Barbituric acids ring stability

Many barbituric acid derivatives have been formulated since the early 1920s. Only a few remain in regular clinical usage and only one, thiopental, is used with any regularity in equine anesthesia. Thiopental is classified as a thiobarbiturate because there is a sulfur atom on the second carbon of the barbituric acid ring. It is also classified as an ultrashort-acting barbiturate. The presence of the sulfur atom decreases the stability of the molecule and shortens the duration of anesthetic action (Branson Booth 1995). 

This subsection examines the hydrolytic stability of cyclic structures containing a ureido link. Schematically, ring closure can be achieved by N-alkylation or by /V-acylation of the second N-atom of the ureido moiety. The former results in the formation of, e.g., hydantoins and dihydropyrimidines. The latter ring closure leads to, e.g., barbituric acids. Taken together, cyclic ureides can also be regarded as ring structures that contain an imido function with an adjacent N-atom. We begin our discussion with the five-membered hydantoins, to continue with six-membered structures, namely dihydropyrimidines, barbituric acids, and xanthines. 

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