The Conformation of Cyclic Fragments

The conformational analysis of cyclic molecular fragments is based on the original idea of Kilpatrick, Pitzer and Spitzer [200], as subsequently developed by various workers [201, 202, 203, 204, 205, 206, 207, 208]. 

Cyclic fragments are common components of molecules and a quantitative description of their conformation is of obvious importance when analyzing the relationship between molecular topology and function. Chemical factors may require rings to be flat, but more often cyclic fragments are non-planar or puckered and can adopt a variety of conformations. 

The diagram illustrates a general mode of puckering that occurs for all even-membered rings. The resulting conformation is known as the crown form and derives from a regular distortion of the planar D h polygon. 

The five-membered ring is the simplest odd-membered ring with pucker. Two symmetrical modes, based on the regular pentagon are represented by  

The former has a av mirror plane and the latter a two-fold axis in the mean plane. For distinguishable atoms five equivalent forms of each type are distinguished by the position of the symmetry elements. Each of these has an enantiomeric form, yielding a total of twenty different puckered forms in two groups of ten conformations with equivalent shapes. For a general iV-membered ring this implies a maximum of 4N related conformations in equal groups with Cs and C2 symmetries respectively. 

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