Synthesis of Enantiopure Carbohydrate Mimetics by Lewis Acid Catalyzed Rearrangement of 1,3-Dioxolanyl-Substituted 1,2-Oxazines

Catalytic asymmetric construction of bridged bicyclo[m.3.1] rings using an intramolecular Diels-Alder reaction

Herein, we presented an enantioselective intramolecular Diels-Alder (IMDA) reaction with vinyl branched vinylidene ortho-quinone methide (VQM). The control of site selectivity in the IMDA reaction led to both chiral bridged bicyclo[4.3.1] and [5.3.1] architectures with high isolated yields (up to 85%) and excellent enantioselectivities (up to 97% ee).

Cooperative Bifurcated Chalcogen Bonding and Hydrogen Bonding as Stereocontrolling Elements for Selective Strain-Release Septanosylation

The exploitation of noncovalent interactions (NCIs) is emerging as a vital handle in tackling broad stereoselectivity challenges in synthesis. In particular, there has been significant recent interest in the harnessing of unconventional NCIs to surmount difficult selectivity challenges in glycosylations. Herein, we disclose the exploitation of an unconventional bifurcated chalcogen bonding and hydrogen bonding (HB) network, which paves the way for a robust catalytic strategy into biologically useful seven-membered ring sugars. Through 13C nuclear magnetic resonance (NMR) in situ monitoring, NMR titration experiments, and density functional theory (DFT) modeling, we propose a remarkable contemporaneous activation of multiple functional groups consisting of a bifurcated chalcogen bonding mechanism working hand-in-hand with HB activation. Significantly, the ester moiety installed on the glycosyl donor is critical in the establishment of the postulated ternary complex for stereocontrol. Through the 13C kinetic isotopic effect and kinetic studies, our data corroborated that a dissociative SNi-type mechanism forms the stereocontrolling basis for the excellent α-selectivity.

One-pot nucleophilic substitution-double click reactions of biazides leading to functionalized bis(1,2,3-triazole) derivatives

The nucleophilic substitution of benzylic bromides with sodium azide was combined with a subsequent copper-catalyzed (3 + 2) cycloaddition with terminal alkynes. This one-pot process was developed with a simple model alkyne, but then applied to more complex alkynes bearing enantiopure 1,2-oxazinyl substituents. Hence, the precursor compounds 1,2-, 1,3- or 1,4-bis(bromomethyl)benzene furnished geometrically differing bis(1,2,3-triazole) derivatives. The use of tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (TBTA) as ligand for the click step turned out to be very advantageous. The compounds with 1,2-oxazinyl end groups can potentially serve as precursors of divalent carbohydrate mimetics, but the reductive cleavage of the 1,2-oxazine rings to aminopyran moieties did not proceed cleanly with these compounds.

Recent advances in the asymmetric synthesis of pharmacology-relevant nitrogen heterocycles via stereoselective aza-Michael reactions

In this review, recent applications of a stereoselective aza-Michael reaction for asymmetric synthesis of naturally occurring N-containing heterocyclic scaffolds and their usefulness to pharmacology are summarized.

Diastereospecific Gold(I)-Catalyzed Cyclization Cascade for the Controlled Preparation of N- and N,O-Heterocycles

The reaction of oxime-tethered 1,6-enynes with a cationic gold(I) catalyst demonstrates a great potential for the synthesis of a range of heterocycles in a diastereospecific fashion. The control of the configuration of the oxime and the alkene of the enyne moiety is the key to selectively obtain dihydro-1,2-oxazines, isoxazolines or dihydropyrrole-N-oxides as single diastereoisomers. As supported by DFT calculations, these cascade reactions proceed stepwise, by the intramolecular addition of the O or N atom of the oxime onto cyclopropyl gold(I) carbene intermediates. In this study, a rare [3,3]-sigmatropic rearrangement of nitrones is also observed.

Design of a Mesoscale Continuous-Flow Route toward Lithiated Methoxyallene

The unique nucleophilic properties of lithiated methoxyallene allow for C-C bond formation with a wide variety of electrophiles, thus introducing an allenic group for further functionalization. This approach has yielded a tremendously broad range of (hetero)cyclic scaffolds, including precursors to active pharmaceutical ingredients. To date, however, its valorization at scale is hampered by the batch synthesis procedure, which suffers from serious safety issues. Hence, the attractive heat- and mass-transfer properties of flow technology were exploited to establish a mesoscale continuous-flow route toward lithiated methoxyallene. An excellent conversion of 94 % was obtained, corresponding to a methoxyallene throughput of 8.2 g h^-1 . The process is characterized by short reaction times, mild reaction conditions and a stoichiometric use of reagents.

Enantioselective synthesis of 3-substituted 1,2-oxazinanes via organocatalytic intramolecular aza-Michael addition

A highly enantioselective intramolecular 6-exo-trig aza-Michael addition was developed to afford chiral 3-substituted 1,2-oxazinanes in high yields (up to 99% yield) and good enantioselectivities (up to 98/2 er). These reactions were enabled by a quinine-derived primary-tertiary diamine as a catalyst and pentafluoropropionic acid (PFP) as a co-catalyst.

The [3 + 3]-cycloaddition alternative for heterocycle syntheses: catalytically generated metalloenolcarbenes as dipolar adducts

The combination of two or more unsaturated structural units to form cyclic organic compounds is commonly referred to as cycloaddition, and the combination of two unsaturated structural units that forms a six-membered ring is formally either a [5 + 1]-, [4 + 2]-, [2 + 2 + 2]-, or [3 + 3]-cycloaddition. Occurring as concerted or stepwise processes, cycloaddition reactions are among the most useful synthetic constructions in organic chemistry. Of these transformations, the concerted [4 + 2]-cycloaddition, the Diels-Alder reaction, is by far the best known and most widely applied. However, although symmetry disallowed as a concerted process and lacking certifiable examples until recently, stepwise [3 + 3]-cycloadditions offer advantages for the synthesis of a substantial variety of heterocyclic compounds, and they are receiving considerable attention. In this Account, we present the development of stepwise [3 + 3]-cycloaddition reactions from virtual invisibility in the 1990s to a rapidly growing synthetic methodology today, involving organocatalysis or transition metal catalysis. With origins in organometallic or vinyliminium ion chemistry, this area has blossomed into a viable synthetic transformation for the construction of six-membered heterocyclic compounds containing one or more heteroatoms. The development of [3 + 3]-cycloaddition transformations has been achieved through identification of suitable and compatible reactive dipolar adducts and stable dipoles. The reactive dipolar species is an energetic dipolar intermediate that is optimally formed catalytically in the reaction. The stepwise process occurs with the reactive dipolar adduct reacting as an electrophile or as a nucleophile to form the first covalent bond, and this association provides entropic assistance for the construction of the second covalent bond and the overall formal [3 + 3]-cycloaddition. Organocatalysis is well developed for both inter- and intramolecular synthetic transformations, but the potential of transition metal catalysis for [3 + 3]-cycloaddition has only recently emerged. The key to the rapid development of the transition metal-based methodology has been recognition that certain catalytically generated vinylcarbenes are effective dipolar adducts for reactions with stable dipolar compounds, including aryl and vinyl ylides. In particular, metallo-enolcarbenes that are generated catalytically from conveniently prepared stable enoldiazoacetates or from donor-acceptor cyclopropenes are highly effective dipolar adducts for [3 + 3]-cycloaddition. The electron-donating oxygen of the silyl ether enhances electrophilic ring closure to the metal-bound carbon of the initial adduct from vinylogous addition, and this enhancement inhibits the alternative [3 + 2]-cycloaddition across the carbon-carbon double bond of the vinylcarbene. Catalytically generated metallo-enolcarbenes react under mild conditions with a broad spectrum of compatible stable dipoles, including nitrones, azomethine imines, ylides, and certain covalent precursors of stable dipoles, to form [3 + 3]-cycloaddition products having the β-ketoester functionality (in dihydrooxazines, tetrahydropyridazines, pyrazolidinone and pyraxole derivatives, dihydroquinolines, and quinolizidines, for example) in high yield. Two ways to access these metallo-enolcarbenes, either by dinitrogen extrusion from enoldiazoacetate esters or by rearrangement of donor-acceptor cyclopropenes, enhance the versatility of the process. The [3 + 3]-cycloaddition methodology is a complementary strategy to [4 + 2]-cycloaddition for the synthesis of heterocyclic compounds having six-membered rings. High levels of enantioselectivity are obtained with the use of chiral ligands on transition metal catalysts that include those on dirhodium(II) and silver(I).

On water: silver-catalyzed domino approach for the synthesis of benzoxazine/oxazine-fused isoquinolines and naphthyridines from o-alkynyl aldehydes

An operationally simple domino approach for the silver-catalyzed synthesis of oxazine/benzoxazine-fused isoquinolines 5a-q and naphthyridines 6a-v by the reaction of o-alkynyl aldehydes 3a-aa with amines having embedded nucleophiles 4a-d under mild reaction condition in water is described. The reaction shows selective C-N bond formation on the more electrophilic alkynyl carbon resulting in the formation of 6-endo-dig cyclized product. The competitive experiments show the viability of an intramolecular nucleophilic attack over an intermolecular attack of the external nucleophile. This methodology accommodates wide functional group variation, which proves to be useful for structural and biological assessment.

Bicyclic pyrazolidinone derivatives from diastereoselective catalytic [3 + 3]-cycloaddition reactions of enoldiazoacetates with azomethine imines

A highly regio- and diastereoselective synthesis of bicyclic pyrazolidinone derivatives by rhodium(II) acetate catalyzed [3 + 3]-annulation with enoldiazoacetates and azomethine imines has been achieved in high yield. A vinylogous reaction of the metal enol carbene with the azomethine imine initiates [3 + 3]-cycloaddition, whereas reaction at the carbene center effects N-N-cleavage of the azomethine imine.

Synthesis of tetrahydropyridazines by a metal-carbene-directed enantioselective vinylogous N-H insertion/Lewis acid-catalyzed diastereoselective Mannich addition

A versatile cascade of reactions, triggered by Rh(II)-catalyzed diazo decomposition followed by a vinylogous N-H insertion/Lewis acid catalyzed Mannich addition, that produces highly substituted 1,2,3,6-tetrahydropyridazines in up to 97 % ee with high yield and diastereocontrol has been developed.

Carbohydrate-auxiliary assisted preparation of enantiopure 1,2-oxazine derivatives and aminopolyols

An approach to enantiopure hydroxylated 2H-1,2-oxazine derivatives is presented utilizing the [3 + 3] cyclisation of lithiated alkoxyallenes and an L-erythrose-derived N-glycosyl nitrone as precursors. This key step proceeded only with moderate diastereoselectivity, but allowed entry into both enantiomeric series of the resulting 3,6-dihydro-2H-1,2-oxazines. Their enol ether double bond was then subjected to a hydroboration followed by an oxidative work-up, and finally the auxiliary was removed. The described three-step procedure enabled the synthesis of enantiopure hydroxylated 1,2-oxazines. Typical examples were treated with samarium diiodide leading to enantiopure acyclic aminopolyols. We also report on our attempts to convert these compounds into enantiopure hydroxylated pyrrolidine derivatives.

Asymmeric formal [3 + 3]-cycloaddition reactions of nitrones with electrophilic vinylcarbene intermediates

With metal carbene access to dipolar intermediates, 3,6-dihydro-1,2-oxazines are produced in high yields by dirhodium(II) carboxylate catalyzed reactions between nitrones and a β-TBSO-substituted vinyldiazoacetate. High enantiocontrol occurs with catalysis by N-phthaloyl-(S)-(amino acid)-ligated dirhodium carboxylates for [3 + 3]-cycloaddition reactions with both acyclic and cyclic nitrones.

Enantiopure aminopyrans by a Lewis acid promoted rearrangement of 1,2-oxazines: versatile building blocks for oligosaccharide and sugar amino acid mimetics

1,3-Dioxolanyl-substituted 1,2-oxazines, such as syn-1 and anti-1, rearrange under Lewis acidic conditions to provide bicyclic products 2-5. Subsequent reductive transformations afforded enantiopure 3-aminopyran derivatives such as 7 and 9 or their protected diastereomers 16 and 18, which can be regarded as carbohydrate mimetics. An alternative sequence of transformations including selective oxidation of the primary hydroxyl groups in 21 and 24 led to two protected beta-amino acid derivatives with carbohydrate-like backbone (sugar amino acids). Treatment of bicyclic ester 23 with samarium diiodide cleaved the N--O bond and furnished the unusual beta-lactam 27 in excellent yield. Alternatively, gamma-amino acid derivative 29 was efficiently prepared in a few steps. Fairly simple transformations gave azides 32 and 35 or alkyne 30 which are suitable substrates for the construction of oligosaccharide mimetics such as 34 by copper iodide catalyzed cycloadditions. With this report we demonstrate that enantiopure rearrangement products 2-5 are protected precursors of a variety of polyfunctionalized pyran derivatives with great potential for chemical biology.

Addition of lithiated methoxyallene to aziridines – a novel access to enantiopure piperidine and β-amino acid derivatives

Addition of lithiated methoxyallene to aziridine derivatives provided the expected primary addition products. The less substituted carbon of the aziridine ring was attacked selectively. The primary adducts could be converted to enantiopure piperidine derivatives or β-amino acid derivatives. The unexpected reactions lead to a tricyclic sulfonamide and to alkynyl-substituted aminoethers. The efficient two-step conversion of a piperidone derivative to a benzomorphan demonstrates the potential of this approach to biologically active compounds.

Sugars, alkaloids, and heteroaromatics: exploring heterocyclic chemistry with alkoxyallenes

As master craftsmen, modern synthetic chemists are challenged to achieve remarkable feats of efficiency and elegance toward molecular targets. The nature of this pursuit necessitates the collection of synthetic repertoires that are tried and true. With methodologies and pathways increasingly scrutinized, the adept chemist must seek out propitious tools to incorporate into the arsenal. With this in mind, this Account highlights the versatility of alkoxyallenes as precursors to valuable heterocyclic building blocks for such efforts as natural product synthesis. Accessed by the etherification of either propargyl alcohols or propargylic halides, alkoxyallenes are obtained after base-catalyzed isomerizations of the propargylic ethers. A host of umpolung synthons are available through this scheme after metalation, generating C(3) nucleophiles synthetically equivalent to vital anionic and zwitterionic synthons. Reactions with a diverse set of heteroatomic electrophiles yield carbohydrates, spiroketals, alkaloids, and heteroaromatics via [3 + 2] or [3 + 3] cyclizations. By employing lithiated alkoxyallenes into transformation routes, the natural product chemist can utilize this methodology as a viable resource in stereoselective synthesis. A survey of our own utilization of alkoxyallenes along synthetic pathways toward natural product targets reveals their suitability for generating advantageous precursors. A set of four stereoisomeric 2,6-dideoxyhexoses were stereoselectively obtained after an initial lithiated alkoxyallene and lactaldehyde cyclization, followed by the oxidative ring opening of the dihydrofurans. Through the addition of a lithiated alkoxyallene to a functionalized benzaldehyde, an essential spiroketal diastereomer was rapidly achieved in a few steps. We greatly benefitted from alkoxyallenes in the construction of complex nitrogen-containing synthetic targets, whether pyrrolidine alkaloids, substituted imidazole derivatives, or functionalized pyridines. A pinnacle example of their utility came from the coupling of alkoxyallenes to nitrones affording 1,2-oxazines, which served as a gateway to an array of novel polyfunctionalized compounds such as aminopolyols, hydroxylated pyrrolidines, or carbohydrate mimetics. Alkoxyallenes have proven themselves to be powerful C(3) building blocks toward complex molecular targets, revealing novel pathways to a variety of desirable highly functionalized heterocycles. In our view, the full extent of their synthetic utility has yet to be truly realized.