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Predict the major product of the following reaction :

6-(Bromomethyl)-5-methyluracil treated with 1. NaN3, DMF, rt; 2. methyl propiolate with CuSO4.5H2O (2 mol%), sodium ascorbate (5 mol%), tBuOH/H2O

  • A.5-Methyluracil with a CH2 at C6 joined to N1 of a 1,2,3-triazole carrying CO2Me at C4
  • B.5-Methyluracil with a CH2 at C6 joined to N1 of a 1,2,3-triazole carrying CO2Me at C5, next to the N-linked nitrogen
  • C.5-Methyluracil with a CH2 at C6 joined to a partly saturated triazoline ring carrying CO2Me
  • D.5-Methyluracil with an exocyclic C=C at C6 joined to N1 of a 1,2,3-triazole carrying CO2Me at C4

Answer: A

The product is (A), the 1,4-disubstituted 1,2,3-triazole with the ester at C4.

Step 1 is an SN2 displacement: azide replaces the bromide, giving the uracil-CH2-N3 azide. Step 2 is the copper-catalysed azide-alkyne cycloaddition (CuAAC, 'click' reaction). CuSO4 with sodium ascorbate generates Cu(I), which forms a copper acetylide from methyl propiolate and directs the cycloaddition so that only the 1,4-regioisomer forms.

(B), the 1,5-isomer, is what ruthenium catalysts give, and a mixture of 1,4 and 1,5 is what the uncatalysed thermal reaction gives. (C) is a non-aromatic dihydrotriazole, and (D) wrongly introduces a C=C at the methylene linker; the triazole product is aromatic and the CH2 link is untouched.

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