Combination of the SNH/aza-Diels–Alder Reactions as Effective Synthetic Approach to 8-Hydroxy(methoxy)-Substituted 2-[6-(1-Methylindol-3-yl)pyridin-2-yl]quinoline Ligands/Fluorophores

8-Hydroxy(methoxy)-substituted 2-[6-(1-methylindol-3-yl)pyridin-2-yl]quinoline ligands were synthesized by means of combination of SNH reaction between 8-methoxy-substituted 2-(6-aryl-1,2,4-triazin-3-yl)quinolones and indole and aza-Diels–Alder reaction of the obtained 1,2,4-triazines with 2,5-norbornadiene. The demethylation of quinoline moiety of 1,2,4-triazine precursor during aza-Diels–Alder reaction in autoclave was demonstrated.