Cycloadducts 4a Research Articles

2,4,6‐Trialkylphenyl‐2H‐phospholes from slightly aromatic 1H‐phospholes and their use in [4 + 2] cycloaddition reactions

Abstract1‐(2,4,6‐Trialkylphenyl)phospholes 1a,b possess a moderate aromatic character. Despite of that they underwent a sigmatropic rearrangement at 150°C to afford 2H‐phospholes 2a,b which by trapping with tolane, or in reaction with another unit of 2 gave [4 + 2] cycloadducts 3a,b, or in a reversible reaction, dimer 6, respectively. Dedimerization of species 6 at 150°C in the presence of tolane, or at 0°C under oxidative circumstances, led to 1‐phosphanorbornadiene 3a, or phosphole oxide dimer 8, respectively. © 2003 Wiley Periodicals, Inc. Heteroatom Chem 14:316–319, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10151

Chemistry of beta-functionalized alpha-nitroso ethylenes. Methyl beta-nitroso acrylate as heterodienophile in

[formula: see text] beta-Functionalized nitroso alkene 2, obtained from methyl beta-nitropropionate 1 and N,O-bis(trimethylsilyl)acetamide, can function as a good heterodienophile in Diels-Alder reactions. Therefore, 2 was trapped by cyclic dienes to give adducts 4 with the corresponding stereoselectivity. Cycloadduct 4a undergoes retro-[4 + 2]-cycloaddition at 33 degrees C in solution; thus 4a can be used to generate nitroso alkene 2 in neutral medium. Cyclopentadiene reacts with adduct 4a according to an endo-(4 + 2]-cycloaddition scheme to give cycloadduct 5 in low yield.

Diels-Alder reaction of (2,4,6-trialkylphenyl)phospholes withN-phenylmaleimide

2,4,6-Trialkylphenylphospholes 3a (R=Me), 3b (R=i–Pr) and 3c (R=t–Bu), with increasing flattening at phosphorus and hence with increasing electron delocalisation, underwent the Diels-Alder reaction with N-phenylmaleimide to give predominantly cycloadducts 4a–c with the trialkylphenyl substituents anti to the phosphanorbornene double bond. With increasing aromaticity, the cycloaddition was slower. The stereostructure of the products (6 and 7) obtained after oxidation was confirmed by stereospecific 2JPC NMR couplings and by an independent synthesis. © 2000 John Wiley & Sons, Inc. Heteroatom Chem 11:271–275, 2000

Diels-Alder Cycloadditions of Ethyl 2-Carbomethoxyethenesulfinates with Cyclopentadiene. Lewis Acid Enhancement and Adduct Identification with the Assistance of Competitive Stereodifferentiating Iodolactonization and Iodosultinization Reactions.

Ethyl (Z)-2-carbomethoxyethenesulfinate (1) and ethyl (E)-2-carbomethoxyethenesulfinate (2) were each subjected to cyclopentadiene (ca. 3 equiv) in order to determine their reactivity in Diels-Alder reactions. Thermal reactions proceeded in less than 4 h, while Lewis Acid enhanced reactions were substantially faster and demonstrated greater selectivity. In particular, the BF(3).Et(2)O-mediated cycloaddition of 1 gave syn/endo cycloadduct 3a as a major diastereomer in about 90% yield. The reactions of 2 were less synthetically useful. The structures of the cycloadducts were established through a combination of X-ray crystal structure determinations and through exposure of the compounds to an electrophilic iodine reagent. Each cycloadduct underwent an iodocyclization reaction with either the carboxylic ester or the sulfinate ester participating in the formation of a polycyclic lactone or sultine, respectively. The iodocyclization reactions allowed further elucidation of the cycloadduct structures.

Stereoselectivity of N-benzyl- C-ethoxycarbonyl nitrone cycloaddition to ( S)-5-hydroxymethyl-2(5 H)-furanone and its derivatives

Stereoselectivity of the reaction of N-benzyl- C-ethoxycarbonyl nitrone ( 3) with ( S)-5-hydroxymethyl-2(5 H)-furanone ( 2a) and its 5-alkoxy substituted derivatives 2b-f was investigated. The reaction proceeds in a highly face-selective manner, the products 4–6 resulting from approach anti to the hydroxymethyl or alkoxymethyl group of the dipolarophile. The exo-stereoselectivity increases as the size of protective group attached to lactone ( 2b-f) increases. Endo-exo diastereoselectivity is affected significantly by the solvent. Microwave irradiation strongly accelerates the reaction with little effect on the diastereoselectivity to give the syn adduct 8a in addition to anti cycloadducts 4a–6a.

Phenylaziridine as a 1,3-dipole. Application to the synthesis of functionalized pyrrolidines

ce:simple-parahenylaziridine 1 in the presence of an appropriate Lewis acid reacts as a 1,3-dipole. The cyclocondensation of 1 with DHP in the presence of BF3·Et2O produced the azaoxa[3.2.0] cycloadducts 4a–4b. The reactivity of the corresponding N-tosyliminium ions was explored.

A Novel Strategy for Cyclobutane Formation. Fine Tuning of Cyclobutanation vs Cyclopropanation

The reactions of (E)-1-(phenylseleno)-2-(trimethylsilyl)ethene (1a) and (E)-1-(phenylseleno)-2-(triethylsilyl)ethene (1b) with dimethyl 1,1-dicyanoethene-2,2-dicarboxylate (2) in the presence of SnCl4 in CH2Cl2 at −78 °C for 1 h exclusively afforded [2 + 2] cycloadducts 3a and 3b in 71% and 78% yields, respectively. The exclusive [2 + 2] selectivity in the reaction of 1 with 2 in the presence of SnCl4 is explained by destabilization of the proposed selenium-bridged intermediate that would lead to the [2 + 1] adduct by the presence of the two electron-withdrawing cyano groups. The reaction of 1a with 2 at 0 °C to room temperature in the presence of ZnBr2 gave a regioisomeric [2 + 2] cycloadduct 7a in low yield along with a corresponding desilylated cyclobutane 8 as the major product in moderate to good yield. The origin of the Lewis acid dependence of the regioselectivity was deduced from a theoretical comparison of stability of Lewis acid complexes.

Transition-Metal-Substituted Acyl Phosphanes and Phosphaalkenes. 32.1 Transformation of an FePC Unit of a Phosphaalkene into a FePCNNC Moiety by Chain Extension through Insertion of the 1,3-Dipole (-NN(Ar)CH+) of Sydnones

The metallophosphaalkene (η5-C5Me5)(CO)2FePC(NMe2)2 (1) underwent reaction with sydnones 3-aryl-NNOC(O)CH (aryl = C6H5 (2a), 4-FC6H4 (2b), 4-ClC6H4 (2c), 4-BrC6H4 (2d)) to afford the novel ferriophosphaalkenes (η5-C5Me5)(CO)2FePCHN(aryl)NC(NMe2)2 (3a−d). Compound 3a forms the [2 + 2] phosphetanyl cycloadduct 4a with dimethyl fumarate. The molecular structure of 3a was determined by single-crystal X-ray diffraction.

Diels–Alder reactivity of trialkyl 2-phosphonoacrylates with N-buta-1,3-dienylsuccinimide

N-Buta-1,3-dienylsuccinimide 1 reacts quantitatively with trimethyl 2-phosphonoacrylate 2a to furnish the ortho-[4 + 2]-cycloadduct 3a as a single stereoisomer. The cis axial/equatorial relationship between the succinimido and phosphonate groups respectively has been established by NMR and X-ray diffraction analyses. Triethyl 2-phosphonoacrylate 2b similarly undergoes cycloaddition with the diene 1 to give a 55∶45 mixture of ortho stereoisomers 3b (cis axial/equatorial) and 4b (trans axial/axial). The activating and directing effect of the phosphonate group is discussed on the basis of a theoretical approach considering the van der Waals complexes.

Synthesis of polycyclic structures from c7h8hydrocarbons via high pressure repetitive cycloaddition reactions

Abstract The synthesis of polycyclic strained molecules from norbornadiene and quadricyclane is described. The procedure uses high pressure and involves concerted [2+2+2] or [2+[sgrave]2+[sgrave]2] processes in the first step followed by Diels-Alder cycloadditions of the primary cycloadducts 4a, 4b, 5a, 5b with cyclopen-tadiene, 1,3-cyclohexadiene, furan, cycloheptatriene. The spectral NMR data of the new polycyclic molecules are given.

( Z)- and ( E)-2-phenyl-4-benzylidene-5(4H)-oxazolones as dienophiles. Improved selectivity by the use of heterogeneous catalysts

Silica gel treated with AlEt 2Cl or TiCl 4 efficiently promotes the reaction of cyclopentadiene with ( Z)-2-phenyl-4-benzylidene-5(4H)-oxazolone ( 1a) with very little Z/E isomerization. Silica gel treated with TiCl 4 and ZnCl 2 supported on silica gel leads to very high yields for the reaction of cyclopentadiene with ( E)-2-phenyl-4-benzylidene-5(4H)-oxazolone ( 1b). Furthermore, E/Z isomerization is almost suppressed, which is not achieved with homogeneous Lewis acids. Whereas for the reaction of 1a with cyclopentadiene the exo cycloadduct ( 3a) is always the major product, for the reaction of cyclopentadiene wth 1b the endo( 2b)/ exo( 3b) selectivity is reversed as a function of the catalyst. Thus, in the reactions catalyzed by silica gel, alumina and exchanged clays, endo ( 2b) is the major product However, the exo cycloadduct ( 3b) is preferably obtained for reactions catalyzed by silica gel and alumina treated with TiCl 4 and ZnCl 2 supported on silica gel.

Unsaturated nitrogen compounds containing fluorine. Part 18. Novel cycloaddition products from the reaction of quadricyclane with fluorinated azines and the azomethine imide 2-hexafluoroisopropylidene-3-methyl-3-phenyl-5,5-bis(trifluoromethyl)tetrahydropyrazol-2-ium-1-ide

Reaction of quadricyclane (6) with the azine CF 3CCl = NN = CClCF 3 (1a) in dichloromethane at 70 °C afforded the expected exo-[2 + 2 + 2] cycloadduct 8a (70%), and the unexpected exo-1,3-dipolar cycloadduct 3a, formed by a non-concerted pathway and isolated as the hydrolysed compound, amide 4a (19%). The corresponding reaction of 6 with hexafluoroacetone azine (CF 3) 2C=NN=C(CF 3) 2 (1b) at 70 °C yielded the exo-[2 + 2 + 2] cycloadduct 8b (54%), the bis-[2 + 2 + 2] 2:1 cycloadduct 9b (8%), formed by further reaction of 8b with 6, and the exo,exo-2:2 cycloadduct 10 (31%), which arose via concerted 1,3-dipolar cycloaddition of azine 1b to the olefinic double bond of 1:1 adduct 8b to give the 1:2 adduct, azomethine imide 14, followed by non-concerted 1,3-dipolar cycloaddition of 14 to quadricyclane 6. In contrast, the corresponding reaction of 6 with the azines CF 3CCl = NN = CFCF 3 (1c) (at 55 °C) and CF 3CF = NN = CFCF 3 (1d) (at 50 °C) gave only the exo-[2 + 2 + 2] 1:1 cycloadducts, i.e. the compounds 8c (65%) (formed by reaction involving the CF 3CF=N grouping in 1c) and 8d (20%) (formed by reaction involving the CF 3CCl = N grouping in 1c) and the compound 8e (88%), as a mixture of two isomers in the ratio 72:16, respectively. Treatment of 6 with the azomethine imide ( CF 3 ) 2 C = N + − N − C ( CF 3 ) 2 CH 2 ⎴ CMePh ( 7) at 70 °C in dichloromethane afforded the exo-1,3-dipolar 1:1 cycloadduct 11 as two stereoisomers [ 11a (12%) and 11b (17%)], the 1-pyrazoline ( CF 3 ) 2 C − N = N − CMePh − ⎴ CH 2 ( 12) (26%) and the symmetrical 1:2 cycloadduct 13b (32%) formed by further reaction of 11b with azomethine imide 7. Reaction of 6 with the azine CF 3(NH 2) = NN = C(NH 2)CF 3 ( 1e) at 100 °C was unsuccessful, as was reaction with 1:1 cycloadduct 8a at 100 °C.

A novel approach to functionalized polycyclic systems; new aspects of the Diels–Alder reactions of 2-acyl derivative of 4H,6H-thieno[3,4-c]furan 5,5-dioxide

The intramolecular Diels–Alder reaction of the 2-acyl derivative of 4H,6H-thieno[3,4-c]furan 5,5-dioxide 2a is achieved by the use of molecular sieves as additive to give the corresponding cycloadduct 4a in high yield. Addition of efeoophiles to this reaction media afforded the polycyclic compounds 5 and 7a–cvia tandem cycloaddition in a single step.

Synthese von Benzo[b]thioxanthenen

Synthesis of Benzo[b]thioxanthenes Benzo[b]thioxanthenes, heterocyclic compounds related to tetracenes and tetracyclines can be obtained by the reaction of 2H-benzo[b]thiete (1) and 1,4-naphthoquinones (3a–g). The primary cycloadducts 4a–g undergo an autoxidation process leading to the quinones 6a–g. The dihydroxy compound 4e shows an additional isomerization by a tetrafold H transfer (4e → 5e′). Another preparative route to benzo[b]thioxanthenes makes use of the cycloadditon reaction of 1 and 1,4-epoxynaphthalenes (7a–d). The primary adducts can be transformed to the title compounds by catalytic dehydration processes (8a–d → 9a-d). An alternate regioselective opening of the oxygen bridge can be performed by the action of trimethyliodosilane (8a → 10a). Methylation of 9a furnishes the sulfonium salt 14a which rearranges in a strongly alkaline medium to 16a; in the presence of oxygen ketone 17a is generated.

Reaction of C60 with Benzocyclobutenol: Expeditious Route to Fullerene Adducts

Reaction of C 60 with benzocyclobutenol (2a) and its methoxy ether (2b) in refluxing toluene gave 1,9-dihydrofullerene cycloadducts (3a) and (3b) in 59% and 50% yields, respectively. The alcohol was converted to pyranyl ether (4), acrylate (5), p-vinylbenzoate (6) and acid succinate esters (7) in good yields under very mild conditions. The adducts exist as two interconverting boat conformers. Acrylate (5) and p-vinylbenzoate (6) should be used for polymerization studies. Acid (7) shows enhanced solubility in polar solvents

Asymmetric base-catalyzed cycloaddition between anthrone and some dienophiles

Anthrone has been found to react with N-methylmaleimide in the presence of catalytic amounts of various chiral β-aminoalcohols. The cycloadduct 3a has been obtained in excellent yield with enantiomeric excess of up to 61%. Its absolute configuration has been assigned by X-ray crystallography. Several features of the reaction have been studied: variation of dienophile; competition between cycloaddition and formation of the optically active Michael adduct 4; solvent and temperature effects. Mechanistic studies are in agreement with a concerted [4+2] process, providing an unique case of a base-catalyzed asymmetric Diels-Alder reaction.

Non-decarboxylative 1,3-dipolar cycloadditions of imines of α-amino acids as a route to proline derivatives.

α-Amino acids react with aryl aldehydes in the presence of N-substituted maleimides to yield stereospecific cycloadducts ( 3a,b) and with dimethyl fumarate to give isomeric mixtures of ( 4a-i) and ( 5a-i). The relatively low yield in the case of dimethylfumarate is presumably due to the steric interaction between the dipolarophile and the substituents at both ends of the dipole.

Utilization of selenium-directed [2 + 2] cycloadditions: concise synthesis of (±)-fragranol

The reaction of 2-(phenylseleno)prop-1ene 1 and methyl vinyl ketone 2 in the presence of EtAlCl2 gave the [2 + 2] cycloadducts 3a and 3b. A radical substitution of the adducts 3a and 3b with allytributyltin gave the allylated cyclobutane product, which was transformed into the cyclobutane natural product, (±)-fragranol. Stereoselective radical substitution using the ethylene glycol ketals of compounds 3a and 3b was also achieved.

Complete assignment of the 1H and 13C NMR spectra of 8‐hydroxy‐7‐methoxy‐4‐methyl‐1α,4α,4aα,8β,8aα‐tetrahydro‐1,4‐methanonaphthalen‐5(1H)‐one

AbstractDiels–Alder cycloaddition of a mixture of 1‐ and 2‐methylcyclopentadienes (1a and 1b, respectively) to 2‐methoxy‐p‐benzoquinone (2) gave a mixture of isomeric endo [4 + 2] cycloadducts (3a–3d). Fractional recrystallization of this product mixture from EtOAc‐hexane gave 6‐methoxy‐1‐methyl‐1α,4α,4aα,8aα‐tetrahydro‐1,4‐methanonaphthalene‐5,8‐dione (3a). Reaction of 3a with NaBH4 in the presence of cerium(III) chloride resulted in the selective reduction of the less hindered CO group in 3a, thereby affording 8‐hydroxy‐7‐methoxy‐4‐methyl‐1α,4α,4aα,8β,8aα‐tetrahydro‐1,4‐methanonaphthalen‐5(1H)‐one (4). The 1H and 13C NMR spectra of 4, a novel tricyclic ketoalcohol, have been completely assigned by using a combination of one‐ and two‐dimensional NMR techniques.

An unexpected reversal in the stereochemistry of transannular cyclizations. A stereoselective synthesis of (±)-epilupinine.

The transannular cyclizations of N-benzylhexahydroazec-5-en-2-one and N-benzylhexahydroazon-5-en-2-one using iodine or phenylselenenyl bromide affords substituted quinolizidines and indolizidines, respectively. In the case of the ten-membered ring lactams an unexpected reversal in stereochemistry was observed. These results were confirmed by elaboration of the iodo cycloadduct 4a into (±)-epilupinine.