Steroids biomimetic total synthesis

Yoder RA, Johnson JN. A case smdy in biomimetic total synthesis polyolefin carbocyclizations to terpenes and steroids. Chem. Rev. 2005 105 4730-4756. 

BIOMIMETIC TOTAL SYNTHESIS OF TERPENES AND STEROIDS THROUGH POLYENE CYCLIZATION... 

Other approaches to the stereoselective total synthesis of nonaromatic steroids include the carbocationic, biomimetic cyclizntion reactions. Generally, these cyclizations begin with the synthesis of an appropriately functionalized cyclopentenol. Acid-catalyzed cyclization forms the B-C-D rings of the steroid nucleus with the natural relative stereochemistry in a single step. 

Stereospecific 2,3-epoxidation of squalene, followed by a nonconcerted carbocationic cyclization and a series of carbocationic rearrangements, forms lanosterol [79-65-0] (77) in the first steps dedicated solely toward steroid synthesis (109,110). Several biomimetic, cationic cydizations to form steroids or steroidlike nuclei have been observed in the laboratory (111), and the total synthesis of lanosterol has been accomplished by a carbocation—olefin cydization route (112). Through a complex series of enzyme-catalyzed reactions, lanosterol is converted to cholesterol (2). Cholesterol is the principal starting material for steroid hormone biosynthesis in animals. The cholesterol biosynthetic pathway is composed of at least 30 enzymatic reactions. Lanosterol and squalene appear to be normal constituents, in trace amounts, in tissues that are actively synthesizing cholesterol. 

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