The reaction of various isocyanates or thiophosgene with zwitterionic pyrimidinylium and 1,3-diazepinylium derivatives has led to the formation of unusually substituted [1,3]oxazolo[3,4-a]pyrimidines and novel heterocyclic system such as [1,3]oxazolo[3,4-a][1,3]diazepine. Oxazolopyrimidine or 1,3-oxazolo[1,4]diazepine moieties are essential in many pharmacologically relevant compounds. Derivatives of 1,3-oxazolo[3,4-a]pyrimidine, such as 2,4-diaza-4-deoxypodophyllotoxin, show significant activity against vincristine-resitant P-388 leukemia, 1 whereas 1,3-oxazolo[3,2-c]pyrimidine derivatives have anti-inflammatory properties. 2 Biological activity of compounds based on the 1,3-oxazolo[3,2-a]pyrimidine moiety is considered to be very significant. 3 Moreover, some derivatives of this system have technical applications 4 in the chemistry of polymers. Some 1,3-oxazolo[3,4-d][1,4]diazepine derivatives have antibacterial activity 5-7 whereas in commercially available drugs, such as mexazolam or oxazolam, the 1,3-oxazolo[3,2-d][1,4]diazepine moiety is present. 8 In this regard, we have now synthesized a series of pharmacologically interesting systems: 1,3-oxazolo[3,4-a]pyrimidines and novel 1,3-oxazolo[3,4-a][1,3]diazepines.At the center of our investigation lie 2-anilino-2-methoxy-3-oxothiobutanoic acid anilides 1, which are readily prepared 9 from 3-oxothiobutanoic acid anilides by the one pot, tandem condensation-addition reaction with nitrosobenzene. The multifunctional compounds 1 have a high synthetic potential.In a recent paper 10 we have described the heterocyclization (at C2) of 2-anilino-2-ethoxy-3-oxothiobutanoic acid anilides 1 with aliphatic 1,3-and 1,4-diamines leading to novel zwitterionic compounds 2 and 3, 11 respectively. Good leaving groups at C2 of compounds 1 facilitate cyclization followed by a novel rearrangement of 3-oxothiobutanoic acid anilides 1 to 2-hydroxythiopropanoic acid zwitterionic derivatives 2 and 3 (Scheme 1). We now wish to report the application of zwitterionic compounds 2 and 3 in the synthesis of saturated fused heterocyclic systems. Treatment of compound 2 or 3 with 2 equivalents of phenyl isocyanate in toluene (100 °C) for 15 minutes furnished a mixture of compounds 4 or 5, respectively, 12 with an equivalent amount of N,N¢-diphenylurea (Scheme 1). Scheme 1 + + toluene / 75 °C toluene / 75 °C 1a Ar = C 6 H 5 1b Ar = C 6 H 4 Cl-p 2 a, b 3 a, b 4 a, b 5 a, b R = Ph, naphthyl, p-chlorophenyl, cyclohexyl