5-Acetylspiro [benzofuran-2 (3H), 1'-cyclopropan]-3-one (3a, AG-629), previously found to be the most potent antiulcer compound in a series of spirocyclopropanes, was obtained by onestep synthesis starting from methyl 5-acetyl-2-[(tetrahydro-2-oxo-3-furanyl) oxy] benzoate (1a). In this reaction, 5, 10'-(diacetyl-5', 6'-dihydrospiro [benzofuran-2 (3H), 4'(3'H)-[2H] oxocino [3, 2-b]-benzofuran]-3-one (5a), which has a new ring system, and 1H, 3H-8-acetyl-4, 5-dihydronaphtho [1, 2-c : 4, 3-b'] difuran-1-one (6) were isolated as by-products. A possible reaction mechanism is presented.