Use of the intramolecular Heck reaction for forming congested quaternary carbon stereocenters. Stereocontrolled total synthesis of (+/-)-gelsemine

Morphological Profiling Identifies the Motor Protein Eg5 as Cellular Target of Spirooxindoles

Oxindoles and iso-oxindoles are natural product-derived scaffolds that provide inspiration for the design and synthesis of novel biologically relevant compound classes. Notably, the spirocyclic connection of oxindoles with iso-oxindoles has not been explored by nature but promises to provide structurally related compounds endowed with novel bioactivity. Therefore, methods for their efficient synthesis and the conclusive discovery of their cellular targets are highly desirable. We describe a selective RhIII -catalyzed scaffold-divergent synthesis of spirooxindole-isooxindoles and spirooxindole-oxindoles from differently protected diazooxindoles and N-pivaloyloxy aryl amides which includes a functional group-controlled Lossen rearrangement as key step. Unbiased morphological profiling of a corresponding compound collection in the Cell Painting assay efficiently identified the mitotic kinesin Eg5 as the cellular target of the spirooxindoles, defining a unique Eg5 inhibitor chemotype.

The Analysis of Two Distinct Strategies toward the Enantioselective Formal Total Synthesis of (+)-Gelsenicine

A full account of a formal enantioselective total synthesis of (+)-gelsenicine is described. Separate strategies based on catalytic cycloisomerization as the central step are considered. One plan involves chirality transfer from enantioenriched substrates, while the other employs asymmetric catalysis. The chirality transfer strategy is less effective, while in the latter, phosphoramidite- and bisphosphine-gold complexes are tested and ultimately provide a key intermediate in high enantiopurity in our Gelsemium alkaloid syntheses.

Synthetic Strategies to Access Heteroatomic Spirocentres Embedded in Natural Products

The spirocyclic motif is abundant in natural products and provides an ideal three-dimensional template to interact with biological targets. With significant attention historically expended on the synthesis of flat-heterocyclic compound libraries, methods to access the less-explored three-dimensional medicinal-chemical space will continue to increase in demand. Herein, we highlight by reaction class the common strategies used to construct the spirocyclic centres embedded in a series of well-studied natural products.

1 Introduction

2 Cycloadditions

3 Palladium-Catalysed Coupling Reactions

4 Conjugate Additions

5 Imines, Aminals, and Hemiaminal Ethers

6 Mannich-Type Reactions

7 Oxidative Dearomatisation

8 Alkylation

9 Organometallic Additions

10 Conclusions

Regio- and Stereoselective Synthesis of Allylic Spiroethers (Spirobenzofuranes) via an Intramolecular Mizoroki-Heck Reaction

The palladium(0)-catalyzed intramolecular annulation of 12 1,3-disubstituted cyclopentenes, derived from (+)-vince lactam, resulted in 5-exo cyclizations which furnished a series of 2,5-dimethyl-1-((3R,4'S)-2H-spiro[benzofuran-3,1'-cyclopentan]-2'-en-4'-yl)-1H-pyrroles in excellent diastereoselectivities and useful isolated yields. The double bond migration process that followed the arylpalladium insertion was controlled by a fine-tuning of the reaction system, which provided regioselectivities of up to 98:2. The selective Mizoroki-Heck reaction was used as the key transformation for preparing two new spirocyclic monoprotected amino acids as single stereoisomers.

Intramolecular Palladium-Catalyzed Ring Opening of Oxabenzonorbornadienes with C1-Tethered Aryl Halides

The novel intramolecular ring opening of oxabenzonorbornadienes with C₁-tethered aryl halides was investigated using palladium catalysts to form fused tetracyclic frameworks. The reaction was generally found to synthesize 1,2-dihydronaphthalen-1-ol products with mild selectivity but was capable of synthesizing dehydrated naphthalene products in excellent yield and selectivity. Substituent effects on oxabenzonorbornadiene and on the iodoarene were explored along with the effects of varying tether length, where an efficient reaction was observed in nearly all cases. A total of 16 examples are reported with yields ranging from 0 to 96%.

Facile Synthesis of Spiro[cyclohexane-1,3’-indoline]-2,2’-diones

Spiro[cyclohexane-1,3’-indoline]-2,2’-diones were easily prepared in good to high yields by the oxidation of N-aryl-N-methyl-2-oxocyclohexane-1-carboxamides in one pot with a short reaction time. The spiroindolinediones could be important for the total synthesis of natural products.

Recent Syntheses and Strategies toward Polycyclic Gelsemium Alkaloids

This Minireview is focused on an in-depth discussion of comparative strategies to construct the gelsemine and gelsedine classes of the gelsemium alkaloids. This document highlights the diversity of strategies used to access specific motifs found within these targets: a) the fused "[3.2.1]bicycle" (in gelsemine) and "oxabicycle" (in gelsedine class); b) the "piroxindole" moiety with C7 quaternary center; c) the "N-heterocycles" and d) the "THP" moiety with C20 quaternary center (in gelsemine).

An unusual stereoretentive 1,3-quaternary carbon shift resulting in an enantioselective RhII-catalyzed formal [4+1]-cycloaddition between diazo compounds and vinyl ketenes

Enantioselective quaternary carbon construction in the assembly of cyclopentenones employing a RhII-catalyzed, formal [4+1]-cycloaddition is described. A Rh2(S-TCPTTL)4-catalyzed cyclopropanation of a vinyl ketene with a disubstituted diazo compound initiates a stereoretentive, accelerated ring expansion to provide the cycloadduct in good to excellent yields and enantioselectivity.

Nine-step total synthesis of (−)-strychnofoline

The asymmetric synthesis of an anti-cancer spirooxindole alkaloid, (−)-strychnofoline, has, for the first time, been accomplished in only nine steps.

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).

Mizoroki-Heck Cyclizations of Amide Derivatives for the Introduction of Quaternary Centers

We report non-decarbonylative Mizoroki-Heck reactions of amide derivatives. The transformation relies on the use of nickel catalysis and proceeds using sterically hindered tri- and tetrasubstituted olefins to give products containing quaternary centers. The resulting polycyclic or spirocyclic products can be obtained in good yields. Moreover, a diastereoselective variant of this method gives access to an adduct bearing vicinal, highly substituted sp3 stereocenters. These results demonstrate that amide derivatives can be used as building blocks for the assembly of complex scaffolds.

Concise Total Synthesis of (+)-Asperazine, (+)-Pestalazine A, and (+)-iso-Pestalazine A. Structure Revision of (+)-Pestalazine A

The concise, enantioselective total syntheses of (+)-asperazine (1), (+)-iso-pestalazine A (2), and (+)-pestalazine A (3) have been achieved by the development of a late-stage C3-C8' Friedel-Crafts union of polycyclic diketopiperazines. Our modular strategy enables the union of complex polycyclic diketopiperazines in virtually their final forms, thus providing rapid and highly convergent assembly at the challenging quaternary stereocenter of these dimeric alkaloids. The significance of this carbon-carbon bond formation can be gauged by the manifold constraints that were efficiently overcome, namely the substantial steric crowding at both reactive sites, the nucleophilic addition of C8' over N1' to the C3 carbocation, and the multitude of reactivity posed by the use of complex diketopiperazine fragments in the coupling event. The success of the indoline π-nucleophile that evolved through our studies is notable given the paucity of competing reaction pathways observed in the presence of the highly reactive C3 carbocation generated. This first total synthesis of (+)-pestalazine A also allowed us to revise the molecular structure for this natural alkaloid.

Enzyme-catalyzed domino reaction: efficient construction of spirocyclic oxindole skeleton using porcine pepsin

Pepsin from porcine gastric mucosa was used as a catalyst in the domino Knoevenagel/Michael/Michael reaction for the synthesis of spirooxindoles.

Mg(ClO4)2-promoted [4 + 3] cycloaddition of oxindole derivatives with conjugated dienes: concise synthesis of spirocycloheptane oxindole derivatives

Novel Mg(ClO₄)₂-promoted [4 + 3] cycloaddition reaction of oxindole derivatives and cyclopentadiene was achieved for the construction of the spirocycloheptane oxindole skeleton.

N,N′-Dioxide/nickel(ii)-catalyzed asymmetric Diels–Alder reaction of cyclopentadiene with 2,3-dioxopyrrolidines and 2-alkenoyl pyridines

A chiral N,N′-dioxide/Ni(OTf)₂ complex-catalyzed asymmetric Diels–Alder reaction of cyclopentadiene with 2,3-dioxopyrrolidines and 2-alkenoyl pyridines has been achieved, leading to the corresponding chiral bridged compounds in up to 97% yield, 95 : 5 dr and 97% ee.

Dinuclear zinc-catalyzed desymmetric intramolecular aldolization: an enantioselective construction of spiro[cyclohexanone-oxindole] derivatives

A bis-ProPhenol zinc complex-catalyzed asymmetric desymmetrizing intramolecular aldol or aldol condensation reaction is reported, which furnishes spiro[cyclohexanone-oxindole] derivatives in good yields with moderate to high enantioselectivities.

Catalytic asymmetric [3 + 2]-cycloaddition for stereodivergent synthesis of chiral indolyl-pyrrolidines

The stereochemical divergent synthesis of indolyl-pyrrolidines was accomplished using an imidazoline-aminophenol (IAP)–Ni(OAc)₂ complex and a bis(imidazolidine)pyridine (PyBidine)–Cu(OTf)₂ complex.

Total synthesis of (+)-gelsemine via an organocatalytic Diels-Alder approach

The structurally complex alkaloid gelsemine was previously thought to have no significant biological activities, but a recent study has shown that it has potent and specific antinociception in chronic pain. While this molecule has attracted significant interests from the synthetic community, an efficient synthetic strategy is still the goal of many synthetic chemists. Here we report the asymmetric total synthesis of (+)-gelsemine, including a highly diastereoselective and enantioselective organocatalytic Diels–Alder reaction, an efficient intramolecular trans-annular aldol condensation furnishing the prolidine ring and establishing the configuration of the C20 quaternary carbon stereochemical centre. The entire gelsemine skeleton was constructed through a late-stage intramolecular S_N2 substitution. The enantiomeric excess of this total synthesis is over 99%, and the overall yield is around 5%.

Gelsamine is a naturally occurring alkaloid, whose complex structure makes it an interesting target for total synthesis. Here, the authors report an enantioselective synthesis of (+)-Gelsemine in 5% overall yield, with an enantioselective organocatalytic Diels–Alder reaction as one of the key steps.

Organocatalytic Highly Enantioselective Substitution of 3-(1-Tosylalkyl)indoles with Oxindoles Enables the First Total Synthesis of (+)-Trigolutes B

A highly enantioselective organocatalytic substitution of 3-(1-tosylalkyl)indoles with oxindoles has been established by using chiral bifunctional organocatalysts, providing an efficient entry to multiply functionalized 3,3'-disubstituted oxindoles, and was exploited as the key step to enable the first asymmetric total synthesis of optically pure (+)-trigolutes B to be accomplished in a concise manner, within seven steps with an 18% overall yield.

A New Method for the Cleavage of Nitrobenzyl Amides and Ethers

A mild and efficient o- and p-nitrobenzyl cleavage protocol was developed. o- and p-Nitrobenzyl groups were easily removed from a variety of substrates using 20% aqueous NaOH in methanol at 75 °C, presumably via oxidation at the benzylic position by oxygen dissolved in the solution. These easily introducible and removable nitrobenzyl groups can serve as valuable protecting groups for the synthesis of multifunctional, complex molecules.