Pfizer Handbook of Microbial Metabolites 



158 



It is likely that the cyclization of squalene to form such struc- 

 tures occurs by a concerted mechanism which proceeds from 

 ring to ring until complete and that this all occurs on one 

 enzyme surface. Thus, isolation of intermediates between 

 squalene and an initial cyclization product such as the one 

 shown is improbable. The cyclization is oxygen-initiated, ex- 

 plaining the frequent occurrence of the 3-hydroxyl groups in 

 natural steroids. 



HO / \ HO 



Proposed initial cyclization 



product of the Squalene —>■ Lanosterol 



route 



Lanosterol 



Transformation of the proposed tetracyclic precursor to lano- 

 sterol involves two 1,2-methyl group migrations (14-^ 13 and 

 8 ^ 14) as shown by tracer experiments." ^^ 



Eburicoic acid has the lanostane skeleton, but with a methyl- 

 ene group attached to carbon atom 24 of the side-chain. Simi- 

 larly, ergosterol has a methyl group in this position. Labeling 



CHo 



HOOC 



Ergosterol 



CH3 CH3 



Eburicoic Acid 



^'■' R. K. Maudgal, T. T. Tchen and Konrad Bloch, J. Am. Chem. 

 Soc. 80 2589 (1958). 



