Dimethyl-t-butylsilyl ether

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). 

Protection of hydroxyl groi. Trimethylsilyl ethers are too susceptible to solvolysis in protic media to be widely useful in synthesis. However, thedimethyl-/-butyl-silyloxy group is about 10 times more stable than the trimethylsilyloxy group. Dimethyl-/-butylchlorosilane reacts only slowly with alcohols in THF in the presence of excess pyridine. However, if imidazole (I, 492-494 2, 220) is used as catalyst and DMF as solvent, dimethyl-t-butylsilyl ethers arc obtained in high yield under mild conditions. N-Dimethyl-r-butylsilylimidazole is probably the actual silylating reagent. 

Dimethyl-t-butylsilyl ethers are cleaved by 2 1 acetic acid-water at 25° or by treatment with tetra-n-butylammonium fluoride (this volume) in THF at 25°. 

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