IntroductionOne of the most fascinating synthetic problems in the chemistry of strained polyspirocyclopropane systems (triangulanes) is the preparation of cyclo[6]triangulane1[so­called davidane (1)] – the first potentially stable member of a family of still elusive cyclic triangulanes. Exploring the possible synthetic approaches to this compound we have prepared several symmetrical 1,3,5,7­tetrasubstituted tricyclo[5.1.0.03,5]octane­2,6­diones (2–5) and determined their crystal structures. Most of the few previously known 2,6­dioxotricyclo[5.1.0.03,5]octanes are 4,8­disubstituted derivatives2and no crystal structures of symmetrically substituted compounds have been described. Compounds2and3possess ananti­configuration of the two cyclopropane rings in the tricyclooctane skeleton; compounds4and5aresyn­isomers. Crystals of4have been obtained as a benzene solvate.The gradual change of substituents and configuration in this series of compounds provides an opportunity to analyse the influence of these alterations upon the packing of the molecules within the crystals. The molecules3–5, which do not have groups, form strong hydrogen bonds which are particularly interesting. Here, the weak intermolecular interactions should determine the packing of the molecules, and in structures3–5a number of possible interactions (C–H⋯O, Cl⋯Cl, C–H⋯Cl,etc.) may be formed. The role of different types of such interactions in structural chemistry is now well established and various aspects of the chemistry of weak hydrogen bonds have been summarised recently by Desiraju and Steiner.3In this communicationwe report the results of single­crystal X­ray structural analyses of2–5and discuss the packing of these molecules within the crystals.