Alkenes activated, reactions with carbohydrates

A simple approach for the formation of 2-substituted 3,4-dihydro-2H-pyrans, which are useful precursors for natural products such as optically active carbohydrates, is the catalytic enantioselective cycloaddition reaction of a,/ -unsaturated carbonyl compounds with electron-rich alkenes. This is an inverse electron-demand cycloaddition reaction which is controlled by a dominant interaction between the LUMO of the 1-oxa-1,3-butadiene and the HOMO of the alkene (Scheme 4.2, right). This is usually a concerted non-synchronous reaction with retention of the configuration of the die-nophile and results in normally high regioselectivity, which in the presence of Lewis acids is improved and, furthermore, also increases the reaction rate. 

Although reaction of carbohydrates with such activated alkenes as acrylonitrile and methyl vinyl sulfone has industrial importance, the... 

Lewis acid with Halides. Ag20 can activate halide as a leaving group by coordination. Methylation of carbohydrates and 5-benzyUdenebarbituric acid have been carried out with Ag20 and iodomethane in DMF. Triphenyltin trifluoroacetate was obtained from the reaction of triphenyltin iodide and trifluoroacetic acid in the presence of silver oxide. Ag20 converts trawi -halohydrins to epoxides(eq 12) or rearranged products (eq 13). It is reported that the reaction is nonstereoselective, and that tra/w-epoxides are the major products when c/s-alkenes are the reactants. ... 

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