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971.
Teresa M.V.D. Pinho e Melo Ana L. CardosoClara S.B. Gomes António M.d'A. Rocha Gonsalves 《Tetrahedron letters》2003,44(33):6313-6315
2H-Azirine-3-carboxylates unsubstituted at C-2 act as dipolarophiles in the reaction with diazomethane giving new 4,5-dihydro-3H-pyrazole derivatives. The synthesis of a pyrimidine was also achieved via 1,3-dipolar cycloaddition of methyl 2-bromo-3-phenyl-2H-azirine-2-carboxylate with an azomethine ylide. 相似文献
972.
Wagner G 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(7):1503-1510
The [2+3] cycloaddition of nitrone PhCHdoublebondN(Me)O to nitriles RCtriplebondN (R=Me, Ph, CF(3)) was studied using quantum chemical calculations at the HF/6-31G* and B3LYP/6-31G* level of theory. With MeCN and PhCN, the reaction occurs through a concerted mechanism and leads selectively to Delta(4)-1,2,4-oxadiazolines rather than Delta(2)-1,2,5-oxadiazolines. Electron withdrawing substituents such as CF(3) at the nitrile provoke a non-synchronous bond formation, with the C-O bond being established on an earlier stage than the C-N bond. Additionally, the reaction becomes thermodynamically and kinetically more favourable. In the reaction of adducts of MeCN with BH(3) or BF(3) as model Lewis acids, the mechanism was found to change from fully concerted in the case of free MeCN towards a two-step reaction in the presence of BF(3), in which C-O bond formation occurs first. The BH(3)-mediated reaction occupies an intermediate stage where ring formation occurs in one-step but non-simultaneously, similar to the reaction of CF(3)CN. Interaction of the Lewis acid with the nitrile in the course of the reaction facilitates the cycloaddition by stabilizing transition states, intermediate and product rather than by activating the nitrile. The solvent influences the organic reaction mainly by lowering the energy of the reagents, thus leading to a higher activation barrier in a more polar solvent. In the Lewis acid mediated reaction, in contrast, the intermediate is strongly stabilised by a polar solvent and with that the synchronicity of the reaction changes in favour of a two-step mechanism. 相似文献
973.
Iva Blanáriková-HlobilováZuzana Kubánová Lubor Fišera Michal K CyranskiPiotr Salanski Janusz Jurczak Nada Prónayová 《Tetrahedron》2003,59(18):3333-3339
Chiral nitrones derived from l-valine react with methyl acrylate to afford the corresponding diastereomeric 3,5-disubstituted isoxazolidines. The dibenzylsubstituted nitrone gave also 3,4-disubstituted isoxazolidine in 4% yield, additionally. The stereoselectivity was dependent on the steric hindrance of the nitrone and reaction conditions. High pressure decreased the reaction time of the cycloadditions. The major products were found to have the C-3/C-6 erythro and C-3/C-5 trans relative configuration. The major cycloadduct undergoes N-O cleavage and deprotection to a chiral diaminodiol derivative. 相似文献
974.
Weimin PengShizheng Zhu 《Tetrahedron》2003,59(24):4395-4404
A series of 5-fluoroalkylated 1H-1,2,3-triazoles were synthesized in good yield by the regiospecific 1,3-dipolar cycloaddition reaction of (Z)-ethyl 3-fluoroalkyl-3-pyrrolidino-acrylates with aryl or benzyl azides. In the cases of benzyl azides, addition of Na2CO3 was crucial for a high yield of the triazoles. Tetrakis(dimethylamino)ethylene (TDAE) promoted reaction of bromodifluoro-methylated triazole with aldehydes affording a new class of β,β-difluoro-β-triazolyl alcohol derivatives, which were lactonized with catalytic amount of p-toluenesulfonic acid in toluene at 80-90°C to give a series of novel bicyclic gem-difluorinated 1H-pyrano[3,4-d][1,2,3]-triazol-4-one compounds in good yield. 相似文献
975.
Richard P Hsung Kevin P ColeLuke R Zehnder Jiashi WangLin-Li Wei Xiao-Fang YangHeather A Coverdale 《Tetrahedron》2003,59(3):311-324
A 20-step total synthesis of (±)-arisugacin A with an overall yield of 2.1% is described here in detail. This synthesis features a formal [3+3] cycloaddition reaction of α,β-unsaturated iminium salts with 6-aryl-4-hydroxy-2-pyrones through a highly stereoselective 6π-electron electrocyclic ring-closure of 1-oxatriene. A strategic dihydroxylation-deoxygenation protocol leading to the desired angular C12a-OH was developed to serve as a critical step in leading to the final total syntheses of arisugacin A. This synthetic endeavor also led to an interesting and unexpected retro-aldol-aldol sequence in the AB-ring. 相似文献
976.
The intrinsic reactivity of eight gaseous, mass-selected 2-azabutadienyl cations toward polar [4(+) + 2] cycloaddition with ethyl vinyl ether has been investigated by pentaquadrupole mass spectrometric experiments. Cycloaddition occurs readily for all the ions and, with the only exception of those from the N-acyl 2-azabutadienyl cations (N-acyliminium ions), the cycloadducts are found to dissociate readily upon collision activation (CID) both by retro-Diels-Alder reaction and by a characteristic loss of an ethanol (46u) neutral molecule. Ethanol loss from the intact polar [4(+) + 2] cycloadduct functions therefore as a structurally diagnostic test: 72 u neutral gain followed by 46 u neutral loss, i.e., as a combined ion-molecule reaction plus CID 'signature' for N-H, N-alkyl and N-aryl 2-azabutadienyl cations. The two N-acyliminium ions tested are exceptional as they form intact cycloadducts with ethyl vinyl ether which dissociate exclusively by the retro-Diels-Alder pathway. 相似文献
977.
The five new silanes C5Me3RSiMenCl3 − n (n = 3, R = i-Pr (5); n = 2, R = i-Pr (6); n = 2, R = s-Bu (7); n = 2, R = cyclohexyl (8); and n = 3, R = t-Bu (9)) were synthesized by reaction of 1-alkyl-2,3,4-trimethylcyclopentadienyl lithium salts with appropriate chlorosilane and characterized by NMR, MS, and IR spectra. At elevated temperatures (250-360 K), all the silanes undergo a non-degenerate sigmatropic silyl rearrangement, which generates non-equivalent structures a and b. The presence of minor structure c was observed in compounds 5 and 7 only. The Diels-Alder cycloaddition of 5 with strong dienophiles tetracyanoethylene (TCNE), and dimethylacetylenedicarboxylate (DMAD) provides compounds 10 and 11, which confirmed isomers a and b, respectively. The free energy of activation of b → a isomerization for compounds 5-8 evaluated from variable temperature NMR spectra show only marginal influence of group R on the 1,2-silyl shift rate. Moreover, in compounds 5 and 7, the process b → a was found significantly faster than b → c process in the above-mentioned temperature range. 相似文献
978.
Sergey M. Lukyanov Igor V. Bliznets Grigory G. Aleksandrov Andrei A. Vasil'ev 《Tetrahedron》2006,62(8):1849-1863
Sterically hindered 2,4-disubstituted 3-(5-tetrazolyl)pyridines were synthesized from corresponding nicotinonitriles using microwave technology. 2-Methylnicotinonitriles were converted into the 2-azidomethyl-3-cyanopyridines via 2-hydroxymethyl and 2-chloromethyl derivatives. Intramolecular [3+2] cycloaddition of an heteroaromatic cyano group to side azido group was carried out to form a novel heterocyclic system containing a (tetrazolo)azaisoindole unit. Condensation of the 2-methylnicotinonitriles and aldehydes gave rise to the corresponding 2-vinyl derivatives, which were then transformed into novel heterocyclic system (5,6-dihydrotetrazolo[5,1-f]-1,6-naphthyridine) by intramolecular N-alkylation reaction of tetrazole ring with olefinic fragment. The 3-(5-tetrazolyl)pyridines obtained were alkylated to give the various N- and C-benzyl derivatives as well as acylated to afford the 3-(1,3,4-oxadiazol-2-yl)pyridines in good yields. A majority of above-mentioned reactions was carried out under microwave irradiation. 相似文献
979.
Paramagnetic nitrile oxide, a derivative of 3-imidazoline-1-oxyl, has been generated by thermolysis of geminal nitrooxime. This compound readily undergoes cycloaddition with alkenes with both activated and nonactivated C=C bonds. The paramagnetic nitrile oxide may be used for introducing a nitroxyl into molecules with multiple carbon-carbon bonds.For the previous communication, see Ref. 1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 289–291, February, 1994. 相似文献
980.