Octatetraene, electrocyclic ring closure

The nucleophilic attack with 2-700 and 2-701 can proceed in a syn or anti manner to provide either 2-702 or 2-703, or both [357]. If2-703 is formed, it follows a charge-driven conrotatory opening of the cyclobutene ring with generation of the coiled 1,3,5,7-octatetraene 2-704. This intermediate is capable of a rapid helical equilibration [357] and a regioselective 8jt electrocyclic ring closure to give 2-705 [358]. 

The direct irradiation of 1,3,5-cyclooctatriene (184) in ether or hydrocarbon solvents leads to the slow formation of two stable isomers corresponding to disrotatory 47T-electrocyclization (185) and bicyclo[3.1.0]pentene (186) formation along with small amounts of the reduced product 187 (equation 69)279-281. Conventional flash photolysis experiments later showed that, in fact, the main primary photochemical process is the formation of a short-lived stereoisomer (r = 91 ms)282, most likely identifiable as ,Z,Z-184. The transient decays to yield a second transient species (r = 23 s) identified as Z,Z-l,3,5,7-octatetraene (188), which in turn decays by electrocyclic ring closure to regenerate 184282 (equation 70). The photochemistry of 184 has been studied on the picosecond timescale using time-resolved resonance Raman spectroscopy49. 

Fig, 4. Transition structure for the electrocyclic ring closure of 1,3,5,7-octatetraene to 1,3,5-cyclooc-tatriene... 

The higher analogs in the series of electrocyclic reactions, the electrocyclic ring closures of 1,3,5-hexatriene and 1,3,5,7-octatetraene, have been studied using HF, MP, and DFT methods. Figure 2 shows the calculated transition structure for the disrotatory ring closure to 1,3-cyclohexadiene, and selected results for this reaction are summarized in Table 2. 

Sol 12. (b) The product, 7,8-dimethyl cycloocta-l,3,5-triene (1) with tmns stereochemistry is obtained upon disrotatory ring closure of octatetraene system, which occurs only under photochemical conditions. Similarly, a bicyclic product (11) with cis stereochemistry at fused carbons is obtained when a triene system undergoes disrotatory electrocyclization under thermal conditions. 

---