Enantioselective synthesis of oseltamivir phosphate

Kinetic Resolution of Nearly Symmetric 3-Cyclohexene-1-carboxylate Esters Using a Bacterial Carboxylesterase Identified by Genome Mining

A new bacterial carboxylesterase (CarEst3) was identified by genome mining and found to efficiently hydrolyze racemic methyl 3-cyclohexene-1-carboxylate (rac-CHCM) with a nearly symmetric structure for the synthesis of (S)-CHCM. CarEst3 displayed a high substrate tolerance and a stable catalytic performance. The enantioselective hydrolysis of 4.0 M (560 g·L^-1) rac-CHCM was accomplished, yielding (S)-CHCM with a >99% ee, a substrate to catalyst ratio of 1400 g·g^-1, and a space-time yield of 538 g·L^-1·d^-1.

Fukuyama reduction, Fukuyama coupling and Fukuyama-Mitsunobu alkylation: recent developments and synthetic applications

Fukuyama reaction for the synthesis of multifunctional aldehydes, secondary amines and ketones has gained considerable importance in synthetic organic chemistry because of mild reaction conditions. The use of thioesters in both Fukuyama aldehydes and ketones synthesis is highly attractive for organic chemists as they are easily accessible from corresponding carboxylic acids. Fukuyama-Mitsunobu reaction utilizes 2-nitrobenzenesulfonyl (Ns) for the protection/activation/deprotection of primary amines to afford secondary amines in good yields and high enantioselectivities. This review presents recent synthetic developments and applications of Fukuyama reaction for the synthesis of aldehydes, secondary amines and ketones.

A novel carboxylesterase from Acinetobacter sp. JNU9335 for efficient biosynthesis of Edoxaban precursor with high substrate to catalyst ratio

A novel carboxylesterase AcEst1 was identified from Acinetobacter sp. JNU9335 with high efficiency in the biosynthesis of chiral precursor of Edoxaban through kinetic resolution of methyl 3-cyclohexene-1-carboxylate (CHCM). Sequence analysis revealed AcEst1 belongs to family IV of esterolytic enzymes and exhibits <40% identities with known carboxylesterases. The optimum pH and temperature of recombinant AcEst1 are 8.0 and 40 °C. Substrate spectrum analysis indicated that AcEst1 prefers substrates with short acyl and alcohol groups. AcEst1 was highly active in the hydrolysis of CHCM with k_cat of 1153 s^-1 and displayed high substrate tolerance. As much as 2.0 M (280 g·L^-1) CHCM could be enantioselectively hydrolyzed into (S)-CHCM by merely 0.08 g·L^-1AcEst1 with ee_s of >99% (S) and substrate to catalyst ratio (S/C) of 3500 g·g^-1. These results indicate that the novel AcEst1 is a promising biocatalyst in the synthesis of chiral carboxylic acids.

Efficient microbial resolution of racemic methyl 3-cyclohexene-1-carboxylate as chiral precursor of Edoxaban by newly identified Acinetobacter sp. JNU9335

Optically active 3-cyclohexene-1-carboxylic acid (CHCA) derivatives are important pharmaceutical intermediates. Due to the special rotatable structure, enantioselective preparation of chiral CHCA is hard to achieve. To identify efficient and enantioselective hydrolases for the biosynthesis of CHCA from methyl 3-cyclohexene-1-carboxylate (CHCM), target-oriented screening from soil samples and gene mining from genome database were explored. All putative hydrolases attempted displayed low enantioselectivity. A hydrolase-producing strain JNU9335 was successfully identified with relatively high enantioselectivity, and was designated as a strain of Acinetobacter sp. according to 16S rDNA sequence and phylogenetic analysis. After optimization, strain JNU9335 could produce 233 U·L^‒1 hydrolase with E value of 21. Isooctane/aqueous biphasic system is favorable for the enzymatic resolution of CHCM, the E value of JNU9335 could further be increased to 36. The newly identified JNU9335 could tolerate as high as 1.0 M CHCM, producing (S)-CHCM with ee_s of 99.6% and isolation yield of 34.7%. This study provides an efficient biocatalyst for the preparation of chiral 3-cyclohexene-1-carboxylic acid derivatives.

Temperature-responsive PNDJ hydrogels provide high and sustained antimicrobial concentrations in surgical sites

Local antimicrobial delivery is a promising strategy for improving treatment of deep surgical site infections (SSIs) by eradicating bacteria that remain in the wound or around its margins after surgical debridement. Eradication of biofilm bacteria can require sustained exposure to high antimicrobial concentrations (we estimate 100-1000 μg/mL sustained for 24 h) which are far in excess of what can be provided by systemic administration. We have previously reported the development of temperature-responsive hydrogels based on poly(N-isopropylacrylamide-co-dimethylbutyrolactone acrylate-co-Jeffamine M-1000 acrylamide) (PNDJ) that provide sustained antimicrobial release in vitro and are effective in treating a rabbit model of osteomyelitis when instilled after surgical debridement. In this work, we sought to measure in vivo antimicrobial release from PNDJ hydrogels and the antimicrobial concentrations provided in adjacent tissues. PNDJ hydrogels containing tobramycin and vancomycin were administered in four dosing sites in rabbits (intramedullary in the femoral canal, soft tissue defect in the quadriceps, intramuscular injection in the hamstrings, and intra-articular injection in the knee). Gel and tissue were collected up to 72 h after dosing and drug levels were analyzed. In vivo antimicrobial release (43-95% after 72 h) was markedly faster than in vitro release. Drug levels varied significantly depending on the dosing site but not between polymer formulations tested. Notably, total antimicrobial concentrations in adjacent tissue in all dosing sites were sustained at estimated biofilm-eradicating levels for at least 24 h (461-3161 μg/mL at 24 h). These results suggest that antimicrobial-loaded PNDJ hydrogels are promising for improving the treatment of biofilm-based SSIs.

Stereoselective Synthesis of Highly Functionalized 5- and 6-Membered Aminocyclitols Starting with a Readily Available 2-Azetidinone

Stereoselective transformations of 4-vinyl-2-azetidinone derivative 4 into a variety of highly functionalized 6- and 5-membered carbocyclic compounds 7 and 9 were carried out using sequences involving sequential C1-N bond cleavage and Ru-catalyzed ring-closing metathesis. The derived carbocycles were further transformed into polyhydroxylated 6- and 5-membered aminocyclitols.

Enantioselective syntheses and application of 4-epi-galiellalactone and the corresponding activity-based probe: from strained bicycles to strained tricycles

The [6,5,5] tricyclic fungal metabolite galiellalactone is a Michael acceptor that has been demonstrated to be a covalent inhibitor for Signal Transducer and Activator of Transcription 3 (STAT3). Recognizing the ring strain associated with the skeleton of this natural product, we utilized 1R-5S-bicyclo[3.1.0]hexan-2-one as the starting material and developed two novel approaches to accomplish the enantioselective total synthesis of the C4 epimer of galiellalactone in 5 and 7 steps, respectively, which capitalized on an efficient radical cyclization/fragmentation cascade reaction. Furthermore, an activity-based probe of 4-epi-galiellalactone with a terminal alkyne tag was successfully prepared to enable the experiments of activity-based protein profiling (ABPP). Through western blot and proteomic analysis, we not only confirmed the known target STAT3, but also identified a new target protein ataxin-7, which formed a covalent bond with the probe in intact cells via the Cys-129 residue.

Enantioselective Halolactonization Reactions using BINOL-Derived Bifunctional Catalysts: Methodology, Diversification, and Applications

A general protocol is described for inducing enantioselective halolactonizations of unsaturated carboxylic acids using novel bifunctional organic catalysts derived from a chiral binaphthalene scaffold. Bromo- and iodolactonization reactions of diversely substituted, unsaturated carboxylic acids proceed with high degrees of enantioselectivity, regioselectivity, and diastereoselectivity. Notably, these BINOL-derived catalysts are the first to induce the bromo- and iodolactonizations of 5-alkyl-4( Z)-olefinic acids via 5- exo mode cyclizations to give lactones in which new carbon-halogen bonds are created at a stereogenic center with high diastereo- and enantioselectivities. Iodolactonizations of 6-substituted-5( Z)-olefinic acids also occur via 6- exo cyclizations to provide δ-lactones with excellent enantioselectivities. Several notable applications of this halolactonization methodology were developed for desymmetrization, kinetic resolution, and epoxidation of Z-alkenes. The utility of these reactions is demonstrated by their application to a synthesis of precursors of the F-ring subunit of kibdelone C and to the shortest catalytic, enantioselective synthesis of (+)-disparlure reported to date.

Catalyst-controlled site-selective asymmetric epoxidation of nerylamine and geranylamine derivatives

Novel catalysts for site- and enantioselective epoxidation of nerylamine and geranylamine derivatives have been developed. Although mCPBA oxidation took place selectively at the more electron-rich double bond to give the 6,7-epoxides, these catalysts provide the 2,3-epoxides in moderate to high enantioselectivity via the oxidation of the relatively electron-deficient double bond.

Influenza Neuraminidase Inhibitors: Synthetic Approaches, Derivatives and Biological Activity

Despite being a common viral disease, influenza has very negative consequences, causing the death of around half a million people each year. A neuraminidase located on the surface of the virus plays an important role in viral reproduction by contributing to the release of viruses from infected host cells. The treatment of influenza is mainly based on the administration of neuraminidase inhibitors. The neuraminidase inhibitors zanamivir, laninamivir, oseltamivir and peramivir have been commercialized and have been demonstrated to be potent influenza viral neuraminidase inhibitors against most influenza strains. In order to create more potent neuraminidase inhibitors and fight against the surge in resistance resulting from naturally-occurring mutations, these anti-influenza drugs have been used as templates for the development of new neuraminidase inhibitors through structure-activity relationship studies. Here, we review the synthetic routes to these commercial drugs, the modifications which have been performed on these structures and the effects of these modifications on their inhibitory activity.

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.

Functional-group specific aptamers indirectly recognizing compounds with alkyl amino group

Aptamers are usually generated against a specific molecule. Their high selectivity makes them only suitable for studying specific targets. Since it is nearly impossible to generate aptamers for every molecule, it can be of great interest to select aptamers recognizing a common feature of a group of molecules in many applications. In this paper, we describe the selection of aptamers for indirect recognition of alkyl amino groups. Because amino groups are small and positive charged, we introduced a protection group, p-nitrobenzene sulfonyl (p-nosyl) to convert them into a form suitable for aptamer selection. Taking N(ε)-p-nosyl-L-lysine (PSL) as a target, we obtained a group of aptamers using the SELEX technique. Two optimized aptamers, M6b-M14 and M13a exhibit strong affinity to PSL with the K(d) values in the range of 2-5 μM. They also show strong affinity to other compounds containing p-nosyl-protected amino groups except those also possessing an α-carboxyl group. Both aptamers adopt an antiparallel G-quadruplex structure when binding to targets. An aptamer beacon based on M6b-M14 showed good selectivity toward the reaction mixture of p-nosyl-Cl and alkyl amino compounds, and could recognize lysine from amino acid mixtures indirectly, suggesting that aptamers against a common moiety of a certain type of molecules can potentially lead to many new applications. Through this study, we have demonstrated the ability to select aptamers for a specific part of an organic compound, and the chemical conversion approach may prove to be valuable for aptamer selection against molecules that are generally difficult for SELEX.

Synthesis of the anti-influenza agent (-)-oseltamivir free base and (-)-methyl 3-epi-shikimate

A new enantioselective synthesis of the anti-influenza agent (-)-oseltamivir free base (7.1% overall yield; 98% ee) and (-)-methyl 3-epi-shikimate (16% overall yield; 98% ee) has been described from readily available raw materials. Sharpless asymmetric epoxidation and diastereoselective Barbier allylation of an aldehyde are the key reactions employed in the incorporation of chirality, while the cyclohexene carboxylic ester core was constructed through a ring closing metathesis reaction.

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).