Stitching Ynones with Nitromethanes: Domino Synthesis of Functionally Enriched Benzofurans and Benzothiophenes

Acid-Promoted Phosphorylation of Ynones for the Synthesis of Phosphoryl Enones

An efficient phospha-aldol/Meyer-Schuster rearrangement cascade reaction between propargylic aldehydes and phosphine oxides has been developed in which various phosphoryl enones were obtained in moderate to excellent yields. A comprehensive series of mechanistic experiments, including the identification of key intermediates and the application of 18O isotope labeling, has confirmed that this cascade reaction proceeds through a phospha-aldol followed by Meyer-Schuster rearrangement cascade reaction.

Benzannulation of Functionally Enhanced Indole-3-carbaldehydes with Ynones and Alkynoates: A Domino Approach to Bioactive Carbazoles─Synthesis of Clauolenzole A, Calothrixin A & B, Methyl Carbazole-3-carboxylate, and Quinocarbazole

A flexible, regioselective, benzannulation strategy toward multifunctional carbazoles from 2-(2-oxo-2-arylethyl)indole-3-carbaldehydes, employing either ynones or alkynoates as reaction partners, has been envisaged and implemented. This enabling access to variegated carbazoles in one-flask operation leads to strategic substituent diversification via reaction partner variation. The efficacy and applications of this methodology are demonstrated through 23 examples and concise syntheses of bioactive clauolenzole A, calothrixin A & B, methyl carbazole-3-carboxylate, and pharmacophoric quinocarbazole.

C3-Chlorination of C2-substituted benzo[b]thiophene derivatives in the presence of sodium hypochlorite

Benzo[b]thiophene rings are common synthons for the development of novel drugs and materials, and thus, the discovery of facile ways for their functionalization is of value. In this work, a new method for the C3-chlorination of C2-substituted benzothiophene derivatives is described. The chlorine source is sodium hypochlorite pentahydrate (NaOCl·5H2O), and optimal transformations occur in aqueous acetonitrile at 65-75 °C to provide the corresponding C3-halogenated products in variable yields (30-65%). The reaction occurs in the presence of vinyl and alkyl groups, while the presence of alcohols leads to competing oxidation reactions at the heterobenzylic position. The presence of a carbonyl group at the C2-position inhibited the halogenation reaction, while the use of benzofuran led to a highly exothermic reaction, presumably via the formation of a peroxide intermediate. Reactions carried out at lower temperatures led to side reactions associated with competing oxidative processes. To gain a better understanding of the mechanism of the reaction, DFT calculations were carried out, where the heteroatom enables the formation of a hypochlorous acidium ion that serves to generate a C2-C3 chloronium ion intermediate in a step-wise manner, which in turn leads to the formation of an S-stabilized C2-carbocation that undergoes re-aromatization to the corresponding C3-chlorinated products. To probe potential synthetic applications, a model C3-chloro derivative was coupled with phenylboronic acid using standard Suzuki-Miyaura coupling conditions.

In Situ-Generated Ammonia Mediates Deep Restructuring of o-Bis-Ynones through a Cascade Process: One-Pot Synthesis of 2-Azafluorenones

One-pot synthesis of 2-azaflorenones from readily accessed o-bis-ynones through Michael addition, orthogonal aldol reaction, dehydrative isomerization, and a 6-endo-dig-cyclization cascade, triggered by in situ-generated ammonia in the presence of a Cu(I) catalyst, has been discovered and its generality scoped. A few selected reactions of a prototypical 2-azafluorenone have been explored for functionality augmentation in its core structure. Overall, this operationally convenient 2-azafluorenone synthesis involves the formation of five new bonds (3 C-N and 2 C-C) in one pot and embodies many green and sustainable features; notably, the reagent ammonia is subsumed into the reactant o-bis-ynones with atom economy, and the only by-product is water.

Tandem Michael-anti-Michael Addition-Mediated Orthogonal Strapping of Diynones: Regioselective Spirocyclopentannulation of Oxindoles and Pyrazolones and DFT Validation

An efficient protocol involving the transformation of sequentially generated recursive anions from heterocyclic precursors to orthogonally strap diynones through one pot transition-metal-free spirocyclopentannulation has been devised, employing oxindoles and pyrazolones as prototypical platforms. Insights into these regioselective tandem Michael-anti-Michael processes have been gleaned through DFT calculations.