- Tuning of α-Silyl Carbocation Reactivity into Enone Transposition: Application to the Synthesis of Peribysin D, E-Volkendousin, and E-Guggulsterone
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A reliable method for enone transposition has been developed with the help of silyl group masking. Enantio-switching, substituent shuffling, and Z-selectivity are the highlights of the method. The developed method was applied for the first total synthesis of peribysin D along with its structural revision. Formal synthesis of E-guggulsterone and E-volkendousin was also claimed using a short sequence.
- Athawale, Paresh R.,Zade, Vishal M.,Rama Krishna, Gamidi,Reddy, D. Srinivasa
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p. 6642 - 6647
(2021/09/02)
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- Copper Hydride Catalyzed Reductive Claisen Rearrangements
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An efficient reductive Claisen rearrangement, catalyzed by in situ generated copper hydride and stoichiometric in diethoxymethylsilane, has been developed. Yields of up to 95 ;% with good to excellent diastereoselectivities were observed in this reaction. Mechanistic studies showed that the stereospecific rearrangement proceeded via a chair transition state of (E)-silyl ketene acetals as intermediates and not via the copper enolates.
- Wong, Kong Ching,Ng, Elvis,Wong, Wing-Tak,Chiu, Pauline
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supporting information
p. 3709 - 3712
(2016/03/08)
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- Chirality Transfer in Gold(I)-Catalysed Direct Allylic Etherifications of Unactivated Alcohols: Experimental and Computational Study
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Gold(I)-catalysed direct allylic etherifications have been successfully carried out with chirality transfer to yield enantioenriched, γ-substituted secondary allylic ethers. Our investigations include a full substrate-scope screen to ascertain substituent effects on the regioselectivity, stereoselectivity and efficiency of chirality transfer, as well as control experiments to elucidate the mechanistic subtleties of the chirality-transfer process. Crucially, addition of molecular sieves was found to be necessary to ensure efficient and general chirality transfer. Computational studies suggest that the efficiency of chirality transfer is linked to the aggregation of the alcohol nucleophile around the reactive π-bound Au-allylic ether complex. With a single alcohol nucleophile, a high degree of chirality transfer is predicted. However, if three alcohols are present, alternative proton transfer chain mechanisms that Erode the efficiency of chirality transfer become competitive.
- Barker, Graeme,Johnson, David G.,Young, Paul C.,Macgregor, Stuart A.,Lee, Ai-Lan
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supporting information
p. 13748 - 13757
(2015/09/22)
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- Dihydrothiophenes containing quaternary stereogenic centres by sequential stereospecific rearrangements and ring-closing metathesis
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Stereospecific [3,3]-sigmatropic rearrangement of O-substituted thiocarbamate derivatives of enantiopure allylic alcohols provides allylic thiocarbamates as single enantiomers. Intramolecular arylation by rearrangement of their allyllithium derivatives provides allylic tertiary thiols. Allylation and ring-closing metathesis gives 2,5-dihydrothiophenes containing sulfur-bearing quaternary centres. This journal is the Partner Organisations 2014.
- Mingat, Ga?lle,McDouall, Joseph J. W.,Clayden, Jonathan
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p. 6754 - 6757
(2014/06/23)
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- Highly stereoselective C-C bond formation by rhodium-catalyzed tandem ylide formation/[2,3]-sigmatropic rearrangement between donor/acceptor carbenoids and chiral allylic alcohols
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The tandem ylide formation/[2,3]-sigmatropic rearrangement between donor/acceptor rhodium carbenoids and chiral allyl alcohols is a convergent C-C bond forming process, which generates two vicinal stereogenic centers. Any of the four possible stereoisomers can be selectively synthesized by appropriate combination of the chiral catalyst Rh2(DOSP)4 and the chiral alcohol.
- Li, Zhanjie,Parr, Brendan T.,Davies, Huw M. L.
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supporting information; experimental part
p. 10942 - 10946
(2012/08/07)
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- Asymmetric catalysis route to anti,anti stereotriads, illustrated by applications
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(Chemical Equation Presented) A short sequence based on asymmetric catalysis, chirality transfer, and an optimized carbometallation protocol gave an anti,anti stereotriad building block in six steps. Both enantiomers of the chirality source, N-methyl ephedrine, are inexpensive, and the auxiliary is recoverable. In one chiral series, the building block was converted to the B-2 intermediate in Miyashita's synthesis of scytophycin C; in the enantiomeric series, it was converted to a key intermediate for aplyronine A and to the polyketide cap for the callipeltins.
- Parker, Kathlyn A.,Xie, Qiuzhe
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supporting information; experimental part
p. 1349 - 1352
(2009/04/18)
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- Stereospecific reaction of α-carbamoyloxy-2-alkenylboronates and α-carbamoyloxy-alkylboronates with grignard reagents - Synthesis of highly enantioenriched secondary alcohols
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Highly enantioenriched secondary alcohols were synthesized by treatment of α-carbamoyloxy-2-alkenylboronates and α-carbamoyloxy-alkylboronates with Grignard reagents. An intermediary boronate complex was transformed stereospecifically to the corresponding secondary 2-alkenyl- and alkylboronates by migration of an introduced residue. Oxidative workup furnished the enantioenriched secondary alcohols.
- Beckmann, Edith,Desai, Vidya,Hoppe, Dieter
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p. 2275 - 2280
(2007/10/03)
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- Stereoselective reactions of acyclic allylic phosphates with organocopper reagents
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A series of acyclic allylic alcohols of general structure R1CH=CHCH(OH)R2 were resolved by Sharpless kinetic resolution. The hydroxyl groups of these enantiomerically enriched alcohols were derivatized to diethyl phosphates, and the derivatives were reacted with organocopper reagents. Cleanest substitution reactions were observed with reagents R32CuCNLi2. With R1 = Me and R3 = n-Bu, the size of R2 affected both the regioselectivity and stereoselectivity of the displacement. Larger R2 groups gave higher regio- and stereoselectivities: with R2 = 3-pentyl, >98% SN2′ regioselectivity and > 98% anti stereoselectivity were observed. Bn2CuCNLi2 gave stereoselectivities comparable to those observed with n-Bu2CuCNLi2 but t-Bu2CuCNLi2 exhibited much lower diastereofacial preference.
- Belelie,Chong
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p. 5552 - 5555
(2007/10/03)
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- Concomitant Epoxide Deoxygenation and Deacetylation of Glycidyl Acetates Induced by Telluride Ion
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Treatment of glycidyl acetates with telluride ion (Te(2-)) produced by reduction of elemental Te with LiEt3BH yields allylic alcohols by loss of the epoxide oxygen atom and the acetyl group from the ester.If the glycidyl acetate is disubstituted at C-3, a rearrangement to an isomeric allylic alcohol competes with the deoxygenation-deacetylation.Triethylborane, a byproduct in the reduction of Te, is believed to play an important role as a Lewis acid since when it is absent or removed by addition of fluoride ion the reaction is extremely slow.
- Dittmer, Donald C.,Zhang, Yanzhi,Discordia, Robert P.
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p. 1004 - 1010
(2007/10/02)
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- A Tellurium Transposition Route to Allylic Alcohols: Overcoming Some Limitations of the Sharpless-Katsuki Asymmetric Epoxidation
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Good yields of enantiomeric allylic alcohols can be obtained in high enantiomeric excess (ee) by combining Sharpless-Katsuki asymmetric epoxidation process (SAE) with tellurium chemistry.The advantages of the tellurium process are as follows: (1) the 50percent yield limitation on the allylic alcohol in the Sharpless kinetic resolution (SKR) can be overcome; (2) allylic tertiary alcohols which are unsatisfactory substrates in the SKR can be obtained in high optical purity; (3) optically active secondary allylic alcohols with tertiary alkyl substituents (e.g. tert-butyl) at C-1 can be obtained in high ee; (4) optically active sterically congested cis secondary alcohols can be obtained in high ee; and (5) the nuisance of the slow SAE of some vinyl carbinols can be avoided.The key step in the reaction sequence is either a stereospecific 1,3-transposition of double bond and alcohol functionalities or an inversion of the alcohol configuration with concomitant deoxygenation of the epoxide function in epoxy alcohols.Trans secondary allylic alcohols can be converted to cis secondary allylic alcohols by way of erythro epoxy alcohols (glycidols); threo glycidyl derivatives are converted to trans secondary allylic alcohols.These transformations are accomplished by the action of telluride ion, generated in situ from the element, on a glycidyl sulfonate ester.Reduction of elemental Te is conveniently done with rongalite (HOCH2SO2Na) in an aqueous medium.This method is satisfactory when Te2- is required to attack at primary carbon site of a glycidyl sulfonate.In cases where Te2- is required to attack a secondary carbon site, reduction of the tellurium must be done with NaBH4 or LiEt3BH.Elemental tellurium is precipitated during the course of the reactions and can be recovered and reused.
- Dittmer, Donald C.,Discordia, Robert P.,Zhang, Yanzhi,Murphy, Christopher K.,Kumar, Archana,et al.
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p. 718 - 731
(2007/10/02)
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- Enantioselective addition of (Z)- And (E)-alkenylzinc bromides to aldehydes: Asymmetric synthesis of sec-allylalcohols
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In situ prepared (Z)- and (E)-1-alkenylzinc bromides 5 were added to various aldehydes 1 in the presence of lithiated (+)-N-methylephedrine or (+)-2-(N,N-dimethylamino)-1,2-diphenylethanol to give sec. allyalcohols 7 in high optical purity with simple rec
- Oppolzer, Wolfgang,Radinov, Rumen N.
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p. 5777 - 5780
(2007/10/02)
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- Stereochemistry of the Michael Addition of N,N-Disubstituted Amide and Thioamide Enolates to α,β-Unsaturated Ketones
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A systematic study of the regio- and diastereoselectivity of the kinetic Michael addition of amide and thioamide enolates to a series of α,β-unsaturated ketones has been carried out.Factors that influence the diastereo- and regiochemical outcome of the reaction include the substitution pattern of the enone and enolate, the enolate counterion, and the solvent.Numerous examples of high selectivity have been discovered.In a number of examples, either the syn or the anti addition products can be obtained by varying the nature of the solvent, donor atom, and/or counterion.These results have correlated in terms of a coherent transition-state model.
- Oare, David A.,Henderson, Mark A.,Sanner, Mark A.,Heathcock, Clayton H.
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p. 132 - 157
(2007/10/02)
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- Catalytic Asymmetric Epoxidation and Kinetic Resolution: Modified Procedures Including in Situ Derivatization
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The use of 3A or 4A molecular sieves ( zeoiltes ) substantially increases the scope of the titanium(IV)-catalyzed asymmetric epoxidation of primary allylic alcohols.Whereas without molecular sieves epoxidations employing only 5 to 10 mol percent Ti(O-i-Pr)4 generally led to low conversion or low enantioselectivity, in the presence of molecular sieves such reactions generally led to high conversion (>95percent) and high enantioselectivity (90-95percent ee).The epoxidations of 20 primary allylic alcohols are described.Especially noteworthy are the epoxidations of cinnamyl alcohol, 2-tetradecyl-2-propen-1-ol, allyl alcohol, and crotyl alcohol-compounds which heretofore had been considered difficult substrates for asymmetric epoxidation.In the case of allylic alcohol, the use of cumene hydroperoxide substantially increases both the reaction rate and the conversion, even in the absence of molecular sieves.In general, enantioselectivities are slightly depressed (by 1-5percent ee) relative to reactions employing 50-100 mol percent Ti(O-i-Pr)4.The epoxidation of low molecular weight allylic alcohols is especially facilitated and, in conjuction with in situ derivatization, provides for the synthesis of many epoxy alcohol synthons which were previously difficult to obtain.The kinetic resolution of four secondary allylic alcohols with 10 mol percent Ti(O-i-Pr)4 is also described.The role of molecular sieves in the reaction and the effects of variation in reaction stoichiometry, oxidant, and tartrate are discussed.
- Gao, Yun,Hanson, Robert M.,Klunder, Janice M.,Ko, Soo Y.,Masamune, Hiroko,Sharpless, K. Barry
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p. 5765 - 5780
(2007/10/02)
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