The tricyclic diester I was prepd. from 2-butyne in 6 steps and 10% overall yield. Thermolysis of I, followed by photolysis gave II, the first persubstituted cis-9,10-dihydronaphthalene. X-ray anal. of II shows the mol. to be considerably twisted along the central bond. In addn., the planes of the ester groups are at an angle of .apprx.60 Deg to the planes of the six-membered rings so that conjugation of the diene systems with the carbonyl functions is interrupted, an effect which is reflected by the unusual spectroscopic properties of II. The same structural feactures can be found in decamethyl-cis-9,10-dihydronaphthalene. When heated slightly, II is converted to the tetracyclic valence isomer III. Dibal redn. of II gave both the expected diol and its 9,10-bis(hydroxymethyl) isomer. Etherification of the redn. products with CH2N2/Et2O-BF3 furnishes a mixt. of three isomeric bis(methoxymethyl) compds. They can be sepd. by chromatog. in the cold, but revert to the mixt. of the three isomers at room temp.