Conformations of Decalin

Exercise 12-26 Use ball-and-stick models to assess the degree of stability to be expected for a decalin with chair-form rings and an axial-axial ring fusion. 

The ramifications of conformational analysis of flexible and rigid ring systems are of considerable importance to the understanding of stability and reactivity in polycyclic systems. This will become increasingly evident in later discussions. 

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The conformation of decalin is discussed on pp. 378-9 of chapter 16 of the textbook. The flattening effect of the alkene is dealt with in chapter 32. 

Examine the structures of trans and cis-decalin. What is the conformation of the two cyclohexane rings in each isomer Obtain the energy of each decalin isomer. Which is more stable and why ... 

Examine the structure of cyclodecane, a molecule which contains the same number of carbons as decalin, but only has one ring (a model of the most stable conformation is provided). Compare it to cis and trans decalin. Make a plastic model of cyclodecane. Is it flexible or locked What conformational properties of cyclodecane can be anticipated from the properties of decalins What properties cannot be anticipated How do you account for this ... 

The problem of the preferred conformation of cyclodecane has been extensively studied by Dunitz et al. (46). In the crystals of seven simple cyclodecane derivatives (mono- or 1,6-disubstituted cyclodecanes) the same conformation was found for the ten-membered ring (BCB-conformation, Fig. 9). It follows from this that the BCB-conformation is an energetically favourable conformation, possibly the most favourable one. Numerous force field calculations support this interpretation Of all calculated conformations BOB corresponded to the lowest potential energy minimum. Lately this picture has become more complicated, however. A recent force field calculation of Schleyer etal. (21) yielded for a conformation termed TCCC a potential energy lower by 0.6 kcal mole-1 than for BCB. (Fig. 9 T stands for twisted TCCC is a C2h-symmetric crown-conformation which can be derived from rrans-decalin by breaking the central CC-bond and keeping the symmetry.) A force field of... 

Dealkylation, heteropoly compunds, 41 160-161, 170-174 Deamination, 27 259 Pd(NH3>/ ions, 39 142 Debye-Waller factor, 21 177 Decahydronaphthalenes conformation of, 18 17-19 isomeric, formation of in hydrogenation of naphthalenes, 18 23-20 rrans-Decalin... 

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),... 

As we have seen, both cyclohexane and decalin are a binary mixture of conformers. As discussed earlier, it is obvious that conformers of the decalin cation-radical act in parallel, whereas conformers of the cyclohexane cation-radical have quite different kinetics of their reactions with uncharged molecules of a solvent and a solute. 

The different conformations of indolo[2,3-a]quinolizidines play an important role in predicting the thermodynamically more stable epimer (see below). The indolo[2,3-a]quinolizidine system can exist in three main conformations one C/D trans ring juncture (conformation a) and two C/D cis ring junctures (conformations b and c). These are in equilibrium by nitrogen inversion and cw-decalin type ring interconversion. Ring C is considered to be in a half chair conformation and ring D in a chair conformation, Scheme (16) [40]. 

At this point, it probably will be helpful to construct models of cis- and trans-decalins to appreciate the following (a) The two compounds cannot interconvert unless C-C or C-H bonds first are broken, (b) traw -Decalin is a relatively rigid system and, unlike cyclohexane, the two rings cannot flip from one chair form to another. Accordingly, the orientation of the substituent is fixed in the chair-chair conformation of trans-decalm, (c) The chair-chair forms of cw-decalin are relatively flexible, and inversion of both rings at once occurs fairly easily (the barrier to inversion is about 14 kcal mole-1). A substituent therefore can interconvert between axial and equatorial conformations (Figure 12-24). 

Figure 12-21 Chair conformations of the decalins. The two drawings of the cis conformation represent the same arrangement of atoms but different perspectives.

The first precise evaluation of the anomeric effect was realized by Descotes and co-workers in 1968 (22). These authors have studied the acid catalyzed isomerization of the cis and trans bicyclic acetals 6 and 6 and found that, at equilibrium, the mixture contains 57% ci s and 43% trans at 80°C. The cis isomer is therefore more stable than the trans by 0.17 kcal/mol. The cis isomer 5 has one (stabilizing) anomeric effect whereas the trans isomer 6 has none. Steric interactions in cis acetal 5 were estimated tobel.65 kcal/mol (one gauche form of ri-butane, 0.85 kcal/mol and an OR group axial to cyclohexane, 0.8 kcal/mol). By subtracting an entropy factor (0.42 kcal/ mol at 80°C) caused by the fact that the cis acetal S exists as a mixture of two conformations (cis decalin system), they arrived at a value of 1.4 kcal/mol for the anomeric effect. 

The protonation leads specifically to the trans-decalin system, though reduction could apparently give rise to two stereoisomeric products. The guiding principle appears to be that protonation of the intermediate allylic anion 12 takes place axially, orthogonal to the plane of the double bond, and to the most stable conformation of the carbanion which allows the best sp3-orbital overlap on the /Tcarbon with the -orbital system of the double bond. 

Figure 17. (a) Conformers of (-)-menthone and then octant projection diagrams, (b) Variable temperature CD spectra of (-)-menthone in hydrocarbon solvents +25° and -74° (decalin) -192° (isopentane-methylcyclohexane). 

Use your models to do a chair-chair interconversion on each ring of the conformation of cis-decalin shown in Figure 3-27. Draw the conformation that results. ... 

First a brief rentin 1 of the conformation of c/s-decalins (see Chapter 18). Unlike frans-decalins, which are rigid, th caii flip rapidly between two all-chair conformations. During the flip, all... 

Transannular hydrogen bonding can also considerably influence the conformations of large rings. Monoprotonation of 1,6-dimethyl-1,6-diazacyclodecane 77 results in the formation of a symmetrical transannular hydrogen bond. An X-ray study of the monoprotonated iodide salt showed that it has a m-decalin type structure 78 with an NN distance of 2.600 e and an NHN angle of 169 <1988CC1528>. 

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