Palladium-Catalyzed Aerobic Oxidative Coupling of ortho-(Alkynyl)styrenes with Allylic Alcohols via 6-endo-dig Cyclization: Regioselective Construction of Polysubstituted Naphthalenes

Advances in Transition Metal-Catalyzed C(sp2)-H Bond Functionalization Using Allyl Alcohols

The transition metal-catalyzed directed site-selective C─H bond functionalization utilizing allyl alcohols as coupling partner has been an intriguing area of research and has made considerable advances during the past decade. Multifunctional coupling characteristics of the allyl alcohol in the regioselective C(sp2)-H functionalization using transition metal-catalysis produces alkyl, alkenyl, allyl, and annulated products. These reactions provide an effective synthetic tool to afford diverse functionalized scaffolds that are of interest in synthetic and medicinal sciences. This review covers the developments of directed site-selective C(sp2)-H functionalization with unactivated allyl alcohols as the coupling partner using the transition metal-catalysis till December, 2024.

Palladium-Catalyzed Bicycloaromatization of o-(Alkynyl)styrenes with Alkynes: Economical Access to Chrysene Derivatives

Herein, a palladium-catalyzed bicycloaromatization of o-(alkynyl)styrenes with alkynes is reported. In this protocol, the 6-endo-dig cyclization of o-(alkynyl)styrenes is followed by deprotonation to complete the first cycloaromatization, and then, a regioselective alkyne insertion/C-H activation occurs to achieve the second cycloaromatization, resulting in atom- and step-economical syntheses of polysubstituted chrysenes. Notably, the products can be further used to construct π-extended arenes using the Scholl reaction.

One-Pot Dual C-C Bond-Forming Cascade Process via Suzuki Coupling and Intramolecular Cyclocondensation: An Access to Functionalized Naphthalenes

Naphthyl ketones encompass a class of essential compounds of biological and industrial significance. The development of synthetic strategies to achieve naphthyl ketones in single-step reactions from readily available materials is highly desirable. Herein, domino one-pot synthesis of functionalized naphthyl ketones via intermolecular Suzuki-Miyaura coupling, followed by intramolecular distal aldol-type condensation with the γ-methyl/methylene groups is reported. The present strategy displayed a comprehensive substrate scope and good functional group tolerance and enabled access to a variety of substituted naphthyl ketones, including aromatic-aliphatic hybrid frameworks of interesting structural features starting from the easily accessible starting materials such as α-iodoenone and 2-formylphenylboronic acids as well as 2-formylphenylboronic acid pinacol esters.

Visible-Light-Enabled Ph3P/LiI-Promoted Tandem Radical Trifluoromethylation/Cyclization/Iodination of 1,6-Enynes with Togni's Reagent

We report the visible-light-induced Ph₃P/LiI-promoted intermolecular cascade trifluoromethyl radical addition/5-exo-dig cyclization/iodination of 1,6-enynes with Togni's reagent using LiI as the iodine source without the need of the transition metal, oxidant, and base. This reaction promises to be a useful method for the preparation of trifluoromethyl-substituted and vinyl C-I bond-containing pyrrolidines and benzofuran products with good regioselectivity and functional-group tolerance under ambient conditions.

Recent Advances in Palladium-Catalyzed Oxidative Couplings in the Synthesis/Functionalization of Cyclic Scaffolds Using Molecular Oxygen as the Sole Oxidant

Over the past years, Pd(II)-catalyzed oxidative couplings have enabled the construction of molecular scaffolds with high structural diversity via C–C, C–N and C–O bond-forming reactions. In contrast to the use of stoichiometric amounts of more common oxidants, such as metal salts (Cu and Ag) and benzoquinone derivatives, the use of molecular oxygen for the direct or indirect regeneration of Pd(II) species presents itself as a more viable alternative in terms of economy and sustainability. In this review, we describe recent advances on the development of Pd-catalyzed oxidative cyclizations/functionalizations, where molecular oxygen plays a pivotal role as the sole stoichiometric oxidant.

1 Introduction

2 Oxidative C–C and C–Nu Coupling

2.1 Intramolecular Oxidative C–Nu Heterocyclization Reactions

2.1.1 C–H Activation

2.1.2 Wacker/Aza-Wacker-Type Cyclization

2.1.3 Tandem Wacker/Aza-Wacker and Cyclization/Cross-Coupling Reactions

2.2 Intermolecular Oxidative C–Nu Heterocoupling Reactions

2.3 Intramolecular Oxidative (C–C) Carbocyclization Reactions

2.4 Intermolecular Oxidative C–C Coupling Reactions

2.4.1 Cyclization Reactions

2.4.2 Cross-Coupling Reactions

2.4.3 Homo-Coupling Reactions

3 Aerobic Dehydrogenative Coupling/Functionalization

4 Oxidative C–H Functionalization

5 Summary

A Metal-Free Path to 2-Iodo-3-alkyl-1-arylbut-2-en-1-ones and Their Application to the Domino Synthesis of Functionalized 2H-Pyran-2-ones

We report a metal-free selective synthesis of 2-iodo-3-alkyl-1-arylbut-2-en-1-ones from propargylic alcohols that is enabled by N-iodosuccinimide. A variety of substituted propargylic alcohols are amenable to delivering the selective 2-iodoenone products in very good yields. The utility of the α-iodoenone derivatives is further extended by developing an efficient, novel, and new synthetic methodology for the synthesis of 3,5,6-trisubstituted 2H-pyran-2-ones. To the best of our knowledge, this protocol is the first of its kind to accomplish 3,5,6-trisubstituted 2H-pyran-2-ones through an unprecedented domino (formation of two C-C bonds and one C-O bond) one-pot process via intermolecular Heck coupling, base-driven Michael addition, and base-mediated double bond isomerization followed by cyclo-condensation. This protocol showed good compatibility with a wide range of iodoenones (18 examples) and 2H-pyran-2-ones (42 examples). Mechanistic studies indicate that palladium is only involved in the Heck coupling; the base solely drives the rest of the steps.

Migratory cycloisomerization of 1,3-dien-5-ynes conjugated with pseudopeptides in assembly of benzo[7]annulenes

A novel domino cycloisomerization of 1,3-dien-5-ynes for the synthesis of 7H-benzo[7]annulenes is reported. The noticeable feature of this domino reaction involves the assembly of the fused bicyclic motifs through a transamidation/5-exo-trig cyclization/8π-electrocyclization sequence in a single step. Finally, mechanistic investigations were conducted experimentally and supported by DFT calculations.

Addition of benzyl ethers to alkynes: a metal-free synthesis of 1H-isochromenes

Bromotrimethylsilane (TMSBr)-promoted intramolecular cyclization of (o-arylethynyl)benzyl ethers to form 1H-isochromenes at room temperature is reported. Further studies indicated that vinyl carbocations are the reaction intermediates which are stabilized by the conjugated aryl groups. Thus, O-addition of benzyl ethers/tetrahydropyrans to alkynes was achieved under metal-free, acidic conditions. These reaction conditions were compatible with an alkynyl Prins reaction; therefore, 1H-isochromenes were produced directly from alkynyl benzaldehydes and alkynyl alcohols using a one-pot procedure.

[Pd]-Catalyzed para-selective allylation of phenols: access to 4-[(E)-3-aryl/alkylprop-2-enyl]phenols

4-[(E)-3-Arylprop-2-enyl]phenols are omnipresent scaffolds and constitute natural products and biologically significant compounds. Obtusastyrene and obtustyrene are two such phenolic-based natural products isolated from Dalbergia retusa. The development of strategies based on a site-selective allylation, particularly protecting group-free substrates and non-activated coupling agents, is indispensable in organic synthesis. Herein, we present a highly regioselective [Pd]-catalyzed para-allylation of phenols using simple, inactivated allylic alcohols as allylating coupling partners. Notably, this strategy is successful in open-air and under mild reaction conditions. Besides, the efficacy of the present protocol was demonstrated by the direct synthesis of obtusastyrene and obtustyrene.

Regioselective Synthesis of Tetrasubstituted Benzenes via Co-Catalyzed Cycloaddition of Alkynyl Ketones and 2-Acetylpyridines

A Co(II)-catalyzed cycloaddition reaction of alkynyl ketones and 2-acetylpyridines using 2,2'-bipyridine as the ligand has been developed. These reactions have been realized through Co-catalyzed reductive coupling of two molecules of 2-acetylpyridine followed by regioselective insertion of the alkynone. It is the first example of regioselective cyclotrimerization of one molecule of alkyne and two molecules of monoketone to polysubstituted benzene derivatives in good to excellent yields. A mechanism involving the formation of a cobaltacyclopentane via homocoupling of 2-acetylpyridines is proposed, and it is supported by the DFT calculations.

Design, synthesis, and evaluation of cytotoxic activities of arylnaphthalene lignans and aza-analogs

A concise and versatile synthetic strategy for the total synthesis of arylnaphthalene lignans and aza-analogs was developed. The main objective was to develop synthetic tactics for the creation of the lactone and lactam unit that would give access to an array of synthetic, natural, and/or bioactive compounds through rather simple chemical manipulation. The flexibility and potentiality of these new processes were further illustrated by the total synthesis of retrojusticidin B (13b), justicidin C (14b), and methoxy-vitedoamine A (22a). In this study, a series of novel aryl-naphthalene lignans and aza-analogs were synthesized, and the cytotoxic activities of all compounds on cancer cell growth were evaluated. The target compounds were structurally characterized by ¹ H NMR (nuclear magnetic resonance), ¹³ C NMR, infrared, high-resolution mass spectrometry, and X-ray crystallography. The IC₅₀ values of these compounds on five tumor cell lines (A549, HS683, MCF-7, SK-MEL-28, and B16-F1) were obtained by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) colorimetric assay. Five of the compounds exhibited excellent activity compared to 5-fluorouracil and etoposide against the five cell lines tested, with IC₅₀ values ranging from 1 to 10 μM.

Synthesis of Highly Conjugated Functionalized 2-Pyridones by Palladium-Catalyzed Aerobic Oxidative Dicarbonation Reactions of N-(Furan-2-ylmethyl) Alkyne Amides and Alkenes as Coupling Partners

A mild, step-economical method for the synthesis of highly conjugated functionalized 2-pyridones from N-(furan-2-ylmethyl) alkyne amides is reported. This method involves Pd-catalyzed aerobic oxidative dicarbonation reactions of alkynes with carbon nucleophiles of a furan ring and an acrylate or styrene as coupling partners. The UV-vis absorption spectra of some of the 2-pyridones indicated that they absorbed shortwave radiation, suggesting their potential utility for filtration of such radiation.

A simple removable aliphatic nitrile template 2-cyano-2,2-di-isobutyl acetic acid for remote meta-selective C–H functionalization

The remote meta-selective C–H functionalization of arenes using first aliphatic nitrile template 2-cyano-2,2-di-isobutyl acetic acid under mild conditions is presented.