Search results for "diastereomer"
showing 10 items of 149 documents
A retro Diels–Alder method for the preparation of pyrrolo[1,2-a]pyrimidinediones from diexo-aminooxanorbornenecarboxamide
2006
Abstract Through the reactions of diexo-3-amino-7-oxanorbornene-2-carboxamide 1 with the oxocarboxylic acids: 4-oxopentanoic acid, p-chlorobenzoylpropionic acid or 2-formylbenzoic acid, the pyrrolo[1,2-a]pyrimidinediones 2 and 3 or pyrimido[1,2-a]isoindoledione 4 were formed on cyclization and thermolysis, when the parent cycles decomposed via the loss of furan to give 2–4 in a retro Diels–Alder reaction. With cis-or trans-2-aroylcyclohexanecarboxylic acids as starting compounds, the 1-aroylhexahydroisoindol-3-ones (5–8) were formed; the phenyl-substituted derivatives gave diastereomeric mixtures. The structures of the new compounds were established by NMR spectroscopy and, for 3 and 6, als…
Bicyclo[6.1.0]nonine
1988
MNDO-Rechnungen zeigen, das fur die drei konstitutionsisomeren cis-Bicyclo[6.1.0]nonine (1–3) jeweils zwei energiearme, zueinander diastereomere Konformere a und b existieren. Zur experimentellen Uberprufung werden die mit der Selenadiazol-Methode oder durch Dehydrobromierung hergestellten, hoch gespannten Bicyclen mit Hilfe von 1H- und 13C-NMR-Messungen auf die Population der Konformeren und auf deren wechselseitige Umwandlung durch Inversion der Achtringe untersucht. Bicyclo[6.1.0]nonine MNDO calculations reveal that two diastereomeric conformations a and b of low energy exist for each of the three isomeric cis-bicyclo[6.1.0]nonynes (1–3). For the experimental proof, the highly strained b…
Chiral complexes of titanium containing a linked amido-cyclopentadienyl ligand: synthesis, structure, and asymmetric imine hydrogenation catalysis
2000
Abstract A series of mono- and disubstituted derivatives (−)-(S)-Ti(η5:η1-C5Me4SiMe2NCHMePh)(X)Cl (X=CH2SiMe3, BH4) and (−)-(S)-Ti(η5:η1-C5Me4SiMe2NCHMePh)X2 (X=OSO2CF3, OiPr, Me, CH2Ph) was prepared from (−)-(S)-Ti(η5:η1-C5Me4SiMe2NCHMePh)Cl2 without significant racemization at the stereogenic center. The monosubstituted complexes are formed as mixtures of diastereomers. One diastereomeric monoalkyl (STi, SC)-Ti(η5:η1-C5Me4SiMe2NCHMePh)(CH2SiMe3)Cl was characterized by X-ray single crystal structure analysis. When the (−)-(S)-NCHMePh group is attached to planar chiral ring moieties 3-tBuC5H3, C9H6, and C9H5(SiMe3)-3 and coordinated at the titanium center, diastereomeric mixtures are formed…
Enantioselective α-alkylation of unsaturated carboxylic acids using a chiral lithium amide
2001
Abstract The regio- and stereochemistry of the alkylation of dienediolates from unsaturated carboxylic acids with benzylic halides, which often results in mixtures of isomers, can be controlled by means of changes in the lithium amide, allowing the α-regioisomer to be obtained as the major diastereoisomer. In addition, when chiral amines are used, moderate enantiomeric excesses can be attained.
ChemInform Abstract: Enantioselective α-Alkylation of Unsaturated Carboxylic Acids Using a Chiral Lithium Amide.
2010
Abstract The regio- and stereochemistry of the alkylation of dienediolates from unsaturated carboxylic acids with benzylic halides, which often results in mixtures of isomers, can be controlled by means of changes in the lithium amide, allowing the α-regioisomer to be obtained as the major diastereoisomer. In addition, when chiral amines are used, moderate enantiomeric excesses can be attained.
Synthesis of a polymer-bound galactosylamine and its application as an immobilized chiral auxiliary in stereoselective syntheses of piperidine and am…
2004
A 2,3,4-tri-O-pivaloylated beta-D-galactopyranosyl azide bearing a hydroxy-functionalized spacer unit at the C-6 position of the galactose was synthesized and immobilized on the solid phase by using a polymer-bound chlorosilane. The azide was reduced to the corresponding galactopyranosylamine, which served as a versatile chiral auxiliary in highly diastereoselective Ugi four-component condensation reactions at ambient temperature. Fluoride-induced cleavage from the polymeric support furnished N-glycosylated N-acylated alpha-amino acid amides. The reaction of the immobilized galactosylamine with aldehydes gave rise to the corresponding aldimines, which underwent a domino Mannich-Michael cond…
Glycosidic juvenogens: derivatives bearing alpha,beta-unsaturated ester functionalities.
2007
Abstract A series of the protected alkyl glycosides 5a / 5b – 12a / 12b was synthesized from the parent isomeric alcohols (insect juvenile hormone bioanalogs; juvenoids), 4-[4′-(2″-hydroxycyclohexyl)methylphenoxy]-3-methyl-but-2-enoic acid ethyl ester ( 1a / 1b – 4a / 4b ; racemic structures) and ( 1a – 4a ; enantiopure structures). Cadmium carbonate was used as a promoter of this Koenigs–Knorr reaction, and the products were obtained in 82–92% yields. Deprotection of the carbohydrate functionality of 5a / 5b – 12a / 12b was carefully performed using ethanolysis in the presence of zinc acetate, due to the presence of another ester functionality in the aglycone part of the molecule of protec…
Enantioselective syntheses of dopaminergic (R)- and (S)-benzyltetrahydroisoquinolines.
2001
Optically pure (1S,R)- and (1R,S)-benzyltetrahydroisoquinolines (BTHIQs), 12a,b as the major diastereomers, were prepared by stereoselective reduction of the isoquinolinium salt possessing (R)- and (S)-phenylglycinol as the chiral auxiliary, respectively. The absolute configurations of (1S,R)-13a hydrochloride (O-debenzoylated derivative from 12a) and (1R,S)-12b diastereomers were unambiguously determined by single-crystal X-ray analysis. Reductive removal of the chiral auxiliary group, subsequent N-propylation, and cleavage of the methylenedioxy group furnished the optically active catecholamines (1S)-16a and (1R)-16b in good overall yield. We have separately prepared for the first time pa…
Bisdihydrodiols, rather than dihydrodiol oxides, are the principal microsomal metabolites of tumorigenic trans-3,4-dihydroxy-3,4-dihydrodibenz[a,h]an…
1994
Several studies on metabolism and biological activity of tumorigenic dibenz[a,h]anthracene (DBA) and its derivatives have led to the conclusion that the M-region dihydrodiol, trans-3,4-dihydroxy-3,4-dihydro-DBA (DBA-3,4-dihydrodiol), is the precursor of the ultimate mutagenic and tumorigenic metabolite of DBA with the presumed structure of a bay-region dihydrodiol oxide. Incubations of DBA-3,4-dihydrodiol (50 microM) with the microsomal hepatic fraction of Sprague-Dawley rats pretreated with Aroclor 1254 yielded more than 13 metabolites upon separation by HPLC. anti-3,4-Dihydroxy-1,2-epoxy-1,2,3,4-tetrahydro-DBA [0.27 nmol/(nmol of P450.15 min)] could be identified for the first time by UV …
Mass spectra of chlorinated esters: 2—Methyl mono- and dichlorobutanoates
1982
The mass spectral fragmentations of methyl mono- and dichlorobutanates have been studied. Deutrium labelling and metastable ion analysis were used to elucidate the fragmentation mechanisms. The molecular ion peaks of the esters are weak and show only in the spectra of the monochloro isomers. A McLafferty rearrangement gives the base peaks in the spectra of methyl 2-chloro-, 4-chloro- and 4,4-dichlorobutanoate; α-cleavage, [COOCH3]+, in methyl 2,2- and 2,4-dichlorobutanoate; [MCl]+, in methyl 3-chlorobutanoate; [MClHCl]+, in methyl 3,4-dichlorobutanoate; [MClCH2CO]+, in methyl 3,3-dichlorobutanoate and [MClCOOCH3]+˙, in methyl erythro- and threo-2,3-dichlorobutanoate. The mass spectra of the…