[10]annulene: Electrocyclization mechanisms.

The mechanism of 6-π electrocyclization of all-cis, mono-trans, and double-trans [10]annulene to yield 4a,8a-dihydronaphthalene has been explored at various quantum-chemical methods. The mono-trans configuration cyclizes preferentially to trans-4a,8a-dihydronaphthalene, in agreement with the experimental results. The cyclization of the all-cis configuration requires firstly a bond-shifting to the naphthalene-like conformation of double-trans [10]annulene, which is the rate-limiting step, and finally its azulene-like conformation electrocyclizes quickly to cis-4a,8a-dihydronaphthalene. Its experimental rate coefficient is consistent with the computed one for the cyclization of the all-cis configuration, unlike the calculated one for the double-trans configuration. These results confirm the configurations assigned by Masamune et al. to the two isomers which they isolated.