A facile synthesis of Tyrian purple based on a biosynthetic pathway

A facile synthesis of Tyrian purple, a valuable purple dye derived from gas tropod mollusks in ancient times, has been accomplished. Tyrian purple, 6,6 '-dibromoindigo, was easily obtained by three steps of reactions from the c ommercially available 6-bromoindole, on the basis of a biosynthetic pathway of indigo and biological precursors of the purple. Iodination of 6-bromoin dole, followed by acetoxylation with silver acetate in acetic acid, afforde d 6-bromo-3-acetoxyindole, whose alkaline hydrolysis accompanying air oxida tion produced Tyrian purple.