Cyclohexane stereoisomerism

The hydrogenation of 2 methyl(methylene)cyclohexane is an example of a stereo selective reaction meaning one m which stereoisomeric products are formed m unequal amounts from a single starting material (Section 5 11)... 

Depending on the orientation of substituents, the disubstituted cyclohexanes can be either equatorial-axial or diequatorial. This is illustrated with 2-methyl cyclohexanol. It exists in two stereoisomeric forms and each of which has two conformations which are readily interconvertible. 

Effects of similar magnitudes have been observed in stereoisomeric cyclohexanes with oxygen functions, e.g., in l-fm-butyl-4-methoxycyclohexane. In the isomeric bicyclo[2.2.1]heptan-2-ols the 170-chemical shift difference is even larger63. 

The cis-trans isomerism of cyclohexane derivatives (Section 5-1 A) is complicated by conformational isomerism. For example, 4-tert-butylcyclohexyl chloride theoretically could exist in four stereoisomeric chair forms, 1, 2, 3, and 4. 

When more than one group is attached to cyclohexane, the stereoisomeric possibilities increase. First, structural isomers of the 1,2, 1,3, or 1,4 type are possible. 

When comparing two stereoisomeric cyclohexane derivatives, the more stable stereoisomer is the one with the greater number of its substituents in equatorial orientations. Rewrite the structures as chair conformations to see which substituents are axial and which are equatorial. 

In this experiment, we will further investigate stereoisomerism by examining a cyclic system, cyclohexane, and several acyclic tetrahedral carbon systems. The latter possess more subtle characteristics as a result of the spatial arrangement of the component atoms. We will do this by building models of representative organic molecules, then studying their properties. 

Reduction of 2,2-dimethyl-cyclohexane-1,3-dione (50) with Baker s yeast gave alcohol (ee 98.3%) whose tetrahydropyranyl derivative on methoxycarbonylation produced (51) quantitatively. Michael addition of (51) with methyl vinyl ketone followed by heating the adduct under reflux with pyrrolidine in benzene yielded (52) in 85% yield as stereoisomeric mixture whose separation presented problems. In order to eliminate the complexity due to a chiral center in tetrahydropyranyl protective group, deprotection of (52) was achieved by treatment with p-toluenesulphonic acid in methanol. The product obtained was a mixture of the lactone (53) and hydroxy ester (54). Probably the stereoisomer of... 

Use analysis of conformations to determine the relative stabilities of stereoisomeric cyclohexane derivatives. (Problems 6.23, 6.24, and 6.26)... 

Exo and endo oxiranes of bicyclo[2.2.1]heptane can be well differentiated on the basis of their spectra.The effect of the molecular asymmetry on the chemical shift of the carbon in 0- and A -glycidyl compounds has been investigated. In the study of stereoisomeric epoxyspirocyclohexane derivatives, the effects of the equatorial and axial oxiranes have been observed on the carbon atoms of the cyclohexane ring. ... 

Utley and coworkers [31,32] reported that the stereoisomeric ratio cis/trans) of 1,4-disubstituted cyclohexanes formed by the hydrogenation of the corresponding activated cyclohexenes at a mercury electrode depended on solvents and proton sources, and discussed the stereochemical mechanism in detail. On the other hand, according to Lessard and coworkers [33,34], the hydrogenation at a Raney nickel electrode always provides the trans isomers in excess. 

PROBLEM 3.9 Based on what you know about disubstituted cyclohexanes, which of the following two stereoisomeric 1,3,5-trimethylcyclohexanes would you expect to be more stable ... 

Stereoisomeric dibenzazepines 577 and 578 were obtained when a cyclohexane solution of the enamine 576 was photoirradiated (78JOC4420). Irradiation of a dilute solution produced the diastereomeric photoadducts 577 and 578 in addition to the enone 579 (78JOC4420) (Scheme 122). 

The mechanism of bromination is discussed more fully in Section 5.3, but the fundamental cause of the stereospecificity is the involvement of the positively charged bromonium ion intermediate. The bromonium ion is opened by an anti approach of the bromide, leading to net anti addition. Entry 1 in Scheme 2.7 illustrates this behavior. Stereoisomeric products are obtained from the E- and Z-isomers, both as the result of anti addition. Stereospecificity is diminished or lost when the bromonium ion is not the only intermediate in the reaction. Entry 2 in Scheme 2.7 shows this behavior for cw-stilbene in nitromethane, where most of the product is the result of syn addition. The addition is anti in less polar solvents such as cyclohexane or carbon tetrachloride. The loss of anti stereospecificity is the result of a change in mechanism. The polar solvent permits formation of a carbocation intermediate. If the bromonium ion can open to a carbocation, a mixture of syn and anti products is formed. In the stilbene case, the more stable anti product is formed. Some loss of stereospecificity is also observed with 1-phenylpropene, where the phenyl group provides stabilization of an open carbocation intermediate. Part of the product from both isomers is the result of syn addition. 

Entry 6 is one of several examples demonstrating enantioselectivity for both the cis and trans isomers of heptane-2,3-epoxide. Entry 7 shows the kinetic resolution of an exocyclic cyclohexane epoxide. The two stereoisomeric monomethyl analogs were only partially resolved and the 3-methyl isomer showed no enantioselectivity. This shows that the steric or hydrophobic effect of the dimethyl substiments is critical for selective binding. 

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