Thermal Isomerization of Isodicyclopentadiene and Its Cycloaddition Reactions

Article abstract of DOI:10.1021/jo00144a018

Isodicyclopentadiene (4) undergoes a <1,5> sigmatropic hydrogen shift at elevated temperatures (170-180 deg C) to form an isomeric, higly reactive diene intermediate 5 which readily undergoes <4 + 2> cycloaddition reactions with dienophiles like dichlorovinylene carbonate (DCVC, 8), vinylene carbonate (VC, 7), and maleic anhydride (9).In all cases addition to 5 occurs exclusively from the exo face.While 7 and 8 under these high-temperature conditions give one major addition product each (12 and 13, respectively), in accord with Alder's rule, 9 at high temperature yields the two isomeric product 15 and 16 initially in the ratio 1:3 of which the isomer 16 (Alder's rule product) on prolonged heating is converted into the stable isomer 15 (anti Alder's rule product).The cycloaddition reactions of 4-cyclopentene-1,3-dione (10), 2,2-dimethyl-4-cyclopentene-1,3-dione (11), and dimethyl acetylenedicarboxylate (28) with 4 and 5 are also discussed.In situ formation of 5 at 0 deg C by photolytic cleavage of diketone 14 is proved by trapping 5 with maleic anhydride.X-ray crystallographic data are provided for the adducts 12, 13, 15, 16, and 21.