t-Butyloxy carbonyl (Boc) is also a widely used amino protecting group in peptide synthesis. In particular, in solid phase synthesis, Boc is used to protect amino groups instead of Cbz. It has the following advantages. First, Boc-amino acids can be crystallized except for a few. Second, the Boc group is easy to be removed by acidolysis, but it has a certain stability, so that the Boc-amino acid can be stored for a relatively long time without decomposition. In addition, acidolysis of the Boc group generally does not bring about side reactions, mainly producing t-butyl cations and then decomposing into isobutylene. Third, the Boc group is stable to alkaline hydrolysis, hydrazinolysis and many nucleophilic reagents. Besides, Boc is stable to catalytic hydrogenolysis, but is much more sensitive to acid than Cbz. When Boc and Cbz exist at the same time, Cbz can be removed by catalytic hydrogenolysis, while Boc remains unchanged, or Boc can be removed by acidolysis while Cbz is not affected, so the two can be well matched.
Introduction of Boc
Free amino groups can easily react with Boc2O in a mixed solvent of dioxane and water under alkaline conditions controlled by NaOH or NaHCO3 to obtain Boc-amino acids. This is one of the common methods for introducing Boc group. The advantage of this method is that its byproducts do not interfere much and are easy to remove. In addition, t-butyl chloroformate and t-butyloxycarbonyl azide can also be used as reagents for the introduction of Boc group.
Removal of Boc
In liquid-phase peptide synthesis, Boc can generally be removed using trifluoroacetic acid (TFA) or 50% TFA (TFA:CH2Cl2 = 1:1, v/v). In solid-phase peptide synthesis, since TFA will bring about some side reactions (such as adding a trifluoroacetyl group to the obtained amine), 1-2M hydrochloric acid/organic solvent is often used. Among them, hydrochloric acid/dioxane is more common.
