3S-squalene-2,3-epoxide

The first enantioselective polyene tetracydization starting with a chiral epoxide was reported by Corey et al. in 1997 [8a]. The silylated enol ether 3 (Scheme 1) was converted into the tetracycle 4 by treatment with the Lewis acid MeAlCl2 at -90 °C. The synthetic route is modeled on the biosynthesis of lanosterol from (3S)-squalene 2,3-epoxide and has also been applied to the biomimetic synthesis of tetracyclic polyprenoids from sediment bacteria [8b]. 

The complex formed on addition of cuprous iodide to a solution of a lithium dialkylamide in ether or tetrahydrofuran is effective in the reductive coupling of allylic halides to give 1,5-dienes with preservation of stereochemistry. This method has been used5 for the stereospecific synthesis of all-trans-squalene and (E,Z,Z,E) squalene from (E,E)- and (Z,JE)-farnesyl bromides, respectively. In an attempted synthesis of (3S)-squalene-2,3-epoxide, 4-[(4R)-2,2,5,5-tetramethyl-l,3-dioxolan-4-yl]butan-2-one (1) and the phosphonium iodide (2) were prepared.6 Unfortu-... 

The sequence of transformations from squalene to lanosterol begins by the enzymatic oxidation of the 2,3-double bond of squalene to form (3S)-2,3-oxidosqualene [also called squalene 2,3-epoxide]. 

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