Hydroxypyridines and Derivatives

Wheland (64) describes 3-hydroxypyridine as a true phenol. Since phenols are the easiest of the substituted benzenes to hydrogenate catalytically (65), it should be of interest to examine some of the reductions of 3-hydroxypyridine for comparison. 

Cavallito (66) reports that neither it nor 3-benzoxypyridine was hydrogenated in the presence of Willstatter s palladium sponge catalyst (67). Under low pressure conditions in ether or dioxane Raney nickel and platinum oxide were ineffective. However, other examples show that reduction takes place readily under a variety of conditions. Biel used Raney nickel at 125° and 50 atm (68), excellent yield of 6-propyl-3-hydroxypiperidine resulted from reduction of the pyridine in acetic acid with platinum oxide (69). Ruthenium in the conversion of 3-hydroxypyridine in aqueous solution gave very high yield of the corre- 

In the reduction of l-phenyl-3-hydroxypyridinium chloride considerable hydrogenolysis took place with the formation of 1-phenyl-piperidine (34%) along with 31% of the desired l-phenyl-3-hydroxy-piperidine (71). While Kao (72) reported very good yield of 3-hydrox-ypiperidine, Biel and his co-workers (73) found that piperidine (50% yield) was obtained in addition to 30-40% of 3-hydroxypiperidine. They postulate that an intermediate allyl type alcohol is formed which is more prone to hydrogenolysis than 3-hydroxypiperidine  

3-Hydroxypiperidine was recovered unchanged when subjected to similar reaction conditions. 

When 2-hydroxypyridine, generally considered as 2-pyridone (64) is hydrogenated only two molecular equivalents of hydrogen are taken up. Grave (74) in some early work suggested that the iV-methyl derivative, l-methyl-2-pyridone, is a true dihydropyridine and should require only two equivalents. Ruzicka (75) in an earlier publication describes 

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