Aldrichimica Acta Volume 30 No 4 (pdf) from Sigma-Aldrich Cycloaddition results obtained with alkene 5 are a better guide to reactivity than those obtained from Smith’s diene (4), as it is the dihydro subunit in 5 which is present in those other polyalicyclic systems produced by catalyzed cycloaddition of DMAD onto norbornene end groups.
Source: wiktionary