The structure of 5-chloro and 5-alkoxy-5,7-dihydro-3//-purine-2,4,6-trio- nes (6 and 7-9) has been corroborated by their spectral properties and X-ray crystallography. The stereochemical model (fì)-10 of the key uricolytic intermediate was prepared using menthol as chiral auxiliary. In acidic solutions, depending on the N-substitution, the ring cleavage occurred either at the 4,9-bond 6b -» 13 (R1 = Me) or at the 3,4-bond 6c -» 14 (R1 = H).
Opening of the 1,6-bond is the dominant process under alkaline conditions. Decarboxylative rearrangement into 1,3-dimethylallantoin (15) was specific for N(7)-unsubstituted derivatives. Evidence for intermediacy of the bicyclol tautomeric form was supplied by isolation of l-menthoxy-2,4-dimethyl-3,7-di- oxo-2,4,6,8-tetraazabicyclo[3.3.0]octane (16); fragmentation of 10 into 5- menthoxy-imidazolidin-2,4-dione (17) also takes place under forcing conditions. Conversely, no allantoin rearrangement was encountered in the R7*H cases; 7d underwent the pyrimidine ring fragmentation to give 5-methoxy-l- methyl-4-methylimino-imidazolidine-2-one (18) or 5-carboxamido-5-methoxy- l-methyl-4-methylimino-imidazolidine-2-one (19). A possible mechanism for these ring transformation reactions is discussed
