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A qualitative hardness scale (N 1 <N 3 <O 4 ) has been found for the conjugated bases of 2-methoxy-4(3H)-pyrimidinones 1-3 and applied to high yielding chemoselective N 3 methylation, ethylation and benzylation reactions. Removal of the 2-methoxy group followed by a second alkylation affords unsymmetrically 1,3-disubstituted uracils.
The lithium salts of the conjugated bases of 4-methoxy- and 4-acetylamino-2(1H)-pyrimidinones 1–3 undergo highly chemoselective N 1 -methylation or ethylation when treated with methyl- or ethylsulfate (hard electrophiles) in dry dioxane, while the use of DMF as solvent results in competitive O 2 -alkylation. Potassium salts of the same bases in DMF undergo prevalent O 2 -attack...
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