5-Methyl-2-phenyl-2H-1,2,3-diazaarsole in reaction with ethyl diazoacetate

5-Methyl-2-phenyl-2H- 1,2,3-diazaarsole 1 reacts with ethyl diazoacetate to form a 1:1 bicyclic product 3a , 8-ethoxycarbonyl-4-methyl-2-phenyl-1-arsa-2,3,6,7-tetraazabicyclo[3.3.0]octa-3.7-diene. The latter isomerizes to a two-coordinate arsenic compound 5 , 3-ethoxycarbonyl-5-[α-(phenylhydrazono)ethyl] 1-1,2,4-diazaarsole. The X-ray crystal structure analyses of both products 3a and 5 have been carried out.