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

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 CH₂- OH. 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.