2-Methoxy-1,3-dioxane, conformational

Sauriol-Lord and St-Jacques (64) have studied 2,4-benzodithiepin 78 and the substituted derivatives 79-82 (Fig. 14). They found that these compounds exist in the chair form 39 (0=S) (Fig. 8), except for the derivative 82 where 13% of the twist-boat 40 (0=S) was detected. The fact that essentially only the chair form was observed in these compounds would simply be due to the long C - S bonds which attenuate the effects of steric repulsions in conformer 39 (0 =S). This is in accord with the fact that in contrast to 2-methoxy-l,3-dioxane which exists as a 7 3 mixture of equatorial conformer 83 and axial conformer 84, 2-methoxy-l,3-dithiane has been found to exist solely in the axial form 85. 

The conformational equilibrium of 2-methoxy-1,3-dioxane was studied again both in the gas state (97JMS335) and in acidic/basic solutions (97JA585). 

The X-ray structures of two anthraquinone derivates of 1,3-dioxane were published. In both 1 -methoxy-4-(2-methylprop-2-enyloxy)-2-[(2i ,6i )-4,4,6-trimethyl-1,3-dioxan-2-yl]-anthraquinone and 4-hydroxy-3-(2-methyl prop-2-enyloxy)-2-[(27 ,67 )-4,4,6-trimethyl-l,3-dioxan-2-yl]anthraquinone the 1,3-dioxane ring adopts the chair conformation and the substituents in positions 2 and 6 are in equatorial conformations (99AX(C)436). Finally, Freeman et al. (02JMS(T)43), employing both ab initio theory and density function theory, calculated the energies of chair, half-chair, sofa, twist, and boat conformers of 1,3-dioxane. 

In contrast to this early empirical solvent scale, one of the more recent, introduced by Eliel and Hofer in 1973 [25] and based on the solvent-dependent conformational equilibrium of 2-z-propyl-5-methoxy-l,3-dioxane, should be mentioned cf. Table 4-9 in Section 4.4.3). In general, polar solvents shift this conformational equilibrium towards the more dipolar axial cis isomer. The authors proposed calling the standard molar Gibbs energy changes associated with this equilibrium, he D scale D for... 

Explain why the most stable chair conformation of 1,3-dioxan-5-ol has an axial hydroxyl group, but the most stable chair conformation of 5-methoxy-l,3-dioxane has an equatorial methoxy group. 

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