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Biomedical subjects

William Tam

Publications and source records attributed to William Tam.

14 recordsLinked to original sources
PubMed2006

Study on the reactivity of the alkyne component in ruthenium-catalyzed [2 + 2] cycloadditions between an alkene and an alkyne.

The ruthenium-catalyzed [2 + 2] cycloadditions of norbornadiene with a variety of alkynes have been investigated. Electronic effect of the alkyne component has shown to play an important role on the rate of the cycloaddition, and the reactivity of the alkyne component increases dramatically as the alkyne becomes more electron deficient. Increase in the steric bulk of the alkyne component decreases the reactivity of the alkyne component. It was also found that chelation effect of propargylic alcohols greatly enhanced the reactivity of the alkyne component in the ruthenium-catalyzed [2 + 2] cycloadditions.

Journal Article↗
PubMed2006

Remote substituent effects in ruthenium-catalyzed [2+2] cycloadditions: an experimental and theoretical study.

The effects of a remote substituent on the regioselectivity of ruthenium-catalyzed [2+2] cycloadditions of 2-substituted norbornenes with alkynes have been investigated experimentally and theoretically using density functional theory. Most of the cycloadditions occurred smoothly at room temperature, giving the exo cycloadducts in excellent yields. Regioselectivities of 1.2:1 to 15:1 were observed with various substituents on the C-2 position of the norbornenes. Exo-C-2-substituents usually showed greater remote substituent effects on the regioselectivities of the cycloadditions than the corresponding endo-C-2-substituents. The regioselectivity of the cycloadditions with C-2 substituents containing an exocyclic double bond (sp2 hybridized carbon at C-2) are much higher than the cycloadditions with the exo and endo 2-substituted norbornenes. Theoretical studies predicted the same trends as experiment and matched the experimental product ratios well. The nature of the regioselectivity in this reaction is discussed. Different strengths of the pi(C5-C6)-->pi(C2-Y) or pi(C5-C6)-->sigma(C2-Y) orbital interactions in 2-substituted norbornenes result in different degrees of C5-C6 double bond polarization. Stronger C5-C6 polarization will increase the difference in the activation energies between the major and minor pathways and thus lead to greater regioselectivities.

Journal Article↗
PubMed2006

Ruthenium-catalyzed [2+2] cycloadditions of alkynyl sulfides and alkynyl sulfones.

Ruthenium-catalyzed [2+2] cycloadditions of bicyclic alkenes with alkynyl sulfides and alkynyl sulfones were investigated. The sulfide and sulfone moieties were found to be compatible with the Ru-catalyzed cycloadditions, giving the corresponding cyclobutene cycloadducts in good yields. The sulfonyl-containing cycloadducts can be transformed into a variety of products that are difficult to obtain via direct cycloaddition.

Journal Article↗
PubMed2005

Rhodium-catalyzed intramolecular [4 + 2] cycloadditions of alkynyl halides.

[reaction: see text] Cationic rhodium(I)-catalyzed intramolecular [4 + 2] cycloadditions of diene-tethered alkynyl halides were found to occur in good yields (70-87%). The halide moiety is compatible with the cycloaddition reactions, and no oxidative insertion to the alkynyl halide was observed. The halogen-containing cycloadducts could be transformed into a variety of products that are difficult or impossible to obtain via direct cycloaddition.

Journal Article↗
PubMed2005

Ruthenium-catalyzed [2 + 2] cycloadditions of ynamides.

Ruthenium-catalyzed [2 + 2] cycloadditions between norbornene and ynamides were investigated. The ynamide moiety was found to be compatible with the ruthenium-catalyzed cycloaddition conditions, giving the corresponding cyclobutene cycloadducts in moderate to good yields (up to 97%). [reaction: see text]

Journal Article↗
PubMed2004

Ruthenium-catalyzed [2 + 2] cycloadditions between 7-substituted norbornadienes and alkynes: an experimental and theoretical study.

The ruthenium-catalyzed [2 + 2] cycloadditions of 7-substituted norbornadienes with an alkyne have been investigated. The cycloadditions were found to be highly regio- and stereoselective, giving only the anti-exo cycloadducts as the single regio- and stereoisomers in good yields. The results on the relative rate of different 7-substituted norbornadienes in the Ru-catalyzed [2 + 2] cycloadditions with an alkyne indicated that the reactivity of the alkene component decreases dramatically as the alkene becomes more electron deficient. Ab initio computational studies on the ruthenium-catalyzed [2 + 2] cycloadditions provided important information about the geometries and the arrangements of the four different groups on the Ru in the initial Ru-alkene-alkyne pi-complex, 14, and in the metallacyclopentene 15. Based on our computational studies, we also found that the first carbon-carbon bond formed in the [2 + 2] cycloaddition is between the C(5) of the alkene and the C(b) (the acetylenic carbon attached to the ester group) of the alkyne 8. Our computational studies on the potential energy profiles of the cycloadditions showed that the activation energy relative to the reactants for the oxidative addition step is in the range of 9.3-9.8 kcal/mol. The activation energy relative to the metallacyclopentene for the reductive elimination step is much higher than for the oxidative addition step (in the range of 25.9-27.6 kcal/mol).

Alkynes↗
PubMed2004

Ruthenium-catalyzed [2 + 2] cycloadditions between bicyclic alkenes and alkynyl halides.

Ru-catalyzed [2 + 2] cycloadditions between norbornadiene and alkynyl halides were found to occur in moderate to good yields (32-89%). The presence of the halide moiety greatly enhances the reactivity of the alkyne component in the cycloaddition and can be transformed into a variety of products that are difficult or impossible to obtain via direct cycloaddition. [reaction: see text]

Journal Article↗
PubMed2004

Prospective randomized controlled trial of argon plasma coagulation ablation vs. endoscopic surveillance of patients with Barrett's esophagus after antireflux surgery.

BACKGROUND: Argon plasma coagulation is one of several techniques used to ablate Barrett's esophagus. This study assessed the efficacy and safety of argon plasma coagulation in the ablation of Barrett's esophagus in patients who have undergone antireflux surgery. METHODS: A total of 40 patients with Barrett's esophagus who had undergone a fundoplication were entered into a prospective, randomized, unblinded study comparing argon plasma coagulation with endoscopic surveillance. Treatment was repeated until either no Barrett's epithelium remained or a maximum of 6 treatment sessions. RESULTS: One month after the final treatment, complete ablation was achieved in 12 patients. In the remaining 8, a reduction of over 95% was observed. One patient died at 9 months of an unrelated cause. At 1 year, one patient with residual Barrett's epithelium regressed completely, while relapse of Barrett's esophagus was seen in another because of fundoplication failure. Buried glands were observed in 35% patients at 1 month, but only 5% at 1 year. Dysplasia was never seen. In the surveillance group, partial regression was observed in 11 patients, and, in 3 with short-segment Barrett's esophagus, regression was complete. The length of Barrett's esophagus increased in two patients. Two had low-grade dysplasia initially, but this was not evident at 1 year. Overall, complete ablation was achieved in 12 of 19 (63%) patients in the ablation group and 3 of 20 (15%) in the surveillance group (p<0.01). CONCLUSIONS: Argon plasma coagulation of Barrett's esophagus is safe and effective. The effects are durable, and buried glands may resolve with time. Long-term follow-up is required to assess the impact of argon plasma coagulation on cancer risk.

Adult↗
PubMed2004

Hepatic hydropericardium.

A 41-year-old man with chronic hepatitis C and cirrhosis presented with pericardial effusion and tamponade requiring pericardiocentesis. Nine liters of pericardial fluid was drained with complete resolution of his ascites. He represented with recurrent pericardial effusions despite salt restriction and diuretic therapy. Subsequent radionuclide scans demonstrated a direct connection between the peritoneal and pericardial spaces. A pericardial window was formed but despite this there was recurrence of pericardial effusion and pleural effusion. The patient underwent orthotopic liver transplantation 7 months later and no recurrence of pleural or pericardial effusion was observed following transplantation. We believe this is the first case report of pericardial effusion secondary to cirrhotic ascites and a communication between the peritoneal and pericardial cavities.

Adult↗
PubMed2002

Molybdenum-mediated cleavage reactions of isoxazoline rings fused in bicyclic frameworks.

The molybdenum-mediated cleavage reactions of isoxazoline rings fused in bicyclic frameworks were investigated. A tandem N-O bond cleavage-retro aldol reaction of an isoxazoline ring fused in a bicyclic framework led to the cleavage of the bicyclic framework. These reactions provide a novel stereoselective synthesis of substituted cyclopentene rings, cyclopentane rings, and attached-ring systems. [reaction: see text]

Journal Article↗
PubMed2002

Oesophageal disorders: future developments.

Elicitation of the characteristic symptom patterns remains the primary approach to the diagnosis of gastro-oesophageal reflux disease, and this may be enhanced by developments in the use of high-resolution endoscopy and oesophageal biopsy. For future therapy, proton pump inhibitors and anti-reflux surgery may compete with reversible acid pump blockers, reflux inhibitor drugs and diverse luminally delivered physical anti-reflux therapies. Short-segment Barrett's oesophagus is known to be highly prevalent, but its impact on the risk of adenocarcinoma remains poorly defined. Biomarkers for Barrett's oesophagus have been proposed to aid in the stratification of cancer risk, and cytology may assume more importance in the future. Endoscopic surveillance for Barrett's oesophagus is widely practised, but more data are needed to demonstrate cost-effectiveness and a positive impact on mortality. Animal and limited human studies suggest that chemoprevention may become an important strategy in reducing the risk of adenocarcinoma. The ablation of Barrett's epithelium results in a reversal of Barrett's epithelium, albeit with an uncertain long-term outcome.

Adenocarcinoma↗