Comparative study on the transition structures of (3,4) and (3,5) ene cyclizations: A theoretical approach

Possible transition structures (TSs) of (3,4) and (3,5) ene cyclizations of 7-methyl-1,6-octadiene and 7-methylocta-1,6-dien-3-one were constructed and optimized by DFT method. Product proportions were calculated using the relative energies of the transition structures and these results are found to be in good agreement with the experimental one. Variation of the product proportions was explained using some model TSs of intermolecular ene reactions. The change of the dihedral angle around the forming carbon-carbon bond in the model transition structures was found to play a crucial role in determining the overall selectivities of cyclized products.