Syntheses and structural properties of distellenes

The syntheses of a mixture of the racemic (a) and the meso form (b) of several derivatives of 2,2'-bi(tricyclo[3.3.0.0(3,7)]octylidene) are described: The 6,6'-diisopropylidene- (4), 6,6'-dimethylidene- (5), 6,6'-dione- (6), syn-4,4'-dione- (7), anti-4,4'-dione- (8), syn-4-methylidene-4'-one- (9), anti-4-methylidene-4'-one- (10), syn-4,4'-dimethylidene- (11), and anti-4,4'-dimethylidene- (12) derivatives. Starting materials for all compounds were tricyclo[3.3.0.0(3,7)]octane-2,4-diol (19) and its 2,6-isomer 20. Key reaction was the McMurry coupling of 6-isopropylidenetricyclo[3.3.0.0(3,7)]octa (23) or 6-methylidenetricyclo[3.3.0.0(3,7)]octan-2-one (24) to yield 4 or 5, respectively. The McMurry coupling of the ketone 26 yielded 7 and 8. From the latter, species 9-12 could be generated via the Wittig reaction. In most cases only the racemates could be isolated. X-ray investigations on 4a revealed a separation of the isopropylidene groups by 10.3 Angstrom. The central C-C bridging bonds of the stellane units in 4a, 7a, 8a, 12a, and 30 C(1)-C(5), C(3)-C(7), C(1)-C(5'), C(3')-C(7') were found to be between 1.596(2) and 1.616(3) Angstrom. In 4a we found a twisting between the terminal isopropylidene groups by a torsion ((CH3)(2)C=C ... C=C(CH3)(2)) of 74 degrees and 83 degrees, respectively, in both independent molecules. The twist angle between the CO groups in 7a amounts to 53 degrees, and in 8a to 118 degrees. The same twist angle was found between the exo-methylidene groups in 12a.