Myers–Saito and Schmittel cyclizations of enyne-(hetero)-1,2,3-trienes: A DFT study on the structure–reactivity relationship

The Myers–Saito (C²–C⁷) cyclization and the alternative Schmittel (C²–C⁶) cyclization of enyne-1,2,3-triene 1R and its hetero analogues 2R–5R were investigated by using pure density functional theory method (BPW91) in connection with the 6-311G(d, p) basis set. It has been shown that heteroatom-containing reactants lower significantly reaction barriers for both cyclization modes and reduce the difference between the barrier of the C²–C⁷ cyclization mode and that of the C²–C⁶ one. The Myers–Saito cyclization of 4R is associated with the smallest reaction barrier and the highest exothermicity. Whereas the Schmittel cyclization of 4R is exothermic, all the others are predicted to be endothermic.