Synthesis of new substituted dihydroheptalene derivatives: SiO2- and base-catalyzed rearrangement of dimethyl trans-3,8-dihydroheptalene-3,8-dicarboxylate

Dimethyl trans-3,8-dihydroheptalene-3,8-dicarboxylate (trans-3) isomerizes to dimethyl cis-3,8-dihydroheptalene-3,8-dicarboxylate (cis-3) upon treatme nt with SiO2. On the other hand, base-catalyzed reaction of trans-3 undergo es a direct 1,3-intramolecular proton shift to give 6 at room temperature i n 5 min. Prolonged reaction time formed isomers 7 and 8 in a ratio of 4:1. AM1 calculations indicate that the isomer 8, which is formed as minor produ ct, has a lower heat of formation (-99.34 kcal mol(1)) than that of the maj or isomer 7 (-92.05 kcal mol(1)). However, when a similar reaction was perf ormed at 100 degreesC, the thermodynamically more stable isomer 8 was forme d as the major product. Furthermore, cycloaddition reactions of these new d ihydroheptalene derivatives 6 and 7 with different dienophiles have been st udied. The mechanism has been discussed.