Cw-decalin

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

Conformational inversion barriers of 5,10-substituted decalins (17) were determined by NMR, and MMI calculations were carried out to see whether the twist-chair (ct) or the boat-boat (bb), correspond to the higher barrier. The ct was found to be higher in cw-decalin (17, R = H), but the introduction of substituents at the ring junction is expected to raise the energy of the bb. The observed AG agreed with the calculated value for ct, which is about 5 kcal/ mol higher than that of bb when R = CH3 or OH (124). 

Another example, which takes place in weakly basic conditions, is provided by the key step of the synthesis of aromadendrene (111 accomplished by Biichi and coworkers [4], in which a cw-decalin system (9) rearranges to a hydroazulene system (10) through a process induced by activated AI2O3 (Scheme 7.3). 

With this idea in mind, Deslongchamps treated the cw-decalin-2,7-dione (12) with a mixture of acetic anhydride-acetic acid in the presence of boron trifluoride ethyl etherate, at room temperature, and the twistanone 20 was isolated in 75% yield. The whole synthetic sequence is outlined in Scheme 13.1.7. 

With this idea in mind, the retrosynthetic analysis (Scheme 13.2.8) leads now to unsaturated cw-decalin 28 which could be reduced to 5-methyl-2-cyclohexenone 31. In the synthetic direction there was initially the uncertainty of whether nitrone 27 would cyclise to 26b rather than to the desired adduct 26a (see below Scheme 13.2.9). However, the authors felt that in fact the risk was "less critical in view of the regioselectivity observed in intramolecular N-alkenylazomethinimine additions involving non-polarised olefinic bonds" [11][12]. 

Becanse of the high concentration of isomeric molecules (>0.1 mol dm ), this equilibrium is established instantaneously. The IP of trans-AtcaYm is 0.02 eV lower than the IP of cw-decalin (9.24 eV versus 9.26 eV). Therefore, the electron-transfer eqnilibrinm is shifted slightly to the left side. Thns, in terms of charge-transfer kinetics, the two ions behave as a single species. It shonld be worth noting that decalin has only two isomeric forms, cis and trans. On the contrary, n-nonane exists in the multitude of conformations. The rate constant of electron exchange between parent nentral molecules of nonane and its cation-radicals is much lower, namely, 2 orders lower than the diffnsion-controlled limit (Borovkov et al. 2007). 

The latter are, indeed, of considerable interest. They have a long history in conformational chemistry [258,259] and deserve attention for the major role they play in the discussion and prediction of stmcmral feamres. Typically, we refer here to gauche interactions exemplified by one of the methyl protons of the axial methylcyclohexane (for instance) interacting with the axial protons at C-3 and C-5 of the ring, or to the three gauche interactions occurring in cw-decalin. ... 

Figure 7 G(Si) value as a function of the carbon atom numbers in the molecules. When more than one measured value was published, we tried to select the most probable value. Alkanes (1) propane, (2) w-butane, (3) w-pentane, (4) cyclopentane, (5) w-hexane, (6) cyclohexane, (7) w-heptane, (8) cycloheptane, (9) methylcyclohexane, (10) w-octane, (11) cyclooctane, (12) isooctane, (13) w-decane, (14) cyclodecane, (15) cw-decalin, (16) trawx-decalin, (17) w-dodecane, (18) dicyclohexyl, (19) n-hexadecane. (From Refs. 18, 29, 65, 92, 148, and 155.)...

Tetralin-cis-Decalin (Syz). Co-oxidations of this system were undertaken because the internally consistent results of Russell and Williamson (32) (rt = 1.89 zb 0.01 rd = 0.01 zb 0.002 rtrd = 0.019) indicated an extraordinary difference in selectivity of the two alkylperoxy radicals. Oxidations of cw-Decalin (7% trans by GLPC, from K and K Laboratories, passed over aluminum before use) and Tetralin were carried out in sealed tubes and analyzed as described for styrene-Tetralin. 

Sumimoto introduced a new sebacic acid process including several catalytic hydrogenation reactions.342 The synthesis starts with naphthalene, which is first partially hydrogenated to tetralin over cobalt oxide or molybdenum oxide, then to decalin over ruthenium or iridium on carbon. The selectivity to cw-decalin is better than 90%. In a later phase of the synthesis 5-cyclododecen-l-one is hydrogenated over Raney nickel to obtain a mixture of cyclododecanone and cyclodode-canol in a combined yield of 90%. The selectivity of this step is not crucial since subsequent oxidation of either compound leads to the endproduct sebacic acid. 

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). 

Trans-decalin is more stable than d.y-decalin because of three 1,3-diaxial interactions present in the cis isomer. You may be able to visualize these interactions by thinking of the circled parts of cw-decalin as similar to axial methyl groups. The gauche interactions that occur with axial methyl groups also occur in cis-decalin. 

The rings of cw-decalin are joined by an axial bond and an equatorial bond. After a ring-flip, the rings are still linked by an equatorial and an axial bond. No additional strain or interaction is introduced by a ring-flip of m-decalin. 

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