Catalyst-Free Chemoselective Synthesis of 3,4-Dihydroquinazoline-2-thiones and 2-Imino[1,3]benzothiazines

Chemodivergent Photocatalyzed Heterocyclization of Hydrazones and Isothiocyanates for the Selectivity Synthesis of 2-Amino-1,3,4-thiadiazoles and 1,2,4-Triazole-3-thiones

A photocatalytic chemodivergent reaction for the selectivity formation of C-S and C-N bonds in a controlled manner was proposed. The reaction medium, either neutral or acidic, is critical to dictate the formation of 2-amino-1,3,4-thiadiazoles and 1,2,4-triazole-3-thiones from isothiocyanates and hydrazones. This is a practical protocol to achieve the chemoselectivity under mild and metal-free conditions.

I2-mediated Csp2–P bond formation via tandem cyclization of o-alkynylphenyl isothiocyanates with organophosphorus esters

A highly efficient molecular-iodine-catalyzed cascade cyclization reaction has been developed, creating a series of 4H-benzo[d][1,3]thiazin-2-yl phosphonates in moderate to excellent yields. This approach benefits from metal-free catalysts and available raw materials.

Base-promoted Cyclization Reaction of o-Isothiocyanato Arylacetylenes and Aroylacetonitriles: Easy Access to Benzo[d][1,3]thiazines

A green and efficient synthesis of benzo[d][1,3]thiazines through base-promoted cyclization reaction of o-isothiocyanato arylacetylenes with aroylacetonitriles has been developed. This protocol displays high step economy and efficiency, and tolerates various...

A catalyst-free method for the synthesis of 1,4,2-dithiazoles from isothiocyanates and hydroxylamine triflic acid salts

A catalyst-free method for the preparation of 1,4,2-dithiazoles is developed by reactions of isothiocyanates with hydroxylamine triflic acid salts. This reaction achieves C-S, C-N, and S-N bond formation, and a range of products are obtained in moderate to good yields. The obvious feature is using shelf-stable hydroxylamine triflic acid salts as a N source to synthesize heterocycles under mild conditions.

A bifunctional iminophosphorane squaramide catalyzed enantioselective synthesis of hydroquinazolines via intramolecular aza-Michael reaction to α,β-unsaturated esters

An efficient synthesis of enantioenriched hydroquinazoline cores via a novel bifunctional iminophosphorane squaramide catalyzed intramolecular aza-Michael reaction of urea-linked α,β-unsaturated esters is described. The methodology exhibits a high degree of functional group tolerance around the forming hydroquinazoline aryl core and wide structural variance on the nucleophilic N atom of the urea moiety. Excellent yields (up to 99%) and high enantioselectivities (up to 97 : 3 er) using both aromatic and less acidic aliphatic ureas were realized. The potential industrial applicability of the transformation was demonstrated in a 20 mmol scale-up experiment using an adjusted catalyst loading of 2 mol%. The origin of enantioselectivity and reactivity enhancement provided by the squaramide motif has been uncovered computationally using density functional theory (DFT) calculations, combined with the activation strain model (ASM) and energy decomposition analysis (EDA).

The activation of both aromatic and aliphatic ureas as N-centered nucleophiles in intramolecular Michael addition reactions to α,β-unsaturated esters was achieved under bifunctional iminophosphorane squaramide superbase catalysis.

Chemodivergent reactions

An important strategy for the efficient generation of diversity in molecular structures is the utilization of common starting materials in chemodivergent transformations. The most studied solutions for switching the chemoselectivity rely on the catalyst, ligand, additive, solvent, temperature, time, pressure, pH and even small modifications in the substrate. In this review article several processes have been selected such as inter- and intramolecular cyclizations, including carba-, oxa-, thia- and oxazacyclizations promoted mainly by Brønsted or Lewis acids, transition metals and organocatalysts, as well as radical reactions. Catalyst-controlled intra- and intermolecular cyclizations are mainly described to give five- and six-membered rings. Cycloaddition reactions involving (2+2), (3+2), (3+3), (4+1), (4+2), (5+2), (6+2) and (7+2) processes are useful reactions for the synthesis of cyclic systems using organocatalysts, metal catalysts and Lewis acid-controlled processes. Addition reactions mainly of carba- and heteronucleophiles to unsaturated conjugated substrates can give different adducts via metal catalyst-, Lewis acid- and solvent-dependent processes. Carbonylation reactions of amines and phenols are carried out via ligand-controlled transition metal-catalyzed multicomponent processes. Ring-opening reactions starting mainly from cyclopropanols, cyclopropenols and epoxides or aziridines are applied to the synthesis of acyclic versus cyclic products under catalyst-control mainly by Lewis acids. Chemodivergent reduction reactions are performed using dissolving metals, sodium borohydride or hydrogen transfer conditions under solvent control. Oxidation reactions include molecular oxygen under solvent control or using different dioxiranes, as well as chemodivergent palladium catalyzed cross-coupling reactions using boronic acids are applied to aromatic and allenic compounds. Other chemodivergent reactions such as alkylations and allylations under transition metal catalysis, dimerization of acetylenes, bromination of benzylic substrates, and A³-couplings are performed via catalyst- or reaction condition-dependent processes.

Iron Complexes Anchored onto Magnetically Separable Graphene Oxide Sheets: An Excellent Catalyst for the Synthesis of Dihydroquinazoline-Based Compounds

In this work, a green, sustainable, and efficient protocol for the syntheses of dihydroquinazoline derivatives is proposed. Initially, three Schiff base complexes of iron containing the ligand (2,2-dimethylpropane-1,3-diyl)bis(azanylylidene)bis(methanylylidene)bis(2,4-Xphenol), where X = Cl (complex 1)/Br (complex 2)/I (complex 3), were synthesized, fully characterized, and used in the desired syntheses. Complex 1 excelled as a catalyst, closely followed by complexes 2 and 3. DFT calculations helped in rationalizing the role of the halide substituent in the ligand backbone as a relevant factor in the catalytic superiority of complex 1 over complexes 2 and 3 for the synthesis of the dihydroquinazoline derivatives. Finally, to facilitate catalyst recoverability and reusability, complex 1 was immobilized on GO@Fe₃O₄@APTES (GO, graphene oxide; APTES, 3-aminopropyltriethoxysilane) to generate GO@Fe₃O₄@APTES@FeL¹ (GOTESFe). GOTESFe was thoroughly characterized through scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy and efficiently used for the synthesis of dihydroquinazoline derivatives. GOTESFe could be magnetically recovered and reused up to five cycles without compromising its catalytic efficiency. Therefore, immobilization of the chosen iron complex onto magnetic GO sheets offers an extremely competent route in providing a blueprint of a readily recoverable, reusable, robust, and potent catalyst for the synthesis of dihydroquinazoline-based compounds.

Base-Promoted Chemodivergent Formation of 1,4-Benzoxazepin-5(4H)-ones and 1,3-Benzoxazin-4(4H)-ones Switched by Solvents

The KOH-promoted chemodivergent benzannulation of ortho-fluorobenzamides with 2-propyn-1-ol can afford either 1,4-benzoxazepin-5(4H)-ones or 1,3-benzoxazin-4(4H)-ones in good yields with high selectivity, depending greatly upon the use of solvents. In the case of using DMSO, the intermolecular benzannulation produced seven-membered benzo-fused heterocycles of 1,4-benzoxazepin-5(4H)-ones, whereas in MeCN, the six-membered benzo-fused heterocycles of 1,3-benzoxazin-4(4H)-ones were formed. The KOH-promoted benzannulation proceeded most probably through the C–F nucleophilic substitution of ortho-fluorobenzamides with 2-propyn-1-ol to give the intermediate of ortho-[(2-propynyl)oxy]benzamide, which underwent the intramolecular hydroamidation in a different manner to afford either seven- or six-membered benzo-fused heterocycles.

A Simple Metal-Free Cyclization for the Synthesis of 4-Methylene-3-Substituted Quinazolinone and Quinazolinthione Derivatives: Experiment and Theory

A new series of 3-substituted 4-methylene-quinazolinthiones and 4-methylene-quinazolinones were synthesized in moderate to excellent yield through a simple reaction of 2-aminoacetophenones with isocyanates or isothiocyanates. The reaction shows good tolerance of many important functional groups in the presence of air and water under metal-free conditions. Only water is produced as a coproduct, rendering this “green” methodology a highly versatile and eco-friendly alternative to the existing methods for the construction of the quinazolinone/quinazolinthione framework. We have interpreted the reaction mechanism by use of quantum chemical calculations on the basis of state-of-the-art computational methods SMD-B3LYP-D3(BJ)/BS1//B3LYP/BS1.

Metal-free multicomponent cascade reactions of homopropargylic amines and acyl chlorides as well as potassium thiocyanate and diiodine: an access to thiazine imides

A novel metal-free multicomponent cascade reaction was developed for the construction of thiazine imides. This four-component cascade reaction had advantages of mild reaction conditions, wide substrate scope and good atom economy. Four new bonds were formed in one pot via a 6-exo-dig iodothiolation cyclization of homopropargylic amines. The corresponding E-configurational thiazine imide products possess an exocyclic vinyliodide functional group.

Chemoselective Synthesis of Structurally Diverse 3,4-Dihydroquinazoline-2(1H)-thiones and 4H-Benzo[d][1,3]thiazines

An efficient, mild, and substrate/catalyst-controlled chemoselective reaction of o-isothiocyanato-(E)-cinnamaldehyde with amines has been established, producing three types of six-membered heterocycles: 2-(4H-benzo[d][1,3]thiazin-4-yl)acetaldehydes, 2-(2-thioxo-1,2,3,4-tetrahydroquinazolin-4-yl)acetaldehydes, and (E)-4-(2-methoxyvinyl)-4H-benzo[d][1,3]thiazines. The reaction scopes were quite broad and excellent yield was achieved. This method is extremely efficient and practical and can be conducted on a gram-scale with slightly inferior reactivity under catalyst-free conditions at low cost, making it an ideal alternative to existing methods.

Unprecedented formation of 3-(tetrahydrofuran-2-yl)-4H-chromen-4-one in a reaction between 3,3a-dihydro-9H-furo[3,4-b]chromen-9-one and malononitrile

Chromone skeletons are widespread among natural products as well as bioactive molecules. Here, we describe an unprecedented reaction of furo[3,4-b]chromen-9-one with malononitrile to afford 3-(tetrahydrofuran-2-yl)-4H-chromen-4-ones. Experimental data suggest that a sequence of Michael/retro-Michael/nucleophilic addition is involved in this unprecedented transformation.