A similar idea is central to a useful bit of chemoselectivity that can be obtained in the react ions of dianions. 1-Propynol can be deprotonated twice by strong bases—first, at the hydroxyl group to make an alkoxide anion (the pKa of the OH group is about 16) and, secondly, at the alkyne (pKa of the order of 25) to make a dianion. When this dianion reacts with electrophiles it always reacts at the alkynyl anion and not at the alkoxide.

●Reactivity of dianions The anion that is formed last reacts first.

Vollhardt used this sort of chemoselectivity in his 1977 synthesis of the female sex hormone oestrone. He needed an alkyl iodide, which could be made by reacting an anion of a bis alkyne with ethylene oxide.

Although anions can often be formed straightforwardly next to alkynes, there are two other more acidic protons (black) in the molecule that would be removed by base before the green proton. However, treatment with three equivalents of butyl lithium removes all three, and the trianion reacts with ethylene oxide at the last-formed anionic centre to give the required compound.

مواضيع ذات صلة

The Boc protecting group—gastrin and aspartame

The Cbz protecting group—oxytocin

Peptide synthesis

A survey of protecting groups

How to react the less reactive group (II): protecting groups

How to react the less reactive group (I): react both then ‘unreact’ one

Competing reactivity: choosing which group reacts

How to oxidize primary alcohols to aldehydes

How to oxidize secondary alcohols to ketones

Birch reduction of arenes

Dissolving metal reductions

Getting rid of functional groups