1,3-Dipolar cycloaddition is a convenient method for construction of various heterocyclic systems. We applied this method for the synthesis 5-hetaryluracil derivatives where substituted uracils played the role of 1,3-dipoles or dipolarophiles. Treatment of the nitrile oxide derived from 5-formyluracil and substituted alkenes gave the appropriate 5-(4,5-dihydroisoxazol-3-yl)pyrimidine-2,4(1H,3H)-diones, which by oxidation with Nbromosuccinimide were transformed into appropriate 5-(isoxazol-3-yl)uracils. When 5-cyanouracil was used as a dipolarophile in the reaction with nitrile oxides, generated from aromatic aldoximes, several 5-(1,2,4-oxadiazol-5-yl)uracils were obtained. An alternative reaction of 5-formyluracil with an excess of nitriles in the presence of cerium ammonium nitrate as an oxidant gave 1,2,4-oxadiazol-3-yl derivatives in moderate yields.
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