Why is the epoxidation of cholesterol stereoselective?

Answer 1

The epoxidation of cholesterol is stereoselective because the C-19 methyl group hinders attack from the top side of the molecule.

Epoxidation involves the concerted syn attack of a peroxy acid on an alkene.

The structure of cholesterol is

There are eight chiral centres. The one of concern to us is C-10 and the methyl group (C-19) attached to it.

The CH₃ group is "up". It is bulky enough to provide much steric hindrance to a reactant trying to attack C-5 and C-6 from the top side.

Hence the epoxide forms exclusively on the unhindered "bottom" side of the molecule.

The only significant product is 5α,6α-epoxycholestan-3β-ol (α is "down"; β is "up").

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Answer 2

Easton Jones

Due to the stereochemistry of the cholesterol molecule, epoxidation occurs selectively at the C5-C6 double bond, resulting in a specific stereochemical arrangement of the epoxide product.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.