Di-terf-butyl dicarbonate

It is also worthwhile to note that the relatively slow catalytic hydrogenolysis of A/ -Z-protected peptide derivatives pemnits some interesting chemical transformations to be performed in situ. For example, direct conversion of Z protection to Boc protection is possible when the hydrogenation is conducted in the presence of di-tert-butyl dicarbonate under neutral conditions.h Alternatively, the same transformation is achieved by the use of triethylsilane and di-tert-butyl dicarbonate in ethanol with catalytic amounts of palladium(II) acetate.h 1 More efficiently this one-pot transformation is achieved by catalytic transfer hydrogenation in the presence of di-terf-butyl dicarbonate (cf. Section 2.1.1.1.3.1.1.6).h l Similarly, peptide cyclization reactions have been performed in situ over Pd/C and the high yields of cyclic monomers are attributed to the high dilution effect as well as to catalysis of the charcoal surface.h l... 

Show the mechanism for formation of a Boc derivative by reaction of an amino acid with di-terf-butyl dicarbonate. 

The reagent most often used to protect the amino group of an amino acid is di-terf-butyl dicarbonate. Notice that the amino group rather than the carboxylate group of the amino acid reacts with di-ferf-butyl dicarbonate because the amino group is a better nucleophile. When glycine reacts with d/-terf-butyl dicarbonate in an addition-elimination reaction, the anhydride bond breaks, forming CO2 and tert-butyl alcohol. 

FIGURE 3.17 Reagents for protection of amino groups as the ferf-butoxycarbonyl derivatives. terf-Butyl chloroformate is rarely used because of its low boiling point. The oxime is 2-ferf-butoxycarbonyloximino-2-phenylacetonitrile,45 Boc20 = di-ferf-butyl dicarbonate, or di-terf-butyl pyrocarbonate.46 (Tarbell et al., 1972 Pozdvev, 1974). Acylations are carried out at pH 9 to avoid dimerization. 

Amino groups are often protected as their terf-hutoxycarbonyI amide (BOC) derivatives. The BOC protecting group is introduced by reaction of the amino acid with di-fert-butyl dicarbonate in a nucleophilic acyl substitution reaction (Section 21.5) and is removed by brief treatment with a strong organic acid such as trifluoroacetic acid, CFjCOOH. 

B0C2O (65) is prepared from potassium tcrt-butoxide and carbon dioxide to form the terf-butyl potassium carbonate, which is converted with phosgene into the di-tert-butyl tricarbonate. The tricarbonate is smoothly converted by basic catalysis with 1,4-diazabicy-clo[2.2.2]octane (DABCO) into the dicarbonate.f ... 

---