164 TERPENOIDS AND STEROIDS 



tion peaks in the visible usually at about 450 mji. Many spectra of individual carotenoids 

 will be found in the appropriate general references and in the book by Strain (61). It 

 should be noted that the exact absorption maximum may vary slightly with the solvent 

 used. Ultraviolet and infrared absorption spectra of steroids have been compiled by 

 Dorfman (93) and Dobriner et al. (94) and infra-red spectra for many steroid sapogenins 

 and their derivatives by Jones et al. (95). Diaz et al. (96) have recorded the absorption 

 spectra observed on treating 16 different sapogenins with concentrated sulfuric acid. 

 Bernstein and Lenhard (97) have correlated steroid structures with their spectra in con- 

 centrated sulfuric acid. Rubber can be distinguished from gutta by infra-red spectra. 

 Rubber has 42% more absorption than gutta at 12/1. 



Optical rotations of all known triterpenoids and steroids have been compiled by 

 Mathieu et al. (98,99). 



METABOLIC PATHWAYS 



There is reasonable agreement regarding the early steps in the biosynthesis of 

 isoprenoid compounds and some scattered evidence regarding the biosynthesis of some 

 of the major categories. These generally acceptable pathways are outlined in Figure 

 8-1. Almost nothing is known for certain about the details of interconversion within any 

 of the major categories, although some working hypotheses are taken for granted in the 

 interests of simplification. 



A. It seems self-evident that cyclic compounds are made from acyclic precursors. 

 Two warnings must however be inserted: 



1. Straight chain structures which do not follow the isoprene rule may be re- 

 garded as derived from cyclic compounds by a ring scission in the middle 

 of an isoprene residue (e. g. elemol); 



2. The straight chain precursors are not necessarily known terpenoids (e. g. 

 phytol as such, although it is the only important acyclic diterpenoid, need 

 not be the precursor of cyclic diterpenoids. Some transient acyclic deriva- 

 tive may be the precursor of both phytol and the cyclic diterpenoids). 



B. Compounds which do not follow the isoprene rule are considered to result from 

 rearrangements late in the biosynthetic sequence rather than from unusual con- 

 densations early in the sequence. 



A general review of this area is presented in a symposium publication (100) and an 

 article by Wright (101). Other reviews are those of Goodwin (102) for carotenoids, 

 Crabbe (103) for tetracyclic triterpenoids, Dutta and Narang (104) for diterpenoids, and 

 Halsall and Theobald for sesquiterpenoids (105). It should be noted that much of the work 

 on steroid biosynthesis has been carried out with animal tissues and fungi. Much work 

 on carotenoid biosynthesis has been done using fungi. It is, of course, uncertain how far 

 such results apply to the higher plants; but where evidence is available, it indicates a 

 similarity of pathways in all organisms. Several specific areas will be discussed below. 



Early in the study of terpene chemistry frequent intra-molecular rearrangements 

 of these compounds were observed. On the basis of such observations it has been as- 

 sumed that molecular rearrangements can account for many in vivo transformations. 

 The most important general type of rearrangement is the Wagner-Meerwein reaction 

 which occurs in the presence of acids. It may be pictured as resulting from the addition 

 of hydrogen ion to a double bond to form a carbonium ion. Electrons from another part 

 of the molecule are attracted to the positively charged carbon atom and a new bond is 

 formed with elimination of a hydrogen ion or addition of a negative ion. This type of re- 



