2- Aryl-1,3-dioxanes, conformational

Uehara et al. (99JOC1436) studied in solution and in the solid state the conformational equilibria of 2,2-diaryl-1,3-dioxanes with aryl substituents... 

How does one then attain an axial orientation of C8 at C9/C11 dioxane ring This requires the aryl-residue Ar in 41 to be placed in an equatorial position, which is unfortunately the thermodynamically less stable situation. As soon as equilibration becomes possible, 41 is converted into 42, which goes over into the twist boat conformation 43 [36] in which all the major residues are equatorially arranged. Erythronolide seco acid derivatives with such an arrangement have a... 

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