Conformations of Mono and Dithioacetals

2-Alkoxytetrahydrothiopyrans were studied by Zefirov and Shekhtman [35] and the axial conformer 233 was found to predominate (90 %) over the equatorial con-former 234 when R was a methyl or a propyl group. The equilibrium of the isomeric mixture of 235 and 236 under acidic conditions was studied by Eliel and Giza [36]. The axial isomer 235 was found to constitute almost 65 % of the mixture. These examples indicate that a monothioacetal function benefits from anomeric effect arising from sulfur, which is possibly a bit weaker than that offered by oxygen. 

The above analysis gains support from the cyclization results of the compounds 238 and 240. The compound 238 furnished the conformer 239a rather than 239b. In the conformer 239a, each heteroatom exerts an anomeric effect on the other heteroatom, whereas only the sulfur atom exerts an anomeric effect on the oxygen atom in the conformer 239b. 

Taking the anomeric effects together with the operating steric factors, the following energy profile emerges  

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Conformation of acetals Conformation of mono and dithioacetals Conformation of 1,3-oxazines and 1,3-diazines Formation and hydrolysis of the acetal function Hydride transfer to cyclic oxonium ion Oxidation of the C —H bond in acetals REFERENCES... 

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