Benzyl ethers bromine

Scheme 4.—Photochemical Cleavage of Benzyl Ethers in the Presence of Bromine.

Oxidative cleavage by means of electrochemically generated cation-radicals is also possible thus benzyl ethers may be cleaved and carboxylates decarboxylated using cation-radicals of brominated triphenylamines in acetonitrile containing a weak base.34 35 Such as indirect reaction makes it... 

Methyl ethers have been employed, but alkyl glycosides may be unstable following hydrogen abstraction from the anomeric center (see Section 11,9). Benzyl ethers and benzylidene (and other aldehyde-based) acetals, which themselves undergo ready radical bromination,27 can be expected to be entirely unsuitable protecting groups. Ketone-derived acetals, on the other hand, should be stable, at least in the absence of acid, and a few examples of successful brominations in their presence are reported in Section 11,4. 

Cyclization of allylic alcohols to form epoxides has been particularly problematical, and the reactions have been more of mechanistic than of synthetic interest. For reactions conducted under basic conditions, it is possible that epoxide formation involves initial halogen addition followed by nucleophilic displacement to form the epoxide. Early examples of direct formation of epoxides from allylic alcohols with sodium hypobromite," bromine and 1.5 M NaOH,12 and r-butyl hypochlorite13 have been reviewed previously.fr Recently it has been shown that allylic alcohols can be cyclized effectively with bis(jym-collidine)iodine(I) perchlorate (equation 3).14 An unusual example of epoxide formation competing with other cyclization types is shown in equation (4).15 In this case, an allylic benzyl ether competes effectively with a -/-hydroxyl group as the nucleophile. 

After debenzylation (Pd-C, cyclohexene) of the hypercores (e.g., 25) by transfer hydrogenation, they were treated with aryl branched, benzyl ether dendrons (Scheme 5.6) that were prepared by similar iterative transformations,1271 i.e., benzylic bromination and phenolic O-alkylation (See Section 5.4.2). Thus, hexaphenol core 27 was reacted with six equivalents of the benzylic bromide building block 28 to give the benzyloxy terminated dendrimer 29. Key features of these dendrimers include cores with flexible alkyl spacers and a three-directional, quaternary carbon branching center. 

Other recent oxidative methods have yet to make a major impact. Photochemical bromination can be used for the oxidative cleavage of a benzyl ether on a substantial scale.290-291 An intermediate a-bromo ether hydrolyses to a hemiace-tal intermediate that collapses to give benzaldehyde and the liberated alcohol [Scheme 4 156] ... 

Lactonization Reaction of 1 with trityl tetrafluoroborate not only effects oxidative debenzylation as expected (4, 567), but also cyclization to 2, d/-aplysis-tatin, a brominated sesquiterpene. Attempted hydrogenolysis of the benzyl ether resulted only in hydrogenation of the double bond. 

Barium Chloride Benzaldehyde Benzenes ulfonic Acid Benzonitrlle Benzyl Chloride Bromine Butyl Acetate Butyl Alcohol Butyl Ether Butyl Phthalate Butylamlne Calcium Chloride... 

Reduction of 202a to the lactol and a subsequent Wittig reaction led to 203 in 52% yield. Protecting-group exchange and tosylation of the resulting primary alcohol provided the cyclization precursor 204 (76%). Base treatment then afforded an 87% yield of cyclopentanes 205 epimeric at the nitrile (an epimerizable position). Hydrolysis and esterification of either isomer led to the trans geminally substituted cyclopentane 206 in 82% yield. Reduction of 206 and protection of the ester produced benzyl ether 207, which was converted to alkyne 208 via a bromination-dehydrobromination sequence in 78% yield. 

Removal of the MOM and Boc groups followed by regioselective bromination with Py-HBr in dichloromethane followed by cyclization through an iminium ion intermediate furnished the bicycle 10 as a single isomer. Deprotection of the benzyl ether and reduction of the azide group afforded the amino diol 11, which was treated with BrCH2C02Ph and propylene oxide and subsequent Pb(OAc)4 oxidation of the amine to provide 12 (80%). Intermolecular addition of the phenolic compound 13 to 12 in the presence of TEA occurred at the sterically less hindered convex face to provide the key intermediate 14 (Scheme 2). ... 

A few miscellaneous oxidations using oxoammonium salts generated from TEMPO or substituted TEMPO analogs have been reported in the literature. These include the electrochemical oxidation of thiols to disulfides by a TEMPO-modified felt electrode [66], the electrochemical oxidation of amines to imines or nitriles [67], the cleavage of benzyl ethers by a single electron transfer mechanism with an oxoammonium bromide salt [68], and the dibromination of propargyl acetates by catalytic oxoammonium tribromide generated from a nitroxide and bromine [69]. 

Aminoethylpiperazine Amyl lactate t-Amyl methyl ether Anisole Benzotrifluoride Benzyl ether Bis (2-chloroethoxy) methane Bromine 1,4-Butanediol Butoxyethyl stearate 3-Butoxy-1,2-propanediol t-Butyl acetate n-Butylamine s-Butylamine Butyl butyrate Butylcyclohexane s-Butylcyclohexane t-Butylcyclohexane Butylcyclopentane s-Butylcyclopentane... 

Selective reduction of benzylidene acetals to form mono-benzyl protected diols was discussed in section 2.3.1, Benzyl ethers but differentiation of the oxygen atoms within benzylidene derivatives can also be achieved in the widely used Hanessian procedure. In this process, benzylic bromination, oxonium ion formation, and nucleophilic attack by bromide ion at the (1°-) 6-position leads to the 4-O-benzoyl-6-deoxy-6-bromo derivative. Figure 2.62 [83]. 

Kita and coworkers use potassium bromide to depolymerize iodosobenzene 17 by generating an I—Br bond. Homolytic cleavage of the I—Br bond to generate the iodine-centered radical allows formation of the benzylic radical 18. The authors propose the benzylic radical is trapped by bromine radical to generate a benzylic bromide 19, a known byproduct of the reaction conditions. Conversion to the alcohol and subsequent oxidation yields the aryl ketone 20. While the ketone products are useful themselves, Ochiai utilizes this mechanism, though with a different hyperva-lent iodine reagent, to deprotect benzylic ethers such as 21 to alcohols 2. ... 

Benzyl ether [857561-04-7] C gHjjBr03 mol.wt. 335.20 - Obtained by bromination of 2-benzyloxy-5-methoxyacetophenone,... 

Acyloxysulfonates have been recommended for ether cleavage, particularly where the usual methods fail. They also show greater specificity in the cleavage of unsym. ethers Bromination-hydro-lysis is a new convenient way to remove selectively 0-benzyl groups from carbohydrates without change in configuration... 

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