Five- and Six-membered rings and related fused systems

Abstract

General.-A brief history of conformational analysis over the past twenty-five years has appeared.' A new text2 on dynamic n.m.r. spectroscopy includes a useful chapter on conformational processes in rings. The concept, first applied to cyclopentane, of defining a unique mean plane for a monocyclic puckered ring, and relating the geometry of the puckering to this plane, has now been generalized to four-, five-, and six-membered carbocyclic and heterocyclic rings by Cremer and P~p l e .~The same authors have studied the problem of pseudorotation in saturated five-membered rings by ab initio molecular orbital theory, applying it notably to oxolan and 1,3-dio~olan.~ Several methods of calculation are proposeds to determine torsional angles in six-membered cyclic compounds, including many heterocyclic systems.Five-membered Rings containing One Heteroatom.-Assignments of relative configurations have been made6 for a number of 2,4-dialkyl-or alkyl-phenyl-ylactones; the cis-isomers are slightly more stable than the trans-isomers. The truns-2-methyl-3-phenyl isomer was more stable than its cis-isomer. Conformations are discussed in terms of an envelope arrangement with C-3 displaced from the plane of the other ring atoms (1)-(3) (R', R2 =Me, Et, But, or Ph). Longrange coupling constants, 4J2,5, are reported' for some 2,5-dihydrofurans and phthalans, which are consistent with changes in pucker angle (4). 2,5-Disubs tituted-3 -0xazo1ines seem to be analogous. ' E. L. Eliel, J. Chem. Educ., 1975, 12, 762.