Search results for "stereochemistry"

Synthesis of nonracemic α-trifluoromethyl α-amino acids from sulfinimines of trifluoropyruvate

2001

We describe a novel and useful method for the synthesis of nonracemic α-trifluoromethyl α-amino acids (α-Tfm-AAs). Key building blocks are the sulfinimines (S)-1a and (S)-1b, prepared by Staudinger reaction from trifluoropyruvate esters and the chiral N-sulfinyl iminophosphorane (S)-8, which were treated with benzyl, allyl, and alkylmagnesium halides. The resulting diastereomeric N-sulfinyl α-Tfm α-amino esters, 12 and 13, were produced with moderate to good stereoselectivity and yields. When alkyl Grignard reagents were used, stereocontrol became progressively higher with increasing steric bulk, while reversed, though poor, stereocontrol was achieved with benzyl/allyl Grignard reagents. An…

Synthesis of N-methyl alkylaminomethane-1,1-diphosphonic acids and evaluation of their complex-formation abilities towards copper(II)

2015

Abstract A series of N -methyl alkylaminomethane-1,1-diphosphonic acids ( 3a – g ) with a common tertiary nitrogen atom (CH 3 –N–R) bearing linear or branched alkyl, cycloheptyl or phenylalkyl R substituents was synthesized in the reaction of diethylphosphonate with triethyl orthoformate and secondary amine followed by hydrolysis, and by the Eschweiler–Clarke methylation of the alkylaminomethane-1,1-diphosphonic acids with formic acid and formaldehyde. Complex-formation abilities of 3a – g towards copper(II) in solution were studied by means of pH-potentiometry, ESI-MS spectrometry, UV–Vis and EPR methods. Evaluation of stability constants for the Cu(II) complexes of 3a – g has revealed the…

Naphthalenophane durch doppelte Photocyclodimerisierung von Distyrylnaphthalinen

1989

Bei der Belichtung von Distyrylnaphthalinen entstehen in regiospezifischen Kopf-Kopf-Cycloadditionen Naphthalenophane. Je nach Stellung der Styryl-Gruppen beobachtet man vollkommen stereospezifische Prozesse (1d 2, 1g 3, 1h 4) und Dimerisierungen mit partiellen Stereoselektivitaten (1b 5–8, 1i 9, 10). Masgeblich sind die Rotamerenpopulation im Ausgangsprodukt 1 und die elektronischen und sterischen Effekte in den Excimeren. Prinzipiell sind Excimere mit paralleler Anordnung moglichst vieler π-Zentren bevorzugt — ein experimenteller Befund, der durch HMO-Rechnungen untermauert wird. Naphthalenophanes by Twofold Photocyclodimerizatio Reactions of Distyrylanaphthalenes Regiospecific head-to-he…

Diels-Alderase Catalyzing the Cyclization Step in the Biosynthesis of Spinosyn A

2015

The conversion of putative macrocyclic lactone into the tricyclic compound, as a key step in the biosynthesis of spinosyn A , has been theoretically investigated using DFT methods. The relatively low activation free energy computed for the cyclization process of the actual macrocyclic lactone (21.0 kcal/mol) furnishes a rationalization for a spontaneous ( i.e. nonenzymatically catalyzed) cyclization process in the biosynthesis of spinosyn A . A geometric analysis of putative macrocyclic lactone indicates that a slight strain on the lactone at the active site of the SpnF gene could decrease the activation free energy to ca . 16 kcal/mol. This nonspecific participation of the enzyme, which ac…

Erratum to “3D-Structure and function of strictosidine synthase – The key enzyme of monoterpenoid indole alkaloid biosynthesis” [Plant Physiol. Bioch…

2008

Isomotiol, a new triterpene from strychnos potatorum

1978

Isomotiol (fern-8-en-3β-ol) was isolated from the leaves of Strychnos potatorum; it was not known previously as a natural product, but it has been obtained by acidic isomerization of compounds with a fern-7-ene or a fern-9(11)-ene skeleton. From the leaves and the bark mixtures of sitosterol, stigmasterol and campesterol were also isolated.

Metallocene sulfide chemistry of niobium and tantalum: new insights into the formation of niobium-polysulfido complexes and synthesis, structure, and…

1992

The previously described reaction of Cp'2NbH3 (Cp' = t-BuC5H4) with S8 was investigated with regard to the kinetic stabilization of as yet elusive intermediates during the formation of the polysulfide complex Cp'4Nb2S9 (1). Therefore, the permethylated complex Cp*2NbS3H (2; Cp* = C5Me5) and the Ta complex Cp'2TaS2H (3) were synthesized in a related reaction and structurally investigated. Despite the problematic localization of the hydride atom during X-ray crystallographic studies, the molecules can be described in a proper manner: Characteristic of 2 and 3 are M(eta(2)-S2) cores to which either an SH (2) or a H (3) ligand is attached. Complex 3 is the first known tantalocene sulfide. It re…

Copper(II) complexes with sulfonamides derived from 2-picolylamine and their use as chemical nucleases

2006

Reaction between 2-picolylamine (2-aminomethylpyridine) with 2-mesitylenesulfonyl and 4- tert -butylbenzene sulfonyl chlorides leads to the formation of 2,4,6-trimethyl- N -[pyridin-2-ilmethyl]benzenesulfonamide (Hpmesa) and 4-ter-butyl- N -[pyridin-2-ilmethyl]benzenesulfonamide (Hptbsa). These compounds react with Cu(II) salts to yield coordination compounds with CuL 2 stoichiometry. The immediate environment of the metal ion is a highly distorted tetrahedron, the sulfonamide ligands acting in a bidentate fashion. Compound Hpmesa crystallizes in monoclinic space group P 2 1 /c (number 14), with Z  = 4; complex [Cu(pmesa) 2 ] crystallizes in monoclinic space group P 1 ¯ (number 2), with Z  …

N-p-Amino- and N-p-nitro-phenylsulfonyl derivatives of dipeptides, a new family of ligands for copper(II). Potentiometric and spectroscopic studies

1995

The co-ordination ability of four dipeptide analogues substituted on the N-terminal amino group with p-nitrophenylsulfonyl (nps-Ala-Ala and nps-Ala-His) and p-aminophenylsulfonyl (aps-Ala-Ala and aps-Ala-His) groups was studied by potentiometric and spectroscopic (UV/VIS absorption, CD and EPR) techniques. The N-terminal sulfonyl substituent drastically changes the acidity of the sulfonamide proton making nitrogen very efficient in binding to CuII. The sulfonamide nitrogen having pK between 9 and 11 does not need any anchoring binding group to form complexes with CuII. The para substituent on the phenyl ring (amino or nitro) influences very strongly the acidity of the sulfonamide proton. Th…

Synthesis of .beta.-ionone in an aldehyde/xanthine oxidase/.beta.-carotene system involving free radical formation

1994

The enzymic cooxidation of β-carotene (BC) by xanthine oxidase (XO) in aqueous solutions leads to β-ionone (BI) and derivatives: epoxy-β-ionone (EPBI), dihydroactinidiolide, β-cyclocitral, pseudoionone, etc. We demonstrate, in this article, that this is due to free radical (R . ) formation during the first hour of stirring and the use of aldehydes as substrate. The bleaching of BC does not occur when the common substrate of XO, xanthine, is used; this proves that the superoxide anion O 2 .- alone is not active on BC. BI formation in this case is not observed