Generation and Trapping of Ketenes in Flow

Continuous Flow Generation of Acylketene Intermediates via Nitrogen Extrusion

A flow chemistry process for the generation and use of acylketene precursors through extrusion of nitrogen gas is reported. Key to the development of a suitable continuous protocol is the balance of reaction concentration against pressure in the flow reactor. The resulting process enables access to intercepted acylketene scaffolds using volatile amine nucleophiles and has been demonstrated on the gram scale. Thermal gravimetric analysis was used to guide the temperature set point of the reactor coils for a variety of acyl ketene precursors. The simultaneous generation and reaction of two reactive intermediates (both derived from nitrogen extrusion) is demonstrated.

Flow Chemistry for Cycloaddition Reactions

Continuous flow reactors form part of a rapidly growing research area that has changed the way synthetic chemistry is performed not only in academia but also at the industrial level. This Review highlights the most recent advances in cycloaddition reactions performed in flow systems. Cycloadditions are atom-efficient transformations for the synthesis of carbo- and heterocycles, involved in the construction of challenging skeletons of complex molecules. The main advantages of translating these processes into flow include using intensified conditions, safer handling of hazardous reagents and gases, easy tuning of reaction conditions, and straightforward scaling up. These benefits are especially important in cycloadditions such as the copper(I)-catalyzed azide alkyne cycloaddition (CuAAC), Diels-Alder reaction, ozonolysis and [2+2] photocycloadditions. Some of these transformations are key reactions in the industrial synthesis of pharmaceuticals.

Exploring the generation and use of acylketenes with continuous flow processes

The generation and use of acyl ketenes under continuous flow reaction conditions is reported. Several reaction classes of these reactive intermediates have been studied. Under zero headspace conditions, a ketone exchange process is possible between volatile ketones. The process can be readily scaled to deliver gram quantities of product.

Synthesis of 1,2,5-oxathiazole-S-oxides by 1,3-dipolar cycloadditions of nitrile oxides to α-oxo sulfines

The generation of novel 1,2,5-oxathiazole-S-oxide cycloadducts from cycloaddition of nitrile oxide dipoles with α-oxo sulfines generated in situ from α-diazosulfoxides is reported.

Infrared Fingerprints of nN → σ*NH Hyperconjugation in Hydrazides

An earlier study demonstrated that hyperconjugation operates in hydrazides by analyzing the N-H stretching mode in gas phase infrared (IR) spectroscopy, and then observing two very distinct bands corresponding to isolated isomers experiencing or not the n_N → σ*_N-H electron delocalization. The present work reports a chemical method to obtain insight on the hyperconjugation in hydrazide derivatives from solution IR spectroscopy. The analogous amides did not show a ν_N-H red-shifted band, as the electron donor orbital in the above hyperconjugative interaction does not exist. In addition, the effect of electron withdrawing groups bonded to a nitrogen atom, namely the trifluoroacetyl and the methanesulfonyl groups, was analyzed on the conformational isomerism and on the ability to induce a stronger hyperconjugation in the resulting compounds.

2-Substituted Aniline as a Simple Scaffold for LuxR-Regulated QS Modulation

The ability of the 2-substituted aniline motif to serve as a scaffold for designing potential LuxR-regulated quorum sensing (QS) modulators has been investigated, using docking experiments and biological evaluation of a series of 15 specially synthesized compounds. Aniline, 2-acetyl-aniline and 2-nitroaniline were considered, as well as their N-acylated derivatives. Docking experiments showed that the 2-substituted aniline motif fits within the LuxR binding site at the place of the lactone moiety of AHL, and the biological evaluation revealed QS antagonisitic activity for several compounds, validating the hypothesis that this scaffold acts on QS. Structure activity relationships are discussed regarding interactions with the key residues of the LuxR binding site, showing significant variations in the H-bonding pattern.

Regio- and Stereoselective Anti-Carbozincation of Alkynyl Ethers Using ZnBr2 toward (Z)-β-Zincated Enol Ether Synthesis

(Z)-β-Aryloxyalkenylzincs are synthesized stereoselectively via anti-carbozincation among alkynyl ethers, silyl ketene acetals, and ZnBr₂. X-ray analysis revealed the structure of the zinc species is a mononuclear two-coordinate dialkenylzinc that is transformed into functionalized enol ethers as a single isomer in the reaction of various electrophiles.

Thermolysis of 1,3-dioxin-4-ones: fast generation of kinetic data using in-line analysis under flow

Rapid acquisition of kinetic data for thermolysis of 1,3-dioxin-4-ones is demonstrated with a commercial meso-scale flow reactor, using a step-change in flow rate or ‘push-out’ from the flow line.

The chemistry of ynol and thioynol ethers

Alkynyl ethers and thioethers are versatile building blocks in organic synthesis allowing various transformations including a number of C–C bond forming processes.

Online quantitative mass spectrometry for the rapid adaptive optimisation of automated flow reactors

An automated continuous reactor for the synthesis of organic compounds, which uses online mass spectrometry (MS) for reaction monitoring and product quantification, is presented.

Microwave-assisted heteropolyanion-based ionic liquid promoted sustainable protocol to N-heteroaryl amides via N-directing dual catalyzed oxidative amidation of aldehydes

A procedure for the synthesis of N-heteroaryl amides directly from oxidative amidation of aldehydes catalyzed by heteropolyanion-based ionic liquids has been reported. The proposed N-directing dual-catalysis mechanism was briefly investigated.