The size of the geminal coupling constant

The 18 Hz geminal coupling constant between the green protons of coniochaetone B is large, but not unusually so for a geminal coupling. A more typical figure in a six-membered ring might be closer to 14 Hz, and we will see shortly why the value in coniochaetone B is bigger than this. The example below provides an opportunity to examine coupling constants in another example where NMR was essential for determining the structure. The compound is pederin, a toxic amide of the blister beetle Paederus fuscipes. After some incorrect early suggestions, the actual structure of the compound was eventually deduced as shown.

We are not going to discuss the full structure elucidation, but will concentrate on the stereo chemistry of the right-hand ring. The five (green) protons on the ring gave the signals listed in the margin. Three of the protons have shifts δH 3–4, and are obviously on carbons attached to oxygen atoms. The other two, δH about 2, must be the diastereotopic pair at C5. The coupling of 12 Hz, which appears in both signals, must be the geminal coupling and the other couplings are found in the signals at δH 3.75 and 3.85. The signal at δH 3.75 has no other couplings and must be from C4 so that leaves δH 3.85 for the hydrogen atom at C6, which is also coupled to the hydrogen in the side chain. The 10 Hz coupling must be axial–axial—the others are all much smaller, mean ing there is just the one axial–axial coupling. The left-hand side chain must therefore occupy an axial position as shown in the margin. This is perhaps a bit surprising—it’s large and branched— but the molecule has no choice but to place one of the two side chains axial. One of the most important compounds from the last 25 years is Taxol, the anti-cancer com pound isolated from the bark of the Pacific yew tree. Taxol’s structure has four rings—with eight, six (twice), and four members—and is too complex to analyse in detail, but the NMR spectrum of the closely related compound in the margin gives us the opportunity to illustrate how much geminal couplings in rings may vary and to analyse some of the factors which control this variation. The coupling between the black Hs is 20 Hz while that between the green Hs is just 6 Hz. 20 Hz is a very large coupling constant, even for geminal coupling, and the reason it is so big H is the adjacent π bond. If a CH₂ group is next to an alkene, aromatic ring, C=O group, CN group, or any other π-bonded functional group, it will have a larger geminal coupling constant. This effect also explains the large 18 Hz coupling in coniochaetone B. But why is the green coupling so small? The reason is the four-membered ring. the same is true of geminal couplings. Another factor comes into play here as well—the adjacent oxygen atom. Electronegative atoms always tend to reduce coupling constants.

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مواضيع ذات صلة

Diastereotopic protons in acyclic compounds

Spotting diastereotopic protons in the NMR spectrum

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

Geminal (2J) coupling

Heteroatoms in rings have axial and equatorial lone pairs

Four-membered rings

Ring size and NMR

Baldwin’s rules and ring opening

Combatting ΔS‡—the Thorpe–Ingold effect

Thermodynamic control over ring size

Making heterocycles: ring-closing reactions

Related effects in other types of compounds