Dimethyl o[4.2.2.0(2,5)]deca-3,7,9-triene-7,8-dicarboxylate (Cookson's
diester) (8) is rearranged thermally via a double Cope reaction to it
s ring-degenerate isomer (14). The structure of diester (8) (indirectl
y) and the rearrangement product (14) (directly) are placed on an uneq
uivocal base by X-Ray structure analysis. AM1 semi-empirical calculati
ons support the preferential formation of (8) from the reaction of DMA
D with transient intermediate diene (7). Epoxidation studies (mCPBA) h
ave been conducted and firm site selectivities determined. A correctio
n is made to an earlier report on the epoxidation selectivity of (18).