Titanocene(III) chloride-mediated reductions of oxazines, hydroxamic acids, and N-hydroxy carbamates

Diversification of Simple Arenes into Complex (Amino)cyclitols

Highly oxygenated cyclohexanes, including (amino)cyclitols, are featured in natural products possessing a notable range of biological activities. As such, these building blocks are valuable tools for medicinal chemistry. While de novo synthetic strategies have provided access to select compounds, challenges including stereochemical density and complexity have hindered the development of a general approach to (amino)cyclitol structures. This work reports the use of arenophile chemistry to access dearomatized intermediates which are amenable to diverse downstream transformations. Practical guidelines were developed for the synthesis of natural and non-natural (amino)cyclitols from simple arenes through a series of strategic functionalization events.

Metal-Free C-H Sulfamidation of 1,4-Naphthoquinone in Water

Direct sulfamidation of 1,4-naphthoquinones using N-methoxy sulfonamides under metal-free conditions in water was developed. Base-mediated nucleophilic addition of N-methoxy sulfonamides, followed by N-O bond cleavage allowed the formation of enesulfonamides. Further, the synthesis of pyrrolonaphthoquinones proved the practicability of the current approach.

Ruthenium(ii)-catalyzed reductive N–O bond cleavage of N-OR (R = H, alkyl, or acyl) substituted amides and sulfonamides

A convenient method for the reductive cleavage of the N–O bonds of amide derivatives was developed using ruthenium(ii)-catalyzed transfer hydrogenation reaction.

Copper-Catalyzed Aqueous N-O Bond Cleavage of 2-Oxa-3-Azabicyclo Compounds to Cyclic cis-1,4-Amino Alcohols

The N-O bond cleavage of 2-oxa-3-azabicyclo substrates, which are readily prepared by the hetero Diels-Alder reaction between nitroso dienophiles and cyclic 1,3-dienes, was effectively catalyzed by heterogeneous copper-on-carbon (Cu/C) under aqueous conditions to give the corresponding cyclic cis-1,4-amino alcohol derivatives. The present method was applied to the direct incorporation of the hydroxy and amino groups derived from a nitroso substrate into cyclic 1,3-dienes with cis-selectivity by the combination of the in situ formation of 2-oxa-3-azabicyclo compounds and following Cu/C-catalyzed N-O bond cleavage. The obtained cis-4-aminocyclohexenols, derived from cyclohexadiene as a cyclic 1,3-diene, could be selectively oxidized by using the ruthenium-on-carbon (Ru/C) catalyst under oxygen atmosphere to the corresponding 4-aminocyclohexenones at 50-65 °C or para-iminoquinones at 100-110 °C as useful reactive synthetic precursors.

Synthetic Studies on Pactamycin: A Synthesis of Johnson's Intermediate

A formal total synthesis of pactamycin (1) has been accomplished by face-selective and regioselective nitroso Diels-Alder (NDA) reaction of acyl nitroso compound 14, which contains a camphorsultam chiral auxiliary, and chiral cyclopentadiene 12. Construction of the chiral secondary alcohol of 12 was performed by (S,S)-Ts-DENEB catalyst-mediated reduction, and the NDA adduct 15a was readily converted to Johnson's intermediate 21.

Nitroso Diels-Alder Cycloadducts Derived From N-Acyl-1,2-dihydropyridines as a New Platform to Molecular Diversity

This review focuses upon the use of nitroso Diels–Alder reactions as a structural complexity generating reaction that has been so far a quite scarcely treated topic, despite its potential. In particular, the use of N-acyl-1,2-dihydropyridines as a non-symmetrical diene component in nitroso Diels–Alder reactions encompasses an initial diversification of pathways giving rise to different cycloadducts (direct and inverse). Selective elaborations of these cycloadducts, basically using a reagent-based approach, deliver a discrete number of structurally diverse compounds, including some original heterobicyclic scaffolds and functionalized heterocycles. This forward synthetic planning allowed the individuation of a new biologically active compound based on a novel oxadiaza-bicyclic-[3.3.1]-nonene scaffold which is still under preclinical evaluation.

Ruthenium-catalyzed ring-opening reaction of a 3-aza-2-oxabicyclo[2.2.1]hept-5-ene with amines — an unexpected mode of ring-opening

The ruthenium-catalyzed ring-opening reaction of a 3-aza-2-oxabicyclo[2.2.1]hept-5-ene with amines was investigated. In the presence of an amine and a ruthenium catalyst, the N–O bond is cleaved, forming a ring-opened allylic alcohol product. Through a possible ruthenium-catalyzed redox isomerization, the allylic alcohol is transformed into a saturated carbonyl product. This unexpected mode of ruthenium-catalyzed ring-opening reaction of a 3-aza-2-oxabicyclo[2.2.1]hept-5-ene leads to the formation of a cyclopentanone derivative in moderate yields.

Efficient Catalytic Enantioselective Hydroxyamination of α-Aryl-α-Cyanoacetates with 2-Nitrosopyridines

The highly enantioselective totally N-selective hydroxyamination reaction of α-aryl-α-cyanoacetates with 2-nitrosopyridines was realized by using a chiral N,N'-dioxide/Mg(OTf)₂ complex as catalyst, which enriches the nitroso chemistry. A variety of 2-cyano-2-[hydroxyl(pyrydin-2-yl)amino]acetates with quaternary stereocenters and potential antibacterial activities were obtained in excellent yields with good to excellent ee values under as low as 0.05 mol % catalyst loading. The products could be easily transformed to useful α-amino amides and 1,2-diamines. Besides, a possible transition state model was proposed to elucidate the origin of the chirality induction.

Total synthesis of (−)-agelastatin A: an SH2′ radical azidation strategy

A new synthetic approach to (−)-agelastatin A has been established through the strategic implementation of brominative olefin transposition of a silyl enol ether and subsequent S_H2′ radical azidation of the resultant allylic bromide.

Generation and Trapping of Nitrosocarbonyl Intermediates

The nitrosocarbonyls (R-CONO) are highly reactive species and remarkable intermediates toward different synthetic targets. This review will cover a research area whose impact in current organic synthesis is constantly increasing in the chemical community. This review represents the first and comprehensive picture on the generation and trapping of nitrosocarbonyls and is solidly built on more than 380 papers. Six different classes of key starting materials such as hydroxamic acids, N-hydroxy carbamates, N-hydroxyureas, nitrile oxides, and 1,2,4-oxadiazole-4-oxides were highlighted. The content of the review surveys all the methods to generate the nitrosocarbonyls through different approaches (oxidative, thermal, photochemical, catalytic, aerobic, and the less common ones) in the light of efficiency, yields, and mildness. The most successful trapping agents employed to catch these fleeting intermediates are reviewed, exploiting their superior dienophilic, enophilic, and electrophilic power. The work is completed by paragraphs dedicated to the detection of the intermediates, theoretical studies, and insights about the challenges and future directions for the field.

Solid-Phase Synthesis of ɤ-Lactone and 1,2-Oxazine Derivatives and Their Efficient Chiral Analysis

Derivatives of 3-methyl-3,6-dihydro-2H-1,2-oxazine-6-carboxylic acid prepared by regioselective hetero Diels-Alder reaction of arylnitroso compounds with sorbic acid were used for solid-phase synthesis of a library of derivatives that included modification of carboxylic group, dihydroxylation of double bond and cleavage of N-O bond. Derivatives of 2,3,4-trihydroxyhexanoic acid obtained from 3,6-dihydro-2H-1,2-oxazines after double bond dihydroxylation and N-O cleavage were used for simple and stereoselective formation of chiral lactones derived from 3,4-dihydroxydihydrofuran-2(3H)-one. The final compounds obtained as a mixture of stereoisomers were analyzed with use of chiral HPLC and SFC. HPLC analyses were not successful for all derivatives or required lengthy chromatography. On the other hand SFC afforded much shorter analyses and was effective for all studied derivatives. The method of synthesis and analysis is thus suitable for future study of stereoselective synthesis of lactones and other derivatives from single oxazine derivatives and application of high-throughput synthesis on solid-support and combinatorial chemistry.

N–O linkage in carbohydrates and glycoconjugates

The synthesis and chemical and physicochemical properties as well as biological and medical applications of various hydroxylamine-functionalized carbohydrate derivatives are summarized.

Fe-Catalyzed reductive NO-bond cleavage – a route to the diastereoselective 1,4-aminohydroxylation of 1,3-dienes

We describe the iron-catalyzed reductive cleavage of N–O-bonds in oxazines using malononitrile as reductant, which – in sequence with a [4 + 2]-cycloaddition – can be considered as an efficient way for the diastereoselective 1,4-aminohydroxylation of 1,3-dienes.

Amant AH, Hein JE, Read de Alaniz J. Cascade rearrangement of furylcarbinols with hydroxylamines: practical access to densely functionalized cyclopentane derivatives

The aza-Piancatelli rearrangement with hydroxylamines to 4-aminocyclopentenones is described. Subsequent transformations highlight the versatility of the cyclopentene scaffold and the value of the hydroxylamine in this transformation.

Synthesis of protected (1-phenyl-1h-pyrrol-2-yl)-alkane-1-amines from phenylnitroso Diels-Alder adducts with 1,2-dihydropyridines

The reductive cleavage of nitrosobenzene-derived cycloadducts with appropriately protected 1,2-dihydropyridines allows a novel and simple obtainment of substituted N-[1-(1-phenyl-1H-pyrrol-2-yl)alkylamides. This synthesis can also be carried out in a very simple, mild, and practical one-pot procedure without isolation of the corresponding nitrosobenzene cycloadduct by means of catalytic amounts of CuCl.

Ruthenium-catalyzed [2+2] cycloaddition reactions of a 2-oxa-3-azabicyclo[2.2.1]hept-5-ene with unsymmetrical alkynes

Ruthenium-catalyzed [2+2] cycloaddition reactions of a 2-oxa-3-azabicyclo[2.2.1]hept-5-ene with unsymmetrical alkynes were investigated. Yields of up to 90% were obtained though regioselectivity was modest. Select cycloadducts could be separated and used to access a highly functionalized [3.2.0] bicyclic structure through reductive cleavage of the N–O bond. These ring-opened products displayed a chemical exchange phenomenon in 1D carbon NMR and required characterization by 2D NMR techniques.

N-O chemistry for antibiotics: discovery of N-alkyl-N-(pyridin-2-yl)hydroxylamine scaffolds as selective antibacterial agents using nitroso Diels-Alder and ene chemistry

The discovery, syntheses, and structure-activity relationships (SAR) of a new family of heterocyclic antibacterial compounds based on N-alkyl-N-(pyridin-2-yl)hydroxylamine scaffolds are described. A structurally diverse library of ∼100 heterocyclic molecules generated from Lewis acid-mediated nucleophilic ring-opening reactions with nitroso Diels-Alder cycloadducts and nitroso ene reactions with substituted alkenes was evaluated in whole cell antibacterial assays. Compounds containing the N-alkyl-N-(pyridin-2-yl)hydroxylamine structure demonstrated selective and potent antibacterial activity against the Gram-positive bacterium Micrococcus luteus ATCC 10240 (MIC(90) = 2.0 μM or 0.41 μg/mL) and moderate activity against other Gram-positive strains including antibiotic resistant strains of Staphylococcus aureus (MRSA) and Enterococcus faecalis (VRE). A new synthetic route to the active core was developed using palladium-catalyzed Buchwald-Hartwig amination reactions of N-alkyl-O-(4-methoxybenzyl)hydroxylamines with 2-halo-pyridines that facilitated SAR studies and revealed the simplest active structural fragment. This work shows the value of using a combination of diversity-oriented synthesis (DOS) and parallel synthesis for identifying new antibacterial scaffolds.

Copper-catalyzed Perkin-acyl-Mannich reaction of acetic anhydride with pyridine: expeditious entry to unconventional piperidines

A regioselective introduction of a methoxycarbonyl methyl group at the C(2) position of unsubstituted pyridine has been accomplished with catalytic amounts of copper(II) triflate in mild reaction conditions. The N-acetyl-1,2-dihydropyridyl acetic acid methyl ester obtained is a valuable building block for the synthesis of new polyfunctionalized piperidine derivatives bearing unconventional substitution patterns.

The nitrosocarbonyl hetero-Diels-Alder reaction as a useful tool for organic syntheses

Organic transformations that result in the formation of multiple covalent bonds within the same reaction are some of the most powerful tools in synthetic organic chemistry. Nitrosocarbonyl hetero-Diels-Alder (HDA) reactions allow for the simultaneous stereospecific introduction of carbon-nitrogen and carbon-oxygen bonds in one synthetic step, and provide direct access to 3,6-dihydro-1,2-oxazines. This Review describes the development of the nitrosocarbonyl HDA reaction and the utility of the resulting oxazine ring in the synthesis of a variety of important, biologically active molecules.

Enantioselective syntheses of carbocyclic nuleosides 5'-homocarbovir, epi-4'-homocarbovir and their cyclopropylamine analogs using facially selective Pd-mediated allylations

Carbocyclic nucleosides (-)-5'-homocarbovir and (+)-epi-4'-homocarbovir were prepared from an acylnitroso-derived hetero Diels-Alder cycloadduct. A kinetic enzymatic resolution generated an enantiopure aminocyclopentenol and Pd(0)-mediated decarboxylative allylations of allyl 2,2,2-trifluoroethyl malonates were used to install the 4'-hydroxyethyl groups. Late stage derivatization gave access to the cyclopropylamine congenors, (-)-5'-homoabacavir and (+)-epi-4'-homoabacavir. All carbonucleoside target molecules were evaluated for antiviral activity.

Cyclopropanation of nitroso Diels-Alder cycloadducts and application to the synthesis of a 2',3'-methano carbocyclic nucleoside

Treatment of nitroso Diels-Alder cycloadducts 1 with diazomethane in the presence of palladium acetate gives synthetically useful exo-6-oxa-7-azatricyclo[3.2.1.02,4]octane derivatives 7 in good to excellent yield. Using this methodology, a conformationally restricted 2',3'-methano carbocyclic nucleoside was efficiently synthesized from nitroso cycloadduct 1a in 7 steps.

Palladium(0)/indium iodide-mediated allylations of electrophiles generated from the hydrolysis of Eschenmoser's salt: One-pot preparation of diverse carbocyclic scaffolds

A formyl equivalent was generated in situ from Eschenmoser's salt in aqueous THF and was reacted with an allylindium species. Acylnitroso-derived hetero-Diels Alder adducts and related allyl acetates were shown to be substrates for Pd(0)/InI-mediated allylations of formaldehyde-related species to provide homoallylic alcohols. Hydroxymethyl groups were installed with regio- and diastereocontrol to provide relevant disubstituted carbocyclic scaffolds. Enantiopure anti-disubstituted cyclopentene products were prepared from a chiral allyl acetate.

Regio- and stereochemically controlled formation of hydroxamic acids from indium triflate-mediated nucleophilic ring-opening reactions with acylnitroso-Diels-Alder adducts

Treatment of acylnitroso-Diels-Alder [2.2.1] bicyclic adducts 2a-b with indium triflate in an alcohol solvent induces ring opening reactions to afford monocyclic anti-1,2-, anti-1,4- and syn-1,4-hydroxamic acids with good to excellent regio- and stereoselectivity (up to 7:86:7). Treatment of [2.2.2] bicyclic nitroso adducts 2c-d under similar reaction conditions generates only anti-1,2- and anti-1,4-hydroxamic acids with anti-1,4-product predominant (up to 17:83).

Palladium-catalyzed decarboxylative rearrangements of allyl 2,2,2-trifluoroethyl malonates: direct access to homoallylic esters

Homoallylic esters are obtained in a single transformation from allyl 2,2,2-trifluoroethyl malonates by using a Pd(0) catalyst. Facile decarboxylation of allyl 2,2,2-trifluoroethyl malonates is attributed to a decrease in pK(a) compared to allyl methyl malonates. Subsequent reduction of the homoallylic 2,2,2-trifluoroethyl ester provides a (hydroxyethyl)cyclopentenyl derivative that represents a key intermediate in the synthesis of carbocyclic nucleosides. A select allyl 2,2,2-trifluoroethyl malonate undergoes a decarboxylative Claisen rearrangement to provide a regioisomeric homoallylic ester.

Regio- and stereoselective indium triflate-mediated nucleophilic ring-opening reactions of 3-aza-2-oxabicyclo[2.2.1]hept-5-ene and -[2.2.2]oct-5-ene systems

Indium triflate-mediated nucleophilic ring-opening reactions of 3-aza-2-oxabicyclo[2.2.1]hept-5-ene and -[2.2.2]oct-5-ene systems with alcohols and water were investigated. Reactions of [2.2.1] bicyclic systems gave anti-1,2-, anti-1,4- and syn-1,4-ring-opened products with moderate to excellent regio- and stereoselectivity. When [2.2.2] bicyclic systems were subjected to similar reaction conditions, only anti-1,2 and anti-1,4-ring-opened products were obtained.

Rhodium-catalyzed ring-opening reactions of a 3-aza-2-oxabicyclo[2.2.1]hept-5-ene with arylboronic acids

Rhodium-catalyzed ring-opening reactions of a 3-aza-2-oxabicyclo[2.2.1]hept-5-ene with arylboronic acids were investigated. The use of [RhCl(COD)](2), (+/-)-BINAP, and NaHCO(3) in MeOH at 60 degrees C was found to be the optimized conditions for the ring-opening reactions to give the 1,2-ring-opened products. Various arylboronic acids were examined, and low to moderate yields were obtained and stereoselectivities (trans/cis) from 50:50 to 100:0 were observed.

Syntheses of carbocyclic uracil polyoxin C analogs: application of Pd(0)/InI-allylation of 4-acetoxy-2-azetidinone

Carbocyclic uracil polyoxin C analogs are prepared from an acylnitroso-derived hetero Diels-Alder cycloadduct in fewer than nine steps. Pd(0)/InI-mediated allylation of 4-acetoxy-2-azetidinone is used to install the beta-amino acid side chain at the C-5' position of the carbocycle.

Synthesis and anticancer activity of new hydroxamic acid containing 1,4-benzodiazepines

By employing an intramolecular Pd(0)-mediated ring opening of an acylnitroso-derived cycloadduct, new hydroxamic acid containing benzodiazepines have been synthesized and have demonstrated biological activity in MCF-7 and PC-3 tumor cell lines. Subsequent N-O bond reduction of the hydroxamate has provided access to amide analogues for SAR studies. During the course of our syntheses, an intermediate oxazoline N-oxide was isolated and gave insight into the mechanism of the key Pd(0)-mediated reaction.