Palladium-Catalyzed Synthesis of Pyrayaquinones, Murraya­quinones, and Murrayafoline-B

Abstract

We describe the total synthesis of murrayafoline-B and seven carbazole-1,4-quinone alkaloids. A palladium(II)-catalyzed oxidative cyclization is used to construct the carbazole skeleton. Pyran annulation and oxidation provide pyrayaquinone-A, -B, and -C. DIBAL-H-promoted reductive ring opening of pyrano[3,2-a]carbazole precursors leads to the prenylated and geranylated carbazole-1,4-quinone alkaloids murrayaquinone-B, -C, -D, and -E and to murrayafoline-B.

Iodine and DMSO as Surrogate of Hazardous Metal and Non‐Metal Reagents in Organic Synthesis

“…Another traditional method involves multistep oxidation of tetrahydrocarbazol‐1‐one (Scheme 57 (a)) [566] . However, there are other methods also which further involve the use of toxic reagents like DDQ and CAN‐SiO 2 [567] and Frémy's salt ((KSO 3 ) 2 NO) supported by DIBAL−H (Scheme 57 (b) and (c)) [568] . To avoid these issues, Humne and colleagues have developed a new method for the preparation of Murrayaquinone derivatives starting from 2‐bromo‐2,3,4,9‐tetrahydro‐1 H ‐carbazol‐1‐one derivatives using 10–100 mol% of iodine and DMSO as the solvent (Scheme 57 (d)) [569] .…”

Trends in carbazole synthesis – an update (2013–2023)

“…the Knölker group being a main contributor in this field. [194][195][196][197][198][199][200][201][202][203][204][205][206][207][208][209][210]211,212 Mechanistically, the carbon-carbon bond formation is initiated by two sequential electrophilic attacks of the palladium catalyst on diarylamine 82, generating six-membered palladacycle 84. Reductive elimination generates the desired carbazole 83, and a palladium(0)-species, which is reoxidised in situ by a co-catalyst (e.g., copper(II) acetate, oxygen, air, tert-butyl hydroperoxide etc.).…”

“…A representative selection of successfully completed total synthesis is shown in Scheme 26, 188–193 with the Knölker group being a main contributor in this field. 194–210,211,212 Mechanistically, the carbon–carbon bond formation is initiated by two sequential electrophilic attacks of the palladium catalyst on diarylamine 82 , generating six-membered palladacycle 84 . Reductive elimination generates the desired carbazole 83 , and a palladium(0)-species, which is re-oxidised in situ by a co-catalyst ( e.g.…”