4.6- Dimethoxy-5-nitropyrimidine

In the case of 2-methoxy-5-nitropyrimidine, the adduct that forms predominantly in DMSO solution has been shown to have the structure of 16, as characterized by two doublets of equal intensities in the NMR spectrum.14,41 The presence of a weak singlet has been indicated as evidence for the formation of minor amounts of the gem-dimethoxy isomer,41 although no further confirmation is available. Thus the preferred attack occurs at a position adjacent to the nitro group, located para to an aza group and other than the methoxyl-bearing position. 

Methoxy-5-nitropyrimidine is also attacked at a CH position, as indicated by the relatively high upheld shift of one of the ring protons.14,41 The formation of the gem-dimethoxy adduct is thus ruled out. Of the two possible isomeric adducts only the one resulting from attack at the 2-position (17) is formed, as unequivocally shown by the singlet at <5 5.75, which is observed starting from 4-methoxy-5-nitropyrimidine-6-rf.41... 

Nitropyrimidine, its 2- and 4-methoxy, and 2,4- and 4,6-dimethoxy derivatives react with acetone in the presence of potassium hydroxide to yield the potassium salts of the anionic adducts 74 and 75, with structures elucidated by spectral ( H-NMR, IR, and UV-visible) methods. The nucleophilic attachment was found to occur only at CH positions, and when there was a choice between 2- and 4(6)-positions, the latter was preferred.125,126 An adduct of the kind corresponding to structure 75 was also obtained by using the conjugate base of acetophenone. The adducts can be converted to the corresponding CH3COCH2- or PhCOCH2-substituted pyrimidines by oxidation, either directly or via the related dihydropyrimidine derivatives.127... 

Dimethoxy-7-a-D-ribofuranosylpyrrolo[3,2-rf]pyrimidine 343 was synthesized through ionic C—C bond formation by coupling the carb-anion of the active methylene group of 6-cyanomethyl-2,4-dimethoxy-5-nitropyrimidine and the D-ribofuranosyl chloride 340. The resulting 341 was cyclized to 342 by catalytic reduction and then deprotected to 343 (86JOC1058) (Scheme 101). 

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