Enantioselective syntheses of (-)- and (+)-monomorine I

Catalytic enantioselective reductive alkynylation of amides enables one-pot syntheses of pyrrolidine, piperidine and indolizidine alkaloids

The primary objective in synthetic organic chemistry is to develop highly efficient, selective, and versatile synthetic methodologies, which are essential for discovering new drug candidates and agrochemicals. In this study, we present a unified strategy for a one-pot, catalytic enantioselective synthesis of α-alkyl and α,α'-dialkyl pyrrolidine, piperidine, and indolizidine alkaloids using readily available amides and alkynes. This synthesis is enabled by the identification and development of an Ir/Cu/N-PINAP catalyzed highly enantioselective and chemoselective reductive alkynylation of α-unbranched aliphatic amides, which serves as the key reaction. This reaction is combined with Pd-catalyzed tandem reactions in a one-pot approach, enabling the collective, catalytic enantioselective total syntheses of eight alkaloids and an anticancer antipode with 90-98% ee. The methodology's enantio-divergence is exemplified by the one-step access to either enantiomer of alkaloid bgugaine.

Au-catalyzed stereodivergent synthesis of 2,5-disubstituted pyrrolidines: total synthesis of (+)-monomorine I and (+)-indolizidine 195B

Stereodivergent synthesis of 2,5-disubstituted pyrrolidines was achieved via a Au-catalyzed tandem intramolecular alkyne hydroamination/iminium formation/Et3SiH reduction. Importantly, the stereochemical outcome could be switched by choosing an appropriate nitrogen protecting group. Total synthesis of a diastereomeric pair of alkaloid natural products, monomorine I and indolizidine 195B, was achieved to showcase the synthetic utility of the tandem reaction.

Modern Photocatalytic Strategies in Natural Product Synthesis

Modern photocatalysis has proven its generality for the development and functionalization of native functionalities. To date, the field has found broad applications in diverse research areas, including the total synthesis of natural products. This contribution covers recent reports of total syntheses involving as a key step a photocatalytic reaction. Among the selected examples, the photocatalytic processes proceed in a highly chemo-, regio-, and stereoselective manner, thereby allowing the rapid access to structurally complex architectures under light-driven conditions.

Selective Electrochemical Oxidation of Functionalized Pyrrolidines

A method for the selective electrochemical aminoxyl-mediated Shono-type oxidation of pyrrolidines to pyrrolidinones is described. These transformations show the high selectivity and functional group compatibility. This chemistry also demonstrates the use of an operationally simple ElectraSyn 2.0 and cost-effective stainless-steel electrode for the electrochemical oxidation of functionalized pyrrolidines.

Synthesis of 1,2,3-triazole-substituted 6,7-dihydroindolizin-8(5H)-one derivatives mediated by Selectfluor

The 1,2,3-triazole and 1 H-1,2,3-benzotriazole-substituted 6,7-dihydroindolizin-8(5 H)-one derivatives were synthesized by the reaction of ( E)-7-(arylmethylidene)-6,7-dihydroindolizin-8(5 H)-ones with 1,2,3-triazole and 1 H-1,2,3-benzotriazole in the presence of Selectfluor in moderate yield. The structures of all the products were characterized thoroughly by nuclear magnetic resonance, infrared, and high-resolution mass spectrometry together with X-ray crystallographic analysis.

Synthesis of 4-Aryl-3,4,5,6-Tetrahydropyrimido[5,4-g]Indolizine-2(1H)-Thione

A number of 4-aryl-3,4,5,6-tetrahydropyrimido[5,4-g]indolizine-2(1H)-thiones were synthesised by the reaction of (7E)-7-arylidene-6,7-dihydroindolizin-8(5H)-one and thiourea in the presence of potassium hydroxide in moderate yields. The structures of all the products were characterised thoroughly by NMR, IR and HRMS together with X-ray crystallographic analysis.

Synthesis of Novel Spiro(Indolizine-Pyrazole) Derivatives via 1,3-Dipolar Cycloaddition of Nitrilimine

The 1,3-dipolar cycloaddition of nitrilimine, generated in situ from N′-phenylbenzohydrazonoyl chloride and triethylamine, to 7-arylmethylidene-6,7-dihydroindolizin-8( 5H)-one afforded novel 4′-aryl-2′,5′-diphenyl-2′,4′,5,6-tetrahydro-8 H-spiro(indolazine-7,3′-pyrazol)-8-ones in moderate yields. The structures of all the products were characterised thoroughly by NMR, infrared spectroscopy and HRMS. The structure and stereochemistry was confirmed by X-ray diffraction analysis.

Synthesis and crystal structures of three novel benzimidazole/benzoindolizine hybrids

Three benzoindolizine derivatives, 1, 2, and 3, were obtained via 1,3-dipolar cycloaddition. The reaction of 1-(2′-benzimidazolylmethyl)isoquinolinium ylides with dimethyl acetylenedicarboxylate gave a mixture of pyrrolo[2,1-a]isoquinoline-1,2-dicarboxylate (1) and 1,10b-dihydropyrrolo[2,1-a]isoquinoline-1,2-dicarboxylate (2) derivatives containing a benzimidazole moiety. The reaction of this isoquinolinium N-ylide with dimethyl maleate gave an unexpected 2,3-dihydropyrrolo[2,1-a]isoquinoline-1,2-dicarboxylate (3). The structures of all reported compounds have been examined by X-ray crystallography, mass spectrometry, and NMR spectroscopy.

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.

Amberlite-IRA-402 (OH) ion exchange resin mediated synthesis of indolizines, pyrrolo [1,2-a] quinolines and isoquinolines: antibacterial and antifungal evaluation of the products

A number of indolizines and pyrrolo[1,2-a]quinolines/isoquinolines were prepared from phenacyl pyridinium, quinolinium and isoquinolinium salts derived from the reaction of the heterocycles with 2-bromo acetophenone with alkynes and alkenes using amberlite-IRA-402 (OH) ion exchange resin as the base. Antibacterial and antifungal studies were carried out against thirteen bacterial and four fungal strains, which revealed that three derivatives (4a, 4b, 7a) out of fifteen are effective against all the thirteen strains and one derivative, 10, showed dual antibactericidal and antifungal efficacy.

Efficient enantio- and diastereodivergent synthesis of poison-frog alkaloids 251O and trans-223B

An efficient and flexible synthesis of poison-frog alkaloids 251O and trans-223B has been achieved by using for both alkaloids an enantiodivergent process starting from the common lactam 1. The relative stereochemistry of 251O and trans-223B was determined to be 7 (R = n-C(7)H(15), R' = n-Pr) and 14 by the present enantioselective synthesis.

Diastereoselective pyrrolidine synthesis via copper promoted intramolecular aminooxygenation of alkenes: formal synthesis of (+)-monomorine

The diastereoselectivity of the copper-promoted intramolecular aminooxygenation of various alkene substrates was investigated. Alpha-substituted 4-pentenyl sulfonamides favor the formation of 2,5-cis-pyrrolidines (dr >20:1) giving excellent yields which range from 76-97% while gamma-substituted substrates favor the 2,3-trans pyrrolidine adducts with moderate selectivity (ca. 3:1). A substrate whose N-substituent was directly tethered to the alpha-carbon exclusively yielded the 2,5-trans pyrrolidine. The synthetic utility of the method was demonstrated by a short and efficient formal synthesis of (+)-monomorine.