Two new heterocyclic [3.3.3.0^1,5]-propellanoid compounds: 2,8-dioxatricyclo[3.3.3.0^1,5]undecane-3,7-dione and a related dimer

2,8-Dioxatricyclo[3.3.3.0^1.5]undecane-3,7-dione, C₉H₁₀O₄. (I), is the dilactone acylal of cyclopentanone-2,2-diacetic acid. Both molecules in the asymmetric unit have conformational chirality and differ principally in the flexing of the carbon ring, which produces a resolvable conformational disorder in one of the molecules. Three intermolecular C-H ⋯ O close contacts exist. 7,7′-Oxybis(2,8-dioxatricyclo[3.3.3.0^1,5]undecan-3-one), C₁₈H₂₂O₇, (II), a racemate, lies on a C₂ axis and is a non-meso furanosyl furanoside dimer derived from the monoacid monoaldehyde corresponding to (I). One intermolecular C - H ⋯ O close contact exists. Diminished intramolecular void space in these small propellanoids generates unusually high crystal density in both species, particularly (I).