An efficient and simple method for synthesis of 2,2-disubstituted-2H-chromenes by condensation of a phenol with a 1,1-disubstituted propargyl alcohol using BF3·Et2O as the catalyst

K2CO3-Promoted oxy-Michael Addition/Cyclization of α,β-Unsaturated Carbonyl Compounds with Naphthols: Synthesis of Naphthopyrans

A potassium carbonate promoted tandem oxy-Michael addition/cyclization of α,β-unsaturated carbonyl compounds with naphthol derivatives for the synthesis of 2-substituted naphthopyrans was developed. Using the readily available, inexpensive potassium carbonate as the promoter, a range of different substituted naphthopyrans were prepared.

Synthesis of 4-trifluoromethyl-2H-chromenes via the reaction of 2-(trifluoroacetyl)phenols with vinyltriphenylphosphonium chloride

Substituted on the benzene ring 4-CF₃-2H-chromenes have been prepared from substituted 2-(trifluoroacetyl)phenols and vinyltriphenylphosphonium chloride according to the Schweizer protocol in moderate to excellent yields. The influence of the type and the position of aromatic ring substituents on yields of 4-CF₃-2H-chromenes have been investigated. It has been shown that 4-CF₃-2H-chromenes are convenient precursors to 4-CF₃-coumarins.

Copper(II)-catalyzed direct dehydrative alkynylation of 2H-chromene hemiketals with terminal alkynes to 2,2-disubstituted 2-alkynylated 2H-chromenes

The first copper-catalyzed direct dehydrative alkynylation of 2H-chromene hemiketals with terminal alkynes has been uncovered. The use of cheap and readily available CuCl₂ as the catalyst allowed the preparation of various 2,2-disubstituted 2-alkynylated 2H-chromenes in moderate to good yields, which compensates for the limitation of the current methods only suited for the synthesis of 2-monosubsituted 2-alkynylated 2H-chromenes.

Y(OTf)3-catalyzed phosphorylation of 2H-chromene hemiacetals with P(O)-H compounds to 2-phosphorylated 2H-chromenes

A facile synthesis of 2-phosphorylated 2H-chromenes has been accomplished herein via a Y(OTf)₃-catalyzed dehydrative coupling of 2H-chromene hemiacetals with P(O)-H compounds. This protocol features low catalyst loading, mild reaction conditions, broad substrate scope and easy elaboration of the products.

Gold(I)-Catalyzed Intramolecular Hydroarylation of Phenol-Derived Propiolates and Certain Related Ethers as a Route to Selectively Functionalized Coumarins and 2H-Chromenes

Methods are reported for the efficient assembly of a series of phenol-derived propiolates, including the parent system 56, and their Au(I)-catalyzed cyclization (intramolecular hydroarylation) to give the corresponding coumarins (e.g., 1). Simple syntheses of natural products such as ayapin (144) and scoparone (145) have been realized by such means, and the first of these subject to single-crystal X-ray analysis. A related process is described for the conversion of propargyl ethers such as 156 into the isomeric 2H-chromene precocene I (159), a naturally occurring inhibitor of juvenile hormone biosynthesis.

Boronic acid/Brønsted acid co-catalyst systems for the synthesis of 2H-chromenes from phenols and α,β-unsaturated carbonyls

Protocols for the synthesis of substituted 2H-chromenes from α,β-unsaturated carbonyls and phenols are described. Optimal combinations of arylboronic acids and Brønsted acids have been identified, such that both can be employed in catalytic quantities to accelerate these condensations. The method has been used to synthesize a variety of substituted 2H-chromenes, as well as photochromic naphthopyrans. The use of pentafluorophenylboronic acid and diphenylphosphinic acid enabled an expansion of the electrophile scope to include α,β-unsaturated ketones. Hall's 'phase-switching' of boronic acids has been exploited to achieve the separation of the two co-catalysts from unpurified reaction mixtures by a simple liquid-liquid extraction.

Ecocatalysis for 2H-chromenes synthesis: an integrated approach for phytomanagement of polluted ecosystems

A direct, general and efficient method to synthesize 2H-chromenes (2H-benzo[b]pyrans), identified as environmentally friendly pesticides, has been developed. This approach lays on the new concept of ecocatalysis, which involves the use of biomass from phytoextraction processes, as a valuable source of metallic elements for chemical synthesis. This methodology is similar or superior to known methods, affording 2H-chromenes with good to excellent yields (60-98%), including the preparation of precocene I, a natural insect growth regulator, with 91% yield. The approach is ideal for poor reactive substrates such as phenol or naphthol, classically transformed into 2H-chromenes by methodologies associated with environmental issues. These results illustrate the interest of combining phytoextraction and green synthesis of natural insecticides.

Synthesis of 2-Acylphenol and Flavene Derivatives from the Ruthenium-Catalyzed Oxidative C-H Acylation of Phenols with Aldehydes

The cationic ruthenium hydride complex [(C₆H₆)(PCy₃)(CO)RuH]⁺BF₄^- has been found to be an effective catalyst for the oxidative C-H coupling reaction of phenols with aldehydes to give 2-acylphenol compounds. The coupling of phenols with α,β-unsaturated aldehydes selectively gives the flavene derivatives. The catalytic method mediates direct oxidative C-H coupling of phenol and aldehyde substrates without using any metal oxidants or forming wasteful byproducts.

Silver salts and DBU cooperatively catalyzed nucleophilic addition/cyclization of propargylic alcohols with trifluoromethyl ketones

A general and efficient synthesis of trifluoromethyl substituted 5-alkylidene-1,3-dioxolane using a silver salt and DBU cooperatively catalyzed nucleophilic addition/cyclization of propargylic alcohols and trifluoromethyl ketones is described.