Intramolecular crossed Claisen ester condensations
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص652-653
2025-07-01
383
In the same way as with intramolecular aldol condensations, we do not have to worry so much about controlling where enolization occurs providing that one product is more stable than the others—for example, it might have a five- or a six-membered ring (rather than a four- or eight-membered one)—and we carry out the reaction under equilibrating conditions. A couple of examples should show what we mean. Although there are two sites for enolate anion formation, one would give a four-membered ring and can be ignored. Only enolization of the methyl group leads to a stable six-membered ring.
In this next example the two possible sites for enolate anion formation would both lead to stable five-membered rings. The product forms a stable enolate under the reaction conditions but the alternative cannot form a stable enolate as there is no hydrogen atom between the two carbonyl groups.
In the next example, there are three possible sites for enolate anion formation, but only one product is formed and in good yield too. If we consider all three possible enolate anions, the choice is more easily made. First, the reaction that does happen. An enolate anion is formed from the ketone at the green site and acylation at carbon follows. The product is a fused rather than a bridged bicyclic structure and can easily form a stable enolate anion.
We could form the enolate anion on the other side of the ketone at the orange site and attack the ester in the same way. The product would be a bridged bicyclic diketone, and is not formed (see above). The third possible enolate site (brown) could give an aldol reaction but the product would again be a bridged bicyclic compound and is not formed.
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