Benzyl ethers acid effect

Selective cleavage, so often a prerequisite in synthesis, of an aryl benzyl ether was effected in 80% yield with the dibenzyl ether of estrane-3,17-diol in chloroform containing N,N-dimethylaniline by the addition of powdered aluminium chloride, stirring during 30 mins, at ambient temperature, and work-up following quenching with M hydrochloric acid (ref.95). 

The general features of the monensin synthesis conducted by Kishi et al. are outlined, in retrosynthetic format, in Scheme 1. It was decided to delay the construction of monensin s spiroketal substructure, the l,6-dioxaspiro[4.5]decane framework, to a very late stage in the synthesis (see Scheme 1). It seemed reasonable to expect that exposure of the keto triol resulting from the hydrogen-olysis of the C-5 benzyl ether in 2 to an acidic medium could, under equilibrating conditions, result in the formation of the spiroketal in 1. This proposition was based on the reasonable assumption that the configuration of the spiroketal carbon (C-9) in monensin corresponds to the thermodynamically most stable form, as is the case for most spiroketal-containing natural products.19 Spiro-ketals found in nature usually adopt conformations in which steric effects are minimized and anomeric effects are maximized. 

The use of trichloroimidates for the preparation of ethers is an effective method for O-alkylation of alcohols [27]. This method has found widespread use in the protection of alcohols as benzyl ethers since the corresponding trichlorobenzylimi-date is inexpensive and commercially available. The mechanism involves activation of the imidate with a catalytic amount of a strong acid (typically TfOH) which leads to ionization of the electrophile and the formation of carbocation which is rapidly trapped by an alcohol. For the preparation of sec-sec ethers, this protocol has been limited to glycosidation reactions, due to the SN1 nature of the reaction which often leads to diastereomeric mixtures of products [26],... 

This result is quite surprizing, but can be ascribed to the acidic components of coal ash, since Bell and coworkers (21) reported that benzyl ether acts as an alkylating reagent TTT the presence of Lewis acid such as ZnCl2- However, the acidic effect due to coal ash may be suppressed considerably by organic base in the coal liquefaction. 

The cleavage of benzyl ethers using hydrobromic acid is promoted by tetra-n-butylammonium bromide [38]. Selective cleavage of aryl silyl ethers can be effected in the presence of aliphatic silyl ethers using solid sodium hydroxide with tetra-n-butyl-ammonium hydrogen sulphate [39]. 

Catalytic Cycle. Attempts to determine the reasons for the improved activity of the base on alumina reagents followed two paths. Being as solubility of base in methanol appeared to greatly effect the production of methyl benzyl ether, we compared the amount of extractable base with selectivity to ester. This concentration of base, presumably alkoxide, was determined by stirring the base in methanol, filtering, and titrating the filtrate with acid. This comparison is given as Table VII. 

Benzyl ethers are readily formed under PTC conditions described in Expt 5.73. These ethers are stable to acidic and basic media, and to oxidising reagents. Deprotection is effected by hydrogenolysis the wide range of conditions appropriate to the structural nature of the alcohol has been reviewed.77... 

Another critical consideration was the development of an effective protecting-group strategy. We chose to employ benzyl ethers as permanent protecting groups, not to be removed during the course of intermittent deprotection events, because of their stability under moderately acidic and... 

The three phenolic furoquinoline alkaloids robustine (18), haplopine (1 R = H, R2 = OH, R3 = OMe), and confusameline (1 R1 = R3 = H, R2 = OH), which were synthesized some time ago (see Vols. 3 and 5), have now been prepared by a modification of the Grundon-McCorkindale method, as illustrated for robustine (Scheme 3).22 Polyphosphate ester was found to be more effective than poly-phosphoric acid for cyclization of the alcohol (17) to the dihydrofuran (19). Dehydrogenation to a furoquinoline was accomplished by reaction of the benzyl ether of dihydrofuran (19) with DDQ. 

Direct 2 - or 3 -0-benzylation of nucleosides has been reported,71,95,96 but removal of the benzyl group by catalytic hydrogen-olysis is not straightforward. The deprotection of methyl-substituted-benzyl ethers of nucleosides has been effected by treatment with acid. An improvement here has been the use of the photosensitive o-nitro-... 

Tin(ll) chloride is an effective reducing agent under acidic conditions. " This inexpensive reagent selectively reduces aromatic nitro compounds in nonacidic and nonaqueous media." Nearly quantitative yields of arylamines are obtained using tin(II) chloride dihydrate in alcohol or ethyl acetate (equation 19). Under these conditions other reducible or acid sensitive groups such as aldehyde, ketone, ester, cyano, halogen and benzyl ethers are not affected. 

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