Alkene stereochemistry in the Claisen rearrangement comes from a chair-like transition state
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص910-911
2025-07-24
581
Alkene stereochemistry in the Claisen rearrangement comes from a chair-like transition state
Stereochemistry may arise if there is a substituent on the saturated carbon atom next to the oxygen atom. If there is, the resulting double bond strongly favours the trans (E) geometry. This is because the substituent prefers an equatorial position on the chair transition state.
The substituent R prefers an equatorial position as the molecule reacts and R retains this position in the product. The new alkene bond is shown in green. Notice that the trans geometry of the alkene in the product is already there in the conformation chosen by the starting material and in the transition state.
The starting material for these aliphatic Claisen rearrangements consists of ethers with one allyl and one vinyl group. We need now to consider how such useful molecules might be made. There is no problem about the allyl half—allylic alcohols are stable, easily made com pounds. But what about the vinyl half? ‘Vinyl alcohol’ is just the enol of acetaldehyde (MeCHO).
The solution is to use an acetal of the aldehyde in an acid-catalysed exchange process with the allylic alcohol. It is not necessary to isolate the allyl vinyl ether as long as some of it is formed and rearranges into the final product.
The acid catalyst usually used, propanoic acid, has a conveniently high boiling point so that the whole mixture can be equilibrated at high temperature. The fi rst step is an acetal exchange in which the allylic alcohol displaces methanol.
The methanol is distilled off as it is the most volatile of the components in this mixture. A second molecule of methanol is now lost in an acid-catalysed elimination reaction to give the vinyl group.
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