Search results for "Stereocenter"
showing 10 items of 123 documents
Organocatalytic enantioselective synthesis of 2,5,5-trisubstituted piperidines bearing a quaternary stereocenter. Vinyl sulfonamide as a new amine pr…
2020
An organocatalytic desymmetrizing intramolecular aza-Michael reaction with vinyl sulfonamides as nucleophilic nitrogen source has been devised for the synthesis of a new family of 2,5,5-trisubstituted piperidines bearing a quaternary sterocenter. The process takes place with excellent levels of enantioselectivity and moderate to good diastereoselectivity. The vinyl sulfonamide moiety can be removed by means of an ozonolysis reaction.
NMR enantiodifferentiation of triphenylphosphonium salts by chiral hexacoordinated phosphate anions
2003
BINPHAT anion—rather than TRISPHAT—is an efficient NMR chiral shift reagent for triphenylphosphonium salts containing stereogenic centers on the aliphatic side-chain. BINPHAT—rather than TRISPHAT—anion is an efficient NMR (1H and 31P) chiral shift reagent for chiral triphenylphosphonium salts.
Palladium-catalyzed Suzuki-Miyaura reaction involving a secondary sp3 carbon: studies of stereochemistry and scope of the reaction.
2007
Palladium-catalyzed C--C bond formation involving secondary sp3-hybridized carbon is described. These reactions occur with secondary 1-bromoethyl arylsulfoxides and different arylboronic acids, to produce the corresponding arylated sulfoxides in moderate to high yields and with complete stereospecificity. Despite the presence of beta hydrogens in the substrate, the competitive beta-hydride elimination is not a significant side reaction when coordinating solvents are used. The reported cross-coupling involves secondary C(sp3)--C(sp2) bond formation: this is the first time that a mechanistic study has been carried out with such substrates. The reaction proceeds with inversion of configuration…
Straightforward Regio- and Diastereoselective Synthesis, Molecular Structure, Intermolecular Interactions and Mechanistic Study of Spirooxindole-Engr…
2021
Straightforward regio- and diastereoselective synthesis of bi-spirooxindole-engrafted rhodanine analogs 5a–d were achieved by one-pot multicomponent [3 + 2] cycloaddition (32CA) reaction of stabilized azomethine ylide (AYs 3a–d) generated in situ by condensation of L-thioproline and 6-chloro-isatin with (E)-2-(5-(4-chlorobenzylidene)-2,4-dioxothiazolidin-3-yl)-N-(2-morpholinoethyl)acetamide. The bi-spirooxindole-engrafted rhodanine analogs were constructed with excellent diastereo- and regioselectivity along with high chemical yield. X-ray crystallographic investigations for hybrid 5a revealed the presence of four contiguous stereocenters related to C11, C12, C19 and C22 of the spiro struct…
N-sulfinyl amines as a nitrogen source in the asymmetric intramolecular aza-Michael reaction: total synthesis of (-)-pinidinol.
2010
N-Sulfinyl amines have been successfully employed as nitrogen nucleophiles for the asymmetric intramolecular aza-Michael reaction. The synthetic strategy involves a cross-metathesis reaction followed by the Michael-type cyclization, either in a base-catalyzed two-step procedure or in a tandem fashion. The developed methodology allows access to chiral substituted pyrrolidines and piperidines bearing one or two stereocenters and it has been applied to the synthesis of the piperidine alkaloid (-)-pinidinol.
Regioselective Epoxide Ring Opening. Steroselective Synthesis of a Tetrahydropyran Ring
1998
The stereoselective synthesis of a 2-substituted tetrahydropyran with adjacent alkoxy-bearing stereogenic centre is described. The key steps of this synthesis were the stereoselective epoxidation of an allylic alcohol and the regioselective epoxide ring opening by lithium aluminum hydride. The regio and stereoselective synthesis of a trihydroxyselenide and a trihydroxysulfide is also described. The latter compounds are not suitable for cyclization to tetrahydrofuran ring.
Crystal structures and absolute configurations of dexmedetomidine and its tosyl derivative
1993
Abstract X-ray diffraction data were used to determine the absolute configuration of dexmedetomidine, a new member of a class of drugs with 4-arylalkyl-1H-imidazole structure. When the anomalous effect of the parent compound proved to be too small for determination of the absolute configuration the tosyl derivative, with the same configuration, was synthesised and used as a reference. Stability of the stereogenic center was verified by HPLC technique. The absolute configuration of the compounds is S. Complete crystal structures are reported for both compounds. Co radiation was used for measurements.
Control of N-Heterocyclic Carbene Catalyzed Reactions of Enals: Asymmetric Synthesis of Oxindole-γ-Amino Acid Derivatives.
2017
A strategy to control the switch between a non‐cycloaddition reaction and a cycloaddition reaction of enals, using N‐heterocyclic carbene (NHC) catalyisis, has been developed. The new scalable protocol leads to γ‐amino‐acid esters bearing a tetrasubstituted stereocenter in good yields and high stereoselectivities by homo‐Mannich reactions of enals and isatin‐derived ketimines. By simply changing the N‐ketimine substituent to an ortho‐hydroxy phenyl group, the corresponding spirocyclic oxindolo‐γ‐lactams are obtained. peerReviewed
Mbandakamine-Type Naphthylisoquinoline Dimers and Related Alkaloids from the Central African Liana Ancistrocladus ealaensis with Antiparasitic and An…
2018
Four new dimeric naphthylisoquinoline alkaloids, michellamine A5 (2) and mbandakamines C–E (4–6), were isolated from the Congolese plant Ancistrocladus ealaensis, along with the known dimer mbandakamine A (3). They represent constitutionally unsymmetric dimers, each consisting of two 5,8′-coupled naphthylisoquinoline monomers. While the molecular halves of michellamine A5 (2) are linked via C-6′ of both of the naphthalene moieties, i.e., via the least-hindered positions, so that the central biaryl axis is configurationally unstable and not an additional element of chirality, the mbandakamines 3–6 possess three consecutive stereogenic axes. Their monomeric units are linked through an unprece…
Noncovalent assembly of functional groups on calix[4]arene molecular boxes
1997
Calix[4]arenes diametrically substituted at the upper rim with two melamine units spontaneously form well-defined box-like assemblies in the presence of two equivalents of 5,5-diethylbarbituric acid. These assemblies, consisting of nine different components, are held together by 36 hydrogen bonds and are stable in apolar solvents at concentrations of up to 10-4M. This paper reports the first X-ray crystal structure, and the MALDI TOF mass spectra together with the complete 1H NMR spectroscopic characterization of these hydrogen-bonded assemblies. The crystal structure clearly shows that the assemblies are stereogenic, as a result of the antiparallel orientation of the two rosette motifs. Fu…