Superacids in protection group chemistry

A newly developed stable dimethyl ether-HF complex (1 15),668 which can be stored at room temperature, is highly effective for the deprotection and cleavage of peptides from Merrifield resins (isolated peptide yields = 88-94%). It can easily be handled, and it is a very useful and convenient hydrogen fluoride equivalent. 

Amorphous and mesostructured Zr02 solid catalysts loaded with various amounts of triflic acid (5-30%) were tested in the acetalization of ethyl acetoacetate to form fructone357 [Eq. (5.235)]. Whereas the amorphous samples [Zr02(a)-TfOH] were nonselective and induced the formation of a byproduct, the mesoporous catalysts [ZrC 2(m)-TfC H] showed complete selectivity to give the desired product in high yields. 

Fluoroboric acid supported on silica (HBF4-silica) has recently been found to be a highly efficient catalyst in the protection of various functional groups. Structurally diverse alcohols, phenols, thiophenols, and anilines can be acylated under solvent-free conditions at room temperature.669 Even acid-sensitive tertiary alcohols (1-alkylcyclo-hexanols) and sterically hindered compounds, such as endo-borneol, give the acylated products in high yields. A triflic acid-silica catalyst also shows high activity in the (9-acetylation with Ac20 of alcohols and phenols.359 

The transformation of carbonyl compounds to (9,5-acetals and 5,5-acetals can be carried out without solvent under mild conditions (1 mol% of catalyst, room temperature, 5 10 min) to afford the products in high yields (70-97%).67°(9,(2- and (9,5-acetals are also readily transformed to 5,5-acetals [Eq. (5.236)]. 

HBF4-silica also exhibits high activity in the protection of aldehydes as 1,1-diacetates (acylals)671 [Eq. (5.237), Table 5.38]. Both aliphatic and aromatic aldehydes react readily to form diacetates in high yields. The catalyst can be reused with marginal decrease in activity. 

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