Total Synthesis of (−)-Hippodamine by Stereocontrolled Construction of Azaphenalene Skeleton Based on Extended One-Pot Asymmetric Azaelectrocyclization

The Development of Reaction Cascades to Synthesize Dimeric Coccinellid Alkaloids

Over the course of the past decade, our group has been intensely interested in achieving the laboratory synthesis of varied members of the coccinellid alkaloid family of natural products. These compounds, produced by varied species of ladybugs throughout the world as defensive agents, include several polycyclic members that can formally be considered as either monomeric or dimeric with architectures that contain between 3 and 7 ring systems along with an array of stereocenters. As a result of their fascinating structures, many groups have achieved syntheses of varied monomeric members using a variety of synthetic strategies and tactics. However, no efforts to synthesize any of the dimeric structures had been reported at the time we began our studies, and only a modest amount of study had been performed as relates to their biosynthesis, with little knowledge of how the larger structures might actually arise in Nature. In this Account, we provide an overview of our general synthetic considerations to achieve a global synthesis of the collection, efforts that have led to date to the formal and total synthesis of 12 different members, 4 at the dimer level. Critical was (1) the identification of a key, common intermediate to enable access to a large number of monomeric substructures in short order, (2) careful thinking as to how the larger structures might arise biosynthetically to fuel building block design, and (3) the development of several reaction cascades that rapidly assembled the majority of their molecular complexity in single-pot operations. Key discoveries in the program include the finding that when efforts to achieve intermolecular dimerizations fail with advanced intermediates, attempts to couple more functionalized fragments earlier and then fold them into the desired structure can be an effective strategy. We also highlight suggestive evidence that a non-natural isomer we originally prepared from one of those cascades may, in fact, be a natural product. And, in particular, we will focus on how two key cascades were developed, as a result of synthetic challenges at varied points in our explorations, which proved capable of forging multiple bonds, rings, and stereocenters in the target structures. One of these includes a designed event that combined 9 different chemical reactions in a single pot and may prove useful for the synthesis of other targets.

Synthesis of 1,2-Dihydro-Substituted Aniline Analogues Involving N-Phenyl-3-aza-Cope Rearrangement Using a Metal-Free Catalytic Approach

An efficient metal-free domino reaction leading to structural/electronically divergent 1,2-dihydropyridines from easily accessible propargyl vinyl anilines via N-phenyl 3-aza-Cope sigmatropic rearrangement is reported with good to excellent yields using 1,2-dichlorobenzene as solvent under thermal conditions. Spirocyclic substitution is also tolerated under the present optimized conditions.

Stereoselective synthesis of 2,6-disubstituted piperidine alkaloids

Among the large number of structurally diverse alkaloids, 2,6-disubstituted piperidine and its analogs have often been targeted when exploiting new synthetic techniques perhaps because of their strong pharmacological properties. This review outlines synthetic strategies to build the 2,6-disubstituted piperidine structural motif with a focus on stereochemical control of two substituents at C2 and C6. The key reactions in this process are then classified on the basis of how the piperidine rings were built with specific examples of natural products that control the stereochemical outcomes and their transition states.

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.

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.

Recent advances in the intramolecular Mannich reaction in natural products total synthesis

This review focuses on selected applications of the intramolecular Mannich reaction as a key step in the total synthesis of natural products (2000–2017).

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.

Synthesis of Enantiopure 3-Hydroxypiperidines from Sulfinyl Dienyl Amines by Diastereoselective Intramolecular Cyclization and [2,3]-Sigmatropic Rearrangement

The highly diastereoselective base-promoted intramolecular cyclization of a variety of enantiopure sulfinyl dienyl amines provides novel sulfinyl tetrahydropyridines that are readily converted to 3-hydroxy tetrahydropyridines via sigmatropic rearrangement. The influence of N- and C- substituents on the process has been studied. Procedures to shorten the sequence such as the tandem cyclization followed by [2,3]-sigmatropic rearrangement, as well as cyclization of the free amine, under Boc- or ArSO- deprotection conditions have been examined. Good to excellent levels of selectivity are generally observed for the reported transformations (dr: 75/25 to >98/2). A novel protocol to access substituted amino dienyl sulfoxides is also reported.

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.

Asymmetric intramolecular aza-Michael reaction in desymmetrization processes. Total synthesis of hippodamine and epi-hippodamine

The use of chiral sulfinyl amines both as nucleophilic nitrogen sources and chiral inducers has been described for the first time in a desymmetrization-type process involving an intramolecular aza-Michael reaction. The resulting product was employed as an advanced intermediate in the total synthesis of the natural product hippodamine and epi-hippodamine, taking advantage of the special symmetry of these molecules. In addition, this is the first asymmetric total synthesis of epi-hippodamine.

A strategy for complex dimer formation when biomimicry fails: total synthesis of ten coccinellid alkaloids

Although dimeric natural products can often be synthesized in the laboratory by directly merging advanced monomers, these approaches sometimes fail, leading instead to non-natural architectures via incorrect unions. Such a situation arose during our studies of the coccinellid alkaloids, when attempts to directly dimerize Nature's presumed monomeric precursors in a putative biomimetic sequence afforded only a non-natural analogue through improper regiocontrol. Herein, we outline a unique strategy for dimer formation that obviates these difficulties, one which rapidly constructs the coccinellid dimers psylloborine A and isopsylloborine A through a terminating sequence of two reaction cascades that generate five bonds, five rings, and four stereocenters. In addition, a common synthetic intermediate is identified which allows for the rapid, asymmetric formal or complete total syntheses of eight monomeric members of the class.