THE SYNTHESIS OF OCTAVALENE (TRICYCLO2,8>OCTA-3,5-DIENE) AND SEVERAL SUBSTITUTED OCTAVALENES

Article abstract of DOI:10.1016/S0040-4020(01)87575-X

The first synthesis of octavalene (1a) is reported.The starting material is homobenzvalene (5), to which monobromocarbene is added.The resulting compound 3a takes up bromine across the central bicyclo<1.1.0>butane bond to form the tribromide 7a which undergoes a cyclopropyl bromide-allyl bromide rearrangement on heating.From the product (10a) HBr is eliminated to give 1,3-dibromocyclobutane with a 1,3-butadiene bridge across its 2- and 4-position (11a).Finally, t-butyllithium removes the two Br atoms from 11a and converts it into 4:1 mixture of 1a and cyclooctatetraene.This reaction sequence represents the first application of protective group strategy in bicyclo<1.1.0>butane chemistry.Octavalene (1a) is shown to rearrange to cyclotetraene at 50 deg.Deuterium-labeled 1a (<1,8-D2>1a) is used to prove that a <1,5>sigmatropic shift does not occur in 1a.Utilizing the above methodology 4-bromooctavalene (1b) and 3-phenyl-5-bromooctavalene (1c) are synthesized from dibromocarbene adducts 3b and c of homobenzvalene (5) and 5-phenylhomobenzvalene (6), respectively.Surprisingly, 1c was accompanied by a small quantity of 3-bromo-1-phenyloctavalene (1d).Possible mechanisms for the addition of bromine to the bicyclo<1.1.0>butane system of compounds 3 and for the formation of the octavalenes 1 are discussed.In the (13)C-NMR spectra of 1 and 11 chemical shifts at unexpectedly high field are observed for C-6 of the 1,3-cycloheptadiene moieties.