Lewis Acid promoted tandem intermolecular Diels-Alder/intramolecular allylation reactions of silyl-substituted 1,3-butadienes leading to multisubstituted 7-norbornenones and related polycyclic compounds

7-Norbornenones of exo,exo-disubstituted patterns were formed highly selectively in good yields from Lewis acid-promoted tandem intermolecular Diels-Alder/intramolecular allylation reactions. The intermolecular Diels-Alder reaction between 1,4-bis(trimethylsilyl)-1,3-butadienes or 1-trimethylsilyl-1,3-butadienes with maleic anhydride in the presence of newly sublimed AlCl3 afforded their corresponding cycloaddition adducts, which underwent AlCl3-mediated intramolecular allylation of the carbonyl group by the in situ generated allylsilane moiety affording 7-norbornenones of exo,exo-disubstituted patterns.     

Synthetic Transformations of Higher Terpenoids: VIII. [4+2]-Cycloaddition Reactions of Lambertianic Acid

The Diels-Alder reaction of lambertianic acid with maleic anhydride occurred in a stereoselective fashion and yielded diastereoisomeric (1R,2S,6R,7R)- and (1S,2R,6S,7S)-exo-adducts. The latter reacted with L-valinol to give the corresponding diterpenoid imides, 4-aza-9-oxabicyclo[2.2.1]dec-8-enes. Reactions of lambertianic acid with N-substituted maleimides in the presence of Lewis acids afforded diastereoisomeric adducts having both exo and endo configuration. Some transformations of the adducts were examined with a view to obtain cantharidin and dihydroisoindole analogs.     

Acylation of N-p-toluenesulfonylpyrrole under Friedel-Crafts conditions: evidence for organoaluminum intermediates

The Friedel-Crafts acylation of N-p-toluenesulfonylpyrrole under Friedel-Crafts conditions has been reinvestigated. Evidence is presented in support of the hypothesis that when AlCl₃ is used as the Lewis acid, acylation proceeds via reaction of an organoaluminum intermediate with the acyl halide. This leads to the production of the 3-acyl derivative as the major product. With weaker Lewis acids (EtAlCl₂, Et₂AlCl) or less than 1 equiv of AlCl₃ the relative amount of 2-acyl product is increased. A mechanistic rationalization is presented to explain these data.     

Racemic and asymmetric Diels-Alder reactions of 1-(2-oxazolidinon-3-yl)-3-siloxy-1,3-butadienes

Achiral and chiral 1-(2-oxazolidinon-3-yl)-3-siloxy-1,3-butadienes were prepared from readily available starting materials. Although more stable than the parent 1-amino-3-siloxy dienes, the 1-(2-oxazolidinon-3-yl)-3-siloxy-1,3-butadienes are still very reactive in Diels-Alder reactions, somewhat more than 1,3-dialkoxy-1, 3-butadienes (e.g., Danishefsky's diene). The cycloadditions of the achiral and chiral dienes with several different dienophiles were examined. The reactions proceeded in good yield, with modest to high endo selectivity. The chiral dienes exhibited excellent facial selectivity in cycloadditions with alpha-substituted acroleins, maleic anhydride and N-phenylmaleimide. Upon reduction and hydrolysis of the cycloadducts, substituted cyclohexenones were obtained with ee's ranging from 22% to >98%.     

A new approach to construction of isoindolo[1,2-a]isoquinoline alkaloids Nuevamine, Jamtine, and Hirsutine via IMDAF reaction

The interaction between 1-furyl-1,2,3,4-tetrahydroisoquinolines and unsaturated acids derivatives (acryloyl, methacryloyl, and crotonoyl chloride, maleic and citraconic anhydride) was studied. It was shown that the reaction proceeds via amide formation and subsequent intramolecular Diels-Alder reaction of the furan (IMDAF). The [4+2] cycloaddition proceeded under mild reaction conditions (25-80 °C) and afforded only the exo-adduct in a high yield. With this method, a new approach to the isoindolo[1,2-a]isoquinoline system, the basic structural element of alkaloids Jamtine, Hirsutine, and Nuevamine, is proposed.     

High pressure and thermal Diels-Alder reactions of 5-nitro[2.2]paracyclophanepyran-6-one

The Diels-Alder reactions of 5-nitro[2.2]paracyclophanepyran-6-one with 1,3-butadienes and 1,2-dihydro-3-vinyl-naphthalene were examined under thermal and high-pressure conditions. The cycloadditions with 1,3-butadienes occurred in good yield and anti-exo diastereoselectively only under high-pressure conditions; the one with 1,2-dihydro-3-vinyl-naphthalene afforded comparable yields of mixtures of anti/syn adducts under normal and high-pressure conditions. A structural analysis of the reaction products by ¹H and ¹³C NMR spectroscopy is presented.     

Poly(vinyl chloride) stabilization with organo-tin compounds: Part VII—Dibutyl tin maleate: Model compound studies

The reaction between dibutyl tin maleate or dibutyl tin bisbutylmaleate and chlorohexene as a model compound for allylic chloride in PVC was studied at 80°C in dichlorobutane solution. From kinetic studies by gas chromatographic analysis of the reaction medium, and also spectroscopic data (infra-red and ¹H NMR) it may be concluded that the main reaction is the substitution reaction which produces mono- and dihexenyl esters, as well as mixed diesters of maleic acid. The elimination reaction plays a minor rôle. Both activities are related to their Lewis acidities. The Diels-Alder condensation between maleates and hexadiene is not observed as a side reaction in these conditions; the main side reaction is the decomposition of monoesters of maleic acid (which is produced by the elimination reaction after one or two steps) into an alcohol and maleic anhydride—a reaction which can be catalysed by organo-tin chlorides. A Diels-Alder reaction between maleic anhydride and hexadiene takes place. The alcohol produced in the side reaction mentioned above may take part in a transesterification reaction with the organo-tin maleate and that reaction probably provides the best explanation of the earlier observation of Frye et al.,¹ according to which there is at least temporary retention of reactivity of tin labelled organo-tin maleate by PVC.     

A novel and facile stereocontrolled synthetic method for polyhydro-quinolines and pyridopyridazines via a diene-transmissive Diels-Alder reaction involving inverse electron-demand hetero Diels-Alder cycloaddition of cross-conjugated azatrienes

Cross-conjugated azatrienes bearing an electron-withdrawing sulfonyl or benzoyl group on the nitrogen atom underwent, on heating or in the presence of a Lewis acid (TMSOTf), an initial inverse electron-demand hetero Diels-Alder reaction with electron-rich dienophiles (vinyl ether, vinyl thioether, and allenyl ether) to produce 1:1 cycloadducts with high endo selectivity. The initial cycloadducts thus obtained underwent a second Diels-Alder reaction stereoselectively on the newly formed diene unit with electron-deficient dienophiles to give the crossed bis-cycloadducts, octahydroquinolines, with high diastereo-π-facial selectivity. The N-sulfonylazatrienes tethering an ortho-cinnamyloxyphenyl dienophile at the triene terminal underwent an initial intramolecular hetero Diels-Alder reaction of the inverse electron-demand type. The subsequent second Diels-Alder reaction of the formed mono-cycloadducts completed the diene-transmissive hetero Diels-Alder protocol to give benzopyrano[3,4-c]quinolines in a highly stereoselective manner.     

New synthetic approach to substituted isoindolo[2,1-a]quinoline carboxylic acids via intramolecular Diels-Alder reaction of 4-(N-furyl-2)-4-arylaminobutenes-1 with maleic anhydride

Acylation of substituted 4-(furyl-2)-4-arylaminobut-1-enes with maleic anhydride provided 2-allyl-6-carboxy-4-oxo-3-aza-10-oxatricyclo[5.2.1.0^1,5]dec-8-enes in high yield under mild reaction conditions. The Diels-Alder adducts are formed via an initial amide formation followed by a stereoselective intramolecular [4+2] exo-cycloaddition reaction. Treatment of the tricyclic compounds with phosphoric acid at high temperatures (70-120 °C) promoted cyclic ether opening, intramolecular cyclization and aromatization to give the corresponding tetracyclic compounds, 5,6,6a,11-tetrahydro-10-carboxyisoindolo[2,1-a]quinolines, in moderate yields. The influence of the acid and the reaction temperature on the cyclization reactions are also discussed.     

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.     

Effect of a Competing n Donor on the Rate of AlCl3-Catalyzed Diels-Alder Reaction of 9,10-Dimethylanthracene with Maleic Anhydride

The catalytic effect of AlCl₃ on the Diels-Alder reaction of 9,10-dimethylanthracene with maleic anhydride is dramatically weakened on addition of a competing n donor, dibutyl ether. The n,v complex of aluminum chloride with dibutyl ether is considerably more stable than the complex with maleic anhydride; therefore, the catalytic effect is suppressed even at comparable concentrations of these n donors.     

New synthetic approach to α-fluoro-β-arylvinyl sulfones and their application in Diels-Alder reactions

A new pathway towards α-fluoro-β-arylvinyl sulfones was elaborated. The reaction of β-bromo-β-fluorostyrenes with sodium 4-methylphenylsulfinate proceeds with maximum 94:6 stereoselectivity and 72-90% yields. The formed α-fluoro-β-arylvinyl sulfones were found to be good dienophiles for Diels-Alder reactions with simple 1,3-dienes. From corresponding (E)-configured dienophiles and cyclopentadiene, cycloadducts bearing the fluorine substituent in exo-position were formed predominantly, while with diphenylisobenzofuran the products with endo-orientation of the fluorine were obtained as the major products. From these cycloadducts, as a proof of principle, p-toluenesulfinic acid was eliminated to give 2-fluoro-3-(4-nitrophenyl)norbornadiene, the formal [4+2]-cycloadduct of cyclopentadiene and 1-fluoro-2-(4-nitrophenyl)acetylene, or the corresponding diphenylisobenzofuran cycloadducts, respectively. This reaction was not successful when other β-hydrogen atoms are accessible for elimination.     

Regio- and stereo-selectivities of the ortho and meta photocycloaddition reactions of ethylenes to benzene and its simple derivatives

The photoreactions of benzene, toluene, anisole, and benzonitrile with acrylonitrile, methacrylonitrile, and vinyl acetate, and of toluene and o- and p-xylene with maleic anhydride are described. The acrylonitriles do not react with benzonitrile but yield mixtures of ortho photocycloadducts with the other arenes. Contrary to previous findings both exo and endo stereoisomers of the ortho cycloadducts of benzene and acryloitrile are formed: the reaction is selective towards the exo isomer but the stereoisomers from methacrylonitrite and benzene are formed with approximately equal efficiencies. Complex mixtures of regio- and stereoisomers of the ortho cycloadducts are formed between toluene and the acrylonitriles but their addition to anisole is more selective and in acetonitrile essentially only 1,2-attack of the ethylene on the arene is observed. The 2:1 photoadducts of maleic anhydride with toluene and o- and p-xylene reflect formation of two regio ortho photocycloadducts in each case. The variation in the ratios of these isomers with temperature and light intensity is interpreted in terms of the differing photolabilities of the 1:1 adducts and their reactivities towards the thermal addition of the second molecule of maleic anhydride. Vinyl acetate undergoes 1,2-cycloaddition to benzonitrile but with the other arenes, meta cycloadducts are favoured. These latter additions are specifically 2,6- with respect to toluene and anisole but there is little regioselectivity with respect to the ethylene although the 7-endo acetate of the meta cycloadduct with benzene does constitute 60% of the reaction mixture.     

An efficient approach to isoindolo[2,1-b][2]benzazepines via intramolecular [4+2] cycloaddition of maleic anhydride to 4-α-furyl-4-N-benzylaminobut-1-enes

Acylation of 4-α-furyl-4-N-benzylaminobut-1-enes with maleic anhydride gave 4-oxo-3-aza-10-oxatricyclo[5.2.1.0^1,5]dec-8-ene-6-carboxylic acid via amide formation followed by intramolecular Diels-Alder reaction of furan (IMDAF). The cycloaddition proceeded under mild reaction conditions (25 °C) and provided only the exo-adduct in quantitative yield. Treatment of this compound with PPA gave isoindolo[2,1-b][2]benzazepine derivatives via ring opening, aromatization and intramolecular electrophilic alkylation. In order to extend the scope of the reaction sequence, 7-oxo-5,11b,12,13-tetrahydro-7H-isoindolo[2,1-b][2]benzazepine-8-carboxylic acids were further transformed into useful synthetic intermediates.     

Alkatrienyl Sulfoxides and Sulfones,VIII: Diels–Alder Reaction of 3‐Methylpenta‐1,2,4‐trienyl Phenyl Sulfoxide with Maleic Anhydride

Regio‐and stereo‐selective Diels–Alder reaction of 3‐methylpenta‐1,2,4‐trienyl phenyl sulfoxide with maleic anhydride in the absence or presence of a Lewis acid as catalyst with formation of sulfinyl‐substituted benzofuran or norbornene cycloadducts, respectively, is described.     

Diels-Alder reactivity of 4-aryl-1-phthalimido-2-siloxy-1,3-butadienes

The reaction of (1Z,3E) and (1E,3E) 4-aryl-1-phthalimido-2-trialkylsiloxy-1,3-butadienes with maleimides and quinones has been studied. The observed exo-stereospecifity can be attributed to the simultaneous presence of the phthalimido and aryl groups, which produce strong hindrance during the endo approach.     

Synthesis of new polyazines by 1,4 photochemical or anionic polymerization of 1,2-diaza-1,3-butadienes

Photochemical and anionic polymerizations of 1,2-diaza-1,3-butadienes are described. Photochemical polymerization was smoothly performed by irradiation of some 1-aminocarbonyl-1,2-diaza-1,3-butadienes with high pressure mercury arc (λ = 300 nm) in the presence of allyltributylstannane. Molecular weights (M_w) in the range 14.6-559 × 10² g/mol were obtained. The TGA curve revealed a first weight loss starting at about 200 °C of some 85%, and a second starting at about 300 °C. The DSC showed the glass transition (T_g) at about −34 °C. Anionic polymerization was performed by treatment of some 1-alkoxycarbonyl-1,2-diaza-1,3-butadienes with n-butyllithium. Molecular weights (M_w) in the range 8.44-242 × 10² g/mol were obtained.     

Synthesis of P-chiral phosphines via chiral metal template promoted asymmetric furan Diels-Alder reaction

The organoplatinum complex containing ortho-metalated (S)-(1-(dimethylamino)ethyl)-naphthalene as the chiral auxiliary has been used to promote the asymmetric [4+2] Diels-Alder reaction between phenyldivinylphosphine and 2-diphenylphosphinofuran. The reaction was complete in 6 days at room temperature, with the formation of four isomeric diphosphino-substituted oxanorbornene metal complexes in the ratio of 4:2:2:1. Only the exo-cycloaddition products were formed. The formation of stereogenic carbon centers within the oxanorbornene skeleton are highly stereoselective, with all four cycloadducts adopting the same absolute configurations. However, the stereocontrol at the external phosphorus stereogenic center is less efficient (S_p:R_p = 2:1 for the template cycloadducts). The chiral naphthylamine auxiliary could be removed chemoselectively by treatment with concentrated hydrochloric acid, and further ligand liberation of the dichloro complexes with aqueous cyanide gave the diphosphino-substituted oxanorbornene ligands. Hydrogenation of the double bonds in the cycloadduct stabilizes the phosphorus stereogenic center of the free diphosphine ligand which otherwise undergoes inversion of absolute configuration.     

New data on the reaction of hexafluorothioacetone with styrenes

Hexafluorothioacetone, generated in situ from its cyclic dimer (1) in the presence of CsF catalyst was shown to react with styrene or 4-alkyl-styrenes (alkyl = c-hexyl, Me, t-Bu) forming the corresponding 2:1 Diels-Alder cycloadducts as major products. All cycloadducts were isolated, and the structure of two of them was established by single crystal X-ray diffraction. While 2-vinylnaphthalene, 4-CF₃- and 4-Cl-styrenes in the presence of CsF catalyst react with 1 producing the corresponding 1:1 Diels-Alder cycloadducts, 4-alkoxy styrenes were found to produce preferentially the corresponding thietanes. The reaction of styrenes with 1 was also found to be sensitive to the source of fluoride ion and the solvent.     

Mechanism of phthalic anhydride formation in the oxidation of n-pentane on a vanadium-phosphorus oxide catalyst

The reaction paths of product formation in the partial oxidation of n-pentane on vanadium-phosphorus oxide (VPO) and VPO-Bi catalysts are considered. The condensed products of n-pentane oxidation were analyzed by chromatography-mass spectrometry, and the presence of C₄ rather than C₅ unsaturated hydrocarbons was detected. It was found that the concentration of phthalic anhydride in the products increased upon the addition of C₄ olefins and butadiene to the n-pentane-air reaction mixture. With the use of a system with two in-series reactors, it was found that the addition of butadiene to a flow of n-butane oxidation products (maleic anhydride, CO, and CO₂) resulted in the formation of phthalic anhydride. The oxidation of 1-butanol was studied, and butene and butadiene were found to be the primary products of reaction; at a higher temperature, maleic anhydride and then phthalic anhydride were formed. The experimental results supported the reaction scheme according to which the activation of n-pentane occurred with the elimination of a methyl group and the formation of C₄ unsaturated hydrocarbons. The oxidation of these latter led to the formation of maleic anhydride. The Diels-Alder reaction between maleic anhydride and C₄ unsaturated hydrocarbons is the main path of phthalic anhydride formation.