Placing Groups on the Chair

Before we can get started, we need to remember what the dashes and wedges mean in the hexagon-style drawing. Remember that wedges are coming out toward you, and dashes are going back away from you. So, at each of the six carbon atoms 

Now let s apply the same terminology to the groups on a chair. Each carbon atom has two groups, one pointing above the ring (up) and one pointing below the ring (down)  

You can do this for every carbon (and you should try on the drawing above), and you will see that each carbon has two groups (up and down). It is important to realize that there is no correlation between up/down and axial/equatorial. Look at the drawing above. For one of the carbon atoms, the up position is axial. For the other carbon atom showing its groups, the up position is equatorial. Take a close look at the two equatorial positions shown above. One of them is up and the other is down. 

Now we are ready to draw a chair when we are given a hexagon. Let s work through the example that we started with  

We begin by placing numbers. These are not the same as the numbers that we used in naming compounds. These are just numbers that help us draw the chair with the groups in the right place. It does not matter where we start or which direction we go in, so let s just say that we will always start at the top and go clockwise  

Now we need to see how to draw a chair with proper placement of the groups when we are given a regular hexagon-style drawing  

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A more striking example of the influence of conformation on the reaction outcome is seen in the nitrous acid deamination of 2-aminocyclohexanols which takes place by rearrangement of a group on the carbinol carbon that is anti to the developing carbocation. The deamination reaction is very fast and the products reflect the population of the chair conformers. The trans isomer exists mainly in the diequatorial conformer thus the only group anti to the amino... 

Piperidinecarboxylic acids, analogs of IV-acetylneuraminic acid, were synthesized by the re-gioselective intramolecular cycloadditions of the suitably substituted optically active azidoalke-nes 133,134. The best results were obtained when the substrate contained protected hydroxy functions. In the cyclization of tert-butyl 2-azido-6-heptenoates 17133, the diastereoselectivity is rationalized by analyzing the steric interactions in the transition states leading to the cycloadducts from different conformations of the azidoalkene. The cycloaddition took place on the / e-face of the alkenyl group in the chair transition state containing all axial substituents. 

There is another aspect to the question of the reactivity of the carbonyl group in r ck)hexanone. This has to do with the preference for approach of reactants from the axial ir equatorial direction. The chair conformation of cyclohexanone places the carbonyl coup in an unsynunetrical environment. It is observed that small nucleophiles prefer to roach the carbonyl group of cyclohexanone from the axial direction even though this is 1 more sterically restricted approach than from the equatorial side." How do the ctfcnaices in the C—C bonds (on the axial side) as opposed to the C—H bonds (on the equatorial side) influence the reactivity of cyclohexanone ... 

Now we know where to put in the groups. Br is on the carbon numbered 1, and Cl is on the carbon numbered 3. This brings us to the up/down system. Draw the chair, showing both positions (up and down) at each of the carbon atoms where we need to place a group ... 

The stereochemical outcome of the reactions shown in Figure B5.6 is accounted for by the chair-like transition states shown in Figure B5.7. These place the aldehyde substituent in an equatorial position rather than an axial position, because in the latter position the substituent would give rise to unfavourable 1,3-diaxial interactions between itself and one of the oxygen substituents on the boron atom. Note that whether or not the methyl group occupies an equatorial or an axial position in the transition state is predefined by the double bond geometry of the allylborane. 

Most molecules tend to favor one conformer over the others based on the stereochemistry of the particular monosaccharide and the steric bulk of the groups that are appended to it. For example, most aldohexoses prefer the chair conformation that places the bulky C5 hydroxymethyl group in the equatorial position. Having said that, the energy barrier between the two possible chair conformations is... 

An E2 elimination can take place on this chair conformation only if the proton and the leaving group can get into a trans-diaxial arrangement. Figure 7-9 shows the E2 dehy-drohalogenation of bromocyclohexane. The molecule must flip into the chair conformation with the bromine atom axial before elimination can occur. 

To confirm the inclusion mode for a sec. amine, the 1 1 piperidine 4 (n = 1) complex was isolated and its molecular structure was determined by X-ray analysis (Fig. 10) [22], The chromophore is planar (except the o-nitro group) within 0.1 A, to which the cyclic polyether ring extends perpendicularly. The protonated piperidine having a chair form structure is placed on the crown ring and stabilized by very short N+ —H—O- (2.65 A) and N+ —H—O (2.93 A) hydrogen bonds, and two N + —O ion-dipole interactions (3.17 and 3.16 A). The remaining ether oxygen, 0(2), seems to be ineffective on the complexation. 

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