1,3,5,7-octatetraenes, electrocyclic reactions

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. 

A number of electrocyclic reactions have been carried out with systems of other sizes, e.g., conversion of the 1,3,5,7-octatetraene 97 to the cyclooctatriene 98.383 The stereochem-... 

With two more electrons in the conjugated system, making eight in all, the octatetraenes 4.51 and 4.54 show conrotatory closure giving the cyclo-octatrienes 4.52 and 4.55, However, the reaction can only just be stopped at this stage, for the products undergo a second electrocyclic reaction giving the bicyclic dienes 4.53 and 4.56 as a result of the allowed disrotatory reaction of the all-m hexatriene units. 

Thomas IV, B. E. Evanseck, 1. D. Houk, K. N. Electrocyclic reactions of 1-substituted 1,3,5,7-octatetraenes. An ab initio molecular orbital study of torqu-oselectivity in eight-electron electrocyclizations, J. Am. Chem. Soc. 1993, 115,... 

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. 

Table 3. Transition structure geometries, activation energies, and reaction energies for transition structures of electrocyclic ring closure of 1,3,5,7-octatetraene...

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