Chair conformation, conformational isomers

Viewed another way, if the axial-equatorial energy difference is mainly a function of steric bulk, then it might be used to assess the relative size of various groups. That is, if the energy difference between the two chair conformational isomers of a monosubstituted cyclohexane were measured, it might serve as a... 

Figure 19. Chair conformational isomers of 2-oxo-p-menthanol, corresponding octant projection diagrams, predicted and observed n - x Cotton effects. From reference 20.

Figure 4.22. A representation of the two chair conformational isomers of methylcyclohexane (a) methyl group axial (b) methyl group equatorial.

All the ring substituents m p D glucopyranose are equatorial m the most stable chair conformation Only the anomenc hydroxyl group is axial m the a isomer all the other substituents are equatorial... 

Certain dimethylcycloalkanes contain a plane of symmetry. For example, both chair conformers of crs-l,3-dimethylcyclohexane possess a plane of symmetry bisecting the molecule through C-2 and C-5. The trans isomer does not have any element of symm and is chiral. 

For example, c/s-4-t-butyleyelohexyl bromide undergoes E2 elimination at a rate about 500 times greater than the tram isomer because only the cis isomer permits anti elimination from the favored chair conformation. ... 

A wyo-Inositol, one of the isomers of 1,2,3,4,5,6-hexahydroxycyclohexane, acts as a growth factor in both animals and microorganisms. Draw the most stable chair conformation of myo-inositol. 

Ketones react with alcohols to yield products called acetals. Why does the all-cis isomer of 4- cvf-butyl-l13-cyclohexanediol react readily with acetone and an acid cataty st to form an acetal while other stereoisomers do not react In formulating your answer, draw the more stable chair conformations of all four stereoisomers and the product acetal. Use molecular models for help. 

Problem 11.19 Which isomer would you expect to undergo E2 elimination faster, f/uus-l-bromo-4-fr7t-butyJcyclohexane or ds-l-bromo-4-tert-butylcyclohexane Draw each mole-, cule in its more stable chair conformation, and explain your answer. 

There are eight diastereomers of 1,2,3,4,5,6-hexachlorocyclohexane. Draw each in its more stable chair conformation. One isomer loses HC1 in an E2 reaction nearly 1000 times more slowly than the others. Which isomer reacts so slowly, and why ... 

Steric hindrance also seems to provide information as to whether yn-aldolates or cmb-adducts predominate in the equilibrium. Considering the chair conformations of the syn- and owz -aldolates, the latter seems to be thermodynamically more stable since it avoids the axial position of the substituent Y which it occupies in the s> -isomer. Indeed, the azzb-diastereomcr is favored in most reversible aldol additions ... 

N.m.r. spectroscopy has played an important part in determining the stereochemistry of the 1,3-dioxaphosphorinanes (52). The presence of the saturated six-membered ring means that there are usually conformational effects to be unravelled before configurational assignments can be made. The chair conformation is generally dominant. Phosphorus substituents which exhibit shielding effects show that in many P " phosphorinanes this substituent occupies an axial position and Sis( has been used to establish the equatorial conformation of a t-butyl substituent at C(5). Even in P" derivatives the isomer possessing the bulkiest P-substituent in an axial... 

Other studies have also established the preference of the chair conformation with the oxygen in the axial position the rationale for this preference is different from the attractive interaction between the sulfoxide oxygen and the syn-axial hydrogens proposed previously . Rather, a repulsion effect is advocated the equatorial oxygen is squeezed between four vicinal hydrogens, while there are only two corresponding repulsions if it is in the axial position. The correlation between the predicted and observed conformational/orientational preferences in 3,3-dimethylthiane oxide (e.g., equatorial preference in the chair conformation) corroborates this interpretation. The axial preferences of the sulfur-oxygen bond in the thiane oxide is reversed in 3,3-dimethylthiane oxide because of the syn-axial interaction. 4,4-Dimethylthiane oxide, however, maintains a predominance of the axial isomers as deduced from the analysis of NMR data . ... 

Size and conformational effects such as those described above have been exploited in a wide range of separation procedures. In a patented method for the separation of cis- and tranj-l-methyl-4-uohexylcyclohexane carboxylic acids only the tram isomer forms a thiourea inclusion compound 831. In solution the cis isomer always has an axial, equatorial arrangement in the chair conformation, while the trans isomer can exist as the diequatorial or diaxial chair conformers. It is not reported which of these is favoured or how the guest packs in the canals. 

The study of the transformation of 5-alkoxyalkyl-5-alkyl-l,3-dioxanes provided the first experimental evidence that the conformation of the reactant molecule plays a determining role regarding the direction of the catalytic reaction. The reason for the differing reaction directions clearly indicates that the conformers adsorb in different ways.32 The 5-alkoxyethyl isomers can exist in their chair conformations (1 and 2 in Scheme 4.12). The main reaction of the adsorbed surface species is the formation of an ester (3) by the rupture of the C-O bond in the ring. In one of the two isomers (1) the R2-0 group can also be adsorbed and this adsorption leads to a smaller ester molecule (4). 

In several cases (including the present example) where diastereoiso-meric aldol products are possible, there is a preference for the formation of the tAreo-diastereoisomer. This stereochemical preference presumably arises because the six-membered cyclic zinc chelate of the ffereo-isomer can exist in a chair conformation with both substituents in equatorial positions. Table I summarizes the results obtained in several aldol condensations performed by the present procedure. 

Cyclic Esters of Phosphorous Acid.—A large number of 2-substituted-4-methyl-l,3,2-dioxaphospholans (88) have been prepared and their stereochemistry and conformations investigated by 1H and 31P n.m.r.69 Unlike the corresponding 1,3-dioxans, the tra/w-isomer (88a) is favoured in all cases, and each isomer is best described in terms of two rapidly equilibrating half-chair conformers with the 4-alkyl group pseudo-axial or pseudo-equatorial. 

The structures of the salts (131)168 and (132),159 a chelated ylide,160 and the insecticide coroxon161 have been determined. The structures of the cis- and trans-isomers of the cyclic phosphonamide (133) differed in that the cw-isomer has a half-chair conformation whereas the trans-isomer has an envelope conformation. Both compounds had hydrogen bonds of approximately 269 pm.162 The phosphadiazane (134)... 

---