Morita-Baylis-Hillman Reaction of β,β-Disubstituted Enones: An Enantioselective Organocatalytic Approach for the Synthesis of Cyclopenta[b]annulated Arenes and Heteroarenes

Morita-Baylis-Hillman Spirannulation under Phosphine- and Anion-Binding Catalysis

We introduce an advancement in Morita-Baylis-Hillman (MBH) chemistry that provides access to α-spirannulated enones. The treatment of enone-tethered azaarenium salts with catalytic amounts of organophosphines provides spiroindenyl dihydropyridines. It represents the α-spirannulation of enones via an intramolecular MBH (IMBH) reaction utilizing dual phosphine- and anion-binding catalysis. The IMBH adducts were subjected to several post-synthetic modifications to access highly functionalized molecules.

Enantioselective Synthesis of Chiral 2-Nitroallylic Amines via Cooperative Cation-Binding Catalysis

Chiral allylic amines are valuable building blocks for biologically important compounds and natural products. In this study, we present the use of cooperative cation-binding catalysis as an efficient method for synthesizing chiral allylic amines. By utilizing a chiral oligoEG and potassium fluoride as a cation-binding catalyst and base, respectively, a wide range of biologically relevant chiral 2-nitroallylic amines are obtained with excellent enantioselectivities (up to >99 % ee) through the organocatalytic asymmetric aza-Henry-like reaction of β-monosubstituted and β,β-disubstituted nitroalkenes with α-amidosulfones as imine precursors. Extensive experimental studies are presented to illustrate plausible mechanisms. Preliminary use of a chiral 2-nitroallylic amine as a Michael acceptor demonstrated its potential application for diversity-oriented synthesis of bioactive compounds.

Conceptual advances in nucleophilic organophosphine-promoted transformations

Catalysis by trivalent nucleophilic organophosphines has emerged as an essential tool in organic synthesis. Several new organic transformations promoted by phosphines substantiate and complement the existing synthetic chemistry tools. Mere design of the substrate and reagent combinations has introduced new modes of reactivity patterns, which are otherwise difficult to achieve. These design considerations have led to the rapid build-up of complex molecular entities and laid a solid foundation to synthesise bioactive natural products and pharmaceuticals. This article presents an overview of some of the conceptual advances, including our contributions to nucleophilic organophosphine chemistry. The scope, limitations, mechanistic insights, and applications of these metal-free transformations are discussed elaborately.

Stereoselective Synthesis of Substituted (Z)-N-Allyl Sulfonamides via a Palladium-Catalyzed Three-Component Tandem Reaction

This paper reports the efficient synthesis of substituted (Z)-N-allyl sulfonamides via a palladium-catalyzed three-component tandem reaction of N-buta-2,3-dienyl sulfonamides with iodides and sulfonyl hydrazide or sulfinic acid sodium salt as nucleophiles. Pd(PPh₃)₄ (2.5 mol %), K₂CO₃, and THF were used as the optimal catalyst, base, and solvent, respectively. The substituted (Z)-N-allyl sulfonamides were obtained in a 30-83% overall yield. Mechanistic investigations revealed that the formation of the single (Z)-isomer was controlled by the formation of a six-membered palladacycle intermediate.

Dual Proton/Silver-Catalyzed Serial (5 + 2)-Cycloaddition and Nazarov Cyclization of (E)-2-Arylidene-3-hydroxyindanones with Conjugated Eneynes: Synthesis of Indanone-Fused Benzo[cd]azulenes

A one-pot step-economic tandem process involving (5 + 2)-cycloaddition and Nazarov cyclization reactions has been reported for the facile synthesis of indanone-fused benzo[cd]azulenes from (E)-2-arylidene-3-hydroxyindanones and conjugated eneynes. This highly regio- and stereoselective bisannulation reaction is enabled by dual silver and Brønsted acid catalysis and opens up a new avenue for the construction of important bicyclo[5.3.0]decane skeletons.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Carbene-Catalyzed Enantioselective Sulfonylation of Enone Aryl Aldehydes: A New Mode of Breslow Intermediate Oxidation

A carbene-catalyzed sulfonylation reaction between enone aryl aldehydes and sulfonyl chlorides is disclosed. The reaction effectively installs sulfone moieties in a highly enantioselective manner to afford sulfone-containing bicyclic lactones. The sulfonyl chloride behaves both as an oxidant and a nucleophilic substrate (via its reduced form) in this N-heterocyclic carbene (NHC)-catalyzed process. The NHC catalyst provides both activation and stereoselectivity control on a very remote site of enone aryl aldehyde substrates. Water plays an important role in modulating catalyst deactivation and reactivation routes that involve reactions between NHC and sulfonyl chloride. Experimental studies and DFT calculations suggest that an unprecedented intermediate and a new oxidation mode of the NHC-derived Breslow intermediate are involved in the new asymmetric sulfonylation reaction.

Synthesis of α,β-unsaturated ketones through nickel-catalysed aldehyde-free hydroacylation of alkynes

α,β-Unsaturated ketones are common feedstocks for the synthesis of fine chemicals, pharmaceuticals, and natural products. Transition metal-catalysed hydroacylation reactions of alkynes using aldehydes have been recognised as an atom-economical route to access α,β-unsaturated ketones through chemoselective aldehydic C-H activation. However, the previously reported hydroacylation reactions using rhodium, cobalt, or ruthenium catalysts require chelating moiety-bearing aldehydes to prevent decarbonylation of acyl-metal-hydride complexes. Herein, we report a nickel-catalysed anti-Markovnikov selective coupling process to afford non-tethered E-enones from terminal alkynes and S-2-pyridyl thioesters in the presence of zinc metal as a reducing agent. Utilization of a readily available thioester as an acylating agent and water as a proton donor enables the mechanistically distinctive and aldehyde-free hydroacylation of terminal alkynes. This non-chelation-controlled approach features mild reaction conditions, high step economy, and excellent regio- and stereoselectivity.

Phosphine-Catalyzed Intramolecular Vinylogous Aldol Reaction of α-Substituted Enones

We demonstrate the first phosphine-catalyzed intramolecular vinylogous aldol reaction (IVAR) of α-substituted enones. This strategy provides access to various pentannulated (hetero)arenes and dibenzocycloheptanones incorporated with two contiguous stereocenters, one of which is an all-carbon quaternary center. The scope of this work is further broadened through elaborations of the IVAR adducts to (i) benzannulated nine-membered carbocyclic systems, (ii) interesting classes of 1,3-dienes, 1,3,5-trienes, and 1-yn-3,5-dienes, and (iii) the analogs of echinolactone D and russujaponol F.

Annulative Morita-Baylis-Hillman reaction to synthesise chiral dibenzocycloheptanes

We describe the first metal-free and organocatalytic strategy to access highly functionalised dibenzocycloheptanes via a phosphine-promoted annulative Morita-Baylis-Hillman (MBH) reaction. The method is manipulated to access to chiral dibenzocycloheptanes as well. This work represents a rare entry for the construction of seven-membered carbocycles via the MBH route. The realisation of several bioactive molecules possessing the dibenzocycloheptane core makes this an attractive strategy.

Divergent Michael/Aldol Cascades Under Semi-Aqueous Conditions: Synthesis of Cyclopenta- and Cycloheptannulated (Hetero)arenes

The synthesis of 3-acetoxyindanones and (hetero)arene-fused dihydrotropones was achieved via divergent annulation cascades. Under mild aqueous and basic conditions, α-substituted enone-aldehydes and 1,3-carbonyls undergo a Michael/aldol/hemiketalization/retro-aldol cascade for the formation of 3-acetoxyindanones possessing two contiguous stereogenic centers, one of which is an all-carbon quaternary center. On the other hand, the same enone-aldehydes generate new classes of fused-dihydrotropones upon reaction with 2,4-dioxobutanoates under merely the same reaction conditions via a Michael/aldol/lactonization/decarboxylation cascade.

Catalytic Enantiodivergent Michael Addition by Subtle Adjustment of Achiral Amino Moiety of Dipeptide Phosphines

Over the past decades, asymmetric catalysis has been intensely investigated as a powerful tool for the preparation of numerous chiral biologically active compounds. However, developing general and practical strategies for preparation of both enantiomers of a chiral molecule via asymmetric catalysis is still a challenge, particularly when the two enantiomers of a chiral catalyst are not easily prepared from natural chiral sources. Inspired by the biologic system, we report herein an unprecedented catalytic enantiodivergent Michael addition of pyridazinones to enones by subtle adjustment of achiral amino moiety of dipeptide phosphine catalysts. These two dipeptide phosphine catalysts, P5 and P8, could deliver both enantiomers of a series of N2-alkylpyridazinones in good yields (up to 99%) with high enantioselectivities (up to 99% ee) via the catalyst-controlled enantiodivergent addition of pyridazinones to enones.

Palladium-catalysed 5-endo-trig allylic (hetero)arylation

A palladium-catalysed intramolecular allylic (hetero)arylation strategy for the synthesis of fused cyclopentenes incorporated with all-carbon quaternary and spiro centres is described. The method is straightforward, shows broad scope, proceeds in synthetically useful yields, and provides a rare means to construct complex cyclopentanoids. The reaction is believed to involve a kinetically unfavourable 5-endo-trig carbocyclisation of the tethered (π-allyl)palladium system. Further, this method was successfully applied as the key step in the total synthesis of diterpene natural products taiwaniaquinone H and dichroanone.

Synthesis of Cyclopropanoids via Substrate-Based Cyclization Pathways

A series of unexpected reactions triggered by the dimethyloxosulfonium methylide led to the discovery of unconventional approaches for the synthesis of cyclopropa-fused tetralones and indeno-spirocyclopropanes. These highly functionalized structures were further elaborated in one step to privileged scaffolds such as tetralones, indenones, and fluorenones. As a whole, the results presented herein establish new diversity-oriented folding pathways.

Phosphine- and water-promoted pentannulative aldol reaction

An intramolecular aldol reaction promoted by tributylphosphine and water is described. Thereby, metal-free access to highly functionalised cyclopenta-fused arenes and heteroarenes is achieved.

Phosphine-Catalyzed Difunctionalization of β-Fluoroalkyl α,β-Enones: A Direct Approach to β-Amino α-Diazo Carbonyl Compounds

An efficient and practical phosphine-catalyzed vicinal difunctionalization of β-fluoroalkyl α,β-enones with TMSN₃ has been developed. Using dppb as the catalyst, the reaction worked efficiently to yield various β-amino α-diazocarbonyl compounds in high yields (up to 94 %). This work marks the first efficient construction of α-diazocarbonyl compounds by phosphine catalysis. Meanwhile, the asymmetric variant induced by the nucleophilic bifunctional phosphine P4 led to various chiral fluoroalkylated β-amino α-diazocarbonyl compounds in high yields and enantioselectivity. NMR and ESI-MS studies support the existence of the key reaction intermediates. In contrast, β-azide carbonyl compounds would be furnished in good yields from β-fluoroalkylated β,β-disubstituted enones.

Organocatalytic Strategies for the Synthesis of Cyclopenta-Fused Arenes and Heteroarenes

Cyclopentanoids are omnipresent in natural products and pharmaceutically relevant compounds. Among them, cyclopenta-fused arenes and heteroarenes possess impressive biological properties and play significant role in materials science. Consequently, several notable methods have been developed for their synthesis over the years. In this review, we mainly described metal-free and organocatalytic approaches that led to the construction of pentannulated arenes and heteroarenes.

Primary Aminothiourea-Catalyzed Enantioselective Synthesis of Rauhut-Currier Adducts of 3-Arylcyclohexenone with a Tethered Enone on the Aryl Moiety at the Ortho-Position

An enantioselective synthesis of Rauhut-Currier (RC) adducts from 3-aryl cyclohexenone with a tethered enone moiety at the ortho-position on the aryl group is accomplished. This method provides a wide range of valuable synthetic building blocks having a unique [6-5-6] all-carbon-fused tricyclic skeleton. A primary amine-containing thiourea, a bifunctional organocatalyst, was found to be an efficient catalyst for this transformation. The primary amine counterpart of the catalyst possibly activates the aliphatic enone via dienamine formation (HOMO activation), whereas the thiourea counterpart activates the tethered enone (LUMO activation). Considering the difficulty in achieving an RC reaction of β,β-disubstituted (alkyl and aryl) enones, this method would be significantly rewarding.

Palladium-Catalyzed Intramolecular Trost-Oppolzer-Type Alder-Ene Reaction of Dienyl Acetates to Cyclopentadienes

A new approach to the synthesis of highly substituted cyclopentadienes, indenes, and cyclopentene-fused heteroarenes by means of the Pd-catalyzed Trost-Oppolzer-type intramolecular Alder-ene reaction of 2,4-pentadienyl acetates is described. This unprecedented transformation combines the electrophilic features of the Tsuji-Trost reaction with the nucleophilic features of the Alder-ene reaction. The overall outcome can be perceived as a hitherto unknown "acid-free" iso-Nazarov-type cyclization. The versatility of this strategy was further demonstrated by the formal synthesis of paucifloral F, a resveratrol-based natural product.

Synthesis of 4-(Trifluoromethyl)cyclopentenones and 2-(Trifluoromethyl)furans by Reductive Trifluoroacetylation of Ynones

Reductive introduction of a fluorine-containing carbon block to readily available conjugated ynones, followed by intramolecular cyclization, successfully gave the corresponding trifluoromethylated cyclopentenones or trifluoromethylated furans in good yields through a simple two-step protocol. The key compound in carbon-carbon bond formation by magnesium-promoted reduction is ethyl trifluoroacetate, which has been rarely used as a fluorine-containing carbon source, especially to electron-deficient carbon atoms in organic synthesis.

Ferrocene Derived Bifunctional Phosphine-Catalyzed Asymmetric Oxa-[4+2] Cycloaddition of α-Substituted Allenones with Enones

An efficient ferrocene-derived bifunctional phosphine-catalyzed enantioselective oxa-[4+2] cycloaddition of α-substituted allenones with a broad range of enones is investigated for the preparation of stereodefined dihydropyrans in good to excellent yields (up to 99 %) and excellent enantioselectivity (up to 99 % ee). Furthermore, a series of valuable chiral polyheterocyclic frameworks can be efficiently achieved in good yields with excellent enantioselectivities.