We report in this paper the syntheses and the results of an investigation of the structure (delocalized homoaromatic or localized Cope system) of 1,5-dimethyl-2,4,6,8-semibullvalenetetracarboxylic dianhydride 1 and the 5-ethyl analog 2. Although many homoaromatic cationic species are known, there is to date no well-accepted example of an electrically neutral homoaromatic molecule. 1 The semibullvalenes have long been recognized as the system most closely approaching neutral homoaromaticity. 1,2 The Cope rearrangements of semibullvalenes (e.g. 1a a 1b) proceed through the homoaromatic transition state (e.g. 1c) often with very low activation barriers. 1,3 A number of attempts to design semibullvalenes having homoaromatic ground states have been considered. 1 One suggestion, by Paquette, is that the bisannelated semibullvalene (3) might be such a system since the "breathing motion" of the Cope process would be inhibited. 4 Semiempirical 5,6 and ab initio 7 quantum mechanical calculations on other bisannelated semibullvalenes support this hypothesis. Müllen et al. subsequently prepared the first example 8 (4) of a bisannelated semibullvalene and demonstrated that its Cope rearrangement is rapid on the NMR time scale even at -160°C. 10 Similarly, our theoretical studies predict the bisanhydride 1 to have a single minimum potential energy surface with a homoaromatic ground state. 6 The known 11 tetraester 5 was viewed as an ideal precursor to bisanhydride 1. Grohmann's route 11 to 5 (Scheme 1A) was modified to give this tetraester more easily and in improved yield (Scheme 1B). The tetraester 5 was converted to the desired anhydride 1 Via the tetraacid 7 (Scheme 1B).1 proved to be remarkably stable; it can be purified by sublimation (200°C at ambient pressure) and only undergoes slight hydrolysis of one of the anhydride moieties upon prolonged exposure to the atmosphere. This is in contrast with other semibullvalenes, which frequently yield cyclooctatetraene upon mild thermolysis 12 and react more or less readily with oxygen. 10,13 The anhydride 1 crystallizes in space group Cc or C2/c, which are crystallographically indistinguishable. In the latter, the molecule uses a crystallographic 2-fold axis, while in the former no symmetry is imposed on the molecule. Data were all taken on the same crystal with Mo KR radiation to 2θ max of 70°at 293, 243, 163, 148, and 123 K. For the three higher temperatures the refinement of coordinates was straightforward in C2/c (Figure 1a) but not in Cc where the ratio of shifts with respect to error remained high, even though the R-factor was somewhat lower in Cc than in C2/c (0.045 and 0.055, respectively). The difference in R-factors is not significant, because the number of parameters in Cc is twice that in C2/c. An analysis of the thermal ellipsoids in C2/c at room temperature ( Figure 1a) did not agree with rigid body motion. The weighted agreement between observed and calculated components of the thermal ellipsoids is 0.22, while for a rigid motion this is expected to be less...