Two new heterocyclic [3.3.3.01,5]‐propellanoid compounds: 2,8‐dioxatricyclo[3.3.3.01,5]undecane‐3,7‐dione and a related dimer

2,8‐Dioxatri­cyclo­3.3.3.0^1,5]­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‐dioxatri­cyclo­3.3.3.0^1,5]­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).