The Phosphinamide-Based Catalysts: Discovery, Methodology Development, and Applications in Natural Product Synthesis

Catalytic Asymmetric Total Synthesis of (+)-Chamaecydin and (+)-Isochamaecydin and their Stereoisomers

A modular approach was developed for the first catalytic asymmetric total syntheses of naturally occurring C30 terpene quinone methides and their non-natural stereoisomers, which feature the presence of an unprecedented spiro[4.4]nonane-containing 6-6-6-5-5-3 hexacyclic skeleton. Resting on a chiral phosphinamide-catalyzed enantioselective reduction of 2,2-disubstituted cyclohexane-1,3-dione, a concise route for the synthesis of enantioenriched 6-6 bicyclic fragment was developed. The 6-6 ring fragment and the five-membered ring fragment were unified via a metal-halogen exchange/intermolecular addition reaction. Subsequently, the central 6-5 bicyclic ring system was constructed through a Michael/aldol cascade. The successful establishment of these strategic transformations allowed for an efficient and rapid construction of spiroannulated 6-6-6-5-5 pentacarbocyclic core via a convergent manner. Finally, the total syntheses of naturally occurring (+)-chamaecydin and (+)-isochamaecydin and their corresponding 1',5'-stereoisomers have been achieved divergently by appropriately orchestrating the reaction sequence including isopropyl incorporation, oxidation state adjustment, and carbonyl group-directed regio- and stereoselective cyclopropanation at a late stage.

Recent advances in the oxidative activation of the C2-C3 π bond of indoles and its applications

The oxidative C2-C3 π bond activation strategy is the most efficient tool to synthesize oxygen-containing indoline, which frequently appears in natural products with various biological activities as structural units. Recently, the oxidation-induced cascade strategy through oxygenation activation of the indolic C2-C3 π bond of indoles has received much attention for its use in efficiently establishing complex indoline with oxygen-containing molecular architectures, and holds tremendous potential in the total synthesis of indole alkaloids. It can be carried out using potential activated indole radical cations or imine cation intermediates produced via oxidative C2-C3 π bond activation of indole with various nucleophiles or ring-forming reagents by employing simple and non-decorated indoles as starting substrates. Herein, we have reviewed recent advances in the oxidation-induced cascade strategies connecting intra-cyclization or inter-annulation reactions, nucleophilic or radical additions and rearrangement via the oxidative C2-C3 π bond activation of indoles over the past two decades, providing diverse oxygen-containing indolines such as indoxyls, indoline oxygen-heterocycles and indolones. The features and mechanisms of different types of oxidation-induced cascade reactions have been summarized and represented, and examples have been given of their asymmetric reactions and applications in the total synthesis of indole alkaloids.

Organocatalytic Enantioselective Arylation to Access Densely Aryl-Substituted P-Stereogenic Centers

Although methods for synthesizing chiral phosphorus scaffolds are available, the potential of this molecular chirality remains largely unexplored. Herein, we present a remote desymmetrization of prochiral biaryl phosphine oxides through an organocatalytic asymmetric arylation. This metal-free approach enables the efficient synthesis of a wide range of densely functionalized P(V)-stereogenic compounds with good to excellent yields and satisfactory enantioselectivities. Mechanistic studies reveal that hydrogen bonding and ion-pairing interactions are crucial for achieving precise stereocontrol in this transformation.

N-Heterocyclic Carbene Enabled Copper Catalyzed Asymmetric Synthesis of Pyrimidinyl Phosphine with both Axial and P-Stereogenicity

P-stereogenic phosphines, renowned for their utility as ligands and catalysts, have been instrumental in the field of asymmetric catalysis. However, the catalytic asymmetric synthesis of chiral ligands possessing both axial and phosphine chirality remains a significant challenge. Here, we present the successful demonstration of a Cu-catalyzed asymmetric C-P construction using in situ generated secondary phosphine and heteroaryl chloride. By introducing a chiral NHC ligand and an achiral diphosphine auxiliary ligand, we effectively alleviated the poisoning effect caused by phosphine(III) compounds and suppressed the nonenantioselective background reaction. The reaction exhibited excellent enantioselectivity, with up to 96 % ee, and good diastereoselectivity, with up to 14 : 1 dr, when employing less sterically hindered secondary phosphines. This particular substrate poses a significant challenge due to its strong poisoning effect in copper catalysis.

Enantioselective Total Synthesis of (-)-Cyathin B2: A Desymmetric Double-Allylboration Approach

A powerful Pt-catalyzed asymmetric diboration/desymmetric double-allylboration cascade reaction has been developed for the construction of synthetically useful, densely functionalized hydrindanes with five stereocenters, including three quaternary ones, in good yields and excellent enantiomeric excess (ee) values within a single synthetic operation. A unified strategy utilizing this key tandem methodology enabled the concise asymmetric total synthesis of cyathane diterpene (-)-Cyathin B2 in 14 steps from commercially available starting materials, thereby demonstrating its remarkable potential in the synthesis of hydrindane-containing natural products and pharmaceuticals.

Chiral Bisphosphine-Catalyzed Asymmetric Staudinger/aza-Wittig Reaction: An Enantioselective Desymmetrizing Approach to Crinine-Type Amaryllidaceae Alkaloids

An unprecedented chiral bisphosphine-catalyzed asymmetric Staudinger/aza-Wittig reaction of 2,2-disubstituted cyclohexane-1,3-diones is reported, enabling the facile access of a broad range of cis-3a-arylhydroindoles in high yields with excellent enantioselectivities. The key to the success of this work relies on the first application of chiral bisphosphine DuanPhos to the asymmetric Staudinger/aza-Wittig reaction. An effective reductive system has been established to address the challenging PV═O/PIII redox cycle associated with the chiral bisphosphine catalyst. In addition, comprehensive experimental and computational investigations were carried out to elucidate the mechanism of the asymmetric reaction. Leveraging the newly developed chemistry, the enantioselective total syntheses of several crinine-type Amaryllidaceae alkaloids, including (+)-powelline, (+)-buphanamine, (+)-vittatine, and (+)-crinane, have been accomplished with remarkable conciseness and efficiency.

Heteropolyacid-Catalyzed Phosphorylation of Secondary Aromatic Alcohols with H-Phosphine Oxides in DMC: A Simple Protocol for C-P Bond Formation

We successfully achieved the phosphorylation of secondary aromatic alcohols with H-phosphine oxides (less developed system) using phosphotungstic acid as a catalyst in dimethyl carbonate. The system was simple and environmentally friendly and showed better activity than traditional Lewis or Brønsted acids such as FeCl3, p-TsOH·H2O, etc., generating up to a 97% isolated yield. Control experiments indicated that the reaction did not occur through the radical pathway, and ethers and carbocation were the key intermediates in the pathway.

Pd(II)-Catalyzed enantioselective C-H olefination toward the synthesis of P-stereogenic phosphinamides

P-Stereogenic phosphorus compounds are important structural elements in chiral ligands or organocatalysts. Herein, we report a Pd(II)-catalyzed enantioselective C-H olefination toward the synthesis of P-stereogenic phosphinamides using cheap commercially available L-pGlu-OH as a chiral ligand. A broad range of P-stereogenic phosphinamides were gained in good yields with high enantioselectivities (33 examples, up to 77% yield, 99% ee) via desymmetrization and kinetic resolution.

A Powerful P-N Connection: Preparative Approaches, Reactivity, and Applications of P-Stereogenic Aminophosphines

For more than five decades, P-stereogenic aminophosphine chalcogenides and boranes have attracted scientific attention and are still in the focus of ongoing research. In the last years, novel transition metal-based synthesis methods have been discovered, in addition to the long-known use of chiral auxiliaries. Enantiomerically pure compounds with N-P+ -X- (X=O, S, BH3 ) motifs served as valuable reactive building blocks to provide new classes of organophosphorus derivatives, thereby preserving the stereochemical information at the phosphorus atom. Over the years, intriguing applications in organocatalysis and transition metal catalysis have been reported for some representatives. Asymmetric reductions of C=C, C=N, and C=O double bonds were feasible with selected P-stereogenic aminophosphine oxides in the presence of hydrogen transfer reagents. P-stereogenic aminophosphine boranes could be easily deprotected and used as ligands for various transition metals to enable catalytic asymmetric hydrogenations of olefins and imines. This review traces the emergence of a synthetically and catalytically powerful functional compound class with phosphorus-centered chirality in its main lines, starting from classical approaches to modern synthesis methods to current applications.

Synthesis of P-Stereogenic Phosphinamides via Pd(II)-Catalyzed Enantioselective C-H Alkynylation

P-Stereogenic phosphinamides represent important structural elements in chiral organocatalysts and bioactive compounds. Herein, we report Pd(II)-catalyzed enantioselective C-H alkynylation using cheap commercially available l-pyroglutamic acid as a chiral ligand. A range of structurally diverse P-stereogenic phosphinamides was prepared in good yields with high enantioselectivities via desymmetrization and kinetic resolution. A tailor-made congested directing group, N-ethyl-N-(3-methylpyridin-2-yl)amino, was crucial for the reactivity.

Synthesis of Phosphinic Amides from Chlorophosphines and Hydroxyl Amines via P(III) to P(V) Rearrangement

Phosphoranyl radicals are essential mediators to bring about new radicals but often produce a stoichiometric amount of phosphine oxide/sulfide waste. Herein, we devised a phosphorus-containing species as a radical precursor, but without the generation of phosphorus waste. Accordingly, a catalyst-free synthesis of phosphinic amides from hydroxyl amines and chlorophosphines via P(III) to P(V) rearrangement is described. Mechanistically, it may involve the initial formation of a R₂N-O-PR₂ species that undergoes homolysis of N-O bonds and subsequent radical recombination.

Copper-Catalyzed Asymmetric Boroprotonation of Phosphinylallenes

Synthesis of chiral phosphorus compounds from readily available substrates by a facile method is an attractive strategy. In this study, an efficient route for copper-catalyzed asymmetric boroprotonation of phosphinylallenes with bis(pinacolato)diboron with high regioselectivity was developed, affording chiral allylphosphine oxides in high yields with high enantioselectivities of up to 98% ee. The synthetic utility was further demonstrated by the facile transformation of the chiral allylphosphine oxides to several stereospecific products.

Synthetic Studies on the Synthesis of Toxicodenane A and 8,11-epi-Toxicodenane A

The diverse synthesis of oxatricyclotridecanes and oxatricyclododecanes, which are the core structures of toxicodenane A and its skeletal analogues, via a unified manner is presented. The stereochemistry at the bridgehead position of the oxa-bridged bicycle could be efficiently controlled through a diastereoselective anti- and syn-Grignard allylation reaction by appropriately tuning the reaction conditions such as the solvent, the counterion of the Grignard reagent, the substrate, or a combination of these. The ring size could be precisely elaborated via a Lewis acid-mediated intramolecular transacetalation and Prins cyclization cascade reaction by varying the steric hindrance of olefin moiety. Namely, substrates bearing a terminally unsubstituted olefinic functionality afforded oxatricyclotridecanes with an overwhelming preference, while those bearing a dimethyl-substituted olefinic group produced exclusively oxatricyclododecanes. The wide utility and generality of the above key transformations are highlighted by the applications in the unified synthesis of (±)-toxicodenance A, (+)-toxicodenane A, (+)-8,11-epi-toxicodenane A, and other oxatricyclic cores with different stereochemistries and ring sizes.