What is it about?
One-pot technique including thermally induced cyclization of amino propargylic alcohols followed by Dess–Martin periodinane-promoted oxidative dearomatization of intermediate 4,5,6,7-tetrahydroindoles grants a novel and distingue entry to 5,6-dihydro-1H-indol-2(4H)ones, which perfectly serve as universal key substrates in the formal total syntheses of numerous Erythrina and Lycorine alkaloids.
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Why is it important?
The main goal of the work wasn’t just to show the novel cyclization process, which, in our opinion, is indeed exciting but too standalone and specialized; but to make the meaningful synthetic use of C2-unsubstituted 4,5,6,7-tetrahydroindoles, which are otherwise useless in isolated form due to their lability and the lack of any novel synthetic implications.
Perspectives
The current work is based on the symbiosis of solvent- and metal-free transformations as well as the chemistry of hypervalent iodine compounds. These fields of organic chemistry have become extensively explored in the last couple of decades, since they provide an attractive alternative for conventional transition metal-mediated cyclization and oxidation reactions. I hope that our work could be of exceptional interest to organic chemists and especially the researchers involved in the field of green chemistry and metal-free and/or polyvalent-iodine promoted transformations.
Mr Ivan A. Andreev
Moskovskij gosudarstvennyj universitet imeni M V Lomonosova
Read the Original
This page is a summary of: Oxidative Dearomatization of 4,5,6,7-Tetrahydro-1H-indoles Obtained by Metal- and Solvent-Free Thermal 5-endo-digCyclization: The Route to Erythrina and Lycorine Alkaloids, Chemistry - A European Journal, April 2016, Wiley,
DOI: 10.1002/chem.201600273.
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Resources
Coral tree
Our synthetic approach to Erythrina alkaloids may in general be envisioned as the growth of the coral tree. In that case, 4,5,6,7-tetrahydroindoles symbolize roots, 5,6-dihydro-1H-indol-2(4H)-ones act as the stem of the tree and, finally, erythrinanes burst into blossom as countless magnificently red flowers.
Synthesis of 4,5,6,7-Tetrahydro-1H-indole Derivatives Through Successive Sonogashira Coupling/Pd-Mediated 5-endo-dig Cyclization
A one-pot Sonogashira cross-coupling/5-endo-dig cyclization procedure leading to 2-aryl-4,5,6,7-tetrahydroindoles was developed. This short (only two steps from commercially available compounds) sequence avoids harsh conditions and expensive catalysts. Our procedure is highly tolerant to a range of functional groups (amino, nitro, carboxy, cyano, hydroxy, and bromo). A family of 21 tetrahydroindoles was synthesized on a gram scale in a good to excellent yields, which is indicative of the general character and scalability of this reaction. This methodology allows access to indoles bearing miscellaneous substituents at the C2 position (by variation of ArI) and at the nitrogen atom (by variation of RNH2, including chiral moieties)
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