The angle strain induced by ring annulation on a spiro-type naphthofuran weakens its C(sp3)-O bond at the opposite peri-position and endows expandability, so that quite different bond lengths [1.493(3)-1.526(6) Å] are determined for the elongated C(sp3)-O bond of the corresponding spiro-acenaphthofuran derivative upon X-ray analyses of its pseudopolymorphs.