Concise total synthesis of (+/-)-salinosporamide A, (+/-)-cinnabaramide A, and derivatives via a bis-cyclization process: implications for a biosynthetic pathway?

Revisiting the 1,3-azadiene-succinic anhydride annulation reaction for the stereocontrolled synthesis of allylic 2-oxopyrrolidines bearing up to four contiguous stereocenters

Polysubstituted 2-oxopyrrolidines bearing at least two contiguous stereocenters constitute the core of several pharmaceuticals, including clausenamide (antidementia). Here, we describe a flexible annulation strategy, which unites succinic anhydride and 1,3-azadienes to produce allylic 2-oxopyrrolidines bearing contiguous stereocenters. The approach is chemoselective, efficient, modular, scalable, and diastereoselective. The scalable nature of the reactions offers the opportunity for post-diversification, leading to incorporation of motifs with either known pharmaceutical value or that permit subsequent conversion to medicinally relevant entities.

Phosphine Catalyzed Michael-Type Additions: The Synthesis of Glutamic Acid Derivatives from Arylidene-α-amino Esters

The reaction of arylidene-α-amino esters with electrophilic alkenes to yield Michael-type addition compounds is optimized using several phosphines as organocatalysts. The transformation is very complicated due to the generation of several final compounds, including those derived from the 1,3-dipolar cycloadditions. For this reason, the selection of the reaction conditions is a very complex task and the slow addition of the acrylic system is very important to complete the process. The study of the variation in the structural components of the starting imino ester is performed as well as the expansion of other electron-poor alkenes. The crude products have a purity higher than 90% in most cases without any purification. A plausible mechanism is detailed based on the bibliography and the experimental results. The synthesis of pyroglutamate entities, after the reduction of the imino group and cyclization, is performed in high yields. In addition, the hydrolysis of the imino group, under acidic media, represents a direct access to glutamate surrogates.

Diastereospecific arylation and cascade deconstructive amidation/thioesterification of readily available lactam-fused bromolactones

An intrinsic goal when designing synthetic methodology is to identify approaches whereby readily accessible precursors are converted into an array of products, which efficiently tap into new 3D-chemical space. In these studies, readily available bicyclic lactam-bromolactones have been interrogated in several fragment growth protocols by utilizing the halogen and lactone motifs as versatile linchpins for strategic construction of C-C, C-N, C-O, and C-S bonds. Diastereospecific C(sp3)-C(sp2) Kumada coupling of sterically imposing [5,5]-bicyclic lactam-bromolactones with several aryl Grignard reagents, under palladium catalysis, furnishes diarylmethane-tethered lactam-lactones in synthetically attractive yields, stereoinvertive fashion, and with a tolerance for many functional groups. When [5,6]-bicyclic lactam-bromolactones, which are prone to β-hydride elimination are employed, efficient arylation is observed only under Co(acac)3-catalyzed conditions. Importantly, these [5,6]-bicyclic lactam-bromolactones undergo retentive arylation, independent of the transition metal catalyst. A base-mediated cascade deconstructive amidation of the [5,6]-bicyclic lactam-bromolactones with primary aliphatic amines proceeds efficiently to afford epoxide-tethered lactam carboxamides, which bear four contiguous stereocenters. Furthermore, an unusual route to homoallylic thioesters has been uncovered through deconstructive contra-thermodynamic thioesterification of the lactam-fused bromolactone precursors.

Isothiourea-Catalyzed [2 + 2] Cycloaddition of C(1)-Ammonium Enolates and N-Alkyl Isatins

Enantioselective [2 + 2] cycloaddition of C(1)-ammonium enolates generated catalytically using the isothiourea HyperBTM with N-alkyl isatins gives spirocyclic β-lactones. In situ ring opening with an amine nucleophile generates isolable highly enantioenriched products in up to 92:8 dr and in >99:1 er.

Enzymatic assembly of the salinosporamide γ-lactam-β-lactone anticancer warhead

The marine microbial natural product salinosporamide A (marizomib) is a potent proteasome inhibitor currently in clinical trials for the treatment of brain cancer. Salinosporamide A is characterized by a complex and densely functionalized γ-lactam-β-lactone bicyclic warhead, the assembly of which has long remained a biosynthetic mystery. Here, we report an enzymatic route to the salinosporamide core catalyzed by a standalone ketosynthase (KS), SalC. Chemoenzymatic synthesis of carrier protein-tethered substrates, as well as intact proteomics, allowed us to probe the reactivity of SalC and understand its role as an intramolecular aldolase/β-lactone synthase with roles in both transacylation and bond-forming reactions. Additionally, we present the 2.85-Å SalC crystal structure that, combined with site-directed mutagenesis, allowed us to propose a bicyclization reaction mechanism. This work challenges our current understanding of the role of KS enzymes and establishes a basis for future efforts toward streamlined production of a clinically relevant chemotherapeutic.

Trans-selective cyclizations of alpha-bromocarboxamides and E/Z-mixed internal olefins catalyzed by a Fe salt

Herein, we describe a protocol for trans-selective cyclizations of α-bromocarboxamides and E/Z-mixed internal olefins catalyzed by a Fe salt.

Enantioenriched α-substituted glutamates/pyroglutamates via enantioselective cyclopropenimine-catalyzed Michael addition of amino ester imines

A procedure for the enantioselective synthesis of α-substituted glutamates and pyroglutamates via a cyclopropenimine-catalyzed Michael addition of amino ester imines is described. Enantioselectivities of up to 94% have been achieved, and a variety of functional groups were found to be compatible. The impact of the catalyst structure and imine substitution is discussed. Compared to other methods, this protocol allows for a broader and more enantioselective access to pyroglutamate derivatives.

Natural product scaffolds as inspiration for the design and synthesis of 20S human proteasome inhibitors

The 20S proteasome is a valuable target for the treatment of a number of diseases including cancer, neurodegenerative disease, and parasitic infection. In an effort to discover novel inhibitors of the 20S proteasome, many reseaarchers have looked to natural products as potential leads for drug discovery. The following review discusses the efforts made in the field to isolate and identify natural products as inhibitors of the proteasome. In addition, we describe some of the modifications made to natural products in order to discover more potent and selective inhibitors for potential disease treatment.

Organo or Metal Complex Catalyzed Synthesis of Five-membered Oxygen Heterocycles

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The reactions involving the formation of C-O bond using metal as a catalyst have emerged to be one of the most influential reactions for the synthesis of heterocycles in modern organic chemistry. Catalysis by metals offers diverse opportunities to invent new organic reactions with a promising range of selectivities such as chemoselectivity, regioselectivity, diastereoselectivity, and enantioselectivity. The methodologies used earlier for synthesis were less approachable to the organic chemist because of their high cost, highly specified instrumentation and inconvenient methods. For both stereoselective and regioselective formation of five-membered O-containing heterocycles, cyclic reactions that are metal and non-metal-catalyzed have known to be very efficient. The present review article covers the applications of metal and non-metal as a catalyst for the synthesis of five-membered O-containing heterocycles.

Total Synthesis of (-)-Salinosporamide A via a Late Stage C-H Insertion

The synthesis of (-)-salinosporamide A, a proteasome inhibitor, is described. The synthesis highlights the assembly of a densely decorated pyrrolidinone core via an aza-Payne/hydroamination sequence. Central to the success of the synthesis is a late-stage C-H insertion reaction to functionalize a sterically encumbered secondary carbon. The latter functionalization leads to an enabling transformation where most of the prototypical strategies failed.

Catalytic enantioselective synthesis of perfluoroalkyl-substituted β-lactones via a concerted asynchronous [2 + 2] cycloaddition: a synthetic and computational study

The enantioselective preparation of a range of perfluoroalkyl-substituted β-lactones through an isothiourea (HyperBTM) catalysed reaction using symmetric anhydrides as ammonium enolate precursors and perfluoroalkylketones (R_F = CF₃, C₂F₅, C₄F₉) is reported. Following optimisation, high diastereo- and enantioselectivity was observed for β-lactone formation using C₂F₅- and C₄F₉-substituted ketones at room temperature (26 examples, up to >95 : 5 dr and >99 : 1 er), whilst -78 °C was necessary for optimal dr and er with CF₃-substituted ketones (11 examples, up to >95 : 5 dr and >99 : 1 er). Derivatisation of the β-lactones through ring-opening, as well as a two-step conversion to give perfluoroalkyl-substituted oxetanes, is demonstrated without loss of stereochemical integrity. Density functional theory computations, alongside ¹³C natural abundance KIE studies, have been used to probe the reaction mechanism with a concerted asynchronous [2 + 2]-cycloaddition pathway favoured over a stepwise aldol-lactonisation process.

Intramolecular nitration–aminocarbonylation of unactivated olefins: metal-free synthesis of γ-lactams

A novel, metal-free synthesis of valuable γ-lactams through the radical-mediated nitration–aminocarbonylation of unactivated olefins is reported.

Versatile Synthetic Approach for Selective Diversification of Bicyclic Aza-Diketopiperazines

Herein, we report a convenient synthesis of unprecedented aza-diketopiperazines (aza-DKPs). The strategy is based on selective diversification of bicyclic aza-DKP scaffolds by click reaction, N-acylation, and/or N-alkylation. These scaffolds containing either azido or amino groups were obtained by a key Rh(I)-catalyzed hydroformylative cyclohydrocarbonylation reaction of allyl-substituted aza-DKP. The methodology is readily amenable to the parallel synthesis of original aza-DKPs to enlarge the chemical diversity of aza-heterocycles.

A Total Synthesis of Salinosporamide A

Salinosporamide A is a β-lactone proteasome inhibitor currently in clinical trials for the treatment of multiple-myeloma. Herein we report a short synthesis of this small, highly functionalized, biologically important natural product that uses an oxidative radical cyclization as a key step and allows for the preparation of gram quantities of advanced synthetic intermediates.

Biologically active γ-lactams: synthesis and natural sources

The γ-lactam moiety is present in a large number of natural and non-natural biologically active compounds. The range of biological activities covered by these compounds is very broad. Functionalized γ-lactams are thus of high interest and have great potential in medicinal chemistry. This review provides a description of the title compounds by focusing on their synthesis, natural sources and biological activities.

A strategy for dual inhibition of the proteasome and fatty acid synthase with belactosin C-orlistat hybrids

The proteasome, a validated cellular target for cancer, is central for maintaining cellular homeostasis, while fatty acid synthase (FAS), a novel target for numerous cancers, is responsible for palmitic acid biosynthesis. Perturbation of either enzymatic machine results in decreased proliferation and ultimately cellular apoptosis. Based on structural similarities, we hypothesized that hybrid molecules of belactosin C, a known proteasome inhibitor, and orlistat, a known inhibitor of the thioesterase domain of FAS, could inhibit both enzymes. Herein, we describe proof-of-principle studies leading to the design, synthesis and enzymatic activity of several novel, β-lactone-based, dual inhibitors of these two enzymes. Validation of dual enzyme targeting through activity-based proteome profiling with an alkyne probe modeled after the most potent inhibitor, and preliminary serum stability studies of selected derivatives are also described. These results provide proof of concept for dual targeting of the proteasome and fatty acid synthase-thioesterase (FAS-TE) enabling a new approach for the development of drug-candidates with potential to overcome resistance.

β-Functionalization of Indolin-2-one-Derived Aliphatic Acids for the Divergent Synthesis of Spirooxindole γ-Butyrolactones

β-Functionalization of indolin-2-one-derived aliphatic acids has been applied in formal [3 + 2] annualtions for catalyst-free and divergent synthesis of two series of structurally interesting 3,3'-spirooxindole γ-butyrolactones that may be attractive for potential drug discovery. These findings also pave the way for further diversity-oriented synthesis of spirooxindoles starting from indolin-2-one-derived aliphatic acids or their derivatives.

6-exo-trig Michael addition-lactonizations for catalytic enantioselective chromenone synthesis

The catalytic enantioselective 6-exo-trig Michael addition-lactonization of enone-acid substrates to form cis-chromenones with high diastereo- and enantiocontrol was developed using the commercially available isothiourea tetramisole. An acidic workup proved necessary to minimize product epimerization and maximize product er, providing cis-chromenones in excellent yield, and with excellent diastereo- and enantioselectivity.

Catalytic Asymmetric Synthesis of Ketene Heterodimer β-Lactones: Scope and Limitations

In this article we describe extensive studies of the catalytic asymmetric heterodimerization of ketenes to give ketene heterodimer β-lactones. The optimal catalytic system was determined to be a cinchona alkaloid derivative (TMS-quinine or Me-quinidine). The desired ketene heterodimer β-lactones were obtained in good to excellent yields (up to 90%), with excellent levels of enantioselectivity (≥90% ee for 33 Z and E isomer examples), good to excellent (Z)-olefin isomer selectivity (≥90:10 for 20 examples), and excellent regioselectivity (only one regioisomer formed). Full details of catalyst development studies, catalyst loading investigations, substrate scope exploration, protocol innovations (including double in situ ketene generation for 7 examples), and an application to a cinnabaramide A intermediate are described. The addition of lithium perchlorate (1-2 equiv) as an additive to the alkaloid catalyst system was found to favor formation of the E isomer of the ketene heterodimer. Ten examples were formed with moderate to excellent (E)-olefin isomer selectivity (74:25 to 97:3) and with excellent enantioselectivity (84-98% ee).

Synthetic approaches towards alkaloids bearing α-tertiary amines

Alkaloids account for some of the most beautiful and biologically active natural products. Although they are usually classified along biosynthetic criteria, they can also be categorized according to certain structural motifs. Amongst these, the α-tertiary amine (ATA), i.e. a tetrasubstituted carbon atom surrounded by three carbons and one nitrogen, is particularly interesting. A limited number of methods have been described to access this functional group and fewer still are commonly used in synthesis. Herein, we review some approaches to asymmetrically access ATAs and provide an overview of alkaloid total syntheses where those have been employed.

Asymmetric Synthesis of Deoxypropionate Derivatives via Catalytic Hydrogenolysis of Enantioenriched Z-Ketene Heterodimers

A diastereoselective approach to deoxypropionate derivatives through Pd/C-catalyzed hydrogenolysis of enantioenriched ketene heterodimers is described. Catalytic hydrogenolysis of the Z-isomer of ketene heterodimers facilitates access to anti-deoxypropionate derivatives (10 examples with dr 7:1 to >20:1). Transfer of chirality from the Z-ketene heterodimer to an acid product was good to excellent in most cases (78-99% ee for 12 examples).

Organocatalytic Lewis base functionalisation of carboxylic acids, esters and anhydrides via C1-ammonium or azolium enolates

This tutorial review highlights the organocatalytic Lewis base functionalisation of carboxylic acids, esters and anhydrides via C1-ammonium/azolium enolates. The generation and synthetic utility of these powerful intermediates is highlighted through their application in various methodologies including aldol-lactonisations, Michael-lactonisations/lactamisations and [2,3]-rearrangements.

Exploring the scope of the isothiourea-mediated synthesis of dihydropyridinones

The exploration and expansion of the scope of the isothiourea-mediated synthesis of dihydropyridinones is presented.

N-heterocyclic carbene-catalyzed cyclocondensation of 2-aryl carboxylic acids and enones: highly enantioselective synthesis of δ-lactones

The NHC-catalyzed [4 + 2] cyclocondensation of 2-aryl carboxylic acids with enones was developed, giving δ-lactones in good yields with high enantioselectivities.

Identification of bioactive metabolites dihydrocanadensolide, Kojic acid, and vanillic acid in soy sauce using GC-MS, NMR spectroscopy, and single-crystal X-ray diffraction

Microbial transformations of intrinsic substrates offer immense potential for generating new bioactive compounds in fermented food products. The aim of this work was to characterize the secondary metabolites in soy sauce, one of the oldest fermented condiments. Ethyl acetate extract (EAE) of soy sauce was separated using flash column chromatography, crystallized, and analyzed by nuclear magnetic resonance (NMR), single-crystal X-ray diffraction (SC-XRD), and mass spectroscopy. Dihydrocanadensolide (DHC), an antiulcer agent, was identified in a food for the first time. The natural stereostructure of DHC, which remained controversial for several decades, was determined as (3S,3aS,6R,6aR)-6-butyl-3-methyltetrahydrofuro[3,4-b]furan-2,4-dione using SC-XRD analysis. Kojic acid (KA) and vanillic acid (VA) were also identified from EAE as bioactive metabolic products of fungi and yeasts. Moreover, a new polymorphic form of KA was determined by SC-XRD.

α-Ketophosphonates as ester surrogates: isothiourea-catalyzed asymmetric diester and lactone synthesis

Isothiourea HBTM-2.1 catalyzes the asymmetric Michael addition/lactonization of aryl- and alkenylacetic acids using α-keto-β,γ-unsaturated phosphonates as α,β-unsaturated ester surrogates, giving access to a diverse range of stereodefined lactones or enantioenriched functionalized diesters upon ring-opening.

Isothiourea-mediated asymmetric functionalization of 3-alkenoic acids

Isothiourea HBTM-2.1 promotes the catalytic asymmetric α-functionalization of 3-alkenoic acids through formal [2 + 2] cycloadditions with N-tosyl aldimines and formal [4 + 2] cycloadditions with either 4-aryltrifluoromethyl enones or N-aryl-N-aroyl diazenes, providing useful synthetic building blocks in good yield and with excellent enantiocontrol (up to >99% ee). Stereodefined products are amenable to further synthetic elaboration through manipulation of the olefinic functionality.

Recent progress towards transition metal-catalyzed synthesis of γ-lactams

The scope of transition metal-catalyzed synthesis of functionalized γ-lactams has broadened tremendously in the last decade (see the picture). These metal-catalyzed processes are reviewed by highlighting their specificity and applicability, and the mechanistic rationale is presented where possible.

(S)-4-Trimethylsilyl-3-butyn-2-ol as an auxiliary for stereocontrolled synthesis of salinosporamide analogs with modifications at positions C2 and C5

Analogs of salinosporamide A with variations of the C2 and C5 substituents are prepared in 8-10 steps using as the first and key transformation a diastereoselective Mukaiyama aldol reaction between the chiral 5-tert-butyldimethylsiloxy-3-methyl-1H-pyrrole-2-carboxylic ester depicted and various aldehyde substrates, promoted by tert-butyldimethylsilyl triflate. In this transformation, the 4-trimethylsilyl-3-butyn-2-ol ester functions to direct the formation of predominantly one of four possible diastereomeric aldol products. Introduction of the C2 appendage by a later-stage, stereocontrolled alkylation reaction permits the construction of analogs variant at this position. Results from in vitro and cell-based assays of proteasomal inhibition are reported. Mass spectrometric studies provide mechanistic details of proteasomal modification by salinosporamide A and analogs.