Question #93782

1 Answer
Nov 5, 2016

Here's what I get.

Here's the structure of cholesterol with the configuration at each chiral centre.

upload.wikimedia.org

And here's another structure showing the numbering.

upload.wikimedia.org

The eight chiral centres (with their attached groups in descending order of priority) are:

1. #"C3 (O, C4, C2, H)"#

#"C4 >C2"# because #"C5 > C1"#.

∴ The configuration at #"C3"# is (#R#).

2. #"C8 (C9, C14, C7, H)"#

#"C9 > C14"# because #"C5 > C17"#.

The configuration at #"C8"# is (#S#).

3. #"C9 (C10, C8, C11, H)"#

The configuration at #"C9"# is (#S#).

4. #"C10 (C5, C9, C1, C19)"#

The configuration at #"C10"# is (#R#).

5.#"C13 (C14, C17, C12, C18)"#

#"C14 >C17"# because #"C9 > C22"#.

∴ The configuration at #"C13"# is (#R#).

6. #"C14 (C13, C8, C15, H)"#

The configuration at #"C14"# is (#S#).

7. #"C17 (C13, C20, C16, H)"#

The configuration at #"C17"# is (#R#).

8. #"C20 (C17, C22, C21, H)"#

The configuration at #"C20"# is (#R#).

A complete stereochemical name would be #(3R,8S,9S,10R,13R,14S,17R,20R)"-10,13-dimethyl-17-(1,3-dimethylhexyl)-"#
#"2,3,4,7,8,9,11,12,14,15,16,17-dodecahydro-"1H"-cyclopenta[a]phenanthren-3-ol"#.

Now you see why we call it "cholesterol".