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3.7.1.5 The Carroll–Claisen Rearrangement

DOI: 10.1055/sos-SD-203-00213

Zeh, J.; Hiersemann, M.Science of Synthesis: Stereoselective Synthesis, (20113359.

In the case of the CarrollClaisen rearrangement, the allyl vinyl ether is generated in situ by enolization of a β-keto allyl ester. The α-allylated β-keto acids, which are the initial products of rearrangement, undergo decarboxylation to afford the corresponding γ,δ-unsaturated ketones.[‌13‌] An instructive example is found in the total synthesis of zincophorin (6) illustrated in Scheme 15.[‌25‌] The β-keto allylic ester 46, prepared by esterification of the corresponding tertiary allylic alcohol with diketene, is cleanly converted into the desired rearrangement product 48 by absorption on neutral alumina and heating to 60°C. Interestingly, the thermal rearrangement gives an intractable mixture of products, and rearrangement under basic conditions (lithium diisopropylamide, refluxing tetrahydrofuran) gives a conjugated diene by elimination. The stereochemical course of the concerted bond reorganization process can be explained by the formation of a chair-like transition-state structure 47 in which an intramolecular hydrogen bond supports the formation of the E-enol. As the keto functionality is not present in zincophorin (6), it is removed in three steps by reduction to the corresponding secondary alcohol, mesylation, and reductive demesylation to give the key intermediate 49.

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