Conformation trans diaxial

The marked ANTI stereoselectivity observed with cyclohexyl systems (see above) reflects the ability to achieve, and the very marked preference to eliminate from, the so-called trans-diaxial conformation (34) ... 

Thus of the geometrical isomers of hexachlorocyclohexane, C6H6C16, one is found to undergo elimination of HC1 at a rate slower, by a factor of 7-24 x 103, than any of the others it is found to be the one (35) that cannot assume the above trans-diaxial conformation. 

The relative configurations and predominant conformations of [l,2,3]oxathiazino[4,3- ]isoquinoline derivatives 177-182 were studied by means of H and 13C NMR spectroscopy with the applications of DNOE, 2D HSC, and 2D-COSY measurements. Their stereostructure includes an equilibrium between the conformers cis-1, trans and cis-2. In the /ra -structure, the B/C rings are trans-ane-WatcA with H-l lb and the N-5 lone electron pair trans-diaxial. In the cis structures, the hetero-rings are air-anellated in the cis-1 conformation C-l is in inside position, while in cis-2... 

The opening of the epoxide in the cij-decalin 24 by acetic acid leads exclusively to the hydroxyacetate 25 (through a kinetically controlled rrani-diaxial opening) rather than to the wanted diastereomer 26 (c/ the stereochemistry of the "southern" part of reserpine). To obtain the correct diastereomer the epoxy-lactone 22 is first formed (Scheme 8.6). Thus the conformation of the cij-decalin system, and therefore that of the substituents, is reversed. The kinetic tran -diaxial opening of the epoxide occurs in a regio- and stereoselective manner to afford compound 28 in which the substituents have the correct position and configuration (a-OH, P-OAc),... 

In cyclic systems, the usual simple requirements of Saytzev or Hofmann rules may be overridden by other special requirements of the system, e.g. the preference for elimination from the trans-diaxial conformation in cyclohexane derivatives (c/. p. 255). Another such limitation is that it is not normally possible to effect an elimination so as to introduce a double bond on a bridgehead carbon atom in a fused ring system (Bredt s rule), e.g. (47)- (48) ... 

Almost all cyclohexane systems are most stable in the chair conformations. In a chair, adjacent axial positions are in an anfi-coplanar arrangement, ideal for E2 eliminations. Adjacent axial positions are said to be in a tran -diaxial arrangement. E2 reactions only proceed in chair conformations from... 

It was finally established131,132 that only those alkaloids possessing, in their preferred conformations), the C-3—H and at least one more adjacent C—H bond trans diaxial to the nitrogen lone pair will exhibit bands in their IR spectra between 2900 and 2700 cm-1, one or more absorbing below 2800 cm-1, whereas those alkaloids possessing in their preferred conformation(s) the C-3—H cis to the nitrogen lone pair will not. 

The resistance of 4,4-dimethylchroman-3-ol to dehydration has been attributed to its existence in a half-chair conformation in which the hydroxy group is equatorial and 1,2-trans diaxial elimination of water is disfavoured (72TL4453). 

While the regiochemistry of simple electrophilic additions to double bonds is controlled by a combination of electronic (Maikovnikov rule), stereoelectronic (trans diaxial addition to cyclohexenes) and steric factors,9 the intramolecular nature of electrophilic heteroatom cyclizations introduces additional conformational, stereoelectronic and entropic factors. The combination of these factors in cyclofunctionalization reactions results in a general preference for exo cyclization over endo cyclization (Scheme 4).310 However, endo closure may predominate in cases where electronic or ring strain factors strongly favor that mode of cyclization. The observed regiochemistry may differ under conditions of kinetic control from that observed under conditions of thermodynamic control. 

For and eliminadon, the trans compound only has a single trans diaxial conformer and that is very little populated. Elimination is slow and gives nonconjugated product... 

The cis compound has a much more populated trans diaxial conformer and the eliminadon can give the more stable conjugated product. Elimination is much faster... 

Inspection of the two sets of chair structures reveals that in one compound the all-equatorial conformer is overwhelmingly favored. In the other compound both chair structures have comparable energies so both will be populated significantly. In the all-equatorial isomer, the carbon-chlorine bond dipole moments reinforce one another leading to a large molecular moment. In the other compound the chlorines are both equatorial part of the time but part of the time they are trans diaxial where the carbon-chlorine bond dipole moments tend to cancel one another. Thus the average dipole moment of these two conformations will be less than the first compound, which exists virtually completely in the all-equatorial conformer. 

The oxyanion must open the epoxide to give a trans diaxial product 48a by attacking the less hindered end of the epoxide. This is inevitably from the endo face but attack at the other end of the epoxide would have to be from right inside the fold. Being a cis decalin, the product can equilibrate to the equatorial conformer 48b and the arrangement for cyclisation is perfect 51. 

Conformation of the keto aldehyde 26 should be A by severe A(l 3) strains[17], but in this conformation both appendages in 2 and 3 positions were not cyclizable trans diaxial. So, the epimerization at the 3 position occurs first, and then cyclization will proceed to afford the desired cis-enone. 

Stereochemistry No preferred conformation Anti-periplanar elimination (trans-diaxial in cyclohexanes)... 

Nearly all cyclohexanes are most stable in chair conformations. In the chair, all the carbon-carbon bonds are staggered, and any two adjacent carbon atoms have axial bonds in an anti-coplanar conformation, ideally oriented for the E2 reaction. (As drawn in the following figure, the axial bonds are vertical.) On any two adjacent carbon atoms, one has its axial bond pointing up and the other has its axial bond pointing down. These two bonds are trans to each other, and we refer to their geometry as trans-diaxial. 

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. 

In a chair conformation of a cyclohexane ring, a trans-diaxial arrangement places the two groups anti and coplanar. 

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