Lewis Acid-Catalyzed 2,3-Dihydrofuran Acetal Ring-Opening Benzannulations toward Functionalized 1-Hydroxycarbazoles

Abstract

The development of a Lewis acid-catalyzed, intramolecular ring-opening benzannulation of 5-(indolyl)2,3-dihydrofuran acetals is described. The resulting 1-hydroxycarbazole-2-carboxylates are formed in up to 90% yield in 1 h. The dihydrofuran acetals are readily accessed from the reactions of enol ethers and α-diazo-β-indolyl-β-ketoesters. To highlight the method’s synthetic utility, a formal total synthesis of murrayafoline A, a bioactive carbazole-containing natural product, was undertaken.

Asymmetric Synthesis of Remotely Chiral Naphthols and Naphthylamines via Naphthoquinone Methides

“…Carbazole 2a was used as a nucleophile for the initial study in view of its wide occurrence in functional materials and biologically important molecules (Table ). Chiral phosphoric acids (CPAs) were evaluated as potential catalysts owing to their demonstrated excellent performance in the in situ generation and one-pot asymmetric transformations of QMs from the corresponding benzylic alcohols. − , To our delight, in the presence of a catalytic amount of CPA A1 , the reaction between 1a and 2a in DCM proceeded exceedingly smoothly at room temperature, providing the desired C–N bond formation product 3a in quantitative yield, albeit in only 29% ee (Table , entry 1). Further screening of other CPAs identified A4 , which bears 2,4,6-tricyclopentylphenyl groups at the 3,3′-positions, as the best catalyst (88% ee , entry 4).…”

Recent synthetic strategies for the construction of functionalized carbazoles and their heterocyclic motifs enabled by Lewis acids

“…It is noteworthy, that the reaction did not work when 3-indolyl dihydrofuran was explored (Scheme 57). 139 Wang and his groups showed a series of new carbazolequinones 90 prepared by a B(C 6 F 5 ) 3-catalyzed [4 + 2] cyclization reaction. This method involved a simple operation, a broad range of substrate variety, and high atomic economy, 1 mol% Lewis acid catalyst loading and avoided using toxic metal catalysts.…”

“…It is noteworthy, that the reaction did not work when 3-indolyl dihydrofuran was explored ( Scheme 57 ). 139 …”