Methyl ether groups

Pyridinium chloride ([PyHjCl) has also been used in a number ofcyclization reactions of aryl ethers (Scheme 5.1-4) [4, 18]. Presumably the reaction initially proceeds by deallcylation of the methyl ether groups to produce the corresponding phenol. The mechanism of the cyclization is not well understood, but Pagni and Smith have suggested that it proceeds by nucleophilic attack of an Ar-OH or Ar-0 group on the second aromatic ring (in a protonated form) [4]. 

A synthetic neuraminic acid derivative having a methyl ether group at 0-4 (Neu5Ac4Me) was synthesized by Beau and coworkers101,102 by using an oxymercuration-demercuration reaction.103 The metabolic behavior of this compound will be described in Sections V and VI. 

Use of this coupling for synthesis of a number of isoquinoline alkaloids also has been published.2 VOF is usually used in this coupling, but this reagent can lead to overoxidation and oxidative demcthylation of aryl methyl ether groups. 

Kauss (39) has shown that the esterification of the carboxyl groups in the D-galacturonic acid chain takes place by a transfer of the methyl groups from S-adenosyl-L-methionine, analogous to the case in which the 4-methyl ether groups are transferred to D-glucuronic acid of hemi-cellulose (40). 

The synthesis of the partially methylated glucoside, methyl 2,3-di-O-methyl-a-D-glucopyranoside (34) (Expt 5.121) also utilises the 4,6-O-benzylidene derivative (27), which is first converted into the 2,3-di-O-methyl derivative (33) by reaction with dimethyl sulphate in the presence of sodium hydroxide. Selective removal of the acetal grouping is achieved by mild acid hydrolysis the methyl ether groups are stable under both acidic and basic conditions. 

Peripheral functionalization (2, 7,12, and 17 positions) of porphycenes has relied on the use of ( > - m e th o x ye thy I - s u b s t i t u ted porphycenes. Synthetic sequences such as removal of a methyl ether group of 3-methoxyethyl-substituted porphycene 48 (M=H2) (Scheme 24) with a Lewis acid such as BBr3 produces the alcohol 49 or other substituted products 50 (R=Br) (1994JMC2797). The hydroxyl-substituted porphycene 49 has further been converted to porphycene sugar derivatives (19730R455). Further, both bromide 50 (R=Br) and mesylate 50 (R=OMs) produced from hydroxyl derivative 49 undergo substitution with NaCN to afford the cyano derivative 50 (R=CN), and then converted to a carboxylic ester 50 (R=COOEt) and its saponified product 50 (R=COOH) (1993MI). 

Demethylation of the two methyl ether groups in 106 1 [Scheme 4.108) was difficult because the benzylic alcohols were labile to both acid and base.197 Protracted treatment of 108 1 with excess lithium diphenylphosphide at room temperature eventually gave the bisdemethylated compound 10fL2 in 95% yield. 

The hemicelluloses of wood contain uronic acid residues which frequently bear methyl ether groups. Hydrolysis of the hemicelluloses yields aldobioLironic and aldotriouronic acids. In researches referred to previously (page 300), O Dwyer " investigated the hemicelluloses from oakwood. She isolated an aldobiouronic acid as a barium salt after combined enzymic and acidic hydrolysis. Analyses of this compound were in agreement with the empirical formula for a barium salt of an 0-methyl-xylo-aldobiouronic acid. This compound was isolated from a polysaccharide material which contained xylose (6 parts), uronic acid (1 part), and methoxyl (1 part). Studies by Anderson on the hemicelluloses of a number of hardwoods also showed that 0-methylhexuronic acid residues are present in glycosidic union with xylose, in ratios extending from 8 to 19 xylose units per uronic acid unit. 

The production of two different fully methylated lactones at once proves that both of the hydroxyl groups at positions (4) and (5) of the hexose chain of cymarose are open, which leaves only position (3) for the methyl ether group, thus confirming the interpretation of the results of the oxidation of cymarose with nitric acid. 

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