Regiospecific 6-Endo-Annulation of in Situ Generated 3,4-Dienamides/Acids: Synthesis of δ-Lactams and δ-Lactones

Lewis Acid-Catalyzed Tandem Reaction Strategy for the Synthesis of Dihydrophenalene-Fused Lactones

A Lewis acid-catalyzed tandem reaction strategy for the construction of a dihydrophenalene-lactone tetracyclic skeleton has been disclosed. Starting with 2-naphthol-tethered ketones and active methylene esters, the tandem reaction catalyzed by Sc(OTf)3 proceeded well to afford an array of dihydrophenalene-fused lactones with moderate to high efficiency and diastereoselectivity. Moreover, the synthetic utility of this protocol was demonstrated by easy gram-scale preparation and diverse product transformations.

Organophotocatalysed synthesis of 2-piperidinones in one step via [1 + 2 + 3] strategy

Six-membered N-containing heterocycles, such as 2-piperidinone derivatives, with diverse substitution patterns are widespread in natural products, drug molecules and serve as key precursors for piperidines. Thus, the development of stereoselective synthesis of multi-substituted 2-piperidinones are attractive. However, existing methods heavily rely on modification of pre-synthesized backbones which require tedious multi-step procedure and suffer from limited substitution patterns. Herein, an organophotocatalysed [1 + 2 + 3] strategy was developed to enable the one-step access to diverse substituted 2-piperidinones from easily available inorganic ammonium salts, alkenes, and unsaturated carbonyl compounds. This mild protocol exhibits exclusive chemoselectivity over two alkenes, tolerating both terminal and internal alkenes with a wide range of functional groups.

Metal-Free, Adjustable, and Recyclable Catalytic Systems for the Construction of C-C Bonds by Activating Propargylic Alcohols

Pyrano[3,2-c]coumarin derivatives and C3-substituted 4-hydroxycoumarins as important skeletal structures of active natural products and pharmaceutically relevant molecules have received increasing attention. However, developing an adjustable system for selectively synthesizing them is still a challenging task. Herein, sulfonic acid-functionalized ionic liquid was successfully used as the catalyst for the alkylation of 4-hydroxycoumarin derivatives with secondary aromatic propargylic alcohols using dimethyl carbonate as the green solvent, giving up to 98% yield. On the other hand, protonated imidazole-based ionic liquid-catalyzed cyclization was also selectively achieved with a nearly quantitative yield. Developed metal-free catalytic systems exhibited well adjustable and recyclable properties, avoiding the contamination of metal and halogen, reducing the neutralization after the reaction, and benefiting the separation between the catalyst and the product. New strategies were applied for performing the gram-scale reaction smoothly. The adjustable systems might occur through two different mechanisms involving propargylic or allenic carbocation and hydrogen bonding effects between the catalysts and the substrates.

FeTPPCl/FeCl3 Co-Catalyzed One-Pot Green Synthesis of α-Diaryl-β-alkynol Derivatives via Propargylic Carbocation Chemistry

A green and highly efficient one-pot method for α-diaryl-β-alkynol derivatives in water at room temperature was developed using the cocatalysis of a Lewis acid and meso-tetraphenylporphyrin iron(III) chloride (FeTPPCl). The unprecedented transformation was promoted by a modulation of the charge properties of propargylic carbocation chemistry and the use of an in situ-generated oxonium ylide as a matching nucleophile. The reaction was performed in water at room temperature with a highly step-economic manipulation in good to excellent yields and with a broad substrate scope. Water also acts as the third reactant for the one-pot transformation. Notably, the FeTPPCl catalyst can be directly reused four times with a slight discount in yields.

Synthesis of Functionalized 9-Substituted Fluorene Derivatives via Boron Trifluoride Catalysed Reaction of Coplanar 9-(Phenylethynyl)-9H-fluoren-9-ols, Aryl Aminoamides and N-Bromosuccinimide

A boron trifluoride catalysed reaction of coplanar 9-(phenyl­ethynyl)-9H-fluoren-9-ols with various 2-aminobenzamides affords a number of highly functionalized, conjugated (Z)-2-((2-(9H-fluoren-9-ylidene)-1-phenylethylidene)amino) benzamides in excellent yield. The reaction in the presence of N-bromosuccinimide affords (E)-5-bromo-2-((2-bromo-2-(9H-fluoren-9-ylidene)-1-phenylethylidene)amino)benz­amides in very good yields. The scope of the reaction is demonstrated by selecting N-aryl substituted 2-aminobenzamides and aminosulfonamides as reaction partners. The structures of representative compounds were established by single-crystal XRD analysis. Based on the structure of the products, a plausible mechanism via formation of allene carbocation intermediates is proposed.

Lewis or Brønsted acid-catalysed reaction of propargylic alcohol-tethered alkylidenecyclopropanes with indoles and pyrroles for the preparation of polycyclic compounds tethered with indole or pyrrole motif

We developed a facile synthetic method to access the cyclopenta[b]naphthalene skeleton from Lewis or Brønsted acid-catalysed propargylic alcohol-tethered alkylidenecyclopropanes with indole and pyrrole derivatives.

Addition of dithi(ol)anylium tetrafluoroborates to α,β-unsaturated ketones

In the presented study, dithi(ol)anylium tetrafluoroborates are added to α,β-unsaturated ketones in a Michael-type reaction yielding diverse substituted ketene diothi(ol)anes. The reactions proceed at room temperature in 1 or 13 h without the need of further additives. The presented procedure is in particular useful for dithi(ol)anylium tetrafluoroborates without electron-withdrawing groups in α-position. This is advantageous with respect to previous approaches, which were limited to the use of ketene dithioacetals substituted with electron-withdrawing groups. Aiming for the systematic investigation of possible steric and electronic influences on the outcome of the reaction, various combinations of electrophiles and nucleophiles were used and the results of the reactions were compared based on the type of the used dithioacetal. The scope of the presented procedure is shown with four additional transformations including the use of additional electrophiles and nucleophiles, the use of a chiral auxiliary and subsequent reduction of selected products. Additionally, we extended the reaction to the synthesis of diene dithiolanes by addition of an ynone to α-alkyl or aryl-substitued dithiolanylium TFBs.

Scope and advances in the catalytic propargylic substitution reaction

Nucleophilic displacement of the propargylic alcohol is one of the sought-after methods in the current scenario. The highly nucleophilic alkyne functional moiety along with its considerably acidic terminal hydrogen atom allows the propargylic unit to play a crucial role in organic synthesis by offering a handle for further synthetic transformations. Until 2000, the most fundamental propargylic substitution reaction was the Nicolas reaction, a multi-step transformation, developed in 1972, which involved cobalt as a stoichiometric promoter. Therefore, the direct catalytic substitution of propargylic alcohols was a highly desirable method for development. The pioneering work on the Ru-catalyzed propargylic substitution reaction in 2000 encouraged many researchers to develop several novel catalytic propargylic substitution reactions, which have made rapid progress since then. The purpose of this review is to emphasise the involvement of diverse types of Lewis acid, transition metal and Brønsted acid catalysts in the propargylic substitution reaction and provide an updated summary of the recent developments in this field. The selected examples presented here are the most significant and relevant ones and we believe that this will help the readers to comprehend the scope of the propargylic substitution reaction with diverse types of catalysts and will envisage the scientific community for the future developments in this field.

Direct nucleophilic displacement of the alpha-hydroxy of the propargylic alcohol is one of the sought-after methods in the current scenario. An updated summary of the recent developments in this field is presented here.

Silver-catalyzed phosphonylation of unprotected propargylic alcohols for the synthesis of allenylphosphoryl compounds

A facile and efficient method for the synthesis of a variety of allenylphosphine oxides by silver-catalyzed phosphonylation of unprotected propargylic alcohols with P(O)H compounds has been established.

Catalytic σ-activation of carbon–carbon triple bonds: reactions of propargylic alcohols and alkynes

A variety of organic transformations have been developed based on conceptually novel catalytic σ-activation of CC bonds of alkynes.

Yb(OTf)3 catalyzed [3 + 2] annulations of D–A cyclopropanes with β-oxodithioesters: a regioselective synthesis of tetrahydrothiophenes

An efficient approach to synthesis tetrahydrothiophenes was developed through the reaction of β-oxodithioesters and D–A cyclopropanes catalyzed by Yb(OTf)₃. This methodology was highly regioselective.