58 



AROMATIC COMPOUNDS 



extract of may apple {Podophyllum peltatum) , has been used as a powerful cathartic. It is 

 a complex mixture (17), but its lignan constituent, podophyllotoxin, is of interest for hav- 

 ing a cytotoxic action like that of colchicine. Podophyllotoxin and other lignans having the 

 partially reduced naphthalene nucleus have shown some promise in treatment of certain 

 types of neoplasms. In plants lignans have been regarded as intermediates in the biosyn- 

 thesis of lignin (see below under "Metabolic Pathways"). 



There are general reviews of the lignans by Erdtman (18) and Hearon and MacMregor 

 (19). Freudenberg and \Veinges (20) have proposed a comprehensive system of nomencla- 

 ture for the lignans. 



LIGNIN 



Lignin is the strengthening material which occurs along with cellulose in the cell 

 walls of all woody plants. It is a high polymer made up of several different types of phenyl- 

 propane units. All lignins contain units related to coniferyl alcohol. In addition, the lig- 

 nin of most dicots has sinapyl groups whereas lignin from grasses may contain p-hydroxy- 

 cinnamyl alcohol units: 



=CHCH20H 



CH=CHCH20H 



Coniferyl alcohol 



p-hydroxycinnamyl alcohol 



CH^CHCHjOH 



Sinapyl alcohol 



These cinnamyl alcohol derivatives also occur in plants as glycosides involving the p- 

 hydroxy groups (e. g. coniferin, syringin). 



Lignin itself as obtained by various isolation procedures (see below) is a brown, 

 amorphous solid which is insoluble in water and most organic solvents. Lignin prepara- 

 tions (which may have suffered some degradation during isolation) have shown molecular 

 weights ranging from 2800 to 6700. Lignin in plants is undoubtedly bound in some way 

 to the polysaccharides which occur with it in the cell walls. However, it is still unclear 

 whether this binding involves covalent bonds such as ether or ester links or whether it is 

 merely through hydrogen bonds. It is probable that there is more than one type of linkage 

 between the repeating units of lignin. The dehydrodiisoeugenol structure shown below at 

 least a likely possibility for a part of the molecule: 



