- Hetero-Diels-Alder Additions of α,β-Unsaturated-Acyl Cyanides. Part 3. Syntheses of 3-Bromo-2-ethoxy-3,4-dihydro-3H-pyran-6-carbonitriles, and about Their Transformation to 2-Ethoxy-2H-pyrans
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Cycloadditions of the α,β-unsaturated-acyl cyanides 1-3 with (Z)-or (E)-1-bromo-2-ethoxyethene (4) may be performed at moderate temperatures and provide in good yields the 3-bromo-2-ethoxy-3,4-dihydro-2H-pyran-6-carbonitriles 5-7, respectively (Scheme 1).Diastereomeric pairs of products result at room temperature merely from the "endo"- and "exo"-transition states; more complex mixtures appear above 60 deg C as a consequence of (Z)/(E)-isomerization of 4.The relative stability of the anomers of 5 and 6 is explored by treatment with BF3*Et2O.Acid alcoholysis (MeOH or EtOH) of 5 leads to acetals 9a, b of 4-bromo-5-oxopentanoate.Alkyl (2Z,4E)-5-ethoxypenta-2,4-dienoates 12, 17, and 20, are formed in alcoholic alkoxide solutions from 5,6,and 7, respectively, which is compatible with the intermediacy of 2-alkoxy-2H-pyrans and their valence tautomers, α,β-unsaturated-acyl cyanides.Methoxide addition to the CN group competes with dehydrobromination in case of 5; it leads to 3-bromo-3,4-dihydro-2H-pyran-6-carboximidate 13 (ca. 50percent at -20 deg C) which can be hydrolyzed to the methyl carboxylate 14.DBU (1,8-diazabicycloundec-7-ene) in benzene converts 5 to 6-ethoxy-2-oxohexa-3,4-dienenitrile (11), the ring-opening product of an obviously unstable 2-ethoxy-2H-pyran; the same reagent dehydrobrominates 6 to 2-ethoxy-4-methyl-2H-pyran-6-carbonitrile (15).HBr Elimination from 7 takes place with great ease in presence of pyridine, or even during chromatography on alumina, and leads to the stable ethyl 6-cyano-2-ethoxy-2H-pyran-4-carboxylate (18); this dimerizes at room temperature to give a 1:3 mixture of tricyclic adducts "endo"-21 and "exo"-21.The structure of the latter is established by an X-ray crystallographic analysis.
- Zhuo, Jin-Cong,Wyler, Hugo,Schenk, Kurt
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- Generation and trapping of ketenes in flow
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Ketenes were generated by the thermolysis of alkoxyalkynes under flow conditions, and then trapped with amines and alcohols to cleanly give amides and esters. For a 10 min reaction time, temperatures of 180, 160, and 140 °C were required for >95% conversion of EtO, iPrO, and tBuO alkoxyalkynes, respectively. Variation of the temperature and flow rate with inline monitoring of the output by IR spectroscopy allowed the kinetic parameters for the conversion of 1-ethoxy-1-octyne to be easily estimated (Ea = 105.4 kJ/mol). Trapping of the in-situ-generated ketenes by alcohols to give esters required the addition of a tertiary amine catalyst to prevent competitive [2+2] addition of the ketene to the alkoxyalkyne precursor.
- Henry, Cyril,Bolien, David,Ibanescu, Bogdan,Bloodworth, Sally,Harrowven, David C.,Zhang, Xunli,Craven, Andy,Sneddon, Helen F.,Whitby, Richard J.
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p. 1491 - 1499
(2015/03/04)
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- 2-Ethoxyvinyllithiums and diethoxyvinyllithiums: What makes them stable or fragile?
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The elusive (E)-2-ethoxyvinyllithium can be readily generated in tetrahydrofuran at -75 °C from (E)-1-bromo-2-ethoxyethylene by halogen/metal exchange and subsequently trapped with electrophiles. Alkylation opens a convenient entry to (E)-configurated enethers. (E)-2-Ethoxyvinyllithium decomposes rapidly at -50 °C whereas its (Z)-isomer, which lacks the possibility to eliminate lithium ethoxide in a favorable anti-periplanar process, is stable under the same conditions. (E)-1,2-Diethoxyvinyllithium even sustains reflux temperatures (approximately 75 °C). 2,2-Diethoxyvinyllithium and (Z)-1,2-diethoxyvinyllithium can be conserved at 0 °C although this time loss of alcoholate can occur in the anti-mode. Obviously it matters whether the energy-rich ethoxyacetylene is formed as the elimination product or simple acetylene, as in the case of (E)-2-ethoxyvinyllithium.
- Schlosser, Manfred,Wei, Heng-Xu
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p. 1735 - 1742
(2007/10/03)
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- AN EFFICIENT SYNTHESIS OF tert-ALKOXYETHYNES
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The synthesis of tert-butoxyethyne (1a) and (1-adamantyloxy)ethyne (1b) has been performed by a three step sequence starting from ethyl vinyl ether.Overall yields are 34 and 67percent, respectively.The process is insensitive to the steric hindrance of the tertiary alcohol used and can be conveniently modified for the preparation of 1-tert-alkoxy-1-alkynes in high yield, without isolation of the intermediate 1-tert-alkoxyethyne.
- Pericas, Miquel A.,Serratosa, Felix,Valenti, Eduard
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p. 2311 - 2316
(2007/10/02)
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- Dehydrobromination of 1,2-Dibromoethoxyethane Using Various Amine Bases
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1,2-Dibromoethoxyethane was treated with a number of basic reagents, mainly tertiary amines, to accomplish dehydrobromination to 1-bromo-2-ethoxyethene, a precursor to an acetaldehyde carbanion equivalent.The yield of this vinyl bromide and the other common byproducts of reaction varied markedly depending on the base and reaction conditions employed.Direct distillation of the product under reduced pressure from a tertiary amine solution was the method of choice, showing little if any effect of temperature and giving reproducible results.Following this procedure, N,N-dimethyldodecylamine was the preffered base for this reaction.This paper presents results of the dehydrobromination reaction using more than 30 different bases and conditions
- Stalick, Wayne M.,Khorrami, Ali,Hatton, Kimi S.
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p. 3577 - 3581
(2007/10/02)
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