Rrans-Decalins

Decalin ring systems appear as structural units in a large number of naturally occurring substances, particularly the steroids. Cholic acid, for example, a steroid present in bile that promotes digestion, incorporates d5-decalin and rran -decalin units into a rather complex tetracyclic structure. 

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... 

Once regioselective and stereoselective controls have been exerted, the cis-decalins must be isomerised to rrans-decalins, the configuration present in the target molecules. Since frans-decalins are thermodynamically more stable than the corresponding cis-decalins, it is possible to isomerise the latter through enolisation, a process that can be favored by the presence of a carbonyl group near to the centre to be inverted. 

The decalin structural feature is contained in steroids, many of which have useful biological activity. Decalin can exist in two isomeric forms depending on whether the rings are trans fused or cis fused, rrans-decalin is constrained to a rigid conformation, whereas cw-decalin is conformationally mobile. 

Note that trans- and cw-decalin are stereoisomers and not conformational isomers cw-Decalin is less stable than rran -decalin by 2.7 kcal/mol. 

Does the angular methyl group in rran. -decalins (a) or in c/.y-decalins (b) have the larger line width Explain. 

In both cis- and rran -decalin, the cyclohexane rings can be in chair conformations. The relative energies will depend on the number of axial substituents. 

The feasibility of cyclizing polyenes with iminium ion initiators has received only scant attention. 72 Cyclization of imine (77) under aprotic conditions with SnCU affords predominantly rran -decalins containing endocyclic unsaturation. This mixture was deduced by H NMR analysis to contain diastereomers (78) to (81) in the indicated abundances as depicted in equation (7). The extent of asymmetric induction in forming the decalin ring system (9S,10iS 9/ ,10/ = 61 39) is significantly lower than that of related cyclizations of chiral acetal substrates. ... 

There is an important difference between the cis- and trans-decalin systems with respect to their conformational properties, rrans-Decalin, because of the nature of the ring fusion, is incapable of ring inversion, cw-Decalin is conformationally mobile and undergoes ring inversion at rates only slightly slower than cyclohexane (AG = 12.3-12.4 kcal/mol in methylene chloride or toluene solution)." ... 

On viewing the drawings of cis- and rrans-decalin in Figure 4-13, which do you think is the more stable isomer Estimate the energy difference between the two isomers. 

Figure 2.3 Sedimentation of390 kDa polystyrene in rran -decalin at temperatures 20 (0)> 25 ( ), 30 (0), 35 (A), and 40 (V) °C, from measurements by Nystrom and Roots showing sedimentation under near-Theta to good solvent conditions(13).

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