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, C 7 H 12 O 4 , (1), C 9 H 12 O 4 , (2), and C 7 H 12 O 2 S 2 , (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.