Cleavages ether

Substitution Reactions on Side Chains. Because the benzyl carbon is the most reactive site on the propanoid side chain, many substitution reactions occur at this position. Typically, substitution reactions occur by attack of a nucleophilic reagent on a benzyl carbon present in the form of a carbonium ion or a methine group in a quinonemethide stmeture. In a reversal of the ether cleavage reactions described, benzyl alcohols and ethers may be transformed to alkyl or aryl ethers by acid-catalyzed etherifications or transetherifications with alcohol or phenol. The conversion of a benzyl alcohol or ether to a sulfonic acid group is among the most important side chain modification reactions because it is essential to the solubilization of lignin in the sulfite pulping process (17). 

Like //-butyUithium, j iZ-butyUithium is infinitely soluble in most hydrocarbons, such as pentane and hexane. Its solutions in hexane are flammable and pyrophoric and therefore should be handled like //-butyUithium (96,100). j iZ-ButyUithium also is very soluble in ethers, but the ether solutions must be kept cold because ether cleavage is more rapid than in the presence of //—butyUithium (122). j iZ-ButyUithium has a t 2 of 2 d at 25°C in di-//-butyl ether and of 1 d at 25°C in di-//-hexyl ether. 

Benzenepentol [4270-96-6] (pentahydroxybenzene) (74) has been prepared by boiling 2,4,6-ttiaminoresorciQol diethyl ether with water, followed by ether cleavage with HI (251). The product is very soluble ia water but spatingly soluble ia organic solvents. Benzenepentol prepared by hydrolysis of... 

Oxidative Reactions. The majority of pesticides, or pesticide products, are susceptible to some form of attack by oxidative enzymes. For more persistent pesticides, oxidation is frequently the primary mode of metaboHsm, although there are important exceptions, eg, DDT. For less persistent pesticides, oxidation may play a relatively minor role, or be the first reaction ia a metaboHc pathway. Oxidation generally results ia degradation of the parent molecule. However, attack by certain oxidative enzymes (phenol oxidases) can result ia the condensation or polymerization of the parent molecules this phenomenon is referred to as oxidative coupling (16). Examples of some important oxidative reactions are ether cleavage, alkyl-hydroxylation, aryl-hydroxylation, AJ-dealkylation, and sulfoxidation. 

Acid catalyzed cleavage of aromatic methyl or ethyl ethers Quantitative methoxy group determination Also ether cleavage with tnmethylsilyl Iodide ... 

The anomeric position of a carbohydrate was protected as its trichloroethyl ether. Cleavage is effected with Zn, AcOH, AcONa (3 h, 92%). ... 

An 6>-nitrobenzyl ether.can be cleaved by photolysis. In tyrosine this avoids the use of acid-catalyzed cleavage and the attendant conversion to S-beirayltyrosine. (Note that this unwanted conversion can also be suppressed by the addition of thioanisole see section on benzyl ether cleavage.)... 

This is a fluorescent benzyl ether used for 2 -protection in nucleotide synthesis. It is introduced using 1 -pyrenylmethyl chloride (KOH, benzene, dioxane, reflux, 2 h, >65% yield). Most methods used for benzyl ether cleavage should be applicable to this ether. 

For an extensive review on selective silyl ether cleavage, see T. D. Nelson and R. D. Crouch, Synthesis, 1031 (1996). 

HBr, AcOH, reflux, 30 min, 85%. The efficiency of this method is significantly improved if a phase transfer catalyst ( -Ci6H33P Bu3 Br ) is added to the mixture. Methods that use HBr for ether cleavage can give bromides in the presence of benzylic alcohols. ... 

Butyldiphenylsiloxy)metbyl]benzamide (SiOMB). " Cleavage is induced by silyl ether cleavage. 

Selective ether cleavage comes about during the substitution step, which obeys an Sn2 mechanism. Therefore, selective cleavage requires selective attack by Y on one of the electrophilic carbons in the protonated ether. Determine if selective attack is likely by examining the shape of the lowest-unoccupied molecular orbital (LUMO) in protonated ethyl propyl ether. Is this orbital larger near one carbon than the other If so, what product combination will result What other atom(s) contribute to the LUMO What would happen if 1 attacked this atom(s) ... 

Repeat this analysis for the reaction of phenyl methyl ether with HI leading to phenol and methyl iodide or methanol and phenyl iodide and involving protonated phenyl methyl ether as an intermediate. (Note In this case, the appropriate empty molecular orbital is LUMO+2 the LUMO is concentrated primarily on the CO bond.) Which reaction, with ethyl propyl ether or phenyl methyl ether, appears to be more likely to give selective ether cleavage ... 

Oxidation of the alkaloid Glaucin (136) resulted in the formation of a yellow alkaloid 137, which seemingly is contained in Glaucium flavum var. vestitum (Scheme 48). Addition of methyl iodide converted this compound via a methylation/ether cleavage sequence into Corunnine (127) and small amounts of Pontevedrine (138) which is not a mesomeric betaine (71TL3093). 

Bischler-Napieralski reaction of 139 to a 3,4-dihydroisoquinoline, oxidation, dehydrogenation and reduction of the nitro to the amino function gave 140 which was subjected to a Pschorr reaction (Scheme 49). Quaternization was accomplished by methyl iodide to furnish the isoquinolininium salt 141 which underwent an ether cleavage on heating a solid sample or benzene or DMF solution to Corunnine (127) (73TL3617). 

Acidic ether cleavages are typical nucleophilic substitution reactions, either SN1 or Sn2 depending on the structure of the substrate. Ethers with only primary and secondary alkyl groups react by an S 2 mechanism, in which or Br attacks the protonated ether at the less hindered site. This usually results in a selective cleavage into a single alcohol and a single alkyl halide. For example, ethyl isopropyl ether yields exclusively isopropyl alcohol and iodoethane on cleavage by HI because nucleophilic attack by iodide ion occurs at the less hindered primary site rather than at the more hindered secondary site. 

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