Site-Selective Diels-Alder Reactions of 1,4,5,8-Naphthodiquinones with Anthracenes and Successively with Cyclopentadiene: Elelctronic Effects vs. Steric Effects

The cycloaddition reactions of naphthodiquinones with anthracene derivatives have been investigated. While anthracene (4) reacted with naphthodiquinones (1,2, and 13) at the internal double bond to give the internal adducts (6, 7, and 14), the reaction of 9,10-substituted anthracenes (5) took place at the terminal double bond to afford the terminal adducts (8, 9, and 15). The successive reaction of adducts 6 and 7 with cyclopentadiene gave the Diels-Alder adducts 16 and 17, respectively, which were photochemically converted into the cage compounds 18 and 19 in high yields. The reaction of 8 with cyclopentadiene afforded terminal adducts 20, whereas the similar reaction of 9 gave internal adducts 21 which were smoothly converted on irradiation to the cage compounds 22. The site selectivity in these reactions was discussed on the basis of the frontier molecular orbital (FMO) analysis as well as consideration of the steric effects of the substituents.