- Rapid methylation of terminal acetylenes by the Stille coupling of methyl iodide with alkynyltributylstannanes: A general protocol potentially useful for the synthesis of short-lived 11CH3-labeled PET tracers with a 1-propynyl group
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The Pd(0)-mediated rapid coupling (trapping) reaction of methyl iodide with an excess amount of alkynyltributyl-stannane has been developed with the aim to incorporate a short-lived 11C-labeled methyl group into biologically active organic compounds with a 1-propynyl structural unit.
- Hosoya, Takamitsu,Wakao, Masahiro,Kondo, Yurie,Doi, Hisashi,Suzuki, Masaaki
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- Highly efficient photochemical generation of a triple bond: Synthesis, properties, and photodecarbonylation of cyclopropenones
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UV irradiation of alkyl-, aryl-, and heteroatom-substituted cyclopropenones results in the loss of carbon monoxide and the formation of quantitative yields of corresponding alkynes. The quantum yield of the photochemical decarbonylation reaction ranges from 20% to 30% for alkyl-substituted cyclopropenones to above 70% for the diphenyl- and dinaphthylcyclorpopenones. Rapid formation (5 ns) and then a somewhat slower decay (ca. 40 ns) of an intermediate in this reaction was observed by using laser flash photolysis. The DFT calculations allowed us to identify this intermediate as a zwitterionic species formed by a cleavage of one of the carbon-carbon bonds of the cyclopropenone ring. The latter then rapidly loses carbon monoxide to produce the ultimate acetylenic product. Despite their high photoreactivity, cyclopropenones were found to be thermally stable compounds with the exception of hydroxy- and methoxy-substituted cyclopropenones. The latter undergo rapid solvolysis in hydroxylic solvents even at room temperature. The application of this reaction to the in situ generation of the enediyne strucutre was illustrated by the photochemical preparation of benzannulated enediyne 12.
- Poloukhtine, Andrei,Popik, Vladimir V.
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p. 7833 - 7840
(2007/10/03)
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- Polylithiumorganic compounds. Part 28. The reaction of allene and alkyl substituted allenes with lithium metal
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The reaction of allene (3a) and alkyl substituted allenes 1,2-hexadiene (3b), cyclopropylallene (3c), and vinylidene cyclopropane (3d) with lithium metal was investigated in order to access 2,3-dilithioalkenes 4a-d. These dilithioalkenes 4a-d are very reactive in polar solvents like THF and act as strong bases, either metalation of the starting allene 3a-d, the solvent, or sufficiently acidic intermediates like 8 a-d is observed. The metalation products 5-7 show follow-up reactions like 1,3-H shift to the corresponding 1-lithio-1-alkynes 8 and subsequent metalation to the dilithioalkynes 9. Additionally, lithium hydride elimination and ring-chain rearrangement (for 5c) are observed. 1,2-Hexadiene (3b) can be brought to reaction with lithium metal in the apolar solvent pentane, here the follow-up reactions are much slower due to the insolubility of 4b. In all cases the elucidation of the reaction pathways is hampered by the formation of complex mixtures of, amongst others, regio- and stereoisomeric products upon quenching with simple electrophiles.
- Maercker, Adalbert,Tatai, Andrea,Grebe, Burkhard,Girreser, Ulrich
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- Methyl Group Migration in the Reactions of Alkynyltrialkylborates
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It is shown that the methyl group cannot be used as a cheap, non-migrating group in the reactions of alkynyltrialkylborates with electrophiles.However, trimethylborane can be used as a methylboronating agent for alkynes, given the right choice of solvent, and this may be of use in terpene synthesis.
- Pelter, Andrew,Drake, Robert A.
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p. 4181 - 4184
(2007/10/02)
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- On 1.2-Shift Reactions and C-H-Insertions of Acyclic Alkylidene Carbenes
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Two series of acyclic terminal vinyl bromides (1...4 and 5...7) were tested in the reaction with potassium tert-butoxide as precursors of alkylidene carbenes.As expected 1 up to 4 only give 1-alkynes whereas the 2-methylated vinyl bromides 5, 6 and 7 yield 1-methylated cyclopentenes predominantly besides 2-alkynes.The formation of cyclopentenes indicates a reaction route via alkylidene carbenes and 1,5-C-H-insertion reactions, that of 2-alkynes is convincing evidence for 1,2-alkyl shift reactions in 2-methyl substituted alkylidene carbenes.The selectivity of 1,5-C-H-insertion depends on the degree of alkyl substitution of the C-5-atom.At 240 deg C the selectivity is 1deg:2deg:3deg ca. 1:54:240.
- Ondruschka, Bernd,Remmler, Matthias,Zimmermann, Gerhard,Krueger, Christian
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- Vinylic Organoboranes. 4. A General, One-Pot Synthesis of 6- and 7-Alkyn-1-ols via Boracyclanes. Influence of Steric Effects in the Iodination of Lithium Alkynyl "Ate" Complexes of Dialkylborinates
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The iodination of the "ate" complexes derived from various B-alkoxyborinane derivatives and 1-alkynyllithium has been investigated.The results indicate the ate complex from B-methoxyborinane is converted into desired 6-alkyn-1-ol in a yield of only 22percent, with much larger amounts, 65percent, of the undesired 1-iodo-1-alkyne.Increases in the steric bulk of the alkoxy group on boron increase the yield of the required 6-alkyn-1-ol with the best results realized with B-(triphenylmethoxy)borinane.Treatment of B-(triphenylmethoxy)borinane with 1-alkynyllithium affords the corresponding "ate" complex.Subsequent iodination induces the migration of one end of the cycloalkyl chain from boron to the adjacent carbon, resulting in the formation of the one-carbon homologated borepane moiety.This then undergoes a rapid deiodoboronation to afford the corresponding (6-alkyn-1-yl)boronate ester.Oxidation of these esters produces the desired 6-alkyn-1-ols in excellent yields (85percent).An attempt to extend this reaction to di-n-alkylborinates to prepare the corresponding unsymmetrical alkynes did not achieve satisfactory results.Alternatively, the iodination of the "ate" complex from B-methylborinane and 1-alkynyllithium, followed by oxidation, provides the required 6-alkyn-1-ols in high yields.This procedure has been successfully extended to the seven-membered borepane moiety to provide the corresponding 7-alkyn-1-ols.Extension of this reaction to the di-n-alkylmethylboranes provides the corresponding unsymmetrical alkynes in good yields.Thus, these procedures constitute a simple, general and one-pot synthesis of the desired alkyn-1-ols, valuable synthons in organic synthesis.Insect pheromones, (Z)-7-tetradecenaland (Z)-7-hexadecenal, were readily prepared in excellent yields by utilizing this convenient procedure.
- Brown, Herbert C.,Basavaiah, D.,Bhat, N. G.
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p. 4518 - 4521
(2007/10/02)
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- Application of Phase Transfer Catalysis, 14.- Preparation of Alkynes from Halides with Solid Potassium tert-Butoxide and Crown Ether
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Preparatively very simple and mild HX eliminations with solid potassium tert-butoxide in petroleum ether in the presence of catalytic amounts of crown-6 are described. 1,2-Dihalides (from alkenes) and 1,1-dihalides (from aldehydes) yield 1-alkynes; internal geminal dihalides (from symmetric ketones) give internal alkynes in excellent yields. 2,2-Dihalides (from methyl ketones) yield homogeneous 1-alkynes only if the 3-position is blocked. (E)-Haloalkenes lead also to alkynes in a syn-elimination process.
- Dehmlow, Eckehard V.,Lissel, Manfred
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