This review describes the synthesis and reactions of pyridine N-oxides within the last ten years. The first part surveys the different synthetic methods which include ring transformation, classical oxidations using peracids, the use of metalloorganic oxidizing agents and cycloaddition reactions. The second part surveys the reactions of pyridine N-oxides including the deoxygenation, nucleophilic reaction and cycloaddition to N-O bond.

Bicyclic and tricyclic fused pyrimidine derivatives have received much attention in connection with biologically significant systems such as purines, 1-7 pteridines 8-14 and alloxazines. The synthesis of the above categories has been accomplished by the cyclisation of 6-aminouracil derivatives and ring transformation of other fused pyrimidine-2,4diones. 15,16 In extending our recent work on simple bicyclic xanthines, 6,7 tricyclic alloxazines 17,18 and pyridodi pyrimidines 19 this paper reports a novel synthesis of 3-benzyl-9-methylxanthine (5), 3,9-dimethyl-8-phenylxanthine (7), 7 dipyrimidopyridines 10a-f and pyrimido [4,5-d]pyrimidines 11a-d via Mannich reactions.The synthesis of 3,9-disubstituted xanthines 5,7 has been performed by various approaches [20][21][22] and we report a more favoured sequence from the 1-substituted-6-chlorouracils 1a, b 23,24 (Scheme 1) by reaction with methylamine and N-methyl benzylamine respectively where the 6-amino compounds 2 and 6 were obtained in a good yield. Nitrosation of 2 afforded the 5-nitroso compound 3, which underwent reductive formylation to 4 which readily undergoes intramolecular cyclization with formamide to furnish the xanthine 5 or refluxing the tertiary amine 6 with sodium nitrite in the presence of acetic acid resulted in ring closure in one step giving the 3,9-dimethyl-8-phenylxanthine (7), probably via a 5-nitroso compound 6a and its tautomer 6b. All the products were assigned by 1 H NMR, UV spectra and microanalysis.It was found that the intramolecular cyclisation of 6-(Nalkylanilino)uracils with dimethylformamide (DMF)-POCl 3 25

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The reactions of enaminones (6-aminouracils) 1a±c and 4 with cyano olefins 2a±b and 5a±f led to the formation of pyrido[2,3-d ]pyrimidines 3a±f and 6a±f in good yield, while the treatment of 4-amino-2-thiouracil 4 with aromatic aldehydes afforded pyridodipyrimidines 7a±c.

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Purine derivatives R 0540Synthesis of 3-Benzylxanthine and Lumazine Analogues. -Xanthine and lumazine derivatives exhibit a wide range of biological activities and they are potential anti-HIV drugs. -(EL ASHRY, E. S. H.; YOUSSIF*, S.; EL AHWANY, M.; EL SANAN, M.; J. Chem. Res. 2005, 4, 262-266; Dep. Chem., Fac. Sci., Zagazig Univ., Zagazig, Egypt; Eng.) -K. Woydowski 45-180

Several fused imidazolopyrimidines were synthesized starting from 6-amino-1-methyl-2-thiouracil (1) followed by nitrosation, reduction and condensation with different aromatic aldehydes to give Schiff's base. The dehydrocyclization of Schiff's bases using iodine/DMF gave Compounds 5a-g. The methylation of 5a-g using a simple alkylating agent as dimethyl sulfate ((CH3)2SO4) gave either monoalkylated imidazolopyrimidine 6a-g at room temperature or dialkylated derivatives 7a-g on heating 6a-g with ((CH3)2SO4). On the other hand, treatment of 1 with different aromatic aldehydes in absolute ethanol in the presence of conc. hydrochloric acid at room temperature and/or reflux with acetic acid afforded bis-5,5-diuracylmethylene 8a-e, which cyclized on heating with a mixture of acetic acid/HCl (1:1) to give 9a-e. Compounds 9a-e can be obtained directly by refluxing of Compound 1 with a mixture of acetic acid/HCl. The synthesized new compounds were screened for antimicrobial activity, and the MIC was measured.

Several xanthines (7–13) are prepared by the cyclisation of 1-benzyl-5,6-diaminouracil with single-carbon inserting agents such as aromatic aldehydes, formamides, acetic anhydride, carbon disulfide, and nitrous acid. Treatment of 6-amino-1-benzyl-5-nitrosouracil with anilinobenzylidene derivatives (14–18) affords 7-hydroxyxanthines (19–23). Cyclisation of the diaminouracil 3 with glyoxal, benzil, and diethyl oxalate leads to lumazines (25–28).