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Chalcones are aromatic ketones found in nature as the central core of many biological compounds. They have a wide range of biological activity and are biogenetic precursors of other important molecules such as flavonoids. Their pharmacological relevance makes them a privileged scaffold, advantageous for seeking alternative therapies in medicinal chemistry. Due to their structural diversity and ease of synthesis, they are often employed as building blocks for chemical transformations. Chalcones have a carbonyl conjugated system with two electrophilic centers that are commonly used for nucleophilic additions, as described in numerous articles. They can also participate in Diels‐Alder reactions, which are [4+2] cycloadditions between a diene and a dienophile. This microreview presents a chronological survey of studies on chalcones as dienes and dienophiles in Diels‐Alder cycloadditions. Although these reactions occur in nature, isolation of chalcones from plants yields very small quantities. Contrarily, synthesis leads to large quantities at a low cost. Hence, novel methodologies have been developed for [4+2] cycloadditions, with chalcones serving as a 2π or 4π electron system. [ABSTRACT FROM AUTHOR]

New and efficient strategies for the conversion of 4‐oxazolin‐2‐ones into 1‐methyl and 1,4‐dimethyl 3‐formylcarbazoles are herein described. Highly convergent cascade and one‐pot processes afforded the corresponding diarylamines, as in situ formed synthetic intermediates or final products in high overall yields. Special attention was given to the synthesis of methylated carbazoles by reacting 4,5‐dimethyl‐4‐oxazolin‐2‐ones with enones under microwave irradiation. The carbazole scaffold was provided by the palladium(II)‐catalyzed double C−H activation to generate oxidative cyclization of diarylamines. This methodology allowed for formal total syntheses of four naturally occurring pyrido[4,3‐b]carbazole alkaloids ellipticine, 9‐methoxyellipticine, olivacine, and 9‐methoxyolivacine. [ABSTRACT FROM AUTHOR]

Enantiopure 3‐((R)‐ and 3‐((S)‐1‐phenylethyl)‐4‐oxazoline‐2‐ones were evaluated as chiral building blocks for the divergent construction of heterocycles with stereogenic quaternary centers. The N‐(R)‐ or N‐(S)‐1‐phenylethyl group of these compounds proved to be an efficient chiral auxiliary for the asymmetric induction of the 4‐ and 5‐positions of the 4‐oxazolin‐2‐one ring through thermal and MW‐promoted nucleophilic conjugated addition to Michael acceptors and alkyl halides. The resulting adducts were transformed via a cascade process into fused six‐membered carbo‐ and heterocycles. The structure of the reaction products depended on the electrophiles and reaction conditions used. Alternative isomeric 4‐methylene‐2‐oxazolidinones served as chiral precursors for a versatile and divergent approach to highly substituted cyclic carbamates. DFT quantum calculations showed that the formation of bicyclic pyranyl compounds was generated by a diastereoselective concerted hetero‐Diels‐Alder cycloaddition. [ABSTRACT FROM AUTHOR]

The N‐substituted exo‐2‐oxazolidinone dienes are versatile molecules that undergo a variety of reactions. To further explore this versatility, Diels‐Alder reactions were carried out with novel naphthalene chalcones. Upon attempting the Diels‐Alder reaction with 2‐hydroxy‐1,4‐naphthoquinone, the formation of chromene unexpectedly took place via a formal [3+3] cycloaddition reaction. The observed reaction was then achieved with other 1,3‐dicarbonyl compounds. Cycloaddition Tales: Versatile N‐substituted exo‐2‐oxazolidinone dienes underwent different cycloaddition reactions, including the famous Diels‐Alder reaction, with novel chalcones to synthesize benzoxazole‐2‐ones and [3+3] formal cycloaddition with 1,3‐dicarbonyl compounds to synthesize chromenes. Both displayed interesting regio‐ and diastereoselectivity. [ABSTRACT FROM AUTHOR]

The process of N-alkylation of several pyrroles, indoles, and derivative heterocycles is herein described, using quaternary ammonium salts as the source of an alkylating agent. These reactions were carried out on several heterocyclic rings with triethylbenzylammonium chloride or tetradecyltrimethylammonium bromide and an NaOH solution at 50%, leading to a chemoselective N-alkylated product and an average yield of 73%. This is an alternative process to the traditional benzylation and methylation of N-heterocycles with direct handling of alkyl halides. [ABSTRACT FROM AUTHOR]

A simple procedure to prepare 1,5-disubstituted 1,2,3-triazoles efficiently from alkyl/aryl azides and β-ketophosphonates in the presence of KOH by an azide-enolate 1,3-dipolar cycloaddition in good yields was developed. [ABSTRACT FROM AUTHOR]

( tert-Butyl)(dimethyl)silyl ( tBuMe2Si; TBS) phenyl/alkyl ethers were efficiently cleaved to the corresponding parent hydroxy compounds in good yields using catalytic amounts of Ce(SO4)2⋅4 H2O by microwave-assisted or conventional heating in MeOH. Intramolecular and competitive experiments demonstrated the chemoselective deprotection of TBS ethers in the presence of triisopropylsilyl (iPr3Si; TIPS) and ( tert-butyl)(diphenyl)silyl ( tBuPh2Si; TBDPS) ethers. [ABSTRACT FROM AUTHOR]