Electrocyclic reactions in a cyclohexadiene‐hexatriene system

In a consecutive series of stereospecific reactions under alternately thermal and photochemical conditions cis‐1,2,3,4,5,6‐hexamethylcyclohexa‐1,3‐diene I, 3,4,5,6‐tetramethyl‐(Z,Z,E)‐octa‐2,4,6‐triene II, transf‐1,2,3,4,5,6‐hexamethylcyclohexa‐1,3‐diene III and 3,4,5,6‐tetramethyl‐(Z,Z,Z)‐octa‐2,4,6‐triene IV have been synthesized from 1,2,3,4,5‐endo,6‐endo‐hexamethylbicyclo[2.2.0] hex‐2‐ene as the starting material. Thermal ring closure of the triene IV did not give the cis‐diene I, but, among other products, the trans‐diene III. An intermediate product in this reaction is 3,4,5,6‐tetramethyl‐(Z,Z)‐octa‐1,3,5‐triene V, formed via a [1,7a] H shift. The structure of compound IV has been assigned on the basis of chemical and spectroscopic (13C and 220 MHz 1H NMR) data. Cis‐diene I can, at room temperature, be converted direct into trans‐diene III under the influence of hydrochloric acid.