Multi-component cascade reaction of 3-formylchromones: highly selective synthesis of 4,5-dihydro-[4,5'-bipyrimidin]-6(1H)-one derivatives

Cascade Annulation for Synthesizing Chromenopyrrolones from o-Hydroxyphenyl Enaminones and 2-Halo-N-alkyloxyacetamides

A cascade cyclization reaction comprising two halogenation reactions and a Michael addition was developed for the synthesis of chromeno[2,3-c]pyrrole-3-ones 4. Additionally, another cascade cyclization reaction, which involves a halogenation reaction followed by two intramolecular Michael additions, was established for the synthesis of chromeno[2,3-b]pyrrole-2-ones 5. Both types of compounds were synthesized from o-hydroxyphenyl enaminones and 2-halo-N-alkyloxyacetamides through a process that facilitated the intramolecular formation of C-C, C-O, and C-N bonds to effectively establish two fused rings in a single operation. This novel protocol is efficient, uses readily accessible starting materials, and operates under mild conditions, demonstrating tolerance for various functional groups while achieving good yields.

Hydroxyl-Directed Rh(III)-Catalyzed C-H Functionalization: Access to Benzo[de]chromenes

A cascade oxidative annulation reaction of heterocyclic ketene aminals (HKAs) with internal alkynes catalyzed by [Cp*RhCl₂]₂ and oxidized by Cu(OAc)₂·H₂O was developed to efficiently synthesize highly functionalized benzo[de]chromene derivatives in good to excellent yields. The reaction proceeded by the sequential cleavage of C(sp²)-H/O-H and C(sp²)-H/C(sp²)-H bonds. These multicomponent cascade reactions were highly regioselective. In addition, all of the benzo[de]chromene products exhibited intense fluorescence emission in the solid state, and they demonstrated concentration-dependent quenching in the presence of Fe³⁺, indicating that these compounds could be used in the recognition of Fe³⁺.

Cu-Catalyzed decarboxylative annulation of N-substituted glycines with 3-formylchromones: synthesis of functionalized chromeno[2,3-b]pyrrol-4(1H)-ones

A novel protocol was developed for preparing functionalized chromeno[2,3-b]pyrrol-4(1H)-ones 3 (CMPOs) from 3-formylchromones with N-substituted glycine derivatives. The method entailed decarboxylative annulation of the acyl group of 3-formylchromones by simply heating a mixture of substrates 1-2 and toluene oxidized by 2-di-tert-butyl peroxide (DTBP) and catalyzed by CuBr. As a result, a series of CMPOs 3 were produced via a cascade reaction. This protocol can be used to synthesize functionalized CMPOs via combinatorial and parallel syntheses in a one-pot reaction rather than a tedious multi-step reaction.

Diastereoselective and E/Z-Selective Synthesis of Functionalized Quinolizine Scaffolds via the Dearomative Annulation of 2-Pyridylacetates with Nitroenynes

An organocatalytic Michael/aza-Michael cascade reaction was developed to build the functionalized quinolizine scaffolds in 60-82% yields, excellent diastereoselectivities, and E/Z selectivities. This protocol involves the [3 + 3] annulations of 2-pyridylacetates with nitroenynes through the dearomative strategy in the presence of an organic base under mild conditions. The versatile late-stage derivatizations further demonstrated the synthetic utility of this methodology.

An Environmentally Benign Multicomponent Cascade Reaction of 3-Formylchromones, 2-Naphthols, and Heterocyclic Ketal Aminals: Site-Selective Synthesis of Functionalized Morphan Derivatives

A novel protocol has been developed for the preparation of highly functionalized 2-azabicyclo[3.3.1]nonane (morphan) derivatives by the interesting three-component cascade reaction of 3-formylchromones, 2-naphthol, and heterocyclic ketal aminals (HKAs) in the ionic liquid [BMIM]PF₆ promoted by the organic base Et₃N. A complex cascade reaction is required, which includes a 1,2-addition, two Michael reactions, two tautomerizations, and an N-alkylation accompanied by a ring-opening reaction and involving the cleavage of one C-O bond and the formation of four bonds (one C-N bond, one C-O bond, and two C-C bonds). As a result, functionalized morphans (5 and 6) bearing naphthalene-structured skeletons were prepared by simple heating of a mixture of 3-formylchromones, 2-naphthols, and HKAs in the environmentally friendly ionic liquid [BMIM]PF₆. This protocol can be used in the synthesis of various morphans and is suitable for combinatorial and parallel syntheses of natural-like morphan derivatives. This approach has several advantages such as the use of an environmentally friendly solvent, simple and practical operation (multicomponent one-pot reaction), and satisfactory yields (65-88%).

A Flexible Synthetic Approach to Fluorescent Chromeno[4,3-b]pyridines and Pyrano[3,2-c]chromenes from Electron-Deficient 3-Vinylchromones

We report a flexible approach to the synthesis of phenanthrene-like heterocycles through organocatalytic ANRORC (Addition of the Nucleophile, Ring Opening, and Ring Closure) reaction of electron-deficient 3-vinylchromones with cyanoacetamide. Addition of highly basic DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) or tetramethylguanidine (TMG) at 80 °C leads to chromeno[4,3-b]pyridines in good yields, whereas Et3 N at 20 °C made it possible to obtain the less accessible pyrano[3,2-c]chromenes and their 2-imines. The synthesis proceeds in mild conditions (EtOH, 20-80 °C), is versatile and applicable for a wide scope of reactants. The obtained compounds show bright fluorescence in the range 460-595 nm with high quantum yields (up to 0.84) in various solvents (MeCN, DMSO, EtOH, H2 O).