X-ray diffraction study of DL-1,4-dimethyl-2,5-dioxabicyclo[2.2.1]heptane-3,6-dione: the sense of twist and folding

The title dilactone exists in a crystalline state (110 K) as enantiomeric synchro-(+,+)-(R,R)- and synchro-(–,–)-(S,S)-twist forms (τ₀ = 1.8°) with the retention of C₂ symmetry, linked by infinite zig-zag heterochiral chains (space group C2/c) by mutual H-bonds of the CH₂–H&z.dotdot;O=C type.