Endo Selectivity in the (4 + 3) Cycloaddition of Oxidopyridinium Ions

An Enantioselective Aminocatalytic Cascade Reaction Affording Bioactive Hexahydroazulene Scaffolds

A novel cascade reaction initiated by an enantioselective aminocatalysed 1,3-dipolar [6+4] cycloaddition between catalytically generated trienamines and 3-oxidopyridinium betaines is presented. The [6+4] cycloadduct spontaneously undergoes an intramolecular enamine-mediated aldol, hydrolysis, and E1cb sequence, which ultimately affords a chiral hexahydroazulene framework. In this process, three new C-C bonds and three new stereocenters are formed, enabled by a formal unfolding of the pyridine moiety from the dipolar reagent. The hexahydroazulenes are formed with excellent diastereo-, regio- and periselectivity (>20 : 1), up to 96 % ee, and yields up to 52 %. Synthetic elaborations of this scaffold were performed, providing access to a variety of functionalised hydroazulene compounds, of which some were found to display biological activity in U-2OS osteosarcoma cells in cell painting assays.

Recent Strategies in the Nucleophilic Dearomatization of Pyridines, Quinolines, and Isoquinolines

Dearomatization reactions have become fundamental chemical transformations in organic synthesis since they allow for the generation of three-dimensional complexity from two-dimensional precursors, bridging arene feedstocks with alicyclic structures. When those processes are applied to pyridines, quinolines, and isoquinolines, partially or fully saturated nitrogen heterocycles are formed, which are among the most significant structural components of pharmaceuticals and natural products. The inherent challenge of those transformations lies in the low reactivity of heteroaromatic substrates, which makes the dearomatization process thermodynamically unfavorable. Usually, connecting the dearomatization event to the irreversible formation of a strong C-C, C-H, or C-heteroatom bond compensates the energy required to disrupt the aromaticity. This aromaticity breakup normally results in a 1,2- or 1,4-functionalization of the heterocycle. Moreover, the combination of these dearomatization processes with subsequent transformations in tandem or stepwise protocols allows for multiple heterocycle functionalizations, giving access to complex molecular skeletons. The aim of this review, which covers the period from 2016 to 2022, is to update the state of the art of nucleophilic dearomatizations of pyridines, quinolines, and isoquinolines, showing the extraordinary ability of the dearomative methodology in organic synthesis and indicating their limitations and future trends.

Enantioselective Synthesis of Tropane Scaffolds by an Organocatalyzed 1,3-Dipolar Cycloaddition of 3-Oxidopyridinium Betaines and Dienamines

Tropane alkaloids constitute a compound-class which is structurally defined by a central 8-azabicyclo[3.2.1]octane core. A diverse bioactivity profile combined with an unusual aza-bridged bicyclic framework has made tropanes molecules-of-interest within organic chemistry. Enantioselective examples of (5+2) cycloadditions between 3-oxidopyridinium betaines and olefins remain unexplored, despite 3-oxidopyridinium betaines being useful reagents in organic synthesis. The first asymmetric (5+2) cycloaddition of 3-oxidopyridinium betaines is reported, affording tropane derivatives in up to quantitative yield and with excellent control of peri-, regio-, diastereo-, and enantioselectivity. The reactivity is enabled by dienamine-activation of α,β-unsaturated aldehydes combined with in situ formation of the pyridinium reaction-partner. A simple N-deprotection protocol allows for liberation of the tropane alkaloid motif, and synthetic elaborations of the cycloadducts demonstrate their synthetic utility to achieve highly diastereoselective modification around the bicyclic framework. DFT computations suggest a stepwise mechanism where regio- and stereoselectivity are defined during the first bond-forming step in which the pyridinium dipole exerts critical conformational control over its dienamine partner. In the second bond-forming step, a kinetic preference toward an initial (5+4) cycloadduct was identified; however, a lack of catalyst turn-over, reversibility, and thermodynamic bias favoring a (5+2) cycloadduct rendered the reaction fully periselective.

Access to 8-Azabicyclo[3,2,1]octanes via the [3 + 2] Cycloaddition of Oxidopyridinium Ions to Maleimides

Herein, we reported a unique and operationally simple method to assemble 8-azabicyclo[3,2,1]octanes by using oxidopyridinium ions and maleimides as synthons. The features of good to high yields and good functional group tolerance are achieved regularly under mild conditions. Of note, oxidopyridinium ions undergo a [3 + 2] cycloaddition on their C2 and C6 positions.

Oxidative bridgehead functionalization of (4 + 3) cycloadducts obtained from oxidopyridinium ions

Treatment of selected (4 + 3) cycloadducts derived from oxidopyridinium ions with N-iodosuccinimide (NIS) in hexafluoroisopropanol (HFIP) resulted in the formation of bridgehead ethers via a net oxidative C-H activation.

Intramolecular (4+3) Cycloadditions of Oxidopyridinium Ions: Towards Daphnicyclidin A

N-alkylation of 5-hydroxynicotinic acid esters with electrophiles containing diene functionality produces salts that undergo intramolecular (4+3) cycloaddition reactions upon heating in the presence of base. Initial studies used a three-carbon tether to join the pyridinium ion and diene, revealing some aspects of the inherent selectivity of the reaction with such substrates. Much more challenging was the synthesis of related species possessing only a two-carbon tether. Nevertheless, the cycloaddition of such compounds was successful, leading directly to the ABC ring system of the alkaloid daphnicyclidin A.

Tropane Skeleta from the Intramolecular Photocycloaddition of (4+3) Cycloadducts of Oxidopyridinium Ions and Dienes

Easily prepared cycloadducts derived from the (4+3) cycloaddition of oxidopyridinium ions with dienes reacted intramolecularly in a [2+2] cycloaddition process to afford complex polycyclic species in which the tropane skeleton was embedded.

Construction of Oxo-Bridged Diazocines via Rhodium-Catalyzed (4+3) Cycloaddition of Carbonyl Ylides with Azoalkenes

Developing efficient strategies for synthesizing novel diazocine compounds is valuable because their use has been limited by their synthetic accessibility. This work describes the catalytic (4+3) cycloaddition reaction of carbonyl ylides with azoalkenes generated in situ. The rhodium-catalyzed cascade reaction features good atom and step economy, providing the first access to oxo-bridged diazocines. The product could be synthesized on a gram scale and converted into diversely substituted dihydroisobenzofurans.