Anomeric effect hyperconjugative nature

The nature of the anomeric effect in the bicyclic /ra r-fused octahydro-l-methyl-l//-2,l-benzothiazine 2,2-dioxide 37 has been examined by single crystal X-ray structure analysis (Figure 5) <1998CJC164>. The crystal structure of 37 shows that the A -Me group assumes an axial position in the solid state. The authors suggest that this conformation is also the most stable in solution and propose that this hyperconjugation effect is >2.0 kcal mol . ... 

Mo, Y. (2010). Computational evidence that hyperconjugative interactions are not responsible for the anomeric effect. Nature Chemistry, 2, 666-671. 

The concept of hyperconjugation relies on a local orbital picture of quantum chemistry that is consistent with the common chemist s view of Lewis structures. Natural bond orbital (NBO) analysis [30] has been used to explain the anomeric effect by showing a favorable interaction between the heteroatom lone pair and the ct orbital of the substituent bonded to the anomeric carbon [16, 31, 32]. This was done most recently by Freitas [33] for a series of 2-substituted tetrahydropyrans, who found that NBO analysis provides a coherent framework in which to analyze the results, as we do in this report. Freitas points out that steric, electrostatic, and hyperconjugative effects play a role whether or not the anomeric effect is observed in his calculations, and sometimes the hypercongjugative effect is not the dominate factor in the preference of one isomer over the other. 

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