- Total syntheses of (-)-vallesamidine and related Aspidosperma and Hunteria type indole alkaloids from the common intermediate
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A new synthetic method of (-)-vallesamidine, including a unique 2,2,3-trialkylindoline skeleton, was developed by reductive radical cyclization reaction from the 2,3-dialkylindole derivative, which has been known to be an intermediate for the synthesis of
- Tanino, Hideo,Fukuishi, Kazuhisa,Ushiyama, Mina,Okada, Kunisuke
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p. 3273 - 3282
(2007/10/03)
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- Cycloaddition Reaction of Mesoionic Betaines as an Approach toward Trialkylindoline Alkaloids
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Intramolecular 1,4-dipolar cycloaddition of an anhydro-4-hydroxy-2-oxo-1,3-thiazium hydroxide across a tethered indole π-bond has been used for the construction of the pentacyclic skeleton of epi-16,17-dihydroeburnamenine. The reaction of 3-ethyl-3-(alkenyl)piperidinones with diketene and trimethylsilyl triflate in benzene at ambient temperature produced annulated pyridones in good yield. The initial reaction involved formation of a N-acetoacetylated amide which was further converted to the pyridone with TMSOTf. The overall process was found to proceed with complete stereospecificity. Treating a sample of 3-ethyl-3-[(E)-4-phenyl-3-butenyl]-2-piperidone with diketene and TMSOTf produced a cycloadduct in 63% yield whose stereochemistry was elucidated by a X-ray crystallographic study. The epimeric Z-isomer produced a different stereoisomer of the annulated dihydropyridone. The mechanism of the annulation involves a TMSOTf induced cyclization followed by proton removal and generation of a cross-conjugated heteroaromatic betaine. This 1,4-dipole undergoes a subsequent intramolecular dipolar cycloaddition across the neighboring π-bond, and the resulting cycloadduct is subsequently converted to the annulated lactam. A related annulation sequence leading to a key intermediate previously utilized in the synthesis of the (±)-vallesamidine has been developed which is based on the intramolecular dipolar cycloaddition of a mesoionic betaine intermediate.
- Padwa, Albert,Harring, Scott R.,Semones, Mark A.
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- An expedient synthesis of epi-eburnamenine via an intramolecular 1,4-dipolar cycloaddition reaction
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The intramolecular 1,4-dipolar cycloaddition of an anhydro-4-hydroxy-2-oxo-1,3-thiazium hydroxide across a tethered indole π-bond has been used for the construction of the pentacyclic skeleton of epi-16,17-dihydroeburnamenine.
- Padwa, Albert,Semones, Mark A.
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p. 335 - 338
(2007/10/02)
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- Synthetic Approach to (+/-)- Vincamine via Cleavage of an α-Diketone Monothioacetal. Alternative Synthesis of (+/-)-Eburnamine, (+/-)-Isoeburnamine, and (+/-)-Eburnamenine
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The half ester (8) prepared from cleavage of 2-(1,3-dithian-2-yl)-4-ethoxycarbonyl-4-ethylcyclohexanone (7) has been converted into (+/-)-eburnamine (20), (+/-)-isoeburnamine (21), and (+/-)-eburnamenine (22) by a stereospecific reaction sequence proceeding via the dithian intermediate (16).However an attempted conversion of (16) into (+/-)-vincamine (6) was unsuccessful.
- Takano, Seiichi,Hatakeyama, Susumi,Ogasawara, Kunio
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p. 457 - 461
(2007/10/02)
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