Mild and expedient asymmetric reductions of α,β-unsaturated alkenyl and alkynyl ketones by TarB-NO2 and mechanistic investigations of ketone reduction

Recent advances in the total synthesis of galantamine, a natural medicine for Alzheimer's disease

Covering: 2006 to 2023(-)-Galantamine is a natural product with distinctive structural features and potent inhibitory activity against acetylcholine esterase (AChE). It is clinically approved for the treatment of Alzheimer's disease. The clinical significance and scarcity of this natural product have prompted extensive and ongoing efforts towards the chemical synthesis of this challenging tetracyclic structure. The objective of this review is to summarize and discuss recent progress in the total synthesis of galantamine from 2006 to 2023. The contents are organized according to the synthetic strategies for the construction of the quaternary center. Key features of each synthesis have been highlighted, followed by a summary and outlook at the end.

Theoretical investigation of nucleophilic substitution reaction of phenyl carbonyl isothiocyanates with pyridines in gas and polar aprotic solvent

This study focuses on the mutual interaction of substituents in the nucleophile and substrate - cross interaction constant, ρXY, in the uncatalyzed aminolysis by substituting pyridine with phenyl carbonyl isothiocyanate. The mechanism was found to be a stepwise process with a rate-limiting breakdown of the -NCS leaving group. This stepwise reaction mechanism considers the cross-interaction constant (CIC) with rate-limiting breakdown of tetrahedral intermediate in gas and solvent phases. The corresponding Hammett coefficients are related to the substituents associated with (1) the nucleophiles (X), ρX (-1.93 to -6.54 for the gas phase and 10.5 to 18.9 in the solvent model), and with (2) the substituents associated with the phenyl ring of the substrate (Y), ρY (0.41-3.48 for the gas phase and 1.83 to -10.70 for the solvent model). It also includes the Brønsted coefficient with X, βX (0.11-1.52 for the gas phase and -2.57 to 3.96 for the solvent model), and CIC values, ρXY (0.69 for the gas phase and 0.87 for the solvent model). In this work, the NBO analysis, reaction potential, reaction electronic flux (REF), dual descriptor, and the structure-energy relationships were considered in interpreting the mechanistic criteria.

Six-step total syntheses of (−)-galanthamine and (−)-

The Amaryllidaceae alkaloid (−)-galanthamine (1) is a reversible, competitive acetylcholinesterase inhibitor deployed clinically to treat the dementia associated with Alzheimer’s disease. Here, we describe a six-step synthesis of this natural product from simple, readily accessible starting materials. Enantioselective 1,2-reduction, Mitsunobu coupling, Heck cyclization and diastereoselective allylic oxidation reactions are used in our approach, which provides the shortest synthetic route to compound 1 reported to date. A simple modification to the closing stages of the sequence allows equally facile access to (−)-N-norgalanthamine (2), a compound with a range of distinctive biological properties. The concise and operationally simple synthetic protocols reported here could obviate the need to manipulate naturally sourced galanthamine in the pursuit of analogues required for pharmacological studies.

Stitching Ynones with Nitromethanes: Domino Synthesis of Functionally Enriched Benzofurans and Benzothiophenes

A convenient one-pot benzannulation of regioisomeric 2- or 3-substituted furan and thiophene ynones with a range of nitromethanes has been discovered to directly access densely and diversely functionalized benzofurans and benzothiophenes. In this protocol, the nitro group in nitromethanes functions as recursive carbanion activator to setup tandem Michael addition-6π-electrocyclization, and its eventual sacrificial elimination facilitates aromatization and overall benzannulation. This benzannulation was also explored with furan/thiophene based o-halo ynones wherein a Michael addition-S_NAr process operates and nitromethanes leave their imprint to deliver nitro substituted benzo-furans and -thiophenes.

Catalytic Enantioselective Addition of Alkylzirconium Reagents to Aliphatic Aldehydes

A catalytic methodology for the enantioselective addition of alkylzirconium reagents to aliphatic aldehydes is reported here. The versatile and readily accessible chiral Ph-BINMOL ligand, in the presence of Ti(OⁱPr)₄ and a zinc salt, facilitates the reaction, which proceeds under mild conditions and is compatible with functionalized nucleophiles. The alkylzirconium reagents are conveniently generated in situ by hydrozirconation of alkenes with the Schwartz reagent. This work is a continuation of our previous work on aromatic aldehydes.

Steric/π-Electronic Insulation of the carbo-Benzene Ring: Dramatic Effects of tert-Butyl versus Phenyl Crowns on Geometric, Chromophoric, Redox, and Magnetic Properties

Hexa-tert-butyl-carbo-benzene (C₁₈ tBu₆ ) and three phenylated counterparts (C₁₈ tBu_m Ph_6-m ; m=4, 2) have been synthesized. The peralkylated version (m=6) provides experimental access to intrinsic features of the insulated C₁₈ core independently from the influence of π-conjugated substituent. Over the series, structural, spectroscopic, and electrochemical properties are compared with those of the hexaphenylated reference (m=0). Anchoring tBu substituents at the C₁₈ macrocycle is shown to enhance stability and solubility, and to dramatically modify UV/Vis absorption and redox properties. Whereas all carbo-benzenes reported previously were obtained as dark-reddish/greenish solids, crystals and solutions of C₁₈ tBu₆ happen to be yellow (λ_max =379 vs. 472 nm for C₁₈ Ph₆ ). In comparison to C₁₈ Ph₆ , the reduction of C₁₈ tBu₆ remains reversible, but occurs at twice as high an absolute potential (E_1/2 =-1.36 vs. -0.72 V). Systematic XRD analyses and DFT calculations show that the C₁₈ ring symmetry is the nearest to D_6h for m=6, which indicates a maximum geometric aromaticity. According to calculated nucleus-independent chemical shifts (NICS), the macrocyclic magnetic aromaticity is also maximum for C₁₈ tBu₆ : NICS(0)=-17.2 ppm versus (-18.0±0.1) ppm for the theoretical references C₁₈ H₆ and C₁₈ F₆ , and -13.5 ppm for C₁₈ Ph₆ . Accurate correlations of NICS(0) with experimentally recorded or calculated maximum UV/Vis absorption wavelengths, λ_max , and chemical hardness, η=E_LUMO -E_HOMO , are evidenced.

Lewis-base-catalysed selective reductions of ynones with a mild hydride donor

Nucleophilic phosphines catalyze efficient 1,2-reductions of ynones employing pinacolborane as a mild hydride donor in the presence of alcohol additives.

Asymmetric transfer hydrogenation–Sonogashira coupling one-pot enantioselective tandem reaction catalysed by Pd(0)–Ru(iii)/diamine-bifunctionalized periodic mesoporous organosilica

Pd(0)–Ru(iii)/diamine-functionalized periodic mesoporous organosilica for asymmetric transfer hydrogenation–Sonogashira coupling of iodoacetophenone and arynes is investigated.

Enantioselective tandem reaction over a site-isolated bifunctional catalyst

An active site-isolated organoruthenium-/organopalladium-functionalized yolk–shell-structured mesoporous silica is developed and its application in the one-pot enantioselective tandem Sonogashira coupling–asymmetric transfer hydrogenation of haloacetophenones and arylacetylenes to various chiral conjugated alkynols is investigated.