SYNTHESIS AND REACTIONS OF MESO-D3-TRISHOMOCUBYLIDENE-D3-TRISHOMOCUBANE AND DL-D3-TRISHOMOCUBYLIDENE-D3-TRISHOMOCUBANE

Titanium-promoted reductive coupling of D3-trishomocubanone affords meso- and dl-D3-trishomocubylidene- D3-trishomocubane (2a and 2b, 60%) and D3-trishomocubanol (3, 25%). The structures of 2a and 2b were elucidated by X-ray crystallographic methods. Reaction of a ca. 1:1 mixture of 2a and 2b with a chloroform solution of trifluoroacetic acid at room temperature for 2.5 h afforded the corresponding 1,2-adduct (4b, 39%) along with isomerically pure, unreacted 2a (46%). The corresponding reaction of 2a, when performed at reflux for 48 h, afforded an extensively rearranged spirocyclic trifluoroacetate adduct, 4a (30%), along with recovered 2a (40%). The structure of 4a was established via X-ray crystallographic analysis of the corresponding alcohol, 5, that resulted via lithium aluminum hydride reduction of 4a. Electrophilic bromination of 2b with Br2-CCl4solution proceeded smoothly at room temperature to produce the corresponding 1,2-adduct, 9 (80%), whose structure was established unambiguously by X-ray crystallography. By way of contrast, the corresponding reaction of 2a, when performed at room temperature for 24 h, gave a spiroketone, 10a (70%). On the basis of results of MM2 and AMI computations, the observed differences between the reactivities of the C=C double bonds in 2a and 2b toward electrophiles can be rationalized in terms of a secondary steric effect that is a consequence of the molecular symmetries of 2a and 2b. Thus, as the degree of pyramidalization of the sp2-hybridized carbon atoms increases with reaction progress, the resulting internal congestion in the region behind the C=C double bond becomes unbearably large in 2a but remains relatively small in 2b. © 1990, American Chemical Society. All rights reserved.