An Azafluorenone Alkaloid and a Megastigmane from<i>Unonopsis lindmanii</i>(Annonaceae)

Yoshida, Norimasa; Siqueira, João Máximo de; Rodrigues, Rossana Lott; Correia, Rodolfo Patussi; Garcez, Walmir Silva

doi:10.5935/0103-5053.20130090

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“…This late-stage functionalization strategy was also employed for the total synthesis of 7-hydroxy-5,8-dimethoxyonychine [53] (8) (Scheme 9). Commercially available carboxylic acid 39 was converted to pyridinemethanol 42 in three steps.…”

Section: Resultsmentioning

confidence: 99%

“…Ester 64 was therefore first converted to the carboxylic acid 65 via alkaline Scheme 9. Total synthesis of alkaloid 5,8-dimethoxy-7-hydroxyonychine [53] ester hydrolysis before reaction thereof with PPA gave 7methoxyonychine (11) in a somewhat higher yield of 15 % over two steps. 7-Methoxyonychine (11) has been previously synthesized by Pan et al in comparable overall yield using a similar methodology where the carboxylic acid 65 was first converted to the acyl chloride prior to AlCl 3 -catalyzed cyclization.…”

Section: Resultsmentioning

confidence: 99%

“…Scheme 9. Total synthesis of alkaloid 5,8-dimethoxy-7-hydroxyonychine[53] (8). Conditions: a) SOCl 2 , ethyl potassium malonate, MgCl 2 , NEt 3 , EtOAc, 76 %; b) benzyltrimethylammonium hydroxide (catalytic), crotonaldehyde, hydroxylammonium chloride, 1,4-dioxane/AcOH (1 : 1), rt -100 °C, 1 h, 45 %; c) LAH, THF, 18 h, 58 %; d) TBHP non , 1,2-DCE, 100 °C, 18 h, 21 % e) NBS, H 2 SO 4 conc.…”

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Collective Total Synthesis of 4‐Azafluorenone Alkaloids

Jourjine

Bracher

2023

Eur J Org Chem

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4-Azafluorenones are typically obtained by acid-mediated cyclization of 2-arylnicotinates. However, this approach fails to give 5-oxygenated 4-azafluorenones due to lactonization of 2-(2-alkoxy)phenylnicotinate intermediates. Herein, we report two modifications of established approaches to 4-azafluorenone synthesis that, either in combination or by themselves, enable the flexible preparation of 4-azafluorenones with diverse oxygenation patterns in the benzenoid ring. Undesired lactonization was circumvented via tert-butyl hydroperoxide (TBHP)mediated radical cyclization of 2-aryl-3-(hydroxymeth-yl)pyridines. In the absence of suitable protecting groups for phenolic intermediates, bromide substituents were regioselectively introduced as latent hydroxy groups and later converted under palladium catalysis. We present the first total syntheses of five 4-azafluorenone alkaloids muniranine, darienine, 5,8dimethoxy-7-hydroxyonychine, 5,6,7,8-tetramethoxyonychine, and 6,8-dihydroxy-7-methoxyonychine in addition to new total syntheses of six 4-azafluorenone alkaloids and one related pyridocoumarin alkaloid.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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