Simple, Mild, and Practical Esterification, Thioesterification, and Amide Formation Utilizingp-Toluenesulfonyl Chloride andN-Methylimidazole

Recent Advances in Continuous-Flow Reactions Using Metal-Free Homogeneous Catalysts

Developments that result in high-yielding, low-cost, safe, scalable, and less-wasteful processes are the most important goals in synthetic organic chemistry. Continuous-flow reactions have garnered much attention due to many advantages over conventional batch reactions that include precise control of short reaction times and temperatures, low risk in handling dangerous compounds, and ease in scaling up synthesis. Combinations of continuous-flow reactions with homogeneous, metal-free catalysts further enhances advantages that include low-cost and ready availability, low toxicity, higher stability in air and water, and increased synthetic efficiency due to the avoidance of the time-consuming removal of toxic metal traces. This review summarizes recently reported continuous-flow reactions using metal-free homogeneous catalysts and classifies them either as acidic catalysts, basic catalysts, or miscellaneous catalysts. In addition, we compare the results between continuous-flow conditions and conventional batch conditions to reveal the advantages of using flow reactions with metal-free homogeneous catalysts.

Stereocomplementary and Parallel Syntheses of Multi-Substituted (E)-, (Z)-Stereodefined α,β-Unsaturated Esters: Application to Drug Syntheses

Ubiquitous α,β-unsaturated esters are well recognized as key structural olefin scaffolds in organic chemistry. (E)- and (Z)-steroselectivity is the most critical issue in their synthesis, however, (E)- and (Z)- stereocomplementary synthetic methods remain quite limited. The present account discloses general (E)-, (Z)-stereocomplementary syntheses of a variety of α,β-unsaturated esters from highly accessible (E)-, (Z)-stereodefined enol tosylates derived from β-ketoesters and α-formyl esters. Step 1 toward the stereocomplementary preparation of (E)-, (Z)-stereodefined enol tosylates is implemented by using inexpensive reagents under mild reaction conditions. Step 2 toward the highly stereoretentive synthesis of (E)- and (Z)-stereodefined α,β-unsaturated esters involves Suzuki-Miyaura, Negishi, Sonogashira, Iron-catalyzed, Mizoroki-Heck, and Buchwald-Hartwig cross-coupling reactions. Notably, this strategy was successfully applied for parallel drug syntheses of (E)- and (Z)-zimelidine, (E)- and (Z)-tamoxifen, and Merck's cyclopropane pharmacophore. Representative successful utilizations by other groups are also introduced.

N-Methylated Peptide Synthesis via Generation of an Acyl N-Methylimidazolium Cation Accelerated by a Brønsted Acid

The development of a robust amide-bond formation remains a critical aspect of N-methylated peptide synthesis. In this study, we synthesized a variety of dipeptides in high yields, without severe racemization, from equivalent amounts of amino acids. Highly reactive N-methylimidazolium cation species were generated in situ to accelerate the amidation. The key to success was the addition of a strong Brønsted acid. The developed amidation enabled the synthesis of a bulky peptide with a higher yield in a shorter amount of time compared with the results of conventional amidation. In addition, the amidation can be performed by using either a microflow reactor or a conventional flask. The first total synthesis of naturally occurring bulky N-methylated peptides, pterulamides I-IV, was achieved. Based on experimental results and theoretical calculations, we speculated that a Brønsted acid would accelerate the rate-limiting generation of acyl imidazolium cations from mixed carbonic anhydrides.

Synthesis and evaluation of novel chromanone and quinolinone analogues of uniflorol as anti-leishmanial agents

Within this work, we describe the design and synthesis of a range of novel chromanones and quinolinones, based on natural products reported to possess anti-leishmanial action. The core heterocycles were obtained either via classical or ionic liquid mediated Kabbe condensation in the case of chromanones, or aqueous Sonogashira based alkynylation followed by acid-catalysed cyclisation in the case of quinolinones. Upon testing in promastigotes, axenic amastigotes and Leishmania-infected macrophages, compound 13c was identified as displaying interesting activity, inhibiting axenic amastigotes and intracellular amastigotes with IC₅₀s of 25.3 and 24.6μM respectively.

Strong Headgroup Interactions Drive Highly Directional Growth and Unusual Phase Co-Existence in Self-Assembled Phenolic Films

Self-assembled materials as surface coatings are used to confer functional properties to substrates, but such properties are highly dependent on molecular organization that can be controlled through tailoring the noncovalent interactions. For monomolecular films, it is well-known that strong, dipolar interactions can oppose line tension generating noncircular domain growth. While many surfactant films exhibit liquid crystalline arrangement of the alkyl chains, there are relatively few reports of crystalline headgroups. Here, we report the self-assembly of phenolic surfactants where the combination of hydrogen bonding and π-stacking leads to a herringbone arrangement of the headgroups, generating a molecular super-lattice that can be observed using grazing incidence X-ray diffraction; such an arrangement has been previously proposed for related phenolic systems but never experimentally observed. We also investigated using pH to modulate the intermolecular interactions and the response of the system in terms of molecular organization. The first hydroxyl deprotonation does not appear to impact the structure but has significant impact on the domain size and morphology. Higher pH generates both strong directional domain growth and a loss of the molecular lattice structure, attributed to a second deprotonation. In contrast, a shorter chain surfactant, lauryl gallate, forms a liquid expanded phase that can contract upon deprotonation. In the condensed phase, the deprotonation kinetics are unusually slow for which an internal charge re-organization is proposed. The slow kinetics leads to the co-existence of three distinct phases for a single component system over relatively long timescales and provides evidence of a liquid-mediated polymorphic transformation process in two-dimensional, soft-matter films. This work has implications for understanding the long-range ordering in aromatic self-assembled structures and the mechanisms underlying Langmuir monolayer polymorphism.

Synthesis and Initial Biological Evaluation of Boron-Containing Prostate-Specific Membrane Antigen Ligands for Treatment of Prostate Cancer Using Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is a therapeutic modality which has been used for the treatment of cancers, including brain and head and neck tumors. For effective treatment via BNCT, efficient and selective delivery of a high boron dose to cancer cells is needed. Prostate-specific membrane antigen (PSMA) is a target for prostate cancer imaging and drug delivery. In this study, we conjugated boronic acid or carborane functional groups to a well-established PSMA inhibitor scaffold to deliver boron to prostate cancer cells and prostate tumor xenograft models. Eight boron-containing PSMA inhibitors were synthesized. All of these compounds showed a strong binding affinity to PSMA in a competition radioligand binding assay (IC₅₀ from 555.7 to 20.3 nM). Three selected compounds 1a, 1d, and 1f were administered to mice, and their in vivo blocking of ⁶⁸Ga-PSMA-11 uptake was demonstrated through a positron emission tomography (PET) imaging and biodistribution experiment. Biodistribution analysis demonstrated boron uptake of 4-7 μg/g in 22Rv1 prostate xenograft tumors and similar tumor/muscle ratios compared to the ratio for the most commonly used BNCT compound, 4-borono-l-phenylalanine (BPA). Taken together, these data suggest a potential role for PSMA targeted BNCT agents in prostate cancer therapy following suitable optimization.

TCFH-NMI: Direct Access to N-Acyl Imidazoliums for Challenging Amide Bond Formations

Challenging couplings of hindered carboxylic acids with non-nucleophilic amines to form amide bonds can be accomplished in high yields, and in many cases, with complete retention of the adjacent stereogenic centers using the combination of N, N, N', N'-tetramethylchloroformamidinium hexafluorophosphate (TCFH) and N-methylimidazole (NMI). This method allows for in situ generation of highly reactive acyl imidazolium ions, which have been demonstrated to be intermediates in the reaction. The reagent delivers high reactivity similar to acid chlorides with the ease of use of modern uronium reagents.

Scale-Up Synthesis and Identification of GLYX-13, a NMDAR Glycine-Site Partial Agonist for the Treatment of Major Depressive Disorder

GLYX-13, a NMDAR glycine-site partial agonist, was discovered as a promising antidepressant with rapidly acting effects but no ketamine-like side effects. However, the reported synthetic process route had deficiencies of low yield and the use of unfriendly reagents. Here, we report a scaled-up synthesis of GLYX-13 with an overall yield of 30% on the hectogram scale with a column chromatography-free strategy, where the coupling and deprotection reaction conditions were systematically optimized. Meanwhile, the absolute configuration of precursor compound of GLYX-13 was identified by X-ray single crystal diffraction. Finally, the activity of GLYX-13 was verified in the cortical neurons of mice through whole-cell voltage-clamp technique.

Polyenylcyclopropane carboxylic esters with high insecticidal activity

BACKGROUND

Pyrethroids are synthetic derivatives of naturally occurring pyrethrum. These molecules are widely used in agriculture for ant, fly and mosquito control and for lawn and garden care. Pyrethroids are the optically active esters of 2,2-dimethyl-3-(2-methylpropenyl)-cyclopropane carboxylic acid, also known as chrysanthemic acid. However, their intense use has resulted in the development of resistance in many insect species. Herein, specific structural modifications of the pyrethroid scaffold and their effect on insecticidal activity, especially on resistant pests strains, are reported.

RESULTS

The exposure to (1R)-trans-(E/Z)-2,3,5,6-tetrafluorobenzyl-3-(buta-1,3-dienyl)-2,2-dimethyl cyclopropanecarboxylate and its diastereomers produced 100% mortality in yellow fever mosquitoes (Aedes aegypti), house mosquitoes (Culex quinquefasciatus) and houseflies (Musca domestica). Moreover, this compound provided complete knockdown within 15 min of exposure against cockroaches (Blattella germanica) and maintained an excellent knockdown activity at 10 days after treatment.

CONCLUSION

Novel pyrethroid derivatives obtained from 2,2-dimethyl-3-(2-methylpropenyl)-cyclopropanecarboxylic acid are described. These derivatives display high insecticidal activity, a wide spectrum of action and no toxicity towards mammalians. The proposed synthetic procedures are highly efficient and inexpensive, and therefore suitable for industrial scale-up.

(E)- and (Z)-stereodefined enol phosphonates derived from β-ketoesters: stereocomplementary synthesis of fully-substituted α,β-unsaturated esters

A versatile, robust, and stereocomplementary synthesis of fully-substituted (E)- and (Z)-stereodefined α,β-unsaturated esters 3 from accessible α-substituted β-ketoesters 1via (E)- and (Z)-enol phosphonates was achieved.

Antileishmanial lead structures from nature: analysis of structure-activity relationships of a compound library derived from caffeic Acid bornyl ester

Bioassay-guided fractionation of a chloroform extract of Valeriana wallichii (V. wallichii) rhizomes lead to the isolation and identification of caffeic acid bornyl ester (1) as the active component against Leishmania major (L. major) promastigotes (IC₅₀ = 48.8 µM). To investigate the structure-activity relationship (SAR), a library of compounds based on 1 was synthesized and tested in vitro against L. major and L. donovani promastigotes, and L. major amastigotes. Cytotoxicity was determined using a murine J774.1 cell line and bone marrow derived macrophages (BMDM). Some compounds showed antileishmanial activity in the concentration range of pentamidine and miltefosine which are the standard drugs in use. In the L. major amastigote assay compounds 15, 19 and 20 showed good activity with relatively low cytotoxicity against BMDM, resulting in acceptable selectivity indices. Molecules with adjacent phenolic hydroxyl groups exhibited elevated cytotoxicity against murine cell lines J774.1 and BMDM. The Michael system seems not to be essential for antileishmanial activity. Based on the results compound 27 can be regarded as new lead structure for further structure optimization.

Amide formation in one pot from carboxylic acids and amines via carboxyl and sulfinyl mixed anhydrides

An efficient method has been developed for the preparation of yet unknown acyclic mixed anhydrides of carboxylic and sulfinic acids. Sterically hindered 2-methylbut-3-ene-2-sulfinyl carboxylates add primary and secondary amines preferentially onto the carbonyl moieties realizing a new method for the one-pot preparation of carboxamides. It uses 1:1 mixtures of carboxylic acids and amines without a base, requires no excess of reagents, and liberates only volatile coproducts. Protected di- and tripeptides have been prepared in solution without epimerization by application of this method.

Synthesis of optically pure norcantharidin analogue NCA-01, a highly selective protein phosphatase 2B inhibitor, and its derivatives

An efficient synthetic route to optically pure norcantharidin analogue NCA-01, a highly selective inhibitor of protein phosphatase 2B (PP2B; calcineurin), has been developed. The absolute stereochemistry of the enantiomers was determined by X-ray crystallographic analysis. Optically pure NCA derivatives that had various substituents at the C1 position were synthesized in a similar manner. The PP2B-inhibitory activities of NCA-01 and its derivatives were independent of the enantiomeric form. NCA-01 dimethyl ester potently inhibited IL-2 production in Jurkat cells.

Syntheses and studies of amamistatin B analogs reveals that anticancer activity is relatively independent of stereochemistry, ester or amide linkage and select replacement of one of the metal chelating groups

A series of analogs of the amamistatin natural products was designed and synthesized to facilitate additional anticancer structure-activity relationships. The results indicate that the anticancer activity is relatively independent of stereochemistry, ester or amide linkage and replacement of the oxazoline/oxazole based iron-binding group with a catechol.

4-Substituted-7-N-alkyl-N-acetyl 2-aminobenzothiazole amides: drug-like and non-xanthine based A2B adenosine receptor antagonists

7-N-Acetamide-4-methoxy-2-aminobenzothiazole 4-fluorobenzamide (compound 1) was chosen as a drug-like and non-xanthine based starting point for the discovery of A(2B) receptor antagonists because of its slight selectivity against A(1) and A(2A) receptors and modest A(2B) potency. SAR exploration of compound 1 described herein included modifications to the 7-N-acetamide group, substitution of the 4-methoxy group by halogens as well as replacement of the p-flouro-benzamide side chain. This work culminated in the identification of compound 37 with excellent A(2B) potency, modest selectivity versus A(2A) and A(1) receptors, and good rodent PK properties.

Synthetic studies on daphnicyclidin A: enantiocontrolled construction of the BCD ring system

An enantiocontrolled entry to the tricyclic core of daphnicyclidin A with five chiral centers including an all-carbon quaternary center is reported. The synthesis features a highly diastereoselective conjugate addition of nitromethane, an Ireland-Claisen rearrangement, and a tandem acyliminium/Mannich-type reaction.

Synthesis and biological evaluation on novel analogs of 9-methylstreptimidone, an inhibitor of NF-kappaB

Synthesis of 9-methylstreptimidone analogs and their inhibitory activities against NF-kappaB (nuclear factor-kappaB) are reported. Among several active derivatives synthesized in this study, 8 with a relatively simple structure, exhibited inhibitory activity against LPS-induced NO production comparable to that of 9-methylstreptimidone.