Enantioselective Synthesis of Nicotine via an Iodine-Mediated Hofmann-Löffler Reaction

Simplified Version of the Eschweiler-Clarke Reaction

The traditional Eschweiler-Clarke reaction is a three-component process involving formaldehyde, amine, and formic acid. In this work, we showed that the reductive potential of formaldehyde was sufficient to provide methylation of secondary amines in the absence of acidic additives. Various acid-sensitive moieties remain intact under developed conditions. The scalability of the elaborated approach was shown for several products. Synthesis of the antifungal agent butenafine demonstrated the preparative utility of the developed approach.

Urea-Promoted Neat Synthesis of Fused Dihydroisoquinolines and Disubstituted Pyridines: A Mechanistic Observation with Molecular-Sensing Studies

An efficient and short synthesis of fused dihydroisoquinolines, diaryl substituted pyridine derivatives in good to high yields has been established by using an environmentally safe, solvent-metal-oxidant-free tandem approach. In this article, we discuss how the electrocyclic reaction is more pronounced in the solid phase in the presence of urea, whereas the typical aza-Michael addition is more prominent in presence of arylamine in the solution phase for 3-(2-formylcycloalkenyl)acrylic ester derivative substrates. The wide range of substrates and urea-promoted neat synthesis made our approach more significant in medical and also analytical science. Moreover, an isoquinoline alkaloid decumbenine B analogue has been synthesized by using our newly developed neat methodology. We have also investigated the photophysical properties of the synthesized fused dihydroisoquinoline derivatives. One of the synthesized molecules was used as a sensor for the selective detection of toxic picric acid. Therefore, the effective neat synthesis and molecular sensing applications of these compounds made our approach more exciting in the field of heterocyclic chemistry.

Synthesis of Six-Membered N-Heterocycle Frameworks Based on Intramolecular Metal-Free N-Centered Radical Chemistry

The formation of intramolecular C-N bond represents a powerful strategy in organic transformation as the great importance of N-heterocycles in the fields of natural products and bioactive molecules. This personal account describes the synthesis of cyclic phosphinamidation, benzosultam, benzosulfoximine, phenanthridine and their halogenated compounds through transition-metal-free intramolecular oxidative C-N bond formation. Mechanism study reveals that N-X bond is initially formed under the effect of hypervalent halogen, which is very unstable and easily undergoes thermal or light homolytic cleavage to generate nitrogen radical. Then the nitrogen radical is trapped by the arene to give aryl radical. Rearomatization of aryl radical under the oxidant to provide corresponding N-heterocycle. Under suitable conditions, the N-heterocycles can be further transformed to halogenated N-heterocycles.

Role of substituents in the Hofmann-Löffler-Freytag reaction. A quantum-chemical case study on nicotine synthesis

The Hofmann-Löffler-Freytag (HLF) reaction can be successfully used to synthesize saturated heterocyclic nitrogen-containing nature-derived pharmaceuticals such as nicotine and its derivatives. In this study the rate-determining hydrogen atom transfer (HAT) step in nicotine synthesis has been analyzed using quantum chemical methods. Through quantification of substituent effects in the HAT step of the reaction on both nitrogen and carbon atoms, optimized synthetic strategies are outlined for the racemic as well as the stereoselective synthesis of nicotine. This latter process can be achieved using common nitrogen protecting groups, such as Ac, TFAc, and Boc. The said protecting groups show superior nitrogen radical activation as compared to the commonly used Tosyl group. Computational results indicate that the 1,5-HAT step is in this case likely to work even for the reaction with primary unactivated carbon centers.

Pushing the boundaries of C–H bond functionalization chemistry using flow technology

C–H functionalization chemistry is one of the most vibrant research areas within synthetic organic chemistry. While most researchers focus on the development of small-scale batch-type transformations, more recently such transformations have been carried out in flow reactors to explore new chemical space, to boost reactivity or to enable scalability of this important reaction class. Herein, an up-to-date overview of C–H bond functionalization reactions carried out in continuous-flow microreactors is presented. A comprehensive overview of reactions which establish the formal conversion of a C–H bond into carbon–carbon or carbon–heteroatom bonds is provided; this includes metal-assisted C–H bond cleavages, hydrogen atom transfer reactions and C–H bond functionalizations which involve an SE-type process to aromatic or olefinic systems. Particular focus is devoted to showcase the advantages of flow processing to enhance C–H bond functionalization chemistry. Consequently, it is our hope that this review will serve as a guide to inspire researchers to push the boundaries of C–H functionalization chemistry using flow technology.