Biomimetic Organocatalytic Approach to 4-Arylquinolizidine Alkaloids and Application in the Synthesis of (-)-Lasubine II and (+)-Subcosine II

The biological activities of quinolizidine alkaloids

Quinolizidine alkaloids isolated from various marine and terrestrial animals and plants are primarily composed of lupinine-, matrine-, and sparteine-type alkaloids. Matrine, phenanthroquinolizidines, bis-quinolizidines, and small molecules from amphibian skins are representative compounds of such alkaloids. Quinolizidine alkaloids harbor anticancer, antibacterial, antiinflammatory, antifibrosis, antiviral, and anti-arrhythmia. In this chapter, we comprehensively outline the biological activity and pharmacological action of quinolizidine alkaloids and discuss new avenues toward the discovery of novel and more efficient drugs based on these naturally occurring compounds. It is urgent for basic research and clinical practice to conduct more targeted comprehensive research based on the lead drugs of quinolizidine alkaloids with significant pharmacological activity.

Synthetic drives for useful drug molecules through organocatalytic methods

The treatment of various pathological conditions in human beings involves the use of safe and efficacious drug substances. But there are different complications associated with the treatment of various disease states including drug resistance, adverse drug reactions, toxicity, etc. To minimize these problems, there is an urgent need to develop new therapeutics with suitable pharmacokinetic and pharmacodynamic properties. So, the organocatalytic methods are emerged as a potential synthetic tool to accelerate the design of new drug candidates with improved physicochemical and pharmacological properties, selectivity, and efficiency for the treatment of life-threatening diseases. Organocatalytic reactions refer to the chemical reaction that is accelerated by organic catalysts instead of using organometallic catalysts. Organocatalysts are more advantageous in comparison to metallic catalysts because organocatalysts are cost-effective, stable, efficient, non-toxic, readily available, and easy to handle. In addition to this, the organocatalysis method involves an eco-friendly reaction by minimizing the formation of by-products and reducing the chemical hazards. Organocatalysts are categorized into four classes such as Lewis acids, Lewis bases, Bronsted acids, and Bronsted bases. These catalysts are generally involved in various reactions mechanisms such as Aldol reaction, Diels–Alder reactions, Michael Addition and Knoevenagal reactions, etc. The utility of organocatalyst in synthetic chemistry results in the development of medicinally active compounds with diverse structural features.

Two Decades of Progress in the Asymmetric Intramolecular aza-Michael Reaction

The asymmetric intramolecular aza-Michael reaction (IMAMR) is a very convenient strategy for the generation of heterocycles bearing nitrogen-substituted stereocenters. Due to the ubiquitous presence of these skeletons in natural products, the IMAMR has found widespread applications in the total synthesis of alkaloids and biologically relevant compounds. The development of asymmetric versions of the IMAMR are quite recent, most of them reported in this century. The fundamental advances in this field involve the use of organocatalysts. Chiral imidazolidinones, diaryl prolinol derivatives, Cinchone-derived primary amines and quaternary ammonium salts, and BINOL-derived phosphoric acids account for the success of those methodologies. Moreover, the use of N-sulfinyl imines with a dual role, as nitrogen nucleophiles and as chiral auxiliaries, appeared as a versatile mode of performing the asymmetric IMAMR.

α-C-H Bond Functionalization of Unprotected Alicyclic Amines: Lewis-Acid-Promoted Addition of Enolates to Transient Imines

Enolizable cyclic imines, obtained in situ from their corresponding lithium amides by oxidation with simple ketone oxidants, are readily alkylated with a range of enolates to provide mono- and polycyclic β-aminoketones in a single operation, including the natural product (±)-myrtine. Nitrile anions also serve as competent nucleophiles in these transformations, which are promoted by BF3 etherate. β-Aminoesters derived from ester enolates can be converted to the corresponding β-lactams.

Diversification of Unprotected Alicyclic Amines by C-H Bond Functionalization: Decarboxylative Alkylation of Transient Imines

Despite extensive efforts by many practitioners in the field, methods for the direct α-C-H bond functionalization of unprotected alicyclic amines remain rare. A new advance in this area utilizes N-lithiated alicyclic amines. These readily accessible intermediates are converted to transient imines through the action of a simple ketone oxidant, followed by alkylation with a β-ketoacid under mild conditions to provide valuable β-amino ketones with unprecedented ease. Regioselective α'-alkylation is achieved for substrates with existing α-substituents. The method is further applicable to the convenient one-pot synthesis of polycyclic dihydroquinolones through the incorporation of a SN Ar step.

The endo-aza-Michael addition in the synthesis of piperidines and pyrrolidines

Intramolecular endo-aza-Michael additions are categorised in various ways.