Effect of hetero atom on the conformational stability of the forming ring in the transition structures of Type-II ene cyclization: a theoretical study

The chair conformation of the forming ring in the transition structure, for its high stability, is usually considered for the determination of the overall stereoselectivity in a type-II ene cyclization reaction. However, present theoretical investigation reveals that the presence of a heteroatom like oxygen or nitrogen in the tether of type-II carbonyl ene cyclization stabilizes the transition structure, in which the forming ring adopts a boat conformation. Due to such stability of the boat conformer the overall stereoselectivity of some type-II reactions may differ from the expected one.