2- -substituted quinolines tautomerism

In another synthesis of quinolines involving imine intermediates, o-oxazoline-substi-tuted anilines (6) react with ketones in dry butanol reflux to give 4-amino-substituted quinolines [e.g. (7)], or 4-quinolones, using tosic acid as catalyst.18 A mechanism involving ketoimine formation with subsequent tautomerization to give an enamine which attacks the oxazoline ring is discussed. 

All bands in the IR spectra of several substituted 1,5-naphthyridines have been assigned the spectrum of 1,5-naphthyridine 1 was compared with the spectrum of 5-substituted quinoline (1963MI1). IR spectroscopy was used to study the keto-enol tautomerism of 4-hydroxy-1,5-naphthyridine in solution and in the solid state it was found that the keto form predominates in polar solvent and the enol form in nonpolar solvent (1967MI2). 

In general, these properties of hydroxy substituted derivatives will be present in pi-deficient nitrogen heterocycles with benzo fusion or with multinitrogen substitution. For example, the 4-OH group on quinoline tautomerizes to the 4-quinolone structure (Scheme 6.36). 

Attempts have been made to deduce the structure of the predominant form of a potentially tautomeric compound from the shifts which occur in the ultraviolet spectrum of the compound in question on passing from neutral to basic or acidic solutions. The fact that no bathochromic shifts were observed for 2- and 4-hydroxy quinoline and 1-hydroxyisoquinoline under these conditions was taken as evidence that they existed in the oxo form [similar work on substituted quinol-4-ones led to no definite conclusions ]. A knowledge of the dissociation constants is essential to studies of this type, and the conclusions can, in any case, be only very tentative. A further dif-... 

Free-Energy Values (AG°, 0.1 kcal moF ) for the Tautomerism of Some Substituted Oxadiazolo[3,4-/ or [3,4-/i]Quinoline V-Oxides 46... 

No enaminone tautomer 124b was observed in CDC13 solutions of substituted 4-(acylmethyl)quinolines 124. The position of the tautomeric equilibrium is greatly affected by both R1 and R2 substituents. Quinolines 124 (R1 = alkyl R2 = H) exist in solution exclusively as keto tautomers 124a, whereas only tautomeric form 124c was observed for diesters 124 (R1 = COOEt R2 = COOMe, COOEt). Both taut-... 

The initially formed imine will tautomerize to a conjugated enamine and cyclization now occurs by electrophilic aromatic substitution. The enamine will normally prefer to adopt the first configuration shown in which cyclization is not possible, and (perhaps for this reason or perhaps because it is difficult to predict which quinoline will be formed from an unsymmetri-cal 1,3-dicarbonyl compound) this has not proved a very important quinoline synthesis. However, the synthetic plan is sound, and we shall describe two important variants on this theme, one for quinolines and one for quinolones. 

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