A novel synthesis of polysubstituted chromenes from various salicylaldehydes and alkynes under mild conditions

Mechanistic Insights on Pd-Catalyzed Three-Component Reactions of Alkynols, Methyl Orthoformate, and Salicylaldehyde Derivatives. Application to the Synthesis of Steroid Chroman Ketals and Spiroketals with Antioxidant Activity

Contrary to our previous report in which a Pd-catalyzed three-component reaction of a steroid alkynol, trimethyl orthoformate, and salicylaldehyde exclusively produced chroman ketals, the same reaction employing 2,5-dihydroxysalicylaldehyde led to a mixture of a chroman ketal and a spiroketal. Provided that both courses of the reaction imply a 4 + 2 inverse demand cycloaddition between an o-quinone methide and an enol ether, density functional theory calculations revealed that the chroman ketal/spiroketal selectivity is governed by both, the rate of the formation of the o-quinone methide and the isomerization of the initially produced exocyclic enol ether─that led to the spiroketal─to its endocyclic partner that produces the chroman ketal. Remarkably, Lewis catalysis is central to the observed reactivity, and the availability of plausible catalytic species controls the overall chemoselectivity. The methodology herein applied and scrutinized enriches the palette of reactions, leading to increased molecular complexity, as demonstrated in the obtained products, whose antioxidant activity and detailed NMR characterization are presented.

A new cycloaddition profile for ortho-quinone methides: photoredox-catalyzed [6+4] cycloadditions for synthesis of benzo[b]cyclopenta[e]oxepines

Visible-light-induced [6+4] cycloaddition reactions of ortho-quinone methides have been developed. The reaction of ortho-quinone methides with pentafulvenes in the presence of a thioxanthylium photoredox catalyst afforded benzo[b]cyclopenta[e]oxepines. The present reaction represents a promising tool for the synthesis of natural products and bioactive compounds that contain a benzoxepine structure.

Redox Potential Controlled Selective Oxidation of Styrenes for Regio- and Stereoselective Crossed Intermolecular [2 + 2] Cycloaddition via Organophotoredox Catalysis

A redox potential controlled intermolecular [2 + 2] cross-cycloaddition has been developed in the presence of a thioxanthylium photoredox catalyst. Electron-rich styrenes such as β-bromostyrene (E_p/2 = +1.61 V vs SCE) were selectively oxidized by a thioxanthylium photoredox catalyst (E_1/2 (C*/C^•-) = +1.76 V vs SCE) to styryl radical cations and reacted with styrene (E_p/2 = +1.97 V vs SCE) to furnish polysubstituted cyclobutanes in high yields. The present reaction can be successfully applied to intermolecular [2 + 2] cross-cycloaddition of β-halogenostyrenes, which cannot be effectively achieved by the hitherto reported representative organophotoredox catalysts.

Visible-light-induced [4 + 2] cycloaddition of pentafulvenes by organic photoredox catalysis

We have developed thioxanthylium photoredox catalyzed [4 + 2] cycloaddition of pentafulvenes at room temperature under green light irradiation, which affords tetrahydrocyclopenta[b]chromenes with high regioselectivities. The present reaction provides a sustainable approach to carry out the cycloaddition of pentafulvenes without the use of transition metal catalysts or high-temperature conditions. This procedure enables a mild and straightforward access to 1,3a,9,9a-tetrahydrocyclopenta[b]chromenes. The quantum yield of the reaction (Φ = 0.15) indicates that the reaction would mainly proceed via photocatalytic pathways.

Access to Electron-Deficient 2,2-Disubstituted Chromanes: A Highly Regioselective One-Pot Synthesis via an Inverse-Electron-Demand [4 + 2] Cycloaddition of ortho-Quinone Methides

We report the one-pot synthesis of 2,2-disubstituted chromanes with electron-withdrawing substituents. This reaction provides a simple yet efficient route to a wide range of electron-deficient chromanes in high yield and excellent regioselectivity. The reaction of salicylaldehyde with 1,1-disubstituted ethylenes smoothly furnishes these electron-deficient chromanes, which can be further transformed into functionalized chromanes or chromene. For example, BW683C was effectively synthesized from 5-chlorosalicylaldehyde with 4-chlorostyrene in two steps in excellent yield. The present reaction thus provides versatile access to functionalized electron-deficient chromanes and chromenes and therefore constitutes a promising tool for the synthesis of biologically and photochemically active molecules.

Friedel-Crafts approach to the one-pot synthesis of methoxy-substituted thioxanthylium salts

An efficient synthesis of methoxy-substituted thioxanthylium salts has been developed. The reaction of diaryl sulfides with benzoyl chlorides in the presence of TfOH smoothly proceeded to give the desired thioxanthylium salts in good yields. In their UV–vis spectra, the maximum absorption wavelengths of methoxy-functionalized thioxanthylium salts were observed at around 460 nm, which show a drastic red shift compared to the parent thioxanthylium salts. The present reaction provides a versatile access to functionalized thioxanthylium salts, and therefore it constitutes a promising tool for the synthesis of biologically and photochemically active molecules.

Organophotoredox-Catalyzed Intermolecular Oxa-[4+2] Cycloaddition Reactions

An intermolecular oxa-[4+2] cycloaddition reaction promoted by a thioxanthylium photoredox catalyst under irradiation with green light has been developed. The reaction of ortho-quinone methides with styrenes smoothly affords the desired cycloadducts. Especially styrenes bearing electron-donating groups are efficiently transformed in this reaction. This method represents a sustainable way to carry out oxa-[4+2] cycloaddition reactions using only a catalytic amount of a photocatalyst and visible light.