Conformation heterocyclic compounds

Saturated five-membered heterocyclic compounds are non-planar, existing in half-chair or envelope conformations. The far-IR spectra of THE and 1,3-dioxolane (127) show both to have barriers of ca. 0.42 kJ moP ... 

The NMR spectra of heterocyclic compounds with seven or more ring members are as diverse as the shape, size and degree of unsaturation of the compounds. NMR is perhaps the most important physical method to ascertain the structure, especially the conformational statics and dynamics, of large heterocycles. Proton-proton coupling constants provide a wealth of data on the shape of the molecules, while chemical shift data, heteroatom-proton coupling constants and heteronuclear spectra give information of the electronic structure. Details are found in Chapters 5.16-5.22. Some data on seven-membered rings are included in Table 10. 

These observations indicated that an intermolecular double condensation to give a bis N-(methylene-4-oxocoumarinyl)-l,4 aromatic diamine had occurred. Data from the elemental analysis indicated that the calculated and observed values were within the acceptable limits ( 0.4%) and in conformity with the assigned structure. In the addition of molar equivalents of 1,4-aromatic binucleophilic compounds to compound 72 we did not observe any heterocyclic compounds resulting from the further intermolecular nucleophilic attack on the single condensation product. Since the condensation of 3-(dimethylaminomethylene)-chromane-2,4-dione with aromatic binucleophilic compounds is the only route to the new coumarinic compounds, this represents a useful synthetic method. 

Romers, C., Geometry and Conformational Properties of Some Five- and Six-Membered Heterocyclic Compounds Containing Oxygen and Sulfur, 4, 39. 

See G.F. Riddell, "The Conformational Analysis of Heterocyclic Compounds", pags. 139-140, Academic Press, London, 1980. 

B-80MI20401 F. G. Ridell The Conformational Analysis of Heterocyclic Compounds ,... 

A group of Italian workers from Modena, led by Professor Taddei, has reviewed published work on the conformations of acyl groups in heterocyclic compounds, including both C-acyl and N-acyl derivatives. The first recent review of the basicity and acidity of azoles, covering both gas-phase and solution measurements, is presented by a group of Spanish workers (Catalan et al.). H. Weber has summarized the considerable recent progress in oxidative transformations of heteroaromatic iminium salts. 

Heterocyclic These are compounds having at least one hetero atom (any other atom but carbon, e.g. O, N and S) within the ring, and conforming to Hiickel s rule. The aromaticity of heterocyclic compounds, e.g. pyridine and pyrrole, can be explained as follows. 

Plots of Aae against the percentage of lone-pair axial conformer (estimated by dipole-moment measurements) for some monocyclic heterocyclic compounds with differing N-alkyl substitution showed no correlation.44 This is to be expected because the nature of the N-alkyl substituent also affects A,e40 Table III). 

Steric effects can alter the reactivity of a heterocyclic compound as compared to its aliphatic analogue. For example, piperidine is less sterically hindered and more strongly nucleophilic than diethylamine. Conformations of these compounds are discussed elsewhere (Section 2.2.4.3). 

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