Swinholide‐A, a new marine macrolide. Complete assignment of ¹H and ¹³C spectra by 2D NMR techniques

¹H and ¹³C NMR spectra of the novel marine macrolide swinholide‐A, which provide the basis for its structure elucidation, have been completely assigned by parallel use of 1D and homo‐ and hetero‐nuclear 2D NMR techniques. Swinholide‐A (1) probably a polyketide, is a 22‐membered macrolide embodying a dihydropyran ring and a 10‐carbon side‐chain ending with a substituted tetrahydropyran ring. The high degree of overlap of eleven CH—O and nine CH₂ signals in the 1D proton NMR spectrum of 1 necessitated the structure elucidation of swinholide‐A tetraformate (2) by 2D homo‐ (up to ⁶J with formyl groups) and hetero‐nuclear NMR correlations. Formyl groups can be good structural probes in complex polyalcohols, as shown by the observed ⁵J and ⁶J formate couplings in 2.