1,5,7,11-Tetraoxaspiro[5.5]undecane, 3,9-Dimethylidene-1,5,7,11-tetraoxaspiro[5.5]undecane and 1,5-Dioxa-7,11-dithiaspiro[5.5]undecane

In all three title compounds, C₇H₁₂O₄, (1), C₉H₁₂O₄, (2), and C₇H₁₂O₂S₂, (3), the spiro C-O distances are substantially shorter than the non-spiro C-O distances as a result of the anomeric effect. In contrast, the spiro C-S distances of compound (3) are similar to the non-spiro C-S distances. In all three compounds, steric repulsions between the lone pairs on the O or S atoms and the axial H atoms of the opposite ring result in a distorted geometry around the spiro-C atoms.

-1