Scarpi D, Bagni F, Faggi C, Carral-Menoyo A, Gómez-Bengoa E, Occhiato EG. Gold(I)-Catalyzed Cycloisomerization/Hetero-Diels-Alder Reaction/Ring Opening Cascade to Functionalized Cyclopentadienes.
J Org Chem 2022;
87:6038-6051. [PMID:
35442687 DOI:
10.1021/acs.joc.2c00296]
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Abstract
Six- and seven-membered ring-fused, functionalized cyclopentadienes can be obtained in moderate to excellent yields by a cascade process entailing the Au(I)-catalyzed propargyl Claisen rearrangement/Nazarov cyclization of propargyl vinyl ethers, the hetero-Diels-Alder reaction with dialkylazodicarboxylates, and the spontaneous conversion of cycloaddition products into cyclopentadienes by a highly regioselective cleavage of a C-N bond. Depending on the treatment of the crude reaction mixtures, two types of products can be obtained: cyclopentadienes with pendant hydrazine and aldehyde moieties that intramolecularly react to form hemiaminals are obtained in 43-52% overall yields when the crude reaction mixtures are left over K2CO3 in a DCM solution. Instead, by reducing in situ the aldehyde group just after addition of the heterodienophile, the regioselective C-N bond cleavage generates the corresponding cyclopentadienes bearing a hydrazine and an alcohol appendage in excellent yields (66-82%) over four steps, all in one pot. Two examples from the latter class of compounds were also converted into ring-fused, functionalized cyclopentadienes, bearing a protected amino group, by the selective N-N cleavage of the hydrazine moiety.
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