V 70)Summary. Measures of the progressive flattening of the chair conformation in a series of constrained cyclohexanols are obtained and compared by proton NMR. coupling constants and by molecular mechanics.The shapes of six membered rings have continued to receive attention both from theoretical [l] and practical [2] viewpoints. It is now recognized that the perfect chair conformation represents an ideal state of affairs since electron diffraction reveals that even cyclohexane itself is flatter than supposed [3], the internal C-C-C angle being 111.55" not 109.4". Furthermore, substitution on the ring causes additional deviations from the chair conformation. When two methyl groups are substituted in a syn-axial disposition, the operation of the Reflex effect brings the trans anti-parallel pair of syn-axial substituents closer together ( fig. 1 a) [4]. Contrariwise, syn-axial placement of an ethane bridge, as exemplified by bicycloj3.2.13 octane, Fig. 1. Operation of the Reflex Effect ( a ) and its inverse (b)
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