Conformation and Pseudorotation of Five-Membered Rings

Murray-Rust and Motherwell [35] searched the CSD for all y8-l -aminofuranoside fragments (12) in nucleosides and nucleotides and made a principal component analysis of the conformations found for the five-membered ring and its direct substituents. A detailed description of this analysis is given in Chapter 4, Section 4.6.3.1. Here we merely summarize the results. 

As mentioned above, the third factor of the principal component analysis describes rotation of the side chain. The factors are by definition mutually orthogonal 

Packing forces alone can change the ring puckering considerably. There are several examples of crystal structures with two symmetry independent molecules that show different ring conformations. For example, in oxanosine (13) [36] one molecule has the C(2 ) exo and the other the C(3 ) exo conformation, a phase change of about 150°. 

The asymmetric distribution in the diagram demonstrates one of the dangers associated with an automated analysis. Inattention to such details or careless handling 

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