A solution of the corresponding epoxyacene and 3,6bis(pyridin-2-yl)-1,2,4,5-tetrazine (BPTZ) in CH2Cl2, is stirred at 45 o C until complete consumption of the starting epoxiacene (TLC monitoring), generating in situ the corresponding isobenzofuran (15-17). To this solution, kept at room temperature in a Schlenk flask, a solution of the aryne precursor (2 or 19, 1 equiv) in CH3CN was added, followed by finely powdered, anhydrous CsF (5-10 equiv; addition of the solid under a positive flow of argon). The mixture is stirred at room temperature for 14 h, then the solvent is evaporated under reduced pressure and the desired endoxide is isolated either by quick chromatography in silica gel (for 21, 23a, 23b and 23c) or by centrifugation and repeated washing with different solvents (for 24, 25a and 25b).