MAGNETIC RESONANCE IN CHEMISTRY Magn. Reson. Chem. 36, 597È600 (1998) Distribution of erythro and threo forms of diþerent types of structures in aspen lignin by using the b-O-4 13 C NMR 2D experiment INADEQUATE Michel Bardet,1 Danielle Robert,1 ¤ Knut Lundquist2 * and Sverker von Unge2 1 De  partement de Recherche Fondamentale sur la Matie ` re Condense  e, SCIB, CEA-Grenoble, F-38041 Grenoble Cedex 9, France 2 Department of Organic Chemistry, Chalmers University of Technology, S-41296 Gothenburg, Sweden Received 23 December 1997 ; revised 7 March 1998 ; accepted 14 March 1998 ABSTRACT : CarbonÈcarbon connectivity spectra of 13C-enriched aspen lignin recorded using the 2D INADE- QUATE experiment revealed cross peaks which can be assigned to four types of arylglycerol b-aryl ethers (b-O-4 structures) : erythro forms of arylglycerol b-syringyl ethers, threo forms of arylglycerol b-syringyl ethers, erythro forms arylglycerol b-guaiacyl ethers and threo forms of arylglycerol b-guaiacyl ethers. The intensities of the cross peaks suggest larger amounts of b-syringyl ethers than b-guaiacyl ethers. The erythro isomers dominate among the b-syringyl ethers. Erythro and threo forms of b-guaiacyl ethers are present in similar amounts. 1998 John Wiley ( & Sons Ltd. KEYWORDS : NMR ; 13C NMR ; 2D INADEQUATE ; lignins ; lignin models ; diastereomer distribution INTRODUCTION Arylglycerol b-aryl ethers constitute the most important type of structural elements in lignins.1 In hardwood lignins there are substantial amounts of guaiacylpro- pane units in addition to syringylpropane units. There- fore, four types of arylglycerol b-aryl ethers (1È4) have to be considered in studies of such lignins. In addition, there is a structural variation due to stereoisomerism since structural elements of the b-aryl ether type can exist as threo or erythro forms (Fig. 1). The distributions of erythro and threo forms of b-O-4 structures in hard- wood lignins have been studied by NMR spectroscopic Figure 1. The erythro and threo forms of arylglycerol b-aryl ethers. * Correspondence to : K. Lundquist, Department of Organic Chem- istry, Chalmers University of Technology, S-41296 Gothenburg, Sweden ¤ Present address : CERMAV/CNRS, Domaine Universitaire, BP 53, F-38041 Grenoble Cedex 9, France. methods. 1H NMR studies of birch lignin suggest an overall predominance of erythro forms, although sub- stantial amounts of threo forms also are present.2h 4 In 13C NMR studies of beech lignin, it could be concluded that erythro forms of b-syringyl ethers (5) are prevalent among the di†erent types of b-ethers present.5 Previous 13C NMR studies of 13C-enriched aspen lignin included the application of the 2D INADE- QUATE experiment.6,7 These studies revealed that some of the cross peaks in the 13C NMR 2D INADE- QUATE spectrum could be attributed to carbon atoms in the side-chains of four classes of b-O-4 structures, namely threo and erythro forms of b-O-4 structures of the syringyl ether (5, 6) and guaiacyl ether (7, 8) types. On the basis of comparisons with model compound data, it could be shown that erythro forms of b-O-4 structures dominated as a consequence of a prevalence of erythro forms of arylglycerol b-syringyl ethers (5). A prevalence of b-ethers of type 5 in hardwood lignins is in accordance with current views on the biosynthesis of lignins.8 In the present work, we extended and corroborated the 2D INADEQUATE studies7 of ( 1998 John Wiley & Sons, Ltd. CCC 0749-1581/98/080597È04 $17.50