1,3-Dipolar cycloaddition of benzyl azide to two highly functionalized alkynes

Benzyl azide undergoes [3 + 2] cycloaddition when reacted with 3,3,4,4-tetraethoxybut-1-yne, a ketal, and the corresponding ketone, 1,1-diethoxybut-3-yn-2-one, in the presence of a Cu(I) salt in various solvents. The outcome is sensitive to the structure of the alkyne and the nature of the metal salt. Both alkynes give the corresponding 1,4-disubstituted 1,2,3-triazoles in up to 70% yield, but the ketone also affords a minor amount of the 1,5-disubstituted analogue. When CuI is used as catalyst, the ketal in addition furnishes some 1-benzyl-5-iodo-4-(1,1,2,2-tetraethoxyethyl)-1,2,3-triazole. On the other hand, when the reaction was carried out under Ru(II) catalysis, no 1,2,3-triazole formation was observed for any of the substrates; the ketal did not react at all, whereas the ketone underwent cyclotrimerization and gave 1,3,5-tris(2,2-diethoxyacetyl)benzene in 60% yield. No reaction occurred when the magnesium acetylide of 3,3,4,4-tetraethoxybut-1-yne was reacted with benzyl azide.