Influence of Anomeric Effect on Conformational Preferences

From the above relative energies, the isomers 162 and 163 must be formed in nearly equal amounts. Further, the isomer 162 should exist, within a small approximation, as a 1 1 mixture of the conformers 162a and 162b due to very small energy difference. Because of very large energy difference (3.7 kcal mol ), the 

Let us now consider the dihydoxy ketone 164 which is enantiomeric to 161 at the secondary carbinol center. Cyclization is expected to lead to the formation of two isomeric acetals 165 and 166. The isomer 165 may be expected to exist as an equilibrium mixture of 165a and 165b. Likewise, the isomer 166 may be expected to exist as an equilibrium mixture of 166a and 166b. Whether both 165 and 166 or just one of them, and also whether each of these could indeed exist as an equilibrium mixture of the two possible conformational isomers will be governed by their relative energy differences, calculated as follows  

We have learnt above enough about the anomeric effect and its implications on conformational preferences. Let us take a look at the examples from the general literature given in Eqs. 42-45 and analyze them in the manner as above. The 

We have discussed above the conformational profile of l,7-dioxaspiro[5.5]un-decane skeleton. There are examples of 1,6-dioxaspiro[4.5]decane skeleton as well, a skeleton that has been encountered in products of natural origin including the ionophores. Cottier et al. [25] have prepared several derivatives of 1,6-dioxaspiro [4.5]decane and shown that 175 exits as the conformer 175a in preference to 175b. These authors have also reported that the compound 176 exists in the conformation as shown, each ring oxygen having an electron pair orbital antiperiplanar to a polar 

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