Procedure for Synthesis of 4,5,6,7 tetrachloro-3 ', 6'-dihydroxy-2', 4 ', 5', 7'-tetraiodo-3H-spiro [isobenzofuran-1,9'-xanthene] -3-one (Rose Bengal) and related xanthenes

A process for the production of a compound of formula 4a: Formula ** ** wherein R1 is independently Cl or Br; R11 and R12 are independently H, Na, K, Li, or a counterion capable of forming a pharmaceutically acceptable salt, which comprises combining a compound of Formula 3a, or an isomer thereof wherein the quinoid compound of Formula 3a, or isomer quinoid thereof, are substantially free of impurities chloride ion and chloride ion impurity species ** ** Formula wherein R1 is independently Cl or Br; R2, R3, R4 and R5 are H; which method it is performed with at least about 4 equivalents of iodine in a substantially free solution of chloride ions and substantially free of impurity species of chloride ions to form the compound of formula 4a, wherein there is substantially no derivatization transhalogenados of compound of Formula 4b, where for transhalogenadas impurities the compound of Formula 4b, R1 is independently Cl or Br; at least one of R2, R3, R4 and R5 is Cl and the rest is I; R11 and R12 are independently H, Na, K, Li, or a counterion capable of forming a pharmaceutically acceptable salt; **Formula**