Alkylation reactions ergosterol

In ergosterol biosynthesis, side chain alkylation of lanosterol normally takes place to build 24-methylenedihydrolanosterol, which itself is then the substrate for demethylation reactions at and C. The C -demethylation has been studied in detail. It is an oxidative demethylation catalyzed by a cytochrome P -system. The first step involved in this reaction is the hydroxylation of the Cj -methy1-group to form the C -hydroxymethyl derivative. A second hydroxylation and loss of water lead to the C -formyl intermediate, which is hydroxylized a third time to form the corresponding carboxylic acid. Decarboxylation does not directly take place, but proceeds instead by abstraction of a proton from C, followed by elimination and formation of a A 4-double bond. The NADPH-dependent reduction of the A14 -double bond finishes the demethylation reaction. Subsequently, demethylation at has to take place twice, followed by a dehydrogenation reaction in A" -position and isomerization from A8 to A7 and A24(28) to A22. respectively. 

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