Stereocontrolled Synthesis of Substituted Chiral Piperidines viaOne-Pot Asymmetric 6π-Azaelectrocyclization: Asymmetric Syntheses of(−)-Dendroprimine, (+)-7-Epidendroprimine, (+)-5-Epidendroprimine, and (+)-5,7-Epidendroprimine

One-Pot Knoevenagel/Imination/6π-Azaelectrocyclization Sequence for the Synthesis of Disubstituted Nicotinonitriles

We report on a copper-catalyzed three-component reaction for the synthesis of disubstituted nicotinonitriles using 3-bromopropenals, benzoylacetonitriles, and ammonium acetate (NH₄OAc). The Knoevenagel-type condensation of 3-bromopropenals with benzoylacetonitriles gives δ-bromo-2,4-dienones that contain strategically placed functional groups that react with the ammonia generated in situ to give the corresponding azatrienes. These azatrienes can then be transformed into trisubstituted pyridines under the reaction conditions via a reaction sequence involving 6π-azaelectrocyclization and aromatization.

Natural Composition and Biosynthetic Pathways of Alkaloids in Medicinal Dendrobium Species

Dendrobium is the second biggest genus in the Orchidaceae family, some of which have both ornamental and therapeutic values. Alkaloids are a group of active chemicals found in Dendrobium plants. Dendrobine has emerged specific pharmacological and therapeutic properties. Although Dendrobium alkaloids have been isolated and identified since the 1930s, the composition of alkaloids and their biosynthesis pathways, including metabolic intermediates, alkaloid transporters, concrete genes involved in downstream pathways, and associated gene clusters, have remained unresolved scientific issues. This paper comprehensively reviews currently identified and tentative alkaloids from the aspect of biogenic pathways or metabolic genes uncovered based on the genome annotations. The biosynthesis pathways of each class of alkaloids are highlighted. Moreover, advances of the high-throughput sequencing technologies in the discovery of Dendrobium alkaloid pathways have been addressed. Applications of synthetic biology in large-scale production of alkaloids are also described. This would serve as the basis for further investigation into Dendrobium alkaloids.

Desymmetrization of Prochiral Cyclobutanones via Nitrogen Insertion: A Concise Route to Chiral γ-Lactams

Asymmetric access to γ-lactams is achieved via a cyclobutanone ring expansion using widely available (1S,2R)-1-amino-2-indanol for chiral induction. Mechanistic analysis of the key N,O-ketal rearrangement reveals a Curtin-Hammett scenario, which enables a downstream stereoinduction (up to 88:12 dr) and is corroborated by spectroscopic, crystallographic, and computational studies. In combination with an easy deprotection protocol, this operationally simple sequence allows the synthesis of a range of optically pure γ-lactams, including those bearing all-carbon quaternary stereocenters. In addition, the formal synthesis of drug molecules baclofen, brivaracetam, and pregabalin further demonstrates the synthetic utility and highlights the general applicability of the presented method.

The 6π-azaelectrocyclization of azatrienes. Synthetic applications in natural products, bioactive heterocycles, and related fields

Covering: 2006 to 2018 The application of the 6π-azaelectrocyclization of azatrienes as a key strategy for the synthesis of natural products, their analogs and related bioactive or biomedically-relevant compounds (from 2006 to date) is comprehensively reviewed. Details about reaction optimization studies, relevant reaction mechanisms and conditions are also discussed.

Stereoselective strategies for the construction of polysubstituted piperidinic compounds and their applications in natural products’ synthesis

An abridged and far-reaching review communication on the construction of the polysubstituted piperidinic core using diverse methodologies for the benefit of organic chemists interested in the total synthesis of biologically active compounds.

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.

Redox cycloisomerization approach to 1,2-dihydropyridines

The phosphine-catalyzed synthesis of 1,2-dihydropyridines via an alkyne isomerization/electrocyclization sequence is described. Propargylidenecarbamate substrates were prepared following a one-pot procedure between a terminal alkyne, a benzonitrile, and a chloroformate in the presence of trimethylaluminum. This methodology gives access to a diverse set of 2,6-disubstituted 1,2-dihydropyridines in high yield. The products can be easily converted into substituted piperidines or pyridines, and this methodology was applied to the synthesis of indolizidines.

A Late Stage Strategy for the Functionalization of Triazolium-based NHC catalysts

A strategy for the diversification of triazolium-based catalysts is presented. This method is based on the reduction to the triazoline, which serves as a suitable and stable substrate for Pd-mediated cross-coupling, followed by trityl cation mediated reoxidation to the triazolium.