Construction of 9-functionalized xanthenes via Diels-Alder reaction of stable ortho-quinone methides and arynes

Synthesis of fused-ring systems and diarylmethane flavonoids via ortho-quinone methide intermediates

In this study, we investigated the ability of ortho-quinone methide (o-QM), an electron-poor diene, to undergo inverse electron-demand Diels-Alder (iEDDA) reaction with electron-rich dienophiles, resulting in fused-ring flavonoid systems. In addition, we explored the Michael-type addition using o-QM and various nucleophiles, providing access to diarylmethane products. The cycloaddition reactions proceeded in a highly regioselective way, depending on the charge distribution of the reacting partners. Overall, the electron-rich dienophiles or nucleophiles reacted with the intermediate o-QM to afford products in reduced to moderate yields. Electron-poor dienophiles or nucleophiles, on the other hand, failed to react with o-QM.

Brønsted Acid Catalyzed Friedel-Crafts Alkylation of Naphthols with In Situ Generated Naphthol-Derived ortho-Quinone Methides: Synthesis of Chiral and Achiral Xanthene Derivatives

We disclose herein an enantioselective protocol for the Brønsted acid catalyzed addition of naphthols to in situ generated naphthol-derived ortho-quinone methides (o-QMs) followed by intramolecular cyclization, which delivers substituted chiral xanthene derivatives, in a one-pot reaction sequence under mild conditions. This process serves to convert naphthol-derived ortho-hydroxyl benzylic alcohols into reactive naphthol-derived o-QMs using a chiral phosphoric acid (CPA) catalyst. Moreover, it is helpful in controlling the enantioselectivity of the carbon-carbon bond-forming event via hydrogen-bonding followed by intramolecular cyclization. Additionally, for the first time, we observe a Brønsted acid catalyzed C(sp²)-C(sp³) bond cleavage of naphthol-derived ortho-hydroxyl benzylic alcohols for the synthesis of achiral xanthene (sigma plane containing) derivatives in good to excellent yields.

Recent Applications of the Diels–Alder Reaction in the Synthesis of Natural Products (2017–2020)

The Diels–Alder reaction has long been established as an extremely useful procedure in the toolbox of natural product chemists. It tolerates a wide spectrum of building blocks of different complexity and degrees of derivatization, and enables the formation of six-membered rings with well-defined stereochemistry. In recent years, many total syntheses of natural products have been reported that rely, at some point, on the use of a [4+2]-cycloaddition step. Among classic approaches, several modifications of the Diels–Alder reaction, such as hetero-Diels–Alder reactions, dehydro-Diels–Alder reactions and domino-Diels–Alder reactions, have been employed to extend the scope of this process in the synthesis of natural products. Our short review covers applications of the Diels–Alder reaction in natural product syntheses between 2017 and 2020, as well as selected methodologies which are inspired by, or that can be used to access natural products.

1 Introduction

2 Syntheses from 2017

3 Syntheses from 2018

4 Syntheses from 2019

5 Syntheses from 2020

6 Conclusion

Electrochemical benzylic C-H arylation of xanthenes and thioxanthenes without catalyst and oxidant

A catalyst-free and oxidant-free C-H arylation of xanthenes and thioxanthenes using electrochemistry has been developed, which affords a number of cross-coupling products in moderate to good yields. This method is...

NaHSO4/SiO2 catalyzed generation of o-quinone/ o-thioquinone methides: synthesis of arylxanthenes/ arylthioxanthenes via oxa-6π-electrocyclization

ortho-Quinone methides, very reactive transient intermediates, are utilized successfully in synthesizing complex organic molecules of natural and biological significance. Among several synthetic protocols, the acid catalyzed generation of ortho-quinone methides from suitably substituted phenols is a promising method for further exploitation in organic synthesis. Such an interesting reactive species is conveniently employed in the synthesis of conformationally restricted triarylmethane derivatives such as 12/9-arylxanthenes/arylthioxanthenes starting from symmetrical/unsymmetrical 2-(hydroxydiarylmethyl)phenol/thiophenol, respectively, using SiO2-NaHSO4. Conformationally restricted 12/9-arylxanthenes/arylthioxanthenes were obtained in 52 to 96% yields using this protocol, which is believed to involve the formation of o-quinone methides followed by electrocyclic ring closure and isomerization at elevated temperature. Photophysical studies of selected examples in acidic media showed turn-on fluorescence by hydride ion transfer mediated π-conjugated xanthylium salt formation and suggested the application potential in bio-imaging and fluorescent sensors.

Iodine-catalyzed efficient synthesis of xanthene/thioxanthene-indole derivatives under mild conditions

An iodine-catalyzed nucleophilic substitution reaction of xanthen-9-ol and thioxanthen-9-ol with indoles has been developed, providing an efficient procedure for the synthesis of xanthene/thioxanthene-indole derivatives with good to excellent yields. This protocol offers several advantages, such as short reaction times, green solvent, operational simplicity, easily available catalyst and mild reaction conditions. Moreover, this method showed good tolerance of functional groups and a wide range of substrates.