Nazarov cyclization of 1,4-pentadien-3-ols: preparation of cyclopenta[b]indoles and spiro[indene-1,4′-quinoline]s

Abstract

The first Lewis acid-catalyzed intramolecular interrupted Nazarov cyclization of 1,4-pentadien-3-ols is described. Using FeBr3 as the catalyst, a series of new substituted cyclopenta[b]indoles was prepared—through a sequence of Nazarov cyclization, nucleophilic amination, and isomerization—with good yields and high diastereo- and regioselectivities. A similar catalytic process was also developed for the synthesis of structurally interesting spiro[indene-1,4′-quinoline]s.

Synthesis of 2-(1H-Indol-2-yl)acetamides via Brønsted Acid-Assisted Cyclization Cascade

“…To test this idea, chalcone 16aa was subjected to the tosylation reaction according to the standard procedure (Scheme 13). 30 Hydrocyanation of the resulting sulfonamide 34 was carried out under the conditions of Procedure 2 to afford ketonitrile 35 in excellent overall yield (Scheme 13). To ensure that the removal of the sulfonyl protection takes place and the featured transformation proceeds smoothly, compound 35 was subjected to the reaction under the conditions of Procedure 5.…”

Total Synthesis of (−)‐Daphenylline

“…Guided by the work of Cao and coworkers, [11o] the intramolecular Friedel-Crafts acylation with simultaneous removal of the methyl group was achieved by treating 27 with thionyl chloride followed by treatment with AlCl 3 in dichloromethane in ao ne-pot fashion. Inspired by aNazarov-type electrocyclization [22] of the divinyl alcohol, we thought that direct reduction 29 may lead to ad ehydrative Nazarov-type electrocyclization or an intramolecular Friedel-Crafts alkylation. After the requisite Suzuki coupling,product 29 was obtained by employing the conditions reported by Zhai and co-workers.…”

Recent metal-catalysed approaches for the synthesis of cyclopenta[b]indoles

“…In 2016, Kwon and co-workers demonstrated an interesting Lewis acid mediated Nazarov cyclisation of 1,4-pentadien-3-ols 106 ( Scheme 26 ). 50 An inexpensive and environmental friendly FeBr 3 was used as a catalyst for the transformation. A diverse range of cyclopenta[ b ]indoles 107 were synthesised in a highly regio- and stereoselective manner under mild Lewis acidic conditions.…”