Dehydrocyclization transannular

Transannular dehydrocyclization of methylcycloalkanes may also be regulated by conformational or other characteristics of the hydrocarbon. In the case of methylcyclononane, the predominant formation of cis-8-methyl-hydrindane with its methyl group in the angular position and of products derived from it (769) suggests that the transannular dehydrocyclization proceeds chiefly with elimination of the intraannular hydrogen atom located on the tertiary carbon atom, and therefore with the methyl group in the extraannular position (Scheme 85). But other bicyclic products are also obtained in appreciable amounts, so that it is not clear whether the direction of dehydrocyclization is determined by the presence of a hydrogen atom at a tertiary carbon atom or by some conformational peculiarities of the molecule. 

Skeletal rearrangements of cycloalkanes containing 9-18 carbon atoms were observed for the first time by Prelog et al. (162) on Pd/C catalysts at 400°C. Under these conditions, polycyclic aromatic and pseudoaromatic hydrocarbons are obtained (indene, azulene, naphthalene, phenanthrene, etc.). By carrying out the reaction on Pt/C under less drastic conditions, Kazanskii et al. (163) could observe the precursors of the aromatics as primary products. The latter are bicycloalkanes resulting from transannular 1-5 or 1-6 dehydrocyclizations (Scheme 84). For instance, cyclooctane yields... 

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