Bpoc group

The 2-(biphenyl-4-yl)prop-2-yloxycarbonyl (Bpoc) group has been proposed by Sieber and Iselint i l as an a-amino protecting group that is cleaved under weakly acidic conditions. It has been successfully used in precedent-setting peptide syntheses of historical note, in particular in the first total synthesis of human insulin.t Despite additional applications in solution synthesisP or in this type of amino protection has not gained... 

Magnesium perchlorate in acetonitrile as a very mild reagent for the cleavage of the Bpoc group has also been reported. With this, neither resins like Siebe- and Rink-amide and SARSIN resins nor extremely acid labile thioxo peptides are cleaved. l... 

The 2-(3,5-dimethoxyphenyl)prop-2-yloxycarbonyl group (Ddz) (Scheme 38) was developed by Birr et al. as an alternative to the Bpoc group which for most synthetic purposes is too acid labile, since A -Bpoc-protected amino acids are occasionally cleaved by their intrinsic acidity.b l Kinetic measurements revealed that the relative cleavage rates of Boc, Ddz, and Bpoc are 1 1400 3000 (for various acid deprotection procedures, see Table 10).b l... 

This group is introduced using , -2,4-hexadienyl-(4-nitrophenyl) carbonate. It is cleaved with 1% TFA in CH2CI2 in 10 min. It is stable to base, milder acids, photolysis, NaBH4 and TBAF, but it is modified with I2 in DMF and unexpectedly it was not cleaved with Pd(0) as are most allyl groups. This group was proposed as a useful alternative to the trityl and Bpoc groups. ... 

Although successful use of the 0-Pfp derivatives in SPS has been reported [35], it remains to be seen whether the Bpoc group regains a place among SPS protocols. 

Further modifications in the same direction culminated in the highly acid labile biphenylylisopropyloxycarbonyl (BPoc) group (Sieber and Iselin 1968)... 

An alternative way to enhance selectivity in the removal of two different blocking groups is to increase the sensitivity of one of them toward acids. This thought led to the development of the already mentioned biphenylylisopropyl-oxycarbonyl (Bpoc) group and to the application of the highly acid sensitive triphenylmethyl (trityl) group. 

Here the p-alkoxy substituent of the benzyl alcohol moiety enhances the acid sensitivity of the ester linkage between peptide and resin and thus the anchoring bond can be cleaved with trifluoroacetic acid. Consequently the blocking groups applied for the protection of the a-amino function must be much more acid-sensitive, like the biphenylylisopropyloxycarbonyl (Bpoc) group ... 

For this reason the biphenyl-isopropyloxycarbonyl (Bpoc) group [101], which can be cleaved completely within 15 min by 0.5% trifluoroacetic acid in dichloromethane (Fig. 35), was developed and reconunended. Yet in the meantime, in several laboratories this protecting moiety was found too labile to be of practical use in extended gel phase n-theses, since the acidity of Bpoc-amino acids themselves is sufficient to deprotect the molecule autoacidolytically. 

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