Cyclooctatriene, thermal cyclization

Extension to the cyclization of higher homologs is straightforward The lower 1 tt orbitals of a polyene with N conjugated double bonds will correlate in C2v with all of the tt MOs of its cyclization product, leaving the Nth out of correlation with cr(ai). If N is even (A = 2, where q = 0,1,2...), xn will be fl2 and a conrotation will be required in order to induce a correspondence with (j if N is odd N = 2q + 1), xn will be bi and a disrotation will be called for. Since all N orbitals are doubly-occupied, the number of electrons involved k) is 2N] the W.-H. Rules for thermal cyclization [1, pp. 38-45] follow directly. Thus, for example, all-czs-octatetraene cyclyzes readily to cyclooctatriene via a conrotatory pathway. [9]... 

The decatetraene cyclizes at — 10°C. by a conrotatory motion, to produce the cyclooc-tatriene with the two methyl groups in a trans orientation. When the cyclooctatriene is warmed to 20°C, it undergoes a thermally allowed disrotatory ring closure to give the final product. 

Four electron pairs undergo reorganization in this electrocyclic reaction. The thermal reaction occurs with conrotatory motion to yield a pair of enantiomeric rra/w-7,8-dimethyI-1,3,5-cyclooctatrienes. The photochemical cyclization occurs with disrotatory motion to yield the cis-1,8-dimethyl isomer. 

Z,6Z,8 )-2,4,6,8-Decatetraene has been cyclized to give 7,8-dimethyl-1,3,5-cyclooctatriene. Predict the manner of ring-closure—conrotatory or disrotatory—for both thermal and photochemical reactions, and predict the stereochemistry of the product in each case. 

---