Organic solvent-free synthesis of Cy39 iodide

The iodide salt of the pyrylium dye Cy39 (Cy39I, 1) has been suggested as an alternative of its perchlorate salt, which is widely used for biolabeling applications. A new efficient organic solvent-free procedure for synthesis of 1, allowing for a greener approach to the cyanine dyes, is suggested. The unique combination of solubility and thermostability of both the starting materials and the target Cy39 salt made the performance of the synthesis in an aqueous solution as well as reusing of unreacted starting materials possible.