Synthesis and structure of methanobenzocyclooctene derivatives

Abstract

10-Oxo-5,9-methanobenzocyclooctene-8-carboxylic acid 4a was prepared by the intramolecular cyclization of 4t-phenylcyclohexane-1r,2c-dicarboxylic acid 1a in concentrated H2SO4 or in the reaction of 4t-phenylcyclohexane-1r,2c-dicarboxylic anhydride 2 in 80% H2SO4. To improve the yield, the esters 3a,b were cyclized to the methanocyclooctene isomers 5a,b, in a 1:5 ratio from 3a, and in a 5:4 mixture (54%) from 3b at elevated temperature. After separation, 5a was hydrolysed, the keto group of 4a was reduced by the Wolff-Kishner method and the resulting cis and trans methylene-bridged benzocyclooctenes 6a,b (1:2) were separated. From 4a with hydrazine, the tetracyclic pyridazinone derivative 7 was obtained. The structures were determined by 1H and 13C NMR methods and for 4a also by X-ray crystallography. © Acta Chemica Scandinavica 1998.