Search results for "Chemical synthesi"
showing 10 items of 292 documents
Synthesis of conformationally restricted 1,2,3-triazole-substituted ethyl β- and γ-aminocyclopentanecarboxylate stereoisomers. Multifunctionalized al…
2010
Abstract Stereoisomers of 1,2,3-triazole-functionalized, conformationally restricted β- or γ-amino esters with a cyclopentane skeleton were efficiently synthetized from the bicyclic β-lactam 6-azabicyclo[3.2.0]hept-3-en-7-one (1) and Vince γ-lactam (15) in five or six steps involving the azide–alkyne 1,3-dipolar cycloaddition of azido-substituted amino ester stereoisomers with nonsymmetric acetylenes. The azide–alkyne click reactions were investigated under thermal and Cu(I)-catalyzed conditions. Surprisingly, the thermally induced cycloaddition furnished the corresponding 1,4-triazoles regioselectively, which also took place selectively in response to Cu(I) catalysis.
Synthetic Studies towards Pectenotoxin-2: Synthesis of the Nonanomeric 10-epi-ABCDE Ring Segment by Kinetic Spiroketalization
2011
The synthesis of the nonanomeric 10-epi-ABCDE ring system of pectenotoxin-2 has been achieved by using a kinetic spiroketalization reaction. The synthesis of the spiroketalization precursor was achieved through a cross-metathesis/hydro-genation sequence. The formation of the epi-C10 isomer resulted from an unexpected anti-Felkin selective addition of organometallic nucleophiles to the advanced CDE ring precursor. This addition reaction was investigated with differently protected α,β-dioxygenated model aldehydes, which displayed similar anti-Felkin selectivities with organometallic nucleophiles.
Base-catalyzed isomerization of 2-isoxazolines enables a two-step enantioselective synthesis of β-hydroxynitriles from enals.
2010
The asymmetric synthesis of β-hydroxynitriles remains a challenge in organic synthesis. Herein we report a convenient synthesis of β-hydroxynitriles from enantiomerically enriched 3-unsubstituted 2-isoxazolines via a base-catalyzed ring-opening reaction that takes place without loss of enantiopurity. In combination with organocatalytic enantioselective synthesis of 3-unsubstituted 2-isoxazolines, the ring-opening enables a short 2-step synthesis of β-hydroxynitriles from α,β-unsaturated aldehydes in high enantiomeric purity.
Modular one-pot synthesis of tetrasubstituted pyrroles from alpha-(alkylideneamino)nitriles.
2007
2,3,4,5-Tetrasubstituted pyrroles have been prepared with high regioselectivity by a formal cycloaddition of alpha-(alkylideneamino)nitriles and nitroolefins followed by elimination of HCN and HNO2. The reaction allows the convergent construction of the pyrrole ring in four steps from a nitroalkane and three aldehydes.
Organocatalyticanti-Selective Mannich Reactions with Fluorinated Aldimines: Synthesis ofanti-γ-Fluoroalkyl-γ-amino Alcohols
2009
The asymmetric Mannich reaction between fluoroalkyl aldimines and aldehydes catalyzed by α,α-diphenylprolinol trimethylsilyl ether is reported. The corresponding Mannich adducts were reduced in situ to afford anti-β-alkyl-γ-fluoroalkyl-γ-amino alcohols in moderate yields and with very high diastereo- and enantioselectivities. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
Synthesis of Spirovetivane Sesquiterpenes from Santonin. Synthesis of (+)-Anhydro-β-rotunol and All Diastereomers of 6,11-Spirovetivadiene
2004
The synthesis of the spirovetivane sesquiterpenes (+)-anhydro-beta-rotunol and all the diastereomers of 6,11-spirovetivadiene in enantiomerically pure form has been achieved starting from santonin. The key step is the silicon-guided acid-promoted rearrangement of a 1-trimethylsilyl-4,5-epoxyeudesmane prepared from santonin in several steps involving lactone reductive opening, conjugate addition of TMSLi-CuCN, deoxygenation of a carbonyl group, and epoxidation. Rearrangement of the epoxide gave a spiro[4,5]decanediol which was used as a synthetic intermediate. From this compound, (+)-anhydro-beta-rotunol was prepared after elimination of the primary hydroxyl group in the side chain, followed…
Photochemical Functionalization of Allyl Benzoates by C-H Insertion
2013
The photoreactivity of allyl benzoates, containing an electron-rich double bond, has been explored by irradiation at 305 nm in different solvents. Solvent addition products arising from an insertion of the alpha H–C bonds of THF, dioxane, and i-PrOH to the allylic double bond was realized. The observed reactivity depended on reaction conditions and substitution pattern of the substrate. A DFT study on this unusual reaction was performed allowing the formulation of two mechanistic pathways.
Mandelamide-zinc-catalyzed enantioselective alkyne addition to heteroaromatic aldehydes.
2006
The (S,S)-mandelamide III catalyzes the additions of both aryl- and alkylalkynylzinc reagents to heteroaromatic aldehydes with good yields and enantioselectivities up to 92%. This catalyst is easily prepared in a one-step procedure, and both enantiomers are available. Unlike most other described methods, using this catalyst does not require the addition of Ti(O(i)Pr)4.
Synthesis and Photoluminescent Properties of 1,1‘-Binaphthyl-Based Chiral Phenylenevinylene Dendrimers
2003
New chiral, soluble binaphthyl derivatives that incorporate stilbenoid dendrons at the 6,6â²-positions have been prepared. The synthesis of the new enantiopure dendrimers was performed in a convergent manner by Horner-Wadsworth-Emmons (HWE) reaction of the appropriately functionalized 1,1â²-binaphthyl derivative (R)-1 and the appropriate dendrons (R)2nGn-CHO. Different electroactive units were incorporated in the peripheral positions of the dendrons in order to tune both the optical and electrochemical behavior of these systems. Fluorescence measurements on the chiral dendrimers reveal a strong emission with maxima between 409 and 508 nm depending upon the substitution pattern. Finally, t…
Syntheses of novelN-([18F]fluoroalkyl)-N-nitroso-4-methyl-benzenesulfonamides and decomposition studies of corresponding19F- and bromo-analogues: pot…
2003
N-([ 18 F]fluoroalkyl)-N-nitroso-4-methyl-benzensulfonamides [n-alkyl = (-CH 2 ) [ 18,19 F]F, n=2-4)] were synthesized in radiochemical yields ranging from 75-90% to provide new secondary labelling precursors for the syntheses of 18 F-labelled compounds. Preliminary decomposition studies utilizing the corresponding non-radioactive 19 F-compounds as well as the bromo containing analogous compounds were performed to evaluate their stability regarding temperature and basicity of the labelling medium. Furthermore, initial difficulties in labelling these compounds led to the development of a modified labelling procedure applying nearly solvent-free conditions. Extensive decomposition experiments…