Short, Enantiospecific Syntheses of Indolizidines 209B and 209D, and Piclavine A from Diethyl-L-Glutamate

Synthesis of Pyrrolidine- and γ-Lactam-Containing Natural Products and Related Compounds from Pyrrole Scaffolds

Polycyclic alkaloid natural products featuring pyrrolidine and pyrrolidinone motifs remain enduring targets of total synthesis endeavors. Pyrrole and its derivatives have been exploited to access many such frameworks, including alkaloids belonging to the Aspidosperma, Stemona, and batzelladine families. In this article, a selection of exemplars that highlight the utility of pyrrole-based approaches to facilitate total syntheses of pyrrolidine- and pyrrolidinone-containing alkaloids and related molecules are showcased.

Aminocatalytic [8+2] Cycloaddition Reactions toward Chiral Cyclazines

An efficient and exceptionally stereoselective synthesis of chiral cycl[3.2.2]azines has been realized by means of the rational design and utilization of novel (E)-3-benzylidene-3H-pyrrolizines in iminium-ion-catalyzed [8+2] cycloaddition reactions. The presented protocol allows for the incorporation of diverse enals, including cinnamaldehydes, enolizable aldehydes, and substrates of extended conjugation. The obtained products contain both an electron-rich alkenyl pyrrole moiety and an electron-deficient carbaldehyde substituent, and both moieties can undergo selective transformations with retention of the stereochemical information established in the [8+2] cycloaddition.

Origin of the Diastereoselectivity of the Heterogeneous Hydrogenation of a Substituted Indolizine

In this work, the stereoselective heterogeneous hydrogenation of a tetrasubstituted indolizine was studied. Partial hydrogenation products were obtained in three steps from a substituted pyridine-2-carboxaldehyde prepared from commercial pyridoxine hydrochloride. The hydrogenation of the indolizine ring was shown to be diastereoselective, forming trans-6b and cis-9. Theoretical calculations (ab initio and DFT) were used to rationalize the unusual trans stereoselectivity for 6b, and a keto–enol tautomerism under kinetic control has been proposed as the source of diastereoselectivity.

Recent advances in the synthesis of azaphenalene alkaloids: first enantioselective approaches

Azaphenalene alkaloids are biosynthesised and segregated by diverse insects of the Coccinellidae family (ladybirds) and are believed to play an important role in the defensive mechanism against their natural predators. The particular unique framework of these alkaloids, along with their potential in the field of biological pest control, has led to several research groups developing synthetic sequences to prepare these compounds. The main purpose of the present review is to provide an update of the more recent synthetic progress towards these alkaloids, including the pioneering enantioselective approaches to chiral congeners.

Methods for the synthesis of annulated pyrroles via cyclisation strategies

In this report, we review the methods that have been employed to synthesise annulated pyrroles.

Synthetic versatility of 2-substituted-6-methyl 2,3-dihydropyridinones in the synthesis of polyfunctional piperidine-based compounds and related β amino acid derivatives

Chiral 2-substituted-6-methyl 2,3-dihydropyidinones 9, which can be facilely obtained from an asymmetric vinylogous Mannich reaction (VMR) with 1,3-bis-trimethysily enol ether, were used as versatile intermediates in constructing chiral polyfunctional piperidine-based compounds. The 6-methyl group of such compounds can be conveniently functionalized via alkylation and acylation reactions to provide efficient entries to the synthesis of a variety of chiral multi-substituted piperidine-based compounds. Further elaboration of the corresponding intermediates also provided access to polyfunctional indolizidine-based compounds. These methods were showcased in an asymmetric synthesis of 2,6-di-substituted piperidine compound 13, reported as the key intermediate in the synthesis of (+)-calvine and a natural alkaloid (-)-indolizidine 209D. Furthermore, selective C5 iodination of compound 9 enabled the installation of additional functional groups at this position. Finally, we demonstrated that the oxidative cleavage of 2-substituted-6-methyl-2,3-dihydropyidinones is a practical and efficient method for the enantioselective synthesis of β-amino acids, which can undergo further intra-molecular cyclization to give the corresponding chiral four-membered β-lactam derivatives.

Simple Indolizidine and Quinolizidine Alkaloids

This review of simple indolizidine and quinolizidine alkaloids (i.e., those in which the parent bicyclic systems are in general not embedded in polycyclic arrays) is an update of the previous coverage in Volume 55 of this series (2001). The present survey covers the literature from mid-1999 to the end of 2013; and in addition to aspects of the isolation, characterization, and biological activity of the alkaloids, much emphasis is placed on their total synthesis. A brief introduction to the topic is followed by an overview of relevant alkaloids from fungal and microbial sources, among them slaframine, cyclizidine, Steptomyces metabolites, and the pantocins. The important iminosugar alkaloids lentiginosine, steviamine, swainsonine, castanospermine, and related hydroxyindolizidines are dealt with in the subsequent section. The fourth and fifth sections cover metabolites from terrestrial plants. Pertinent plant alkaloids bearing alkyl, functionalized alkyl or alkenyl substituents include dendroprimine, anibamine, simple alkaloids belonging to the genera Prosopis, Elaeocarpus, Lycopodium, and Poranthera, and bicyclic alkaloids of the lupin family. Plant alkaloids bearing aryl or heteroaryl substituents include ipalbidine and analogs, secophenanthroindolizidine and secophenanthroquinolizidine alkaloids (among them septicine, julandine, and analogs), ficuseptine, lasubines, and other simple quinolizidines of the Lythraceae, the simple furyl-substituted Nuphar alkaloids, and a mixed quinolizidine-quinazoline alkaloid. The penultimate section of the review deals with the sizable group of simple indolizidine and quinolizidine alkaloids isolated from, or detected in, ants, mites, and terrestrial amphibians, and includes an overview of the "dietary hypothesis" for the origin of the amphibian metabolites. The final section surveys relevant alkaloids from marine sources, and includes clathryimines and analogs, stellettamides, the clavepictines and pictamine, and bis(quinolizidine) alkaloids.

Clauson Kaas Pyrrole Synthesis Catalyzed by Acidic Ionic Liquid under Microwave Irradiation

A new procedure to synthesize the N-substituted pyrrole derivatives by Clauson Kaas reaction catalyzed by acidic ionic liquid under microwave irradiation was developed. This procedure provides several advantages such as high yield, clean product formation, and short reaction time.

Ultrasound-assisted synthesis of 2,5-dimethyl-N-substituted pyrroles catalyzed by uranyl nitrate hexahydrate

An efficient synthesis of different novel 2,5-dimethyl-N-substituted pyrrole derivatives by the Paal-Knorr condensation has been accomplished using uranyl nitrate hexahydrate as catalyst under soft conditions and ultrasonic irradiation. The synthesized compounds were confirmed through spectral characterization using IR, (1)H NMR, (13)C NMR and mass spectra.

Indolizidine (-)-235B' and related structural analogs: discovery of nicotinic receptor antagonists that inhibit nicotine-evoked [3H]dopamine release

Although several therapeutic agents are available to aid in tobacco smoking cessation, relapse rates continue to be high, warranting the development of alternative pharmacotherapies. Nicotine-evoked dopamine release from its presynaptic terminals in the central nervous system leads to reward which maintains continued tobacco use. The ability of indolizidine (-)-235B' and a sub-library of structurally related analogs to inhibit nicotine-evoked [(3)H]dopamine release from rat striatal slices was determined in the current study. Indolizidine (-)-235B' inhibited nicotine-evoked [(3)H]dopamine release in a concentration-dependent manner (IC(50)=42 nM, I(max)=55%). Compound (-)-237D, the double bond-reduced analog, afforded the greatest inhibitory potency (IC(50)=0.18 nM, I(max)=76%), and was 233-fold more potent than indolizidine (-)-235B'. The des-8-methyl aza-analog of indolizidine (-)-235B', ZZ-272, also inhibited nicotine-evoked [(3)H]dopamine release (IC(50)=413 nM, I(max)=59%). Concomitant exposure to maximally effective concentrations of indolizidine (-)-235B', ZZ-272 or (-)-237D with a maximally effective concentration of α-conotoxin MII, a selective antagonist for α6β2-containing nicotinic receptors, resulted in inhibition of nicotine-evoked [(3)H]dopamine release no greater than that produced by each compound alone. The latter results suggest that indolizidine (-)-235B', (-)-237D, ZZ-272 and α-conotoxin MII inhibit the same α-conotoxin MII-sensitive nicotinic receptor subtypes. Thus, indolizidine (-)-235B' and its analogs act as antagonists of α6β2-nicotinic receptors and constitute a novel structural scaffold for the discovery of pharmacotherapies for smoking cessation.

Total synthesis and study of myrmicarin alkaloids

The myrmicarins are a family of air- and temperature-sensitive alkaloids that possess unique structural features. Our concise enantioselective synthesis of the tricyclic myrmicarins enabled evaluation of a potentially biomimetic assembly of the complex members via direct dimerization of simpler structures. These studies revealed that myrmicarin 215B undergoes efficient and highly diastereoselective Brønsted acid-induced dimerization to generate a new heptacyclic structure, isomyrmicarin 430A. Mechanistic analysis demonstrated that heterodimerization between myrmicarin 215B and a conformationally restricted azafulvenium ion precursor afforded a functionalized isomyrmicarin 430A structure in a manner that was consistent with a highly efficient, non-concerted ionic process. Recent advancement in heterodimerization between tricyclic derivatives has enabled the preparation of strategically functionalized hexacyclic structures. The design and synthesis of structurally versatile dimeric compounds has greatly facilitated manipulation of these structures en route to more complex myrmicarin derivatives.

Total synthesis of indolizidine alkaloid (-)-209D: overriding substrate bias in the asymmetric rhodium-catalyzed [2+2+2] cycloaddition

CO! You had me at hello: The use of chiral biphenyl-based phosphoramidite ligands on rhodium provides an efficient [2+2+2] cycloaddition between terminal alkyl alkynes and alkenyl isocyanates (see scheme). The cycloaddition proceeds through a CO migration pathway, and facilitates a rapid four-step asymmetric synthesis of indolizidine (-)-209D.

Facile synthesis of two diastereomeric indolizidines corresponding to the postulated structure of alkaloid 5,9E-259B from a Bufonid toad (Melanophryniscus)

A short synthesis of the postulated structure for indolizidine alkaloid 259B with the hydrogens at C5 and C9 entgegen has been achieved with complete control of stereochemistry at C5. Both diastereoisomers at C8 were obtained, but neither proved to be the natural product. The comparison of the mass and FTIR spectral properties of the synthetic compounds to those of the natural material strongly suggest that the gross structure is correct and that the difference may be a branch in the C5 alkyl side-chain. The GC-retention times of the two synthetic compounds were markedly longer than that of the natural 5,9E-259B.

Knorr-Rabe partial reduction of pyrroles: application to the synthesis of indolizidine alkaloids

Background

The Birch reduction of electron rich pyrroles does not occur readily. However, dissolving metal reduction with zinc under acidic conditions gives 3-pyrrolines (2,5-dihydropyrroles) in reasonable yield. This dissolving metal reduction was first reported by Knorr and Rabe in 1901 but since then has only been reported for the reduction of electron rich pyrroles.

Results

The partial reduction of bicyclic α-ketopyrrole derivatives has been performed under dissolving metal conditions with zinc and hydrochloric acid to give excellent yields of hexahydroindolizidines. This reduction method has been utilised for the diastereoselective synthesis of 5-alkylindolizidines and the stereoselectivity obtained is opposite to that of catalytic hydrogenation.

Conclusion

An efficient stereoselective synthesis of indolizidine alkaloids has been developed from α-ketopyrrole intermediates using a modified version of Knorr and Rabe's pyrrole reduction.

An asymmetric synthesis of all stereoisomers of piclavines A1-4 using an iterative asymmetric dihydroxylation

The asymmetric synthesis of both enantiomers of piclavines A1, A2, A3, and A4 has been achieved using an iterative asymmetric dihydroxylation with enantiomeric enhancement.

Preparation of Six Isomeric Bis-acylporphyrins with Chromophores Reaching the Near-Infrared via Intramolecular Friedel−Crafts Reaction

We describe the preparation of six diketones based on the frameworks of five bis-naphthoporphyrins and one perinaphthoporphyrin. All diketones derive from meso-tetraarylporphyrins having incorporated two carbonyl groups, each one connected to one beta-pyrrole carbon and one ortho carbon atom from a meso-aryl group. These compounds were all produced in good yield by intramolecular Friedel-Crafts acylation, either from porphyrins meso-substituted by o-carboxyphenyl or o,o'-dicarboxyphenyl substituents or from porphyrins bearing carboxy groups attached to the pyrrolic beta-positions. Although the former reaction does not show significant regioselectivity when run on nickel complexes, the opposite is true for the corresponding free bases. All diketones show a spectacular bathochromic shift of the UV-vis absorption, the longest wavelength bands absorbing in the 700-825 nm range. Two compounds were structurally characterized by X-ray diffraction. In the case of the diketone, whose carbonyl groups are attached to vicinal pyrrolic beta-positions, a significant intermolecular interaction between the two carbonyl groups and an aromatic hydrogen atom was detected.

The total synthesis of piclavines A1-4 and their biological evaluation

The total synthesis of piclavines A1, A2, A3, and A4 has been achieved starting from compound 10 with enantiomeric enhancement. Their biological activities (antibacterial, antimicrobacterial, and antiviral activities and inhibition of cell growth) were evaluated.

New aromatase inhibitors. Synthesis and biological activity of aryl-substituted pyrrolizine and indolizine derivatives

We report herein the design and the synthesis of some aryl-substituted pyrrolizine and indolizine derivatives, on the basis of a hypothetical pharmacophore structure designed to fit the catalytic site of the human cytochrome P450 aromatase. The in vitro biological evaluation of these compounds allowed us to point out two new potent non-steroidal aromatase inhibitors, MR 20494 and MR 20492, with IC50 values in the range of 0.1 microM.