Pyridine compounds rings

Coordination compounds containing bidentate ligands are often thermally more stable than those comprised of related monodentate ligands, e.g. ethylenediamine (en) complexes dissociate at a higher temperature than those of ammonia or pyridine. Compounds containing a ring structure, such as coordinated salicylaldehyde (sal) and acetyl-acetonate (acac), are particularly stable, and may often be sublimed... 

Houghton C, RB Cain (1972) Microbial metabolism of the pyridine ring. Formation of pyridinediols (dihydroxypyridines) as intermediates in the degradation of pyridine compounds by microorganisms. Biochem J 130 879-893. 

Pyridine compounds in which phosphorus is directly attached to a ring carbon are relatively rare. Phosphorus nucleophiles are not able to replace ring hydrogen atoms in pyridines and pyridine 1-oxides. Some time ago it was found that pyridine yields a zwitterion (170) when it is heated under reflux in the presence of tetraphosphorus decasulfide (Scheme 105) (68MI20500). Recently, a product that contains a C—P bond was isolated after extended heating under the same conditions followed by treatment with hydrochloric acid (81 JCR(S)285). However, it is uncertain whether free pyridine undergoes reaction in this case. Attack by phosphorus nucleophiles on salts is well established. Af-Methylacridinium methosulfate affords a stable isolable 9,10-dihydro adduct (171) that readily forms a... 

Ring closure of (17) under Vilsmeier conditions led to pyridine compound (18) (Scheme 12) <95JHC(32)505>. 

Pyridine rings are frequently used in combination with other functional pesticides. These pyridine compounds have structures which are determined by the main function involved, the pyridine ring being an attachment to the main function. 

While the existence of such molecules would clarify the formation and the observed mutarotation of the pyridine compound, it would also seem that in the usual acetylation and methylation reactions an acidic orthoester would behave as though it posse.ssed a normal structure. This would make impo.ssible the exact proof of the structure of the acidic orthoesters by means of chemical transformations. The situation concerning this problem was not further clarified by Helferich and Muller s contention that, contrary to the opinion of Haworth and coworkers, it is possible to construct a strainless orthoacid ring between positions 4 and 6 in the pyranose ring. 

We speculated that the electron-withdrawing nature of the pyridine ring was responsible for the lack of reactivity of 23 toward benzyne. In troubleshooting the benzyne addition we examined the use of a methoxy group to offset the electron-withdrawing properties of the pyridine. Compounds 24-28 (Fig. 8) were prepared in the order indicated. As each was... 

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