A concise stereoselective synthesis of Preussin, 3-epi-Preussin, and analogues

Intramolecular Palladium-Catalyzed Carboamination for the Stereoselective Synthesis of Five-Membered Iminosugar C-Glycosides

We report here a new method for the stereoselective synthesis of five-membered iminosugar C-glycosides using an intramolecular palladium-catalyzed carboamination. We have prepared efficiently two sugar-derived aminoalkenes, which were submitted to the carboamination conditions in the presence of different aryl bromides. A small library of protected iminosugars carrying a 1-C-arylmethyl substituent was obtained, and some of them were fully deprotected to yield original iminosugar C-glycosides. This methodology provides one of the shortest pathways to this family of molecules.

Stereoselective Synthesis of δ- and ε-Amino Ketone Derivatives from N-tert-Butanesulfinyl Aldimines and Functionalized Organolithium Compounds

The addition of functionalized organolithium compounds derived from 5-chloro-2-methoxy-1-pentene and 6-chloro-2-methoxy-1-hexene to N-tert-butanesulfinyl aldimines imines, and a subsequent hydrolysis of the enol ether moiety, yielded different δ- and ε-amino ketone derivatives, respectively, in moderate yields and diastereoselectivities. The application of these compounds in organic synthesis was demonstrated by the preparation of 2-substituted 6-methylpiperidines in a stereoselective manner, among them natural alkaloids (+)- and (−)-isosolenopsin A.

Addition of Lithium Anion of (Acetylmethylene)triphenylphosphorane to Nonracemic Sulfinimines: Total Synthesis of (+)-241D and Formal Total Synthesis of (+)-Preussin

The addition of lithium anion of (acetylmethylene)triphenylphosphorane to nonracemic sulfinimines was investigated. It was found that the addition proceeded with good diastereoselectivity and further reaction of the formed sulfinimidophosphorane with several aldehydes afforded the β-sulfinamido substituted enones in good yields. The resultant enones were elaborated to the synthesis of alkaloid (+)-241D, to the formal total synthesis of (+)-preussin, and to the synthesis of aminocyclopentenol.

Enantioselective Rhodium-Catalyzed Allylation of Aliphatic Imines: Synthesis of Chiral C-Aliphatic Homoallylic Amines

Reported herein is a method for the efficient syntheses of optically active 1-alkyl homoallylic amines in yields up to 95%, 13.5:1 dr, and 98% ee under mild, aqueous reaction conditions, via the Rh-catalyzed asymmetric allylation of aliphatic aldimines. This method provides a streamlined synthetic platform for the preparation of indolizidine and piperidine alkaloids, thus demonstrating its usefulness.

Reactions of Allylmagnesium Reagents with Carbonyl Compounds and Compounds with C═N Double Bonds: Their Diastereoselectivities Generally Cannot Be Analyzed Using the Felkin-Anh and Chelation-Control Models

This review describes the additions of allylmagnesium reagents to carbonyl compounds and to imines, focusing on the differences in reactivity between allylmagnesium halides and other Grignard reagents. In many cases, allylmagnesium reagents either react with low stereoselectivity when other Grignard reagents react with high selectivity, or allylmagnesium reagents react with the opposite stereoselectivity. This review collects hundreds of examples, discusses the origins of stereoselectivities or the lack of stereoselectivity, and evaluates why selectivity may not occur and when it will likely occur.

Enantiodivergent Approach to the Synthesis of Cis-2,6-Disubstituted Piperidin-4-ones

Enantiopure β-amino ketone derivatives were synthesized by decarboxylative Mannich reaction of chiral N- tert-butanesulfinyl imines with β-keto acids and were subsequently transformed into cis-2,6-disubstituted piperidin-4-ones through an organocatalyzed condensation with aldehydes. Both enantiomers were accessible from the same precursors by inverting the order in the reaction sequence of the aldehydes involved in the imine formation and the intramolecular Mannich condensation. The synthesis of the piperidine alkaloids (+)-241D, (-)-epimyrtine, and (-)-lasubine II demonstrated the utility of this methodology.

Alkoxyallene-based syntheses of preussin and its analogs and their cytotoxicity

Short syntheses of oxa-preussin, racemic preussin and (-)-preussin are reported. Starting from a racemic 3-nonyl-substituted methoxyallene derivative, its lithiation and addition to phenylethanal provided the corresponding allenyl alcohol that was converted into two diastereomeric dihydrofuran derivatives by silver nitrate-catalyzed 5-endo-trig cyclization. The acid hydrolysis of the enol ether moiety gave heterocyclic ketones and subsequent highly stereoselective reductions with l-selectride furnished 2-benzyl-5-nonylfuran-3-ol derivatives in good overall yield. The major all-cis-diastereomer has the skeleton and relative configuration of preussin and is hence called oxa-preussin. An analogous sequence with the same allene, but an N-sulfonyl imine as the electrophile, finally led to racemic preussin. The stereoselectivities of the individual steps are discussed in detail. With an enantiopure 2-benzyl-5-nonylpyrrolidin-3-one intermediate the preparation of (-)-preussin with an enantiomeric ratio of >95 : 5 could be accomplished in a few steps. The sign of the optical rotation of this product finally proved the absolute configurations of its precursors and demonstrated that our chiral auxiliary-based route led to the antipode of the natural product. The cytotoxicity of several of the prepared heterocycles against MCF-7 tumor cells was investigated and five compounds, including racemic and enantiopure (-)-preussin, were identified as highly cytotoxic with IC50 values in the range of 3-6 μM.

Preussins with Inhibition of IL-6 Expression from Aspergillus flocculosus 16D-1, a Fungus Isolated from the Marine Sponge Phakellia fusca

New pyrrolidine alkaloids, preussins C-I (1-7) and (11 R)/(11 S)-preussins J and K (8 and 9), were isolated from the sponge-derived fungus Aspergillus flocculosus 16D-1. The structures and configurations of these preussins were elucidated by detailed spectroscopic analysis, modified Mosher's method, and comparisons with literature data. These compounds showed strong to moderate inhibitory activity toward IL-6 production in lipopolysaccharide-induced THP-1 cells with IC₅₀ values ranging from 0.11 to 22 μM, but were inactive against normal tumor cell lines and fungi.

Stereoselective Coupling of N-tert-Butanesulfinyl Aldimines and β-Keto Acids: Access to β-Amino Ketones

The reaction of chiral N-tert-butanesulfinyl aldimines with β-keto acids under basic conditions at room temperature proceeds with high levels of diastereocontrol, leading to β-amino ketones in high yields. Based on DFT calculations, an eight-membered cyclic transition state involving coordination of the lithium atom to the oxygens of carboxylate and sulfinyl units was proposed, being in agreement with the observed experimental diastereomeric ratios. The synthesis of the piperidine alkaloid (-)-pelletierine was successfully undertaken in order to demonstrate the utility of this methodology.

An efficient approach to trans-4-hydroxy-5-substituted 2-pyrrolidinones through a stereoselective tandem Barbier process: divergent syntheses of (3R,4S)-statines, (+)-preussin and (−)-hapalosin

Natural products of (+)-preussin and (−)-hapalosin have been asymmetrically synthesized through the stereoselective tandem Barbier process.

Isolation and biosynthesis of preussin B, a pyrrolidine alkaloid from Simplicillium lanosoniveum

A new pyrrolidine alkaloid, preussin B (1), was isolated from the culture extract of the fungus Simplicillium lanosoniveum TAMA 173 along with the known congener preussin (2). The structure and absolute configuration of 1 were determined by spectroscopic analysis and spectral comparison with 2. Feeding experiments with 13C-labeled precursors revealed that the pyrrolidine ring of 1 was assembled from acetate and l-phenylalanine by a PKS-NRPS hybrid biosynthetic pathway.

Base-dependent stereodivergent intramolecular aza-Michael reaction: asymmetric synthesis of 1,3-disubstituted isoindolines

The nucleophilic addition (A(N)) / intramolecular aza-Michael reaction (IMAMR) process on Ellman's tert-butylsulfinyl imines, bearing a Michael acceptor in the ortho position, is studied. This reaction affords 1,3-disubstituted isoindolines with a wide range of substituents in good yields and diastereoselectivities. Interestingly, careful choice of the base for the aza-Michael step allows either the cis or the trans diastereoisomers to be exclusively obtained. This stereodivergent cyclization has enabled the synthesis of C2-symmetric bisacetate-substituted isoindolines. In addition, bisacetate isoindolines bearing two well-differentiated ester moieties are also noteworthy because they may allow for the orthogonal synthesis of β,β'-dipeptides using a single nitrogen atom as a linchpin.

A concise and stereoselective synthesis of hydroxypyrrolidines: rapid synthesis of (+)-preussin

A convergent and stereoselective synthesis of 2,5-disubstituted 3-hydroxypyrrolidines has been developed that involves reductive annulation of β-iminochlorohydrins, which are readily available from β-ketochlorohydrins, and provides rapid access to a variety of 2,5-syn-pyrrolidines. Application of this process to the concise (three-step) synthesis of the fungal metabolite (+)-preussin and analogues of this substance is reported.

Asymmetric palladium-catalyzed carboamination reactions for the synthesis of enantiomerically enriched 2-(arylmethyl)- and 2-(alkenylmethyl)pyrrolidines

The enantioselective synthesis of 2-(arylmethyl)- and 2-(alkenylmethyl)pyrrolidine derivatives via Pd-catalyzed alkene carboamination reactions is described. These transformations generate enantiomerically enriched products with up to 94% ee from readily available alkenyl or aryl bromides and N-boc-pent-4-enylamines. The application of this method to a concise asymmetric synthesis of (-)-tylophorine is also discussed.

Pd-catalyzed carboamination of oxazolidin-2-ones: a stereoselective route to trans-2,5-disubstituted pyrrolidines

Palladium-catalyzed carboamination reactions between aryl bromides and 4-(but-3-enyl)-substituted oxazolidin-2-ones are described. These transformations afford bicyclic oxazolidin-2-one derivatives that can be converted to trans-2,5-disubstituted pyrrolidines in one step. The starting materials are easily prepared from amino acid precursors, and products that contain up to three stereocenters are generated with >20:1 dr.

Use of aryl chlorides as electrophiles in Pd-catalyzed alkene difunctionalization reactions

The development of conditions that allow use of inexpensive aryl chlorides as electrophiles in Pd-catalyzed alkene carboamination and carboetherification reactions is described. A catalyst composed of Pd(OAc)(2) and S-Phos minimizes N-arylation of the substrate and prevents formation of mixtures of regioisomeric products. A number of heterocycles, including pyrrolidines, isoxazolidines, tetrahydrofurans, and pyrazolidines, are efficiently generated with this method.

Palladium-catalyzed alkene carboamination reactions for the synthesis of substituted piperazines

A strategy for the stereoselective preparation of enantiomerically enriched cis-2,6-disubstituted piperazines from amino acid precursors is described. The target compounds are generated in 95-99% ee with good to excellent levels of diastereoselectivity (usually 14:1 to >20:1) using Pd-catalyzed carboamination reactions between aryl or alkenyl halides and substituted ethylenediamine derivatives to form the heterocyclic rings. The synthesis requires only 4-5 steps from commercially available amino acids, and allows for the modular construction of piperazines bearing different substituents at N(1), N(4), C(2), and C(6). The use of this strategy for the construction of 2,3-disubstituted piperazines, fused bicyclic piperazines, and tetrahydroquinoxalines is also reported. In addition, the mechanism of the key carboamination reactions are discussed, and new models that predict and explain the stereochemical outcome of these transformations are presented.

Influence of hydroxylamine conformation on stereocontrol in Pd-catalyzed isoxazolidine-forming reactions

Palladium-catalyzed carboamination reactions between N-Boc-O-(but-3-enyl)hydroxylamine derivatives and aryl or alkenyl bromides afford cis-3,5- and trans-4,5-disubstituted isoxazolidines in good yield with up to >20:1 dr. The diastereoselectivity observed in the formation of cis-3,5-disubstituted isoxazolidines is superior to selectivities typically obtained in other transformations, such as 1,3-dipolar cycloaddition reactions, that provide these products. In addition, the stereocontrol in the C-N bond-forming Pd-catalyzed carboamination reactions of N-Boc-O-(but-3-enyl)hydroxylamines is significantly higher than that of related C-O bond-forming carboetherification reactions of N-benzyl-N-(but-3-enyl)hydroxylamine derivatives. This is likely due to a stereoelectronic preference for cyclization via transition states in which the Boc group is placed in a perpendicular orientation relative to the plane of the developing ring, which derives from the conformational equilibria of substituted hydroxylamines.

Mild conditions for Pd-catalyzed carboamination of N-protected hex-4-enylamines and 1-, 3-, and 4-substituted pent-4-enylamines. Scope, limitations, and mechanism of pyrrolidine formation

The use of the weak base Cs2CO3 in Pd-catalyzed carboamination reactions of N-protected gamma-aminoalkenes with aryl bromides leads to greatly increased tolerance of functional groups and alkene substitution. Substrates derived from (E)- or (Z)-hex-4-enylamines are stereospecifically converted to 2,1'-disubstituted pyrrolidine products that result from suprafacial addition of the nitrogen atom and the aryl group across the alkene. Transformations of 4-substituted pent-4-enylamine derivatives proceed in high yield to afford 2,2-disubstituted products, and cis-2,5- or trans-2,3-disubstituted pyrrolidines are generated in good yield with excellent diastereoselectivity from N-protected pent-4-enylamines bearing substituents at C1 or C3. The reactions tolerate a broad array of functional groups, including esters, nitro groups, and enolizable ketones. The scope and limitations of these transformations are described in detail, along with models that account for the observed product stereochemistry. In addition, deuterium labeling experiments, which indicate these reactions proceed via syn-aminopalladation of intermediate palladium(aryl)(amido)complexes regardless of degree of alkene substitution or reaction conditions, are also discussed.

Stereoselective synthesis of imidazolidin-2-ones via Pd-catalyzed alkene carboamination. Scope and limitations

A method for the synthesis of imidazolidin-2-ones from N-allylureas and aryl or alkenyl bromides via Pd-catalyzed carboamination reactions is described. The N-allylurea precursors are prepared in one step from readily available allylic amines and isocyanates, and the Pd-catalyzed reactions effect the formation of a C-C bond, a C-N bond, and up to two stereocenters in a single step. Good diastereoselectivities are obtained for the conversion of substrates bearing allylic substituents to 4,5-disubstituted imidazolidin-2-ones, and excellent selectivity for the generation of products resulting from syn-addition across the alkene is observed when substrates derived from cyclic alkenes or E-1,2-disubstituted alkenes are employed. A brief discussion of reaction mechanism and product stereochemistry is presented.

A concise asymmetric synthesis of cis-2,6-disubstituted N-aryl piperazines via Pd-catalyzed carboamination reactions

A concise, modular, asymmetric synthesis of cis-2,6-disubstituted piperazines from readily available amino acid precursors is described. The key step in the synthesis is a Pd-catalyzed carboamination of a N1-aryl-N2-allyl-1,2-diamine with an aryl bromide. The products are obtained in 14-20:1 dr, with >97% ee, and the key cyclizations are the first examples of six-membered ring formation via Pd-catalyzed carboamination reactions of unsaturated amines with aryl halides.

Palladium-catalyzed synthesis of cyclopentane-fused benzocyclobutenes via tandem directed carbopalladation/C-H bond functionalization

A new Pd-catalyzed reaction for the stereoselective synthesis of cyclopentane-fused benzocyclobutenes is described. These transformations likely proceed via carbamate-directed carbopalladation followed by intramolecular C-H activation of an alkylpalladium intermediate. The mechanistic relationship between these transformations and Pd-catalyzed reactions of gamma-(n-Boc-amino)alkenes with aryl bromides that afford pyrrolidines is discussed. Differences in reactivity between Pd-amino and Pd-amido complexes appear to play a key role in the outcome of these transformations.

Pyrrolidine and piperidine formation via copper(II) carboxylate-promoted intramolecular carboamination of unactivated olefins: diastereoselectivity and mechanism

An expanded substrate scope and in-depth analysis of the reaction mechanism of the copper(II) carboxylate-promoted intramolecular carboamination of unactivated alkenes is described. This method provides access to N-functionalized pyrrolidines and piperidines. Both aromatic and aliphatic gamma- and delta-alkenyl N-arylsulfonamides undergo the oxidative cyclization reaction efficiently. N-Benzoyl-2-allylaniline also underwent the oxidative cyclization. The terminal olefin substrates examined were more reactive than those with internal olefins, and the latter terminated in elimination rather than carbon-carbon bond formation. The efficiency of the reaction was enhanced by the use of more organic soluble copper(II) carboxylate salts, copper(II) neodecanoate in particular. The reaction times were reduced by the use of microwave heating. High levels of diastereoselectivity were observed in the synthesis of 2,5-disubstituted pyrrolidines, wherein the cis substitution pattern predominates. The mechanism of the reaction is discussed in the context of the observed reactivity and in comparison to analogous reactions promoted by other reagents and conditions. Our evidence supports a mechanism wherein the N-C bond is formed via intramolecular syn aminocupration and the C-C bond is formed via intramolecular addition of a primary carbon radical to an aromatic ring.

Mild conditions for the synthesis of functionalized pyrrolidines via Pd-catalyzed carboamination reactions

[reaction: see text] The palladium-catalyzed carboamination of N-protected gamma-aminoalkenes with aryl bromides and triflates has been achieved under new, mild reaction conditions using the weak base Cs(2)CO(3) in dioxane solvent. These reactions tolerate a wide variety of functional groups, including enolizable ketones, nitro groups, methyl esters, and acetates, which are not compatible with previously described conditions.

Palladium-Catalyzed Carboetherification and Carboamination Reactions of γ-Hydroxy- and γ-Aminoalkenes for the Synthesis of Tetrahydrofurans and Pyrrolidines

Substituted tetrahydrofuran and pyrrolidine moieties are displayed in a wide range of interesting biologically active molecules. The Pd-catalyzed carboetherification or carboamination of γ-hydroxy- and γ-aminoalkenes is a powerful tool for the construction of these heterocycles, as it is convergent and can allow access to a variety of analogs from a single γ-hydroxy- or γ-aminoalkene starting material. This microreview describes the current state of this field.

Two palladium-catalyzed domino reactions from one set of substrates/reagents: efficient synthesis of substituted indenes and cis-stilbenoid hydrocarbons from the same internal alkynes and hindered Grignard reagents

Two types of domino reactions from the same internal alkynes and hindered Grignard reagents based on carbopalladation, Pd-catalyzed cross-coupling reaction, and a C-H activation strategy are described. The realization of these domino reactions relied on the control of the use of the ligand and the reaction temperature. Our study provides efficient access to useful polysubstituted indenes and cis-substituted stilbenes and may offer a new means of development of tandem/domino reactions in a more efficient way. [reaction: see text].

Intramolecular aminopalladation of alkenes as a key step to pyrrolidines and related heterocycles

Palladium catalysis for the intramolecular amination of alkenes is a powerful approach in heterocycle synthesis. The initial common step in this approach consists of an aminopalladation reaction. This tutorial review describes the synthesis of 2-amino alkyl-palladium compounds and renders special attention on the subsequent manipulation of the alkyl-palladium group. By carefully choosing the appropriate conditions, a wide variety of different heterocyclic structures are accessible which arise from reactions such as hydroamination, aza-Heck coupling, aminocarbonylation or oxidative processes such as aza-Wacker reaction and 1,2-difunctionalization.