Interesting Contrasts in Electrocyclic Reactions for Thieno- and -pyrans with Chromenes

Article abstract of DOI:10.1021/jo00069a026

Thienopyrans 3, 4, and 5 were synthesized and found to have remarkable differences in stability.Systems 3 and 5 undergo electrocyclic ring-opening to 8 and 18, respectively.In acid milieu at room temperature, system 5 exists in an equilibrium with ring-opened thienopyranone 18.Ground state and activation parameters for the open and closed forms of thienopyrans 3 and 5 as well as the isosteric chromene 2 were calculated using the AM1 Hamiltonian.The Gibbs' free energy differences between the open and closed isomers in each system were found to be predictive of the observed equilibria.The mechanism for ring-opening in both thienopyran systems is proposed to be acid-catalyzed based on the calculated temperatures required for ring-opening as well as experimentally determined results.