Ethyl (3-hydroxyisoamyl) barbituric acid as the principal metabolite of amytal.

An earlier study (1) from this laboratory proved conclusively that Amytal, 5-ethyl-5-isoamylbarbituric acid (I), is not detoxified by simple hydrolysis of the barbituric acid ring and indicated that the alkyl side chain (or chains) must suffer alteration in viva. Recently (2), it was discovered that the isomeric drug, pentobarbital, is excreted as the diastereoisomers of 5-ethyl-5-(3-hydroxy-1-methylbutyl)barbituric acid. This finding, reflecting an unusual pattern of biological oxidation, initiated further work to identify the end-products of Amytal. The present report describes the isolation, proof of structure, and quantitative determination in the urine of dogs of the principal metabolite of the drug. From extracts of urine of dogs after the administration of Amytal it was possible to isolate only one metabolite of the drug, but this was obtained in relatively good yield (35 per cent). The compound had an elementary composition corresponding to Amytal with 1 additional oxygen atom. It had the characteristic ultraviolet absorption spectrum of 5,5dialkylbarbituric acids. The presence of a hydroxyl group was indicated by the infra-red spectrum and proved by the preparation of a crystalline acetate. The metabolite was optically inactive. Although there are six different monohydroxy derivatives of Amytal, it was reasoned that 5-ethyl-5-(3-hydroxyisoamyl)barbituric acid (II) was the most likely structure for the metabolite. Special consideration of this isomer was based largely on two facts. First, ethyl groups attached to the barbituric acid ring are very stable in viva (3, 4). Secondly, primary alcohols resulting from w oxidation are rapidly oxidized to acids (5, 6), whereas secondary or tertiary alcohols from non-terminal carbon oxidation appear to be fairly stable in V&JO, as judged from the metabolites of pentobarbital and hydrolapachol (discussed below). The surmise was proved to be correct by synthesis. The metabolite of Amytal was synthesized by two different methods. The first procedure involved the addition of water to 5-ethyl-5-(3-methyl-