Synthesis of Chromeno[2,3-d]pyrimidin-5-one Derivatives from 1,3,5-Triazinanes via Two Different Reaction Pathways

Additive-Free Construction of Tetrahydropyrimidine Skeleton by using 1,3,5-Triazinane as Four-Atom Synthon

Herein, an unprecedented [4 + 2] cycloaddition of enaminone with 1,3,5-triazinane has been developed. The representative semihydrogenated aromatic heterocycle 1,2,3,4-tetrahydropyrimidines have been synthesized with a broad substrate scope, demonstrating potential antitumor activity. This approach has been smoothly conducted under additive-free and environmentally friendly conditions that are compatible with various functional groups. Furthermore, the condition optimization process reveals that the tetrahydropyrimidine product is regulated via the reaction temperature.

Palladium-Catalyzed Cycloaddition Reactions of π-Allylpalladium 1,4-Dipoles with 1,3,5-Triazinanes: Access to Hexahydropyrimidines, 1,3-Oxazinanes, and 1,5-Diazocanes

Palladium-catalyzed decarboxylation of 5-methylene-1,3-oxazinan-2-ones and 5-methylene-1,3-dioxan-2-ones to generate aza-π-allylpalladium and oxa-π-allylpalladium 1,4-dipoles for [4 + 2] cycloaddition reaction with 1,3,5-triazinanes was developed, affording a wide range of hexahydropyrimidine and 1,3-oxazinane derivatives in good to excellent yields (up to 99%). The acyclic sulfonamido-substituted allylic carbonates as aza-π-allylpalladium 1,4-dipole precursors also apply to the developed synthesized strategy, achieving the synthesis of hexahydropyrimidines. Moreover, the in situ-generated aza-π-allylpalladium 1,4-dipoles undergoing dimeric [4 + 4] cycloaddition were also demonstrated by the construction of 1,5-diazocane derivatives.

Reaction of 1,3,5-Triazinanes with Phosphoryl Diazomethanes: Access to 5-Phosphoryl-1,2,3,4-tetrahydropyrimidines

Conversion of 1,3,5-triazinanes into 5-phosporyl-1,2,3,4-tetrahydropyrimidines is achieved efficiently through the microwave-assisted reaction with phosphoryl diazomethanes. Both trialkyl and triaryl 1,3,5-triazinanes were converted by diazomethyldiarylphosphine oxides, dialkyl diazomethylphosphonates, and alkyl diazomethyl(aryl)phosphinates and functionalized simultaneously in good to excellent yields. The reaction is a sequence of 1,3,5-triazinane fragmentation, tandem nucleophilic addition of the generated formaldimines and phosphoryl diazomethanes, and final N,N-acetalization.

Synthesis of 2H-Chromeno[2,3-b]tetrahydropyridine Frameworks through Cs2CO3-Promoted Annulation of 2-Amino-4H-chromen-4-ones and 4-Benzylideneoxazol-5(4H)-ones

This innovative reaction involved the [3+3] annulation of 2-amino-4H-chromen-4-ones and 4-benzylideneoxazol-5(4H)-ones. The process provided quick and easy access to a broad range of structurally diverse and highly functionalized 1,3,4,5-tetrahydro-2H-chromeno[2,3-b]pyridines in moderate to good yields, which possess trans-form C3 and C4 substituents.

Synthesis of 2-amino-9H-chromeno[2,3-d]thiazol-9-ones with anti-inflammatory activity via cascade reactions of 2-amino-3 iodochromones with amines and carbon disulfide

A simple and efficient synthetic approach to 2-amino-9H-chromeno[2,3-d]thiazol-9-ones via copper-promoted cascade reactions was developed. The reaction employed easily available 2-amino-3-iodochromones and amines as substrates and the targeting tricyclic compounds could be obtained with moderate to good yields. Even more important, several synthesized compounds exhibited potent anti-inflammatory activities, which suggested that this protocol may provide valuable hits for drug development in the future.

ZnCl2-Promoted Three-Component Reaction of 2-Aminochromenones, Aromatic Aldehydes, and Quinone Monoketals: Access to 5,6-Dihydro-12H-chromeno[2,3-c]isoquinolin-12-one Derivatives

A three-component reaction of 2-amino-4H-chromen-4-ones, aromatic aldehydes, and 4,4-dialkoxycyclohexa-2,5-dien-1-ones for the concise synthesis of chromeno[2,3-c]dihydroisoquinoline derivatives has been investigated. This reaction involved consecutive ZnCl2-promoted Micheal addition and intramolecular Friedel-Crafts alkylation. This synthetic protocol offered several advantages, including the readily accessible starting materials, good functional group tolerance, and simplicity of operation. Additionally, the structures of products obtained were determined based on X-ray diffraction studies.

Catalyst-free inverse-electron-demand aza-Diels-Alder reaction of 4,4-dicyano-2-methylenebut-3-enoates and 1,3,5-triazinanes: access to polysubstituted tetrahydropyridines

An inverse-electron-demand aza-Diels-Alder reaction between 4,4-dicyano-2-methylenebut-3-enoates and 1,3,5-triazinanes under catalyst-free and additive-free conditions was developed, which provided a highly convenient and straightforward method to construct a series of polyfunctionalized tetrahydropyridines in high yields. This strategy features numerous advantages, including high efficiency, good functional group tolerance, broad substrate scope, and environmentally friendly conditions.

One-pot domino synthesis of five- and six-membered fused dihydropyridines promoted by PPh3-NBS in aqueous medium

A facile one-pot synthesis of five- and six-membered fused dihydropyridines such as chromenodihydropyridines, pyrazolodihydropyridines and isoxazolopyridines was accomplished for the first time by employing PPh₃-NBS via a formal [3 + 2 + 1] cycloaddition of 1,3-bisnucleophiles (i.e., 2-aminochromone, 4-aminochromone, 5-aminopyrazole and 5-aminoisoxazole), β-enaminones and aldehydes in aqueous medium. The present approach involves a Michael type addition followed by intramolecular cyclization leading to the formation of two new C-C bonds and one C-N bond. High compatibility and excellent yields are the advantages of this protocol.

Construction of Diverse N-Heterocycles by Formal (3 + 3) Cycloaddition of Naphthol/Thionaphthol/Naphthylamine and 1,3,5-Triazinanes

Here, we report a facile and metal-free method for the construction of dihydrooxazine derivatives via a formal (3 + 3) annulation reaction of naphthols and 1,3,5-triazinanes. The 1,3,5-triazinanes were utilized as a formal three-atom synthon (C-N-C) for cycloaddition. In addition, dihydrothiazine and tetrahydrobenzoquinazoline derivatives could also be produced in good yields by this strategy under catalyst-free and additive-free conditions.

DBU-Mediated Domino Annulation Reaction of 2-Amino-4H-chromen-4-ones and Aromatic Aldehydes for Synthesis of Polysubstituted 3-Hydroxy-5H-chromeno[2,3-b]pyridinones

A pseudo-three-component annulation reaction of substituted 2-amino-4H-chromen-4-ones with aromatic aldehydes promoted by DBU was investigated to access polysubstituted 5H-chromeno[2,3-b]pyridines. This reaction included a sequential intermolecular nucleophilic addition/Michael cyclization/intramolecular epoxidation/ring opening/aromatization sequence, which possessed excellent step and atom economy in a single operation for generating 3-hydroxy-5H-chromeno[2,3-b]pyridines from readily available substrates.