Synthesis of isoxazoloazaborines via gold(i)-catalyzed propargyl aza-Claisen rearrangement/borylative cyclization cascade

Isoxazoloazaborines 3 and 5 have been synthesized from 4-N-propargylaminoisoxazole 1via gold(I)-catalyzed propargyl aza-Claisen rearrangement followed by electrophilic borylative cyclization in 27-86% yields. In situ generation of isoxazole 2 having an amino group and allenyl functionality is essential to give highly substituted isoxazoloazaborines 3 and 5, although the conventional propargyl aza-Claisen rearrangement readily affords the corresponding nitrogen-containing heterocycles, such as pyridines and pyrroles. The resulting isoxazoloazaborine 5a underwent the N-O bond insertion of zinc carbenoid to give oxazine-fused azaborine 6 in 48% yield.