The conformation of six-membered rings

Abstract

Although an indefinite number of conformations exists for six-mem- bered rings, and although there is general consensus about the six classical forms (Bucourt and Hainaut, 1965; Cano et al., 1977; Romers et al., t974), there is no generally accepted nomenclature or symbolic formalism to distinguish among them. A quantitative basis for a classification exists in the literature (Cremer and Pople, 1975), and the purpose of this communication is to propose a logical nomenclature adapted to the classification. The analysis starts out from crystallographic fractional coordinates and involves transformation first to a set of Cartesian coordinates, and eventually to Cartesian coordinates of pucker for the ring system under consideration. From these coordinates, a set of three parameters of pucker in the form of polar coordinates (Q, O, ~) is obtained. These coordinates map out the conformation of the ring on the surface of a sphere, radius Q, and with poles at O = 0, 180% as suggested by Hendrickson (1967). Figure 1 depicts the surface of this sphere in two-dimensional polar projection. Each of the hexagons represents a canonical conformation in terms of the six classical forms. The symbols on the sides of the hexagons indicate the signs of the endocyclic torsion angles: the torsion angle definition of Klyne and Prelog (1960) is used. The established terminology for chair, boat, and half-chair forms is retained. Of the variant forms twist-boat/skew-boat and envelope/half- boat/sofa, the first form is preferred in each case. For the 1,3-diplanar form, the name screw-boat is proposed. This scheme provides a set of names with six different initials which can conveniently be used to represent the various types. For a unique description of ring conformation, a well-defined atomic numbering scheme is needed next. Whenever a single atom in a ring (e.g., 317