Photochemically , an anion of S( +)-5-ethyl-l-methyl-5-phenylbarbituric acid is converted stereospecifically to R(-)-5-ethyl-3-methyl-5-phenylhydantoin at room temperature. The R configuration is proposed for (-)-5-allyl-3-methyl-5-phenylhydantoin. In our earlier investigation on photolysis of methylphenobarbital') (la) it was found that the barbituric acid ring undergoes contraction yielding 5-ethyl-3-methyl-5-phenylhydantoin (24. This conversion took place in an alkaline medium where la is ionized and was accompanied by the evolution of CO. Similar contraction was found als0 for the photolysis of N-methylbarbital but in this case the product of ring opening -N-(2-ethylbutyryl)-N-methylurea -was als0 isolated'). Stereospezifizität der photoindnzierten Umwanäiung von Methylphenobarbitai zu Mephenytoin EinSince methylphenobarbital is a chirai compound and its enantiomers were separated and their configuration e~tablished~'~'~), we decided to investigate the stereochemid course of the above mentioned photochemical reaction leading to the chiral hydantoin derivative with the absolute configuration als0 recently defined6).Preparative photolysis of S( +)-methyiphenobarbital (la) in the alkaline ethanol-water medium at ca. 15°C yielded the appropriate hydantoin denvative which showed the optica1 activity characteristic for i?-mephenytoin (2a) : [a]Do = -101.6", lit .6) -105.6'. The optical purity of the isoiated mephenytoin was checked by the HPLC method according to Sybilska et al.') using f3-cyclodextrin as a chiral component of the mobile phase and an RP-18 column. It was found that the hydantoin obtained does not contain enantiomeric S(+)-mephenytoin and that the remaining substrate does not change its steric structure (Fig. 1).Chromatographic results als0 indicated that i?(-)-la was photolysed yielding S( +)-2a. Similar experiments for S( +)-and R(-)-5-aliyl-l-methyi-5-phenyibarbituric acids (lb) revealed that both enantiomers are photolysed at room temperature to appropriate, enantiomeric hydantoins. Preparative photolysis of S( +)lb yielded 5-allyl-3-methyl-

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Die Photolyse der Barbiturate (I) in Gegenwart von Basen liefert unter Decarbonylierung Hydantoine (II).