KONRAD BLOCH 



Much experimental work along enzymatic lines remains 

 to be done to establish the principle and the details of the 

 cyclization process. Nevertheless it is exceedingly useful to 

 draw on the broad chemical background which already exists 

 in the terpene field and to attempt a rationalization of the 

 squalene-cholesterol conversion. Taking the squalene hypoth- 

 esis as a basis, Ruzicka and his collaborators have recendy 



Fig. 3. Ionic mechanism for the cyclization of squalene to 

 steroids according to Ruzicka (20). 



presented a comprehensive reaction scheme which connects 

 squalene with the steroids as well as with the various pentacyclic 

 triterpenes, and serves as a highly stimulating working hypoth- 

 esis for future biochemical work (20) . According to Ruzicka 

 the cyclization of squalene is initiated by a cationic attack at the 

 carbon atom which corresponds to C3 of the steroid skeleton, one 

 of the two positions which are most susceptible to attack by 

 electrophilic reagents. Thereafter, the tetracyclic ring system is 



480 



