Organic Reactions Volume 4 - Sciencemadness Dot Org

DIENE SYNTHESIS II 83
covered as the main product 9 (a small amount of £rcms~3,6-methanohexahydrophthalic
acid is also formed). The anhydride of 3,6-methano-A^tetrahydrophthalic
acid proved to be identical with that derived
(three steps) from dibromomaleic anhydride and cyclopentadiene. 12
The ethano group of the 3;6-ethano~A 1,4 -dihydrophthalic esters
(LI, n K 2), like that of the ethanodihydrobenzene system of 1,4ethano~l,4-dihydroanthraquinone
(LXI), is sensitive to heat. The
esters of acetylenedicarboxyUc acid react smoothly with cyclohexadiene
at 0°, and a yield of about 84% of the dimethyl ester of 3,6-ethano-
A 1,4 -dihydrophthalic acid is obtained. 50 Heating the reaction mixture
to 200° pyrolyzes the primary adduct to ethylene and the ester of
phthalic acid. Thus, by identifying the ethylenic products of pyrolysis
it has been possible to locate the conjugated systems of a-phellandrene
(LXII), a-terpinene (LXIII), a-pyronene (LXIV), and /5-pyronene
(LXV).
The propano bridge of the cycloheptadiene adduct (LI, n «= 3), like
the methano bridge (LI7 n = 1), is thermally stable. A conjugated
CO2C2H5
JC2H5
XXVI
(2 steps)
H