Altho,gh very prominant in bicycloC2.2.llheptadiene photochemistry; the difficulty of observing Cristol closure (internal 2 + 2 cycloaddition) with bicyclo[2.2.2]octodienes has been surprising to many workers.1 Recently, however, Prinzbach, Eberbach, and PhillipposianZ have isolated the Cristol closure product from the 2,3-bis(carbomethoxy) derivative and its anti-7,8methylene derivative, and Liu 3 isolated the closed forms of 2,3-bis(trifluoromethyl) derivatives. We here report our work on an example of this photochemical ring closure reaction to point out the remarkable instability of the closed form.
Reaction of dimethyl acetylenedicarboxylate with 4,5-dimethyl-1,2-dihydrophthalic anhydride (1:l ratio) at room temperature for three days in Ccl4 gave a 30$ yield of the Diels-Alder adduct la,, mp 119-120Β° (crystallized from CHCls-pentane) in addition to higher adducts. nmr (-30", CDCls) 64.36 (dd, J = i.5, 2.II-Iz, 2 bridgehead H), 3.91 (S,6 methoxy H), 3.50 (dd, J = 1.5, 2.1, 2R), 1.83 (9, 6 methyl H). The addition to 1,2_dihydrophthalic anhydride was very slow at room temperature; at 45" for two days, the yield of g was quite iow , most of the material being converted to higher adducts. nmr: (CDCls, 45") 66.62 (dd, J = 4.4, 3.2, 2 vinyl H), 4.63 (broad mult, 2 bridgehead H), 3.81 (S, 6 methoxy H), 3.48 (dd, J = 1.5, 1.9, 2H).