4- Methoxybenzyl ether over oxidation

Six protective groups for alcohols, which may be removed successively and selectively, have been listed by E.J. Corey (1972B). A hypothetical hexahydroxy compound with hydroxy groups 1 to 6 protected as (1) acetate, (2) 2,2,2-trichloroethyl carbonate, (3) benzyl ether, (4) dimethyl-t-butylsilyl ether, (5) 2-tetrahydropyranyl ether, and (6) methyl ether may be unmasked in that order by the reagents (1) KjCO, or NH, in CHjOH, (2) Zn in CHjOH or AcOH, (3) over Pd, (4) F", (5) wet acetic acid, and (6) BBrj. The groups may also be exposed to the same reagents in the order A 5, 2, 1, 3, 6. The (4-methoxyphenyl)methyl group (=MPM = p-methoxybenzyl, PMB) can be oxidized to a benzaldehyde derivative and thereby be removed at room temperature under neutral conditions (Y- Oikawa, 1982 R. Johansson, 1984 T. Fukuyama, 1985). 

The overhydrogenation of aromatic aldehydes beyond benzylic alcohols is rarely important with copper-chromium oxide and Raney nickel unless the reaction conditions are too vigorous. Thus, over copper-chromium oxide o- and p-methoxybenzaldehydes were hydrogenated to the corresponding methoxybenzyl alcohols in high yields in methanol at 110-125°C and 22-24 MPa H2 (eq. 5.18).42 At 185°C, however, / -methoxybenzyl alcohol was hydrogenolyzed to give p-cre-sol methyl ether (eq. 5.18). 

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