Physiology of the upper segment, body, and lower segment of the esophagus.

The following discussion on the physiology of the esophagus includes commentaries on the function of the muscularis mucosa and submucosa as a mechanical antireflux barrier in the esophagus; the different mechanisms of neurological control in the esophageal striated and smooth muscle; new insights from animal models into the neurotransmitters mediating lower esophageal sphincter (LES) relaxation, peristalsis in the esophageal body (EB), and motility of esophageal smooth muscle; differentiation between in vitro properties of the lower esophageal circular muscle, clasp muscle, and sling fibers; alterations in the relationship between pharyngeal contraction and relaxation of the upper esophageal sphincter (UES) in patients with dysphagia; the mechanical relationships between anterior hyoid movement, the extent of upper esophageal opening, and aspiration; the application of fluoroscopy and manometry with biomechanics to define the stages of UES opening; and nonpharmacological approaches to alter the gastroesophageal junction (GEJ).

A comparative study of structure and function of the longitudinal muscle of the anal canal and the internal anal sphincter in pigs.

Purpose: This study aims to compare the physiology of the longitudinal muscle of the anal canal with the internal anal sphincter in pigs.

Methods: Histology and in vitro studies were performed to compare the effect of neural responses induced by electric stimulation and through nicotinic, purinergic, and serotoninergic receptors.

Results: The longitudinal muscle and the internal anal sphincter are constituted exclusively by smooth muscle.

Theoretical study on the mechanism of the [2+1] thermal cycloaddition between alkenes and stable singlet (phosphino)(silyl)carbenes.

The mechanism and the origins of the stereocontrol observed in the reaction between differently substituted alkenes and stable (phosphino)(silyl)carbenes giving cyclopropanes have been studied computationally. These cyclopropanation reactions proceed via asynchronous concerted mechanisms involving early transition structures with a significant charge transfer from the carbene to the alkene moiety. The geometric features of these transition structures preclude a significant overlap between the orbitals required for secondary orbital interactions between the reactants.

Reaction of N-vinylic phosphazenes with alpha,beta-unsaturated aldehydes. Azatriene-mediated synthesis of dihydropyridines and pyridines derived from beta-amino acids.

Aza-Wittig reaction of N-vinylic phosphazenes (1,2 addition), derived from diphenylmethylphosphine or derived from trimethylphosphine with alpha,beta-unsaturated aldehydes, leads to the formation of 3-azatrienes through a [2 + 2]-cycloaddition-cycloreversion sequence. The presence of an alkyl substituent in position 3 of N-vinylic phosphazenes increases the steric interactions, and [4 + 2] periselectivity (1,4 addition) is observed. Reaction of azatrienes with alpha,beta-unsaturated aldehydes yields pyridines.

Mechanism and stereoselectivity of the aza-Wittig reaction between phosphazenes and aldehydes.

The mechanism of the aza-Wittig reaction between phosphazenes and aldehydes has been studied computationally, using DFT methods (B3LYP/6-31G level), and experimentally. It has been found that the reaction consists of a tandem [2+2] cycloaddition-cycloreversion sequence in which pi and sigma orbitals as well as lone pairs are involved. Both [2+2] processes take place via thermally allowed supra-supra mechanisms.

Pharmacologic characterization of intrinsic mechanisms controlling tone and relaxation of porcine lower esophageal sphincter.

The neurotransmitters mediating relaxation of lower esophageal sphincter (LES) were studied using circular LES strips from adult pigs in organ baths. LES relaxation by sodium nitroprusside (1 nM-3 microM), vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP; 1 nM-1 microM), ATP (10 microM-30 mM), and tricarbonyldichlororuthenum dimer (1 microM-1 mM) was unaffected by tetrodotoxin (1 microM) or l-N(G)-nitroarginine methyl ester (l-NAME; 100 microM). Calcitonin gene-related peptide (CGRP; 1 nM-1 microM) did not affect LES tone.

Substituent effects in eight-electron electrocyclic reactions.

The main features of transition structures associated with eight-electron electrocyclic reactions have been studied with Density Functional Theory. It is found that conrotatory electrocyclization reaction of (3Z,5Z)-octa-1,3,5,7-tetraenes takes place via Möbius aromatic transition structures of helical conformation. The reaction is completely periselective.

Light-induced aminocarbene to imine dyotropic rearrangement in a chromium(0) center: an unprecedented reaction pathway.

1,2-Dyotropic rearrangement can be induced by irradiation of properly functionalized Fischer carbenes. This novel reaction takes place by a stepwise mechanism and with double inversion of configuration at the static scaffold. Good yields are obtained with both cyclic and acyclic structures, thus suggesting that this unprecedented transformation can be extended to other organometallic systems.

[4+3] versus [4+2] mechanisms in the dimerization of 2-boryl-1,3-butadienes. A theoretical and experimental study.

The thermal dimerization of 2-boryl-1,3-butadienes and the scope of this reaction to prepare six-membered rings difficult to synthesize by other methodologies have been studied. In addition, the nature of this dimerization has been studied theoretically. It has been found that the reaction coordinate associated with the formation of the cycloadduct of lowest energy has significant [4+3] character.

Competitive Mechanisms and Origins of Stereocontrol in the [2 + 2] Thermal Cycloaddition between Imines and Keteniminium Cations. A Complementary Entry to 2-Azetidinones (beta-Lactams) and Related Compounds.

The different reaction paths associated with the formal [2 + 2] thermal cycloaddition between several keteniminium cations and imines has been studied computationally. It is found that the reaction takes place via stepwise mechanisms that involve the sequential formation of the N1-C2 and the C3-C4 bonds. In some cases, the second step of the reaction is subjected to torquoelectronic effects and determines its stereochemical outcome.

Origins of the Stereodivergent Outcome in the Staudinger Reaction between Acyl Chlorides and Imines.

Calculations using density functional theory (DFT, B3LYP/6-31G level) provide an explanation for the stereodivergent outcome observed in the Staudinger reaction between acyl chlorides and imines to form 2-azetidinones (beta-lactams). When the ketene is formed prior to the cycloaddition stages, preferential or exclusive formation of cis stereomers is predicted. When the imine reacts directly with the acyl chloride, the step that determines the stereochemical outcome of the reaction is an intramolecular S(N)2 displacement.

On the Aromatic Character of Electrocyclic and Pseudopericyclic Reactions: Thermal Cyclization of (2Z)-Hexa-2,4-5-trienals and Their Schiff Bases.

Pericyclic or pseudopericyclic? Although both mechanisms lead to the same product, they are deeply different in nature. The ring-current model proves to be a useful tool to define different kinds of aromaticity and to distinguish between pericyclic and pseudopericyclic reactions.

Solvent and Substituent Effects in the Periselectivity of the Staudinger Reaction between Ketenes and alpha,beta-Unsaturated Imines. A Theoretical and Experimental Study.

The outcome of the cycloaddition between activated ketenes and alpha,beta-unsaturated imines has been investigated both experimentally and theoretically. Our results indicate that activated monosubstituted ketenes yield exclusively [2 + 2] cycloadducts. Disubstituted activated ketenes yield [2 + 2] and/or [4 + 2] cycloadducts.