Cycloaddition dipolaire-1,3 des arylnitriloxydes sur les dihydro-1,2 quinoleines n-substituees

The 1,3-dipolar cycloaddition reaction of arylnitrile oxideson 1,2- dihydroquinolines gives two adducts if the nitrogen atom of the dipolarphile carries benzoyl group. On the other hand the reaction provides only one adduct in the N-substituentis benzyl group. The structure of the cycloadducts has been established by spectroscopic and chemical method.     

Elektrochemische Bildung von Δ1,2-Norbornen

Electrochemical Formation of Δ^1,2-Norbornene The electrochemical reduction of 1,2-dihalogen norbornanes in tetrahydrofuran/furan leads to a mixture of two isomeric cycloadducts 6 and 7. The ratio of these adducts corresponds to those which have been found in reductive bisdehalogenation of 1 and 2 by butyllithium.     

Stereoselectivity in Diels-Alder reactions of diene-substituted N-alkoxycarbonyl-1,2-dihydropyridines

Diene substituent effects on the regiochemical and stereochemical outcomes of uncatalyzed Diels-Alder reactions of N-alkoxycarbonyl-1,2-dihydropyridines with both styrene and methyl vinyl ketone (MVK) were studied. Alkyl substitution on the diene in all cases examined resulted in a kinetic preference for 7-endo isomers (7-phenyl 51-96% exo and 7-acetyl 54-96% exo). For both dienophiles, the highest stereoselectivities (>or=89% endo) were observed with 5-methyl or 6-methyl substituents in the dihydropyridine. Theoretical calculations of the energies of gas phase endo and exo transition states at the RHF/3-21G(*) predict that total entropy, DeltaStotal, considerations favor endo cycloadducts for both dienophiles with DHP, while total energy considerations, DeltaEo, favor endo cycloadducts for styrene and exo cycloadducts for MVK. At this level, favored endo-phenyl isomers are correctly predicted for styrene reactions, but the calculation of 7-acetyl exo or endo isomer dominance is diene-substituent-dependent for MVK reactions. The preference for endo addition of MVK to the parent, 5-methyl, and 6-methyl-DHPs was successfully predicted by calculations at the B3LYP/6-31G* theory level.     

Nitrone cycloadditions to 1,2-diphenylcyclopropenes and subsequent transformations of the isoxazolidine cycloadducts

1,3-Dipolar cycloaddition of C-aryl,N-aryl (or N-methyl) nitrones with a number of 1,2-diphenylcyclopropenes substituted at the C(3) position occurs with the formation of expected "normal" cycloadducts (with N-methylnitrones) and products of their subsequent transformations. Among them are corresponding alpha-acetophenyl aziridines and tetra (or penta) -arylpyrroles. Aziridines and the normal cycloadducts can be also thermally converted to such arylpyrroles with moderate to good yields. Substitution at the C(3( position of cyclopropenes by an electron acceptor group decreases the reactivity of cyclopropenes.     

1,2-Homoheptafulvene: Synthesis and cycloaddition

1,2-Hemoheptafulvene ($\text{2}$) was synthesized and reacted with 4-phenyl-1,2,4-triazoline-3,5-dione to yield [4π + 2π] and [6π + 2σ + 2π] cycloadducts.     

Synthesis of Delta(1)-1,2-diazetines via a Diels-Alder cycloaddition approach

[reaction: see text] A novel method for the synthesis of Delta(1)-1,2-diazetines is presented. Diels-Alder cycloaddition of dienophile 4 with five dienes afforded cycloadducts in good to excellent yields. Four of the obtained cycloadducts were converted to the corresponding diazetines.     

Addition reactions of heterocycles. VI. Reactions of 1,2-dimethylpyrrole and 1-methyl-2-carbomethoxypyrrole with nitrilimines

Addition reactions of 1,2-dimethylpyrrole and 1-methyl-2-carbomethoxypyrrole with C-acetyl-N-phenylnitrilimine, have been investigated. 1,2-Dimethylpyrrole gives three different types of adducts: i.e. bis-cycloadducts (Vc) and (VIc), spirocycloadduct (IX), and non cyclic bis-adduct (XII). On the other hand, 1-methyl-2-carbomethoxypyrrole gives the bis-cycloadduct (VIb) only. Compound XII arises probably through a double 1,3-addition reaction, whereas the formation of cycloadducts Vc, VIc, and IX depends on the substituents present at C₂ of the pyrrole ring and consequentially on the intermediary occurence of mono-cycloadduct (IIIc), its methylenic tautomer VII, VIc, and XL The behaviour of the cycloadducts towards heating or acidic treatment showed a termal cleavage of the pyrazoline ring and acidic cleavage of the pyrrolidine ring.     

Diels-alder cycloadditions of diene-substituted N-ethoxycarbonyl-2-methyl-1,2-dihydropyridines with N-phenylmaleimide

The ten possible substitution patterns for N-Ethoxycarbonyl-2-methyl-1,2-dihydropyridines 5 in which one or two olefinic sites are alkyl substituted were synthesized and reacted with N-phenylmaleimide 2 to provide cycloadducts 6. N-Ethoxycarbonyl-5,6-cyclohexyl-2-methyl-1,2-dihydropyridine 51 provided the novel spirocycle 61.     

1,2‐Bis(trifluoromethyl)ethene‐1,2‐dicarbonitrile: Enol Ethers and Ketene Acetals as Cycloaddition Partners

(E)‐ and (Z)‐1,2‐bis(trifluoromethyl)ethene‐1,2‐dicarbonitrile ((E)‐ and (Z)‐BTE, resp., =(E)‐ and (Z)‐2,3‐bis(trifluoromethyl)but‐2‐enedinitrile) were used as a stereochemical probe in studying (2+2) cycloadditions of acceptor with donor alkenes. The additions to methyl (E)‐ and (Z)‐propenyl ether gave rise to the eight conceivable cyclobutanes 8 , although in different ratios in reactions of (E)‐ and (Z)‐BTE. The ¹⁹F‐NMR data served the structural assignment and the quantitative analysis. The mechanistic discussion is based on rotations and ring closures of the assumed 1,4‐zwitterionic intermediates. Dimethylketene dimethyl acetal, methylketene dimethyl acetal, and ketene diethyl acetal show an increasing rate in their reactions with BTE as well as in the equilibration of the cycloadducts.     

1,2,4-triazolium-ylures—V: Réactions des triazolium-phénacylures avec les alcynes dis-symétriques

4-Aryl-1,2,4-triazolium-phenacylides and ethyl propiolate give 3-substituted 4-aryl-1,2,4-triazoles via cycloadducts.     

Stereochimie de l'action des organomagnesiens et des organolithiens sur les dimethyl-1,2 alcoxy-1 silacyclobutanes

The reaction of organomagnesium organolithium compounds (alkyl, aryl, allyl and benzyl derivatives) with 1,2-dimethyl-1-alkoxy-l-silacyclobutanes proceeds with retention of configuration at the silicon atom. Stereochemical results are discussed in terms of the S_N2Si mechanism. The proposed configurations are supported by ¹H and ¹³C NMR data.     

Photocycloisomerization of Boc‐Protected 5‐Alkenyl‐2,5‐dihydro‐1H‐ pyrrol‐2‐ones

On irradiation (254 nm), the newly synthesized Boc‐protected 5‐alkenyl‐2,5‐dihydro‐1H‐pyrrol‐2‐ones 13 undergo regioselective intramolecular [2+2] photocycloadditions. While the allyl derivatives 13a – 13c afford mainly azatricyclo[3.3.0.0^2,7]octanones, i.e., crossed cycloadducts, the butenyl‐ and pentenyl‐substituted compounds 13d and 13e isomerize preferentially to straight cycloadducts.     

Specific synthesis of 1,2- and 1,3-dialkylidenecycloheptanes by [3+2+2] cyclization of alkenyl Fischer carbene complexes and allenes

The 1,2- and 1,3-dialkylidenecycloheptane rings are specifically assembled from chromium alkenyl Fischer carbene complexes and allenes via [3+2+2] cyclization reactions. The former cycloadducts are obtained when the cyclization is performed in the presence of 1 equiv of [Ni(cod)2], while the [Rh(cod)Cl]2-catalyzed cyclization leads to the latter cycloadducts.     

1,3‐Dipolar Cycloaddition of 4‐Chlorobenzonitrile Oxide with Some Dipolarophiles: Theoretical Analysis of Regioselectivity

Reaction of 4‐chlorobenzonitrile oxide ( 2 ) which was generated in situ with acrylo nitrile ( 3 ), vinyl acetate ( 4 ) and allyl bromide ( 5 ) as dipolarphile afforded the new cycloadducts 6a , 7a , and 8a compounds, respectively. Reactivity and regiochemistry of these reactions were investigated using activation energy calculations and density functional theory‐based reactivity indexes. The theoretical ¹³C NMR chemical shifts of the cycloadducts which were obtained by gauge‐invariant atomic orbital method were comparable with the observed values.     

Reactions of 1,3,4-triphenyl-1,2-dihydrophosphete

In contrast to previously reported reactivity of the tungsten pentacarbonyl complex of a 2-substituted 1,2-dihydrophosphete, which apparently undergoes electrocyclic ring opening to the corresponding 1-phospha-1,3-butadiene and subsequent [4+2] cycloaddition reactions with dienophiles, the reaction chemistry of 1,3,4-triphenyl-1,2-dihydrophosphete is dominated by its nucleophilic nature. Although low to modest yields of cycloadducts are obtained with some dienophiles, the reactions forming these products are apparently stepwise, as indicated by the loss of stereochemistry in the reaction of dimethyl maleate and in the competitive formation of a phosphorus-free dimer in the reaction of N-methylmaleimide. Dimethyl acetylenedicarboxylate affords three major products, each of which incorporates two equivalents of the acetylene, again apparently a result of initial nucleophilic addition of the dihydrophosphete to the “dienophile.” © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:9–19, 1998     

Synthesis and stereochemistry of some novel dihydropyrrolo[3,4-c]pyrazoles

Abstract  

Eleven novel dihydropyrrolo[3,4-c]pyrazole derivatives were obtained by the reaction of chiral (1R)-N-(1-phenylethyl)maleimide and C,N-aryl-substituted nitrilimines. The reaction afforded the cycloadducts as a regioisomeric mixture which can be separable in some cases. The structure and stereochemistry of cycloadducts were assigned on the basis of infrared (IR), ¹H and ¹³C nuclear magnetic resonance (NMR), mass and X-ray spectra, optical rotation measurements, and CHN analyses.     

4+2] Cycloaddition of 2-substituted 1,2-dihydropyridines with nitrosobenzene: asymmetric synthesis of trans-2-substituted 3-amino-1,2,3,6-tetrahydropyridines

[reaction: see text] A new methodology for the stereoselective synthesis of trans-2-substituted 3-amino-1,2,3,6-tetrahydropyridines is reported. The preparation of these 3-aminopiperidines is achieved by cycloaddition of nitrosobenzene with 2-substituted 1,2-dihydropyridines followed by chemoselective reduction of the cycloadducts. Enantioenriched 1,2-dihydropyridine derivatives are easily prepared from pyridine and a chiral amide following a previous report from our laboratories. Moreover, the in situ hydrogenation of these cycloadducts over palladium in a solution of hydrogen chloride in methanol led to tetrahydropyrroloimidazoles.     

Synthesis,absolute configuration and conformation of optically active 1,2-homoheptafulvalene

An optically active 1,2-homoheptafulvalene was successfully synthesized and subjected to spectroscopic investigation. The cycloaddition of the optically active hydrocarbon with tetracyanoethylene (TCNE) and 4-phenyl-1,2,4-triazoline-3,5-dione(PTAD) gave a [4 + 2] cycloadduct and a mixture of [8 + 2] cycloadducts, respectively, which are both optically active.     

1,3-Dipolar cycloaddition of nitrones with α,β-unsaturated sulfones

C,N-Diphenyl nitrones react with substituted α,β-unsaturated phenylsulfones yielding isoxazolidinic cycloadducts whose structure and stereochemistry were assigned on the basis of ¹H and ¹³C nmr data. The ycloaddition regioselectivity is discussed in accordance with frontier orbital considerations.     

1,3-Dipolar Cycloadditions to Strained Olefins

The Bredt olefins bicyclo [3.3.1]non-1-ene (2) , bicyclo [4.2.1]non-1 (8)-ene (3) , and bicyclo [4.2.1]non-1 (2)-ene (4) react rapidly with 1,3-dipoles such as diazomethane, phenyl azide, and mesitonitrile oxide to yield mixtures of two regioisomeric cycloadducts 10, 11 and 12 , respectively. On the contrary, cycloaddition to the comparable monocyclic 1-methyl-(E)-cyclooctene (5) is fairly regioselective. 2-Methylnorborn-2-ene (6) gives one isomer with mesitonitrile oxide (as do less strained olefins), but mixtures with diazomethane and phenyl azide. ¹H-NMR. and ¹³C-NMR. spectra of the cycloadducts are reported. The results are discussed in the light of frontier molecular orbital theory.