Decalin symmetry

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... 

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. 

When we look at the structures of trans- and cis-decalin, it is apparent that a further plane of symmetry, through the ring fusion, is present in both structures. This means that each isomer is superimposable on its mirror image consequently, there are only two configurational isomers of decalin, one trans and one cis. 

The situation in trans- and ciG-decalin is complicated by the symmetry elements. If this symmetry is destroyed, e.g. by introducing dimethyl substituents, we get back to reassuringly familiar territory in which two chiral centres lead to four configurational isomers. The same is true in the trans- and cw-1,2-dimethylcyclohexane series. 

The fusion of rings of different sizes reduces symmetry in the structures instead of the rather unusual situation with the decalins, where there are only two configurational isomers, the hydrindanes exist in the... 

Heats of isomerization can be calculated directly from the equilibrium constants for the reactions shown above because there are no changes in symmetry and the other conformational isomers (boat conformations) of sufficient stability to introduce significant entropy-of-mixing terms. The liquid-phase heats of isomerization in Table XX for aa ial-l-methyl-fm s-decalin (cis-anti isomer) to equa-anti isomer) decrease at lower temperature consistent with the greater volatility of the more stable isomer, appropriate to the 5° difference in boiling points. At the highest experimental temperature, the liquid-phase heats of isomerization closely approach the 2.10 kcal/mole observed in the vapor phase. The analytical data are in Table XXI. 

A new synthesis of nojigiku alcohol (773) (Vol. 4, p. 554) starts from tricyclene (774) and we should perhaps recall the preparation of the latter from camphor (40) tosylhydrazone and sodium methoxide in decalin. [Tricyclene is also produced in other reactions leading to camphene (775), such as the reaction of 3-bromocamphor with methylaniline. ] TTie new synthesis consists in anodic oxidation with triethylamine in acetic acid, and yields 76% of 773, with 8% of the endo-isomer and 11% of diols. Oxidation of tricyclene (774) with lead tetraacetate also gives nojigiku alcohol (773), but as a minor (16%) product, the main product being camphor (40). Tricyclene having a plane of symmetry, these methods all lead to racemic 773. 

Another crucial finding was the realization that rapid spin-lattice (Ti) relaxation in the high-symmetry cycloalkane radical cations precludes their detection with optically-detected magnetic resonance (ODMR) [39-48], the technique which was routinely used to study radical cations in radiolysis of hydrocarbons [38, 50]. For example, trans-decalin + isolated in room-temperature cyclohexane has Tj 7 ns [50] while typical solute radical cations have Ti 1 ps. Since it takes several tens of nanoseconds to flip the electron spin with the microwave radiation (which is required for the magnetic resonance detection) radical cations of some cycloalkanes cannot be detected by ODMR. 

A preliminary analysis of the 5a-steroid nucleus indicates that an interaction of axial methyl groups with planar zigzag structures may have to be taken into account as well as the contributions they may make in simpler four-atom interactions. Thus the D-homogonane nucleus (79) has a center of symmetry and is, therefore, achiral. Contraction of ring D to form gonane (80) should be rotationally equivalent to converting trans-decalin (achiral, [M]d0°) to traus-hydrindane (59) ([M]d 7.4°). For this to be... 

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