You may think that this method is rather too trivial to be called a method for controlling the geometry of double bonds, as it’s only of any use for making cyclic alkenes. Well, chemists are more ingenious than that! It is not necessary to have an all-carbon ring to preserve the cis geometry of a double bond. Lactones (cyclic esters) and cyclic anhydrides are useful too.

A double bond in a five- or six-membered compound must have a cis configuration and com pounds like these are readily made. Dehydration of this hydroxylactone can give only a cis double bond and ring-opening with a nucleophile (alcohol, hydroxide, amine) gives an open chain compound also with a cis double bond.

He realized that the essential Z double bond would be easy to make if he were to start with a cyclic molecule (in which only cis double bonds are possible) that could be ring-opened to the com pound he needed. This is how he did it.

Birch reduction of a simple aromatic ether generated two cis double bonds. The more reactive (because it is more electron-rich) of these reacts with ozone to give an aldehyde-ester in which the Z geometry is preserved. NaBH₄ reduces the aldehyde group to a hydroxyl group, which needs to be got rid of: a good way to do this is to tosylate and reduce with LiAlH4, which substitutes H for OTs. The LiAlH₄ also does the job of reducing the ester to an alcohol, giving the Z-configured compound that Corey needed.

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شعبة الحرم الشريف تواصل تنفيذ خطتها الخاصة بشهر المحرّم داخل مرقد أبي الفضل العباس (عليه السلام)

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مواضيع ذات صلة

Predominantly E alkenes can be formed by stereoselective elimination reactions

Addition of nucleophiles to alkynes

E-selective reduction of alkynes using sodium in liquid ammonia

Using light to make Z alkenes from E alkenes

Equilibration of alkenes

Lewis acids promote couplings via oxonium ions

Allyl silanes are more reactive than vinyl silanes but also react through β-silyl cations

Allyl silanes as nucleophiles

Vinyl silanes offer a regio- and stereoselective route to alkenes

Aryl silanes undergo ipso substitution with electrophiles

Silicon stabilizes a positive charge on the β carbon

Alkynyl silanes are used for protection and activation