Efficient Synthesis of 1H-Benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-one Derivatives Using Ag2CO3/TFA-Catalyzed 6-endo-dig Cyclization: Reaction Scope and Mechanistic Study

A small library of 1H-benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-one derivatives was prepared in good to excellent yields, involving a Ag₂CO₃/TFA-catalyzed intramolecular oxacyclization of N-Boc-2-alkynylbenzimidazole substrates. In all experiments, the 6-endo-dig cyclization was exclusively achieved since the possible 5-exo-dig heterocycle was not observed, indicating the high regioselectivity of this process. The scope and limitations of the silver catalyzed 6-endo-dig cyclization of N-Boc-2-alkynylbenzimidazoles as substrates, bearing various substituents, were investigated. While ZnCl₂ has shown limits for alkynes with an aromatic substituent, Ag₂CO₃/TFA demonstrated its effectiveness and compatibility regardless of the nature of the starting alkyne (aliphatic, aromatic or heteroaromatic), providing a practical regioselective access to structurally diverse 1H-benzo[4,5]imidazo[1,2-c][1,3]oxazin-1-ones in good yields. Moreover, the rationalization of oxacyclization selectivity in favor of 6-endo-dig over 5-exo-dig was explained by a complementary computational study.

Electrophile-Promoted Nucleophilic Cyclization of 2-Alkynylindoles to Give 4-Substituted Oxazinoindolones

A method for the synthesis of 4-organoselanyl oxazinoindolone derivatives by the cascade cyclization of N-(alkoxycarbonyl)-2-alkynylindoles using iron(III) chloride and diorganyl diselenides as promoters was developed. This protocol was applied to a series of N-(alkoxycarbonyl)-2-alkynylindoles containing different substituents. The reaction conditions also tolerated a variety of diorganyl diselenides having both electron donating and electron withdrawing groups. However, the reaction did not work for diorganyl disulfides and ditellurides. The reaction mechanism seems to proceed via an ionic pathway and the cooperative action between iron(III) chloride and diorganyl diselenides is crucial for successful cyclization. We also found that using the same starting materials, by simply changing the electrophilic source to iodine, led to the formation of 4-iodo-oxazinoindolones. The high reactivity of Csp² -Se and Csp² -I bonds were tested under cross-coupling conditions leading to the preparation of a new class of functionalized indole derivatives. In addition, the absorption, emission and electrochemical properties of 4-organoselanyl oxazinoindolones showed an important relationship with the substituents of the aromatic rings. The advantages of the methodology include the use of electrophilic to promote the cyclization reaction and functionalization of the indole ring, and the electronic properties presented by the prepared compounds can be exploited as probes, analyte detectors and optical materials.

A Ag2CO3/TFA-catalyzed intramolecular annulation approach to imidazo[1,2-c][1,3]oxazin-5-one derivatives

A series of 2,7-disubstituted 3-methylimidazo[1,2-c][1,3]oxazin-5-ones were synthesized in good yields via Ag₂CO₃/TFA-mediated intramolecular annulation of N-Boc-2-alkynyl-4-bromo(alkynyl)-5-methylimidazoles. This methodology was carried out in the presence of a catalytic amount of silver carbonate and trifluoroacetic acid in dichloroethane at 60 °C. In all experiments, only the six-membered ring product was obtained since the possible five-membered compound was not observed, proving the high regioselectivity of this approach. A complementary computational study was performed in order to rationalize the mechanism of 6-endo-dig heterocycle formation. In addition, 2-bromo-3-methyl-7-phenylimidazo[1,2-c][1,3]oxazin-5-one was used as a building block to synthesize a small library of new 2-substituted imidazo[1,2-c][1,3]oxazin-5-one derivatives through the Suzuki, Sonogashira and Heck cross coupling reactions.

Synthesis of New Highly Functionalized 1H-Indole-2-carbonitriles via Cross-Coupling Reactions

An approach for the preparation of polysubstituted indole-2-carbonitriles through a cross-coupling reaction of compounds 1-(but-2-ynyl)-1H-indole-2-carbonitriles and 1-benzyl-3-iodo-1H-indole-2-carbonitriles is described. The reactivity of indole derivatives with iodine at position 3 was studied using cross-coupling reactions. The Sonogashira, Suzuki–Miyaura, Stille and Heck cross-couplings afforded a variety of di-, tri- and tetra-substituted indole-2-carbonitriles.

Cycloisomerization of π-Coupled Heteroatom Nucleophiles by Gold Catalysis: En Route to Regiochemically Defined Heterocycles

Since the dawn of millennium, catalytic gold chemistry is at the forefront to set off diverse organic reactions via unique activation of π-bonded molecules. Within this purview, cycloisomerization of heteroatom nucleophiles linked to a π-system is one of the well recognized chemistry for the construction of numerous heterocyclic cores. Though the rudimentary aspects of this transformation are reviewed by several groups in different timeline, a holistic view on regiochemistry of such reactions went largely overlooked. Hence, this account emphasizes the gold catalyzed regioselective cycloisomerization of structurally distinctive π-connected hetero-nucleophiles leading to different heterocycles documented in the last two decades. From an application perspective, this account also highlights those methodologies which find a role in the total synthesis of natural products. Wherever appropriate, mechanistic details and contributing factors for selectivity are also discussed.

Rh(III)-Catalyzed Chemodivergent Annulations between Indoles and Iodonium Carbenes: A Rapid Access to Tricyclic and Tetracyclic N-Heterocylces

Herein, we report an acid-controlled highly tunable selectivity of Rh(III)-catalyzed [4 + 2] and [3 + 3] annulations of N-carboxamide indoles with iodonium ylides lead to form synthetically important tricyclic and tetracyclic N-heterocycles. Here, iodonium ylide serves as a carbene precursor. The protocol proceeds under operationally simple conditions and provides novel tricyclic and tetracyclic scaffolds such as 3,4-dihydroindolo[1,2-c]quinazoline-1,6(2H,5H)-dione and 1H-[1,3]oxazino[3,4-a]indol-1-one derivatives with a broad range of functional group tolerance and moderate to excellent yields. Furthermore, the protocol synthetic utility was extended for various chemical transformations and was easily scaled up to a large-scale level.