases.

Cinchona-alkaloid-based zwitterionic chiral stationary phases as potential tools for high-performance liquid chromatographic enantioseparation of cationic compounds of pharmaceutical relevance.

Enantiomers of cationic compounds of pharmaceutical relevance, namely tetrahydro-ß-carboline and 1,2,3,4-tetrahydroisoquinoline analogs, were separated by high-performance liquid chromatography. Separations were performed on Cinchona-alkaloid-based zwitterionic ion exchanger type chiral stationary phases applied as cation exchangers using mixtures of methanol and acetonitrile or tetrahydrofuran as bulk solvent components containing triethylammonium acetate or ammonium acetate as organic salt additives. On the zwitterionic ZWIX(+) and ZWIX(-) columns investigated, retention and enantioseparation of the studied basic analytes were influenced by the nature and concentration of the organic components of the mobile phase. The effect of organic salt additives on the retention behavior of the studied analytes can be described by the stoichiometric displacement model related to the counterion concentration. Investigations on the structure-retention relationships were performed applying different mobile phase systems for the two types of cationic analytes. For the thermodynamic characterization, parameters such as changes in standard enthalpy (Δ(ΔH°)), entropy (Δ(ΔS°)), and free energy (Δ(ΔG°)) were calculated on the basis of van't Hoff plots derived from the ln α versus 1/T curves. In most cases, enthalpy-driven enantioseparations were observed, with a consistent dependence of the calculated thermodynamic parameters on the mobile phase composition. Elution sequences of the studied compounds were determined in all cases.

High-performance liquid chromatographic evaluation of strong cation exchanger-based chiral stationary phases focusing on stationary phase characteristics and mobile phase effects employing enantiomers of tetrahydro-ß-carboline and 1,2,3,4-tetrahydroisoquinoline analogs.

In this study, we present results obtained on the enantioseparation of some cationic compounds of pharmaceutical relevance, namely tetrahydro-ß-carboline and 1,2,3,4-tetrahydroisoquinoline analogs. In high-performance liquid chromatography, chiral stationary phases (CSPs) based on strong cation exchanger were employed using mixtures of methanol and acetonitrile or tetrahydrofuran as mobile phase systems with organic salt additives. Through the variation of the applied chromatographic conditions, the focus has been placed on the study of retention and enantioselectivity characteristics as well as elution order. Retention behavior of the studied analytes could be described by the stoichiometric displacement model related to the counter-ion effect of ammonium salts as mobile phase additives. For the thermodynamic characterization parameters, such as changes in standard enthalpy Δ(ΔH°), entropy Δ(ΔS°), and free energy Δ(ΔG°), were calculated on the basis of van't Hoff plots derived from the ln α vs. 1/T curves. In all cases, enthalpy-driven enantioseparations were observed with a slight, but consistent dependence of the calculated thermodynamic parameters on the eluent composition. Elution sequences of the studied compounds were determined in all cases. They were found to be opposite on the enantiomeric stationary phases and they were not affected by either the temperature or the eluent composition.

Efficient Synthesis of New Fluorinated ß-Amino Acid Enantiomers through Lipase-Catalyzed Hydrolysis.

An efficient and novel enzymatic method has been developed for the synthesis of ß-fluorophenyl-substituted ß-amino acid enantiomers through lipase PSIM (Burkholderia cepasia) catalyzed hydrolysis of racemic ß-amino carboxylic ester hydrochloride salts 3a-e in iPr2O at 45 °C in the presence of Et3N and H2O. Adequate analytical methods were developed for the enantio-separation of racemic ß-amino carboxylic ester hydrochlorides 3a-e and ß-amino acids 2a-e. Preparative-scale resolutions furnished unreacted amino esters (R)-4a-e and product amino acids (S)-5a-e with excellent ee values (≥99%) and good chemical yields (>48%).

Effects of N-methylation and amidination of cyclic ß-amino acids on enantioselectivity and retention characteristics using Cinchona alkaloid- and sulfonic acid-based chiral zwitterionic stationary phases.

In the present work the effects of N-methylation and N-amidination of ampholytic cyclic ß-amino acids on their retention and enantioseparation characteristics on Cinchona alkaloid- and sulfonic acid-based zwitterionic chiral selectors, namely Chiralpak ZWIX(+)™ and ZWIX(-)™ columns are described. In a polar-ionic mobile phase, a double ion-pairing interaction mechanism takes place between the charged sites of the chiral analytes (the selectands) and the chiral selector moieties. As a support to correlate the chromatographic results with the structural details of the analytes, pKa values and van der Waals volumes of the substituted amino groups were calculated. In order to ensure better understanding of the mechanistic details of the chromatographic system the composition of the bulk solvent, the role of acid base additives, the concentration of the counter-ions, temperature and the structures of the ampholytic analytes have been investigated. Applying N-Fmoc protection, the ampholytic character of the analytes diminished, leading to a marked loss of retention. In the temperature range studied (5-40 °C) thermodynamic parameters, such as the difference in the standard enthalpy change Δ(ΔH°), entropy Δ(ΔS) and Gibbs energy Δ(ΔG°) were calculated from linear van't Hoff plots derived from the ln α vs. 1/T curves. The values of the thermodynamic parameters depended on the structures of the chiral selectors applied and the analytes studied.

Efficient Enzymatic Routes for the Synthesis of New Eight-membered Cyclic ß-Amino Acid and ß-Lactam Enantiomers.

Efficient enzymatic resolutions are reported for the preparation of new eight-membered ring-fused enantiomeric ß-amino acids [(1R,2S)-9 and (1S,2R)-9] and ß-lactams [(1S,8R)-3, (1R,8S)-3 (1S,8R)-4 and (1R,8S)-7], through asymmetric acylation of (±)-4 (E > 100) or enantioselective hydrolysis (E > 200) of the corresponding inactivated (±)-3 or activated (±)-4 ß-lactams, catalyzed by PSIM or CAL-B in an organic solvent. CAL-B-catalyzed ring cleavage of (±)-6 (E > 200) resulted in the unreacted (1S,8R)-6, potential intermediate for the synthesis of enantiomeric anatoxin-a. The best strategies, in view of E, reaction rate and product yields, which underline the importance of substrate engineering, are highlighted.

Enzymatic Strategy for the Resolution of New 1-Hydroxymethyl Tetrahydro-ß-carboline Derivatives in Batch and Continuous-Flow Systems.

Many alkaloids containing a tetrahydro-ß-carboline skeleton have well-known therapeutic effects, leading to increased interest in the synthesis of these natural products. Enantiomers of N-Boc-protected 1-hydroxymethyl-1,2,3,4-tetrahydro-ß-carboline [(±)-7], 1-hydroxymethyl-6-methoxy-1,2,3,4-tetrahydro-ß-carboline [(±)-8], and 1-hydroxymethyl-6-fluoro-1,2,3,4-tetrahydro-ß-carboline [(±)-9] were prepared through enzymecatalyzed asymmetric acylation of their primary hydroxyl group. The preliminary experiments were performed in a continuous-flow system, while the preparative-scale resolutions were done as batch reactions. Excellent enantioselectivities (E>200) were obtained with Candida antarctica lipase B (CAL-B) and acetic anhydride in toluene at 60 °C. The recovered alcohols and the produced esters were obtained with high enantiomeric excess values (ee≥96 %). The O-acylated enantiomers [(S)-10-(S)-12)] were transformed into the corresponding amino alcohols [(S)-7-(S)-9)] with methanolysis. Microwave-assisted Boc removals were also performed and resulted in the corresponding compounds (R)-4-(R)-6 and (S)-4-(S)-6 without a drop in the enantiomeric excess values (ee≥96 %).

Enantioseparation of ß-carboline derivatives on polysaccharide- and strong cation exchanger-based chiral stationary phases. A comparative study.

In this study we attempted to describe in a comparative manner the enantioselectivity performance of six different polysaccharide- and two strong cation exchanger-type chiral stationary phases (CSPs) for the resolution of free and N-protected ß-carboline derivatives. On commercially available cellulose- or amylose-based CSPs, the enantioseparations were carried out in normal-phase mode by variation of the nature and the concentration of the alcohol modifier in n-hexane as mobile phase. With the application of strong cation exchanger-type CSPs, the enantioseparations were optimized by the variation of methanol-acetonitrile bulk solvent compositions in the presence of various amounts of acid and base additives acting as counter-ions. Detailed thermodynamic investigations revealed that in all cases the enantioseparations observed were enthalpically driven, i.e. the retention and selectivity decreased with increasing temperature. Elution sequences were determined routinely; no general rule was found on polysaccharide-based CSPs, while on the two enantiomeric strong cation exchanger-type CSPs the predicted reversal of the elution sequence could be confirmed on switching from one enantiomeric CSP to the other form.

Combinatorial effects of the configuration of the cationic and the anionic chiral subunits of four zwitterionic chiral stationary phases leading to reversal of elution order of cyclic ß3-amino acid enantiomers as ampholytic model compounds.

In a systematic way enantioseparations of non-methylated and the corresponding N-monomethylated ampholytic cyclic ß3-amino acids were carried out on four zwitterionic chiral stationary phases (CSPs; ZWIX(+)™, ZWIX(-)™, ZWIX(+A), ZWIX(-A)). CSPs were based on the combinations of quinine and quinidine as the cationic and of (R,R)- and (S,S)-aminocyclohexane sulfonic acid as the anionic sites. In polar-ionic mobile phase systems, the effects of the composition of the bulk solvents, the additives, the concentration of the co- and counter-ions, the temperature, and the structures of the ampholytic analytes were investigated. The changes in standard enthalpy, Δ(ΔH°), entropy, Δ(ΔS°), and free energy, Δ(ΔG°), were calculated from the linear van't Hoff plots derived from the ln α vs 1/T curves in the studied temperature range (5-40°C). Unusual temperature behavior was observed on the ZWIX(-)™ column: decreased retention times were accompanied by increased separation factors with increasing temperature, and separation was entropically-driven. For the other three CSPs, enthalpically-driven enantioseparations were observed. Via the consequent determination of the elution order of the resolved enantiomers, the effects of the absolute configuration of the chiral anionic and cationic subunits of the zwitterionic CSPs could be elucidated. N-methylation of the amino acids led unexpectedly to a reversal of the elution sequence, which can be interpreted by a subtle shift of the hierarchical order of the sterically most important driving interaction sites from the cationic to the anionic units, and vice versa.

High-performance liquid chromatographic enantioseparation of fluorinated cyclic ß(3) -amino acid derivatives on polysaccharide-based chiral stationary phases. Comparison with nonfluorinated counterparts.

The stereoisomers of five fluorinated cyclic ß(3) -amino acid derivatives and their nonfluorinated counterparts were separated on chiral stationary phases containing as chiral selectors cellulose tris-(3,5-dimethylphenyl carbamate), cellulose tris-(3-chloro-4-methylphenyl carbamate), cellulose tris-(4-methylbenzoate), cellulose tris-(4-chloro-3-methylphenyl carbamate), amylose tris-(3,5-dimethylphenyl carbamate) or amylose tris-(5-chloro-2-methylphenyl carbamate). The enantioseparations were carried out in normal-phase mode with n-hexane/alcohol/alkylamine mobile phases in the temperature range 5-40 °C. The effects of the mobile phase composition, the nature and concentration of the alcohol and alkylamine additives, the structures of the analytes and temperature on the separations were investigated. Thermodynamic parameters were calculated from plots of ln α vs. 1/T. The Δ(ΔH°) values ranged between -5.0 and +1.6 kJ/mol, while Δ(ΔS°) varied between -12.6 and +5.7 J/mol/K. The enantioseparation was enthalpically controlled, the retention factor and the separation factor decreasing with increasing temperature, but entropically controlled separation was also observed. The elution sequence was determined for all of the investigated analytes. Copyright © 2016 John Wiley & Sons, Ltd.

Stereo- and Regiocontrolled Syntheses of Exomethylenic Cyclohexane ß-Amino Acid Derivatives.

Cyclohexane analogues of the antifungal icofungipen [(1R,2S)-2-amino-4-methylenecyclopentanecarboxylic acid] were selectively synthesized from unsaturated bicyclic ß-lactams by transformation of the ring olefinic bond through three different regio- and stereocontrolled hydroxylation techniques, followed by hydroxy group oxidation and oxo-methylene interconversion with a phosphorane. Starting from an enantiomerically pure bicyclic ß-lactam obtained by enzymatic resolution of the racemic compound, an enantiodivergent procedure led to the preparation of both dextro- and levorotatory cyclohexane analogues of icofungipen.

Utilization of (18-crown-6)-2,3,11,12-tetracarboxylic acid as a chiral NMR solvating agent for diamines and ß-amino acids.

The compound (18-crown-6)-2,3,11,12-tetracarboxylic acid was evaluated as a chiral nuclear magnetic resonance (NMR) solvating agent for a series of diamines and bicyclic ß-amino acids. The amine must be protonated for strong association with the crown ether. An advantage of (18-crown-6)-2,3,11,12-tetracarboxylic acid over many other crown ethers is that it undergoes a neutralization reaction with neutral amines to form the protonated species needed for binding. Twelve primary diamines in neutral and protonated forms were evaluated. Diamines with aryl and aliphatic groups were examined. Some are atropisomers with equivalent amine groups. Others have two nonequivalent amine groups. Association equilibria for these systems are complex, given the potential formation of 2:1, 1:1, and 1:2 crown-amine complexes and given the various charged species in solution for mixtures of the crown ether with the neutral amine. The crown ether produced enantiomeric differentiation in the (1) H NMR spectrum of one or more resonances for every diamine substrate. Also, a series of five bicyclic ß-amino acids were examined and (18-crown-6)-2,3,11,12-tetracarboxylic acid caused enantiomeric differentiation in the (1) H NMR spectrum of three or more resonances of each compound.

Novel stereocontrolled syntheses of tashiromine and epitashiromine.

A novel stereocontrolled approach has been developed for the syntheses of tashiromine and epitashiromine alkaloids from cyclooctene ß-amino acids. The synthetic concept is based on the azetidinone opening of a bicyclic ß-lactam, followed by oxidative ring opening through ring C-C double bond and reductive ring-closure reactions of the cis- or trans-cyclooctene ß-amino acids.

High-performance liquid chromatographic separation of paclitaxel intermediate phenylisoserine derivatives on macrocyclic glycopeptide and cyclofructan-based chiral stationary phases.

High-performance liquid chromatographic methods were developed for the separation of enantiomers of four unnatural paclitaxel precursor phenylisoserine analogs on chiral stationary phases containing macrocyclic glycopeptides and cyclofructans as chiral selectors. The effects of the mobile phase composition, the nature and concentration of different mobile phase additives (alcohols, amines and acids) in different chromatographic modes, temperature and the structures of the analytes on the separations were investigated. Separations were carried out at constant mobile phase compositions in the temperature range 10-50°C on macrocyclic antibiotic-based and 5-35°C on cyclofructan-based columns and the changes in enthalpy, Δ(ΔH°), entropy, Δ(ΔS°), and free energy, Δ(ΔG°), were calculated. The elution sequence was determined in most cases; no general rule could be observed.

High-Performance Liquid Chromatographic Enantioseparation of Cyclic ß-Amino Acids on Zwitterionic Chiral Stationary Phases Based on Cinchona Alkaloids.

Stereoselective high-performance liquid chromatographic separations of eight sterically constrained cyclic ß-amino acid enantiomer pairs were carried out using the newly developed Cinchona alkaloid-based zwitterionic chiral stationary phases Chiralpak ZWIX(+) and ZWIX(-). The effects of the mobile phase composition, the nature and concentrations of the acid and base additives, the counterions and temperature on the separations were investigated. The changes in standard enthalpy, Δ(ΔH°), entropy, Δ(ΔS°), and free energy, Δ(ΔG°), were calculated from the linear van't Hoff plots derived from the ln α vs. 1/T curves in the studied temperature range (10-50°C). The values of the thermodynamic parameters depended on the nature of the selectors and the structures of the analytes. Unusual temperature behavior was observed on the ZWIX(-) column: decreased retention times were accompanied by increased separation factors with increasing temperature. On the ZWIX(+) column only enthalpically, whereas on the ZWIX(-) column both enthalpically and entropically driven separations were observed. The elution sequence was determined in all cases and was observed to be the opposite on ZWIX(+) and on ZWIX(-).