Conformational Analysis of 5-Substituted 1,3-Dioxanes. 7. Effect of Lithium Bromide Addition^†^,1

The position of equilibria, established by means of
BF3, between diastereomeric cis- and
trans-5-substituted-2-phenyl-1,3-dioxanes, in solvents THF and
CHCl3, and in the presence of 0, 1, and 10
equiv of LiBr has been determined. The observed ΔG°
values show that the addition of salt to the
reaction medium influences the position of equilibrium. Lithium
bromide effects on the conformational behavior are discussed in terms of lithium ion complexation
events that lead to increased
stability of the axial isomers when the substituent at C(5) is
CO2H, CO2CH3,
CONHCH3, and CH2OH. By contrast, disruption of the intramolecular hydrogen bond
present in the axial 5-acetamido
derivative (cis-9 substituent equal to
NHCOCH3) modifies the preference for the axial
conformation
in salt-free 9 to a net dominance of the equatorial isomer
in the presence of LiBr. Interpretation
of the experimental observations was based on models that are
apparently supported by
semiempirical AM1 calculations. The results derived from the
present study contribute to our
understanding of the processes involved in molecular recognition and
may model salt effects in
physiological events.