Enantioselective enzymatic desymmetrizations in organic synthesis

Highly efficient synthesis of enantiomerically enriched 2-hydroxymethylaziridines by enzymatic desymmetrization

Both enantiomers of protected and unprotected 2-hydroxymethylaziridines are efficiently and enantiospecifically synthesized by using a combination of enzymatic and synthetic methods. PPL was used for lipase-catalyzed desymmetrization of N-protected serinol. [reaction: see text].

Remote desymmetrization at near-nanometer group separation catalyzed by a miniaturized enzyme mimic

The chirality of biological receptors often requires syntheses of therapeutic compounds in single enantiomer form. The field of asymmetric catalysis addresses enantioselective synthesis with chiral catalysts. Chemical differentiation of sites within molecules that are separated in space by long distances presents special challenges to chiral catalysts. As the distance between enantiotopic sites increases within a substrate, so too may the requirements for size and complexity for the catalyst. The extreme of catalyst complexity could be defined by macromolecular enzymes and their amazing capacity to effect stereospecific reactions over long distances between reactive sites and enzyme-substrate contacts. We report here a synthetic, miniaturized enzyme mimic that catalyzes a desymmetrization reaction over a very long distance.

Biocatalysis for pharmaceutical intermediates: the future is now

Biocatalysis is continuing to gain momentum and is now becoming a key component in the toolbox of the process chemist, with a place alongside chemocatalysis and chromatographic separations. The pharmaceutical industry demands a speed of development that must be on a parallel with conventional chemistry and high optical purity for complex compounds with multiple chiral centres. This review describes how these demands are being addressed to make biocatalysis successful, particularly by the use of micro-scale technology for high-speed catalyst screening and process development alongside discipline integration of biology and engineering with chemistry. Developments in recombinant technology will further expand the repertoire of biocatalysis in the coming years to new chemistries and enable catalyst design to fit the process. Further development of biocatalysis for green chemistry and high productivity processes can also be expected.

Lipase-mediated purification of methyl nonactate, an important natural product building block for diversity-oriented synthesis

Methyl nonactate is a valuable starting material for the production of natural product-like combinatorial libraries and ketide amino acids, a series of novel conformationally constrained amino acid analogues. Fermentation of Streptomyces griseus generates high titers of macrotetrolide antibiotics from which methyl nonactate can be generated by methanolysis. Unfortunately, this approach generates mixtures of homologues that are not economically separable. We report a specific lipase-mediated hydrolysis of nonactate derivatives that discriminates between homologues and is the foundation of an economically tractable preparation of methyl nonactate in scale.

Enantioselective desymmetrization of prochiral cyclohexanone derivatives via the organocatalytic direct Aldol reaction

Asymmetric desymmetrization of 4-substituted cyclohexanones using proline amide-catalyzed direct aldol reaction afforded beta-hydroxyketones with three stereogenic centers in high enantioselectivities of up to >99% ee.

Asymmetric Reduction and Oxidation of Aromatic Ketones and Alcohols Using W110A Secondary Alcohol Dehydrogenase fromThermoanaerobacterethanolicus

An enantioselective asymmetric reduction of phenyl ring-containing prochiral ketones to yield the corresponding optically active secondary alcohols was achieved with W110A secondary alcohol dehydrogenase from Thermoanaerobacter ethanolicus (W110A TESADH) in Tris buffer using 2-propanol (30%, v/v) as cosolvent and cosubstrate. This concentration of 2-propanol was crucial not only to enhance the solubility of hydrophobic phenyl ring-containing substrates in the aqueous reaction medium, but also to shift the equilibrium in the reduction direction. The resulting alcohols have S-configuration, in agreement with Prelog's rule, in which the nicotinamide-adenine dinucleotide phosphate (NADPH) cofactor transfers its pro-R hydride to the re face of the ketone. A series of phenyl ring-containing ketones, such as 4-phenyl-2-butanone (1a) and 1-phenyl-1,3-butadione (2a), were reduced with good to excellent yields and high enantioselectivities. On the other hand, 1-phenyl-2-propanone (7a) was reduced with lower ee than 2-butanone derivatives. (R)-Alcohols, the anti-Prelog products, were obtained by enantiospecific oxidation of (S)-alcohols through oxidative kinetic resolution of the rac-alcohols using W110A TESADH in Tris buffer/acetone (90:10, v/v).

New Concept for the Separation of an Anomeric Mixture of α/β-d-Nucleosides through Regioselective Enzymatic Acylation or Hydrolysis Processes

An efficient and high yield protocol for the synthesis and separation of 3'- and/or 5'-protected alpha-2'-deoxynucleosides has been developed through regioselective acylation/deacylation processes catalyzed by enzymes. Pseudomonas cepacia lipase (PSL-C) was found to be highly chemo- and regioselective toward the 3'-position of the beta-2'-deoxynucleoside derivatives, whereas PSL-C displayed opposite selectivity toward the 5'-position for the corresponding alpha-anomer. The successful application of this protocol was demonstrated by a convenient separation of an alpha/beta-mixture of thymidine derivatives from an industrial waste stream. Furthermore, this technique was also applied for the separation of an anomeric mixture of 2'-deoxy-2'-fluoro-alpha/beta-arabinonucleosides that are useful building blocks for the antisense constructs.

Kinetic resolution of 4-chloro-2-(1-hydroxyalkyl)pyridines using Pseudomonas cepacia lipase

Here we report a detailed procedure for the enzymatic kinetic resolution of 4-chloro-2-(1-hydroxyalkyl)pyridines, valuable precursors for the preparation of enantiomerically pure catalysts derived from 4-(N,N-dimethylamino)pyridine. Pseudomonas cepacia lipase shows excellent enantioselectivity in the acylation of the (R)-enantiomer at 30 degrees C and 250 r.p.m., with vinyl acetate as the acyl donor and tetrahydrofuran as the solvent. The reaction times for resolution of the pyridine derivatives depend on the structure of the selected substrate.

Chiral Oxazolopiperidone Lactams: Versatile Intermediates for the Enantioselective Synthesis of Piperidine-Containing Natural Products

Phenylglycinol-derived oxazolopiperidone lactams are exceptionally versatile building blocks for the enantioselective construction of structurally diverse piperidine-containing natural products and bioactive compounds. These lactams are readily available in both enantiomeric series by cyclocondensation of the chiral amino alcohol with a delta-oxo acid derivative and allow the substituents to be introduced at the different ring positions in a regio- and stereocontrolled manner, providing access to enantiopure polysubstituted piperidines bearing virtually any type of substitution pattern, and also quinolizidines, indolizidines, perhydroquinolines, hydroisoquinolines, as well as complex indole alkaloids. Of particular interest are cyclocondensation reactions with racemic or prochiral delta-oxo (di)acid derivatives in processes involving dynamic kinetic resolution and/or differentiation of enantiotopic or diastereotopic ester groups, as they directly lead to lactams that already incorporate the carbon substituents on the heterocyclic ring. The use of (S)-3,4-dimethoxyphenylalaninol or (S)-tryptophanol in the above cyclocondensation reactions expands the potential and the scope of the methodology, providing a straightforward route to enantiopure benzo[a]- and indolo[2,3-a]quinolizidines.

Dynamic Kinetic Resolution and Desymmetrization Processes: A Straightforward Methodology for the Enantioselective Synthesis of Piperidines

A straightforward procedure for the synthesis of enantiopure polysubstituted piperidines is reported. It involves the direct generation of chiral non-racemic oxazolo[3,2-a]piperidone lactams that already incorporate carbon substituents on the heterocyclic ring and the subsequent removal of the chiral auxiliary. The key step is a cyclocondensation reaction of (R)-phenylglycinol or other amino alcohols with racemic or prochiral delta-oxo (di)acid derivatives in highly stereoselective processes involving dynamic kinetic resolution and/or desymmetrization of diastereotopic or enantiotopic ester groups.

In search of sustainability: process R&D in light of current pharmaceutical industry challenges

Is there a need for a paradigm shift in the pharmaceutical industry? Many researchers think so and take as examples the eroding corporate reputation, a regulatory environment that is harsher than ever, and the request for cheaper drugs from patient organizations and authorities. Process R&D, which interfaces medicinal chemistry and production, has taken on this challenge by increasing the delivery focus early on to ensure timely availability of desired compounds. The quest for lower costs of goods has forced the design of best synthetic routes that, given the molecular complexity, often lead to catalytic methodologies. Applying these methodologies will enable not only the cost element, but also the increasingly important aspects of environmental friendliness, and atom and stage efficiency, to be addressed.

Enzymatic synthesis of (R)-cyanohydrins by a novel (R)-oxynitrilase from Vicia sativa L

The defatted seed meal of common vetch (Vicia sativa L.) served as a source of (R)-oxynitrilase which catalyzed the enantioselective addition of HCN to aromatic, heteroaromatic, fluoro-substituted aromatic aldehydes to produce the corresponding enantiomeric pure cyanohydrins in yields of 52-100% and 88-99% ee at 12 degrees C under micro-aqueous medium (diisopropyl ether) without addition of buffer solution.

Enantioselective Synthesis of γ-Hydroxyenones by Chiral Base-Catalyzed Kornblum DeLaMare Rearrangement

A cinchona-alkaloid catalyzed asymmetric Kornblum DeLaMare rearrangement has been developed. Thus, enantioenriched 4-hydroxyenones are prepared from dienes by a two-step sequence involving photochemical dioxygenation and chiral base-catalyzed desymmetrization of the resulting endoperoxides.

Desymmetrization of glycerol derivatives with peptide-based acylation catalysts

[reaction: see text] Nucleophile-loaded peptides have been evaluated as catalysts for the desymmetrization of glycerol derivatives through an enantioselective acylation process. Enantiomeric excesses of up to 97% have been obtained for the monoacylated products. A range of other substrates have been examined that shed light on the mechanistic basis of the desymmetrizations.

Synthesis of jasplakinolide analogues containing a novel omega-amino acid

The synthesis of the omega-amino acid 4 is described utilizing a two-dimensional synthesis strategy combined with an enzymatic differentiation of homotopic ester groups. The amino acid 4 features two non-bonded interactions that result in conformational constraints on a cyclic construct. This amino acid was incorporated into the four macrolactams 17, 22, 31, and 37. The ring in 17 and 22 is 18-membered, whereas 31 and 37 have a 19-membered ring. The pairs with the same ring size differ in a N-methyl group. For the larger macrolactams (31 and 37) conformational analysis showed that the macrocyclic rings are somewhat more rigid than in the natural lead, the depsipeptide jasplakinolide. Nevertheless, their conformations are comparable to the natural product. There are no intramolecular hydrogen bonds, neither is the cis-rotamer populated in the N-methyl compound 37. Due to the increased flexibility of the smaller macrolactams 17 and 22 and signal overlap, a distinct solution structure could not be obtained for these compounds. The amino acid 4 should be useful for restricting the conformation of other small peptides.

Resolution of Methyl Nonactate by Rhodococcus erythropolis under Aerobic and Anaerobic Conditions

[reaction: see text]. An efficient resolution of methyl nonactate is reported by biotransformation in shake flask cultures of Rhodococcus erythropolis. The equilibrium of the reaction redox system can be manipulated by switching from aerobic to anaerobic growth, thereby generating both enantiomers of the target in excellent yield and enantiomeric purity.

Electrostatic repulsion as an additional selectivity factor in asymmetric proline catalysis

The metal free, single amino acid-catalyzed asymmetric desymmetrization (ADS) of meso-compounds with nitrosobenzene has been investigated using DFT. In this communication, we describe the role of electrostatic and dipole-dipole interactions in amino acid-catalyzed reactions, which has not previously been invoked in discussions of these important reactions.

Baker's yeast-mediated enantioselective reduction of substituted fluorenones

In the presence of DMSO as co-solvent and under vigorous agitation, baker's yeast in water was found to reduce substituted fluorenones to the corresponding fluorenols in good to excellent enantioselectivities.

Novel immobilization method of enzymes using a hydrophilic polymer support

A novel immobilization of an enzyme with a hydrophilic polymer support in organic solvents has been developed utilizing the "polymer-incarcerated (PI) method", which has been used to immobilize metal catalysts; the kinetic resolution of secondary alcohols was found to proceed more smoothly using immobilized lipases (CALB) than free lipases.

Accessing tetrahydrofuran-based natural products by microbial Baeyer–Villiger biooxidation

A heterobicyclic lactone obtained by stereoselective Baeyer-Villiger biooxidation with recombinant whole-cells expressing cyclopentanone monooxygenase from Comamonas sp. NCIMB 9872 was used for formal total syntheses of various natural products containing a tetrahydrofuran structural motif.

A Biocatalytic Route to P-Chirogenic Compounds by Lipase-Catalyzed Desymmetrization of a Prochiral Phosphine−Borane

[reaction: see text] Available methods for synthesis of P-chirogenic compounds are limited. We set out to find biocatalytical means to introduce asymmetry in a phosphine-borane. After screening different lipases, Candida antarctica lipase B was found to give excellent results in the desymmetrization of prochiral phosphine-boranes. Both enantiomers can be obtained in up to >98% optical purity via acetylation or hydrolysis in processes that allow recycling of the substrates.

Catalytic asymmetric synthesis of chiral allylic esters

Trichloroacetimidate derivatives of prochiral (Z)-2-alken-1-ols react at room temperature with carboxylic acids to give chiral 3-acyloxy-1-alkenes in high enantiopurity in the presence of di-mu-acetatobis[(eta5-(S)-(pR)-2-(2'-(4'-methylethyl)oxazolinyl)cyclopentadienyl,1-C,3'-N)(eta4-tetraphenylcyclobutadiene)cobalt]dipalladium (COP-OAc) or its enantiomer. This reaction has broad scope, proceeds with predictable high stereoinduction, is accomplished at room temperature using high substrate concentrations and low catalyst loadings, and likely proceeds by a novel mechanism.

Direct Amino Acid-Catalyzed Asymmetric Desymmetrization of meso-Compounds: Tandem Aminoxylation/O−N Bond Heterolysis Reactions

[reaction: see text] A practical organocatalytic process for the synthesis of optically active, highly substituted alpha-hydroxy-ketones was achieved through asymmetric desymmetrization (ADS) of prochiral ketones. The ADS and O-N bond reduction reaction of prochiral ketone with nitrosobenzene in the presence of a catalytic amount of chiral amine or amino acid produced the tandem ADS/O-N bond reduced products as single diastereomers with good yields and excellent enantiomeric excesses.