The contribution of non-classical CHax⋯OC hydrogen bonds to the anomeric effect in fluoro and oxa-methoxycyclohexanes

In this theory study the dominance of non-classical 1,3-diaxial CHax⋯OC hydrogen bonds (NCHBs) dictating anomeric effects in fluorinated methoxycyclohexanes and 2-methoxytetrahydropyrans is demonstrated, a phenomenon which is most often described as a consequence of hyperconjugation.

Heteroatom Substitution at Amide Nitrogen—Resonance Reduction and HERON Reactions of Anomeric Amides

This review describes how resonance in amides is greatly affected upon substitution at nitrogen by two electronegative atoms. Nitrogen becomes strongly pyramidal and resonance stabilisation, evaluated computationally, can be reduced to as little as 50% that of N,N-dimethylacetamide. However, this occurs without significant twisting about the amide bond, which is borne out both experimentally and theoretically. In certain configurations, reduced resonance and pronounced anomeric effects between heteroatom substituents are instrumental in driving the HERON (Heteroatom Rearrangement On Nitrogen) reaction, in which the more electronegative atom migrates from nitrogen to the carbonyl carbon in concert with heterolysis of the amide bond, to generate acyl derivatives and heteroatom-substituted nitrenes. In other cases the anomeric effect facilitates SN1 and SN2 reactivity at the amide nitrogen.

Heteroatom Substitution at Amide Nitrogen — Resonance Reduction and HERON Reactions of Anomeric Amides

This review describes how resonance in amides is greatly affected upon substitution at nitrogen by two electronegative atoms.  Nitrogen becomes strongly pyramidal and resonance stabilisation, evaluated computationally, can be reduced to as little as 50% that of N,N-dimethylacetamide.  However, this occurs without significant twisting about the amide bond, which is borne out both experimentally and theoretically.  In certain configurations, reduced resonance and pronounced anomeric effects between heteroatom substituents are instrumental in driving the HERON (Heteroatom Rearrangement On Nitrogen)† reaction, in which the more electronegative atom migrates from nitrogen to the carbonyl carbon in concert with heterolysis of the amide bond, to generate acyl derivatives and heteroatom-substituted nitrenes.  In other cases the anomeric effect facilitates S­N1 and SN2 reactivity at the amide nitrogen.

Ab initio modelling of the anomeric and exo anomeric effects in 2-methoxytetrahydropyran and 2-methoxythiane corrected for intramolecular BSSE

Oxygen substitution in 2-methoxytetrahydropyran by sulphur in 2-methoxythiane approximately doubles the anomeric effect, which slows down enzymatic processing of 2-methoxythiane.