Conformation of heterocycles

We noted earlier that bonds around nitrogen and oxygen atoms occupied some of the tetrahedral array, lone pairs taking up other orbitals. This means that we can use essentially the same basic principles for predicting the shape and conformation of heterocycles as we have used for carbocycles. A substituent... 

Although the conformations of heterocycles are governed by the same principles that apply to carbocycles, where appropriate, additional factors, such as the anomeric effect, can have a significant influence upon the energies of the isomers in equilibrium. 

CONFORMATIONS OF HETEROCYCLIC EIGHT-MEMBERED RINGS 5.19.9.1 General Considerations... 

This section presents X-ray diffraction, dynamic NMR and theoretical data on the conformations of heterocyclic eight-membered rings. Ideally, it would be desirable to know the whole conformational potential energy surface of the ground electronic state of a molecule. In practice, the lowest (and perhaps the second lowest) energy conformation can be more or less characterized, and some conformational energy barriers can be determined and calculated. 

Use of NMR Spectroscopy to Study the Steric Structure of Hydrogenated Heterocycles II, Configuration and Conformations of Heterocycles Yu. Yu. Samitov, Khim. Geterotsikl. Soedin., 1980, 1443-1470. 

Conformations of heterocyclic fragments of 1,3,5,2,4-trithiadiazepines and 1,3,5,2,4,6-trithiatriazepines are planar. Cyclic polysulfides possess as a rule in the chair conformation both in crystal (Section 13.17.5.4) and in solution... 

The most complete picture of conformational flexibility of pyrimidine rings in nucleic acid bases has been provided by molecular dynamics study of isolated molecules using ab initio Carr-Parinello method [45]. According to these studies, the population of planar conformation of heterocycle does not exceed 20% for thymine, cytosine, and guanine and amounts to about 30% for adenine (Table 21.4). These values are considerably smaller as compared to estimations based on vibrational frequencies mentioned above. Such difference is quite natural because in the case of vibrational analysis, only the lowest ring out-of-plane normal mode is considered. However, there are also smaller contributions of the other ring out-of-plane vibrations not included in this analysis. Therefore, such estimation should be considered as an upper limit for assessment of population of planar conformation of ring. 

Conformations of heterocyclic derivatives of hydroxylamine 81T849. Electron spectroscopy for chemical evaluation (ESCA) investigation of... 

The x-ray investigation of (27) demonstrated that the conformation of heterocycle may be described as a plane of Cj, Sj, Sj (with 0.05-0.7 A atomic deviations) with S3, S4, S5 inclined at 79°. This seven-membered heterocycle is highly distorted <83JOUi33>. 

CONFORMATIONS OF HETEROCYCLIC RINGS 10.4.1. Four-Membered Rings... 

The structures and conformations of heterocycles have been well-studied computational ) and in some cases conformational analysis has been used to predict reactivity. Although not central to this article, many properties of the equilibrium structures of heterocycles have been calculated (such as dipole moments, infrared and Raman spectroscopy, NMR spectroscopy, hyperpolarizabilities, electronic transitions, nuclear quadrupole coupling constants, basicities and acidities, HOMO-LUMO gaps, and simple relative energies) and used to infer information about possible reaction mechanisms. 

Nuclear magnetic resonance was applied to study conformational dynamics of a-tocopherol in solution and in model membranes. The rapid equilibrium between two or more conformers of heterocyclic ring was observed in H NMR spectra. 

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