Benzyl ethers, ozone

When the solvent is or contains an alcohol, it is often incorporated in the ozonation products For example, Kratzl et al (1976) obtained about one mole of ethyl formate per mole of veratrole ozonized when the solvent was 10% ethanol in chloroform Tanahashi et al (1975) obtained methyl esters and, on hydrogenolysis of some ozomde/methanol adducts, methyl benzyl ethers using... 

Few synthetically useful examples of the oxidation of ethers by oxygen or ozone have been publish-ed.7 96 Q0 In 1978, Ourisson and coworkers reported that ozonization of the natural product cedrane oxide (43) on silica gel at -78 °C led to the formation of the corresponding lactone (44) in 30% yield (equation 32).A small amount of the tertiary alcohol (45) was also produced. Later, in the course of a chiral total synthesis of compactin, Hirama examined the ozonolysis of the alkene (46 equation 33). ° Under carefully controlled conditions, selective ozonolysis of the double bond could be achieved in 88% yield. However, when excess ozone was employed, significant amounts of the benzoate (47) were obtained, even at -78 C. In subsequent studies, benzyl ethers of primary and secondary alcohols,and carbohydrates were oxidized to the corresponding benzoates in excellent yields. Surprisingly, no further synthetic rqrplications of this reaction have been reported. 

Enders and Jegelka [88] have used l,3-dioxan-5-one 122, a protected dihydroxyacetone derivative, to construct enantiomerically pure C5- to C9-deoxycarbohydrates. For example, reaction of 122 with SAMP gives the hydrazone 123, which is deprotonated and alkylated with methyl iodide to yield 124. The monoalkylated hydrazone is then alkylated in the same manner with chloromethyl benzyl ether to form 125. Cleavage of the hydrazone with ozone furnishes the protected ulose 126 (>98% de, >98% ee), which is deprotected to (—)-5-deoxy-L-r/ir o-3-pentulose 127. Reduction of 126 with L-Selectride, followed by deprotection, provides 5-deoxy-D-arabinitol 128 (>95% de, >95% ee) (Scheme 13.46). 

The cost of raw materials for use in phenolic foams is low, but since this foam has the weak points listed above, it cannot exceed polyurethane foam in cost performance. Resol-type foams and benzylic ether-type foams use chlorofluorocarbon (CFC) as a foamimg agent. It is necessary to develop a new foaming agent which does not destroy the ozone layer in the atmosphere. When these problems are solved phenolic foam applications will make further rapid progress. 

The ozonation method has provided novel insight into the stereo structure of lignin side chains. It has other important biological and industrial outcomes, such as the analysis of benzylic ether bonds established between lignins and polysaccharides [112],... 

O Karlsson, T Ikeda, T Kishimoto, K Magara, Y Matsumoto, S Hosoya. Ozonation of a lignin-carbohydrate complex model compound of the benzyl ether type. J Wood Sci 46 263-265, 2000. 

Ozonation of benzyl ethers affords high yields of benzoate esters (eq 32). Coupled with deacylation by NaOMe, this... 

New methods for cleavage of benzyl ethers have been reported. Catalytic transfer hydrogenolysis using ammonium formate as donor over palladlum-on-charcoal has been found to work better than with formic acid or cyclohexene over palladium catalysts. Trityl ethers were also cleaved but not benzylldene acetals. Ozone (0 /02,... 

Finally, benzylic oxidation offers a third method for deprotection, as an alternative to hydrogenolysis or electrophilic activation. For example, ozone converts benzyl ethers into benzoate esters at around 0 °C (Figure 2.13) [23], and ruthenium tetraoxide generated in situ has been used for the same purpose the conditions are sufficiently mild that labile trimethylsilyl ethers remain intact (Figure 2.14) [24]. 

A portion of the olefin (3.5 grams) dissolved in pentane (10 ml) was treated with ozone at —178°C for 8 hrs. The product was treated with excess LiAlH4 in ether at 0°C and refluxed. After usual work up, benzyl alcohol (1.7 grams) and (—)(S)-2-methyl-l-butanol [1.1 grams [< ]D25 —5.50, op 94.5% (16, 17)] were recovered. An optical purity of 94.5% is attributed to ( + )(S)-l-phenyl-3-methyl-l-pentene having D25 +44.95°. 

For GC analysis, the salts of the lowest molecular weight acids present in ozonation products subjected to base-promoted hydrolysis have been converted to their benzyl esters by reaction with benzyl bromide (Bonnet et al. 1989). The salts of all acids produced have commonly been converted to the free acids, usually with the aid of a cation exchange resin. The acids have then been converted to methyl esters by reaction with diazomethane (Bonnet et al. 1989) or, more often, have been converted to trimethylsilyl (TMS) esters (Matsumoto et al. 1986, Taneda et al. 1989, Habu et al. 1990). Trimethylsilylation has the major advantage that alcoholic and phenolic hydroxyl groups are simultaneously converted to TMS ethers, thus greatly facilitating GC analysis. 

Oxidation of benzyl ketone hydrazones with Hg2 (O2CCF3) in ether or dioxane gives a direct synthesis of phenylacetyl-enes (CeHsC CR). Olefin formation occurs with saturated aldehydes and carboxylic acids when they are allowed to react with ruthenium and rhodium complexes. Olefins can be converted to aldehydes in high yields by ozonization followed by treatment with dimethylsulfide. ... 

---