- EFFECT OF AGGREGATION ON STEREOCHEMISTRY AND MECHANISM OF ASYMMETRIC OXIDATION OF THE LITHIUM ENOLATE OF METHYL 3,3-DIMETHYLBUTANOATE IN THE SOLID STATE AND IN SOLUTION
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Oxidation of the title enolate by enantiopure (camphorylsulfonyl)oxaziridines in THF afforded α-hydroxyester with 53-73percent ee, but the product from the solid state reactions was racemic.The results suggest that the enolate exists and reacts as an aggregate in the solid state reactions.
- Wei, Yen,Bakthavatchalam, R.,Jin, Xian-Ming,Murphy, Christopher K.,Davis, Franklin A.
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- Chiral propargylic cations as intermediates in SN1-type reactions: Substitution pattern, nuclear magnetic resonance studies, and origin of the diastereoselectivity
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Nine propargylic acetates, bearing a stereogenic center (-C*HXR 2) adjacent to the electrophilic carbon atom, were prepared and subjected to SN1-type substitution reactions with various silyl nucleophiles employing bismuth trifluoromethanesulfonate [Bi(OTf)3] as the Lewis acid. The diastereoselectivity of the reactions was high when the alkyl group R2 was tertiary (tert-butyl), irrespective of the substituent X. Products were formed consistently with a diastereomeric ratio larger than 95:5 in favor of the anti-diastereoisomer. If the alkyl substitutent R2 was secondary, the diastereoselectivity decreased to 80:20. The reaction was shown to proceed stereoconvergently, and the relative product configuration was elucidated. The reaction outcome is explained by invoking a chiral propargylic cation as an intermediate, which is preferentially attacked by the nucleophile from one of its two diastereotopic faces. Density functional theory (DFT) calculations suggest a preferred conformation in which the group R2 is almost perpendicular to the plane defined by the three substituents at the cationic center, with the nucleophile approaching the electrophilic center opposite to R2. Transition states calculated for the reaction of allyltrimethylsilane with two representative cations support this hypothesis. Tertiary propargylic cations with a stereogenic center (-C* HXR2) in the α position were generated by ionization of the respective alcohol precursors with FSO3H in SO2ClF at -80 C. Nuclear magnetic resonance (NMR) spectra were obtained for five cations, and the chemical shifts could be unambiguously assigned. The preferred conformation of the cations as extracted from nuclear Overhauser experiments is in line with the preferred conformation responsible for the reaction of the secondary propargylic cations.
- Nitsch, Dominik,Huber, Stefan M.,Poethig, Alexander,Narayanan, Arjun,Olah, George A.,Prakash, G. K. Surya,Bach, Thorsten
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p. 2851 - 2857
(2014/03/21)
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- Stereospecific α-methallylation of hydroxyaldehydes by silatropic ene cyclisation
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We describe the thermal rearrangement of aldehydes bearing an α-(allyl- or crotylsilyl)oxy substituent. The transformations are best described mechanistically as intramolecular silatropic ene reactions based on stereoselectivity, kinetic and computed transition state data. The overall process constitutes a stereospecific (meth)allylation of α-hydroxyaldehydes, under neutral conditions, in which the hydroxyl protecting group is also the (meth)allylating agent.
- Robertson, Jeremy,Hall, Michael J.,Green, Stuart P.
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experimental part
p. 5541 - 5551
(2009/12/09)
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- Remarkably Facile Solvolyses of Triflates via Carbocationic Processes in Dimethyl Sulfoxide
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A number of triflates have been shown to undergo clean pseudo-first-order solvolysis reactions in DMSO-d6 to give products derived from carbocationic intermediates. Thus, t-BuCH(OTf)CO-t-Bu (5) and t-BuCH 2OTf (9) react readily in DMSO-d6 at 25 °C to give a rearranged oxosulfonium salts, and subsequent alkene products where methyl migration to the incipient cationic center occurs. t-BuCH(OTf)CO 2CH3 (14) gives analogous rearranged products, and 1-methylcyclopropyl triflate (21) gives a ring-opened allylic oxosulfonium salt. These triflates react primarily via kΔ pathways. 6-Methylbicyclo[3.1.0]hex-6-yl triflate (23), bicyclo[2.2.1hept-1-yl triflate (24), 1,6-methano[10]-annulen-11-yl triflate (25), (CH3) 2C(OTf)CO2CH3 (26), and (CH3) 2CCN(OTf) (29) all react in DMSO-d6 to give carbocation-derived products. PhCH(OTf)CF3 (33) and substituted analogues also react readily in DMSO-d6, and the Hammett ρ + value is -3.7. This suggests a "borderline" mechanism where the transition state has substantial charge development. The primary feature of these solvolyses is the high reactivity of all of these triflates in DMSO-d6. Thus, these triflates are all more reactive in DMSO-d 6 than in HOAc, and for most, rates are faster than in CF 3CH2OH. Triflates 5, 21, 29, and 33 are 10 8-109 times more reactive in DMSO-d6 than the corresponding mesylates. It is suggested that the decreased need for electrophilic solvation of trifiate anion, and the high cation solvating ability of DMSO, are the reasons for the high triflate reactivity in DMSO-d 6.
- Creary, Xavier,Burtch, Elizabeth A.
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p. 1227 - 1234
(2007/10/03)
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- Rate acceleration of nucleophilic substitution of 2-chloro-4,6- dimethoxypyrimidine by sulfinate catalysis
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The use of sulfinates greatly enhances the rate of substitution in the reaction of 2-chloro-4,6-dimethoxypyrimidine with alkoxy or aryloxy nucleophiles. Pyrimidinyloxy derivatives as intermediates for potent herbicides have been prepared in good yields fr
- Bessard, Yves,Crettaz, Roger
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p. 4739 - 4745
(2007/10/03)
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- Asymmetric Induction in the Diels-Alder Reactions of α-Hydroxy Acyl Nitrones
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The hydroxamic acids 7, derived from a series of α-hydroxy acids 5, have been oxidised with periodate to form transient, chiral acyl nitroso compounds 8, which were trapped in situ with cyclopentadiene and cyclohex-1,3-diene to give mixtures of diastereoi
- Kirby, Gordon W.,Nazeer, Muhammad
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p. 1397 - 1402
(2007/10/02)
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- Studies on the rearrangement of (trichloromethyl)carbinols to α-chloroacetic acids
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Phenyl(trichloromethyl)carbinol undergoes an unimolecular, predominantly intramolecular conversion into potassium α-chlorophenylacetate on stirring with 10percent aqueous potassium hydroxide at 0 deg C for several days.Besides providing an interesting example of a 1-2 chlorine shift, the reaction is of potential importance for the synthesis of α-chloro acids.The study of a variety of (trichloromethyl)carbinols shows the reaction is general for secondary (trichloromethyl)carbinols as well as trichloroethanol.The mechanism of the reaction involves the preliminary formation of an epoxide.Several mechanisms are considered for the conversion of the epoxide to the α-chloroacetate anion, but none accounts for all of the experimental facts.Tertiary carbinols break down at the epoxide stage into a ketone and carbon monoxide.
- Reeve, Wilkins,McKee, James R.,Brown, Robert,Lakshmanan, Sitarama,McKee, Gertrude A.
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p. 485 - 493
(2007/10/02)
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