1.3- dioxane derivatives, conformational equilibria

Intermediate-intensity positive n -+ r Cotton effects (all in dioxane) of the 3-oxotriterpenoids 54, 57-59 were attributed to an A-ring chair-boat conformational equilibrium. The contribution of the boat form (like in 2/3-methyl model derivatives 53 and 56) was estimated to be ca 30%140. 

As in the case of the 1,2-dioxins, the 1,2-dithiins exist in various states of saturation, oxidation, and benzoannelation (cf. Scheme 1, 17-27) and they have been studied in detail both theoretically and experimentally. Not only were the conformations of the ring and attached substituents investigated, but the valence isomerism of 1,2-dithiin by both NMR and high-level ab initio molecular orbital (MO) calculations and the dithiol/disulfide equilibrium by MP2 calculations were also examined. The latter equilibrium has been applied successfully as a luminescent molecular switch (cf. Section 8.10.2.1). Finally, as a very interesting 1,2-dithiin derivative, the synthesis, structure, and reactivity of the (-l-)-camphor-derived analog 25 and its sulfoxide 26 and sulfone 27 have been reported. Both the synthesis and the antimalarial activity of the 2,3-dioxabicyclo[3.3.1]nonane pharmacophore 28, which contains the 1,2-dioxane moiety, have been reviewed recently <2006BML2991>. 

The control of anomeric stereochemistry continues to fuel the investigation into the synthetic utility of (x-oxygenated radical intermediates. Moreover, it has proven to be a valuable tool in organic synthesis, especially in the stereoselective synthesis of various substituted tetrahydropyrans, y>>n-l,3-dioxanes, and carbohydrate derivatives. The recent discovery of non-equilibrium radical reactions and conformation-induced self-regeneration of stereocenters should provide new opportunities in the ever-expanding field of a-oxygenated radical chemistry. 

In this context, the positions of equilibrium between diastereoisomeric cis- and trans-5-(t-butylthio)-, 5-(r-butylsulfinyl)-, and 5-(t-butylsulfonyl)-2-isopropyl-l,3-dioxanes were reported recently [90,91] (Table 3.3). Although AG° values for the sulfides are very similar, the difference in conformational behaviour for the sulfoxides is significant, and the effect of changing from methyl to t-butyl in the sulfones is quite dramatic the large preference of the methyl analogue for the axial position (1.19 kcal mol, Table 3.3) is reversed in the t-butyl derivative, where the equatorial isomer is more stable by 1.14 kcal mol. Thus, the /-butyl ligand on... 

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