Dedicated to Professor Albert Eschenmoser (24.11.87) Structures of a,a' -bridged spiro [4.4]nonanes and [5.5.5.5]fenestranes, obtained from X-ray analysis and MNDO calculations, are investigated in terms of symmetry-deformation coordinates. The central C(C), fragments of these molecules show strong preference for compression, which is due to opening of opposite bond angles. The planoid deformations and strain energies of the fenestranes are discussed.It has been recognized that the configuration of the spiro centre in annulenes with a central C-atom and in unsaturated as well as saturated fenestranes show planoid distortions positions between the two rings will be discussed (Fig. I , I and 11)'). As before [4], the planarizing deformations in the central C(C), fragment of these tri-and tetracyclic compounds are related to the principal C, axis, which in these cases bisects the bond angles O,, and O,, (Fig. I ) . To assess these deformations, crystal structures as well as MNDO results of these spiro molecules are considered. The spiroalkanes of type I and I1

The configuration of the spiro centre in polycyclic compounds containing a spiro [4.4]nonane substructure is analyzed in terms of symmetry coordinates. As revealed by X-ray structures of such compounds, the local distortions around the quaternary C-atom are dominated by a decrease of the ring-bond angles at the spiro centre and a twist of one ring relative to the other. The planoid deformations given by the planarization index PC are comparatively small for spiro [4.4]nonanes, which contain either no additional bridges between the rings or only GI$'-or @'-polymethylene bridges. This indicates that the local strain in the spiro C(C), fragments is rather small.The theory of van't HOff[l] and Le Be1 [2] requires the four ligands of a saturated C-atom to have tetrahedral geometry [3]. A regular tetrahedral structure is not implied, and indeed, only few molecules contain a tetracoordinate C-atom with exact T or Td symmetry. Distortions from regular tetrahedral coordination have been investigated ever since u. Baeyer first discussed bond-angle deformations in small carbocyclic ring systems and correlated them with strain energy [4].A survey of polycyclic molecules [5] shows that bond-angle distortions at tertiary or quaternary C-atoms are often associated with local C,, symmetry as in cubane [6], dodecahedrane [7], and the propellanes; an extreme example of this kind is [ 1.1. llpropellane, where strain is related to two inverted C-atoms [8].Strained molecules containing quaternary C-atoms with local fourfold rotation-inversion (DJ or orthorhombic (D,) symmetry have been much less investigated. Examples are the spiro hydrocarbons and the fenestranes (which may be considered as bridged spiro compounds')). The deformations from local Td symmetry in the bond angles of these molecules indicate a tendency towards planar tetracoordinate C-atoms. In spiropentane the two ring-bond angles at the spiro centre decrease, whereas the other four angles increase. In the fenestranes on the other hand, the four ring angles decrease and the remaining two angles increase. To assess the type and extent of deformation, the angles are analyzed in terms of symmetry-deformation coordinates. On the basis of these coordinates, a simple measure of the degree of planarization of tetracoordinate C-atoms, i.e. of their planoid deformation, may be defined.') Paddlanes [9] and pyramidanes also belong to the class of strained hydrocarbons with local geometry of twofold and fourfold symmetry, respectively [5].

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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.

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