The synthesis of octavalene (tricyclo[5.1.0.02,8]octa-3,5-diene) and several substituted octavalenes

Abstract 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 ( 1Oa ) HBr is eliminated to give a 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 a 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 cyclooctatetraene at 50°. Deuterium-labeled 1a ([1,8-D 2 ] 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 the 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 bomine 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.