THE ZINCKE REACTION. A REVIEW

An efficient ultrasonic-assisted synthesis of imidazolium and pyridinium salts based on the Zincke reaction

A mild and efficient method has been developed using ultrasound irradiation for the synthesis of imidazolium and pyridinium salts based on the Zincke reaction. Tertiary nitrogen nucleophiles such as pyridines and imidazoles can be alkylated with primary amine by simply using their ammonium form Zincke salts. In almost all cases, a clear yield increase results and a dramatic reduction of the reaction time accompanied by an improved quality of the products occurs.

Complex polycyclic lactams from pericyclic cascade reactions of Zincke aldehydes

In the course of a simple mechanistic study on the rearrangement of Zincke aldehydes to Z-alpha,beta,gamma,delta-unsaturated amides, a thermally induced pericyclic cascade rearrangement that converts Zincke aldehydes derived from allylic and homoallylic amines into polycyclic lactams was discovered. The key reaction involves an E-Z alkene isomerization, a 6pi electrocyclic ring closure, a [1,5]-sigmatropic shift of hydrogen, a 6pi electrocyclic ring-opening, and a Diels-Alder cycloaddition, and proceeds with excellent stereoselectivity. The unusual observation that furans and an indole serve as dienophiles in this cascade reaction permitted the synthesis of complex, functional group-rich tri- and tetracyclic lactams. In all cases, the rigid polycyclic products are available in only two steps from pyridinium salts and the allylic or homoallylic secondary amines.

Synthesis of N-arylpyridinium salts bearing a nitrone spin trap as potential mitochondria-targeted antioxidants

The generation of excess reactive oxygen species (ROS) in mitochondria is responsible for much of the oxidative stress associated with ageing (aging), and mitochondrial dysfunction is part of the pathology of neurodegeneration and type 2 diabetes. Lipophilic pyridinium ions are known to accumulate in mitochondria and this paper describes a general route for the preparation of nitrone-containing N-arylpyridinium salts having a range of lipophilicities, as potential therapeutic antioxidants. The compatibility of nitrones with the Zincke reaction is the key to their synthesis. Their trapping of carbon-centred radicals and the EPR spectra of the resulting nitroxides are reported.

Mild lanthanide(iii) catalyzed formation of 4,5-diaminocyclopent-2-enones from 2-furaldehyde and secondary amines: a domino condensation/ring-opening/electrocyclization process

Lewis acid catalyzed reaction of 2-furaldehyde and secondary amines results in the formation of 4,5-diaminocyclopent-2-enones exclusively as the trans diastereomers: computational studies support the stereochemistry of the adducts as arising from a thermal conrotatory pi4a electrocyclization of an initial ring-opened intermediate (a deprotonated Stenhouse salt).

Convenient, scalable and flexible method for the preparation of imidazolium salts with previously inaccessible substitution patterns

A high yielding and modular approach to N,N'-disubstituted imidazolium salts is described, providing access to substitution patterns that are beyond the reach of established methodology.