Substituent-Guided Palladium-Ene Reaction for the Synthesis of Carbazoles and Cyclopenta[b]indoles

Abstract

An efficient palladium-catalyzed intramolecular Trost–Oppolzer type Alder-ene strategy was developed for the synthesis of carbazoles and cyclopenta­[b]­indoles from easily accessible­(3-allyl-1H-indol-2-yl)­methyl acetates. This strategy was extended for the synthesis of naphthalenes and dibenzobenzofurans as well. In addition, a short synthesis of antibacterial and antifungal natural product glycozoline and its analogues was also achieved.

Palladium‐Catalyzed Intramolecular Alder‐Ene Type Cycloisomerization Reactions

“…The idea of merging electrophilic features of the Tsuji-Trost reaction with the nucleophilic features of Alder-ene reaction led us to design 36 as starting compounds (Scheme 19). [23] It was hypothesized that the π-allylpalladium species AR could be trapped by the tethered olefin in an Alder-ene fashion to generate the carbocation AS which might undergo deprotonation to give a plausible mixture of regioisomers AT. The dihydrocarbazoles AT could then be oxidized to obtain benzenoids of the type 37.…”

“…An interesting observation was made during the evaluation of the substrate scope (Scheme 19). [23] When unsubstituted allyl side chain was employed (when R=H in 36), the formation of fused cyclopentenes 38 was observed. Thereby, a variety of cyclopenta [b]indoles were prepared in good to excellent yields.…”

Synthesis of Heteroarene‐Fused Cyclopentadienes and Related Compounds Suitable for Metallocene Preparation

“…Halogen-substituted indoles and 3-(4-buten-1-yl)indole derivative also formed dihydrocyclopenta[b]indole derivatives 332-334 with average yields (Scheme 51C). [190] Original catalytic approach involving Au (I)/ Brønsted acid to polysubstituted cyclopenta[b]indoles from easily accessible 1-(2-aminophenyl)prop-2-ynols and 1,3-dicarbonyl compounds had been developed by Ramasastry et al [191] The mechanism of this reaction involved Au (I) mediated cyclization to 2-alkylideneindolin-3-ol, formation of allyl cation and interaction with 1,3-dicarbonyl compound with cyclization at the final step (Scheme 52A). Representative sample of 21 compounds 335-355 (Scheme 52B) was prepared to confirm high efficiency of this method.…”

“…[196] FeCl 3 -mediated Nazarov cyclization was used in the synthesis of 2-carbethoxysubstituted benzo[b]cyclopenta[d]thiophene derivative 399 (Scheme 57C). [181] Organocatalytic method, based on Morita-Baylis-Hillman reaction, and proposed by Satpathi and Ramasastry for the synthesis of 2-alkylidene-1H-benzo [b]cyclopenta[d]thiophen-3(2H)-ones, [190,[197][198][199] was applicable only to the obtaining of 3-hydroxy or 1,3diketo derivatives which are outside the scope of our review. At the same time, 2-alkylidene-2,3-dihydro-1H-benzo[b]cyclopenta[d]thiophen-1-ones 400-403 were obtained by organocatalytic cyclization of 2methyl-3-alkynoylbenzo[b]thiophenes (Scheme 58A).…”

“…Synthesis of dihydrocyclopenta[b]indoles by tandem Tsuji-Frost/Alder-ene reaction (A), proposed reaction mechanism (B) and increase in the range of substrates (C). [190] Scheme 52. Synthesis of substituted dihydrocyclopenta[b]indoles by Au (I) catalyzed cyclization of 1-(2-aminophenyl)prop-2-ynols followed by the reaction with 1,3-dicarbonyl compounds: reaction mechanism (A) and compounds prepared (B).…”