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The preparation and the X-ray structure analysis of ci.~,truns,cis,cis-lO-hydroxy-[4.5.5.5]fenestrane-lmethanol (8b) is reported. The measured bond angles and bond lengths of the central C(C), fragment are better reproduced by calculations with the AM1 than by the MNDO method.Introduction. -The tetracyclic fenestranes, where four rings share a central C-atom, are a unique class of hydrocarbons, because two opposite bond angles in the central C(C), substructure are consistently larger than 109" 28'. This type of deformation may be enhanced by ring contraction and by incorporation of trans-fused bicyclo[m.n.o]alkane subunits. Whereas computational analyses of the deformation space of C(C), fragments in fenestranes and related hydrocarbons are readily available [I], the investigation of their structure and their chemistry depends strongly on efficient syntheses of this class of compounds. Our recent success in the preparation of a variety of highly functionalized fenestranes provided crystalline compounds amenable to a detailed structural analysis. We report here the X-ray structure analysis of a [4.5.5.5]fenestrane containing a transfused bicyclo[3.3.0]octane subunit. The results are compared with data from AM1 calculations.