On the regioselectivity of Wittig reactions with unsymmetrically substituted succinic anhydrides

Phosphorane ylids react readily with succinic anhydrides to give enol-lactones. With highly substituted succinic anhydrides, condensations occur at the less substituted carbonyl group, suggesting that the reaction is sterically controlled. This, however, is not the case in monosubstituted anhydrides where effects other than steric become dominant. Condensation of phosphorane 1a with methoxysuccinic anhydride occurs selectively at the carbonyl group adjacent to the substituent. Stabilization of the transition state through complexation between the oxygen atom of the substituent and an electron-deficient phosphorus of the ylid is proposed. Keywords: cyclic anhydrides, Wittig reaction, regioselectivity, stabilized ylid.

On the regioselectivity of the condensation of stabilized phosphoylids with 3-substituted phthalic anhydrides

Condensations of stabilized phosphorane 1 with 3-substituted phthalic anhydrides were investigated. The importance of various effects influencing regio- and stereoselectivity of these reactions is discussed. It is proposed that the oxygen atom on the substituents in position 3 can act as a Lewis base toward the electron-deficient phosphorus of the ylid. The resulting complexation stabilizes the transition state for the reaction at the ortho carbonyl group, thus offsetting the usual steric and "push" effects, which favour attack at the meta carbonyl function. Keywords: Wittig condensations, phthalic anhydrides, regioselectivity, stereoselectivity.

Morphometric analysis of the effects of phosphorus and chromium on a diatom

Nutrients, particularly phosphorus, may influence, and in some cases, mediate toxicity effects of heavy metals on algae. Further, heavy metal and phosphorus uptake can be coupled and the metal may be incorporated into polyphosphate bodies thereby reducing the amount of metal available to intracellular sites. A series of experiments was designed to evaluate the effects of chromium on a diatom and to determine if these effects are modified by the phosphorus status of the cell.A cultured diatom, Cyclotella aff. meneghiniana was the test organism and was grown in WC medium at 20°C. The experimental regime consisted of control cells with the following phosphorus status: phosphorus deficient, phosphorus sufficient, and an overplus of phosphorus which was achieved by placing cells in phosphate free medium and then adding double the standard amount of phosphorus. Hexavalent chromium at a concentration of 0.2 μg-at/L was added to each of the above phosphorus categories. The addition of chromium to the overplus cells occurred before overplus and following the period of overplus.