Abstract
This review describes the synthesis and reactions of pyridine N-oxides within the last ten years. The first part surveys the different synthetic methods which include ring transformation, classical oxidations using peracids, the use of metalloorganic oxidizing agents and cycloaddition reactions. The second part surveys the reactions of pyridine N-oxides including the deoxygenation, nucleophilic reaction and cycloaddition to N-O bond.
Highlights
Spectroscopic properties 1 Synthesis of pyridine N-oxides 1.1 From the esters of N-hydroxy-2-thiopyridone 1.2 By ring transformation of isooxazoles. 1.3
A variety of pyridine derivatives 15a-d were converted into their corresponding N-oxides 16a-d in good yields and high chemoselectivity in the presence of hydrogen peroxide as oxygen donor, catalytic amount of manganese tetrakis(2,6-dichlorophenyl) porphyrin [Mn(TDCPP)Cl] and ammonium acetate as cocatalyst in CH2Cl2 / CH3CN.[18]
The best results were reported for the deoxygenation of pyridine N-oxides, when the reaction of compound was carried out using zinc (4.5 eq.)[66] gave
Summary
The chemistry and applications of N-oxides have recently received much attention due to their usefulness as synthetic intermediates and their biological importance[1]. The N-O moiety of pyridine N-oxides possesses a unique functionality which can act effectively as a push electron donor and as a pull electron acceptor group. The study of 3-halo-2,6-dimethylpyridine N-oxides has shown that apart from the strong 272-278 nm band, two or three more bands are observed in the regions 220-240 and 310-330 nm. It might originate from the n-π* transition, i.e. excitation from HOMO to either the LUMO or higher MO This band is observed at a significantly higher energy (363.3 nm) for 4-chloro-2,6-dimethyl-3iodopyridine N-oxide than for 3-iodo-2,6-dimethyl-pyridine N-oxide (329 nm). The irradiation of compounds 2 in acetonitrile with a medium pressure mercury lamp at room temperature for 1-4 h led to the formation of the corresponding dimers 3 as well as the expected disulfides 4 as shown in (Scheme 1). 12
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