Imidazoles, to protect

Imidazoles, to protect, 385-397 Imidazolidines, to protect carbonyl groups, 218-219... 

KOH, H2O, reflux, 12 h, 64-92% yield. This group is more stable to n-BuLi than is the benzyl group when used to protect imidazoles. 

CF3CO2H, Me2S, 40-60 min, 100% yield, [Imidazole = His(Tos)]." The related phenylsulfonyl group has been used to protect pyrroles and indoles, and is cleaved with NaOH/H20/dioxane, it, 2 h. " ... 

The dinitrophenyl group has been used to protect the imidazole — NH group in histidines (45% yield)" by reaction with 2,4-dinitrofluorobenzene and potassium carbonate. Imidazole —NH groups, but not a-amino acid groups, are quantitatively regenerated by reaction with 2-mercaptoethanol (22°, pH 8, 1 h)." The 2,4-... 

Finally, in imidazole-4,5-dicarboxaldehydes groups such as — CH2OCH2 CH2SiMc3 were used to protect the heterocyclic NH group during aldol condensation (91MI1). 

Cu—Zn superoxide dismutases (SODs) [87,88] are abundant in eukaryotic cells and may serve to protect cells against the toxic effects of superoxide or deleterious oxy-products derived from 02 . The active site copper and zinc ions are 6.3 A apart and are bridged by a histidine imidazolate. In the oxidized form Cu(II) is roughly pentacoordinate, with four His N s and a water molecule. A highly conserved Arg residue is thought to stabilize Cu(II)-bound anions (e.g., Cu(II)—02 ) a redox reaction releases 02, generating Cu(I), which can reduce more 02 substrate to give peroxide and Cu(II). 

The answer is to protect the hydroxyl group, and the group chosen here was a silyl ether. Such ethers are made by reacting the alcohol with a trialkylsilyl chloride (here f-butyl dimethyl silyl chloride, or TBDMSCl) in the presence of a weak base, usually imidazole, which also acts as a nucleophilic catalyst (Chapter 12). 

MO-Acetals afford robust protection for indoles and pyrroles with SEM and BOM ethers being particular favourites. A SEM group has also been used to protect imidazole.534 They are introduced in much the same way as already discussed for amines and amides in section 8.6. L Hence treatment of Indolelactam derivative 274.1 with SEMC and sodium hydride in THF at —15 °C afforded the M 0-acetal 274 in >80% yield 535... 

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