Asymmetric 1,3-dipolar cycloaddition of N-metalated azomethine ylides to methyl (S)-2-(p-tolylsulfinyl)acrylate. Synthesis of optically pure 2,4,5-trisubstituted 2,5-dihydro-1H-pyrroles

The first 1,3-dipolar reaction of azomethine ylides with optically pure vinyl sulfoxide are reported. The presence of the sulfinyl group increase the reactivity of the acrylate moiety as a dipolarophile, and the reactions evolve with complete regio- and endo-selectivities. Nevertheless, mixtures of the two diastereoisomers 4 and 5 (75-88% de) resulting from the anti dipole/s-cis dipolarophile and syn dipole/s-trans dipolarophile approaches, respectively, are obtained. The stereoselectivity can be controlled by using THF or MeCN as solvents or by changing the reaction temperature in MeCN. After separation of the cycloadducts, optically pure 2,5-dihydro-1H-pyrroles are easily obtained by pyrolytic desulfinylation.     

A chiral auxiliary derived from levoglucosenone in asymmetric Diels-Alder transformations

The Diels-Alder reaction of the acrylate derived from levoglucosenone with cyclopentadiene was studied under several conditions, in the presence and absence of a Lewis acid. The results showed satisfactory diastereomeric excess and the ratio of cycloadducts was found to depend on the reaction conditions. A reversal in stereoselectivity was observed when EtAlCl₂ or Et₂AlCl were employed as Lewis acids.     

Equilibrium between a vinylogous ylide and a phosphonium dienolate zwitterion: vinylogous Wittig olefination versus vinylogous aldol-type reaction

This paper describes the equilibrium established between a phosphonium dienolate zwitterion and a vinylogous phosphorus ylide, and their reactions with aldehydes. The reactions between ethyl 2-methyl-2,3-butadienoate and various aldehydes occur through either a phosphonium dienolate or a vinylogous ylide intermediate, depending on the presence/absence of a Lewis acid and the nature of the phosphine. We observed a rare vinylogous Wittig olefination from the reaction between ethyl 2-methyl-2,3-butadienoate and an electron-deficient aromatic aldehyde in the presence of a stoichiometric amount of an electron-deficient triarylphosphine and a catalytic amount of a Lewis acid (e.g., BF₃·Et₂O). On the other hand, the use of triphenylphosphine, in the absence of a Lewis acid, facilitated vinylogous aldol addition, accompanied by a rare 1,2-aryl phosphorus-to-carbon migration.     

Binaphthol-derived bisphosphoric acids serve as efficient organocatalysts for highly enantioselective 1,3-dipolar cycloaddition of azomethine ylides to electron-deficient olefins

A variety of chiral bisphosphoric acids derived from binaphthols have been evaluated for enantioselective 1,3-dipolar cycloaddition reactions, revealing that the feature of the linker in the catalysts exerted great impact on the stereoselectivity. Among them, the oxygen-linked bisphosphoric acid 1a provided the highest level of stereoselectivity for the 1,3-dipolar cycloaddition reaction tolerating a wide range of substrates including azomethine ylides, generated in situ from a broad scope of aldehydes and α-amino esters, and various electron-deficient dipolarophiles such as maleates, fumarates, vinyl ketones, and esters. This reaction actually represents one of the most enantioselective catalytic approaches to access structurally diverse pyrrolidines with excellent optical purity. Theoretical calculations with DFT method on the formation of azomethine ylides and on the transition states of the 1,3-dipolar cycloaddition step showed that the dipole and dipolarophile were simultaneously activated by the bifunctional chiral bisphosphoric acids through the formation of hydrogen bonds. The effect of the bisphosphoric acids on reactivity and stereochemistry of the three-component 1,3-dipolar cycloaddition reaction was also theoretically rationalized. The bisphosphoric acid catalyst 1a may take on a half-moon shape with the two phosphoric acid groups forming two intramolecular hydrogen bonds. In the case of maleates, one phosphate acts as a base to activate the 1,3-dipole, and simultaneously, the two hydroxyl groups in the catalyst 1a may respectively form two hydrogen bonds with the two ester groups of maleate to make it more electronically deficient as a much stronger dipolarophile to participate in a concerted 1,3-dipolar cycloaddition with azomethine ylide. However, in the cases involving acrylate and fumarate dipolarophiles, only one hydroxyl group forms a hydrogen bond with the ester functional group to lower the LUMO of the C-C double bond and another one is remained to adjust the acidity and basicity of two phosphoric acids to activate the dipole and dipolarophile more effectively.     

Chiral Lewis acid-catalyzed asymmetric Baylis-Hillman reactions

An effective chiral Lewis acid-catalyzed asymmetric Baylis-Hillman reaction is described. Good to high enantioselectivities were obtained using 3 mol % chiral catalyst. Novel camphor-derived dimerized ligands were prepared from the condensation of (+)-ketopinic acid with the corresponding diamines and hydrazine under acidic conditions. When alpha-naphthyl acrylate was used as a Michael acceptor, the reaction is complete within 20 min with high stereoselectivity and in reasonable chemical yields.     

Regioselective hydrosilylations of propiolate esters with tris(trimethylsilyl)silane

Lewis acid and substituent dependency on the regioselectivity of hydrosilylation of propiolate esters 1a-c with tris(trimethylsilyl)silane (2a) was found. The reaction of methyl and ethyl propiolate esters and 2a without Lewis acid and in the presence of EtAlCl2 and Et2AlCl gave beta-silicon-substituted Z-alkenes 3 selectively. On the other hand, reaction in the presence of AlCl3 in dichloromethane gave alpha-silicon-substituted alkenes 4. In the case of trifluoroethyl propiolate ester 1c, reaction with aluminum chloride-based Lewis acids gave alpha-silicon-substituted alkenes 4 exclusively. Two competitive mechanisms, free-radical and ionic, are proposed as the source of the complementary regioselectivity displayed in these reactions. A transition state of the radical-forming step was obtained computationally. The reaction of various reactive acetylene substrates and 2a without Lewis acid and without solvent at room temperature gave beta-silicon-substituted Z-alkenes 3 selectively.     

Investigations on regio‐ and stereoselectivities in cycloadditions involving α‐ (3‐pyridyl)‐N‐phenylnitrone: Development of an efficient route to novel nicotine analogs

Thermal reactions of hitherto α‐(3‐pyridyl)‐N‐phenylnitrone ( 1 ) with mono‐substituted electron‐rich and electron‐neutral dipolarophiles are regio‐, and stereo‐selective (exo‐selective), controlled by LUMO ‐ dipole ‐ HOMO‐ dipolarophile interaction, and furnish syn‐5‐substituted‐3‐(3‐pyridyl)‐isoxazolidines ( 5 ) in high yields. With electron deficient dipolarophiles such as acrylonitrile there is observed a loss of regioselectivity as well as stereoselectivity and the regioselectivity is reversed in reactions with methyl vinyl ketone and methyl acrylate, due to intervention of HOMO‐dipole ‐ LUMO‐dipolarophile interaction, affording 4‐substi‐tuted‐3‐(3‐pyridyl)‐isoxazolidines ( 7 ) as major products. Reactions of nitrone ( 1 ) with disubstituted dipolarophiles such as methyl methacrylate and ethyl coronate furnish methyl syn‐5‐methy‐3‐pyridyl‐1‐phenyl‐isoxazolidine‐5‐carboxylate ( 8 ) and ethyl anti‐5‐methy‐3‐pyridyl‐1‐phenyl‐isoxazolidine‐4‐carboxylate ( 10 ), respectively, in high yields. Reaction with N‐Phenylmaleimide affords novel isoxazolidino‐pyrro‐lidinediones bearing a 3‐pyridyl moiety ( 11, 12 ). A mechanistic rationalization of the obtained results in terms of electronic, steric and secondary interactions is proffered.     

Synthesis,spectral analysis,and <Emphasis Type="Italic">in vitro</Emphasis> microbiological evaluation of ethyl 7,9-diaryl-1, 4-diazaspiro[4.5]dec-9-ene-6- carboxylates as a new class of antibacterial and antifungal agents

A series of novel ethyl 7,9-diaryl-1,4-diazaspiro[4.5]dec-9-ene-6-carboxylates was synthesized by the reaction of ethyl 4,6-diaryl-2-oxocyclohex-3-ene-1-carboxylates with ethylenediamine in the presence of p-toluenesulfonic acid without a solvent under focused microwave irradiation. The title compounds were screened for their antimicrobial activities against a spectrum of clinically isolated microorganisms.     

Chalcogenide-Lewis acid mediated reactions of electron-deficient alkynes with aldehydes

Reactions of but-3-yn-2-one (2) with aldehydes 1 in the presence of a Lewis acid and dimethyl sulfide (3 a) predominantly gave (E)-alpha-(halomethylene)aldols 4-5 in high yields, while reactions of methyl propiolate (6 a) mainly afforded (Z)-3-halogeno-2-(hydroxymethyl)acrylates 7-8 in low to moderate yields. A reaction of dimethyl acetylenedicarboxylate (10) with 1 a in the presence of TiCl(4) and 1,1,3,3-tetramethylthiourea (3 c) produced maleate (E)-11 (40 %) and butenolide 12 (40 %). When a reaction of 6 a with 1 a was carried out in the presence of TiBr(4) and 3 a (0.2 equiv) at -20 degrees C for 60 h, 3-(methylthio)-2-(hydroxyalkyl)acrylate 9 a was obtained in an 8 % yield. Experiments were conducted in order to elucidate the formation mechanism of 9 a, and it was made clear that 9 a was formed via the processes of the Michael addition of sulfide 3 a to alkynoate 6 a and an aldol reaction with 1 a and demethylation.     

Chemistry of o-Benzoquinones and Masked o Benzoquinones VII.: Regioselective and Stereoselective Cycloadditions of 6, 7-Dipropyl- and 8, 9-Eipropyl-1, 4-Dioxaspiro[4.5] deca-6, 8-Di-En-2, 10-Dione with Monosubstituted Ethylenes

The cycloadditions of the titled two masked o-benzoquinones, 2 and 3 , with monosubstituted ethylenes including ethyl acrylate, styrene, ethyl vinyl ether and 1-hexene were studied. The reactions proceeded with high stereoselectivity and regioselectivity to give endo head-to-head adducts when ethyl acrylate, styrene and ethyl vinyl ether were used as addenda. In the case of 1-hexene, the reaction with 2 took place with high regioselectivity but low stereoselectivity to afford endo as well as exo head-to-head adducts while the reaction with 3 occurred with less regioselectivity to produce presumably all the eight possible isomers. The regiochemistry of the adducts were determined by the ¹H nmr analysis of their hydrolysis products, bicyclo[2,2,2]oct-5-en-2,3-diones 6 , and the subsequent photolysis products, 1,3-cyclohexadienes 7 . The stereochemistry was established by the study of the lanthanide induced shifts of compounds 6a-6f with Fu(fod)₃. The regioselectivity and stereoselectivity of these cycloaddition reactions were explained in terms of frontier molecular orbital theory and steric effect. The present study provides also a facile method to prepare regioselectively bicyclo[2, 2, 2]oct-5-en-2,3-diones (stereo-selectively also) and 1,3-cyclohexadienes from unsymmetric catechols via masked o-benzoquinones.     

Facial selectivity in the nucleophilic additions of vinylmagnesium bromide to bicyclo[2.2.2]oct-5-en-2-one derivatives

The levels of diastereoselection attainable by addition of vinylmagnesium bromide to a selection of bicyclo[2.2.2]octenone derivatives 1-6 in the presence of various Lewis acids such as LiBr, CeCl3, TiCl4, ZnBr2, MgBr2, and Et2AlCl have been determined. The 1,2-addition of ketone 1 with vinylmagnesium bromide in THF provided a mixture of anti- and syn-isomers. The reactions of 2 with vinylmagnesium bromide at room temperature afforded anti- and syn-isomers with preference to anti-isomers in most cases. These reactions in the presence of Lewis acids afforded anti-isomers as the major product with an excellent stereoselectivity or as single isomers in some cases. The ketones 3 gave surprisingly different results providing anti-isomers predominantly even in the presence of Lewis acids. The bicyclic ketones 4 and 5 and all-carbon tricyclic ketone 6 furnished the syn-isomer as the main product. There is no significant effect of Lewis acid catalysis in the nucleophilic addition reactions of 1, 4, 5, and 6. The use of a preformed vinylmagnesium bromide-CeCl3 reagent for the addition reactions of 2d-f and 3d-f provided almost exclusively syn-isomers. The substituents and reaction conditions can influence facial selectivity in the nucleophilic additions to the bicyclo[2.2.2]oct-5-en-2-one derivatives.     

Lewis acid-promoted Kharasch-Curran additions: a competition kinetics study of bromine atom transfer addition of N-alpha-bromoacetyl-oxazolidinone to 1-hexene

Lewis acids can efficiently promote free radical atom transfer reactions of an oxazolidinone imide substrate, 1, derived from alpha-bromo acetic acid. Thus, 1 undergoes a radical chain addition to 1-hexene giving the atom transfer addition compound, 6, in the presence of scandium or ytterbium triflate in 1,2-dichloroethane or a cosolvent mixture of 1/9 THF/dichloromethane. In 1,2-dichloroethane the solution is heterogeneous, while the cosolvent mixture gives a homogeneous solution, even at temperatures of -78 degrees C. Competition experiments were carried out in both solvent systems with added carbon tetrachloride to study how Lewis acid affected the product distribution. In the presence of carbon tetrachloride, chloride 7 is formed in addition to 6 and the ratio of these two products depends on the amount of Lewis acid present. In the presence of ytterbium triflate, in the cosolvent system, the reaction rate of bromine atom transfer was enhanced up to 400-fold compared to the reaction without added Lewis acid. Significant rate enhancements were also obtained in the solvent 1,2-dichloroethane, although the analysis of the system is complicated by the heterogeneous nature of the medium. Computation of C-Br bond dissociation energies (BDE) of the complexed and uncomplexed oxazolidinone bromide suggest that complexation lowers the BDE due to the effect of the strong electron-withdrawing group on the C-Br bond dipole.     

Facile Method for 1,3‐Dipolar Cycloaddition Reaction of Azomethine Ylides: Highly Stereoselective Synthesis of Substituted Pyrrolidine Derivatives

The substituted pyrolidine derivatives with high stereoselectivity were obtained through cycloaddition reaction in the presence of a Lewis acid namely diethylzinc by solvent‐free grinding method.     

Asymmetric Synthesis of Chromans Through Bifunctional Enamine-Metal Lewis Acid Catalysis

Cooperative enamine-metal Lewis acid catalysis has emerged as a powerful tool to construct carbon-carbon and carbon-heteroatom bond forming reactions. A concise synthetic method for asymmetric synthesis of chromans from cyclohexanones and salicylaldehydes has been developed to afford tricyclic chromans containing three consecutive stereogenic centers in good yields (up to 87 %) and stereoselectivity (up to 99 % ee and 11 : 1 : 1 dr). This difficult organic transformation was achieved through bifunctional enamine-metal Lewis acid catalysis. It is believed that the strong activation of the salicylaldehydes through chelating to the metal Lewis acid and the bifunctional nature of the catalyst accounts for the high yields and enantioselectivity of the reaction. The absolute configurations of the chroman products were established through X-ray crystallography. DFT calculations were conducted to understand the mechanism and stereoselectivity of this reaction.     

Investigation of Lewis acid-catalyzed asymmetric aza-Diels-Alder reactions of 2H-azirines

Asymmetric Diels-Alder reactions with 2H-azirines as dienophiles have been studied. Diastereoselective reactions with an enantiopure azirine 1b, bearing a chiral auxiliary, gave substituted bi- and tricyclic tetrahydropyridines in high yield and stereoselectivity, under the influence of a Lewis acid. The novel enantioselective [4+2] cycloaddition reaction of 3-benzyl-2H-azirine carboxylate with cyclopentadiene was investigated with various chiral Lewis acid complexes and provided the corresponding tetrahydropyridines in moderate to low yield and enantioselectivity.     

A DFT study of the domino inter

The molecular mechanism of the domino inter [4 + 2]/intra [3 + 2] cycloaddition reactions of nitroalkenes with enol ethers to give nitroso acetal adducts has been characterized using density functional theory methods with the B3LYP functional and the 6-31G basis set. The presence of Lewis acid catalyst and solvent effects has been taken into account to model the experimental environment. These domino processes comprise two consecutive cycloaddition reactions: the first one is an intermolecular [4 + 2] cycloaddition of the enol ether to the nitroalkene to give a nitronate intermediate, which then affords the final nitroso acetal adduct through an intramolecular [3 + 2] cycloaddition reaction. The intermolecular [4 + 2] cycloaddition can be considered as a nucleophilic attack of the enol ether to the conjugated position of the nitroalkene, with concomitant ring closure and without intervention of an intermediate. For this cycloaddition process, the presence of the Lewis acid favors the delocalization of the negative charge that is being transferred from the enol ether to the nitroalkene and decreases the activation energy of the first cycloaddition. The [4 + 2] cycloaddition presents a total regioselectivity, while the endo/exo stereoselectivity depends on the bulk of the Lewis acid used as catalyst. Thus, for small Lewis acid catalyst, modeled by BH(3), the addition presents an endo selectivity. The [3 + 2] cycloaddition reactions present an total exo selectivity, due to the constraints imposed by the tether. Inclusion of Lewis acid catalyst and solvent effects decrease clearly the barrier for the first [4 + 2] cycloaddition relative to the second [3 + 2] one. Calculations for the activation parameters along this domino reaction allow to validate the results obtained using the potential energy barriers.     

Chiral phosphine lewis bases catalyzed asymmetric aza-Baylis-Hillman reaction of N-sulfonated imines with activated olefins

In the aza-Baylis-Hillman reaction of N-sulfonated imines (N-arylmethylidene-4-methylbenzenesulfonamides and others) with methyl vinyl ketone, ethyl vinyl ketone, and acrolein, we found that, in the presence of a catalytic amount of chiral phosphine Lewis base such as (R)-2'-diphenylphosphanyl-[1,1']binaphthalenyl-2-ol LB1 (10 mol %) and molecular sieve 4A, the corresponding aza-Baylis-Hillman adducts could be obtained in good yields with good to high ee (70-95% ee) at low temperature (approximately -30 to -20 degrees C) or at room temperature in THF, respectively. In CH2Cl2 upon heating at 40 degrees C, the aza-Baylis-Hillman reaction of N-sulfonated imines with phenyl acrylate or naphthyl acrylate gave the adducts in good to high yields (60-97%) with moderate ee (52-77%). The mechanistic insight has been investigated by 31P and 1H NMR spectroscopic measurements. The key enolate intermediate, which has been stabilized by intramolecular hydrogen bonding, has been observed by 31P and 1H NMR spectroscopy. An effective bifunctional Lewis base and Bronsted acid phosphine Lewis base system has been disclosed in this catalytic, asymmetric aza-Baylis-Hillman reaction.     

Highly efficient aza-Baylis-Hillman reaction of N-tosylated imines with MVK, acrolein, and phenyl acrylate or alpha-naphthyl acrylate: Lewis base effects and a convenient method to synthesize alpha,beta-unsaturated beta-amino carbonyl compounds

This paper describes several highly efficient aza-Baylis-Hillman reactions of N-tosylated imines with MVK, acrolein, and phenyl acrylate or alpha-naphthyl acrylate in the presence of a Lewis base. In most cases, the reaction can be completed within 1 h using the appropriate Lewis base catalyst. An efficient method to synthesize beta-amino ketones, aldehydes and esters in high yields and short reaction time has been developed.     

The influence of water on the rates of 1,3-dipolar cycloaddition reactions: trigger points for exponential rate increases in water-organic solvent mixtures. Water-super versus water-normal dipolarophiles

The nature of the rate enhancements caused by gradually increasing the mole fraction of water in the solvent (from 0 to 1) for the cycloaddition reactions of pyridazinium-dicyanomethanide 1,3-dipole, 2, with the dipolarophiles ethyl vinyl ketone (a water-super dipolarophile) and methyl acrylate (a water-normal dipolarophile) in the organic solvents acetonitrile, acetone, methanol, ethanol, and tert-butyl alcohol at 37 degrees C are explored. In each case as the mole fraction of water surpassed ca. 0.9, exponential rate enhancements were triggered. When methanol replaced water, the rate enhancements were smaller, and no triggering effect was observed. The dramatic rate enhancement triggered for the water-super dipolarophile was significantly reduced as the temperature was raised in the range 29-64 degrees C. The results suggest that a dominant hydrogen-bonding effect operates in water-induced rate enhancements of 1,3-dipolar cycloaddition reactions with water-super dipolarophiles as well as the hydrophobic effect. The hydrogen-bonding effect involves secondary bridging hydrogen bonding from the primary water-solvation shell of the transition state and the growth of structured water clusters. Theoretical calculations strongly support these conclusions.     

Four-component tandem protocol for the stereoselective synthesis of highly functionalized [1,4]-thiazines

The one-pot, four-component tandem reaction of ethyl 2-[(2-oxo-2-arylethyl)sulfonyl]acetate/ethyl 2-[(2-ethoxy-2-oxoethyl)sulfonyl]acetate, an aromatic aldehyde and pyrrolidine provides a rapid and facile access to new ethyl 3-aroyl-1-benzyl-2,2-dioxo-4-aryloctahydro-2-pyrrolo[2,1-c][1,4]thiazine-1-carboxylates/diethyl 1-benzyl-2,2-dioxo-4-aryloctahydro-2-pyrrolo[2,1-c][1,4]thiazine-1,3-dicarboxylates. This reaction shows high stereoselectivity and proceeds in good yields.