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. 2009 Jan 16;74(2):545-53.
doi: 10.1021/jo8017846.

The effect of electrostatic interactions on conformational equilibria of multiply substituted tetrahydropyran oxocarbenium ions

Michael T Yang  1 , K A Woerpel
Affiliations

The effect of electrostatic interactions on conformational equilibria of multiply substituted tetrahydropyran oxocarbenium ions

Michael T Yang et al. J Org Chem. .

Abstract

The three-dimensional structures of dioxocarbenium ions related to glycosyl cations were determined by an analysis of spectroscopic, computational, and reactivity data. Hypothetical low-energy structures of the dioxocarbenium ions were correlated with both experimentally determined (1)H NMR coupling constants and diastereoselectivity results from nucleophilic substitution reactions. This method confirmed the pseudoaxial preference of C-3 alkoxy-substituted systems and revealed the conformational preference of the C-5 alkoxymethyl group. Although the monosubstituted C-5 alkoxymethyl substituent preferred a pseudoequatorial orientation, the C-5-C-6 bond rotation was controlled by an electrostatic effect. The preferred diaxial conformer of the trans-4,5-disubstituted tetrahydropyranyl system underscored the importance of electrostatic effects in dictating conformational equilibria. In the 2-deoxymannose system, although steric effects influenced the orientation of the C-5 alkoxymethyl substituent, the all-axial conformer was favored because of electrostatic stabilization.

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Figures

Figure 1
Figure 1
Preferred conformations of the oxocarbenium ion intermediates in nucleophilic substitution reactions of monosubstituted tetrahydropyran oxocarbenium ions.
Figure 2
Figure 2
Monosubstituted lactones
Figure 3
Figure 3
Relative energy of conformers at different level of theory.
Figure 4
Figure 4
Theoretical and experimental J-values for C-3 alkoxy dioxocarbenium ion (B3LYP/6-31G*)
Figure 5
Figure 5
Relative low-energy conformers of C-5 alkoxymethyl dioxocarbenium ion (B3LYP/6-31G*).
Figure 6
Figure 6
Comparison of theoretical and experimental J-values of C-5 alkoxymethyl dioxocarbenium ion.
Figure 7
Figure 7
Theoretical and experimental J-values of C-5 alkoxymethyl rotamers.
Figure 8
Figure 8
Relative low-energy conformers of the 4,5-disubstituted dioxocarbenium ion (B3LYP/6-31G*).
Figure 9
Figure 9
Comparison of theoretical and experimental J-values for 4,5-disubstituted dioxocarbenium ion.
Figure 10
Figure 10
Calculated low-energy conformers of the 2-deoxymannosyl oxocarbenium ion (B3LYP/6-31G*).
Figure 11
Figure 11
Comparison of theoretical and experimental J-values of the 2-deoxymannosyl oxocarbenium ion.
Figure 12
Figure 12
Nucleophilic addition to oxocarbenium ions derived from 2-deoxymannolactone.
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