INFLUENCE OF ALKYL SUBSTITUTION ON THE INTRAMOLECULAR IONIC DIELS-ALDER REACTION OF TETRAENES

Fifteen tetraenes, comprised primarily of methylated analogs of (3E,8E)-1,3,8,10-undecatetraene, were synthesized and treated with acid to study the influence of alkyl substitution on the intramolecular ionic Diels-Alder reaction. Depending on methyl substitution patterns of these tetraenes, bicyclo[4.3.0]nonyl, bicyclo[4.4.0]decyl, and bicyclo[5.4.0]undecyl ring systems were produced. For two tetraenes, the ring preference between two different Diels-Alder-derived ring skeletons could be controlled using different temperatures and acid catalysts. Mechanistically, a stepwise process was postulated for at least some of these Diels-Alder reactions. The irreversible and stereoselective formation of the same Diels-Alder product from two noninterconverting tetraenes, which differed only by the cis-trans relationship of a terminal methyl group, was best explained by a stepwise process. A stepwise process was best demonstrated when intermediate triene products isolated from a reaction mixture irreversibly cyclized to Diels-Alder products when resubmitted to the reaction conditions. Although several of the tetraenes failed to give significant amounts of Diels-Alder products, certain alkyl substitution patterns were identified which generally led to high yields of cyclized products.