Acid-promoted Reaction Research Articles

Stereoselective syntheses of substituted pterocarpans with anti-HIV activity, and 5-aza-/5-thia-pterocarpan and 2-aryl-2,3-dihydrobenzofuran analogues

Oxygenated pterocarpans and 5-azapterocarpans are prepared utilizing Lewis acid-promoted reactions of 2-alkoxy-1,4-benzoquinones with 2 H-chromenes and N-tosyl-1,2-dihydroquinolines, respectively. Similarly, benzannulated analogues are prepared via reactions of 5-alkoxy-1,4-naphthoquinones with chromenes, and related 2-aryl-2,3-dihydrobenzofurans result from reactions of styrenes with the quinones. Syntheses of 5-thiapterocarpans are also described utilizing Pd(0)-coupling of o-chloromercuriophenols with 2 H-chromenes.

Synthesis of (±)-licarin B and eupomatenoids-1 and -12: A general approach to 2-aryl-7-alkoxy-benzofuranoid neolignans

New syntheses of the title compounds are described using Lewis acid-promoted reactions of styrenes with N-phenylsulfonyl-1,4-benzoquinone monoimines to regioselectively form the 2-arylbenzofuranoid ring system followed by conversion of the aromatic N-phenylsulfonyl moiety into a propenyl substituent.

The Lewis acid-promoted reactions of o-phthaladehyde with trialkyl phosphites: Formation of 1-dialkoxyphosphorylisobenzofuran

The Lewis acid-promoted reaction of o-phthaladehyde with trialkyl phosphites led to the formation of 1-dialkoxyphosphorylisobenzofurans, which underwent Diels-Alder reaction with dienophiles such as N-phenylmaleimide, maleic anhydride to give the corresponding adducts.

Chemo- and regioselectivity in the Lewis acid-induced reaction of sterically unhindered isocyclic allylsilanes with 3-butyn-2-one

The primordial influence of the ring size in the Lewis acid-promoted reaction of sterically unhindered isocyclic allylsilanes with 3-butyn-2-one is reported. In all the cases, the classical Sakurai-Hosomi reaction is, for the most part or fully, suppressed. With cyclohexanic frameworks, allyltrimethylsilanes like 1a-c afford mainly or solely H-ene components. This chemoselectivity is quite different from the one of cyclopentanic core compounds 1d-f which act primarily as [2 + 2] cycloaddition agents at the expense of the H-ene products procedure. Furthermore, the regioselectivity of the H-ene products formation is opposite for the two skeletons. Using Me 2AlCl, the [2 + 2] cyloaddition of five carbon atoms isocyclic allylsilanes proceeds with complete chemoselectivity and provides an efficient synthesis of functionalized bicyclo [3. 2. 0] heptenes.

Substituent-control of stereoselectivity in the reaction of allylic tins. Anti-selective lewis acid-promoted reaction toward aldehydes

In the reaction of cis-2-alkenyltins (allylic tins) toward aldehydes, unusual anti-homoallyl alcohols were selectively obtained when the substituent at the 2-position of the alkenyltin reagent was a bulky one such as a tert-butyl or trialkylsilyl group. This reaction is assumed to proceed via the inverse antiperiplanar acyclic transition state.

Why Do Catalytic Quantities of Lewis Acid Generally Yield More Product than 1.1 Equiv in the Intramolecular Diels-Alder Reaction with a Furan Diene? 2.(1) AM1 Calculations and Mathematical Simulation of the Equilibria.

The results presented here provide additional support for our hypothesis based on the relative basicity of the reaction controlling functional group to rationalize experimental observations on intramolecular Diels-Alder reactions with a furan diene (IMDAF) regarding the quantity (0.1 or 1.1 equiv) of Lewis acid required to facilitate the reaction most effectively. Heats of formation, DeltaH(f), and heats of reaction, DeltaH(R), have been obtained using AM1 calculations for 26 IMDAF reactions. These DeltaH(R) are generally exothermic (indicating that these IMDAF reactions are favorable) and can be qualitatively correlated with experimental yields of adduct, thereby providing a means of predicting the feasibility of the IMDAF promoted by 0.1 equiv of Lewis acid. The equilibria involved in the Lewis acid-promoted IMDAF reaction have been qualitatively interpreted using reaction coordinate diagrams and quantitatively investigated by generating a mathematical simulation of the reaction scheme. This demonstrates that the experimental behavior of the IMDAF reaction is well represented by the relative basicity hypothesis and that the LA concentration-dependent behavior should also be observed for other Lewis acid-promoted organic reactions.

Uncatalysed (thermal) and Lewis acid-promoted asymmetric hetero-Diels–Alder reaction of 1-thiabuta-1,3-dienes (thiochalcones) with di-(–)-menthyl fumarate. Configuration determination by X-ray crystallographic analysis of (2S,3R,4R)-(+)-2,3-bis[(–)-menthoxycarbonyl]-4,6-diphenyl-3,4-dihydro-2H-thiopyran and conversion of cycloadducts into optically pure diols

1-Thiabuta-1,3-dienes 1 (thiochalcones) underwent asymmetric hetero-Diels–Alder reaction with di-(–)-menthyl fumarate 2 to afford a mixture of 3,4-cis-(3 and 4) and 3,4-trans-dihydrothiopyrans (5 and 6) in good-to-excellent chemical yield with fair diastereoselectivities, the stereoisomer 3 predominating. An endo(3,4-cis): exo(3,4-trans) selectivity was observed in the ratio 98–85:2–15. The uncatalysed reaction at 20–40°C showed 10–48% de in diastero-π-facial selectivity, while the selectivity was improved to 62–71% de by using a suitable Lewis acid under appropriate reaction conditions. The 3,4-cis-stereoisomer formed favourably in both thermal and Lewis acid-promoted reactions, had the same configuration 2S,3R,4R(3). The configuration was unequivocally established by X-ray crystallographic structural analysis using the isomer 3a (R1= R2= Ph). The isomer 3, which could be obtained stereochemically homogeneous by chromatography and recrystallization, was consecutively transformed to optically pure diols 7 and 8 or 9 by LiAlH4 reduction, followed by reductive desulfurization with Raney Ni.

Contrasting reactivity in Lewis acid-promoted reactions of thio- and silyl-allenes with 1,4-benzoquinones

In Ti(IV)-promoted reactions with 2-methoxy-1,4-benzoquinones 3, thioallenes 1 give 2+2 and/or 3+2 products via attack on a CC moiety of the quinone, whereas silylallene 2 gives products derived from attack on a carbonyl group of the quinone.

Lewis Acid-Promoted Reactions of Styrenyl Systems with Benzoquinone Bisimines: New Regioselective Syntheses of Substituted 2-Aryl-2,3-dihydroindoles and 2-Arylindoles

BF3-promoted reactions of 2-alkoxy-1-(N-benzoyl)-4-(N-benzenesulfonyl)-1,4-benzoquinone bisimines (2/3) with β-methylstyrenes yield trans-5-(N-benzoylamino)-6-alkoxy-2-aryl-3-methyl-2,3-dihydroindoles (4/5) or 3-(N-benzoylamino)-6-aryl-7-methyl-4,8-dioxobicyclo[3.2.1]oct-2-enes (6) regio- and stereoselectively. The former are produced in reactions with styrenes bearing alkoxy substituents, whereas with neutral or electron deficient styrenes, the latter are found. As applications of this new methodology to the synthesis of biologically interesting molecules, reactions of 2H-chromenes and 7-methoxy-N-toluenesulfonyl-1,2-dihydroquinoline with the bisimines have been developed as routes to substituted aza- and diazapterocarpans. Syntheses of 2-arylindoles via oxidation of the dihydroindoles 4 are also reported.

Studies on the Synthesis of Acanthodoral and Nanaimoal: Evaluation of Cationic Cyclization Routes

Intramolecular Lewis acid-promoted reactions of α,β-unsaturated ketone 6 and aldehydes 7 and 8 were examined as potential routes to acanthodoral (1), a structurally interesting natural product. Ketone 6 afforded ene product 22 exclusively, and both 7 and 8 gave mixtures of bicyclic aldehydes 3 and 26 and tricyclic aldehyde 25. The latter most likely results from 7 by intramolecular cyclization of the alkene onto the Lewis acid-activated carbonyl moiety affording carbocation 31 followed by a 1,2-hydride shift and ring closure. Starting from 8, tricyclic aldehyde 25 apparently forms by cyclization to cation 35 and ring closure to cyclobutane 36, followed by ring opening to 31, the same cation as formed in reactions of 7. Nanaimoal (3) results from loss of H+ from 31, and bicyclic aldehyde 26 may be formed in a similar manner or by a concerted ene reaction. The configuration of 25 establishes that the stereochemistry of the initial cyclization to 31 precludes the possible use of this strategy for the synthes...

Switchable regioselectivity in lewis acid-promoted reactions of 1,4-benzoquinone monoimides with styrenyl systems: Selective syntheses of either 2-aryl-2,3-dihydrobenzofurans or 2-aryl-2,3-dihydroindoles

Reactions of 4-(N-phenylsulfonyl)-2-alkoxy-1,4-benzoquinone monoimines with electron-rich propenylbenzenes promoted by BF 3 yield 7-alkoxy-2-aryl-3-methyl-5-(N-phenylsulfonyl)amino-2,3-dihydrobenzofurans nearly exclusively; whereas, promotion of the reactions by >2 equiv of Ti(IV) gives mainly N-phenylsulfonyl-6-alkoxy-2-aryl-5-hydroxy-3-methyl-2,3-dihydoindoles.

Lewis Acid-Promoted Reactions of Unsymmetrically Substituted Stilbenes with 2-Methoxy-1,4-benzoquinones: Stereoselective Synthesis of trans-2,3-Diaryl-2,3-dihydrobenzofurans

Lewis Acid-Promoted Reactions of Unsymmetrically Substituted Stilbenes with 2-Methoxy-1,4-benzoquinones: Stereoselective Synthesis of trans-2,3-Diaryl-2,3-dihydrobenzofurans

Cycloaddition reactions of 1,4-benzoquinone mono- and bisimides with styrenyl systems: New syntheses of nitrogen substituted azapterocarpans, pterocarpans, 2-aryl-2,3-dihydroindoles and -dihydrobenzofurans

Lewis acid-promoted reactions of 1,4-benzoquinones and 1,4-benzoquinone bis- and monoimides with various 2 H-chromenes, N-tosyl-1,2-dihydroquinolines and styrenes regio- and stereoselectively produce the title compounds in good yields.

Synthesis of Binaphthol-Derived Podands Based on Selective, Acid-Promoted Reaction of Benzylic Alcohols with Ethylene Glycols

3-Hydroxymethyl and 3,3’-bis(hydroxymethyl) binaphthols (1,2) react under acidic conditions smoothly with ethylene glycols to afford corresponding ethers 3 and 4, mostly in good to excellent yields. Screening experiments revealed that the reaction occurs efficiently with several different Lewis acids. Glycerine as glycol component gives a 7:1 mixture of 1o-OH- and 2o-OH-derived products 5 and 6. Monoalkyl derivatives of ethylene glycols are behaving in a similar way the reaction times being substantially longer.

Stereospecific Lewis Acid-Promoted Reactions of Styrenyl Systems with 2-Alkoxy-(6-Alkyl)-1,4-Benzoquinones: Scope, Limitations, and Synthetic Applications

Stereospecific Lewis Acid-Promoted Reactions of Styrenyl Systems with 2-Alkoxy-(6-Alkyl)-1,4-Benzoquinones: Scope, Limitations, and Synthetic Applications

Evaluation of a synthetic route to ε-viniferin based on a new method for the stereoselective preparation of 2,3-diaryl-2,3-dihydrobenzofurans

Lewis acid-promoted reactions of unsymmetrically substituted ( E)-stilbenes with 2-methoxy-1,4-benzoquinones regio- and stereoselectively produce trans 2,3-diaryl-2,3-dihydrobenzofurans.

The Stereoselective Synthesis of Trisubstituted Olefins by the Reaction of (2-Phenylthiocyclobutyl)methyl Benzoates and Methyl Ethers with Silyl Nucleophiles

Abstract The Lewis acid-promoted reaction of 2-phenylthiocyclobutanemethanol derivatives with silyl nucleophiles gave the corresponding trisubstituted olefins with high stereoselectivity in good yields. It is suggested that the reaction proceeds via the thionium ion intermediate.

Studies on Conjugated Nitriles. VII. Lewis Acid-Promoted Reaction of Active Methylene Compounds with Diethyl Phosphorocyanidate; Preparation of .ALPHA.,.BETA.-Unsaturated .ALPHA.-Aminophosphonates.

Reaction of diethyl phosphorocyanidate (DEPC) with dimethyl malonate (1a) and ethyl cyanoacetate (1b) in the presence of zinc chloride and triethylamine resulted in selective addition of 1a, b to the cyano group of DEPC to give α, β-unsaturated α-aminophosphonates (2a, b). In contrast, similar treatment of enolizable methyl acetoacetate (1c) and acetylacetone (1d) with DEPC gave the corresponding enolphosphates (4c, d) as a result of nucleophilic displacement on the phosphorus atom of DEPC. Conversion of the resulting α-aminophosphonate (2a) to uracil-6-phosphonates (6a, b) was achieved by treatment with phenyl isocyanate (5a) and isothiocyanate (5b), respectively.

1,5-Asymmetric induction in Lewis acid-promoted reactions of (5-alkoxy-2,4-dimethylpent-2-enyl)tributylstannanes and aldehydes

Allyltin trihalides generated in situ from (5-alkoxy-2,4-dimethylpent-2-enyl)tributylstannanes 8–10 and tin(IV) halides react with aldehydes with excellent 1,5-stereocontrol.

Influence of the chalcogenocarbonyl group on the structure of heterocyclic analogues of β-tricarbonyl compounds. Synthesis and structural features of Schiff bases derived from 3-formyl-4-thio(seleno)coumarin

The equilibrium between Z and E isomers of Schiff bases derived from 3-formyl-4-thio(seleno)coumarin 1, 2 resulting from the hindered rotation around the exocyclic carbon–carbon bond in the stable ketoamine tautomeric form has been studied by means of IR and 1D and 2D (NOESY)1H NMR spectroscopy and ab initio and semiempirical calculations. The ratio between the Z and E forms in solution was found to lie in the range 3.5:1–5.8:1 (XS) and 2.4:1–3.1:1 (XSe). The activation energy for Z⇌E interconversion was estimated for 1a and 2a using dynamic 1H NMR spectroscopy to be 45.7 and 35.3 kJ mol–1, respectively. The process was found to be catalysed by trifluoroacetic acid. The relative stability of both tautomeric and conformational isomers calculated theoretically at MP2/6–31G* level for the model molecules of 3-methylimino-4-hydroxy(thio, seleno)-2-pyrone is in good agreement with experimental data. Estimated activation barriers for the rotational mechanism are higher than experimentally obtained which may indicate a contribution from acid-promoted reaction pathways.