Extended tandem reactions of 2H-1,2,3-triazole N-oxides with dialkyl acetylenedicarboxylates and N-phenylmaleimide: substituted monocyclic 2,5-dihydro-1,2,3-triazines and new tetrahydrofuro[2,3-d]-1,2,3-triazoles. Azolium 1,3-dipoles part 5