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Ylidenenorbornadienes (YNDs), prepared by [4 + 2] cycloadditions between fulvenes and acetylene carboxylates, react with thiol nucleophiles to yield mixtures of four to eight diastereomers depending on the symmetry of the YND substrate. The mixtures of diastereomers fragment via a retro-[4 + 2] cycloaddition with a large variation in rate, with half-lives ranging from 16 to 11,000 min at 80 °C. The diastereomer-enriched samples of propane thiol adducts [YND-propanethiol (PTs)] were isolated and identified by nuclear Overhauser effect spectroscopy (NOESY) correlations. Simulated kinetics were used to extrapolate the rate constants of individual diastereomers from the observed rate data, and it correlated well with rate constants measured directly and from isolated diastereomer-enriched samples. The individual diastereomers of a model system fragment at differing rates with half-lives ranging from 5 to 44 min in CDCl 3 . Density functional theory calculations were performed to investigate the mechanism of fragmentation and support an asynchronous retro-[4 + 2] cycloaddition transition state. The computations generally correlated well with the observed free energies of activation for four diastereomers of the model system as a whole, within 2.6 kcal/mol. However, the observed order of the fragmentation rates across the set of diastereomers deviated from the computational results. YNDs display wide variability in the rate of fragmentation, dependent on the stereoelectronics of the ylidene substituents. A Hammett study showed that the electron-rich aromatic rings attached to the ylidene bridge increase the fragmentation rate, while electron-deficient systems slow fragmentation rates.

Ylidenenorbornadienes (YNDs), prepared by [4 + 2] cycloadditions between fulvenes and acetylene carboxylates, react with beta -mercaptoethanol to yield a mixture of four diastereomers. These four diastereomers fragment via a retro-[4 + 2] cycloaddition at differing rates. A simulated kinetics approach extrapolated the rate constants of the diastereomers from the observed rate data. YNDs display wide variability in rate of fragmentation, dependent on the stereoelectronics of the ylidene substituents. A substrate containing one carboxylic ester proved exceptionally stable to fragmentation.

Splenic function in patients with sickle B+ (SB+) thalassemia has been poorly documented. We evaluated the clinical course and splenic function in 12 children with SB+ thalassemia with simultaneous technetium sulfur colloid spleen scans and determination of pitted erythrocytes by direct interference contrast microscopy (DICM). All patients displayed normal uptake of radiocolloid. Mean percentage of pitted erythrocytes was 2.2% compared to 0.06% in 10 normal eusplenic controls and 13.8% in 10 sickle cell patients. In this group of children, who were carefully monitored for 136 patient years, there was no episode of bacteremia/sepsis, and a low prevalence of vaso-occlusive episodes. The slight increase in percentage of pitted erythrocytes of SB+ thalassemia patients does not seem to herald any clinically relevant loss of splenic function. SB+ thalassemia children younger than 10 years of age who do not seem to present a higher risk of invasive bacterial infections than eusplenic children, should receive conservative treatment for isolated febrile episodes and should not be submitted to prophylactic penicillin.