3b) and (4b) were obtained in a total yield of 93 % and a ratio 6:94. Replacement of the KOC(CH3)3 by the stronger base lithium piperidide altered this ratio to 60:40. This reversal, and the marked dependence of the isomer ratio on temperature, piperidine concentration, and solvent, suggest that reaction by way of the dehydro compound (2) is in competition with direct exchange of substituents to give (36) ; this is being studied further. The viscous reddish-yellow mixture (b.p. 114-116 "C/ 0.04 mm Hg) of isomers (36) and (46) was separated by column chromatography. The UV spectrum of (36) [Amax = 255 (E = 31000), 275 (19500, sh), 353 (8200), and 410 mp (1400, sh)] is very similar to that of 1,6-methano[lO]annulene-2-amine[sl, whereas that of (46) is shifted towards shorter wavelengths. This observation and the fine splitting of the AB doublets in the N M R spectrum unambiguously prove the structure shown. Experiments designed to trap the postulated intermediate 2, 3didehydro-1,6-methano [lO]annulene (2) in Diels-Alder re- actions [61 were successful on use of tetracyclone (tetraphenylcyclopentadienone) : we obtainedagain by dehydrohalogenation of ( I ) with potassium t-butoxide -a cu. 30 "/: yield of 1,2,3,4-tetraphenyl-7,12-methanobenz[lO]annuler~e (5) (m.p.
258 "C).