CHEMICAL STRUCTURE 



accumulated considerable evidence to show that squalene is 

 converted to cholesterol in the animal body (15), and even 

 though it may turn out that squalene itself is not the acyclic 

 precursor, the principle of the postulated conversion is probably 

 correct. The specific mechanism which Robinson had proposed 

 (19) (Figure 2 A) seemed at first a reasonable one to adopt, 

 since it conformed with the distribution pattern which at that 



LANOSTEROL 

 VOSER ET AL. 1952 



ISOSOUALENE 

 MONDON 1953 



Fig. 2. Structural relation between squalene and lanosterol; A, 

 cyclization of squalene according to Robinson (19); B, cyclization of 

 squalene according to Woodward and Bloch (25); M, C: derived 

 biologically from methyl and carboxyl carbon atoms of acetate, respec- 

 tively. 



time had been established for 1 7 of the 27 carbon atoms in the 

 steroid molecule (9,26). Nevertheless, it was realized that this 

 analytical information was too incomplete to restrict the number 

 of possible cyclization mechanisms. Robinson's formulation 

 had the virtue of rationalizing the presence of the angular methyl 

 groups in the positions in which they appear in fact in all the 

 steroids. On the other hand, to judge from the chemical 

 behavior of the terpenes a ring closure along the suggested lines 

 did not seem probable. For example, the acid-catalyzed transi- 

 tion of an isoprene derivative to the cyclic isomer proceeds in 



477 



