1- uracil 5-nitro-, preparation

Later, a completely different and more convenient synthesis of riboflavin and analogues was developed (34). It consists of the nitrosative cyclization of 6-(A/-D-ribityl-3,4-xyhdino)uracil (18), obtained from the condensation of A/-D-ribityl-3,4-xyhdine (11) and 6-chlorouracil (19), with excess sodium nitrite in acetic acid, or the cyclization of (18) with potassium nitrate in acetic in the presence of sulfuric acid, to give riboflavin-5-oxide (20) in high yield. Reduction with sodium dithionite gives (1). In another synthesis, 5-nitro-6-(A/-D-ribityl-3,4-xyhdino) uracil (21), prepared in situ from the condensation of 6-chloro-5-nitrouracil (22) with A/-D-ribityl-3,4-xyhdine (11), was hydrogenated over palladium on charcoal in acetic acid. The filtrate included 5-amino-6-(A/-D-ribityl-3,4-xyhdino)uracil (23) and was maintained at room temperature to precipitate (1) by autoxidation (35). These two pathways are suitable for the preparation of riboflavin analogues possessing several substituents (Fig. 4). 

It may be said in conclusion that the reactivity of position 5 (i.e., 6 of the triazine ring) is similar to that of uracil. The only difference seems to be in the failure to prepare 5-nitro-6-azauracil although this reaction proceeds readily with uracil. 

Uracil derivatives can be nitrated at C-5 under conditions that allow retention of a sugar residue at N-1 Nitration at N-3 can also be achieved N-3-nitro-compounds react with amines via an ANRORC mechanism, with displacement of nitramide and incorporation of the amine as a substituted N-3, as shown below. This seqnence has been ntilised to prepare N-3-labelled pyrimidines (31.2.2). An analogous ANRORC process takes l-(2,4-dinitrophenyl)-uracils to 1-arylamino-uracils by reaction with arylamines at room temperatnre. ... 

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