A quantitative study on the stereoselectivity of sigmatropic shift reactions in acyclic systems

The semiempirical MINDO/3 method has been used to study sigmatropic [1,3] shifts in propene, [1,5] shifts in pentadiene and [1,7] shifts in heptatriene, occurring in the suprafacial and antarafacial way. Hydrogen, fluorine and methyl (with retention or inversion of configuration) were taken as the shifting goups. For the [1,3] shifts some STO-3G and 4-31G calculations have also been performed. Good correspondence has been obtained with the stereoselectivity predictions of the Woodward-Hoffmann theory. The activation energy for the allowed reactions is 8–21 lower than the value for the forbidden modes. The shift of a F atom proceeds via an inversion-type mechanism

Model study for steroidal D/C ring closure in olefinic cyclization reactions; a MINDO/3 study

MINDO/3 molecular orbital calculations are performed for the ring closure of (Z)- and (E)-allkenes as a model for the D/C ring closure to steroids. It is shown that a lower activation energy is needed for cyclizations in the chair configuration, than for cyclizations in the boat configuration. An analogous picture is found by comparison of (E)- and (Z)-alkenes. Cyclization of the (E)-alkene in the chair or boat configuration is energetically favoured over the ring closure of (Z)-alkenes. These results are in agreement with the Stork-Eschenmoser hypothesis

Model study for steroidal D/C ring closure in olefinic cyclization reactions; a MINDO/3 study

MINDO/3 molecular orbital calculations are performed for the ring closure of (Z)- and (E)-allkenes as a model for the D/C ring closure to steroids. It is shown that a lower activation energy is needed for cyclizations in the chair configuration, than for cyclizations in the boat configuration. An analogous picture is found by comparison of (E)- and (Z)-alkenes. Cyclization of the (E)-alkene in the chair or boat configuration is energetically favoured over the ring closure of (Z)-alkenes. These results are in agreement with the Stork-Eschenmoser hypothesis

Formation of the 7-triphenylphosphonio-norbornen-7-ylium dication : a non-classical dication

While the norbornen-7-ylium cation dissolved in liquid SO2 does not undergo 7-substitution with PPh3, the analogous reaction with the 7-methoxynorbornen-7-ylium cation produces the 7-methoxynorbornen-7-yl-triphenylphosphonium cation; the intermediate 7-triphenylphosphonio-norbornen-7-ylium dication could be detected

Decreased carotenoid concentrations due to dietary sucrose polyesters do not affect possible markers of disease risk in humans

Excessive consumption of energy and fat increases the risk for obesity. Snacks containing sucrose polyesters (SPE) as a dietary fat replacer are on the market in the United States. SPE products have been shown to lower concentrations of serum carotenoids in short-term studies. Experimental studies on the longer-term effects on health of decreased carotenoid concentrations are lacking. A 1-y randomized, double-blind, placebo-controlled parallel trial was performed. Subjects (n = 380) with a habitual low or high fruit and vegetable intake were assigned to the treatments (0, 7, 10 or 17 g/d SPE). SPE was given in the form of spreads, chips or both. The groups were compared for serum carotenoids, vitamins and markers of oxidative damage, eye health, cardiovascular health and immune status. After 1 y, serum lipid-adjusted carotenoids showed the largest decrease in the SPE chips and spread group (17 g/d) compared with the control group [-carotene 33%; ß-carotene 31%, lycopene 24%, ß-cryptoxanthin 18%, lutein 18% (all P <0.001) and zeaxanthin 13% (P <0.05)]. Consumption of SPE spread (10 g/d SPE) decreased carotenoid concentrations by 11¿29% (all P <0.05). SPE chips (7 g/d SPE) decreased zeaxanthin (11%), ß-carotene (12%) and -carotene (21%; all P <0.05). Serum lipid adjusted -tocopherol decreased significantly by 6¿8% (all P <0.001) in all SPE groups. No negative effects were observed on markers of oxidation, eye health, cardiovascular health or immune status. This study shows that decreases in serum carotenoid concentrations do not affect possible markers of disease risk