Second Generation Catalytic Asymmetric Synthesis of Tamiflu: Allylic Substitution Route

Isolation and Structural Characterization of Stable Carbamic-Carbonic Anhydrides: An Experimental and Computational Study

Carbamic-carbonic anhydrides are elusive species that have been only indirectly detected under controlled conditions. This functional group is transiently formed during the reaction of secondary amines with anhydrides in the...

Recent progress in chemical approaches for the development of novel neuraminidase inhibitors

Influenza virus is the main cause of an infectious disease called influenza affecting the respiratory system including the throat, nose and lungs. Neuraminidase inhibitors are reagents used to block the enzyme called neuraminidase to prevent the influenza infection from spreading. Neuraminidase inhibitors are widely used in the treatment of influenza infection, but still there is a need to develop more potent agents for the more effective treatment of influenza. Complications of the influenza disease lead to death, and one of these complications is drug resistance; hence, there is an urgent need to develop more effective agents. This review focuses on the recent advances in chemical synthesis pathways used for the development of new neuraminidase agents along with the medicinal aspects of chemically modified molecules, including the structure-activity relationship, which provides further rational designs of more active small molecules.

Development of New Synthetic Reactions Using Hetero-Heavy Atoms and Their Application to Synthesis of Biofunctional Molecules

Novel reactions using hetero-heavy atoms (P, S, Si, Se, and Sn) were developed and applied to the synthesis of biofunctional molecules and some medicine-candidates, in which the following items are covered. 1) Development of introduction of C₁-unit using cyanophosphates (CPs). 2) Carbene-generation under neutral condition from CPs and its application to organic synthesis. 3) [3,3]Sigmatropic rearrangement-ring expansion reactions of medium-sized cyclic thionocarbonates containing a sulfur atom and their application to natural product synthesis. 4) Stereoselective synthesis of novel β-imidazole C-nucleosides via diazafulvene intermediates and their application to investigating ribozyme reaction mechanism. 5) Developments of novel histamine H₃- and H₄-receptor ligands using new synthetic methods involving Se or Sn atoms.

Cycloaddition reactions for antiviral compounds

Prominent in the current stage of drug development, antiviral compounds can be efficiently prepared through cycloaddition reactions. The chapter reports the use of classical Diels–Alder and their hetero version for the design and synthesis of compounds that were tested for their antiviral activities against a variety of viruses. Furthermore, 1,3-dipolar cycloaddition reactions of selected 1,3-dipoles, such as azides, nitrones, and nitrile oxides, are reviewed in the light of their application in the preparation of key intermediates for antiviral synthesis. A few examples of [2+2] cycloaddition reactions are also presented. The products obtained from these pericyclic reaction approaches were all tested for their activities in terms of blocking the virus replication, and the relevant biological data are highlighted.

Enantioselective Synthesis of Oseltamivir Phosphate (Tamiflu) via the Iron-Catalyzed Stereoselective Olefin Diazidation

We herein report a gram-scale, enantioselective synthesis of Tamiflu, in which the key trans-diamino moiety has been efficiently installed via an iron-catalyzed stereoselective olefin diazidation. This significantly improved, iron-catalyzed method is uniquely effective for highly functionalized yet electronically deactivated substrates that have been previously problematic. Preliminary catalyst structure-reactivity-stereoselectivity relationship studies revealed that both the iron catalyst and the complex substrate cooperatively modulate the stereoselectivity for diazidation. Safety assessment using both differential scanning calorimetry (DSC) and the drop weight test (DWT) has also demonstrated the feasibility of carrying out this iron-catalyzed olefin diazidation for large-scale Tamiflu synthesis.

Application of Heterogeneous Catalysts in the First Steps of the Oseltamivir Synthesis

The first steps of oseltamivir synthesis from quinic acid involve acetalization and ester formation. These reactions are catalyzed by either acids or bases, which may be accomplished by heterogeneous catalysts. Sulfonic solids are efficient acid catalysts for acetalization and esterification reactions. Supported tetraalkylammonium hydroxide or 1,5,7-triazabicyclo[4.4.0]dec-5-ene are also efficient base catalysts for lactone alcoholysis and in this work, these catalysts have been applied in two alternative synthetic routes that lead to oseltamivir. The classical route consists of an acetalization, followed by a lactonization, and then a lactone alcoholysis. This achieves a 66% isolated yield. The alternative route consists of esterification followed by acetalization and is only efficient when an acetone acetal is used.

Nonclassical Routes for Amide Bond Formation

The present review offers an overview of nonclassical (e.g., with no pre- or in situ activation of a carboxylic acid partner) approaches for the construction of amide bonds. The review aims to comprehensively discuss relevant work, which was mainly done in the field in the last 20 years. Organization of the data follows a subdivision according to substrate classes: catalytic direct formation of amides from carboxylic and amines ( section 2 ); the use of carboxylic acid surrogates ( section 3 ); and the use of amine surrogates ( section 4 ). The ligation strategies (NCL, Staudinger, KAHA, KATs, etc.) that could involve both carboxylic acid and amine surrogates are treated separately in section 5 .

Enantioselective Syntheses of Lignin Models: An Efficient Synthesis of β-O-4 Dimers and Trimers by Using the Evans Chiral Auxiliary

We describe an efficient five-step, enantioselective synthesis of (R,R)- and (S,S)-lignin dimer models possessing a β-O-4 linkage, by using the Evans chiral aldol reaction as a key step. Mitsunobu inversion of the (R,R)- or (S,S)-isomers generates the corresponding (R,S)- and (S,R)-diastereomers. We further extend this approach to the enantioselective synthesis of a lignin trimer model. These lignin models are synthesized with excellent ee (>99 %) and high overall yields. The lignin dimer models can be scaled up to provide multigram quantities that are not attainable by using previous methodologies. These lignin models will be useful in degradation studies probing the selectivity of enzymatic, microbial, and chemical processes that deconstruct lignin.

Intramolecular cyclization of alkynyl α-ketoanilide utilizing [1,2]-phospha-Brook rearrangement catalyzed by phosphazene base

A novel catalytic cyclization reaction of alkynyl α-ketoanilide was developed by utilizing the [1,2]-phospha-Brook rearrangement. This reaction involves the generation of an amide enolate via the umpolung process, that is the addition of dialkyl phosphite to a keto moiety followed by the [1,2]-phospha-Brook rearrangement, and the subsequent intramolecular addition of the enolate to an alkyne to afford 3,4-dihydro-2-quinolone derivatives. Under high-temperature reaction conditions, further rearrangement of the allylic phosphate moiety occurs to provide 2-quinolone derivatives.

A concise synthetic approach toward tamiflu (oseltamivir phosphate): cis-aziridine as the key synthon and RCM

The key synthon cis-aziridine has been efficiently utilised for the synthesis of tamiflu (oseltamivir phosphate), using Wittig olefination, Barbier addition, Mitsunobu reaction and ring closing metathesis (RCM) as key essentials.

Stereospecific synthetic approach towards Tamiflu using the Ramberg–Backlund reaction from cysteine hydrochloride

The stereospecific formal synthesis of Tamiflu from l-cysteine hydrochloride as the chiral source is described. The notable feature of the present strategy is the Ramberg–Backlund reaction and Sharpless–Reich protocol as the key chemical transformations to access the cyclohexene skeleton of Tamiflu.

Exploring naphthyl-carbohydrazides as inhibitors of influenza A viruses

A library of hydrazide derivatives was synthesized to target non-structural protein 1 of influenza A virus (NS1) as a means to develop anti-influenza drug leads. The lead compound 3-hydroxy-N-[(Z)-1-(5,6,7,8-tetrahydronaphthalen-2-yl)ethylideneamino]naphthalene-2-carboxamide, which we denoted as "HENC", was identified by its ability to increase the melting temperature of the effector domain (ED) of the NS1 protein, as assayed using differential scanning fluorimetry. A library of HENC analogs was tested for inhibitory effect against influenza A virus replication in MDCK cells. A systematic diversification of HENC revealed the identity of the R group attached to the imine carbon atom significantly influenced the antiviral activity. A phenyl or cyclohexyl at this position yielded the most potent antiviral activity. The phenyl containing compound had antiviral activity similar to that of the active form of oseltamivir (Tamiflu), and had no detectable effect on cell viability.

Two approaches toward the formal total synthesis of oseltamivir phosphate (Tamiflu): catalytic enantioselective three-component reaction strategy and L-glutamic acid strategy

Two independent formal total syntheses of oseltamivir phosphate were successfully achieved: the first utilized a copper-catalyzed asymmetric three-component reaction strategy, and the second utilized L-glutamic acid γ-ester as a chiral source to install the correct stereochemistry. Both strategies used Dieckmann condensation to construct a six-membered ring core, after which manipulation of the functional groups and protecting groups accessed Corey's intermediate for the synthesis of oseltamivir phosphate. While the first synthesis was accomplished via four purification steps in 25.7% overall yield, albeit with moderate optical purity (76% ee), the second strategy achieved the synthesis via six purification steps in 19.8% overall yield with perfect enantiocontrol.

Synthesis of (-)-oseltamivir phosphate (Tamiflu) starting from cis-2,3-bis(hydroxymethyl)aziridine

Oseltamivir phosphate (Tamiflu) has been synthesized from cis-2,3-bis(hydroxymethyl)aziridine. After protection of the cis-2,3-bis(hydroxymethyl)aziridine with a Boc group, desymmetrization provided a chiral aziridine, which was a key intermediate to install the required stereogenic center containing a nitrogen atom. Allylation and ring closing metathesis are the key reactions to obtain the cyclic product that was successfully converted to the desired oseltamivir phosphate.

A simple and straightforward approach toward selective C=C bond reduction by hydrazine

A simple and straightforward method for reducing the C=C double bond with hydrazine is described. A number of representative C=C bonds in various steric and electronic environments were examined. Substituted alkenes can be selectively reduced in EtOH in the presence of hydrazine to give the corresponding products in up to 100% yields.

Achieving regio- and enantioselectivity of P450-catalyzed oxidative CH activation of small functionalized molecules by structure-guided directed evolution

Directed evolution of the monooxygenase P450-BM3 utilizing iterative saturation mutagenesis at and near the binding site enables a high degree of both regio- and enantioselectivity in the oxidative hydroxylation of cyclohexene-1-carboxylic acid methyl ester. Wild-type P450-BM3 is 84% regioselective for the allylic 3-position with 34% enantioselectivity in favor of the R alcohol. Mutants enabling R selectivity (>95% ee) or S selectivity (>95% ee) were evolved, while reducing other oxidation products and thus maximizing regioselectivity to >93%. Control of the substrate-to-enzyme ratio is necessary for obtaining optimal and reproducible enantioselectivities, an observation which is important in future protein engineering of these mono-oxygenases. An E. coli strain capable of NADPH regeneration was also engineered, simplifying directed evolution of P450 enzymes in general. These synthetic results set the stage for subsequent stereoselective and stereospecific chemical transformations to form more complex compounds, thereby illustrating the viability of combining genetically altered enzymes as catalysts in organic chemistry with traditional chemical methods.

Several generations of chemoenzymatic synthesis of oseltamivir (Tamiflu): evolution of strategy, quest for a process-quality synthesis, and evaluation of efficiency metrics

Four generations of chemoenzymatic approaches to oseltamivir are presented. The first two generations relied on the use of cyclohexadiene-cis-diol derived enzymatically from bromobenzene. The third and fourth generation used the corresponding diol obtained from ethyl benzoate by fermentation with E. coli JM109(pDTG601a). Oseltamivir was obtained from ethyl benzoate by intersecting intermediate 39 (third-generation synthesis) and intermediate 45 (fourth-generation synthesis). Both of these advanced approaches benefited from symmetry considerations and translocation of the acrylate double bond with concomitant elimination of the C-1 hydroxyl. The syntheses are evaluated for overall efficiency by the use of efficiency metrics and compared with other syntheses of oseltamivir (both academic and industrial).

Development of a concise synthesis of (-)-oseltamivir (Tamiflu)

We report a full account of our work towards the development of an eight-step synthesis of anti-influenza drug (-)-oseltamivir (Tamiflu) from commercially available starting materials. The final synthetic route proceeds with an overall yield of 30 %. Key transformations include a novel palladium-catalyzed asymmetric allylic alkylation reaction (Pd-AAA) as well as a rhodium-catalyzed chemo-, regio-, and stereoselective aziridination reaction.