Alternative chair conformation

A A trisubstituted cyclohexane with three substituents—red, yellow, and blue—undergoes a ring-flip to its alternative chair conformation. Identify each substituent as axial or equatorial, and show the positions occupied bv the three substituents in the ring-flipped form. 

The 3-hydroxyl group, which was shown to be uniquely important in the sweetness of sugars in the Cx conformation,does not seem to have the same importance in the alternative chair conformation. For example,... 

The alternative chair conformation, should we draw it instead, would be less favoured than that shown because of the increased number of axial substituents. The conformation of o-glucose is the easily remembered one, in that all the substituents are equatorial. 

N.m.r. evidence has indicated that vitamin D3 exists in solution as a mixture of the alternative chair conformers in ring A. X-Ray data now show that the same is true of the crystalline vitamin, which comprises an equimolar ratio of the con-formers with 3-OH equatorial and axial, respectively. Strong hydrogen-bonding between hydroxy-groups results in an extended helix of molecules about an axis... 

X-ray crystallographic studies have shown that crystalline D-glucose as commonly isolated exists in the a-D-glucopyranose form. Furthermore the stable chair conformation (1), in which the hydroxyl groups on C-2, C-3 and C-4, and the hydroxymethyl group on C-5 are equatorial, is preferred to the alternative chair conformation (6) in which these groups occupy axial positions. 

In the limited number of examples where the trans- 1,2-diols would be diaxial if the compound adopted a chair conformation, mixtures are also usually obtained. To form dibutylstannylene acetals, conformational changes to skew, boat, or alternative chair conformations are required. Conversion to the boat is easy for l,6-anhydro-/3-D-glucopyranose149 and methyl a-D-idopyranoside, but may be more difficult for methyl 4,6-0-benzylidene-a-D-altropyranoside. 

It should be pointed out, however, that in some cases alternative chair conformations may be accessible in which the 1,3-ftanj-axial condition is removed and litde hindrance to normal 5 n2 replacement... 

The spectrum of the radical (72), from the oxidation of dioxan, exhibits splitting from the a-proton (doublet), the two y-protons (small triplet), and the two j8-protons, which give rise to a triplet in which the central line is broadened (Fig. 5) (Dixon and Norman, 1964a). If flipping between (72) and the alternative chair conformer were very fast, the two /S-protons would be averaged to give a 1 2 1 triplet with equal line-... 

In X, all the chlorine atoms are equatorial, so there is no hydrogen attached to an adjacent carbon atom in the anti conformation (the ring bonds are occupying this 180° dihedral angle orientation). Thus, with no favourable anti conformation available, reaction will be slow. The situation would not be improved if the molecule were to flip to the alternative chair conformation with all Cl atoms axial, since here all the anti positions would be occupied by chlorine atoms. Y has two axial chlorine atoms each with an ad jacent axial hydrogen atom in an anti conformation, so reaction is faster. Z has two axial chlorine atoms, each with two adjacent axial hydrogen atoms in an anti conformation, so there are more possibilities for anti elimination, and reaction is faster still. 

The favored conformations of a number of peracetylated 1-thioal-dopyranose derivatives have been examined by use of 220-MHz n.m.r. spectroscopy. For the D-aldohexose derivatives investigated, the Ci(D) conformation was strongly favored, most probably because of the large steric effect of the acetoxymethyl group with the D-aldopentoses, the possibility that the alternative chair conformation was present in appreciable proportion at equilibrium with the (apparently favored) forms having the 1-SAc group equatorially attached could not be discounted the conformational equilibria were measured in a subsequent study (see p. 91). ... 

Draw the structure of ambroxol in the alternative chair conformation. Which of the two conformations is more stable ... 

We can also draw the cis and trans isomers of 1,4-dimethylcyclohexane as nonplanar chair conformations. In working with alternative chair conformations, it is helpful to remember that all groups axial on one chair are equatorial in the alternative chair, and vice versa. In one chair conformation of tran l, 4-dimethylcyclohexane, the two methyl groups are axial in the alternative chair conformation, they are equatorial. Of these chair conformations, the one with both methyls equatorial is considerably more stable. 

The alternative chair conformations of os-1,4-dimethylcyclohexane are of equal energy. In each chair conformation, one methyl group is equatorial and the other is axial. 

Following is a planar hexagon representation of one isomer of 1,2,4-trimethylcyclohexane. Draw the alternative chair conformations of this compound, and state which is the more stable. 

Draw the alternative chair conformations for the cis and trans isomers of 1,2-dimethylcyclohexane, 1,3-dimethylcyclohexane, and 1,4-dimethylcyclohex-ane. (See Examples 3.10, 3.11)... 

Draw alternative chair conformations for each substi- 3.48 How many six-membered rings exist in adamantane ... 

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