Cyclooctatriene, electrocyclic formation

The predicted conrotatory cyclization of octatetraenes was confirmed for the case of the methyl-substituted compounds, which above 16 °C readily formed the cyclooctatrienes shown in equations 13 and 14)14. We conclude this section with an electrocyclic reaction involving ten TT-electrons, that is, the formation of azulene (17) when the fulvene 16 is heated (equation 15)15,16. 

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. 

The ruthenium-catalyzed cycloaddition of 1,6-diynes to 1,3-dienes forming 1,3,5-cyclooctatrienes and vinylcyclohexadienes can formally be assigned to [4+2+2] cycloaddition. This reaction is actually a tandem process catalyzed by Ru(II) resulted in the formation of Z-tetraenes or vinyl-Z-trienes, followed by the ring closure in pure thermal conrotatory Sir- or disrotatory 6ir-electrocyclization processes, respectively (Scheme 2.88 and Table 2.13) [141]. 

---