ORIGIN OF THE CONFORMATIONAL MOBILITY FOR 4-TETRAHYDROPYRANONE - AN AB-INITIO MO STUDY

Studies by the ab initio MO methods indicate a rather flat ring system and a small barrier for ring inversion in 4-tetrahydropyranone (4-THPN). The crystal structure of syn-6-(p-bromophenyl)-2-methoxy-5-methyl-4-THPN determined by X-ray diffraction supports the flattened ring structure predicted by the ab initio calculations. The relationship between the endocyclic torsional angle phi and the bond length C-X (X = O, N, C, and S) in a 4-X-cyclohexanone system was studied by ab initio MO methods. The size of phi is in the order of 4-THPN (phi = 44.5 degrees) < 4-piperidinone (phi = 47.1 degrees) < cyclohexanone (phi = 49.1 degrees) < 4H-thiopyran-4-one(phi = 53.6 degrees). The flattening of the ring is inversely proportional to the C-X bond distance.