- Second-Generation Synthesis of the Northern Fragment of Mandelalide A: Role of π-Stacking on Sharpless Dihydroxylation of cis-Enynes
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The development of a π-stacking-based approach for increased stereoselectivity in Sharpless asymmetric and diastereomeric dihydroxylation of cis-enynes is disclosed. The use of neighboring, electron-rich benzoate esters proved key to the success of this process. Density functional theory study suggests that the substrate benzoate ester group rigidifies the dihydroxylation transition states by forming a favorable π-stacking interaction in both Major-TS and Minor-TS. The energetic preference for the Major-TS was found in part because of the favorable eclipsing conformation of the alkene substituent as opposed to the disfavored bisecting conformation found in the Minor-TS. The application to a second-generation synthesis of the C15-C24 northern portion of mandelalide A is demonstrated.
- Ghosh, Ankan,Brueckner, Alexander C.,Cheong, Paul Ha-Yeon,Carter, Rich G.
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p. 9196 - 9214
(2019/08/12)
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- A New Entry of Practical and Chemoselective Iodocarbenoids for Carbonyl Iodomethylenation
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Direct oxidative addition of CHI3 to the Mg-TiCl4 bimetallic species resulted in the generation of a highly chemoselective and practically convenient iodomethylenetitanium complex, which efficiently effected condensation even with enolizable or inert carbonyl compounds, such as sterically congested ketones, to provide vinyl iodide compounds.
- Yan, Tu-Hsin,Chen, Mei-Yuan
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p. 390 - 394
(2017/04/24)
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- Synthesis of cyclic alkenylsiloxanes by semihydrogenation: A stereospecific route to (Z)-alkenyl polyenes
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Cyclic alkenylsiloxanes were synthesized by semihydrogenation of alkynylsilanes - a reaction previously plagued by poor stereoselectivity. The silanes, which can be synthesized on multigram scale, undergo Hiyama-Denmark coupling to give (Z)-alkenyl polyene motifs found in bioactive natural products. The ring size of the silane is crucial: five-membered cyclic siloxanes also couple under fluoride-free conditions, whilst their six-membered homologues do not, enabling orthogonality within this structural motif.
- Elbert, Bryony L.,Lim, Diane S. W.,Gudmundsson, Haraldur G.,O'Hanlon, Jack A.,Anderson, Edward A.
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supporting information
p. 8594 - 8598
(2014/07/21)
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- Synthetic utility of tribenzyltin hydride and its derivatives as easily accessible, removable, and decomposable organotin reagents
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Radical reactions using tribenzyltin hydride (Bn3SnH) easily prepared from tin and benzyl chloride were studied. The Et3B- initiated reduction and cyclization of haloalkanes and haloalkenes with Bn 3SnH proceeded efficiently. Homolytic hydrostannylation of alkynes with Bn3SnH followed by treatment with electrophiles gave functionalized alkenes in good to high yields. The organotin byproducts formed could be easily removable by filtration and silica-gel column chromatography without any pretreatment. It was also found that tribenzyltin chloride (Bn 3SnCl) easily decomposed to benzyl alcohol in a basic solution of H2O2.
- Yamakawa, Takeshi,Kinoshita, Hidenori,Miura, Katsukiyo
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supporting information
p. 129 - 134
(2013/03/13)
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- Terminal alkynes from aldehydes via dehydrohalogenation of (Z)-1-iodo-1-alkenes with TBAF
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Terminal alkynes were prepared in near quantitative yields via dehydrohalogenation of (Z)-1-iodo-1-alkenes with tetrabutylammonium fluoride (TBAF) under mild conditions. The methodology was expanded to include a one-pot, direct synthesis of terminal alkynes from aldehydes without the necessity of isolating and purifying the intermediate iodoalkene.
- Beshai, Mira,Dhudshia, Bhartesh,Mills, Ryan,Thadani, Avinash N.
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supporting information; experimental part
p. 6794 - 6796
(2009/04/07)
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- Synthesis of (E)-α,β-unsaturated esters and (Z)-vinyl halides with total or high diastereoselectivity by using samarium metal
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A highly diastereoselective β-elimination of 2-halo-3-hydroxy esters 1 or O-acetylated 1,1-dihaloalkan-2-ols 4 is achieved with samarium in the presence of diiodomethane, yielding α,β-unsaturated esters 2 or vinyl halides 5, respectively. The β-elimination reaction was promoted by samarium diiodide, which was generated in situ. The starting halohydrins 1 or 4 are easily prepared by reaction of the corresponding lithium enolates of α-halo esters or dihalomethyllithium with aldehydes at -78°C. The influence of the reaction conditions and the structure of the starting compounds on the diastereoselectivity of the β-elimination reactions is discussed, A comparative study of the β-elimination reaction with preformed SmI2 and metallic samarium is also performed. These elimination reactions can be explained by the proposed mechanism. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002).
- Concellon, Jose M.,Rodriguez-Solla, Humberto,Huerta, Monica,Perez-Andres, Juan A.
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p. 1839 - 1847
(2007/10/03)
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- Samarium-mediated β-elimination in dihalo alcohols: Diastereoselective synthesis of (Z)-vinyl halides
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High diastereoselectivity in β-elimination reactions of O-acetyl 1,1- dihaloalkan-2-ols to give (Z)-vinyl halides was achieved by using samarium diiodide [Eq. (1)]. The reaction was also highly diastereoselective and totally chemoselective when a mixture
- Concellon, Jose M.,Bernad, Pablo L.,Perez-Andres, Juan A.
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p. 2384 - 2386
(2007/10/03)
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- A practical transformation of aldehydes into (E)-iodoalkenes with geminal dichromium reagents
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A catalytic cycle of a chromium salt in a stereoselective transformation of aldehydes to (E)-l-iodoalkenes using a geminal dichromium reagent, is assembled with zinc, Me3SiCl, and NaI in dioxane.
- Takai, Kazuhiko,Ichiguchi, Tetsuya,Hikasa, Shintaro
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p. 1268 - 1270
(2007/10/03)
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- IPy2BF4 is also a useful reagent for stereospecific iodine-silicon exchange in open chain trimethylsilylalkenes
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Reaction of the title monosubstituted vinylsilanes with IPy2BF4/HBF4 gives iodoalkenes through the ipso-substitution process with stereospecific retention of configuration.
- Barluenga, Jose,Alvarez-Garcia, Lorenzo J.,Gonzalez, Jose M.
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p. 2153 - 2156
(2007/10/02)
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- Stereospecific preparation of polyfunctional olefins by the carbometalation of alkynes with polyfunctional zinc-copper organometallics. Stereospecific preparation of five-membered carbocycles by intramolecular carbocupration
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The addition of copper reagents prepared by transmetalation from polyfunctional organozinc iodides to reactive alkynes such as 1-(methylthio)-1-hexyne, phenylacetylene and acetylene, leads to stereochemically well-defined (syn addition), highly functionalized alkenylcopper reagents that after protonation, allylation, iodolysis, or stannylation afford stereochemically pure di-, tri-, and tetrasubstituted olefins. A similar intramolecular carbocupration has allowed the preparation of highly substituted five-membered carbocycles. Compared to lithium- or magnesium-copper reagents, the highly functionalized copper reagents can contain an ester, nitrile, or chloride function. However, they display a lower reactivity than the lithium- or magnesium-copper reagents and do not react with unactivated terminal alkynes.
- Achyutha Rao,Knochel, Paul
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p. 5735 - 5741
(2007/10/02)
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- Vinylic Organoboranes. 13. A Convenient Stereospecific Synthesis of (Z)-1-Halo-1-alkenes from 1-Alkynes via (E)-1-Alkenylborane Derivatives with Halogens
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The reaction of (E)-1-alkenylborane derivatives with bromine under various conditions was investigated, and a simple stereospecific synthesis of (Z)-1-bromo-1-alkenes has been developed.Addition of bromine to (E)-1-alkenyldisiamylborane, derived from the
- Brown, Herbert C.,Subrahmanyam, Chitti,Hamaoka, Tsutomu,Ravindran, Nair,Bowman, Donald H.,et al.
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p. 6068 - 6075
(2007/10/02)
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- Vinylic Organoboranes. 12. Synthesis of (Z)-1-Halo-1-alkenes via Hydroboration of 1-Halo-1-alkynes Followed by Protonolysis
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Practical conditions have been developed for synthesizing (Z)-1-halo-1-alkenes from 1-halo-1-alkynes.The reaction of 1-halo-1-alkynes with dicyclohexylborane (Chx2BH) or with 9-borabicyclononane (9-BBN) gives exclusively the corresponding B-((Z)-1-halo-1-alkenyl)dialkylboranes 1.No dihydroboration products were found.A general procedure is reported for protonolyzing 1 with AcOH, yielding the (Z)-1-halo-1-alkenes.The synthesis of these haloalkenes is particularly convenient using Chx2BH in pentane.In this solvent, the synthesis of Chx2BH, the subsequent hydroboration and protonolysis, and the removal of the borane residue with ethanolamine all go smoothly and rapidly with nearly quantitative yields.
- Brown, Herbert C.,Blue, Clarence D.,Nelson, Donna J.,Bhat, Narayan G.
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p. 6064 - 6067
(2007/10/02)
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- A STEREOSELECTIVE SYNTHESIS OF (Z)-1-IODO-1-ALKENES
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(Z)-1-iodo-1-alkenes can be synthesized stereoselectively via the Wittig reaction of iodomethylenetriphenylphosphorane with aldehydes.
- Stork, Gilbert,Zhao, Kang
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p. 2173 - 2174
(2007/10/02)
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- Z-1-VINYLIODIDE DURCH WITTIG-REAKTION
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A wide spectrum of Z-1-iodo-alkenes is available by Wittig reaction of iodomethylenetrimethylphosphorane with aldehydes.The formation of 1,1-diiodo-1-alkenes as byproducts is explained.
- Bestmann, Hans Juergen,Rippel, Hans Christoph,Dostalek, Roman
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p. 5261 - 5262
(2007/10/02)
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- Reactions of Alkylmercurials with Heteroatom-Centered Acceptor Radicals
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The relative reactivities of alkylmercury halides toward PhS., PhSe., or I. decrease drastically from R = tert-butyl to R = sec-alkyl to R = n-butyl, indicative that R. is formed in the rate-determining step in the attack of these radicals upon RHgCl.The alkyl radicals thus formed will enter into chain reactions in which a heteroatom-centered radical (A.) is regenerated from substrates such as RS-SR, ArSe-SeAr, ArTe-TeAr, PhSe-SO2Ar, Cl-SO2Ph; ZCH=CHA (A = Cl, I, SPh, SO2Ph); or PhCCHA (A = I, SPh, SO2Ph). β-Styrenyl (PhCH=CHA, Ph2C=CHA) and β-phenethynyl (PhCCA) systems with A = I, Br, SO2Ph also enter into chain reactions with mercury(II) salts with the ligands PhS, PhSe, PhSO2, or (EtO)2PO.The relative reactivities of a series of reagents toward t-Bu. and of PhCH=CHA, Ph2C=CHA, and PhCCA toward c-C6H11. are reported as well as the regioselectivity of t-Bu. attack observed for 1,2-disubstituted ethylenes (ZCH=CHA) with Z and A from the group Ph, Cl, Br, I, SO2Ph, SPh, Bu3Sn.Reactions of (E)- and (Z)-PhCH=CHI or MeO2CCH=CHI with t-Bu. or c-C6H11. occurred in a regioselective and stereospecific (retention) manner.Reactions of (E)- and (Z)-ClCH=CHCl occurred in a nonstereospecific manner in which the E/Z product ratio increased with the bulk of the attacking radical.A similar effect on the E/Z product ratios was observed for (Z)-MeO2CCH=CHCl.
- Russell, Glen A.,Ngoviwatchai, Preecha,Tashtoush, Hasan I.,Pla-Dalmau, Anna,Khanna, Rajive K.
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p. 3530 - 3538
(2007/10/02)
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