Decalin conformation

The data of Mewis et al. (1989) for poly(methyl methacrylate) spheres stabilized by poly(12-hydroxy stearic acid) and dispersed in decalin conform... 

The formation of the intermediate iminium ion (23) in the reaction thus involves loss of the 60-H which is trans diaxial, i.e. antiperiplanar, to the N—O bond of (22). Although the IV-oxide (22) could conceivably undergo the Polonovski reaction via its alternative c -decalin conformer, this is highly improbable because in this conformation the three ring substituents would be axially oriented. Accordingly, the structure of an A(-oxide cannot always be correlated with that of the firee base because of the possibility of nitrogen inversion (compare 21 with 22). 

The trioxadecalin core 113 of mycalamide B 112 (which is structurally related to pederin) has been synthesized from D-mannitol (by Roush ) and D-sorbitol (by Hoffinann ). Hoffmann s approach is outlined in Scheme 23 (see next page), but both pieces of work incorporate interesting points. The unusual (and unstable) C(10) aminal unit of 112 has the opposite stereochemistry to that required by the natural product, but prior work has shown that equilibration at this centre is feasible. Also, both groups found that 112 exists as the ring-flipped c/s-decalin conformer in contrast to the conformational preference expressed by mycalamide B itself. 

The Birch reductions of C C double bonds with alkali metals in liquid ammonia or amines obey other rules than do the catalytic hydrogenations (D. Caine, 1976). In these reactions regio- and stereoselectivities are mainly determined by the stabilities of the intermediate carbanions. If one reduces, for example, the a, -unsaturated decalone below with lithium, a dianion is formed, whereof three different conformations (A), (B), and (C) are conceivable. Conformation (A) is the most stable, because repulsion disfavors the cis-decalin system (B) and in (C) the conjugation of the dianion is interrupted. Thus, protonation yields the trans-decalone system (G. Stork, 1964B). 

The decalin (bicyclo[4.4.0]decane) ring system provides another important system for study of conformational effects in cyclohexane rings. Equilibration of the cis and trans isomers reveals that the trans isomer is favored by about 2.8 kcal/mol. Note that this represents a change in configuration, not conformation. The energy difference can be analyzed by noting that the cis isomer has three more gauche butane interactions that are... 

The /rans-fiised decalin system is conformationally rigid, and the stereochemistry of the product indicates that the initial addition of the trichloromethyl radical is from the axial direction. This would be expected on stereoelectronic grounds, because the radical should initially interact with the n orbital. The axial trichloromethyl group then shields the adjacent radical position enough to direct the bromine abstraction in the trans sense. 

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 ... 

Make a sketch of each decalin isomer, and label the orientation of the bridgehead hydrogens with respect to each ring (equatorial or axial). Build a plastic model of each isomer and determine its conformational flexibility (a flexible molecule can undergo a ring flip, but a locked molecule cannot). Is flexibility responsible for stabihty ... 

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 ... 

Polycyclic compounds are common in nature, and many valuable substances have fused-ring structures. For example, steroids, such as the male hormone testosterone, have 3 six-membered rings and 1 five-membered ring fused together. Although steroids look complicated compared with cyclohexane or decalin, the same principles that apply to the conformational analysis of simple cyclohexane lings apply equally well (and often better) to steroids. 

Left section of one layer two rings with chair conformation and relative arrangement as in cw-decalin are emphasized. 

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... 

We were mesmerized by the X-ray structure of hydroxy ketone 56 with its AB-ring junction being both. -hybridized, revealing a very different conformation from that of 48. The structure showed near-perfect chairs for the m-fused l-a /-decalinic. With this conformational preference, the p-Me group is pseudo-equatorial with the a-Me group now being pseudo-axial position. We believe this new a-Me orientation is responsible for the fact that the C5a-carbonyl is not reduced by NaBFLt it is sterically inaccessible, even by a nucleophile as small as a hydride. 

The trans-decation has a center of symmetry, midway between C9 and C10 and is therefore an optically inactive molecule. The cis-decation is dissymmetric and has two interconvertible conformations, but the trans-decalin is a rigid molecule. 

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