Beat Wirz

The synthetic development of the anti-influenza neuraminidase inhibitor oseltamivir phosphate (Tamiflu®): A challenge for synthesis & process research

The evolution of the synthesis of oseltamivir phosphate (Tamiflu®), used for the oral treatment and prevention of influenza virus infections (viral flu) is described. Oseltamivir phosphate is the ethyl ester prodrug of the corresponding acid, a potent and selective inhibitor of influenza neuraminidase. The discovery chemistry route and scalable routes used for kilo laboratory production as well as the technical access to oseltamivir phosphate from (-)-shikimic acid proceeding via a synthetically well-developed epoxide building block followed by azide transformations are reviewed. Synthesis and process research investigations towards azide-free conversions of the key epoxide building block to oseltamivir phosphate are discussed. The search for new routes to oseltamivir phosphate independent of shikimic acid including Diels-Alder approaches and transformations of aromatic rings employing a desymmetrization concept are presented in view of large-scale production requirements.

New, Efficient Synthesis of Oseltamivir Phosphate (Tamiflu) via Enzymatic Desymmetrization of a meso-1,3-Cyclohexanedicarboxylic Acid Diester

A new, enantioselective synthesis of the influenza neuraminidase inhibitor prodrug oseltamivir phosphate 1 (Tamiflu) and its enantiomer ent-1 starting from cheap, commercially available 2,6-dimethoxyphenol 10 is described. The main features of this approach comprise the cis-hydrogenation of 5-(1-ethyl-propoxy)-4,6-dimethoxy-isophthalic acid diethyl ester (6a) and the desymmetrization of the resultant all-cis meso-diesters 7a and 7b, respectively. Enzymatic hydrolysis of the meso-diester 7b with pig liver esterase afforded the (S)-monoacid 8b, which was converted into cyclohexenol 17 via a Curtius degradation and a base-catalyzed decarboxylative elimination of the Boc-protected oxazolidinone 14. Introduction of the second amino function via S(N)2 substitution of the corresponding triflate 18 with NaN3 followed by azide reduction, N-acetylation, and Boc-deprotection gave oseltamivir phosphate 1 in a total of 10 steps and an overall yield of approximately 30%. The enantiomer ent-1 was similarly obtained via an enzymatic desymmetrization of meso-diester 7a with Aspergillus oryzae lipase, providing the (R)-monoacid ent-8a.

Identification of (S)-selective transaminases for the asymmetric synthesis of bulky chiral amines

The use of transaminases to access pharmaceutically relevant chiral amines is an attractive alternative to transition-metal-catalysed asymmetric chemical synthesis. However, one major challenge is their limited substrate scope. Here we report the creation of highly active and stereoselective transaminases starting from fold class I. The transaminases were developed by extensive protein engineering followed by optimization of the identified motif. The resulting enzymes exhibited up to 8,900-fold higher activity than the starting scaffold and are highly stereoselective (up to >99.9% enantiomeric excess) in the asymmetric synthesis of a set of chiral amines bearing bulky substituents. These enzymes should therefore be suitable for use in the synthesis of a wide array of potential intermediates for pharmaceuticals. We also show that the motif can be engineered into other protein scaffolds with sequence identities as low as 70%, and as such should have a broad impact in the field of biocatalytic synthesis and enzyme engineering.

Novel versatile approach to an enantiopure 19-nor, des-C,D vitamin D3 derivative ☆

Abstract A short and efficient de novo route to the des-C,D vitamin D3 derivative 3 (Ro 65-2299), a potential antipsoriatic, has been developed. This route features an assembly strategy so far unexplored in vitamin D chemistry involving a modified Julia olefination of the A-ring ketone 30 and the 2-benzothiazolyl sulfone 60. Construction of the A-ring building block was accomplished by an efficient three-step route starting from the meso trans-1,3,5-cyclohexane triol (26), which was desymmetrized by a highly selective enzymatic mono-hydrolysis of the corresponding triacetate 27 followed by oxidation of the alcohol 29 to give the homochiral diacetoxy ketone 30 (ee=99.5%) in 83% overall yield. Furthermore, we found efficient and practical syntheses of the 5-acetoxy-2-cyclohexenone (31) and its enantiomer 32, both new building blocks useful for natural product synthesis.

Expanding the Imine Reductase Toolbox by Exploring the Bacterial Protein-Sequence Space.

Recent investigations on imine reductases (IREDs) have enriched the toolbox of potential catalysts for accessing chiral amines, which are important building blocks for the pharmaceutical industry. Herein, we describe the characterization of 20 new IREDs. A C‐terminal domain clustering of the bacterial protein‐sequence space was performed to identify the novel IRED candidates. Each of the identified enzymes was characterized against a set of nine cyclic imine model substrates. A refined clustering towards putative active‐site residues was performed and was consistent both with our screening and previously reported results. Finally, preparative scale experiments on a 100 mg scale with two purified IREDs, IR_20 from Streptomyces tsukubaensis and IR_23 from Streptomyces vidiochromogenes, were carried out to provide (R)‐2‐methylpiperidine in 98 % ee (71 % yield) and (R)‐1‐methyl‐1,2,3,4‐tetrahydroisoquinoline in >98 % ee (82 % yield).

Asymmetric Reductive Amination of Ketones Catalyzed by Imine Reductases

Biocatalysis employing imine reductases is a promising approach for the one‐step generation of chiral amines from ketones. The enzymes reported for this process suffer from low activity and moderate stereoselectivity. We identified a set of enzymes that facilitate this reaction with high to quantitative conversions from a library of 28 imine reductases. This enabled the conversion of ketones with ammonia, methylamine, or butylamine into the corresponding amines. Most importantly, we performed preparative (>100 mg) scale syntheses of amines such as (1S,3R)‐N,3‐dimethylcyclohexylamine and (R)‐N‐methyl‐2‐aminohexane with excellent stereochemical purities (98 % de, 96 % ee) in good yields.

Enzymatic preparation of chiral 3-(hydroxymethyl)piperidine derivatives

Abstract t-Butyl (R)-3-(hydroxymethyl)-1-piperidinecarboxylate was prepared with lipase P in up to 98 % ee by means of enantioselective esterification of the racemic alcohol as well as by enantioselective hydrolysis of the corresponding butyryl ester and subsequent chemical hydrolysis of the retained (R)-ester. A work-up procedure feasible on the kg-scale is described.

Protein-engineering of an amine transaminase for the stereoselective synthesis of a pharmaceutically relevant bicyclic amine

Application of amine transaminases (ATAs) for stereoselective amination of prochiral ketones represents an environmentally benign and economically attractive alternative to transition metal catalyzed asymmetric synthesis. However, the restrictive substrate scope has limited the conversion typically to non-sterically demanding scaffolds. Recently, we reported on the identification and design of fold class I ATAs that effect a highly selective asymmetric synthesis of a set of chiral aromatic bulky amines from the corresponding ketone precursors in high yield. However, for the specific amine synthetic approach extension targeted here, the selective formation of an exo- vs. endo-isomer, these biocatalysts required additional refinement. The chosen substrate (exo-3-amino-8-aza-bicyclo[3.2.1]oct-8-yl-phenyl-methanone), apart from its pharmacological relevance, is a demanding target for ATAs as the bridged bicyclic ring provides substantial steric challenges. Protein engineering combining rational design and directed evolution enabled the identification of an ATA variant which catalyzes the specific synthesis of the target exo-amine with >99.5% selectivity.

Identification of Potent and Selective Cathepsin S Inhibitors Containing Different Central Cyclic Scaffolds

Starting from the weakly active dual CatS/K inhibitor 5, structure-based design supported by X-ray analysis led to the discovery of the potent and selective (>50,000-fold vs CatK) cyclopentane derivative 22 by exploiting specific ligand-receptor interactions in the S2 pocket of CatS. Changing the central cyclopentane scaffold to the analogous pyrrolidine derivative 57 decreased the enzyme as well as the cell-based activity significantly by 24- and 69-fold, respectively. The most promising scaffold identified was the readily accessible proline derivative (e.g., 79). This compound, with an appealing ligand efficiency (LE) of 0.47, included additional structural modifications binding in the S1 and S3 pockets of CatS, leading to favorable in vitro and in vivo properties. Compound 79 reduced IL-2 production in a transgenic DO10.11 mouse model of antigen presentation in a dose-dependent manner with an ED50 of 5 mg/kg.

Large scale preparation of chiral building blocks for the P3 site of renin inhibitors

Racemic ethyl 2-benzyl-3-(tert-butylsulfonyl)propionate (1) and racemic ethyl 2-benzyl-3-[[1-methyl-1-((morpholin-4-yl)carbonyl)ethyl]sulfonyl] propionate (3) were enantioselectively hydrolyzed by subtilisin Carlsberg generating the respective (S)-acids used as building blocks for renin inhibitors. The esters were readily converted as emulsions at elevated temperature, in a suspended form or a two-phase-liquid system. The enzyme maintained its excellent selectivity and a good activity also at high initial substrate concentrations (up to 50% w/w). The enzymatic reaction and work-up were optimized and scaled up. Emulsion problems during work-up encountered with these highly concentrated mixtures were solved by application of a disk separator for phase separation.