Antiallergenic 8-azapurines. 3. Structural characterization of 2-(2-propoxyphenyl)-8-azahypoxanthine, 2-[2-propoxy-5-(propylsulfonyl)phenyl]-8-azahypoxanthine, and 2-[2-propoxy-5-(N-methyl-N-isopropylsulfamoyl)phenyl]-8-azahypoxanthine