Conformational equilibria in saturated

Nitrogen heterocycles see also N-Heterocyclic compounds) advances in amination of, 49, 117 aromatic six-membered. regioselective substitution in, 44, 199 conformational equilibria in saturated six-membered rings, 36, I covalent hydration in, 20, 117 photochemistry of, 30, 239 reactions of acetylenecarboxylic esters with, 23, 263... 

Saturated aliphatic amines are similar to the alkanes, the main differences being the short C—N bond (C—C 1.523 A, C—N 1.450 A) and the replacement of one substituent by a lone pair of electrons (LP). The pyramidal structure of acyclic amines is quite labile, with barriers to inversion in the range 4-8 kcal mol (Rauk et al, 1971). The problem of the steric requirements of the lone pair has been much discussed in relation to conformational equilibria in saturated six-membered N-heterocycIes (Lambert and Feather-man, 1975 Blackburne et al, 1975). 

PE spectroscopy has also been applied to the study of the conformational equilibria of saturated heterocyclic six-membered rings, and in particular of hexahydropyridazines. The... 

The general principle that the protons in the alicyclic six-membered rings of saturated six-five-, six-six-, and six-seven-membered fused ring systems give the smallest tJ values (ca. 8-11 Hz) for 4a- and 8a-H in the Z-in form, intermediate values (ca. 20 Hz) in the Z-out form for the ds-fused isomers, and clearly higher values (ca. 25-30 Hz) for the trans-fused isomers [71JCS(C)3222] is applied next to estimate the conformational equilibria in 4-oxo-(thioxo) and 2,4-dioxo(thioxo) derivatives. 

One of the present authors has extensively used the dipole-moment method to calculate conformational equilibria of saturated heterocycles. In hindsight this has been a frustrating experience not so much because of the assumptions and approximations that must be made, but because the results in some cases are in good agreement with those derived from other methods, whereas for other groups of compounds the dipole-moment conclusions are clearly incorrect. In this discussion we first discuss the method, using piperidines as an example, and then attempt to assess its areas of applicability and causes of failure. 

Conformational Equilibria in Nitrogen-Containing Saturated Six-Membered Rings... 

Conformational equilibria in nitrogen-containing saturated six-membered rings, 36, 1... 

Condensed heterocyclo[n,m-n, b, or cjquinazolines. 52, I Condensed 4-thiazolidinones, 49, I Condensed thiophene systems, tetra- and pentacyclic, 32, 127 Condensed 1,2,4-triazines I. Fused to heterocycles with three-, four-, and five-membered rings, 59, 39 Condensed l,2,4-triazolo[3,4-z) heterocycles, synthesis, 49, 277 Conformational equilibria in nitrogen-containing saturated six-membered rings, 36, I... 

Crabb, T. A., Katritzky, A. R., Conformational Equilibria in Nitrogen-Containing Saturated Six-Membered Rings,... 

NMR spectral measurements have been applied to the investigation of conformational equilibria in many saturated systems. Because of the sensitivity of the P nucleus to NMR detection, P NMR is of great practical importance, providing valuable information supporting the H and C spectral measurements. 

The reason for this is most likely due to the fact that the saturated five-membered rings are very flexible due to pseudorotation and are involved in complex conformational equilibria in which the individual conformations were very difficult if not impossible to identify, whereas the pyranoid rings are rather stiff and consequently have much simpler conformational equilibria. 

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