Part II: Diastereoselective Reactions Involving β-Mono- and β,β′-Disubstituted Alkylidene Oxindoles: Pondering Alkene Geometry

Expeditious entry into carbocyclic and heterocyclic spirooxindoles

Spirocyclic frameworks have attracted synthetic practitioners due to their unique three-dimensional assembly, improved metabolic stability, solubility, and increased molecular complexity with regard to planar architectures. A recent surge in the number of spirocyclic oxindoles inhibiting enzymes, moderating unique protein-protein interactions, modulating receptors and transporters is testament to their prevalence. Against this background, the construction of spirocyclic frameworks containing an oxindole moiety as a torsional switch via stereoselective methods is in great demand. Herein we present a summary of the past three years in the progress of metal, organic molecule, nanostructured particle mediated, and even uncatalyzed versions of the highly diastereo- and enantioselective pathways leading to oxindole spirocycles.

Tosylhydrazine-promoted self-conjugate reduction–Michael/aldol reaction of 3-phenacylideneoxindoles towards dispirocyclopentanebisoxindole derivatives

An efficient tosylhydrazine-mediated conjugate reduction of 3-phenacylideneoxindole and sequential Michael/intramolecular aldol reaction is reported under base-catalyzed conditions towards the formation of densely substituted dispirocyclopentanebisoxindole derivatives. The reaction proceeded in a diastereoselective manner to afford four chiral stereocenters. The method also has advantages of wide substrate scope, readily available starting materials and operational simplicity through one pot reaction.

Ultrasound-Engineered fabrication of immobilized molybdenum complex on Cross-Linked poly (Ionic Liquid) as a new acidic catalyst for the regioselective synthesis of pharmaceutical polysubstituted spiro compounds

A novel supported molybdenum complex on cross-linked poly (1-Aminopropyl-3-vinylimidazolium bromide) entrapped cobalt oxide nanoparticles has been successfully fabricated through two different procedures, i.e. ultrasound (US) irradiations (100 W, 40 kHz) and reflux. The efficiency of the two different methods was comparatively investigated on the fundamental properties of proposed catalyst using diverse characterization techniques. Based on the obtained results, the ultrasonication method provides controlled polymerization process; as a result, well connected polymeric network is formed. In addition, the use of ultrasound waves turned out to be able to increase the particles uniformity, specific surface area (from 79.19 to 223.83 m2/g), and the onset thermal degradation temperature (Td) value (from 248 to 400 °C) of the prepared catalyst which intensifies the catalytic efficiency. Besides, US-treated catalyst demonstrated high chemical stability and maintained its cross-linked network after eight cycles recovery, while the cross-linked network of catalyst obtained under silent condition was completely disrupted. Furthermore, the ultrafast multi-step fabrication procedure was performed in less than 6 h under ultrasonic condition while a similar process promoted by a mechanical stirring method came to a conclusion after 5-6 days. Accordingly, the utility of the ultrasound irradiation was proved, and US-treated catalyst was applied for improved synthetic methodology of spiro 1,4-dihydropyridines and spiro pyranopyrazoles through different acidic active sites. Due to the significant synergistic influence between the proposed catalyst and US irradiation, a variety of novel and recognized mono-spiro compounds were fabricated at room temperature in high regioselectivity.

Highly regio- and diastereoselective [3 + 2]-cycloadditions involving indolediones and α,β-disubstituted nitroethylenes

A highly diastereoselective [3 + 2]-cycloaddition strategy involving multiple oxindoles and several α,β-disubstituted nitroethylenes is developed to access tetra-substituted α-spiropyrrolidine frameworks. A variety of α-amino acids were employed for the first time in order to generate azomethine ylides under thermal conditions, affording regioisomers 13 and 14 merely by changing the α-substituents (R = H and substituted carbons) of the α-amino acids. The reaction tolerates various sterically demanding, electron-rich and electron-deficient aryl and nitrogen substituents on glycines, oxindoles and nitroethylenes. The operational simplicity, such as the use of a metal-free and non-inert environment, the utilization of non-halogenated solvents and the ease of isolation, adhering to the principles of green chemistry, makes this process attractive for scale-up opportunities. The reaction delivers good yields (80-94%) and diastereoselectivities (up to 98 : 2) in favor of (cis,cis)-spirooxindoles, with opposite regioselectivity compared to β-nitrostyrenes under identical conditions. Two spiropyrrolidine cycloadducts with unprotected amides exhibited significant activity against Gram-positive MRSA.