Stereo- and Regioselective [3+2] Cycloaddition of Acetoxy Allenoates with Azides: Metal-Free Synthesis of Multisubstituted Triazoles

We have developed a regio- and stereoselective thermal [3+2]-cycloaddition protocol involving acetoxy allenoates as 1,2-dipoles under metal-free conditions for the synthesis of 1,4,5-tri/1,5-disubstituted 1,2,3-triazoles. delta-Acetoxy allenoates act as alpha- and beta-carbon donors and lead to trisubstituted 1,2,3-triazoles with an alkenyl functionality at the 5-position. In sharp contrast to this, beta- and gamma-carbons participate in the case of beta'-acetoxy allenoates to afford 1,5-disubstituted triazole cores. This [3+2] cycloaddition shows a broad substrate scope concerning acetoxy allenoate as well as azide and offers essentially E-stereoisomers in good to high yields. Divergently, the reaction of delta-acetoxy allenoate with trimethylsilyl azide gives an acyclic, nitrogen-inserted product with the cleavage of C-C bonds.