N.M.R. Solutions to Problems of Connectivity in the Structural Elucidation of Natural Products

Abstract

Structural elucidation in general involves determining an extended sequence of bond connectivities, and as someone who is not specifically an organic chemist I am very interested in methods which enable one to build up a structure from scratch in a logical manner as one then requires a minimum of biogenetic knowledge and chemical intuition. Indeed we have been quite successful in solving structures where initially we had no idea what class of compound was involved. Scapaniapyrone A (1) is an example where the combined use of the proton-coupled ¹³C n.m.r. spectrum and 2D long-range ^δC/^δH correlation led inexorably to the novel naphthyl pyrone structure,¹ without having first to propose a working structure and then test it to see if it fit. However the connectivity problem that I wish to discuss in this chapter is the more common situation where one has to work out how to fit together several pieces whose structures individually have been relatively easy to elucidate. To illustrate the various types of problem that can arise I wish to describe the structural elucidation of oligosaccharides from several sources and of some compounds isolated from liverworts (Hepaticae).