Benzyl ethers reviews

The reagent most commonly used for oxidation of ethers is RuO. The subject is well summarized in an early review by Gore [75], Primary methyl ethers RCHjOCHj are oxidised to esters RCOOCH, and secondary methyl ethers R R CjHjOCHj to ketones R COR while with benzyl ethers PhCH OR the esters PhCOOR are formed. For cyclic ethers, the carbon atoms adjacent to the O atom are oxidised, and if there are two secondary carbon atoms the main products are lactones, sometimes with partial hydrolysis to carboxylic acids [75], There is a short review on oxidation of ethers by RuO, principally on the mechanisms involved [76],... 

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. 

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... 

Although a number of different reagents have been discovered for the selective oxidation of ethers, e.g. halogens, iodine tris(trifluoroacetate), trichloroisocyanuric acid, UFs, A(,N-dibromobenzenesul-fonamide and lead tetraacetate, few have assumed any synthetic importance. Of these, the most significant are the metallic oxidants chromic acid and ruthenium tetroxide. DDQ has also been widely used for the oxidative d rotection of benzyl ethers. It is the aim of this chapter to review the latest developments in ether oxidation by these, and other reagents, with particular emphasis on chemo- and regio-selectivity. Several reviews on the subject have appeared previously. " The related oxidation of acetals has been reviewed recently" and will not be dealt with here. 

Alkyl aryl ethers are qnite stable on heating. Phenyl benzyl ether isomerizes slowly at 250 °C to afford 4-benzylphenol and its ortho-i ovasc as a minor prodnct" . The conditions of isomerizations of O-aUcylated and O-aralkylated phenols were reviewed . 

The fj-nitrobenzyl and p-nitrobenzyl ethers can be prepared and cleaved by many of the methods described for benzyl ethers. In addition, the o-nitrobenzyl ether can be cleaved by irradiation (320 nm, 10 min, quant, yield of carbohydrate 280 nm, 95% yield of nucleotide ). This is one of the most important methods for cleavage of this ether. These ethers can also be cleaved oxidatively (DDQ or electrolysis) after reduction to the aniline derivative." Clean reduction to the aniline is accomplished with Zn(Cu) (acetylacetone, rt, >93% yield).Hydrogenolysis is also an effective means for cleavage. A polymeric version of the o-nitrobenzyl ether has been prepared for oligosaccharide synthesis that is also conveniently cleaved by photolysis. An unusual selective deprotection of a bis-o-nitrobenzyl ether has been observed. The photochemical reaction of o-nitrobenzyl derivatives has been reviewed. ... 

Organic Reactions contains a review of the fission of benzyl ethers, including cyclic benzyl ethers, by hydrogenation.43 Ring fission of furans has been reviewed by Jones and Taylor.44 Burwell45 has written an excellent and detailed report on the cleavage of ethers, in which he discusses also the theoretical problems involved he concludes with some indications for the fission of sulfides, a subject that is treated also by Tarbell and Hamish.46... 

Numerous syntheses of swainsonine s synthetic diastereoisomers have been pubHshed. Detailed discussion of these (unless they occur in routes that produce swainsonine as well) would swell this review to unmanageable lengths but for completeness, the following references are provided (—)-2-ept -swainsonine (379), its 8-O-benzyl ether, and its enantiomer, which appears in the literature as (4-)-l,8,8a-tri-q) -swainsonine (cni-379) " (—)-8-ept-swainsonine (380) swainsonine (381) ... 

Although slightly outside the scope of this review, an interesting case of stereoselection should be presented here. It has been observed by Gibson (nee Thomas) and coworkers during the deprotonation of tricarbonylchromium complexes of benzyl alkyl ethers by means of the chiral bis(lithiumamide) base 234 (equation 54) . The base removes the benzylic pro-R-H atom in 233 from the most reactive conformation to form the planary chiral intermediate 235. The attack of the electrophile forming 236 proceeds exclusively from the upper face in 235, because the bulky chromium moiety shields the lower face. Simpkins and coworkers extended the method to the enantioselective substitution of the chromium complexes of 1,3-dihydroisobenzofurans . 

Bromotrimethylsilane has proven to be useful for a wide variety of applications most of them being reviewed.5 8 Other recent applications are mild cleavage of oxiranes,9 the synthesis of glycosyl bromides,10 the selective cleavage of tetrahydro-2,5-dimethoxyfuran and tetrahydro-2,6-dimethoxy-pyran,11 the cleavage of esters and ethers,12 and the synthesis of benzyl bromides.13... 

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