Strategic approaches to the synthesis of pyrrolizidine and indolizidine alkaloids

Advances in the Total Synthesis of Pharmacologically Important Fused Indolizidine Alkaloids: Securinine, Gephyrotoxin and Lepadiformine

Indolizidine alkaloids represent a diverse group of naturally occurring compounds which are derived from various sources and possess a wide range of pharmacological activities. Fused indolizidine alkaloids represent a distinct subset of these compounds, where additional rings are fused with the indolizidine core. When combined with the specific stereochemistry typically required for their biological activity, the fused ring structure complicates the synthesis of this important class of compounds. Among the well-studied fused indolizidine alkaloids are securinine, gephyrotoxin, and lepadiformine, which have all exhibited potential in important therapeutic areas. Due to their complex structures, over the years numerous approaches have been proposed to synthesize these compounds. In this article, we review the progress made in synthetic routes for these key fused indolizidine alkaloids throughout history, providing a broad overview and the distinct advantages of the various strategies that could be employed in their synthesis.

Oxidative [4+2] Annulation of Pyrrole-2-carbaldehyde Derivatives with o-Hydroxyphenyl Propargylamines: Syntheses of 5,6,7-Trisubstituted Indolizines

A base promoted oxidative [4+2] annulation of pyrrole-2-carbaldehyde derivatives with o-hydroxyphenyl propargylamines for the synthesis of highly substituted indolizines has been developed. Using DBN as base, a broad range of 5,6,7-trisubstituted indolizines have been prepared in good to excellent yields under mild conditions, and many useful functional groups can be tolerated.

Synthesis and evaluation of tetrahydropyrrolo[1,2-a]quinolin-1(2H)-ones as new tubulin polymerization inhibitors

Here we explored new 1,5-disubstituted pyrrolidin-2-ones 1, 2 and 5-aryl-3,3a,4,5-tetrahydropyrrolo[1,2-a]quinoline-1(2H)-ones 3 as inhibitors of tubulin polymerization. We evaluated their effects on microtubule dynamics in vitro and on the proliferation of A549 cells, using flow cytometry-based cell cycle analysis. The results were verified with phase-contrast microscopy in three cancer cell lines: A549, HeLa and MCF-7. Guided by molecular modeling of the interactions between tubulin and the most active of the identified compounds, we designed, synthesized, and tested the 3-hydroxyphenyl-substituted compound 3c. This compound was further shown to bind to the colchicine site of tubulin and reduce microtubule growth rates in vitro. Moreover, compound 3c arrested division of the A549 cells in the low micromolar range (IC50 = 5.9 μM) and exhibited cytotoxicity against four different cell lines in the MTT assay for cell proliferation. Our findings demonstrate that 5-aryltetrahydropyrrolo[1,2-a]quinoline-1(2H)-one is a promising scaffold for the development of novel tubulin polymerization inhibitors.

Stereocontrolled Construction of Spirooxindole-Containing 5,6,7,8-Tetrahydropyrrolo[1,2-a]pyridine via Michael/Friedel-Crafts Domino Reaction Promoted by Secondary Amine-Squaramide

An asymmetric Michael/Friedel-Crafts cascade reaction with Morita-Baylis-Hillman (MBH) nitroallylic esters and 3-pyrrolyloxindoles has been developed for the stereoselective construction of spirooxindole-containing tetrahydroindolizines. A range of tetracyclic scaffolds possessing three consecutive chiral centers, including an all-carbon quaternary stereocenter, were generated in 53-85% isolated yields with high diastereoselectivities and enantiopurities (≥3:1 dr, 50-98% ee). A newly synthesized bifunctional secondary amine/squaramide organocatalyst was demonstrated to exhibit better stereochemical control than their tertiary analogues.

Recent advances in the total synthesis of polyhydroxylated alkaloids via chiral oxazines

This review summarizes recently established methodologies developed for the enantioselective and diastereoselective synthesis of chiral 1,3-oxazines. These compounds are of interest as advanced synthetic intermediates in the total synthesis of structurally complex and biologically active polyhydroxylated alkaloids such as (+)-1-deoxynojirimycin, (-)-anisomycin, (+)-castanospermine, (+)-casuarine, (-)-conduramine F-1, (-)-sphingofungin B, Neu5Ac methyl ester, and other natural products. The devised synthetic approach aims to offer a direct, efficient, and adaptable method for obtaining both pure enantiomers and pure diastereomers. It revolves around utilizing chiral building blocks like syn,syn-, syn,syn,anti-, syn,anti-, syn,anti,syn-, anti,syn-, anti,syn,syn-, and anti,syn,anti-oxazines. By integrating oxazine chemistry with established and innovative transformations, this approach enabled the synthesis of 30 polyhydroxylated amines across various studies conducted between 2007 and 2022.

A Simple Entry to the 5,8-Disubstituted Indolizidine Skeleton via Hetero Diels-Alder Reaction

The 5,8-disubstituted indolizidines are the largest family of indolizidines isolated from the skin of amphibians. These compounds exhibit interesting biological activities such as noncompetitive blockers of nicotinic receptors. In this paper, we present a short, simple, and general synthesis of these alkaloids based on the hetero Diels-Alder reaction between suitable monoactivated dienes and Δ1-pyrroline as the dienophile. The selectivity of the process is explained based on computational studies. Concise synthesis of the indolizidine alkaloid 181B from a hetero Diels-Alder reaction was accomplished in four steps.

Assembly of spirocyclic pyrazolone-pyrrolo[4,3,2-de]quinoline skeleton via cascade [1,5] hydride transfer/cyclization by C(sp3)-H functionalization

Herein, a highly efficient, scalable, and cascade [1,5] hydride transfer/cyclization method for constructing unique spirocyclic pyrazolone-pyrrolo[4,3,2-de]quinoline structures via C(sp3)-H functionalization is achieved, using pyrazolones and oxindoles attached to C4 amines. This strategy represents a limited approach utilizing C-H activation to construct spirocyclic pyrazolone scaffolds with moderate to excellent reaction performance.

Enantioselective Synthesis of Fluorinated Indolizidinone Derivatives

The enantioselective synthesis of fluorinated indolizidinone derivatives has been developed. The process involved an enantioselective intramolecular aza-Michael reaction of conjugated amides bearing a pendant α,β-unsaturated ketone moiety, catalyzed by the (S)-TRIP-derived phosphoric acid, followed by dimethyltitanocene methylenation and ring closing metathesis (RCM). Final indolizidine-derived products comprise a fluorine-containing tetrasubstituted double bond generated by the RCM reaction, which is a challenging task. The whole synthetic sequence took place in acceptable overall yields with excellent enantioselectivities.

Enantio- and diastereodivergent synthesis of fused indolizines enabled by synergistic Cu/Ir catalysis

Highly diastereo-/enantioselective assembly of 2,3-fused indolizine derivatives could be easily available through a cascade allylation/Friedel-Crafts type reaction enabled by a synergistic Cu/Ir catalysis. This designed protocol provides an unprecedented and facile route to enantioenriched indolizines bearing three stereogenic centers in moderate to high yields with excellent stereoselective control, which also featured broad substrate generality. Remarkably, four stereoisomers of the 2,3-fused indolizine products could be efficiently constructed in a predictable manner through the pairwise combination of copper and iridium catalysts. The synthetic utility of this method was readily elaborated by a gram-scale reaction, and synthetic transformations to other important chiral indolizine derivatives. Quantum mechanical explorations constructed a plausible synergetic catalytic cycle, revealed the origins of stereodivergence, and rationalized the protonation-stimulated stereoselective Friedel-Crafts type cyclization to form the indolizine products.

An Intramolecular Hydroaminomethylation-Based Approach to Pyrrolizidine Alkaloids under Microwave-Assisted Heating

A general method for the synthesis of pyrrolizidine derivatives using an intramolecular hydroaminomethylation protocol (HAM) under microwave (MW) dielectric heating is reported. Starting from a 3,4-bis(benzyloxy)-2-[(benzyloxy)methyl]-5-vinylpyrrolidine, MW-assisted intramolecular HAM in the presence of gaseous H2 and CO gave the natural alkaloid hyacinthacine A2 protected as benzyl ether. The same approach gave a lentiginosine analogue starting from the corresponding vinyl N-hydroxypyrrolidine. The nature of the reaction products and the yields were strongly influenced by the relative stereochemistry of the starting pyrrolidines, as well as by the catalyst/ligand employed. The use of ethanol as a solvent provides environmentally friendly conditions, while the ligand/catalyst system can be recovered by separating the alkaloid product with an SCX column and recycling the ethanolic solution. HAM worked up to three times with the recycled catalyst solution without any significant impact on yield.

Diastereoselective Synthesis of Tetrahydrofurano[2,3-g]indolizidine and 8-Aminoindolizidines from L-Asparagine

8-Aminoindolizidines were synthesized from L-asparagine as the chiral starting material. The key dibenzylamino succinimide intermediate was synthesized in two steps. Three homologs of chiral hydroxylactams tethered with hydroxyalkene were synthesized from the succinimide via a sequence involving N-alkylation, cross olefin metathesis and hydride reduction. The dibenzylamino group gave the stereocontrol of the key N-acyliminium ion cyclization of these hydroxylactams. 5-Substituted-aminoindolizidines were synthesized with high diastereoselectivity at C6. A tandem cyclization of N-(6-hydroxyhex-3-en-1-yl)-g-hydroxylactam resulted in formation of tetrahydrofurano[2,3-g]indolizidine system.

The Chemistry and Chemical Ecology of Lepidopterans as Investigated in Brazil

The interdisciplinary field of Chemical Ecology in Brazil is currently composed of groups that emerged through the pioneering studies of Keith Spalding Brown Jr. and José Tércio Barbosa Ferreira. Following Keith Brown 's steps, José Roberto Trigo continued investigating the role of plant natural products in mediating the association among insects and their host plants, mainly in the Order Lepidoptera. The role of pyrrolizidine alkaloids in those associations was investigated extensively by Brown and Trigo, and most of what is currently known on this subject is based on their studies. The present work acknowledges their contribution to the Brazilian chemical ecology field and on insect-plant communication studies mediated by different chemical compounds.

Acyclic Twisted Amides

In this contribution, we provide a comprehensive overview of acyclic twisted amides, covering the literature since 1993 (the year of the first recognized report on acyclic twisted amides) through June 2020. The review focuses on classes of acyclic twisted amides and their key structural properties, such as amide bond twist and nitrogen pyramidalization, which are primarily responsible for disrupting nN to π*C═O conjugation. Through discussing acyclic twisted amides in comparison with the classic bridged lactams and conformationally restricted cyclic fused amides, the reader is provided with an overview of amidic distortion that results in novel conformational features of acyclic amides that can be exploited in various fields of chemistry ranging from organic synthesis and polymers to biochemistry and structural chemistry and the current position of acyclic twisted amides in modern chemistry.

Double asymmetric intramolecular aza-Michael reaction: a convenient strategy for the synthesis of quinolizidine alkaloids

A new methodology to access the quinolizidine skeleton in an asymmetric fashion was devised. It involves two consecutive intramolecular aza-Michael reactions of sulfinyl amines bearing a bis-enone moiety, in turn generated by a monodirectional cross metathesis reaction. The sequence, which takes place with excellent yields and diastereocontrol, was applied to the total synthesis of alkaloids lasubine I and myrtine.

Extraction of bioactive compounds from Senecio brasiliensis using emergent technologies

Several plant species synthesize biologically active secondary metabolites. Pyrrolizidine alkaloids are a large group of biotoxins produced by thousands of plant species to protect against the attack of insects and herbivores, but they are highly toxic for humans and animals. In this study, extracts from the aerial part of Senecio brasiliensis were obtained using different technologies: ultrasound-assisted extraction (UAE), pressurized liquid extraction (PLE), and microwave hydrodiffusion and gravity (MHG). The study aimed to evaluate the effectiveness of these technologies for the extraction of chemical compounds found in this plant, focusing on two pyrrolizidine alkaloids: integerrimine and senecionine. Influential parameters on yield and chemical composition were also evaluated: for UAE and MHG, temperature and pressure; for PLE, temperature, and percentage of ethanol. All the extraction techniques were efficient for the extraction of integerrimine and senecionine. The UAE and PLE stood out for the higher yields and number of compounds. The PLE presented a maximum yield of 18.63% for the matrix leaf and the UAE a maximum yield of 11.82% for the same matrix. These two techniques also stood out in terms of the number of compounds, once 36 different compounds were found via PLE and 17 via UAE.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02845-1.

α,γ-Dioxygenated amides via tandem Brook rearrangement/radical oxygenation reactions and their application to syntheses of γ-lactams

Pyrrolidones are common heterocyclic fragments in various biologically active compounds. Here, a two-step radical-based approach to γ-lactams bearing three to four stereocenters starting from epoxides, N-allylic silylacetamides and TEMPO is reported. The sequence starts with a new tandem nucleophilic substitution/Brook rearrangement/single electron transfer-induced radical oxygenation furnishing orthogonally protected α,γ-dioxygenated N-allylamides with wide scope, mostly good yields, and partly good diastereo- and enantioselectivity for defined combinations of chiral epoxides and chiral amides. This represents a very rare example of an oxidative geminal C-C/C-O difunctionalization next to carbonyl groups. The resulting dioxygenated allylic amides are subsequently subjected to persistent radical effect-based 5-exo-trig radical cyclization reactions providing functionalized pyrrolidones in high yields as diastereomeric mixtures. They converge to 3,4-trans-γ-lactams by base-mediated equilibration, which can be easily further diversified. Stereochemical models for both reaction types were developed.

Synthesis of legonmycins A and B, C(7a)-hydroxylated bacterial pyrrolizidines

A one-flask, two-step procedure from 3-amino-2-methyl-5,6,7,7a-tetrahydro-1H-pyrrolizin-1-one affords the Streptomyces secondary metabolites legonmycins A and B - three operations overall from methyl N-Boc-prolinate. The key step proceeds in each case via N,O-diacylation, then selective oxidative hydrolysis of the intermediate bicyclic pyrrole and establishes a precedent for the synthesis of related C(7a)-hydroxylated pyrrolizidines.

Enantioselective Synthesis of Pyrrolizidinone Scaffolds through Multiple-Relay Catalysis

A triple-tandem protocol for the synthesis of the pyrrolizidinone skeleton has been devised. It involves a cross metathesis-intramolecular aza-Michael reaction-intramolecular Michael addition tandem sequence, starting from N-pentenyl-4-oxo-2-alkenamides and conjugated ketones. In the presence of two cooperative catalysts, namely the second-generation Hoveyda-Grubbs catalyst and (R)-TRIP-derived BINOL phosphoric acid, this multiple-relay catalytic process takes place in good yields and outstanding levels of diastero- and enantioselectivity with the simultaneous generation of three contiguous stereocenters.

Enantioselective Synthesis of 3-Allylindolizines via Sequential Rh-Catalyzed Asymmetric Allylation and Tschitschibabin Reaction

The first highly regio- and enantioselective synthesis of 3-allylindolizines has been developed by the sequential Rh-catalyzed asymmetric allylation and Tschitschibabin reaction. Above the 20:1 branch/linear ratio, up to a 96% yield and 99% ee could be obtained with the help of tert-butyl-substituted chiral bisoxazolinephosphine ligand. In situ generated highly nucleophilic 2-alkylpyridinium ylides are utilized to undergo the asymmetric alkylation reaction before cyclization.

Microwave-Assisted Synthesis of Quinazolines and Quinazolinones: An Overview

Microwave irradiation (MWI), as a unique, effective, sustainable, more economic, and greener source of energy compared to conventional heating, is applied in different organic transformations to result in the rapid formation of desired compounds due to thermal/kinetic effects. In this review, we try to underscore the applications of microwave irradiation (MWI) in the synthesis of quinazoline and quinazolinone derivatives that have been achieved and reported on in the last two decades.

Inhibitory activities of indolizine derivatives: a patent review

INTRODUCTION

Indolizines are structural isomers with indoles. Although several indole-based commercial drugs are available in the market, none of the indolizine-based drugs are available up-to-date. Natural and synthetic indolizines have a wide-range of pharmaceutical importance such as antitumor, antimycobacterial, antagonist, and antiproliferative activities. This prompted us to search and collect all possible data about the pharmacological importance of indolizine to open an avenue to the researchers in exploring more medicinal applications of such biologically important compounds.

AREAS COVERED

The current review article covers the advancements in the biological and pharmacological activities of indolizine-based compounds during the last decade. The covered areas of this work involved anticancer, anti-HIV-1, anti-inflammatory, antimicrobial, anti-tubercular, larvicidal, anti-schizophrenia, CRTh2 antagonist's activities in addition to enzymatic inhibitory activity.

EXPERT OPINION

The discovery of indolizine drugs will be a major breakthrough as compared with their widely available drug-containing indole isosteres. Major work collected here was focused on anticancer, anti-tubercular, anti-inflammatory, and enzymatic inhibitory activities. The SAR study of the reported biologically active indolizines is summarized throughout the review whenever highlighted to the rationale the behavior of inhibitory action. Several indolizines with certain functions provided great enhancement in the therapeutic activities comparing with reference drugs.