Oxypalladation Initiating the Oxid­ative Heck Reaction with Alkenyl ­Alcohols: Synthesis of Isocoumarin-Alkanones

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

3-Amino-4-(diphenylamino)-1H-2-benzopyran-1-one

Various synthetic methodologies to obtain 3,4-diaminoisocumarin nucleus have been reported and described. However, mechanistic analysis based on experimental evidence is lacking. Herein, we report the synthesis of the novel 3-amino-4-(diphenylamino)-1H-2-benzopyran-1-one using a two-step methodology with a new mechanistic proposal to explain the formation of the latter based on previously reported precursors and the established conditions. This compound was afforded in 80% yield.

Unexpected Formation of 4-[(1-Carbamoyl-3-oxo-1,3-dihydro-2-benzofuran-1-yl)amino]benzoic Acid from 4-[(3-Amino-1-oxo-1H-2-benzopyran-4-yl)amino]benzoic Acid

With the aim of obtaining derivatives belonging to 2′,3′-diphenyl-3H-spiro[[2]benzofuran-1,4′-imidazole]-3,5′(3′H)-dione nucleus, we synthesized 4-[(3-amino-1-oxo-1H-2-benzopyran-4-yl)amino]benzoic acid (a 3,4-diaminoisocoumarine derivative), a known precursor of 4-[(1-carbamoyl-3-oxo-1,3-dihydro-2-benzofuran-1-yl)amino]benzoic acid (a phthalide–carboxamide-bearing system) by a novel methodology that we report here. The reaction conditions were optimized to afford the latter in 62% yield.

Emergence of 2-Pyrone and Its Derivatives, from Synthesis to Biological Perspective: An Overview and Current Status

Pyrone moieties are present in natural products and can be synthesized by a diverse range of synthetic methods, resulting in the formation of various derivatives through chemical modifications. Many pyrone-based derivatives are commercially available and are biocompatible. They are building blocks of various intermediates in organic synthesis. They possess remarkable biological properties including antimicrobial, antiviral, cytotoxic, and antitumor activity. These characteristics have made them valuable for the development of drugs. We have summarized recent developments in the synthesis of 2-pyrone and its derivatives and their potential applications. With regard to synthetic approaches, the focus has been on metal-free and transition metal-catalyzed reactions.

Enantioselective Synthesis of Fused Isocoumarins via Palladium-Catalyzed Annulation of Alkyne-Tethered Malononitriles

An enantioselective palladium-catalyzed annulation of alkyne-tethered malononitriles for the synthesis of 3,4-ring-fused isocoumarins is described. This cascade strategy involves oxypalladation of ortho-alkynylbenzoates and desymmetrizing addition onto one cyano group of the pendant malononitriles, which enables the concurrent construction of two rings and an all-carbon quaternary stereocenter in a single operation.

Palladium-Catalyzed Aminocyclization-Coupling Cascades: Preparation of Dehydrotryptophan Derivatives and Computational Study

Dehydrotryptophan derivatives have been prepared by palladium-catalyzed aminocyclization-Heck-type coupling cascades starting from o-alkynylaniline derivatives and methyl α-aminoacrylate. Aryl, alkyl (primary, secondary, and tertiary), and alkenyl substituents have been introduced at the indole C-2 position. Further variations at the indole benzene ring, as well as the C-2-unsubstituted case, have all been demonstrated. In the case of C-2 aryl substitution, the preparation of the o-alkynylaniline substrate by Sonogashira coupling and the subsequent cyclization–coupling cascade have been performed in a one-pot protocol with a single catalyst. DFT calculations have revealed significant differences in the reaction profiles of these reactions relative to those involving methyl acrylate or methacrylate, and between the reactions of the free anilines and their corresponding carbamates. Those calculations suggest that the nature of the alkene and of the acid HX released in the HX/alkene exchange step that precedes C–C bond formation could be responsible for the experimentally observed differences in reaction efficiencies.

Palladium-Catalyzed Oxidation Reactions of Alkenes with Green Oxidants

Transition-metal-catalyzed oxidative functionalization of alkenes has emerged as a powerful and valuable tool in modern organic synthesis. Recently, many methods have been established for the assembly of C-C and C-heteroatom bonds, which provides tremendous possibility for application in biology, medicine, and materials science. However, the use of stoichiometric amounts of strong oxidants leads to poor selectivity, low atom economy, and a series of undesired waste products. By contrast, green oxidants, such as O₂ , H₂ O₂ , or tert-butyl hydroperoxide (TBHP), have bright prospects due to their attributes of mild, low cost, and great sustainability in transition-metal-catalyzed oxidation reactions. Based on the great and unique potential for the development of aerobic reactions, this review mainly highlights homogenous palladium-catalyzed green oxidations of alkenes that have been reported in recent years. These methods provide new strategies for the transformation and functionalization of alkenes; some of them have also been successfully applied to the synthesis of the core structures of drugs and natural products. Additionally, through in-depth studies of the reaction mechanisms in this field, it is believed that palladium-catalyzed green oxidation reactions of alkenes with O₂ , H₂ O₂ , or TBHP will create added value for organic synthetic chemistry.

Palladium-Catalyzed Cascade Intramolecular Cyclization and Allylation of Enynoates with Allylic Alcohols

A Pd(II)-catalyzed mild and highly regioselective 6- endo cyclization/allylation reaction of enynoates with simple allylic alcohols has been developed. Under mild reaction conditions, the vinyl palladium species generated in situ after cyclization could insert C-C double bond of allylic alcohol through cross-coupling reaction and lead to the formation of allyl pyrone via β-OH elimination. This cascade cross-coupling reaction represents a direct and atom economic methodology for the construction of novel allyl pyrones in moderate to good yields.