The efficient synthesis of all-cis-[5.5.5.5]fenestrane (2) from the readily available intermediate 3 allowed the electron-diffraction analysis of 2. This structure analysis revealed long C-C bonds in the central C(C), fragment and a twist-envelope conformation for the four cyclopentane substructures. The four bridgehead H-atoms are in a synclinal rather than an ecliptic position with an approximate D, symmetry of 2. Planarizing distortions are evident from the opposite bond angles at the central C-atom being 116.2 0.5" with the remaining four being 103.7 i 0.2".

The conditions for the novel tandem Pauson‐Khand reaction have been explored. The highly functionalized tetracyclic compounds 11c, 11d, and 16 were prepared from the ene‐diynes 4c, 4d, and 10 by treatment with 2 equiv. of [Co2(CO)8] and 4‐methylmorpholine N‐oxide (NMO) or Me3NO in yields of 24, 22, and 53%, respectively (Table). In the presence of 1–3 equiv. of H2O added to the NMO used for induction of the Pauson‐Khand reaction of 6d, a mixture of cyclopentanones 17/18 and cyclopentenones 12/13 was obtained (Scheme 5). The first example of a [Co2(CO)6]‐induced highly stereoselective ene reaction is described. To account for these results, the formation of intermediates are proposed (Schemes 6 and 7) which hitherto have not been considered in the mechanistic description of the Pauson‐Khand reaction.

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Dedicated to Hans-Dieter Scharfon the occasion of his 65th birthday (28.X11.95)The conditions for the novel tandem Pauson-Khand reaction have been explored. The highly functionalized tetracyclic compounds l l c , l l d , and 16 were prepared from the ene-diynes 4c, 4d, and 10 by treatment with 2 equiv. of [Co,(CO),] and 4-methylmorpholine N-oxide (NMO) or Me3N0 in yields of 24, 22, and 53%, respectively (Table). In the presence of 1-3 equiv. of H,O added to the NMO used for induction of the Pauson-Khandreaction of 6d, a mixture of cyclopentanones 17/18 and cyclopentenones 12/13 was obtained (Scheme 5). The first example of a [Co,(CO),]-induced highly stereoselective ene reaction is described. To account for these results, the formation of intermediates are proposed (Schemes 6 and 7) which hitherto have not been considered in the mechanistic description of the Pauson-Khand reaction. ). Usually, this transformation is initiated by heating in solution or oxidation using 4-methylmorpholine N-oxide (NMO), trimethylamine oxide (Me,NO) or DMSO. In addition, special methods were described [5], e.g. the use of solid adsorbing materials [6] or catalysts [7]. Intramolecular cyclization-carbonylations, by which hept-1-en-6-ynes and oct-1 -en-7-ynes were transformed into bicyclo[3.3.0]oct-1-en-3-ones and bicyclo[4.3.0]non-6-en-8-ones [8], could also be achieved under these conditions. The choice of the method for activating the cyclization reaction depends on the substrate. Thus, the tetracyclic [5.5.5.5]fenestrane 2 was only formed by treatment of 1 with a trialkylamine N-oxide like NMO or Me,NO [9] (Scheme I, a).A very attractive approach to fenestranes is given by a tandem Pauson-Khandreaction of easily accessable ene-diynes. Thus, we had observed that the open-chain ene-diyne 4c reacted with 2 equiv. of [Co,(CO),] in the presence of NMO via a tandem reaction directly