- A Convenient Stereoselective Reduction of Gem-Dibromides with a Combination of Dimethyl Phosphite and Potassium Carbonate
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An efficient and highly stereoselective reduction of a gem-dibromocyclopropane to the corresponding monobromocyclopropane under mild reaction conditions was developed using a combination of dimethyl phosphite and potassium carbonate. This reaction provided a simple and practical way for the synthesis of the valuable monobromocyclopropanes and β-monobromoalkenes.
- Zhao, Yalei,Chen, Tieqiao,Wang, Xiang-Bo,Han, Li-Biao
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- Thermally induced cyclobutenone rearrangements and domino reactions
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(Chemical Equation Presented) Four thermal-rearrangement pathways and a domino reaction leading to quinones arise from the thermolysis of cyclobutenones. The course of vinylcyclobutenone rearrangements is dictated by the nature of the substituent, R (see
- Harrowven, David C.,Pascoe, David D.,Guy, Ian L.
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p. 425 - 428
(2008/02/02)
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- Halodephosphorylation of α,β-unsaturated phosphonic acid monoesters
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Reaction of α,β-ethylenic and acetylenic phosphonic acid monoesters with (biscollidine)iodine(I) or (biscollidine)bromine(I) hexafluorophosphate led to α,β-unsaturated halides by, without precedent, dephosphorylation reactions.
- Lahrache, Hind,Robin, Sylvie,Rousseau, Gérard
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p. 1635 - 1637
(2007/10/03)
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- 4-endo-trig cyclization processes using bis(collidine)bromine(i) hexafluorophosphate as reagent: Preparation of 2-oxetanones, 2-azetidinones, and oxetanes
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Reaction in methylene chloride of bis(collidine)bromine(I) hexafluorophosphate with α,β-unsaturated acids and α,β-unsaturated N- sulfonamides was found to lead diastereospecifically to the corresponding 2- oxetanones and 2-azetidinones in moderate yields (23-60%), by an almost unknown 4-endo cyclization. This process allow the synthesis of these interesting classes of products in one step from common substrates. Similarly, the reaction of cinnamic alcohols led, by the same cyclization procedure, to oxetanes (20-36%); the presence of a gem-dimethyl group in α of the alcohol function appeared beneficial.
- Homsi, Fadi,Rousseau, Gerard
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- The regiochemistry and stereochemistry of α,β-epoxysilane ring opening with silyl halides or pseudohalides
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The reaction of trimethylsilyl halides with α,β-epoxysilanes leads to either a vinyl halide by addition/elimination or a vinyl enol ether by rearrangement; the route depends upon the nature of the groups attached to the epoxyde ring.With groups β to the silicon that can stabilise an adjacent positive charge the directive effect of silicon towards substitution at the α position is overcome such that the enol ether is obtained.Trimethylsilyl pseudohalides behave similarly except that the intermediate addition product can be isolated.The stereoselectivity of ring opening is found to vary with the reagent. epoxyde / epoxysilane / ring opening / regiochemistry / halosilane
- Bassindale, Alan R.,Soobramanien, Marie-Claire,Taylor, Peter G.
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p. 604 - 608
(2007/10/02)
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- New strategies for the synthesis of vitamin D metabolites via Pd-catalyzed reactions
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The invention of new palladium-catalyzed reactions offers new insights into synthetic strategies directed toward the vitamin D system. The palladium-catalyzed cycloisomerization of 1,6- and 1,7-enynes to dialkylidenecycloalkanes permits a lynchpin approach to the A ring of vitamin Ds. Using the thioacetal of formaldehyde, the proper subunits containing the olefin and the acetylene were attached. Pd(2+) effected cycloisomerization to an A ring subunit. A more effective strategy evolved from the evolution of a Pd-catalyzed alkylative cyclization of enynes. Whereas prior work established the feasibility of this process for 1,6-enynes, model studies reported herein demonstrate the feasibility of its extension to 1,7-enynes. This reaction permits the creation of a new concept for vitamin D synthesis wherein A ring formation is concomitant with its attachment to an appropriate CD fragment. An asymmetric synthesis of the requistite 1,7-enyne required six steps. Bromomethylenation of Grundmann's ketone and its side chain hydroxylated derivative proceeded with excellent geometrical selectivity (>30:1) using the Wittig reaction. A Pd catalyst generated from (dba)3Pd2·CHCl3 and triphenylphosphine stitched together these two units in a single step resulting in syntheses of alphacalcidiol and calcitriol.
- Trost, Barry M.,Dumas, Jacques,Villa, Marco
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p. 9836 - 9845
(2007/10/02)
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- Efficient Syntheses of Multisubstituted Methylenecyclopropanes via Novel Ultrasonicated Reactions of 1,1-Dihaloolefins and Metals
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Ultrasonicated reactions of 1,1-dihaloolefins with lithium, magnesium or sodium in the presence of alkenes rapidly afford multisubstituted methylenecyclopropanes.
- Xu, Linxiao,Lin, Guoying,Tao, Fenggang,Brinker, Udo H.
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p. 650 - 653
(2007/10/02)
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- Synthesis of Mono and gem-Dihalogeno-olefins from Carbonyl Compounds and in situ Generated Lithium Carbenoids
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The treatment of carbonyl compounds (5) with dihalogenomethane and lithium dicyclohexylamide at -78 deg C leads after acid hydrolysis to the crude alcohols (6), which by silylation with trimethylchlorosilane-hexamethyldisilazane-pyridine affords the crude
- Barluenga, Jose,Fernandez-Simon, Jose L.,Concellon, Jose M.,Yus, Miguel
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p. 691 - 694
(2007/10/02)
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- A General Method for the Synthesis of 3,5-Cyclovitamin D3 and Derivatives. A Stereoselective Synthesis of Vitamin D3
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A stereoselective synthesis of vitamin D3 (1) was achieved via 3,5-cyclovitamin D3 (33) which was synthesized from the chiral aldehyde (3) and the vinyl bromide (27) derived from Grundmann's ketone (25). (+/-)-(Z)-5-(2-Cyclohexylidene-ethylidene)-4-methylenecyclohexane-r-1, t-3-diol (24) as a model compound of 1α-hydroxyvitamin D3 was also stereoselectively synthesized via the solvolysis of (+/-)-α-cyclohexylidenemethyl-3β-methoxymethoxy-2-methylenebicyclohexane-1-methanol (21) which was prepared from the aldehyde (12) and the organostannane (16).
- Nemoto, Hideo,Wu, Xiao-Ming,Kurobe, Hiroshi,Minemura, Kayoko,Ihara, Masataka,at. al.
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p. 1185 - 1190
(2007/10/02)
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- Photochemistry of Alkyl Halides. 10. Vinyl Halides and Vinylidene Dihalides
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The photobehavior of the acyclic vinyl iodide 2, the 1-iodocycloalkenes 11-14 and 39, the (halomethylene)cycloalkanes 45-48, and the (dihalomethylene)cyclohexanes 62-64 has been studied.Except for the dichloride 64, which exhibited only radical behavior, each of the halides afforded a mixture of ionic and radical products.The two bromides studied, 48 and 63, afforded lower ratios of ionic to radical products than the corresponding iodides 45 and 62.Irradiation of vinyl iodides was found to be a convenient and powerful method for the generation of vinyl cations, including the highly strained 1-cyclohexenyl and 1-cyclopentenyl cations and the unstabilized α-unsubstituted cations 51 and 54.The latter cations underwent rearrangement to the ring-expanded 1-cycloalkenyl cations 28 and 36, respectively.Lowering the temperature of the irradiation of iodides 13, 14, and 45 resulted in an increased ratio of ionic to radical products.However, iodide 47, which underwent principally fragmentation to enyne 61, showed little temperature effect.
- Kropp, Paul J.,McNeely, Steven A.,Davis, Robert Drummond
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p. 6907 - 6915
(2007/10/02)
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- Photochemistry of Alkyl Halides. 9. Geminal Dihalides
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The photobehavior of the geminal dihalides (diiodomethyl)cyclohexane (7), (bromoiodomethyl)cyclohexane (11), (dibromomethyl)cyclohexane (17), (diiodomethyl)cyclopentane (22), 3,3-dimethyl(diiodomethyl)cyclobutane (27), and 8,8-diiodo-2,6-dimethyl-2-octene (31) has been studied and compared with that previously observed for diiodomethane.In all solvents the corresponding vinyl halides (iodomethylene)cyclohexane (13), (bromomethylene)cyclohexane (21), (iodomethylene)cyclopentane (23), 3,3-dimethyl(iodomethylene)cyclobutane (28), or cis- and trans-3,7-dimethyl-1-iodo-1,6-octadiene (33) were obtained, which are thought to arise from α-halo cationic intermediate formed via initial light-induced homolytic cleavage of the carbon-iodine bond followed by electron transfer within the resulting caged radical pair, as shown in Schemes II and III.In the case of diiodide 31 competing intramolecular trapping of the α-iodo cation afforded in addition the cyclized isopulegyl iodide (34).In polar solvents the vinyl iodides were accompanied by the nonhalogenated products methylenecyclohexane (15), 1-methylcyclopentene (25), cyclohexene (26), 4,4-dimethylcyclopentene (29), and cis- and trans-carane (35), which are thought also to arise from the α-halo cationic intermediate. 1.1-Diiodo-2,2-dimethylpropane (1b) afforded 2-methyl-2-butene (6b).Except for carane (35) from diiodide 31 there was no detectable formation of cyclopropanes.In methanol the nucleophilic substitution products (dimethoxymethyl)cyclohexane (14), (dimethoxymethyl)cyclopentane (24), and 1,1-dimethoxy-2,2-dimethylpropane (30) were obtained.It is concluded that geminal dihalides undergo predominant, if not exclusive, photoreaction via initial clea vage of a single carbon-halogen bond in analogy with monohalides and that carbene intermediates are not formed.A similiar conclusion has been reached previously for diiodomethane in the photocyclopropanation of alkenes.
- Kropp, Paul J.,Pienta, Norbert J.
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p. 2084 - 2090
(2007/10/02)
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- Vinyl Cations, 36. Solvolysis of Cycloalkylidenemethyl and 1-Cyclopenten-1-yl Triflates
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Cyclohexylidenemethyl triflate (2), cyclobutylidenemethyl triflate (5), 1-cyclopenten-1-yl triflate (7), bicyclohex-2-en-2-yl triflate (9), 1-cyclobutylideneethyl triflate (19), and 2-methyl-1-cyclopenten-1-yl triflate (29) were solvolyzed in solvents of various ionizing power and nucleophilicity and the solvolysis products were identified.The cyclobutylidenealkyl triflates solvolyze via a vinyl cation mechanism involving ion pairs with rearrangement to cyclopentene and cyclopentanone compounds.The 1-cyclopenten-1-yl triflates do not produce vinyl cation intermediates but give only the corresponding ketones via an O - S bond cleavage of the triflate group.
- Hanack, Michael,Maerkl, Rainer,Martinez, Antonio Garcia
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p. 772 - 782
(2007/10/02)
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