**4. Segment condensation of peptides using** *O***-isoacylpeptides**

Although NCL by Kent et al. allowed to preparing the long chain peptides, in general, conventional segment condensation other than NCL often involves the

#### **Figure 5.**

*Segment condensation of peptides. (A) Conventional segment condensation between peptides. (B) Segment condensation using O-isopeptide method.*

**39**

*Isoacylpeptide Method for Long-Chain and Difficult Sequence-Containing Peptide Preparation*

racemization of amino acid as shown in **Figure 5A**. Especially, it is a serious problem in SPPS because of no purification of intermediates. It is well-known that the urethane structure, such as Boc and Fmoc protecting group, on the α-amino group of amino acid can prominently reduce the racemization of amino acid in peptide bond coupling reaction. Hence, segment condensation between peptides other than NCL must not be in peptide chemistry. We noticed the protected *O*-isoacylpeptide structure that possesses a urethane structure on the α-amino group of Ser or Thr residue. When an *O*-isoacylpeptide possessing a urethane-type protecting group at the C-terminus was coupled with another peptide, we speculated that the condensed peptide may be not racemized at the C-terminal amino acid of *O*-isoacylpeptide. Hence we designed segment condensation method as shown in **Figure 5B**. The segment condensation based on the *O*-isoacylpeptide method showed no racemization [24], and could release the original peptide similar to the other *O*-isoacylpeptides under physiological condition. This method appears to be

Recently, some important synthesis methods such as NCL and pseudoproline method for preparation of long chain and difficult sequence-containing peptides had been reported. Although these approaches allow to preparing some long chain peptides without a genetic engineered approach. However, these methodologies are not a panacea for a long chain and difficult sequence-containing peptides. We supply alternative solution for the long chain and difficult sequence-containing peptide preparation. Namely, we have developed the *O*-isoacylpeptide method for peptide preparation. *O*-isoacylpeptides that have a/some ester bonds can be converted into the parent peptides under physiological condition *via* the *O*-*N* intramolecular acyl migration. Moreover, we developed segment condensation with no racemization using the *O*-isoacylpeptide method. The segment condensation based on *O*-isoacylpeptides may be alternative choice to NCL method. We desire to apply to these methodologies of various peptide preparations for drug, diagnosis,

This study was supported in part by the Grants-in-Aid for Scientific Research from MEXT (Ministry of Education, Culture, Sports, Science and Technology), Japan (KAKENHI No. 23590137, No. 26460163 and No.18K06562), and a donation from Mrs. Kazuko Fujita with the cherished desire of the late Tetsuro Fujita,

*DOI: http://dx.doi.org/10.5772/intechopen.84248*

alternative choice to NCL method.

**5. Conclusion**

and molecular research.

**Acknowledgements**

**Conflict of interest**

Professor Emeritus of Kyoto University.

We confirmed independence from the funding source.

*Isoacylpeptide Method for Long-Chain and Difficult Sequence-Containing Peptide Preparation DOI: http://dx.doi.org/10.5772/intechopen.84248*

racemization of amino acid as shown in **Figure 5A**. Especially, it is a serious problem in SPPS because of no purification of intermediates. It is well-known that the urethane structure, such as Boc and Fmoc protecting group, on the α-amino group of amino acid can prominently reduce the racemization of amino acid in peptide bond coupling reaction. Hence, segment condensation between peptides other than NCL must not be in peptide chemistry. We noticed the protected *O*-isoacylpeptide structure that possesses a urethane structure on the α-amino group of Ser or Thr residue. When an *O*-isoacylpeptide possessing a urethane-type protecting group at the C-terminus was coupled with another peptide, we speculated that the condensed peptide may be not racemized at the C-terminal amino acid of *O*-isoacylpeptide. Hence we designed segment condensation method as shown in **Figure 5B**. The segment condensation based on the *O*-isoacylpeptide method showed no racemization [24], and could release the original peptide similar to the other *O*-isoacylpeptides under physiological condition. This method appears to be alternative choice to NCL method.
