- The barrier to enantiomerization and dynamic resolution of N-Boc-2-lithiopiperidine and the effect of TMEDA
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The kinetics of enantiomerization and dynamic thermodynamic resolution (DTR) of N-Boc-2-lithiopiperidine have been measured, revealing significant differences in enthalpy and entropy for these processes and a role for the achiral ligand TMEDA; the racemization and the DTR are catalytic and first order in [TMEDA], and this has implications for asymmetric synthesis with chiral organolithiums.
- Coldham, Iain,Leonori, Daniele,Beng, Timothy K.,Gawley, Robert E.
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- [3+2]-Cycloaddition of nonstabilized azomethine ylides, Part 9: A general approach for the construction of X-azabicyclo[m.2.1]alkanes in optically pure form by asymmetric 1,3-dipolar cycloaddition reactions
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A general strategy for the construction of X-azabicyclo[m.2.1]alkane frameworks in optically pure form is reported by the asymmetric [3+2]- cycloaddition reaction of cyclic azomethine ylides with Oppolzer's acryloyl camphor sultam.
- Pandey, Ganesh,Laha, Joydev K.,Mohanakrishnan
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- INDOLIZIDINE AND QUINOLIZIDINE RING FORMATION IN THE SET-PHOTOCHEMISTRY OF α-SILYLAMINES
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The scope and limitations of indolizidine and quinolizidine ring forming, SET-photoinduced, α-amino radical cyclization reactions were explored.
- Hoegy, Susan E.,Mariano, Patrick S.
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- Dynamics of catalytic resolution of 2-lithio-N-Boc-piperidine by ligand exchange
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The dynamics of the racemization and catalytic and stoichiometric dynamic resolution of 2-lithio-N-Boc-piperidine (7) have been investigated. The kinetic order in tetramethylethylenediamine (TMEDA) for both racemization and resolution of the title compound and the kinetic orders in two resolving ligands have been determined. The catalytic dynamic resolution is second order in TMEDA and 0.5 and 0.265 order in chiral ligands 8 and 10, respectively. The X-ray crystal structure of ligand 10 shows it to be an octamer. Dynamic NMR studies of the resolution process were carried out. Some of the requirements for a successful catalytic dynamic resolution by ligand exchange have been identified.
- Beng, Timothy K.,Tyree, William S.,Parker, Trent,Su, Chicheung,Williard, Paul G.,Gawley, Robert E.
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supporting information
p. 16845 - 16855
(2013/01/15)
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- Dynamic kinetic and kinetic resolution of N-Boc-2-lithiopiperidine
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Asymmetric substitution of 2-lithiopiperidines can be achieved by dynamic resolution; the organolithium is formed as a racemic mixture by proton abstraction (or tin-lithium exchange) and is resolved in the presence of a chiral ligand. The Royal Society of
- Coldham, Iain,Patel, Jignesh J.,Raimbault, Sophie,Whittaker, David T. E.
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p. 4534 - 4536
(2008/09/16)
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- Stereoselective construction of X-azabicyclo[m.2.1]alkanes by [3+2]-cycloaddition of non-stabilized cyclic azomethine ylides: Synthesis of enantiopure constrained amino acids and formal total synthesis of optically active epibatidine
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A new and general strategy for the stereoselective construction of X-azabicyclo[m.2.1]alkanes has been developed by the [3+2]-cycloaddition of cyclic azomethine ylides with suitable achiral dipolarophiles. The cyclic azomethine ylides, where the whole of the ylide conjugation is in the ring, have been generated by the sequential double desilylation of the N-alkyl-α,α′-bis(trimethylsilyl) cyclic amines utilizing Ag(I)F as one electron oxidant. The structural rigidity of cyclic azomethine ylides has allowed preferential facial discrimination by the dipolarophile resulting into very good exolendo selectivity. The exolendo selectivity associated with these cycloadditions has been further exploited to access optically pure X-azabicyclo[m.2.1]alkanes by carrying out the cycloadditions with the Oppolzer's acryloyl dipolarophile. Application of this methodology is demonstrated by the construction of few constrained amino acids related to azabicyclic structural framework and the formal total synthesis of optically active epibatidine.
- Pandey, Ganesh,Laha, Joydev K,Lakshmaiah
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p. 3525 - 3534
(2007/10/03)
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- [3 + 2] Cycloaddition of nonstabilized azomethine ylides. 7. Stereoselective synthesis of epibatidine and analogues
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Epibatidine (1) is synthesized by employing a [3 + 2] cycloaddition strategy as a key step via nonstabilized azomethine ylide 10, generated by one-electron oxidative double desilylation of N-benzyl-2,5- bis(trimethylsilyl)pyrrolidine (12). Cycloaddition of 10 with trans-ethyl-3- (6-chloro-3-pyridyl)-2-propenoate (22a) gives 26 in which the 6-chloro-3- pyridyl moiety is endo-oriented. Decarboxylation followed by debenzylation gives unnatural epimer 30 of 1. The required cycloadduct 33, in which 6- chloro-3-pyridyl moiety is exo-oriented, is obtained stereoselectively utilizing cis-ethyl-(6-chloro-3-pyridyl)-2-propenoate (22b) as dipolarophile. 30 is also converted to 1 by epimerization reaction using KO(t)Bu. An alternative route involving conjugate addition of 6-chloro-3-iodo pyridine (37) to 36, obtained by cycloaddition of 10 with ethyl propiolate, is also suggested for the stereoselective synthesis of 1. A number of substituted epibatidines (38, 39, 40, 41, and 42) are synthesized through this strategy using appropriate dipolarophiles. Formal synthesis of the N-methyl homoepibatidine 48 and its epimer 46 is suggested from the cycloaddition of homologous azomethine ylide 44, derived from 43, with 22a and 22b, respectively.
- Pandey, Ganesh,Bagul, Trusar D.,Sahoo, Akhil K.
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p. 760 - 768
(2007/10/03)
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- Stereoselectivity in the Photoinduced Electron Transfer (PET) promoted intramolecular cyclisations of 1-alkenyl-2-silyl-piperidines and -pyrrolidines: Rapid construction of 1-azabicyclo[m.n.0] alkanes and stereoselective synthesis of (±)-isoretronecanol and (±)-epilupinine
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PET promoted cyclisations of 1-alkenyl-2-silyl-pyrrolidines and -piperidines 9a-d to 1-aza-bicyclo[m.n.0]alkanes have been found to be stereoselective. The five-membered ring formation gives predominantly cis products while six-membered rings are trans. Application of such cyclisations to the synthesis of (±)-isoretronecanol 22a, (±)-epilupinine 29 and related alkaloids has been demonstrated. Copyright 1996 by the Royal Society of Chemistry.
- Pandey, Ganesh,Reddy, Gottimukkula Devi,Chakrabarti, Debasish
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p. 219 - 224
(2007/10/03)
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- Sequential two-electron oxidation of α,α′-disilylmethylamines to generate non-stabilized azomethine ylide: An ideal approach for the construction of substituted and fused pyrrolidine ring systems
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α,α′-Di(trimethylsilylmethyl)amines undergo sequential double desilylation processes, by two-electron oxidation initiated either by photoinduced electron transfer (PET) or Ag(I)F, to produce non-stabilized azomethine ylides efficiently which upon trapping with appropriate dipolarophiles give the corresponding pyrrolidines. Application of this strategy to cyclic analogue for the rapid construction of biologically important 1-azabicyclo[m,3.0]alkane framework is discussed.
- Pandey, Ganesh,Lakshmaiah,Gadre, Smita R.
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- α-Lithioamine Synthetic Equivalents: Synthesis of Diastereoisomers from Boc Derivatives of Cyclic Amines
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Sequences of α'-lithiations and electrophilic substitutions of Boc-pyrrolidines, Boc-piperidines, and Boc-hexahydroazepines that provide compounds which are substituted adjacent to nitrigen are reported, and the pathways of the reactions are discussed.By this methodology monosubstituted 2 and disubstituted 2,4, 2,6, and 2,5 Boc-piperidines are obtained as single or separable diastereoisomers consistent with equatorial lithiations and retentive electrophilic substitution in chair conformations.Both cis and trans 2,6-disubstituted diastereoisomers can be prepared, and control of diastereoselectivity is demonstrated by syntheses of solenopsin A, a 2,6-trans-disubstituted piperidine, and of Boc-dihydropinidine, a 2,6-cis-disubstituted piperidine.In the case of 3-methoxy-Boc-piperidine elimination of methoxide occurs upon lithiation, and with cis-2,4-disubstituted Boc-piperidines the electrophile is introduced with trans stereochemistry at C-6.These reactions are suggested to involve twist boat conformations consistent with an X-ray crystal structure of 2-methyl-6-(trimethylstannyl)-4-phenyl-N-Boc-piperidine.Boc-pyrrolidine lithiates more rapidly than Boc-piperidine, provides 2-substituted products with electrophiles, and on further lithiation-substitution gives 2,5-cis- and -trans substituted products.Boc-perhydroazepine provides 2-substituted products by the sequence and on further lithiation-substitution gives 2,7-trans-disubstituted products.
- Beak, Peter,Lee, Won Koo
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p. 1109 - 1117
(2007/10/02)
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- Ag(I)F as one electron oxidant for promoting sequential double desilylation : An ideal approach to non-stabilized azomethine ylides for the rapid construction of 1-azabicyclo (m:3:0) alkanes
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A novel methodology for effecting sequential double desilylation by Ag(I)F for the generation of non-stabilized azomethine ylide and its application for the synthesis of 1-azabicyclo(m.3.0) alkane systems are described.
- Pandey, Ganesh,Lakshmaiah
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p. 4861 - 4864
(2007/10/02)
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- Stereoselectivity in the cyclisation of photoinduced electron transfer (PET) generated cyclic α -amino radicals: First general stereoselective entry to 1-azabicyclo (m:n:o) alkane systems
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Stereoselectivity in the intramolecular cyclisation of PET-generated cyclic α-amino radicals and its application to the synthesis of 1-azabicyclo (m:n:o) alkane systems is reported.
- Pandey, Ganesh,Reddy, G. Devi
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p. 6533 - 6536
(2007/10/02)
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- α-LITHIOAMINE SYNTHETIC EQUIVALENTS FROM DIPOLE-STABILIZED CARBANIONS: THE t-BOC GROUP AS AN ACTIVATOR FOR α'-LITHIATION OF CARBAMATES
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The t-Boc group activates the α'-lithiation of piperidinyl and related carbamates to give lithium reagents which add to electrophiles to provide α'-elaborated carbamates.
- Beak, Peter,Lee, Won-Koo
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p. 1197 - 1200
(2007/10/02)
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