Anfi-Periplanar conformation

Treatment of neomenthyl chloride with base rapidly produces two different alkenes, i.e. 2-menthene and 3-menthene. If one considers the three-dimensional shape of neomenthyl chloride, it can be seen that, in the preferred conformer with the two alkyl groups equatorial (see Section 3.3.2), the chlorine is an axial substituent. This means there are two different hydrogen atoms adjacent that are also axial and anfi-periplanar to the chlorine. As a conseqnence, two different E2 eliminations can occnr hence the two observed prodncts. That the two prodncts are not formed in eqnal amonnts will be considered in the next section. 

The preference for a gauche relationship between fluorine and the N-CO moiety observed in 7 and 8 is of broader significance as a conformational tool. In the solid state, the conformation of 2-fluoro-A -(2-flnoroethyl)-propionamide (13) reflects a gauche relationship between the fluorine and NH-CO moieties, torsion angle = -69.9°, whilst the F atom a- to the amide carbonyl is aligned in a fashion consistent with the preferred dipole interactions in which the two are anfi-periplanar, torsion angle = 2° [29]. A similar phenomenon is observed with 14 [35]. 

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