Equatorial heteroatom substituted

NMR and CD spectroscopy have been used to determine the equatorial axial equilibrium in /1-heteroatom-substituted cyclohexanones89. 

Four- and five-membered ring systems will preferentially span the axial-equatorial sites in a trigonal bipyramid, even at the expense of forcing more electronegative substituents to occupy the electronically less favourable sites. As a result of -interaction, the axial-equatorial placement is relatively more favourable for ring systems with heteroatoms substituted at phosphorus than for those with P-C bonds. 

The incorporation of heteroatoms can result in stereoelectronic effects that have a pronounced effect on conformation and, ultimately, on reactivity. It is known from numerous examples in carbohydrate chemistry that pyranose sugars substituted with an electron-withdrawing group such as halogen or alkoxy at C-1 are often more stable when the substituent has an axial, rather than an equatorial, orientation. This tendency is not limited to carbohydrates but carries over to simpler ring systems such as 2-substituted tetrahydropyrans. The phenomenon is known as the anomeric ect, because it involves a substituent at the anomeric position in carbohydrate pyranose rings. Scheme 3.1 lists... 

Experimentally, it has been shown that the introduction of a heteroatom such as oxygen into the 4-position of piperidine has no appreciable influence on the conformational equilibrium348. Similarly, infrared overtone measurements suggest that in tetrahydro-1,2 oxazine the NH-equatorial conformer predominates349. On the other hand, many piperidines substituted by heteroatoms in the 3 position are found to exhibit a NR axial preference. For example, the preferred conformation for the molecules shown below is NR axial350,35... 

Spherical polar coordinates are used for conformational representation of pyranose rings in the C-P system. Unlike the free pseudorotation of cyclopentane, the stable conformations of cyclohexane conformers are in deeper energy wells. Even simong the (less stable) equatorial (6 = 90 ) forms, pseudorotation is somewhat hindered. Substitutions of heteroatoms in the ring and additions of hydroxylic or other exocyclic substituents further stabilize or destabilize other conformers compared to cyclohexane. A conformational analysis of an iduronate ring has been reported based on variation of < ) and 0 (28), and a study of the glucopyranose ring... 

In the heteroatom O-substituted cyclohexane 22 equatorial attack, to give 23a, is stereoelec-tronically even more favored than in the analogous carbocyclic case [d.r. (eq/ax) 85 15]81. 

We have coloured the lone pairs green or black according to whether they are axial or equatorial, but you can also consider the colour coding in a different way black lone pairs are parallel with C-C or C-heteroatom bonds in the ring green lone pairs are parallel with axial C-H bonds outside the ring, or, if the ring has substituents, with the bonds to those substituents. This substituted tetrahydropyran illustrates... 

In cyclohexane chemistry, a t-butyl group that occupies an equatorial position can function as a holding group. Substitution of a t-butyl group in the 4-position relative to the heteroatom has been successfully utilized to prepare conformationally stable derivatives of both sila-... 

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