Preparation of intercalating dye thiazole orange and derivatives

An improved method for the preparation of asymmetric monomethine cyanines by the condensation of 6-substituted-2-iminobenzothiazolines and 1-alkyl-4-methylquinolinium salts is described. The preparation can be considered as being more environmentally friendly than the more usual method involving condensation of 2-methylthio-3-methylbenzothiazolium salts and 1-alkyl-4-methylquinolinium salts. The synthesis results in the important fluorogenic intercalating dye 1-methyl-4-[(3-methylbenzothiazoline-2-ylidene)methine]-quinolinium salts, or its derivatives with substituents at the 6-position in the benzothiazole nucleus. The starting materials are commercially available or can be easily synthesized. The procedure for the preparation of these biological non-covalent labels is simple and reliable.