Bond Fixation in Annulenes. 16. Synthesis of the 1,2-Dimethyl-3-phenylcyclooctatetraene ->/<- 1,8-Dimethyl-2-phenylcyclooctatetraene Bond Shift Isomer Pair. Probe of the Relative Size of a Flanking Phenyl Substituent by means of Racemization Kinetics

Article abstract of DOI:10.1021/ja00364a031

Two synthetic approaches to the cyclooctatetrene bond shift isomers 3 ->/<- 4 were developed.The first began with a phenyl-substituted starting material (phenylsuccinic acid) and proceeded to introduce the two methyl groups by desulfonylative ring contraction of a bicyclic α,α'-disubstituted sulfone.The second had its origins in citraconic anhydride.The two carbonyl groups were differentiated by regiospecific conversion to bromo lactone 20.Following stepweise introduction of the phenyl and second methyl substituents, a titanium(0)-promoted coupling was utilized to prepare 26.Both pathways were dependent upon regioselective bromination-debromination of a bicyclo<4.2.0>octadiene to generate the cyclooctatetraene nucleus.Although 3 dominates over 4 in the equilibrium, it is possible to separate these bond shift isomers by cycloaddition with a triazolinedione.When recourse was made to the endo-bornyl-substituted dienophile, it was ultimately possible to obtain (-)-3 and to determine its rate of racemization.The data indicate that the flanking phenyl group in 3 causes significant rate retardation relative to the trimethyl derivative.This finding contrasts with the behavior of 2 which racemizes more rapidly despite an interstitial phenyl substituent.The source of these dissimilar effects has been clarified by MM2 calculations, the details of which are presented.