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

Abstract The 80 MHz 1H NMR and 20 MHz 13C NMR spectra of five 4‐methylcoumarins, six 4‐methyl‐2(1H)‐quinolones and nine 3‐methylbenzo[b]furans, including six new compounds, were fully assigned. Homonuclear 1H{1H} NOEs and selective heteronuclear 13C{1H} 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 13C1H{1H} 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.