J coupling constants in other ring sizes

Vicinal J) coupling constants in other ring sizes 

Three-membered rings are flat with all bonds eclipsed so the dihedral angle is 0 for cis Hs and 109 for trans Hs. Looking at the Karplus curve, we expect the cis coupling to be larger, and it is. A good example is chrysanthemic acid, which is part of the pyrethrin group of insecticides found in the pyrethrum plant. Both cis and trans chrysanthemic acids are important. 

The epoxides have much smaller coupling constants because (1) the C-C bond is longer (2) there is an electronegative element and (3) the spreading out effect of the small ring comes Into play. 

The natural product cerutenin Is an antibiotic containing a cis epoxide. The coupling constant between the black hydrogens is 5.5 Hz. 

The compound has been made from an unsaturated lactone by epoxidation and ring opening. Follow what 

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