MOLECULAR LADDERS CONSTITUTED OF LATERALLY-FUSED 1,4-CYCLOHEXADIENE SUBUNITS - OPEN-CHAIN MODELS OF THE [N]BELTENES

A practical synthesis of 2,3,6,7-tetramethylene-1,4,5,8-tetrahydronaphthalene (11) has opened up several avenues for the elaboration of molecules consisting largely or exclusively of 1,2:4,5-fused 1,4-cyclohexadiene subunits. Diels-Alder addition of dienophiles to both 1,3-diene segments of this reactive hydrocarbon leads directly to 1,4,5,6,7,10,11,12-octahydrotetracene and substituted derivatives of this polyolefin. A route to the hexacene homolog is available by twofold [4 + 2] cycloaddition of a benzyne followed by Birch reduction. For reasons of solubility, the benzyne should carry alkyl groups in its 4- and 5-positions. Finally, a two-step cyclohomologation scheme is described that allows for the systematic controlled extension of the structural features present in 11 to its tetra- and hexacyclic congeners. The insolubility of the latter hydrocarbon limits its further exploitation in synthesis.