Design and Application of Peptide-Mimic Phosphonium Salt Catalysts in Asymmetric Synthesis

Chiral phosphonium salt catalysis, traditionally classified as a type of phase transfer catalysis, has proven to be a powerful strategy for the stereoselective preparation of diverse optically active molecules. However, there still remain numerous forbidding issues of reactivity and selectivity in such well-known organocatalysis system. Accordingly, the development of new and high-performance phosphonium salt catalysts with unique chiral backbones is highly desirable, yet challenging. This Minireview describes the prominent endeavours in the development of a new family of chiral peptide-mimic phosphonium salt catalysts with multiple hydrogen-bonding donors and their applications in a plethora of enantioselective synthesis during the past few years. Hopefully, this minireview will pave a way for further developing much more efficient and privileged chiral ligands/catalysts featuring exclusively catalytic ability in asymmetric synthesis.

Structurally divergent enantioselective synthesis of benzofuran fused azocine derivatives and spiro-cyclopentanone benzofurans enabled by sequential catalysis

An important objective in organic synthesis and medicinal chemistry is the capacity to access structurally varied and complex molecules rapidly and affordably from easily available starting materials. Herein, a protocol for the structurally divergent synthesis of benzofuran fused azocine derivatives and spiro-cyclopentanone benzofurans has been developed via chiral bifunctional urea catalyzed reaction between aurone-derived α,β-unsaturated imine and ynone followed by switchable divergent annulation reactions by Lewis base catalysts (DBU and PPh3) with concomitant epimerization. The skeletally diversified products were formed in high yields with high diastereo- and enantioselectivities. Computational analysis with DFT and accurate DLPNO-CCSD(T) has been employed to gain deeper insights into mechanistic intricacies and investigate the role of chiral and Lewis base catalysts in skeletal diversity.

PPh3-Triggered Tandem Synthesis of 2,3-Disubstituted Benzofuran Derivatives from o-Quinone Methides with Acyl Chlorides

A PPh₃-triggered tandem strategy for the efficient synthesis of valuable 2,3-disubstituted benzofuran derivatives in generally good to high yields from aryl or alkyl acyl chlorides and o-quinone methides has been developed. This method features mild reaction conditions, simple operation, and a broad substrate scope.

Palladium-Catalyzed [4+4] Cycloadditions for Highly Diastereo- and Enantioselective Synthesis of Functionalized Benzo[b]oxocines

Asymmetric cycloaddition reactions represent a powerful strategy for building up complex molecular architectures, especially those with medium-sized rings. Herein, we disclose a highly diastereo- and enantioselective cycloaddition strategy that involves...

Highly Enantioselective Synthesis of Phosphorus-Containing ϵ-Benzosultams by Bifunctional Phosphonium Salt-Promoted Hydrophosphonylation

ϵ-Benzosultam derivatives are potential drug candidates with diverse biological activities. A series of chiral ϵ-benzosultams bearing phosphorus functionalities was synthesized by catalytic asymmetric hydrophosphonylation in the presence of a bifunctional phosphonium salt catalyst. The desired hydrophosphonylation products were obtained in good yields with high enantioselectivities, and scale-up reactions and further derivations were successfully accomplished. Some control experiments were also conducted to elucidate the plausible reaction mechanism of this chemical transformation.

Unusual Application for Phosphonium Salts and Phosphoranes: Synthesis of Chalcogenides

A novel strategy for the synthesis of sulfides and selenides from phosphonium salts and thio- or selenesulfonates, commercially available compounds, is described. When phosphoranes were used in the reaction, different products were obtained. The methodology does not require the use of metals, reactive species, or anhydrous conditions to be performed.

Catalytic Asymmetric Conjugate Addition/Hydroalkoxylation Sequence: Expeditious Access to Enantioenriched Eight-Membered Cyclic Ether Derivatives

A sequential enantioselective conjugate addition/hydroalkoxylation between in situ generated ortho-quinomethanes and ynones by combining bifunctional squaramide and DBU catalysis has been developed. A variety of eight-membered cyclic ethers with two contiguous tertiary stereocenters were obtained in high yields with excellent stereoselectivities. This reaction not only provides a new strategy for constructing enantioenriched eight-membered cyclic ethers but also demonstrates the practicability of ynones as C4-syntons for the synthesis of chiral medium-membered rings.

Non-hydrogen bond catalyst-mediated diastereoselective conjugate additions of 5H-oxazol-4-ones to o-hydroxyphenyl-substituted p-quinone methides

An efficient DBU-catalyzed conjugate addition of 5H-oxazol-4-ones to o-hydroxyphenyl-substituted p-quinone methides has been developed, affording the valuable diarylmethanes in high yields with excellent diastereoselectivity. This strategy demonstrates a robust access to a wide range of diarylmethane derivatives possessing biologically significant o-hydroxyphenol and p-hydroxyphenol moieties under mild reaction conditions.

Enantioselective one-pot synthesis of 4H-chromene derivatives catalyzed by a chiral Ni(ii) complex

A Ni(ii)–bis(oxazoline) complex and p-TSOH are used to form enantioenriched 4H-chromenes from ortho-quinone methides (o-QMs) and dicarbonyls, providing the desired products in up to 95% ee. The method is compatible with various β-ketoester substrates, and the products obtained could be converted into biologically active 4H-chromene derivatives.

A Ni(ii)–bis(oxazoline) complex and p-TSOH are used to form enantioenriched 4H-chromenes from ortho-quinone methides (o-QMs) and dicarbonyls, providing the desired products in up to 95% ee.

Copper-catalyzed double C–S bond formation for the synthesis of 2-acyldihydrobenzo[b]thiophenes and 2-acylbenzo[b]thiophenes

Efficient domino methods are developed for the synthesis of 2,3-dihydrobenzo[b]thiophenes and 2-acylbenzo[b]thiophenes from 2-iodoketones using in situ generated iodine from by-product KI with a Cu-catalyst and xanthate.

Palladium-catalyzed enantioselective [5 + 4] annulation of ortho-quinone methides and vinylethylene carbonates

Highly enantioselective [5 + 4] annulations of ortho-quinone methides with vinylethylene carbonates are enabled by asymmetric palladium catalysis for the synthesis of chiral nine-membered benzo-heterocycles.

Asymmetric synthesis of benzothiazolopyrimidines with high catalytic efficiency and stereoselectivity under bifunctional phosphonium salt systems

Bifunctional phosphonium salt-mediated formal [4+2] annulation towards chiral benzothiazolopyrimidine compounds with excellent yields and stereoselectivities.