- The Pentamethoxyallyl Cation
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The pentamethoxyallyl cation is readily generated from pentamethoxychlorocyclopropane by treatment with AlBr3 or SnCl4 in CD2Cl2 or with SnCl4 in SO2.
- Moss, Robert A.,Guo, Wenjeng,Hagedorn, Alfred,Beveridge, Richard
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- Modulation of carbene spin state population through precursor photophysics
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Dicarbomethoxycarbene can be generated by photolysis of S-C sulfonium ylides derived from thiophene. By manipulating the thiophene (leaving group) portion of the ylide, the initial spin distribution of the carbenes can be strongly influenced. With certain carbene traps, product distributions from dicarbomethoxycarbene depend on the initial spin state distribution in which the carbene is generated and this is used as a means to report on the initial spin state distributions. This approach should be general for other carbenes generated from analogous precursors.
- Jenks, William S.,Heying, Melanie J.,Rockafellow, Erin M.
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supporting information; experimental part
p. 955 - 958
(2009/07/11)
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- On the most powerful chemical traps for bis(methoxycarbonyl)carbene (=2-methoxy-1-(methoxycarbonyl)-2-oxoethylidene)
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The efficiency and validity of different chemical substrates for trapping bis(methoxycarbonyl)carbene (=2-methoxy-1-(methoxycarbonyl)-2-oxoethylidene; 1) is dependent on the conditions of carbene generation. On conventional photolysis of dimethyldiazomalonate (=2-diazopropanedioic acid dimethyl ester; 2) by long-wave UV light (through a Pyrex filter, λ > 290 nm), the most powerful trap for carbene 1 in the series of substrates Me2S, MeOH, cyclohexane, and pyridine is Me2S (with an efficiency ratio of ca. 6 :4 : 2 : 1, resp.). When short-wave decomposition of diazomalonate 2 is employed (through a quartz filter, λ > 210 nm), more reliable and useful chemical traps for bis(methoxycarbonyl)carbene (1) are pyridine and cyclohexane, whose adducts with 1 are rather stable under short-wave-irradiation conditions. Application of alcohols for the trapping of 1 proves to be preferential when simultaneous monitoring of carbene and oxoketene formation during photolysis is necessary.
- Shevchenko, Vladimir V.,Zhegalova, Natalya G.,Borzenko, Andey O.,Nikolaev, Valerij A.
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experimental part
p. 501 - 509
(2009/02/07)
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- S,C-sulfonium ylides from thiophenes: Potential carbene precursors
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Photolysis of S,C-sulfonium ylides derived from thioanisol, thiophene, benzothiophene, or dibenzothiophene provides products deriving from dicarbomethoxycarbene. In methanol, no rearrangement of the carbene to the ketene derivative is observed. Formation of dibenzothiophene and benzothiophene is quantitative. For the thiophene-based ylide, insertion of the carbene into the α-CH bond of thiophene is observed. Evidence is presented that supports formation of both singlet and triplet carbene. Copyright
- Stoffregen, Stacey A.,Heying, Melanie,Jenks, William S.
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p. 15746 - 15747
(2008/09/19)
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- Enzymatic Synthesis of Chiral Monosubstituted Malonates in Organic Solvents
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Prochiral stereospecificity of enzymes in organic solvents was used to develop a strategy for the formation of heretofore unknown chiral monosubstituted malonate diesters with high enantiomeric excess.The enzymatic reaction involved transesterification of symmetrical monosubstituted dimethyl malonates with benzyl alcohol, exploiting the ability of lipases to discriminate between the enantiotopic ester groups of the symmetrical malonate molecule.This enzymatic approach is not feasible in aqueous solutions because the activated malonic hydrogen invariably undergoes fast exchange accompanied by racemisation.The synthetic utility of this method was further demonstrated by converting the configurationally unstable mixed methyl benzyl diesters into the corresponding half esters, which were in turn selectively reduced into configurationally stable and synthetically useful hydroxyesters.
- Shapira, Michal,Gutman, Arie L.
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p. 1689 - 1700
(2007/10/02)
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- A CONVENIENT METHOD FOR ENZYMATIC BENZYL-ALKYL TRANSESTERIFICATION UNDER MILD NEUTRAL CONDITIONS
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Lipases from Candida cylindracea and from Pseudomonas fluorescens efficiently catalyse the benzyl to alkyl transesterification in organic solvents under mild conditions in nearly quantitative yields.
- Gutman, Arie L.,Shkolnik, Eleonora,Shapira, Michal
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p. 8775 - 8780
(2007/10/02)
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- Reactions of Carbenes with Oxetane and with Oxetane/ Methanol Mixtures
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Ethoxycarbonylcarbene, bis(methoxycarbonyl)carbene, phenylcarbene (17a), diphenylcarbene (17b), fluorenylidene (17c), 2-furylcarbene (31a), 2-furyl(phenyl)carbene (31b), 4-oxo-2,5-cyclohexadienylidene (40), and 4,4-dimethyl-2,5-cyclohexadienylidene (53) were generated by photolysis of the appropriate diazo compounds.With neat oxetane, most of these carbenes react by competitive C-H insertion (B -> A, Scheme 1) and ylide formation (B -> C). 31a and 40 do not insert into C-H bonds; 31b does not attack oxetane but rearranges exclusively with formation of 26.The ylides undergo Stevens rearrangement to give tetrahydrofurans (C -> D) and α',β-elimination, leading to allyl ethers (C -> E).With oxetane/ methanol mixtures, the intervention of oxonium ions (H) is indicated by the formation of 1,3-dialkoxypropanes (I).The oxonium ions arise either by protonation of the ylides (C -> H) or by protonation of the carbenes (B -> G), followed by electrophilic attack of the carbocations (G) at oxetane (G -> H).The former route is followed by the alkoxycarbonylcarbenes and by 40; the ylides derived from the remaining carbenes do not react with methanol, owing to their rapid Stevens rearrangements.Protonation of the carbenes 17b, 31, and 53 is clearly indicated by their product ratios and, for 31, by the formation of isomeric ethers (33, 36).The more electrophilic carbenes discriminate but slightly between oxetane and methanol while the more nucleophilic carbenes (17b, 31, 53) prefer the protic methanol strongly over the aprotic oxetane. Key Words: Carbenes/ Oxygen ylides/ Stevens rearrangement/ Oxonium ions/ Insertion, O-H/ Ylides
- Kirmse, Wolfgang,Lelgemann, Rudolf,Friedrich, Klaus
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p. 1853 - 1863
(2007/10/02)
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- Reaction of Dicarbomethoxycarbene with Acetaldehyde and Simple Ketones
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Singlet dicarbomethoxycarbene reacts with acetaldehyde to give dioxolane 8, the ultimate product of formation of ylide 10 and subsequent addition of a second molecule of aldehyde.Photosensitized generation of the carbene gives increased hydrogen abstraction and decreased products derived from the ylide.Replacement of the aldehyde with a ketone changes the course of the reaction and dioxolanes (11, 20, and 26) become the major products.Suggestions are made for the mechanisms of the singlet and triplet reactions and for the peculiar behavior of acetone in which generation of singlet and triplet carbene gives the same product slate.
- L'Esperance, Robert P.,Ford, Thomas M.,Jones, Maitland
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p. 209 - 213
(2007/10/02)
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- Isolation, Structure, and Synthesis of Hermidin, a Chromogen from Mercurialis perennis L.
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Hermidin has been isolated from Mercurialis perennis L. as a colourless, crystalline compound, shown by synthesis to be 5-hydroxy-4-methoxy-1-methylpyridine-2,6(1H,3H)-dione (2), which in aqueous solution exist as a dihydroxy-4-methoxy-1-methylpyridin-2(1H)-one (3) or (4).Oxidation of hermidin with nitric acid affords 4-methoxy-1-methylpyridine-2,3,6-trione (16).When equimolecular amounts of the latter and hermidin in aqueous solution are mixed an immediate deep blue solution containing an ion-radical (17) results.Oxidation of hermidin by air in methanolic solution gives 5,5'-dihydroxy-4,4'-dimethoxy-1,1'-dimethyl-3,3'-bipyridine-2,2',6,6'-(1H,1'H,3H,3'H)-tetraone (18), and in pyridine solution 4,4'-dimethoxy-1,1'-dimethyl-3,3'-bipyridine-2,2',5,5',6,6'-hexaone (23).Knackmuss' conclusions on the oxidation of 3,6-dihydroxy-4-methylpyridin-2(1H)-one (26) have been re-interpreted. 3,4-Dihydroxy-5-methoxy-1-methylpyridin-2(1H)-one (7) has been synthesized.
- Swan, George A.
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p. 1757 - 1766
(2007/10/02)
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- Catalytic Role of Copper Triflate in Lewis Acid Promoted Reactions of Diazo Compounds
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Copper(II)trifluoromethanesulfonate is often a superior catalyst for transformations of diazo compounds that are normally promoted by boron trifluoride etherate.Formal carbon-oxygen insertion occurs when ortho esters are reacted with diazocarbonyl compounds in the presence of Cu(OTf)2, although the use of BF3*Et2O provides higher yields of insertion products.In contrast, the β,γ-unsaturated diazo ketone 6 undergoes intramolecular cyclization to produce the corresponding cyclopentenone in higher yield with Cu(OTf)2 catalysis than with the use of BF3*Et2O.Intamolecular cyclopropanation of the γ,δ-unsaturated diazo ketone 8 occurs in exceptionally high yield in the presence of Cu(OTf)2, whereas with BF3*Et2O intramolecular cyclization occurs with extensive rearrangement anf fluoride transfer.Copper(II)triflate is unique among transition-metal catalysts that are normally employed for reactions with diazo compounds in its effectiveness for these transformations.
- Doyle, Michael P.,Trudell, Mark L.
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p. 1196 - 1199
(2007/10/02)
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- LITHIOPENTAMETHOXYCYCLOPROPANE: PRECURSOR FOR THE SYNTHESIS OF HIGHLY OXYGENATED CYCLOPROPANES
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Pentamethoxychlorocyclopropane (1) affords the corresponding lithiocyclopropane (2) upon treatment with t-butyllithium.Reagent 2 can be quenched with H2O, MeI, Me3SiCl, or MeSSMe to give excellent yields of highly oxygenated cyclopropanes 3-6.
- Moss, Robert A.,Hui, Henry K.
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p. 305 - 312
(2007/10/02)
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