Kuhn methylation

Methylation of a carbohydrate is most practically achieved by using the Hakomori procedure,9 which has largely replaced the Purdie, Haworth, and Kuhn methylation methods. 

Kuhn methylation of narciclasine led to the pentamethyl derivative 350 as well as a smaller amount of the aromatized product 357 whereas, upon acid treatment, narciclasine was transformed into a phenan-thridone which is identical with narciprimine. Structure 355 was assigned to narciprimine rather than the isomeric 356, equally acceptable on the basis of the three adjacent protons of the NMR spectrum, but because of its derivation from narciclasine which had assigned a structure bearing the three hydroxy group at positions 1, 2, and 3. 

Vilkas and Lederer411 used the procedure for the O- and N-methylation of a natural peptide glycoside and report that it is superior to the Kuhn methylation procedure. Methylation of peptides is useful in the analysis of structures by mass spectrometry. 

Although reaction in JV,N-dimethyIformamide greatly facilitates methylation, the method suffers from the limitation that it often gives low recoveries of methylated polysaccharides, and the effectiveness of the methylation appears to depend on the volume of N,M-dimethylformamide used. Even with monosaccharides, incomplete methylation has been reported. D. A. Rees and coworkers have given detaUs of a modified Kuhn methylation for carrageenan, and they also described the methylation of dextran and of a xylan from red seaweed in N-methyl-2-pyr-rolidinone. A micro Kuhn method suitable for oligosaccharides has been described by Perila and Bishop. ... 

In applying the Kuhn methylation to a pentasaccharide isolated from amylopectin a-limit dextrin, the maximum methoxyl content that French, E. E. Smith, and Whelan were able to obtain was 41.8X (theoretical, 49.5%). Quantitative analysis of the hydrolyzate showed that there was no undermethylation, and they concluded that it is essential to wash the product with cyanide after each methylation in order to eliminate chloroform-soluble impurities containing iodine. Kabat and coworkers found that a Kuhn methylation of hexosamines using silver oxide did not give interpretable results, but no problem was experienced when barium oxide was employed. 

The reaction of 2,3,4,6-tetra-O-acetyl-a-D-glucopyranosyl bromide with methyl (methyl 2-0-methyl-a-D-galactopyranosid)uronate in the presence of silver carbonate has yielded the crystalline pseudoaldobiouronic acid derivatives methyl [methyl 2-0-methyl-3-0- and 4-0-(2,3,4,6-tetra-0-acetyl-)3-D-gluco-pyranosyl)-a-D-galactopyranosid]uronate. Deacetylation of the compounds followed by Kuhn methylation afforded mainly crystalline, fully methylated products. 

The selective benzoylation of N-acetyl-iV-aryl-/3-D-ribopyranosylamines has been investigated. " The relative reactivity of the hydroxy-groups was HO-4 > HO-2 > HO-3, whereas that for analogous jS-D-xylopyranosylamines (Z. Smiatacz, Carbohydrate Res., 1974, 38, 117) and a-L-arabinopyranosylamines (Z. Smiatacz, Roczniki Chem., 1974, 48, 947) was HO-3 HO-4 > HO-2. Methyl ethers of iV-acetyl-A -aryl-jS-D-xylo- and -a-L-arabino-pyranosylamines have been prepared by Kuhn methylation. ... 

ATBN - amine terminated nitrile rubber X - Flory Huggins interaction parameter CPE - carboxylated polyethylene d - width at half height of the copolymer profile given by Kuhn statistical segment length DMAE - dimethyl amino ethanol r - interfacial tension reduction d - particle size reduction DSC - differential scanning calorimetry EMA - ethylene methyl acrylate copolymer ENR - epoxidized natural rubber EOR - ethylene olefin rubber EPDM - ethylene propylene diene monomer EPM - ethylene propylene monomer rubber EPR - ethylene propylene rubber EPR-g-SA - succinic anhydride grafted ethylene propylene rubber... 

Finally, with the aim of discovering novel chiral oxomolybdenum catalysts able to perform enantioselective alkene epoxidations, Kuhn et al. have reported the exploration of the catalytic behaviour of a series of dioxomolybdenum(VI) complexes with chiral cw-8-phenylthiomenthol ligands derived from ( + )-pulegone. Therefore, the epoxidation of c -p-methylstyrene using t-butyl-hydroperoxide as the oxidant and performed in the presence of ( + )-(2i ,5i )-2-[1-methyl-l-(phenylthio)ethyl]-5-methylcyclohexanone oxime as the ligand, did not produce, however, a significant optical induction in these conditions. 

Bhattachaijee and coworkers101 reported the first synthesis of the methyl a- and /3-ketoside carboxylates, 63 and 66, to be used as model compounds for the n.m.r.-spectroscopic analysis of the Neisseria meningitidis exopolysaccharide. The method used was that employed by Kuhn and coworkers109 for the synthesis of the analogous methyl /3-and a-ketosides of NeuAc (compounds 67 and 68 compare Section IV,5). 

The degradation pathway of p-cresol in groundwater appears to proceed by oxidation of the methyl group to first give the corresponding benzaldehyde, then benzoic acid (Kuhn et al. 1988 Smolenski and Suflita 1987 Suflita et al. 1988, 1989). The hydroxybenzoic acid then can be either decarboxylated or dehydroxylated to phenol or benzoic acid, respectively. 

Mischnick, R, Kuhn, G. (1996). Model studies on methyl amyloses correlation between reaction conditions and primary structure. Carbohydr. Res., 290, 199-207. 

Ring and aryl methyl group oxidation were the initial toluene-degradation routes speculated on for the nitrate-reducing enrichment obtained by Kuhn etal. (1988) and the metabolically diverse iron-reducing bacterium Geobacter metallireducens (Lovley Lonergan, 1990). The speculation was consistent with the fact that both of these cultures could metabolize the appropriate suite of putative intermediates. However, conclusive evidence as to which pathway was actually involved was not obtained. 

Table I. Comparative Study of C-Methyl Determinations on Normal and Corresponding Reduced Coals by Kuhn-Roth Method...

Reduction apparently creates fresh hydroaromatic structure (at the expense of the aromatics), and thus the methyl groups attached to the aromatic structures are likely to become amenable to quantitative estimation (by the Kuhn-Roth procedure), provided that the particular aromatic ring is reduced to hydroaromatic. Significantly, the reduced samples of the lower rank coals did not yield much higher values for methyl groups than the original samples. Thus, it would appear that Kuhn-Roth estimation does not completely measure the true C-methyl content in coals, especially in high rank coal samples. 

A correlation between the hydroaromatic carbon and the C-methyl carbon as determined by Kuhn>Roth method is shown in Figure 1. The relationship is more or less linear. It had been determined earlier (14) that the proportion... 

---