4 INTRODUCTION 



what the extreme cases are, the reader can presumably interpolate other structures 

 which probably occur in nature. For example, if two natural benzene derivatives are 

 indicated differing in that one has only a single phenolic hydroxyl group while the other 

 has three methoxyl groups, it may be assumed that other compounds probably exist with 

 intermediate structures. Reference to the organic chemical literature or such books as 

 that of W. Karrer will then confirm or deny the validity of such interpolation. 



As with structures chosen, examples of the natural occurrence of plant products 

 are illustrative rather than exhaustive. In general, coverage has been restricted to the 

 higher plants (botanically the Embryophyta), but occasional references to algae and fungi 

 have been made when it seemed pertinent to do so. Where a plant is named as the source 

 of a compound, it is almost never true that it is the only plant which has this constituent 



though it will usually be the one richest in it. Almost every natural product is found 



in more than one species although there is often a taxonomic pattern restricting a com- 

 pound or a class of compounds to a certain genus, family, or order. In other cases 

 such restriction is statistical rather than absolute or quantitative rather than qualitative. 

 Usually no indication is given as to the part of a plant richest in a constituent or the stage 

 of maturity at which the highest concentration may be found, although these variables are 

 of crucial importance to anyone wishing to isolate a compound. Reference to the books 

 of Paech and Tracey or W. Karrer will usually lead rapidly to such information if it is 

 required. 



Only very brief indications are given as to the functions of compounds in plants or 

 their pharmacological properties. These two extremely interesting areas fall just beyond 

 the scope of the present book, but they are worthy of mention since it is because of such 

 properties that workers in botany or pharmacy are led to seek information on the chem- 

 istry of plant constituents. 



ISOLATION METHODS 



Procedures for isolating substances from plants are nearly as varied as the sub- 

 stances themselves, but we have tried to indicate methods which are generally useful for 

 a group of compounds rather than methods which have been used for specific members 

 of the group. Often particular compounds continue to be isolated by tried and true meth- 

 ods rather than by adopting recent methods of more general utility. Thus, while the rec- 

 ommended way to purify an unknown alkaloid might well be chromatography on a cation 

 exchange resin, it would be unusual to purify strychnine in this way. 



In preparing plant materials for isolation of individual compounds the most impor- 

 tant precaution is the avoidance of artifacts. If living tissue is processed too slowly, 

 enzymatic action may cause profound changes in certain constituents. Oxidation and hy- 

 drolysis are the most common degradative processes; and if constituents are sought which 

 are subject to them, care must be taken to avoid such effects. On the other hand, if tis- 

 sues are heated to prevent enzymatic action, certain heat-labile substances may undergo 

 change. Probably the safest general method for all eventualities is immersion in liquid 

 nitrogen followed by freeze-drying, and extraction of dried material with solvents which 

 do not permit degradative changes to occur. Such drastic measures are seldom neces- 

 sary, however, and a consideration of the properties of the substances to be isolated will 

 usually point the way to simpler yet adequate procedures for preparing the plant material. 

 General methods for preparing extracts are reviewed by Pirie (1). Newer methods of 

 isolating natural products have recently been reviewed (2). 



Chromatographic methods have become more and more prominent in the isolation 

 and purification of unknown compounds. A full discussion of such methods is beyond the 

 scope of the present book. Several helpful reviews are available on methods of prepara- 



