TiCl4/Et3N-Mediated Condensation of Acetate and Formate Esters: Direct Access to β-Alkoxy- and β-Aryloxyacrylates

Activation-Free Sulfonyl Fluoride Probes for Fragment Screening

SuFEx chemistry is based on the unique reactivity of the sulfonyl fluoride group with a range of nucleophiles. Accordingly, sulfonyl fluorides label multiple nucleophilic amino acid residues, making these reagents popular in both chemical biology and medicinal chemistry applications. The reactivity of sulfonyl fluorides nominates this warhead chemotype as a candidate for an external, activation-free general labelling tag. Here, we report the synthesis and characterization of a small sulfonyl fluoride library that yielded the 3-carboxybenzenesulfonyl fluoride warhead for tagging tractable targets at nucleophilic residues. Based on these results, we propose that coupling diverse fragments to this warhead would result in a library of sulfonyl fluoride bits (SuFBits), available for screening against protein targets. SuFBits will label the target if it binds to the core fragment, which facilitates the identification of weak fragments by mass spectrometry.

Photostable Abscisic Acid Agonists with a Geometrically Rigid Cyclized Side Chain

The plant hormone abscisic acid (ABA) plays a central role in adaptive responses to abiotic stresses that adversely affect crop growth and productivity. However, ABA photoinstability limits its use in agriculture. To overcome this drawback, in this study, we developed photostable ABA analogues, the (+)-BP2A compound series (compounds 5-9), in which the dienoic acid side chain of ABA was replaced with phenylacetic acid. All BP2A analogues showed higher stability against UV-B irradiation at 302 nm than ABA, and compounds 6 and 7 barely decomposed even under sunlight. In physiological assays, (+)-BP2A and (+)-compound 7, in which the α,β-unsaturated carbonyl group of BP2A was reduced, exhibited ABA-like activities, including inhibition of seed germination and induced drought tolerance in Arabidopsis. Biochemical studies revealed that (+)-compound 7, unlike (+)-BP2A, did not activate pyrabactin resistance-like (PYL) receptors in vitro and was converted to (+)-BP2A in plants, suggesting that it functions as a prodrug PYL agonist. Furthermore, (+)-compound 7 inhibited seed germination of tomato, lettuce, and rice. Thus, this compound represents a potential plant growth regulator that induces ABA-type responses in agricultural fields.

Synthesis of Indoles by Palladium-Catalyzed Reductive Cyclization of β-Nitrostyrenes with Phenyl Formate as a CO Surrogate

The reductive cyclization of suitably substituted organic nitro compounds by carbon monoxide is a very appealing technique for the synthesis of heterocycles because of its atom efficiency and easiness of separation of the only stoichiometric byproduct CO2, but the need for pressurized CO has hampered its diffusion. We have recently reported on the synthesis of indoles by reductive cyclization of o-nitrostyrenes using phenyl formate as a CO surrogate, using a palladium/1,10-phenanthroline complex as catalyst. However, depending on the desired substituents on the structure, the use of β-nitrostyrenes as alternative reagents may be advantageous. We report here the results of our study on the possibility to use phenyl formate as a CO surrogate in the synthesis of indoles by reductive cyclization of β-nitrostyrenes, using PdCl2(CH3CN)2 + phenanthroline as the catalyst. It turned out that good results can be obtained when the starting nitrostyrene bears an aryl substituent in the alpha position. However, when no such substituent is present, only fair yield of indole can be obtained because the base required to decompose the formate also catalyzes an oligo-polymerization of the starting styrene. The reaction can be performed in a single glass pressure tube, a cheap and easily available piece of equipment.

Metal-Free Stereoselective Synthesis of (E)- and (Z)-N-Monosubstituted β-Aminoacrylates via Condensation Reactions of Carbamates

N-monosubstituted β-aminoacrylates are building blocks, which have been used in the preparation of amino acids and pharmaceuticals. Two efficient, stereoselective methods of preparation, via acid- or base-promoted condensation reactions of carbamates, are described. The base-promoted reaction is E-selective, while acid catalysis can, through the choice of solvent, selectively form E or Z. The acid-catalyzed E-selective process proceeds through a crystallization obviating the need for chromatographic purification.

Nucleophilic Vinylic Substitution (SNV) of Trisubstituted Monofluoroalkenes for the Synthesis of Stereodefined Trisubstituted Alkenes and Divinyl Ethers

We herein describe a nucleophilic vinylic substitution (S_NV) of trisubstituted monofluoroalkenes with excellent stereocontrol (d.r. > 99:1). Starting from (E)-β-monofluoroacrylates, various trisubstituted (E)-alkenes containing O/N/S-substituent groups at the vinylic position can be obtained under simple conditions. Furthermore, (E,E)-divinyl ethers can be generated through dimerization of the monofluoroalkenes, triggered by adventitious water in the reaction mixture.

Increasing the steric hindrance around the catalytic core of a self-assembled imine-based non-heme iron catalyst for C-H oxidation

Sterically hindered imine-based non-heme complexes 4 and 5 rapidly self-assemble in acetonitrile at 25 °C, when the corresponding building blocks are added in solution in the proper ratios. Such complexes are investigated as catalysts for the H2O2 oxidation of a series of substrates in order to ascertain the role and the importance of the ligand steric hindrance on the action of the catalytic core 1, previously shown to be an efficient catalyst for aliphatic and aromatic C-H bond oxidation. The study reveals a modest dependence of the output of the oxidation reactions on the presence of bulky substituents in the backbone of the catalyst, both in terms of activity and selectivity. This result supports a previously hypothesized catalytic mechanism, which is based on the hemi-lability of the metal complex. In the active form of the catalyst, one of the pyridine arms temporarily leaves the iron centre, freeing up a lot of room for the access of the substrate.

An enzymatic acetal/hemiacetal conversion for the physiological temperature activation of the alkoxyamine C–ON bond homolysis

Enzymatic trigger. Upon enzymatic hydrolysis by Subtilisin A, highly stable alkoxyamines are transformed into highly labile alkoxyamines able to homolyze spontaneously in less than 500 seconds, at 37 °C.

Catalytic Reductions Without External Hydrogen Gas: Broad Scope Hydrogenations with Tetrahydroxydiboron and a Tertiary Amine

Facile reduction of aryl halides with a combination of 5% Pd/C, B₂(OH)₄, and 4-methylmorpholine is reported. Aryl bromides, iodides, and chlorides were efficiently reduced. Aryl dihalides containing two different halogen atoms underwent selective reduction: I over Br and Cl, and Br over Cl. Beyond these, aryl triflates were efficiently reduced. This combination was broadly general, effectuating reductions of benzylic halides and ethers, alkenes, alkynes, aldehydes, and azides, as well as for N-Cbz deprotection. A cyano group was unaffected, but a nitro group and a ketone underwent reduction to a low extent. When B₂(OD)₄ was used for aryl halide reduction, a significant amount of deuteriation occurred. However, H atom incorporation competed and increased in slower reactions. 4-Methylmorpholine was identified as a possible source of H atoms in this, but a combination of only 4-methylmorpholine and Pd/C did not result in reduction. Hydrogen gas has been observed to form with this reagent combination. Experiments aimed at understanding the chemistry led to the proposal of a plausible mechanism and to the identification of N,N-bis(methyl-d ₃)pyridine-4-amine (DMAP-d ₆) and B₂(OD)₄ as an effective combination for full aromatic deuteriation.

Interception of amide ylides with sulfonamides: synthesis of (E)-N-sulfonyl amidines catalyzed by Zn(OTf)2

Through the interception of amide ylides with sulfonamides, we herein report an intermolecular condensation reaction between sulfonamides and common amides.

Selective C–H bond electro-oxidation of benzylic acetates and alcohols to benzaldehydes

A method for the electrosynthesis of aldehydes is presented.

TiCl4/DMAP mediated Z-selective knovenagel condensation of isatins with nitroacetates and related compounds

A Z-selective Knovenagel condensation reaction of isatins with nitroacetates and related compounds mediated by TiCl₄ and DMAP was described.