13C N M R shielding data for about 300 compounds are tabulated, and critical spectral features are discussed as a guide to the use of the technique in structure elucidation and stereochemical allocations of lignan, neolignan and their derivatives. Unanalysed reported data have been analysed and, wherever necessary, -C signal assignments have been revised on the basis of more recent evidence and to obtain consistency throughout the series.The term lignan was introduced by Haworth' around 1940 for a category of plant products having a common constitutional feature of two c&3 (npropylbenzene) residues linked by a bond connecting the central ( p ) carbon atoms of each side-chain. In 1969, McCredie et al., proposed that the definition of lignans should be extended to cover all natural products of low molecular weight that arise primarily from the oxidative coupling of p-hydroxyphenylpropane units. The term neolignan was introduced to designate compounds in which the two C6-C, units are not linked by a p-p' bond? According to the most recent definition, lignans are formed by oxidative coupling of cinnamyl alcohols and/or cinnamic acids, whereas neolignans are formed by oxidative coupling of 1-propenylphenols and/or allylphen~ls.~ Lignans and neolignans have attracted much interest over the years, both on account of their widespread occurrence in nature6 and their broad range of biological activities7 (anti-tumour substances, hypertensive, sedative and anti-bacterial activity, plant germination inhibitors, growth inhibitors and antifungal agents). Of possibly even greater importance is the recent isolation of lignans from animals, including humans.' As a consequence, various reviews' have appeared, including a book,6 covering the literature up to 1976 and dealing with various aspects of this class of natural products. The most recent review deals with the synthesis of these compounds."From a survey of the literature it is evident that, after the appearance of the first report '' in 1974, I3C NMR spectroscopy has played a distinctive role in structure elucidation and assignment of stereochemistry, and has also contributed to the revision of some structures which were derived from limited information gained from 'H NMR and MS data. I3C shielding data, however, are still scattered in the literature and so far there is no systematic compilation. This may be one of the reasons why most publications dealing with *I3C NMR spectral Investigations, Part 9. For Part 8, see Ref. 1; for Part 5 , see Ref. 63.Author to whom correspondence should be addressed.the isolation and chemical investigation of such compounds ignore I3C NMR spectral analysis, despite its potential. A review of the available I3C NMR literature for this important class of natural products was therefore considered to be timely. The assignments in some cases have been revised in the light of more recent evidence. Similarly, based on various facts, the stereochemistry of some of the compounds has been assigned and/or revised. Instrumental and theoretical aspects...
