INTRODUCTION 9 



it is possible to limit consideration to this nucleus and speak of proton resonance spec- 

 troscopy. Spectra are plotted as signal strength vs. magnetic field strength. Although 

 only the hydrogen nuclei of a molecule are responsible for the spectrum, a variety of 

 peaks is obtained depending on the environments of the different hydrogen nuclei. Thus, 

 indirectly, functional groups may be detected since, for example, the hydrogen of a hy- 

 droxyl group will show a peak different from the hydrogen of an aldehyde group. The 

 technique is especially valuable for indicating molecular configuration and is often the 

 best, if not the only, method available for this purpose. Additional information may be 

 found in a short article (35) and two books (36, 37). 



Other very specialized techniques which at present are likely to be of only rare in- 

 terest to the reader of this book are mass spectrometry and optical rotatory dispersion. 

 These methods have been the subject of recent reviews (38, 39). 



METABOLIC PA THW^A YS 



Although metabolism as such is outside the central scope of the book, division into 

 chapters has been made on the basis of metabolic pathways. The brief sections on metab- 

 olism are therefore intended to demonstrate the unity which a knowledge of biosynthesis 

 can bring to the diversity of natural products. Organization on this basis is novel and 

 has placed together compounds which are traditionally discussed separately. For example, 

 terpenoids (which are usually included with "essential oils") and steroids are covered in 

 the same chapter since their biosynthesis proceeds along almost identical pathways. For 

 some natural products the pathways of biosynthesis remain unknown, but by analogy it is 

 often possible to arrive at reasonable hypotheses. For instance, the stimulating ideas of 

 Birch have introduced a unity into Chapter 6 which is very attractive although extensive 

 metabolic studies have not been carried out to establish the complete reliability of this 

 apparent unity. 



Presentation of many metabolic pathways may be found in the valuable books of 

 Umbreit (40) and Greenberg (41). However, both of these books ignore many natural 

 products which are prominent in higher plants but not found elsewhere. An abbreviated, 

 overall scheme of the metabolic pathways which we have assumed is shown in Figure 1-1. 

 The numbers in parentheses indicate the chapters in which the various classes of com- 

 pounds are discussed. More detailed schemes are presented in each chapter. 



BIBLIOGRAPHY 



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