A simple and mild synthesis of 1H-isochromenes and (Z)-1-alkylidene-1,3-dihydroisobenzofurans by the iodocyclization of 2-(1-alkynyl)benzylic alcohols

Cu(II)-Catalyzed Annulative Sulfonylimination of α-Carbonyl-γ-alkynyl Sulfoxonium Ylides through Sulfoxonium Ylide Elimination for Access to (Z)-1H-Isochromenes

1H-Isochromene scaffolds are ubiquitous in natural products and significant bioactive molecules. Although several methods for these molecular syntheses have been developed, reports on the efficient construction of iminated isochromenes are still rather limited. Herein, we report a new Cu(II)-catalyzed annulation and sulfonylimination cascade of α-carbonyl-γ-alkynyl sulfoxonium ylides with sulfamides, enabling direct C-C σ-bond elimination to furnish iminated (Z)-1H-isochromenes in 51-97% yields. During the reaction process, the sulfoxonium ylide unit as a traceless eliminative group is the key to successful sulfonylimination, which enables Cu(II)-catalyzed N-H insertion and subsequent acetic acid-mediated C-C σ-bond elimination.

Cascade Cyclization of o-(2-Acyl-1-ethynyl)benzaldehydes with Amino Acid Derivatives: Synthesis of Indeno[2,1-c]pyran-3-ones and 1-Oxazolonylisobenzofurans via the Erlenmeyer-Plöchl Azlactone Reaction

A highly regioselective divergent approach is reported for the synthesis of both indeno[2,1-c]pyran-3-one and 1-oxazolonylisobenzofuran derivatives using the Erlenmeyer-Plöchl azlactone (EPA) reaction. This approach involves the synthesis of o-(2-acyl-1-ethynyl)benzaldehydes, which reacted with various amino acids. Reaction with N-acylglycines resulted in the formation of indeno[2,1-c]pyran-3-ones, involving the sequential formation of two C-C bonds and two C-O bonds. Conversely, when the same conditions were applied to free amino acids, 1-oxazolonylisobenzofurans were obtained. This reaction involved the formation of a C-C bond between oxazolone and o-(2-acyl-1-ethynyl)benzaldehyde, followed by the formation of a C-O bond through a selective 5-exo-dig cyclization.

Quick Access to Protoberberine and Tetrahydroprotoberberine Derivatives/Analogues with One-Pot Sequential Isochromenylium-Mediated Annulation

A one-pot access to the protoberberine and tetrahydroprotoberberine scaffold has been developed from easily available primary amine-functionalized o-alkynylbenzaldehydes. The core skeleton of protoberberine was afforded via in situ generation of isochromenylium by Ag(I) catalysis, subsequent cyclization to isoquinolinium, and final hydride reduction by a continuous operation procedure. The newly developed step-economic protocol shows excellent regio- and stereoselectivity and is capable of achieving those previously unavailable derivatives/analogues with electron-deficient substituents, providing a robust tool for medicinal investigations of protoberberines and derivatives.

ortho-Alkynyl Benzyl Alcohols as C6 Synthons in Regioselective Construction of Polysubstituted Naphthalenes

A straightforward methodology for the assembly of polysubstituted naphthalenes from ortho-alkynyl benzyl alcohols, enabled by using catalytic amounts of Tf2O, has been developed. This transformation not only features transition-metal free and without using other bases and additives but also provides a new synthetic application for ortho-alkynyl benzyl alcohols, i.e., as C6 synthons for the construction of PAHs.

Time and Temperature Dependent Palladium-Catalyzed Stereo- and Regioselective Alkoxy-arylation of Triple Bonds: Synthesis of (E)/(Z)-1,1-Disubstituted-3-(1-Phenylalkylidene)-1,3-dihydroisobenzofurans

The development of synthetic strategies enabling the stereo- and regio-selective synthesis of organic molecules is indispensable in the pharmaceutical industry. This work describes a stereo- and regioselective synthesis of (E)/(Z)-1,1-disubstituted-3-(1-arylalkylidene)-1,3-dihydroisobenzofurans catalyzed by palladium. The DHIBFs are achieved from readily available aryl bromides and ortho-alkyne tertiary alcohols via intermolecular aryl Heck coupling and intramolecular oxo-cyclization of the suitably situated hydroxyl group. Significantly, it was demonstrated that a Z-isomer was formed as a substantial isomer at 80 °C for 6 h, whereas the stable E-isomer was the predominant one at slightly vigorous conditions (100 °C for 12 h). In addition, careful observation has also led to the realization that this double isomerization from Z- to E-isomer was triggered in the NMR tube itself in CDCl₃ at room temperature. Significantly, this protocol, for the first time, enabled the synthesis of heavily substituted dihydroisobenzofurans, especially, bearing a tetra-substituted double bond.

Synthesis of 2-Aryl Acetophenones via Hydrobromination and Oxy-isomerization of (o-Arylethynyl)benzyl Alcohols

Hydrobromination and oxy-isomerization of (o-arylethynyl)benzyl alcohols to yield brominated aryl ketones were achieved with bromotrimethylsilane. The substrate scope suggested that vinyl carbocations, stabilized by the conjugated aryl groups, are the reaction intermediates. 1H-Isochromene was also detected by ¹H NMR, and an isolated 1H-isochromene was converted to the product when retreated with TMSBr. The formation of 1H-isochromene is equivalent to a 6-endo-dig cyclization and contrasts with the corresponding reactions under basic conditions, in which the 5-exo-dig process dominated.

The synthesis of indeno[de]isochromene derivatives from arylvinyl epoxides and carbonyl compounds via tandem Nazarov and oxa-Pictet–Spengler cyclizations

The conversion of aryl vinyl epoxides and carbonyl compounds in the presence of a catalytic amount of BF3·OEt2 to indeno[de]isochromenes via tandem Nazarov and oxa-Pictet–Spengler cyclizations is described.

Gold-catalyzed synthesis of 1H-isochromene-4-carbaldehydes via oxidative cascade cyclization

An efficient gold-catalyzed formation of indenylidene-derived 1H-isochromene-4-carbaldehydes from substituted 1,5,10-triyne-O-silanes was developed under mild reaction conditions. In this reaction, gold-catalyzed selective oxidation, 1,2-migration, nucleophilic addition and then 5-endo-dig cyclization took place regioselectively. The indenylidene-derived isochromene-4-carbaldehydes were synthesized in moderate to very good yields via the formation of new C-C and C-O bonds in one pot.

Ynamides enabled 6-, 7-, and 8-endo-dig iodocyclization of ethoxyethyl ethers: rapid construction of medium-sized oxacycles at room temperature

Iodocyclization of ynamides enabled the construction of six, seven-, and eight-membered oxacycles in very short reaction times at room temperature.

Palladium mediated domino reaction: synthesis of isochromenes under aqueous medium

Isochromenes have been synthesized using palladium-catalyzed C–C and C–O bond forming reactions starting from ortho-bromo tertiary benzylic alcohols and internal acetylenes. Notably, this domino process is feasible by using the green solvent, water. The protocol exhibited a broad substrate scope and afforded various isochromenes.

Isochromenes have been synthesized using palladium-catalyzed C–C and C–O bond forming reactions starting from ortho-bromo tertiary benzylic alcohols and internal acetylenes.

Iodine mediated oxidative cross coupling of 2-aminopyridine and aromatic terminal alkyne: a practical route to imidazo[1,2-a]pyridine derivatives

A novel, transition-metal free route leading to imidazo[1,2-a]pyridine derivatives via iodine mediated oxidative coupling between 2-aminopyridine and aromatic terminal alkyne has been demonstrated. This newly developed method discloses an operationally simple way for the construction of imidazoheterocycles. Commercially available antiulcer drug zolimidine may readily be synthesized employing this method.

Boron-Heteroatom Addition Reactions via Borylative Heterocyclization: Oxyboration, Aminoboration, and Thioboration

Organoboron compounds and heterocycles are powerful building blocks and precursors for organic synthesis, including for drug discovery and agrochemical and material synthesis. The common strategy for the synthesis of borylated heterocycles involves two separate synthetic steps: first, synthesis of the heterocyclic core, and second, borylation of the core through established methods such as transition-metal-catalyzed C-H or C-X activation/borylation or lithiation/borylation. In this Account, we describe our laboratory's development of borylative heterocyclization reactions that access the heterocyclic core and install boron in one synthetic step. These methods provide complementary bond disconnections, regiochemistry, and functional-group compatibility to current methods. We describe our methods with two categories: a direct borylation method that refers to addition reactions starting from a preformed B-element σ bond, which is essential in the mechanistic route to product formation, and a formal borylation method that refers to addition reactions that do not require formation of a B-element bond but instead proceed through carbon-carbon π-bond activation by an electrophilic boron source followed by dealkylation or deacylation. Through electrophilic activation of the alkyne rather than activation of the B-element bond, formal borylation provides a complementary strategy toward neutral organoboron reagents. We first studied direct oxyboration toward the formation of borylated benzofurans, where a preformed boron-oxygen σ bond is added across an alkyne activated by a carbophilic gold catalyst. We describe detailed mechanistic and kinetic studies of this class of reactions. Application of the knowledge gained from these studies aided in the future development of additional direct borylation reactions involving boron-nitrogen and boron-oxygen σ bonds to form borylared indoles and isoxazoles, respectively. Formal addition of boron/oxygen equivalents to effect oxyboration to form borylated lactones from o-alkynyl esters is then described. This class of reactions takes advantage of bifunctional ClBcat as a carbophilic carbon-carbon π-bond activator and eventual dealkylating agent. We describe our motivation in developing this new class of catalyst-free borylation reactions and subsequently applying the formal borylation strategy to the thioboration of o-alkynylthioanisole substrates to form borylated benzothiophenes. We then proceed to describe our investigations into the details of the mechanism of the formal thioboration reaction. These collaborative mechanistic studies included experimental and computational findings that elucidated the rate-determining step and intermediates of the reaction. These studies further compared different boron sources as electrophiles, including those used in other known reactions, providing fundamental knowledge about the capabilities of commercially available boron reagents toward borylative heterocyclization. Our findings provide guiding principles for reaction design and information leading toward the design of a diverse set of boron-heteroatom addition reactions and their formal equivalents that proceed through borylative heterocyclization.

Cu-Catalyzed Cascade Annulation of Alkynols with 2-Azidobenzaldehydes: Access to 6H-Isochromeno[4,3-c]quinoline

A copper-catalyzed cascade reaction of alkynols and 2-azidobenzaldehydes has been achieved, giving 6H-isochromeno[4,3-c]quinoline in yields of 40-81%. This reaction provides a novel, concise strategy for rapidly constructing compounds with fused N- and O-containing heterocycles. In contrast to previously reported reactions of alkynols in which the first step is intramolecular cycloisomerization, the first step in this novel reaction of alkynols is entropically unfavorable intermolecular addition. The resulting hemiacetal intermediate then undergoes intramolecular cyclization and aromatization to afford the product.

One-pot synthesis of 1H-isochromenes and 1,2-dihydroisoquinolines by a sequential isocyanide-based multicomponent/Wittig reaction

A one-pot synthesis of 1H-isochromenes and 1,2-dihydroisoquinolines by a I-MCR/Wittig sequence was developed. The reaction of phosphonium salt , an acid, an amine (or without), and an isocyanide gave the 1H-isochromenes or 1,2-dihydroisoquinolines in good yields by a sequential Passerini or Ugi condensation and an intramolecular Wittig reaction in the presence of K2CO3.

Selective synthesis of (Z)-2-enynyl-2-hydroxy-imidazolidine-4,5-diones via Cu(i)-mediated multicomponent coupling of terminal alkynes, carbodiimides and oxalyl chloride

(Z)-2-Enynyl-2-hydroxy-imidazolidine-4,5-diones are synthesized via Cu(i)-mediated (Z)-selective geminal coupling among two terminal alkynes, carbodiimides, and oxalyl chloride. Further transformation is explored.

Molecular iodine-mediated reaction of 2-(2-phenylethynyl)-Morita–Baylis–Hillman adducts: an easy route to naphthyl ketones and iodo-substituted isochromenes

The iodine mediated reaction of 2-(2-phenylethynyl)-MBH gives corresponding naphthyl ketones and iodo-substituted isochromenes.