Showing papers on "Nucleophile published in 2005"

Organic Reactions in Aqueous Media with a Focus on Carbon−Carbon Bond Formations: A Decade Update

Abstract: 4.2.8. Reductive Coupling 3109 5. Reaction of Aromatic Compounds 3110 5.1. Electrophilic Substitutions 3110 5.2. Radical Substitution 3111 5.3. Oxidative Coupling 3111 5.4. Photochemical Reactions 3111 6. Reaction of Carbonyl Compounds 3111 6.1. Nucleophilic Additions 3111 6.1.1. Allylation 3111 6.1.2. Propargylation 3120 6.1.3. Benzylation 3121 6.1.4. Arylation/Vinylation 3121 6.1.5. Alkynylation 3121 6.1.6. Alkylation 3121 6.1.7. Reformatsky-Type Reaction 3122 6.1.8. Direct Aldol Reaction 3122 6.1.9. Mukaiyama Aldol Reaction 3124 6.1.10. Hydrogen Cyanide Addition 3125 6.2. Pinacol Coupling 3126 6.3. Wittig Reactions 3126 7. Reaction of R,â-Unsaturated Carbonyl Compounds 3127

Enantioselective organo-cascade catalysis.

Abstract: A new strategy for organocatalysis based on the biochemical blueprints of biosynthesis has enabled a new laboratory approach to cascade catalysis. Imidazolidinone-based catalytic cycles, involving iminium and enamine activation, have been successfully combined to allow a large diversity of nucleophiles (furans, thiophenes, indoles, butenolides, hydride sources, tertiary amino lactone equivalents) and electrophiles (fluorinating and chlorinating reagents) to undergo sequential addition with a wide array of α,β-unsaturated aldehydes. These new cascade catalysis protocols allow the invention of enantioselective transformations that were previously unknown, including the asymmetric catalytic addition of the elements of HF across a trisubstituted olefin. Importantly, these domino catalysis protocols can be mediated by a single imidazolidinone catalyst or using cycle-specific amine catalysts. In the latter case, cascade catalysis pathways can be readily modulated to provide a required diastereo- and enantiosele...

Selectfluor: mechanistic insight and applications.

Abstract: The replacement of hydrogen atoms with fluorine substituents in organic substrates is of great interest in synthetic chemistry because of the strong electronegativity of fluorine and relatively small steric footprint of fluorine atoms. Many sources of nucleophilic fluorine are available for the derivatization of organic molecules under acidic, basic, and neutral conditions. However, electrophilic fluorination has historically required molecular fluorine, whose notorious toxicity and explosive tendencies limit its application in research. The necessity for an electrophilic fluorination reagent that is safe, stable, highly reactive, and amenable to industrial production as an alternative to very hazardous molecular fluorine was the inspiration for the discovery of selectfluor. This reagent is not only one of the most reactive electrophilic fluorinating reagents available, but it is also safe, nontoxic, and easy to handle. In this Review we document the many applications of selectfluor and discuss possible mechanistic pathways for its reaction.

Gold(I)-Catalyzed Intramolecular Acetylenic Schmidt Reaction

Abstract: Substituted pyrroles were prepared by a gold(I)-catalyzed acetylenic Schmidt reaction of homopropargyl azides. The reaction allows for regiospecific substitution at each position of the pyrrole ring under mild conditions. A mechanism in which azides serve as nucleophiles toward gold(I)-activated alkynes with subsequent gold(I)-aided expulsion of dinitrogen is proposed.

Quaternary Stereocenters: Challenges and Solutions for Organic Synthesis

Abstract: Foreward. Preface. List of Contributors. Symbols and Abbreviations. 1 Important Natural Products (Hirokazu Arimoto and Daisuke Uemura). 1.1 Introduction. 1.2 Alkylation of Tertiary Carbon Centers. 1.3 Cycloaddition to Alkenes. 1.4 Rearrangement Reactions. 1.5 Carbometallation Reactions. 1.6 C-H Functionalization Reactions. 1.7 Asymmetric Modification of Enantiotopic/Diastereotopic Substituents of Quaternary Carbon Centers. 1.8 Summary. 2 Important Pharmaceuticals and Intermediates (Johannes G. de Vries). 2.1 The Chirality of Drugs and Agrochemicals. 2.2 Steroids. 2.3 Pharmaceuticals and Agrochemicals Based on a-Dialkylated Amino Acids. 2.4 Azole Antimycotics. 2.5 Alkaloids. 2.6 HIV Inhibitors. 2.7 &beta -Lactam Antibiotics. 2.8 The Tetracyclines. 2.9 Summary and Outlook. 3 Aldol Reactions (Bernd Schetter and Rainer Mahrwald). 3.1 Introduction. 3.2 Metal Enolates. 3.3 Catalytic Aldol Additions. 3.4 Conclusions. 3.5 Note Added in Proof 79 4 Michael Reactions and Conjugate Additions (Angelika Baro and Jens Christoffers). 4.1 Introduction. 4.2 Chiral Bronstedt Bases. 4.3 Chiral Metal Complexes. 4.4 Chiral Auxiliaries. 5 Rearrangement Reactions (Annett Pollex and Martin Hiersemann). 5.1 Introduction. 5.2 Applications. 5.3 Summary. 6 Cycloaddition Reactions (Giovanni Desimoni and Givseppe Faita). 6.1 Introduction. 6.2 [2+1] Cycloaddition Reactions. 6.3 [2+2] Cycloaddition Reactions. 6.4 1,3-Dipolar Cycloaddition Reactions. 6.5 Diels-Alder Reactions. 6.6 Hetero-Diels-Alder Reactions. 6.7 Consecutive Cycloaddition Reactions. 7 Asymmetric Cross-coupling and Mizoroki-Heck Reactions (Louis Barriault and Effiette L. O. Sauer). 7.1 The Asymmetric Heck Reaction. 7.2 Metal-catalyzed Cross-coupling Reactions. 7.3 Summary. 8 Alkylation of Ketones and Imines (Diego J. Ramon and Miguel Yus). 8.1 Introduction. 8.2 Diastereoselective Additions. 8.3 Enantioselective Additions by Modulated Processes. 8.4 Enantioselective Additions by Promoted Processes. 9 Asymmetric Allylic Alkylation (Manfred Braun). 9.1 Introduction. 9.2 Electrophilic Allylic Alkylation. 9.3 Nucleophilic Allylic Alkylation. 9.4 Miscellaneous Methods. 9.5 Outlook. 10 Phase-Transfer Catalysis (Takashi Ooi and Keiji Maruoka). 10.1 Introduction. 10.2 Carbon-Carbon Bond Formation Through PTC. 10.3 Carbon-Heteroatom Bond Formation Through PTC. 10.4 Conclusion. 11 Radical Reactions (Kalyani Patil and Mukund P. Sibi). 11.1 Introduction. 11.2 Radical Cyclization. 11.3 Atom- and Group-transfer Cyclizations. 11.4 Intermolecular Radical Allylations. 11.5 Other Metallic Reagents. 11.6 Radical Reactions in the Solid State. 11.7 Conclusion. 11.8 Experimental. 12 Enzymatic Methods (Uwe T. Bornscheuer, Erik Henke, and Jurgen Pleiss). 12.1 Introduction. 12.2 Strategies for the Kinetic Resolution of Sterically Demanding Substrates. 12.3 Conclusion. Index.

Catalytic, Enantioselective, Vinylogous Aldol Reactions

Abstract: In 1935, R. C. Fuson formulated the principle of vinylogy to explain how the influence of a functional group may be felt at a distant point in the molecule when this position is connected by conjugated double-bond linkages to the group. In polar reactions, this concept allows the extension of the electrophilic or nucleophilic character of a functional group through the pi system of a carbon-carbon double bond. This vinylogous extension has been applied to the aldol reaction by employing "extended" dienol ethers derived from gamma-enolizable alpha,beta-unsaturated carbonyl compounds. Since 1994, several methods for the catalytic, enantioselective, vinylogous aldol reaction have appeared, with which varying degrees of regio- (site), enantio-, and diastereoselectivity can be attained. In this Review, the current scope and limitations of this transformation, as well as its application in natural product synthesis, are discussed.

Asymmetric multicomponent domino reactions and highly enantioselective conjugated addition of thiols to alpha,beta-unsaturated aldehydes.

Abstract: An organocatalytic asymmetric multicomponent domino and a conjugated addition reaction to α,β-unsaturated aldehydes are presented. The development is based, first, on an organocatalyzed highly enantioselective nucleophilic thiol addition to the β-carbon atom in the iminium ion intermediate, followed by an electrophilic amination of the α-carbon atom to the enamine intermediate. The multicomponent reactions proceed to give enantiopure amino−thiols in moderate to good yields. Furthermore, the organocatalyzed thiol addition to α,β-unsaturated aldehydes takes place in good yields and excellent enantioselectivities.

Palladium-catalyzed ring-forming aminoacetoxylation of alkenes.

Abstract: A mild, palladium(II)-catalyzed ring-forming aminoacetoxylation of alkenes is described. Treatment of a range of nitrogen nucleophiles with catalytic palladium(II) in the presence of PhI(OAc)2 as oxidant resulted in alkene aminoacetoxylation, affording a variety of nitrogen-containing heterocycles. Our studies indicate the possibility for high levels of reaction regio- and stereocontrol. It appears that this is a stereoselective trans alkene difunctionalization and thus a useful alternative to related cis-selective, metal-catalyzed alkene aminohydroxylation processes.

Thiourea-catalyzed enantioselective cyanosilylation of ketones.

Abstract: The new chiral amino thiourea catalyst 3d promotes the highly enantioselective cyanosilylation of a wide variety of ketones. The hindered tertiary amine substituent plays a crucial role with regard to both stereoinduction and reactivity, suggesting a cooperative mechanism involving electrophile activation by thiourea and nucleophile activation by the amine.

Synthesis of Highly Substituted Furans by the Electrophile-Induced Coupling of 2-(1-Alkynyl)-2-alken-1-ones and Nucleophiles

Abstract: [reaction: see text] The coupling of 2-(1-alkynyl)-2-alken-1-ones with nucleophiles, either catalyzed by AuCl3 or induced by an electrophile, provides highly substituted furans in good to excellent yields under very mild reaction conditions Various nucleophiles, including functionally substituted alcohols, H2O, carboxylic acids, 1,3-diketones, and electron-rich arenes, and a range of cyclic and acyclic 2-(1-alkynyl)-2-alken-1-ones readily participate in these cyclizations Iodine, NIS, and PhSeCl have proven successful as electrophiles in this process The resulting iodine-containing furans can be readily elaborated to more complex products using known organopalladium chemistry

Brønsted acid-catalyzed imine amidation.

Abstract: A new method for the Bronsted acid-catalyzed addition of amide nucleophiles to imines to produce protected aminal products is described. Simple Bronsted acids (phenyl phosphinic acid and trifluoromethanesulfonimide) were shown to be excellent catalysts, providing high yields of the aminal product. A catalytic asymmetric imine amidation using sulfonamides as nucleophiles was successful when a hindered biaryl phosphoric acid catalyst derived from 2,2‘-diphenyl-[3,3‘-biphenanthrene]-4,4‘-diol (VAPOL) was used. Excellent yields and enantioselectivities were found in these additions (up to 99% ee).

Palladium‐Catalyzed Vinylation of Organic Halides

Abstract: The palladium-catalyzed vinylation of organic halides provides a very convenient method for forming carboncarbon bonds at unsubstituted vinylic positions. Generally the reaction does not require anhydrous or anaerobic conditions although it is advisable to limit access of oxygen when arylphosphines are used as a component of the catalyst. The transformation is valuable because it cannot be carried out in a single step by any other known method (except in certain Meerwein reactions). The organic halide employed is limited to aryl, heterocyclic, benzyl, or vinyl types, with bromides and iodides seen most often. Halides with an easily eliminated beta-hydrogen atom (i.e., alkyl derivatives) cannot be used since they form only olefins by elimination under the normal reaction conditions. The base needed may be a secondary or tertiary amine, sodium or potassium acetate, carbonate, or bicarbonate. When nucleophilic secondary amines are used as coreactants with most vinylic halides, a variation occurs that often produces tertiary allylic amines as major products. The catalyst is commonly palladium acetate, although palladium chloride or preformed triarylphosphine palladium complexes, as well as palladium on charcoal, have been used. A reactant, product, or solvent may serve as the ligand in reactions involving organic iodides, but generally a triarylphosphine or a secondary amine is required when organic bromides are used. The reaction, which occurs between ca. 50° and 160° proceeds homogeneously. Solvents such as acetonitrile, dimethylformamide, hexamethylphosphoramide, N-methylpyrrolidinone, and methanol have been used, but are often not necessary. The procedure is applicable to a very wide range of reactants and yields are generally good to excellent. Several variations of the reaction are known in which the organic halide is replaced by other reagents such as organometallics, diazonium salts, or aromatic hydrocarbons. These reactions are not discussed in detail, but are only briefly compared with the halide reaction. Other related reactions such as the palladium-catalyzed replacement of allylic substituents with carbanionic reagents, the palladium-promoted nucleophilic substitutions at olefinic carbons, and the numerous palladium-catalyzed coupling reactions of halides and organometallics are also beyond the scope of this review. The palladium-catalyzed vinylic substitution reaction has not yet received much attention from organic chemists, but its broad scope and simplicity demonstrate that it is a useful method for the synthesis of a variety of olefinic compounds. Keywords: palladium catalyst; vinylation; organic halides; scope; limitations; experimental procedures; vinylic substitution; ethylene; acrylic acid; butenol; methyl acrylate; styrene; pentadiene; acrolein dimethyl acetal; olefins

The DMAP-catalyzed acetylation of alcohols- : A mechanistic study (DMAP = 4-(dimethylamino)pyridine)

Abstract: The acetylation of tert-butanol with acetic anhydride catalyzed by 4-(dimethylamino)pyridine (DMAP) has been studied at the Becke3 LYP/6-311 + G(d,p)//Becke3 LYP/6-31G(d) level of theory. Solvent effects have been estimated through single-point calculations with the PCM/UAHF solvation model. The energetically most favorable pathway proceeds through nucleophilic attack of DMAP at the anhydride carbonyl group and subsequent formation of the corresponding acetylpyridinium/acetate ion pair. Reaction of this ion pair with the alcohol substrate yields the final product, tert-butylacetate. The competing base-catalyzed reaction pathway can either proceed in a concerted or in a stepwise manner. In both cases the reaction barrier far exceeds that of the nucleophilic catalysis mechanism. The reaction mechanism has also been studied experimentally in dichloromethane through analysis of the reaction kinetics for the acetylation of cyclohexanol with acetic anhydride, in the presence of DMAP as catalyst and triethylamine as the auxiliary base. The reaction is found to be first-order with respect to acetic anhydride, cyclohexanol, and DMAP, and zero-order with respect to triethyl amine. Both the theoretical as well as the experimental studies strongly support the nucleophilic catalysis pathway.

Enantioselective Friedel−Crafts Alkylations of α,β-Unsaturated 2-Acyl Imidazoles Catalyzed by Bis(oxazolinyl)pyridine−Scandium(III) Triflate Complexes

Abstract: An enantioselective Friedel−Crafts alkylation with α,β-unsaturated 2-acyl imidazoles and electron-rich aromatic nucleophiles catalyzed by bis(oxazolinyl)pyridine−scandium(III) triflate complexes has been accomplished. These α,β-unsaturated 2-acyl imidazoles are effective electrophiles for the Friedel−Crafts reaction. The resulting adduct 2-acyl imidazole is easily converted to amides, esters, carboxylic acids, ketones, and aldehydes by methylation and subsequent displacement of the imidazole residue.

Kinetics of electrophile-nucleophile combinations: A general approach to polar organic reactivity

Abstract: Benzhydrylium ions (Ar 2 CH + ) and structurally related quinone methides are em- ployed as reference electrophiles for comparing the nucleophilicities of a large variety of compounds, e.g., alkenes, arenes, alkynes, allylsilanes, allylstannanes, enol ethers, enamines, diazo compounds, carbanions, transition-metal π-complexes, hydride donors, phosphanes, amines, alkoxides, etc., using the correlation equation log k (20 °C) = s(N + E), where s and N are nucleophile-dependent parameters and E is an electrophilicity parameter. The same equation was employed to derive the electrophilicity parameter E for different types of car- bocations, cationic transition-metal π-complexes, typical Michael acceptors, and electron-de- ficient arenes. The E, N, and s parameters thus obtained can be used for predicting rates and selectivities of polar organic reactions.

Oxidative cyclizations in a nonpolar solvent using molecular oxygen and studies on the stereochemistry of oxypalladation.

Abstract: Oxidative cyclizations of a variety of heteroatom nucleophiles onto unactivated olefins are catalyzed by palladium(II) and pyridine in the presence of molecular oxygen as the sole stoichiometric oxidant in a nonpolar solvent (toluene). Reactivity studies of a number of N-ligated palladium complexes show that chelating ligands slow the reaction. Nearly identical conditions are applicable to five different types of nucleophiles: phenols, primary alcohols, carboxylic acids, a vinylogous acid, and amides. Electron-rich phenols are excellent substrates, and multiple olefin substitution patterns are tolerated. Primary alcohols undergo oxidative cyclization without significant oxidation to the aldehyde, a fact that illustrates the range of reactivity available from various Pd(II) salts under differing conditions. Alcohols can form both fused and spirocyclic ring systems, depending on the position of the olefin relative to the tethered alcohol; the same is true of the acid derivatives. The racemic conditions served as a platform for the development of an enantioselective reaction. Experiments with stereospecifically deuterated primary alcohol substrates rule out a “Wacker-type” mechanism involving anti oxypalladation and suggest that the reaction proceeds by syn oxypalladation for both mono- and bidentate ligands. In contrast, cyclizations of deuterium-labeled carboxylic acid substrates undergo anti oxypalladation.

Rhenium-catalyzed formation of indene frameworks via C-H bond activation: [3+2] Annulation of aromatic aldimines and acetylenes

Abstract: A rhenium complex, [ReBr(CO)3(thf)]2, catalyzes the reaction of an aromatic aldimine with an acetylene to give an indene derivative in a quantitative yield. The reaction proceeds via C-H bond activation, insertion of the acetylene, intramolecular nucleophilic cyclization, and reductive elimination. In contrast to ruthenium and rhodium catalysts, which are usually employed in this type of reaction, the rhenium catalyst promotes the intramolecular nucleophilic cyclization of the alkenylmetal species generated by insertion of the acetylene.

Copolymerization of cyclohexene oxide with CO2 by using intramolecular dinuclear zinc catalysts.

Abstract: The intramolecular dinuclear zinc complexes generated in situ from the reaction of multidentate semi-azacrown ether ligands with Et2Zn, followed by treatment with an alcohol additive, were found to promote the copolymerization of CO2 and cyclohexene oxide (CHO) with completely alternating polycarbonate selectivity and high efficiency. With this type of novel initiator, the copolymerization could be accomplished under mild conditions at 1 atm pressure of CO2, which represents a significant advantage over most catalytic systems developed for this reaction so tar. The copolymerization reaction was demonstrated to be a living process as a result of the narrow polydispersities and the linear increase in the molecular weight with conversion of CHO. In addition, the solid-state structure of the dinuclear zinc complex was characterized by X-ray crystal structural analysis and can be considered as a model of the active catalyst. On the basis of the various efforts made to understand the mechanisms of the catalytic reaction, including MALDI-TOF mass analysis of the copolymers' end-groups. the effect of alcohol additives on the catalysis and CO2 pressure on the conversion of CHO, as well as the kinetic data gained from in situ IR spectroscopy. a plausible catalytic cycle for the present reaction system is outlined. The copolymerization is initiated by the insertion of CO2 into the Zn OEt bond to afford a carbonate ester-bridged complex. The dinuclear zinc structure of the catalyst remains intact throughout the copolymerization. The bridged zinc centers may have a synergistic effect on the copolymerization reaction: one zinc center could activate the epoxide through its coordination and the second zinc atom may be responsible for carbonate propagation by nucleophilic attack by the carbonate ester on the back side of the cis-epoxide ring to afford the carbonate. The mechanistic implication of this is particularly important for future research into the design of efficient and practical catalysts for the copolymerization of epoxides with CO2.

On the formation of aliphatic polycarbonates from epoxides with chromium(III) and aluminum(III) metal-salen complexes.

Abstract: A DFT-based description is given of the CO2/epoxide copolymerization with a catalyst system consisting of metal (chromium, iron, titanium, aluminum)-salen complexes (salen = N,N'-bis(3,5-di-tert-butylsalicyliden-1,6-diaminophenyl) in combination with either chloride, acetate, or dimethylamino pyridine (DMAP) as external nucleophile. Calculations indicate that initiation proceeds through nucleophilic attack at a metal-coordinated epoxide, and the most likely propagation reaction is a bimolecular process in which a metal-bound nucleophile attacks a metal-bound epoxide. Carbon dioxide insertion occurs at a single metal center and is most likely the rate-determining step at low pressure. The prevalent chain terminating/degradation-the so-called backbiting, a reaction leading to formation of cyclic carbonate from the polymer chain-would involve attack of a carbonate nucleophile rather than an alkoxide at the last unit of the growing chain. The backbiting of a free carbonato chain end is particularly efficient. Anion dissociation from six-coordinate aluminum is appreciably easier than from chromium-salen complexes, indicating the reason why in the former case cyclic carbonate is the sole product. Experimental data were gathered for a series of chromium-, aluminum-, iron-, and zinc-salen complexes, which were used in combination with external nucleophiles like DMAP and mainly (tetraalkyl ammonium) chloride/acetate. Aluminum complexes transform PO (propylene oxide) and CO2 to give exclusively propylene carbonate. This is explained by rapid carbonate anion dissociation from a six-coordinate complex and cyclic formation. CO2 insertion or nucleophilic attack of an external nucleophile at a coordinated epoxide (at higher CO2 pressure) are the rate-determining steps. Catalysis with [Cr(salen)(acetate/chloride)] complexes leads to the formation of both cyclic carbonate and polypropylene carbonate with various quantities of ether linkages. The dependence of the activity and selectivity on the CO2 pressure, added nucleophile, reaction temperature, and catalyst concentration is complex. A mechanistic description for the chromium-salen catalysis is proposed comprising a multistep and multicenter reaction cycle. PO and CO2 were also treated with mixtures of aluminum- and chromium-salen complexes to yield unexpected ratios of polypropylene carbonate and cyclic propylene carbonate.

A stable aminyl radical metal complex

Abstract: Metal-stabilized phenoxyl radicals appear to be important intermediates in a variety of enzymatic oxidations. We report that transition metal coordination also supports an aminyl radical, resulting in a stable crystalline complex: [Rh(I)(trop2N.)(bipy)]+OTf– (where trop is 5-H-dibenzo[a,d]cycloheptene-5-yl, bipy is 2,2′-bipyridyl, OTf– is trifluorosulfonate). It is accessible under mild conditions by one-electron oxidation of the amide complex [Rh(I)(trop2N)(bipy)], at a potential of –0.55 volt versus ferrocene/ferrocenium. Both electron paramagnetic resonance spectroscopy and density functional theory support 57% localization of the unpaired spin at N. In reactions with H-atom donors, the Rh-coordinated aminyl behaves as a nucleophilic radical.

Direct amino acid catalyzed asymmetric synthesis of polyketide sugars.

Abstract: The ability of amino acids to form nucleophilic enamines with aldehydes and ketones has been used in the development of asymmetric α-oxidation reactions with electrophilic oxidizing agents. Singlet molecular oxygen has for the first time been asymmetrically incorporated into aldehydes and ketones, and the products were isolated as their corresponding diols in good yields and ee’s. Organocatalytic α-oxidations of cyclic ketones with iodosobenzene and N-sulfonyloxaziridine were also possible and furnished after reduction the product diols in generally low yields and in low to good ee’s. Amino acids have also been shown to catalyze the formation of carbohydrates by sequential aldol reactions. For example, proline and hydroxy proline mediate a highly selective trimerisation of α-benzyloxyacetaldehyde into allose, which was obtained in >99 % ee. Non linear effect studies of this reaction revealed the largest permanent nonlinear effect observed in a proline-catalyzed reaction to date. Moreover, polyketides were also assembled in a similar fashion by an amino acid-catalyzed one-pot reaction, and was successful for the trimerisation of propionaldehyde, however the sequential cross aldol reactions suffered from lower selectivities. This problem was overcome by the development of a two-step synthesis that enabled the formation of a range of polyketides with excellent selectivities from a variety of aldehydes. The method furnishes the polyketides via the shortest route reported and in comparable product yields to most multi-step synthesis. All polyketides were isolated as single diastereomers with >99 % ee. Based on the observed amino acid-catalysis, amino acids are thought to have taken part in the prebiotic formation of tetroses and hexoses.

Enantioselective synthesis of stephacidin B.

Abstract: We describe an enantioselective synthetic route to the antiproliferative alkaloid stephacidin B (1) proceeding in 18 steps and 4.0% yield from 4,4-(ethylenedioxy)-2,2-dimethylcyclohexanone (3). Key features of the synthetic sequence include the use of the Corey-Bakshi-Shibata (CBS) reduction to introduce asymmetry early in the synthetic route, use of the novel electrophile N-(tert-butoxycarbonyl)-5-(isopropylsulfonyloxymethyl)-2,3-dihydropyrrole in a stereoselective enolate alkylation, a diastereoselective Strecker-type addition of hydrogen cyanide to an N-Boc enamine substrate in the solvent hexafluoroisopropanol, platinum-catalyzed nitrile hydrolysis under neutral conditions, cyclization of an acylamino radical intermediate to form the diketopiperazine core of stephacidin B, and implementation of a convergent procedure for introduction of the key 3-alkylidene-3H-indole 1-oxide functional group in the final stage of the route to prepare the structure 2, previously proposed to be the fungal metabolite avrainvillamide (17 steps, 4.2% yield). We observed that synthetic (-)-2 dimerized in the presence of triethylamine to form (+)-stephacidin B (>95%). We also obtained evidence that 2 can form 1 under mild conditions, and that 2 reacts with nucleophiles, such as methanol, by conjugate addition.

Bifunctional Lewis Acid-Nucleophile-Based Asymmetric Catalysis: Mechanistic Evidence for Imine Activation Working in Tandem with Chiral Enolate Formation in the Synthesis of β-Lactams

Abstract: We report a mechanistically based study of bifunctional catalyst systems in which chiral nucleophiles work in conjunction with Lewis acids to produce β-lactams in high chemical yield, diastereoselectivity, and enantioselectivity. Chiral cinchona alkaloid derivatives work best when paired with Lewis acids based on Al(III), Zn(II), Sc(III), and, most notably, In(III). Homogeneous bifunctional catalysts, in which the catalyst contains both Lewis acidic and Lewis basic sites, were also studied in detail. Mechanistic evidence allows us to conclude that the chiral nucleophiles form zwitterionic enolates that react with metal-coordinated imines. Alternative scenarios, which postulated metal-bound enolates, were disfavored on the basis of our observations.

Enantioselective organocatalytic Michael additions of aldehydes to enones with imidazolidinones: cocatalyst effects and evidence for an enamine intermediate.

Abstract: An enantioselective intermolecular Michael addition of aldehydes to enones catalyzed by imidazolidinones has been achieved. Chemoselectivity (Michael addition vs aldol) can be controlled through judicious choice of hydrogen-bond-donating cocatalysts. The optimal imidazolidinone/hydrogen-bond-donor pair affords Michael addition products in excess of 90% ee. Furthermore, we have isolated and characterized an enamine intermediate and demonstrated its efficacy as a nucleophile in the observed Michael addition reactions.