CASCADE REARRANGEMENT OF 5-CYCLOPENTYLIDENECYCLOOCTANONES

Acid-catalyzed rearrangement of 5-cyclopentylidenecyclooctanone deriva tives 9a-c was examined to obtain polyspiropolyquinanes 11a-c, conside red to have a unique helical structure, through cascade rearrangement pathways consisting of continuous transannular cyclization followed by successive 1,2-alkyl shifts. The substrates were prepared easily by u se of the Wittig or McMurry reaction, Reaction of the 5-cyclopentylide necyclooctanone (9a) with acid gave the expected dispirotriquinane ket one 11a in high yield. The precise mechanism was elucidated by a deute rium-labeling experiment, In the case of the ketone 9b, having another spiroannulated cyclopentane ring attached on 9a, the trispirotetraqui ane 11b was not obtained but the bis-propellane-type tetrahydrofuran 2 5 was produced exclusively, The 5-(5'-cyclopentylidenecyclooctylidene) cy (9c) afforded the polycyclic compounds 27-31, depending on the acid used, instead of the desired tetraspiropentaquinane 11c. The structur es of the products were determined by NMR spectral data including 2D C -13 INADEQUATE spectra and X-ray crystallographic analyses. The unexpe cted rearrangement pathways are also discussed.