Reactions of cis-decalins

cis-decalins can flip rapidly between two all-chair conformations. During the flip, all substituents change their conformation. The substituent R is axial on ring B in the fi rst conformation of cis-decalin shown below but equatorial in the second. The ring junction Hs are always axial on one ring and equatorial on the other. The green hydrogen is equatorial on ring A and axial on ring B in the fi rst conformation and vice versa in the second. Of course, they are cis in both. Because R gets equatorial, the second conformation is preferred in this case.

A standard reaction that gives substituted decalins is the Robinson annelation. A Robinson annelation product available in quantity is the keto-enone known sometimes as the Wieland–Miescher ketone and used widely in steroid synthesis. The non-conjugated keto group can be protected or reduced without touching the more stable conjugated enone.

If either of these products is reduced with hydrogen and a Pd catalyst (the alcohol is fi rst made into a tosylate), the cis-decalin is formed because the enone, although 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 con verted into an alkene by Grignard addition and E1 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.

Treatment of the other product, the keto-tosylate, with base leads to an intramolecular enolate alkylation—a cyclization on the inside of the folded molecule that actually closes a four-membered ring. The reaction is easily seen in conformational terms and the product can not readily be drawn in conventional diagrams.

A similar reaction happens on the epoxide to produce a beautiful cage structure. This time it is a five-membered ring that is formed, but the principle is the same—the molecule closes across the fold rather easily. The new stereogenic centres can only be formed with this con figuration: no other stereoisomer would be a feasible structure.

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مواضيع ذات صلة

cis-Fused rings

Reactions that break open bridged molecules can preserve stereochemistry

Bridged bicyclic rings

Regard five-membered rings with two or more sp3 carbons as flat

Five-membered ketones are flexible

Only axial attack is possible with cyclohexenes

Axial or equatorial attack is possible on a cyclohexanone

Diastereotopic protons in acyclic compounds

Spotting diastereotopic protons in the NMR spectrum

How to tell if protons are homotopic, enantiotopic, or diastereotopic

The size of the geminal coupling constant

Geminal (2J) coupling