The first of the four chapers in Volume 10, by Benzion Fuchs, is
about the stereochemistry, and, in particular, the conformation of five￾membered rings, both carbocyclic and heterocyclic. In contrast to the cyclo￾hexane ring and its heterocyclic analogs, which lie in deep energy wells
represented by the chair conformations, five-membered rings are known
to be highly mobile, conformationally speaking. It was recognized early
that cyclopentanes are puckered and that there are two nonplanar confor￾mations more symmetrical than the others: the half-chair or Cz and the
envelope or C, conformation; it was also recognized that cyclopentane
rapidly “pseudorotates” between these conformations and an infinity of
other, less symmetrical ones. Nevertheless there arose a tendency to over￾interpret the behavior of substituted cyclopentanes or heterocyclopentanes
in terms of the two symmetrical conformations of the parent compound. In
his chapter, Fuchs has critically examined the conformational behavior of
a number of such compounds both on a theoretical and on an experimen￾tal basis, with the emphasis of the experimental work being on spectro￾scopic and diffractional behavior. While the chapter does not purport to
be exhaustive, it hopefully deals with enough systems to allow workers in
the field to generalize the findings to whatever saturated five-member ring
they may have encountered.