Selective heteronuclear NOE enhancements in benzoheterocycles. Effect of ring size on indirect three‐spin effects

Abstract The 80 MHz 1 H NMR and 20 MHz 13 C NMR spectra of five 4‐methylcoumarins, six 4‐methyl‐2(1 H )‐quinolones and nine 3‐methylbenzo[ b ]furans, including six new compounds, were fully assigned. Homonuclear 1 H{ 1 H} NOEs and selective heteronuclear 13 C{ 1 H} NOEs were measured after low‐power pre‐saturation of the methyl protons. Indirect, negative heteronuclear NOE enhancements were found in suitable three‐spin systems of the 13 C 1 H{ 1 H} type, and their magnitude was found to be dependent on ring size. The first examples of indirect, heteronuclear NOE enhancements on non‐protonated carbons are described.