Introduction to Heterocyclic Chemistry 13 the net result is that the O-C bond is strengthened and shortened,where- as the C-Y bond is weakened and lengthened.However,for other Y atoms (e.g.oxygen or nitrogen)the anomeric effect can operate in both directions,i.e.Y can be a donor as well as an acceptor. Anomeric effects are cumulative,and can cause a potentially flexible ring to adjust to a more rigid conformation in order to maximize the overlap of suitable lone pair and o*orbitals.It has been particularly instructive in explaining 'anomalous'preferences for substituent orien- tations in tetrahydropyrans and related compounds.In the case of 2- compounds and a full discussion methoxytetrahydr opyran,for exam ple.the axial conformer is three times the ce forr more populated than the equatorial form(Scheme 1.2) matons where the best donor lone pair.or bond. axial (75% equatorial (25 2-Methoxytetrahydropyran(Y=OMe) Scheme 1.2 Heteroatom Replacement Nitrogen and oxygen are found in level 2 of the Periodic Table,and a further alteration in ring topology may arise when the heteroatom is replaced by an element from a lower level.Here,apart from an increase the shape of the molecule,it can also modify the chemical properties. Worked Problem 1.1 Q Which of the following heterocycles conform to the Huckel rule (4n+2)for aromaticity:(i)furan;(ii)1H-azepine;(iii)pyrylium per- chlorate [chlorate(VID]: Furan 1H-Azepine Pyrylium perchlorateIntroduction to Heterocyclic Chemistry 13 the net result is that the 0-C bond is strengthened and shortened, where￾as the C-Y bond is weakened and lengthened. However, for other Y atoms (e.g. oxygen or nitrogen) the anomeric effect can operate in both directions, i.e. Y can be a donor as well as an acceptor. Anomeric effects are cumulative, and can cause a potentially flexible ring to adjust to a more rigid conformation in order to maximize the overlap of suitable lone pair and o* orbitals. It has been particularly instructive in explaining ‘anomalous’ preferences for substituent mien￾The simply restricted to ring effect is not tations in tetrahydropyrans and related compounds, In the case of 2- compounds and a full discussion methoxytetrahydropyran, for example, the axial conformer is three times ~e~~~o~~e~ (?~~~~‘;e~nce for more populated than the equatorial form (Scheme 1.2). ?Me OMe axial (75%) equatorial (25%) 2-Methoxytetrahydropyran (Y = OMe) Heteroatom Replacement Nitrogen and oxygen are found in level 2 of the Periodic Table, and a further alteration in ring topology may arise when the heteroatom is replaced by an element from a lower level. Here, apart from an increase in atomic diameter, the replacement element may use a hybridization state different than that of the earlier elements. Not only can this affect the shape of the molecule, it can also modify the chemical properties. conformations where the best donor lone pair, or bond, is orientated antiperiplanar to the best acceptor bond’.g Scheme 1.2