STUDIES ON QUINONES .27. DIELS-ALDER REACTION OF 8,8-DIMETHYLNAPHTHALENE-1,4,5(8H)-TRIONE

The Diels-Alder reaction of the title quinone 1 with various symmetrical and unsymmetrical dienes in ethanol solution is reported. The cycloaddition takes place, in all cases, across the external quinone double bond affording with cyclopentadiene, buta-1,3-diene and 2,3-dimethylbuta-1,3-diene, the corresponding adducts 3, 4 and 5. The cycloaddition of 1 with 2-methylbuta-1,3-diene and penta-1,3-diene provided 90:10 and 65:35 mixtures of regioisomers 6-7 and 17-18, respectively. Enolisation of these adduct mixtures afforded the corresponding anthracenones 10-11 and 21-22. Compounds 11 and 22, the minor components of the anthracenone mixtures, were synthesized from acetylnaphthalenes 14 and 23. The reaction of 1 with (E)-1-trimethylsilyloxybuta-1,3-diene yielded exclusively adduct 19 and with (E)-1-methoxybuta-1,3-diene gave a mixture of compounds 28, 31 and 32. The regioselectivity of the Diels-Alder reactions of quinone 1 with 2-methylbuta-1,3-diene and penta-1,3-diene is in agreement with that predicted by frontier molecular orbital (FMO) theory. On the basis of frontier molecular orbital interactions, compound 19 is proposed as the regioisomer generated in the reaction of 1 with the (E)-1-trimethylsilyloxybuta-1,3-diene.