Search results for "MOLECULE"

Stannacycloalkyl and stannepinyl derivatives of dipeptides

1994

Stannacyclohexyl and stannacycloheptyl derivatives of dipeptides (CH2)nSnAA · xH2O (H2AA = H2GlyGly, with n = 5, x = 2 or n = 6, x = 1; H2AA = H2GlyAla, H2GlyVal, H2GlyMet, with n = 5, 6, x = 1) and stannepinyl glycylglycinate monohydrate have been obtained by the reaction of (CH2)nSnCl2 or stannepinyl dichloride with Na2AA. According to infrared and 119Sn Mossbauer data of the solid compounds, AA acts as a tridentate ONN ligand and tin has a trigonalbipyramidal environment. An analogous structure has been inferred from 1H, 13C, and 119Sn NMR data for the undissociated molecules in methanol solution.

Synthesis, crystal structure and molecular conformation of the tBuCO-D,L-Ala-Δz-Phe-NhiPr α,β-unsaturated dipeptide

2009

The crystal structure of the tBuCO-D,L-Ala-delta Z-Phe-NHiPr dipeptide has been solved by X-ray diffraction. The peptide crystallizes in monoclinic space group P2(1)/c with a = 13.445 (3) A, b = 35.088 (4) A, c = 14.755 (3) A, beta = 116.73 (1) degree, Z = 12 and dc = 1.151 g.cm-3. The three independent molecules per asymmetric unit accommodate a beta II-folded conformation, but only one of them contains the typical i + 3----i interaction characterizing a beta-turn. In the other two molecules, the N...O distance exceeds 3.2 A, a value generally considered the upper limit for hydrogen bonds in peptides. In solution, the beta II-turn conformation is largely predominant.

1983

The first dissociation constant of oligonuclear phenolic compounds consisting of one nitrophenol unit besides further alkyl phenol units (10 dinuclear, 15 trinuclear, 6 tetranuclear compounds, and 4 nitrophenols as model) was determined spectroscopically in water/methanol (50/50 by vol.) at 25°C. For the ortho-linked title compounds with the nitrophenol unit at the end of the molecule a decrease of pK1 is observed which becomes stronger with increasing chain length. The introduction of bulky groups in ortho-position of the dissociating phenolic unit, as well as at the opposite end of the molecule causes a further decrease of pK1. The lowest value is found for a p-nitrophenol unit in the mid…

Synthesis of fluorinated piperidine and azepane β-amino acid derivatives

2016

Abstract A convenient and robust stereocontrolled procedure has been developed for the synthesis of novel trifluoromethyl-containing piperidine and azepane β-amino ester stereoisomers. The synthesis started from readily available unsaturated bicyclic β-lactams and was based on oxidative cleavage of the ring C C double bond followed by ring closure of the diformyl intermediates in the presence of 2,2,2-trifluoroethylamine hydrochloride through reductive amination. The synthetic procedure has efficiently been extended towards the synthesis of mono- and difluorinated analogs.

Synthesis of densely functionalized cispentacin derivatives through selective aziridination and aziridine opening reactions: Orthogonally protected d…

2014

A novel substrate-directed synthetic route to a series of highly functionalized, orthogonally protected di- or triaminocyclopentanecarboxylate derivatives with multiple chiral centres from an unsaturated bicyclic β-lactam has been accomplished by applying stereoselective ring C–C double bond aziridination with chloramine-T and phenyltrimethylammonium tribromide, followed by regioselective aziridine opening with different N,O nucleophiles and hydrides. The functionalization strategy was successfully extended for access to enantiomerically pure orthogonally protected triaminocarboxylates.

From linear quaterthiophene to sulflower: A comparative theoretical study

2009

Abstract In this paper, we report a theoretical study of four types of thiophene-based oligomers showing the same number of C C double bonds and very different molecular structures. The comparative study has been performed on the basis of B3LYP/6-31G∗∗ calculations. The way the thiophene rings are linked together has a remarkable influence on the molecular and electronic properties. Linear quaterthiophene and heptathienoacene show similar aromatic structures but a loss of π-conjugation is detected for the latter due to the condensation of thiophene rings. A blue shift of the most intense electronic transition is predicted for fused heptathienoacene compared with non-fused quaterthiophene. C…

Bonding in the heavy analogue of hydrogen cyanide: the curious case of bridged HPSi.

2010

The bonding of firstand second-row elements differ dramatically. The simplest unsaturated silicon hydrides Si2H2 and Si2H4 exhibit quite unusual geometries [1] compared to the analogous hydrocarbon molecules. For example, the most stable form of Si2H2 is nonplanar with C2v symmetry and two bridging H atoms, in sharp contrast to linear acetylene, HC! CH. Phosphorus and nitrogen share many of the same bonding characteristics, but P prefers single over multiple bonds. For these reasons, it may be difficult to predict the most stable isomeric arrangement, even for a small molecule with a single Por Si atom and especially when it contains both. Silicon–phosphorus bonds are important in materials…

Structure and properties of N-methyl-N-(4-pyridyl)-nitramine and 1,4-dihydro-1-methyl-4-nitriminopyridine

2002

Abstract The molecular structure and spectral properties of N -methyl- N -(4-pyridyl)-nitramine ( 1 ) and 1,4-dihydro-1-methyl-4-nitriminopyridine ( 3 ) were investigated by the X-ray diffraction, FTIR, nuclear magnetic resonance (NMR) methods and DFT theoretical calculations. N -methyl- N -(4-pyridyl)-nitramine ( 1 ) crystallises in the orthorhombic Pbca space group. It is composed of two planar fragments; nitramino (N 2 O 2 ) group and the pirydyl ring. These two groups are twisted at about 70° with respect to each other, along the Ar–N bond. On the contrary, in compound 3 , they are coplanar to each other. Twist of the nitramino group in 1 with respect to the pirydyl ring leads to a sign…

The conformation cis of N-acetyl-N-methyl-α,β-dehydroalanine N′-methylamide and saturated analogues

2007

A series of three homologous amino acids derivatives: N-acetyl-N-methyl-α,β–dehydroalanine N′-methylamide (1), N-acetyl-N-methyl-L-alanine N′-methylamide (2), and N-acetyl-N-methyl-DL-alanine N′-methylamide have been synthesised. The racemic species undergoes spontaneous separation into L and D-enantiomers. From these two chiral forms, the structure of L-enantiomer (3) was analysed. The molecules of 1 – 3 adopt the cis arrangement of the N-terminal amide bond. The molecular conformations are similar for 1 (φ, ψ = 94.6(1)°, −1.7(1)°) and 3 (φ, ψ = 111.5(1)°, −23.8(1)°), and also 2 (φ, ψ = −114.8(2)°, 29.5(2)°), if inversion through the chiral C2 carbon is considered. They are stabilised by i…

Methyl 3-(4-methoxyphenyl)prop-2-enoate

2001

The title molecule, C(11)H(12)O(3), is almost planar, with an average deviation of the C and O atoms from the least-squares plane of 0.146(4)A. The geometry about the C=C bond is trans. The phenyl ring and -COOCH(3) group are twisted with respect to the double bond by 9.3(3) and 5.6(5) degree, respectively. The endocyclic angle at the junction of the propenoate group and the phenyl ring is decreased from 120 degree by 2.6(2) degree, whereas two neighbouring angles around the ring are increased by 2.3(2) and 0.9(2) degree. This is probably associated with the charge-transfer interaction of the phenyl ring and -COOCH(3) group through the C=C double bond. The molecules are joined together thro…