“…The cyclization of the enamines 16 is realized by the action of heat in high-boiling solvents such as biphenyl [44], biphenyl ether [1, 26,32,34,36,37,41,42,[45][46][47], Dowtherm A [35,38,39,48,49], and mineral oil [1] or by using effective cyclization agents such as polyphosphoric acid or its esters [1, 2], the halogen derivatives of phosphorus [1], ZnCl 2 [1], conc. sulfuric acid in mixture with acetic anhydride [1], and Eaton's reagent (a mixture of phosphorus(V) oxide and methanesulfonic acid) [37,40].…”