Thioester resins

Among the electrophilic handles proposed for head-to-tail and side-chain-to-tail cyclization of peptides on solid support by intrachain aminolysis with concurrent detachment of the product from the resin in the protected form (see Section 6.8.3.1.3), generally the oxime resin (also called Kaiser resin)1364 365 and a thioester resin[363l are recommended (see Scheme 14). In addition to the classical head-to-tail cyclization,[3431 the oxime resin is used for side-chain cyclizations as well as for the synthesis of multicyclic peptides vide infra). Due to its dual functions, the oxime resin can be employed only with Boc/Bzl chemistry it is not compatible with Fmoc/tBu chemistry where the basic N -deprotection leads to free amino groups and thus to premature cyclization reactions. To avoid this premature cleavage of the... 

In a similar manner, the thioester resin[363l is used only with Boc/Bzl chemistry, and the intramolecular aminolysis of the thioester by the amino group occurs upon deprotection and neutralization with DIPEA (3.0 equiv) and is catalyzed by DMAP (0.1 equiv) in DMA (cyclization time 2-7 d). 

The helix peptide was synthesized on a Gly-thioester resin 103 using standard V°-Boc chemistry SPPS. 104105 The peptide was deprotected and simultaneously cleaved from the resin by treatment with HF plus 2% PhOMe, 0 °C, 1 h. The crude peptide was taken up in neat TFA, diluted with double deionized H20 to 0.5% TFA and lyophilized to remove residual PhOMe. The helix was purified by semipreparative RP-HPLC (buffer A 0.1% TFA in H20 buffer B 90% MeCN+10% buffer A gradient 0-67% buffer B over 60 minutes at 3 mL min-1) and characterized by IS-MS calcd, 1372.6 found, 1372.24 (average isotope composition). 

In this method, the cysteine-thioester cyclization generates a cyclic peptide 86a (see Scheme 23) with a Xaa-Cys bond whose thiol moiety is then used for tethering to the core through an S-alkylation reaction.191 The requirement for the cyclization reaction is a linear precursor 84 containing both an N-terminal Cys and a C-terminal thioester. Such a peptide precursor can be conveniently synthesized by a stepwise solid-phase synthesis on a thioester resin 81 using Boc chemistry (Scheme 22). Cleavage by HF after assembly of the peptide sequence will produce the desired precursor with an N-terminal Cys and a C-terminal thioester 84. The crude peptide is then purified by RP-HPLC and the purified unprotected peptide is then circularized in aqueous conditions buffered at pH > 7.0. 

Canne LE, Walker SM, Kent SBH. A general method for the synthesis of thioester resin linkers for use in the solid phase synthesis of peptide-a-thioacids. Tetrahedron Lett. 1996 36 1217-1220. 

Figures. The methods for preparation of peptide aldehydes. Methods 1 and 2 prepare an N-terminal aldehyde peptide by oxidation of an N-terminal Ser-peptide or hydrolysis of a dimethoxyacetate-peptide. Methods 3-7 prepare C-terminal aldehyde peptides. Methods 3 and 4 are known as the n + 1 method. The peptide alkyl ester or alkyl thioester is obtained from different types of resins then a masked amino acid glycodiol ester is introduced by enzymatic synthesis (3a, 3b) or by a chemical method (4) finally, the peptide is treated with TFA to give the aldehyde peptide. In methods 5 and 6, aldehyde peptides are obtained from oxidation of a peptide glycol diol ester. In method 7, treatment of an N-protected peptide thioester resin with Pd" and EtjSIH gives the cleaved C-terminal peptide aldehyde.

The final method (Figure 5, method 7) is an on-resin Pd -catalysed reduction of thioesters. Treatment of N-protected peptide thioester resin with Pd ° in the presence of Et3SiH gives the cleaved C-terminal peptide aldehyde. The reaction is performed typically under N2 in THF or DCM using a 20 30-fold excess of EtsSiH at 4°C to avoid racemization. The catalyst is obtained by in situ reduction of Pd(OAc)2- The yield ranges from 80 to 89% (33). 

Collectively, these results support the use of basic pH to inhibit hydrolysis of the peptidyl thioester, and the strong reducing environment of a smaU thiol and trialkylphosphine to prevent A ,5-diacyl by-products and to reduce disulphide formation. Using these optimized conditions, peptides ranging from 9 to 88 amino adds with 60-88% yield were prepared. The methods for preparation of the thioester resin and ligation are described below. 

Canl995 Canne, L.E., Walker, S.M. and Kent, S.B.H., A General Method for the Synthesis of Thioester Resin Linkers for Use in the Solid Phase Synthesis of Peptide-a-Thioacids, Tetrahedron Lett., 36 (1995) 1217-1220. 

Scheme 5 Synthesis of thioacids via acidic cleavage from thioester resins...

For thioester resins, the TAMPAL resin is prepared by coupling commercially available S-Tritylmercaptopropionic acid to Boc-Leu-OCH2 PAM resin, followed by deprotection with TFA / TIPS / water (90 5 5) [11]. As previously indicated, a wealth of different, appropriately protected amino acid building block are commercially available the most frequendy used are listed in Table 2, and Albericio and colleagues have recendy discussed this in great detail [8]. 

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