CONFORMATIONAL AND ELECTRONIC INTERACTION STUDIES OF ALPHA-SUBSTITUTED CARBONYL-COMPOUNDS .12. S-ETHYL ALPHA-HETERO-SUBSTITUTED THIOACETATES

The analysis of the nu(CO) bands in the IR spectra of the S-ethyl alpha-heterosubstituted thioacetates helped by Molecular Mechanics Calculations indicated the existence of the cis/gauche rotational isomerism. In the thioacetates series the gauche rotamers predominate over the cis ones. This has been ascribed to the large stabilization of the pi(CO)* orbital leading to a stronger pi(CO)*/sigma(C-X) and pi(CO)*/n(X) orbital interactions and to a larger stabilization of the gauche rotamers. The positive carbonyl cis shifts (DELTAnu(c)) ascribed to the Repulsive Field Effect are smaller than the ones for the heterosubstituted-acetones, -acetophenones, -methyl acetates and -N,N-diethylacetamides. This trend is discussed in terms of the lower carbonyl dipole moment for the thioesters which is mainly due to the pi(CO)/3d(s) interaction in the carboxythioethyl group. The larger negative carbonyl gauche shifts (DELTAnu(g)) for the heterosubstituted thioacetates in relation to the other referred carbonyl compounds along with the higher Non Additivity Effect of the alpha-methylene carbon chemical shifts for the majority of the title compounds in comparison with the other hetero-substituted carbonyl compounds are further support for the larger contribution of the pi(CO)*/sigma(C-X) and pi(CO)*/n(X) orbital interactions in the thioesters than the same interactions for the other carbonyl compounds. This has been attributed to a great stabilization of the pi(CO)* orbital in the carboxythioethyl group of the thioester due to its interaction with a 3d sulfur orbital.714173010712

CONFORMATIONAL AND ELECTRONIC INTERACTION STUDIES OF ALPHA-SUBSTITUTED CARBONYL-COMPOUNDS .10. METHYL ALPHA-HETERO-SUBSTITUTED ACETATES

Nu(CO) frequencies and relative intensities, and HE(I) photoelectron ionization energies were measured for some methyl alpha-hetero-substituted acetates (XCH2C(O)OMe: X = F, OMe, NMe2, Cl, Br, SMe, and I). The IR data indicate a cis-gauche rotational isomerism, where the gauche rotamers' stabilities should be due to a hyperconjugative interaction. The carbonyl frequency shifts of the cis rotamers have been ascribed to the Repulsive Field Effect, while for the gauche rotamers a competition between, the inductive and hyperconjugative effects should occur. The anomalous behaviour of some substituted acetates (X = F, OMe, and SMe) was attributed to orbital interactions. A good linear correlation between the carbonyl oxygen lone pair (n'o) ionization energy and the substituent sigma(I) inductive parameter (excluding X = Br and OMe) was found, as a consequence of the predominance of the inductive effect over the through-bond and through-space orbital interactions.372374

ELECTRONIC INTERACTIONS STUDIES OF TRANS-3-(2'-CHLOROCYCLOPENTYLTHIO)-THIOCHROMONES AND E-3-(2'-CHLORO-1'-METHYLPROPEN-1'-YL-THIO)-THIOCHROMONES AND THEIR 3-SULFINYL AND 3-SULFONYL DERIVATIVES

The analysis of the nu(CO) bands in the IR spectra of the 3-(2'-chlorocyclopentylthio)- and 3-(2'-chloro-1'-methylpropen-1'-ylthio)-thiochromones and their 3-sulfinyl and 3-sulfonyl derivatives. supported by molecular mechanics calculations of 3-methylthio-, 3-methylsulfinyl- and 3-methylsulfonyl-thiochromones, taken as model compounds, indicates the existence of a single conformer for the whole series except for the case of the 3-(2'-chloro-1'-methylpropen-1'-ylsulfinyl)-thiochromone where three conformers can be observed. The negative carbonyl frequency shifts (DELTA(nu)) for the 3-sulfinyl- and 3-thiothiochromone derivatives and the positive carbonyl shifts for the 3-sulfonyl-thiochromone derivatives are interpreted on the grounds of an interplay of the pi(CO) --> 3d(SO(n)) orbital interaction with the inductive and field effects of the 3-substituents. The orbital interaction is responsible for the shorter S . . . O intramolecular contact observed previously from X-ray data of some 3-thio-thiochromone derivatives. The bathochromically shifted n --> pi(CO)* band for the 3-thio-and 3-sulfinyl-thiochromone derivatives and the slight hypsochromic effect for the 3-sulfonyl-thiochromone derivatives are discussed in terms of the relative energy level stabilization of the pi(CO)* orbital (due to the pi(CO)*/3d(SO(n)) orbital interaction) and the n(O(CO)) lone pair (due to the electron-attracting inductive effect of the 3-sulfur-substituent). The IR and UV spectra of thiochroman-4-one show the existence of a mixture of two conformers, the twist-boat being the one where the transannular interaction between the 3p(s) --> pi(CO)* orbitals takes place.824173072

H-1 and C-13 NMR of some alpha-halo derivatives of o-xylene

C-13 and H-1 NMR data for a series of alpha-halo derivatives of o-xylene are presented. A dynamic H-1 NMR investigation of alpha,alpha,alpha',alpha'-tetrabromo-o-xylene (5) was performed and the spectrum of the most stable conformer at 213 K is presented. The free energy of activation for the rotation of the CHBr2 groups in 5 are determined for the first time. (C) 1997 Elsevier Science Ltd

Dieulafoy lesion: Endoscopic and surgical management

Dieulafoy lesion is characterized by exteriorization of a large pulsatile arterial vessel through a minimal mucosal tear Surrounded by normal mucosa, causing massive and recurrent tipper digestive bleeding in previously healthy patients. More frequently presented than diagnosed, with the increase of its knowledge among endoscopists, a large number of cases are expected in the literature. Data from patients with upper gastrointestinal bleeding Submitted to endoscopy at the Federal University of São Paulo, Gastrointestinal Endoscopy Unit from 199) through 2002 were reviewed for Dieulafoy lesion. We found 15 patients with typical Dieulafoy gastric lesion. Their ages ranged from IS to 78 years (mean age 49.9): 5 patients were female and 10 were male. Bleeding presented as hematemesis and melena in 7 cases (46.6%), hematemesis alone in 4 cases (26.6%), and melena alone in the other 4 cases (26.6%). Initial hemostatic approaches employed were: alcoholization (2 cases), epinephrine associated with alcohol injection (5 cases), sclerosis in 7 cases and surgery in 1 case. Dieulafoy lesion is a distinct nosologic entity that must be Suspected in patients with massive digestive bleeding. Endoscopy became the procedure of choice for diagnosis and treatment of this disease.Universidade Federal de São Paulo, EPM, Dept Surg Gastroenterol, São Paulo, BrazilUniversidade Federal de São Paulo, EPM, Dept Surg Gastroenterol, São Paulo, BrazilWeb of Scienc

Protein cutoff scanning: A comparative analysis of cutoff dependent and cutoff free methods for prospecting contacts in proteins

In this study, we carried out a comparative analysis between two classical methodologies to prospect residue contacts in proteins: the traditional cutoff dependent (CD) approach and cutoff free Delaunay tessellation (DT). In addition, two alternative coarse-grained forms to represent residues were tested: using alpha carbon (CA) and side chain geometric center (GC). A database was built, comprising three top classes: all alpha, all beta, and alpha/beta. We found that the cutoff value at about 7.0 angstrom emerges as an important distance parameter. Up to 7.0 angstrom, CD and DT properties are unified, which implies that at this distance all contacts are complete and legitimate (not occluded). We also have shown that DT has an intrinsic missing edges problem when mapping the first layer of neighbors. In proteins, it may produce systematic errors affecting mainly the contact network in beta chains with CA. The almost-Delaunay (AD) approach has been proposed to solve this DT problem. We found that even AD may not be an advantageous solution. As a consequence, in the strict range up to 7.0 angstrom, the CD approach revealed to be a simpler, more complete, and reliable technique than DT or AD. Finally, we have shown that coarse-grained residue representations may introduce bias in the analysis of neighbors in cutoffs up to 6.8 angstrom, with CA favoring alpha proteins and GC favoring beta proteins. This provides an additional argument pointing to the value of 7.0 angstrom as an important lower bound cutoff to be used in contact analysis of proteins.74372774