Highly Efficient Copper-Catalyzed Domino Ring Opening and Goldberg Coupling Cyclization for the Synthesis of 3,4-Dihydro-2H-1,4-benzoxazines

Synthesis of 1,4-benzoxazines via Y(OTf)3-catalyzed ring opening/annulation cascade reaction of benzoxazoles with propargylic alcohols

A convenient Y(OTf)₃-catalyzed cascade formal [4 + 2] cyclization approach for the formation of 1,4-benzoxazine scaffolds from benzoxazoles and propargyl alcohols through a ring-opening and regioselective ring-closure process has been developed. By using this mild and practical protocol, a broad range of aldehyde-containing 1,4-benzoxazine compounds were prepared in moderate to excellent yields with good functional group tolerance. Mechanistic studies indicated that an S_N1 nucleophilic substitution of benzoxazole with a propargyl cation was involved in this transformation.

Recent Advances in the Tandem Copper-Catalyzed Ullmann-Goldberg N-Arylation–Cyclization Strategies

N‒Aryl bond formation under copper catalysis has been playing a pivotal role and has been extensively used as a key step in the total syntheses of several therapeutic molecules. The...

Domino ring opening/cyclization of oxiranes for synthesis of functionalized 2H-pyran-5-carboxylate

A simple method for the synthesis of functionalized 2H-pyrans via a catalytic reaction of an oxirane, an alkyne, and a malonate has been developed in which a 6-exo-dig cyclization pathway is observed. In this transformation, the attack of in situ generated copper acetylides on oxiranes formed homopropargylic alcohol intermediates which further transferred to 2H-pyrans with the help of malonates.

Synthesis of 2-Aminophosphates via SN2-Type Ring Openings of Aziridines with Organophosphoric Acids

The synthesis of 2-aminophosphates is achieved by a S_N2-type ring opening reaction of various N-protected or free aziridines with phosphoric acids in a regiospecific and/or enantiospecific way. A one-pot, two-step procedure is also developed enabling direct access to 2-aminophosphates from olefins without isolation of the aziridine intermediates.

Synthesis of Nonracemic 1,4-Benzoxazines via Ring Opening/Cyclization of Activated Aziridines with 2-Halophenols: Formal Synthesis of Levofloxacin

Novel 3,4-dihydro-1,4-benzoxazine derivatives have been synthesized by an efficient and simple method in excellent enantio- and diastereospecificity (ee > 99%, de > 99%). The reaction proceeds via Lewis acid-catalyzed S_N2-type ring opening of activated aziridines with 2-halophenols followed by Cu(I)-catalyzed intramolecular C-N cyclization in a stepwise fashion under one-pot conditions to furnish the 3,4-dihydro-1,4-benzoxazine derivatives in excellent yields (up to 95%). The strategy offers a short and efficient synthesis to ( S)-3-methyl-1,4-benzoxazine ( S)-3v, a late stage intermediate in the synthesis of levofloxacin.

One-Pot Synthesis of Benzo[b][1,4]oxazins via Intramolecular Trapping Iminoenol

A highly atom-efficient "one-pot" protocol has been developed to construct multisubstituted 2-hydroxy-benzo[b][1,4]oxazins starting from N-(2-hydroxylaryl)enaminones. This procedure comprises a PIDA-mediated intramolecular iminoenol tautomer trapping reaction, followed by Et₃N-promoted aerobic oxidative ring construction. In particular, an O₂ molecule from air served as the oxygen source of the hydroxyl group in the titled products. This reaction proceeded smoothly at room temperature under air atmosphere and metal-free conditions.

Phenolate-induced intramolecular ring-opening cyclization of N-tosylaziridines: access to functionalized benzoxacycles

Phenolate-induced, diastereo- and regioselective intramolecular exo-tet ring-opening cyclization of N-tosylaziridines has been achieved for the first time.

Synthesis of Benzo[1,4]heterocycles using Palladium Catalyzed Heck Reaction to Vinylogous Carbonates/Carbamates: Unexpected Formation of Indoles via Carbopalladation Intercepted by Nucleopalladation

An efficient protocol for the stereoselective synthesis of benzo[1,4]heterocycles via palladium catalyzed Heck reaction on o-halo-aryl-oxa/thia/aza tethered vinylogous carbonates/carbamates/esters has been developed. Unexpected formation of indoles is observed when unprotected 2-iodoaniline tethered vinylogous carbonates are subjected to the Heck reaction. Mechanistic studies indicate that formation of these indoles is an outcome of the interception of the carbopalladation step by nucleopalladation. The method can be used to gain rapid access to the core skeleton of abacopterin A-C.

An efficient synthesis of pyrido[1,2-a]indoles through aza-Nazarov type cyclization

Transition metal free Brønsted acid mediated synthesis of pyrido[1,2-a]indole scaffolds has been developed through aza-Nazarov type cyclization of readily available diaryl(2-pyridyl)methanol using formic acid for the synthesis of biologically and medicinally important pyrido[1,2-a]indole, indolo[1,2-a]quinoline and pyrimido[1,2-a]indole derivatives.

Regiospecific synthesis of neuroprotective 1,4-benzoxazine derivatives through a tandem oxidation–Diels–Alder reaction

Highly functionalized 1,4-benzoxazine derivatives have been synthesized at room temperature, with complete regiochemical control, through a tandem oxidation–Diels–Alder reaction.

Synthetic route to chiral indolines via ring-opening/C-N cyclization of activated 2-haloarylaziridines

A practical approach for the synthesis of 3-substituted indolines via regio- and stereoselective SN2-type ring-opening of 2-(2-halophenyl)-N-tosylaziridines with heteroatomic nucleophiles (O, N, and S) followed by palladium-catalyzed intramolecular C-N cyclization is reported in excellent yields (up to >99%) and enantiomeric excess (ee 99%).

Copper-catalyzed synthesis of purine-fused polycyclics

A novel protocol for a Cu-catalyzed direct C((sp(2)))-H activation/intramolecular amination reaction of 6-anilinopurine nucleosides has been developed. This approach provides a new access to a variety of multiheterocyclic compounds from purine compounds via Cu-catalyzed intramolecular N-H bond tautomerism which are endowed with fluorescence.

Synthesis of functional acetylene derivatives from calcium carbide

AHA Erlebnis: CaC(2), used to produce acetylene until several decades ago, is re-emerging as a cheap, sustainable resource synthesized from coal and lignocellulosic biomass. We report efficient catalytic protocols for the synthesis of functional acetylene derivatives from CaC(2) through aldehyde, alkyne, and amine (AAA) as well as alkyne, haloalkane, and amine (AHA) couplings, and in addition demonstrate its use in click and Sonogashira chemistry, showing that calcium carbide is a sustainable and cost-efficient carbon source.

Copper-mediated domino synthesis of pyrimido[4,5-b]carbazolones via Ullmann N-arylation and aerobic oxidative C-H amidation

New pyrimido[4,5-b]carbazolone derivatives have been synthesized through cascade Ullmann N-arylation and aerobic oxidative C-H amidation reactions catalyzed by CuBr under air and ligand-free conditions.

Synthetic route to chiral tetrahydroquinoxalines via ring-opening of activated aziridines

A highly regio- and stereoselective route for the synthesis of racemic and nonracemic tetrahydroquinoxalines via the S(N)2-type ring-opening of activated aziridines with 2-bromoanilines followed by the Pd-catalyzed intramolecular C-N bond formation is described.

Alumina-supported Cu(II), a versatile and recyclable catalyst for regioselective ring opening of aziridines and epoxides and subsequent cyclization to functionalized 1,4-benzoxazines and 1,4-benzodioxanes

An easily accessible catalyst, alumina-supported copper(II), efficiently catalyzes the ring opening of aziridines and epoxides followed by cyclization of the corresponding intermediate to produce a variety of functionalized 1,4-benzoxazines and 1,4-benzodioxanes, respectively, in one pot without any ligand in high yields. The ring cleavages of aziridines and epoxides are highly regioselective. The catalyst is inexpensive, non-air-sensitive, environmentally friendly, and recyclable. The function of the catalyst and the reaction pathway are postulated. This protocol is successfully utilized for the formation of three carbon-heteroatom bonds, namely, C-O, C-N, and C-S, in one pot.