Synthesis and crystal structures of 5,17-dibromo-26,28-dihydroxy-25,27-dipropynyloxycalix[4]arene, 5,17-dibromo-26,28-dipropoxy-25,27-dipropynyloxycalix[4]arene and 25,27-bis(2-azidoethoxy)-5,17-dibromo-26,28-dihydroxycalix[4]arene
Acta crystallographica. Section E, Crystallographic communications