The First Modular Route to Core-Chiral Bispidine Ligands and Their Application in Enantioselective Copper(II)-Catalyzed Henry Reactions

Bispidine coordination chemistry

Bispidines are extremely rigid ligands, easy to prepare in a large variety, with denticities of four to ten, various donor sets and charges, for mono- and oligonuclear transition metal, main group and rare earth complexes. In the last approx. 20 years significantly more than 50 new bispidine based ligands were prepared and their coordination chemistry studied. Biological probes and medicinal applications is one main area in bispidine coordination chemistry, where fast complex formation, high stability, metal ion selectivity and inertness are of utmost importance. Oxygen activation and oxidation catalysis is another main focus in bispidine coordination chemistry, with catalyst efficiency and stability as well as product selectivity as important requirements. Particularly successful applications in these areas are presented and discussed in detail, in addition to fundamental principles that show the importance of ligand rigidity, cavity size and shape as overarching fundamental properties.

Stereoselective Intramolecular [2+2] Trapping of 1,2-Cyclohexadienes: a Route to Rigid, Angularly Fused Tricyclic Scaffolds

1,2-Cyclohexadienes generated under mild fluoride-mediated desilylative conditions undergo efficient intramolecular [2+2] trapping, providing tricyclic alkylidene cyclobutanes with complete diastereoselectivity for the cis-fused products. Pendent styrenes or electron-deficient olefins can trap simple 1,2-cyclohexadienes or their oxygenated counterparts, with 14 substrates being disclosed. Reactions proceed at ambient temperature using just cesium fluoride in up to 91 % yield, and the necessary precursors are easily accessed from substituted 2-bromocyclohexenones. Multiple synthetic routes have been developed to install the appropriate functional groups required for [2+2] trapping.

Combined structural and theoretical investigation on differently substituted bispidine ligands: predicting the properties of their corresponding coordination polymers

Pyridine-based bispidine ligands L1–L7 have been studied by single crystal X-ray diffraction and solid-state DFT calculations in an attempt to predict the dynamic properties of their corresponding Mn(ii) coordination polymers.

Asymmetric catalysis in direct nitromethane-free Henry reactions

This review summarizes the current state and applications of catalytic Henry reactions involving complex nitroalkanes coupling with various carbonyl compounds to generate chiral β-nitro alcohol scaffolds with four adjacent stereogenic centers.

Functionalized Magnetic Nanoparticles as Catalysts for Enantioselective Henry Reaction

With the aim to easily recover and reuse the catalyst, an efficient amino alcohol catalyst previously tested in the asymmetric addition of diethylzinc to several aromatic aldehydes has been immobilized on proper functionalized superparamagnetic core-shell magnetite-silica nanoparticles and employed in the Henry reaction in the semi-homogeneous phase. The nanocatalyst exhibits a promising catalytic activity that remains unchanged in the three catalytic cycles performed. The results prove that highly efficient catalysts, by being immobilized on suitable magnetic nanosupports, can be easily recovered and reused, maintaining their catalytic behavior.

Photothermal conversion triggered thermal asymmetric catalysis within metal nanoparticles loaded homochiral covalent organic framework

For seeking high enantiopurity, the previously reported thermal asymmetric catalysis is usually carried out at low temperature sometimes with limited yield, that is, the high enantiomeric excess (ee) usually at the cost of high yield. Thus, the achieving both high stereoselectivity and yield is an enormous challenge. We report herein two metal nanoparticle (M NP)-loaded and porphyrin-containing homochiral covalent organic framework (CCOF)-based composite catalysts, and their application in the thermally-driven asymmetric one-pot Henry and A³-coupling reactions. All the reactions are conducted at elevated temperatures with both excellent stereoselectivity and yield which resulted from the synergy of CCOF confinement effect and M NP catalytic activation. Notably, the needed thermal energy for the asymmetric reactions herein is derived from the photothermal conversion via porphyrin-based CCOF upon irradiation with visible light. Remarkably, the CCOF confinement effect can be effectively maintained up to 100 °C for the asymmetric one-pot Henry and A³-coupling reactions herein.

Achieving both high stereoselectivity and yield is a challenge for conventional asymmetric catalysis. Here, the authors report two metal nanoparticle-loaded and porphyrin-containing homochiral covalent organic framework-based composite catalysts that exhibit high stereoselectivity and yield in the thermally-driven asymmetric reactions.

The Enantioselective Total Synthesis of Bisquinolizidine Alkaloids: A Modular "Inside-Out" Approach

Bisquinolizidine alkaloids are characterized by a chiral bispidine core (3,7-diazabicyclo[3.3.1]nonane) to which combinations of an α,N-fused 2-pyridone, an endo- or exo-α,N-annulated piperidin(on)e, and an exo-allyl substituent are attached. We developed a modular "inside-out" approach that permits access to most members of this class. Its applicability was proven in the asymmetric synthesis of 21 natural bisquinolizidine alkaloids, among them more than ten first enantioselective total syntheses. Key steps are the first successful preparation of both enantiomers of C₂ -symmetric 2,6-dioxobispidine by desymmetrization of a 2,4,6,8-tetraoxo precursor, the construction of the α,N-fused 2-pyridone by using an enamine-bromoacrylic acid strategy, and the installation of endo- or, optionally, exo-annulated piperidin(on)es.

Gram-Scale Synthesis of the (-)-Sparteine Surrogate and (-)-Sparteine

An 8-step, gram-scale synthesis of the (-)-sparteine surrogate (22 % yield, with just 3 chromatographic purifications) and a 10-step, gram-scale synthesis of (-)-sparteine (31 % yield) are reported. Both syntheses proceed with complete diastereocontrol and allow access to either antipode. Since the syntheses do not rely on natural product extraction, our work addresses long-term supply issues relating to these widely used chiral ligands.

Conjugate substitution and addition of α-substituted acrylate: a highly efficient, facile, convenient, and versatile approach to fabricate degradable polymers by dynamic covalent chemistry

Degradable poly(conjugated ester)s with various backbones were synthesized via conjugate substitution (S_N2′) polymerization of bis[α-(chloromethyl)acrylate] and nucleophilic monomers under ambient conditions.

Application of chiral bi- and tetra-dentate bispidine-derived ligands in the copper(ii)-catalyzed asymmetric Henry reaction

Rationalized bispidinone-based ligands were reacted with copper(ii) salts, which afforded effective catalysts for asymmetric Henry reactions.

Bispidines for radiopharmaceuticals

Radiometal based radiopharmaceuticals for imaging and therapy require selective ligands (bifunctional chelators, BFCs) that form metal complexes, which are inert against trans-chelation under physiological conditions, linked to a biological vector, directing them to the targeted tissue. Bispidine ligands with a very rigid backbone and widely variable donor sets are reviewed as an ideal class of BFCs, and recent applications are discussed.

Chiral copper-salen complex grafted over functionalized mesoporous silica as an efficient catalyst for asymmetric Henry reactions and synthesis of the potent drug (R)-isoproterenol

A new chiral Cu(ii)@AFS-1 catalyzed asymmetric Henry reaction (ee = 94%, yield 96%) and diastereoselective Henry reaction have been reported. Additionally, drug (R)-(−)-isoproterenol was synthesized using this protocol.

Efficient synthesis of chiral benzofuryl β-amino alcohols via a catalytic asymmetric Henry reaction

A reliable and generally applicable method to access bioactive chiral benzofuryl β-amino alcohols via a catalytic asymmetric Henry reaction.

Silver comes into play: Henry reaction and domino cycloisomerisation sequence catalysed by [Ag(i)(Pc-L)] complexes

The Ag(i) complexes of pyridine-containing ligands with an active pendant arm are new catalysts for an old reaction: the nitroaldol condensation. When the substrates are 2-alkynylarylaldehydes, a smart cascade cycloisomerisation process can occur.