Photochromism of 2H-Naphtho[1,2-b]pyran-4-carboxylates

New 2H-naphthopyrans substituted at C-4 by a carboxylate group were synthesized through the one-pot acid-catalyzed reaction of naphthols with ethyl 4,4-diaryl-4-hydroxybut-2-ynoate which was easily obtained from ethyl propiolate. These colorless compounds show photochromic properties under UV irradiation. The carboxylate group exerts a strong effect on the thermal fading speed of the colored species leading to a fast switch between the colored and colorless states, which, however, can be appropriately counterbalanced leading to a noticeable color change at room temperature. The mechanism of this reversible transformation was studied by NMR.