1. The chromogen formation from N-acetylneuraminic acid in the periodate-thiobarbituric acid reaction was investigated. Measurement of periodate consumption showed an uptake of approx. 3moles/mole of substrate in neutral as well as in strongly acidic solution. Therefore the chromogen beta-formylpyruvic acid is not a direct product of the periodate oxidation; it is presumed to be formed from the true oxidation product, a hexos-5-uluronic acid, by aldol splitting during the reaction in hot acidic solution with thiobarbituric acid. 2. Methyl (methyl beta-l-threo-hexos-4-enepyranosid)uronate, an analogue of the pre-chromogen, has been shown to yield with thiobarbituric acid in acidic solution a pigment exhibiting an identical absorption spectrum and showing the same behaviour on paper chromatography as the pigment obtained from N-acetylneuraminic acid in the periodate-thiobarbituric acid assay. 3. The substitution at C-2 of methoxyneuraminic acid does not inhibit the periodate-thiobarbituric acid reaction. In neutral solution methoxyneuraminic acid is oxidized by periodate to a substance that reacts readily with thiobarbituric acid in acidic solution. When periodate oxidation is attempted in acidic solution, protonation of the amino group protects this group against oxidation, rendering methoxyneuraminic acid negative in the assay systems of Warren (1959a,b) and Aminoff (1959, 1961).
Furthermore, no 1 : 2-adduct was formed by the reaction of trimethyl phosphite with p-nitrobenzaldehyde in the presence of an excess of 2-methyl-2-nitrosopropane in CH,CI, at room temperature during 40 hours. This means, that radical reactions are stopped by the scavenger, because no alternative reaction path is available. ExperimentalESR spectra were recorded at room temperature on a Varian E-4 spectrometer. The dimer of 2-methyl-2-nitrosopropane was prepared as described by Stowel16 from t-butylamine, Na,WO,. 2H,O and hydrogen peroxide. 3-Benzylidene-acetylacetone', 2,7-dinitrophenanthrenequinone8and tris(dimethylamino)phosphine9 were prepared by standard methods. Trimethyl phosphite and triethyl phosphite were commercially available (BDH) and were distilled from sodium before use. Methyl vinyl ketone (Schuchardt, Miinchen) was distilled before use and 9,lO-phenanthrenequinone (Fluka AG) was crystallized from benzene.J . C. Stowell, J. Org. Chem. 36, 3055 (1971).' M . E. McEntee and A . R. Pinder, J. Chem. SOC. 4426 (1957). ' J . Schmidt, Ber. 35, 3 122 (1 902).R. Burgada, Ann. Chim. (France) 8, 348 (1963). IntroductionSelective halogenation using molecular sieves J. van Abstract. On treatment of aromatic compounds, entirely adsorbed in type X molecular sieves, with bromine or chlorine a selective halogenation is found to take place. Alkylbenzenes give mainly -and halogenobenzenes almost exclusively -the para isomers. The bromination of alkylbenzenes takes place only in the ring. Phenyl ethers undergo a strong deactivation of the aromatic ring. In halogenations of toluene and halogenobenzenes, the use of catalytic amounts of CaX in which in most cases part of the cation is exchanged by Fe+++, result also in a significantly higher paralortho ratio, than in reactions homogeneously catalyzed by FeCI,.Although the molecular sieves (synthetic zeolites) have found extensive use in separations and as high temperature catalysts' of isomerization, cracking or alkylation reactions, only a few applications in preparative organic synthesis have been reported. Examples include the use of molecular sieves as selective adsorbents in the preparation of acetals and enamines, as described by Roelofson and van Bekkum'. We have now studied the use of molecular sieves in the halogenation of aromatic compounds, especially the halogenation of bromobcnzene, chlorobenzene, several alkylbenzenes and methoxybenzene. The halogenation of benzene with a zeolite containing Fe is described by Levina et aL3, but without any mention of the quantitative composition of the reaction products. We considered it possible that molecules adsorbed in the welldefined, relatively narrow cavities of the zeolite crystals might be restricted in their orientations. Under this influence or on account of the electrostatic forces within the crystal lattice the substitution pattern might be changed in comparison with reactions in solution. A related example of this, published during our investigation, is found in the selective (para) halogenation of anisole cn...
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