Drawing decalins

Using molecular models as well as structural drawings, explain why trans-decalin is rigid and cannot ring-flip, whereas ds-decalin can easily ring-flip. 

Probtem 9.48 Decalin, C,oH,g, has cis and trans isomers that differ in the configurations about the two shared C s as shown below. Draw their conformational structural formulas. 

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 samples examined were prepared by the method developed by Smith et al. [70, 71] and by Matsuo [72]. Sample films of a thickness of ca. 100 pm were obtained by drying a gel which was obtained by quenching a 0.4 g/dl decalin solution of linear polyethylene with a molecular weight of 3 x 106 from 140 °C in ice-water. The samples thus obtained could be drawn to a very high extent because of very few intermolecular chain entanglements. However, since they could not be drawn highly in one step, they were drawn 10 times at the first step in a silicon oil bath at 145 °C at a rate of 1.6 times/min and then at the second step they were drawn so that the final draw ratio was 50,100, and 150 times. 

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. ... 

This may look like a long stretch for the enol to reach across the ten-membered ring to reach the other ketone, but the conformational drawing in the margin shows just how close they can be. You should compare this conformation with that of a decalin (Chapter 18),... 

The as-decalin is formed because the enone, though flattened, is already folded to some extent. A conformational drawing of either molecule shows that the top surface is better able to bind to the flat surface of the. catalyst. Each of these products shows interesting stereoselective reactions. The ketal can be converted into an alkene by Grignard addition and El elimination and then epoxidized. Everything happens from the outside as expected with the result that the methyl group is forced inside at the epoxidation stage. 

Note In working with decalins, it is essential to use models. Many structural features of decalins that are obvious with models are not easily visualized with drawings. 

Draw three-dimensional structures for each decalin derivative. 

The two isomers have ds and trans ring junctions so we should first make conformational drawings. The trans compound is easy as it has a fixed conformation like a trans-decalin (p. 463). The ds mpound can have two conformations as both rings can flip. 

The mechanism is easy (intramolecular Sn2) and the conformation of a trans decalin is fixed so we can start with the conformational drawings. 

The opening of cyclohexene epoxides is controlled by the need to get the trans diaxial products. To get the right answer we need merely to draw the only possible trans diaxial (i.e with CN and O" diaxial) product from each of these conformationally fixed trans decalins. Cyanide must, of course, open the epoxide with inversion so the OH group in the products is on the same side as the oxygen atom in the original epoxides. 

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