Thioester linkers

A more versatile route to synthesize peptide thioesters is by the preparation of a peptide thioacid via the solid-phase method and then reaction with a halo derivative to form the thioester. This route can be achieved via the preparation of thioester linkers that can be applied in stepwise solid-phase peptide synthesis. The general structure of the N-protected amino thioacid attached to the linker is shown in Scheme 14. t65-80 ... 

In order to facilitate the generation of peptide thioesters, several groupst have developed generalized versions of the thioester linker pioneered by Hojo and Aimoto.f A 3-sulfanylpropanoic acid residue is generated on an acid-labile linker such as Boc-Leu-PAM-resin or Boc-Leu-MBHA-resin. It is important to have a one-residue spacer between the MBHA linker and the thiol for optimal acid stability of the amide bond.f All twenty Boc-protected amino acids can be coupled to this thiol on the solid support to generate the thioester.Despite the potential reactivity of the thioester to the amino terminus, the formation of a dioxopiperazine is not generally observed when using in situ neutralization protocols. However, when the sequence was Leu-Tyr-Arg-Ala-Pro, 20% of a dipeptide deletion product, Leu-Tyr-Arg, was observed. It is likely that sequences such as C-terminal Pro-Gly would also be subject to this side reaction.t ... 

Sulfur analogs of some of the described hydroxy linkers and resins (Figure 15.2) have been employed to synthesize resin-bound thioesters that can then be cleaved with alcohols, amines and organometallic reagents to furnish esters, amides, ketones, aldehydes and alcohols [50]. Secondary amines react sluggishly with linker 9 and, therefore, more reactive thioester-linker derivatives were developed, such as 17 and 18. Yields for a hindered amine such as Pro-OMe were in the order of 60-70%. Thiol-containing PS-DVB resin 7 has been used for the obtention of P-hydroxyacids by hydrolysis with 0.2 M NaOH in aqueous dioxane [51],... 

Fig. 10.1-2 Generation of peptide cr-thioesters by Fmoc-based SPPS using sulfonamide safety catch linker resin (a), a masked thioester equivalent incorporated post-SPPS (b), and a masked thioester linker strategy (c).

Scheme 2 Cleavage possibilities of ester and thioester linkers...

In general, thioester Unkers can be called multifunctional as carboxylic ester linkers. They are discussed within this work because cleavage strategies and products differ only slightly from those of esters, hi fact, thioester linkers disclose possibilities where ester linkers traditionally fail. 

Thioesters are known to be cleaved by addition of 20% 1 M aqueous NaOH in dioxane to give carboxylic acids and thio-Wang resins [80] but they have also been cleaved under acidic conditions. An approach of Villain et al. describes the synthesis of thioacids by acidic cleavage of thioesters on solid support [81]. This procedure depends on a special type of linker (38) which is synthesized via addition of a mercapto-benzophenone derivative to aminomethyl-polystyrene. Cleavage of the resulting C-terminal thioacids 41 is induced by addition of HP (Scheme 5, for another application of thioester linkers for thioacid synthesis see Canne et al. [82]). 

Scheme 8 Cleavage of thioester linker yielding ketones, primary and tertiary alcohols...

To our knowledge no possibility exists to cleave ester linkers with carbon nucleophiles to give ketones. In contrast, thioester linkers can be ap-pUed for these purposes as shown by Bradley et al. [89]. The thioester 55 in Scheme 8 was prepared easily by addition of an arylthioamide and sodiiun iodide to Merrifield resin in aqueous DMF. It was shown that thioesters without the need of special Hnker demands can be transformed into ketones 57 as well as into primary (60) and tertiary alcohols (59). The addition of Grignard reagents to thioesters gives tertiary alcohols whereas the addition of C-nucleophUes via organocuprates provides ketones in 53% yield. These results differ from those found by Vlattas et al. in 1997 who observed the... 

Applications for this lactone formation chemistry are manifold. Kobayashi et al. for example found a method to synthesize monosaccharide derivatives via cleavage of thioester linkers [139]. The resulting lactones had to be reduced with DIBAL-H to get glucose derivatives. 

In our laboratory we use two different SPPS-based approaches to synthesize cyclic disulfide-rich peptides. Most commonly we use the intramolecular native chemical ligation (NCL) reaction between a thioester group at the C-terminus and a cysteine residue at the N-terminus [16, 26-29]. For this approach the coupling of the thioester linker requires Boc-SPPS protection chemistry as the thioester is unstable in the basic conditions used for Fmoc chemistry. The first amino acid coupled after the thioester linker could be any residue however, Gly, Cys, and His are most favorable, and Leu, Thr, Val, He, and Pro are the least favorable amino acids, as reflected in their ligation rates with the N-terminal cysteine [26]. The second SPPS approach used in our laboratory to synthesize the cyclic disulfide-rich peptides involves an Fmoc-based method, in... 

---