Symmetric anhydride synthesis

The two main resin linkers developed so far are shown in Scheme 18, i.e. tris(alk-oxy)benzylamide- 412 and 4-alkoxybenzylamide-type linkers)341 the former being TFA labile and thus fully compatible with Fmoc/tBu and the latter strongly acid labile and correspondingly compatible with Boc/Bzl chemistry. As shown in the case of the tris(alk-oxy)benzaldehyde handle such handles may be introduced into the C-terminal amino acid ester by reductive amination, and after suitable N -protection coupled to amino-functionalized resins (see Scheme 18). Alternatively, the tris(alkoxy)benzaldehyde-functionalized resin, BAL resin, (see Scheme 14) is used to link the C-terminal amino acid ester by reductive amination. To overcome the difficult acylation of the V -arylamino acid ester derivative on resin (best results with 10 equivalent symmetrical anhydrides), synthesis in solution of the C-terminal dipeptide building block containing the amide handle followed by its attachment to the resin has been proposed)341 ... 

One of the first dedicated applications of microwaves in solid-phase chemistry was in the synthesis of small peptide molecules, as described by Wang and coworkers [22]. As a preliminary test, the authors coupled Fmoc-Ile and Fmoc-Val, respectively, with Gly-preloaded Wang resin using the corresponding symmetric anhydrides (Scheme 7.1). 

In a more recent study using dedicated multimode microwave reactors for chemical synthesis, which enable temperature and power control, it was demonstrated that microwave irradiation could be effectively employed to couple aromatic carboxylic acids to polystyrene Wang resin [25], if the symmetrical anhydride procedure was used, and not the three-component O-acylisourea activation method [19]. Almost quantitative loading was achieved in l-methyl-2-pyrrolidone (NMP) at 200 °C within 10 min under... 

F Weygand, P Huber, K Weiss. Peptide synthesis with symmetrical anhydrides I. Z Naturforsch 22B, 1084, 1967. 

FMF Chen, K Kuroda, NL Benoiton. A simple preparation of symmetrical anhydrides of iV-alkoxycarbonylamino acids. Synthesis 928, 1978. 

EJ Heimer, C Chang, T Lambros, J Meienhofer. Stable isolated symmetrical anhydrides of (Va-9-fluorenylmethyloxycarbonylamino acids in solid-phase peptide synthesis. hit J Pept Prot Res 18, 237, 1981. 

Symmetrical anhydrides are stable enough to be isolated but not stable enough to be stored for future use. They can be purified by repeated crystallization or by washing a solution of the anhydride that has been obtained using a soluble carbo-diimide with aqueous solutions (see Section 1.16). The use of symmetrical anhydrides allows for synthesis of a peptide with less possibility of generation of side products. The anhydrides are particularly effective for acylating secondary amines (see Section 8.15).9 11... 

During the first decade when solid-phase synthesis was executed using Fmoc/tBu chemistry, the first Fmoc-amino acid was anchored to the support by reaction of the symmetrical anhydride with the hydroxymethylphenyl group of the linker or support. Because this is an esterification reaction that does not occur readily, 4-dimethylaminopyridine was employed as catalyst. The basic catalyst caused up to 6% enantiomerization of the activated residue (see Section 4.19). Diminution of the amount of catalyst to one-tenth of an equivalent (Figure 5.21, A) reduced the isomerization substantially but did not suppress it completely. As a consequence, the products synthesized during that decade were usually contaminated with a small amount of the epimer. In addition, the basic catalyst was responsible for a second side reaction namely, the premature removal of Fmoc protector, which led to loading of some dimer of the first residue. Nothing could be done about the situation,... 

Rearrangement of symmetrical anhydrides occurs even in the absence of base, but at a much slower rate. Insertion of two residues during synthesis occurs by... 

FIGURE 8.7 Synthesis of a protected tripeptide containing a 2-hydroxy-4-methoxybenzyl-protected peptide bond.38 (A) Acylation of the carboxy-terminal residue, (B) removal of both protecting groups, (C) O-acylation of the benzyl-protector by the symmetrical anhydride of the amino-terminal residue, and (D) migration of the protected amino-terminal residue from the oxygen atom to the amino group of the dipeptide ester. 

Synthesis via symmetrical anhydrides of N-Fmoc amino acids... 

This synthesis can produce either a symmetric anhydride (both acids the same) or an asymmetric anhydride (different acids). 

The utility of carbodiimide reagents was too important to consider abandoning them. Instead, it rapidly became obvious that carbodiimide activation could be used to prepare in situ active esters. Numerous nucleophilic additives were discovered and prepared for use in these reactions. The most important of these additives was 1,2,3-benzotriazol-l-ol (HOBt, 1), first reported for use in peptide synthesis by Konig and Geiger in 1970.[2(l As an example of a typical result, 1.2 equivalents of HOBt was added to the DCC coupling of Boc-Leu-Phe-OH to H-Val-OtBu in DMF as a solvent. Less than 1% of the l-d-l epimerized tripeptide was formed. When the reaction was carried out in the absence of HOBt, the amount of l-d-l product formed was 14.3%. Addition of HOBt to DCC reactions converts the intermediate G-acylisourea (2) (and any symmetrical anhydride) into the HOBt active ester 3 (Scheme 8). 

In addition to the more or less popular methods of depsipeptide synthesis discussed vide supra, there are also a limited number of complementary and effective synthetic procedures that have been described for this purpose. Among these, the well-known method of symmetric anhydrides from N-protected amino acids has to be considered. This method has found successful use in the esterification of hydroxy acids in the presence of some catalyst additives. Initially, the addition of pyridine11091 or 1-hydroxybenzotriazole in pyridine1 101 to a symmetric anhydride was utilized for ester bond formation. As an example, Katakai has prepared a number of didepsipeptides in 85-96% yield by means of a 2-nitrophenylsulfenyl /V-carboxy anhydride with lactic acid derivatives in the presence of pyridine.1 09 ... 

The synthesis of the peptide surrogate Boc-Phe(l,2,4-oxadiazole)-OEt 55 is shown in Scheme 17J102 Boc-Phe-OH (53) was converted into its symmetric anhydride 54 by stirring with DCC at 0°C. The anhydride (without isolation) was then reacted with ethyl ami-no(hydroxyimino)acetate[103 in refluxing pyridine. The product, 1,2,4-oxadiazole 55, was thus obtained in 72% yield and 93% ee. In general, this method gives good to excellent enantiomeric excesses (Table 3).11(121 The use of a mild base such as pyridine is critical for the suppression of unwanted epimerization at the a-carbon attached to the heterocycle. 

Tertiary aliphatic alcohol linkers have only occasionally been used in solid-phase organic synthesis [73], This might be because of the vigorous conditions required for their acylation. Esterification of resin-bound linker 4 with /V-Fmoc-prolinc [72,74] could not be achieved with the symmetric anhydride in the presence of DMAP (20 h), but required the use of /V-Fmoc-prolyl chloride (10-40% pyridine in DCM, 25 °C, 10-20 h [72]). A further problem with these linkers is that they can undergo elimination, a side reaction that cannot occur with benzyl or trityl linkers. Hence, for most applications in which a nucleophile-resistant linker for carboxylic acids is needed, 2-chlorotri-tyl- or 4-acyltrityl esters will probably be a better choice than ferf-alkyl esters. 

For the solid-phase synthesis of amides, it makes a significant difference whether the amine or the acid is linked to the support. Resin-bound amines are readily acy-lated by adding first a carboxylic acid and then a carbodiimide (Table 13.3). The acid/ carbodiimide ratio is not critical, because both the O-acylisourea (ratio 1 1) and the symmetric anhydride (ratio 2 1) will lead to N-acylation. It should, however, be borne in mind that the half-lives of O-acylisoureas are shorter than those of anhydrides, and for difficult couplings it might be advantageous to acylate with a symmetric anhydride (two equivalents of acid and one of carbodiimide). 

Carboxylic mixed anhydrides are very important for the rapid synthesis of peptides by the stepwise procedure,however the use of carboxylic mixed anhydrides,e.g.those derived from pivalic acid and a protected amino acid (1),suffers from two disadvantages. Firstly,regiospecificity of attack at the desired carboxyl function is largely determined by steric effects and will not be 100% for all coupling reactions.Secondly,such mixed anhydrides have a propensity towards disproportionation to symmetric anhydrides which is highly undesirable in terms of reaction efficiency.This latter process can be depressed by operation of the reaction at -15 °C, but with the concurrent decrease in reaction rate and,on large scale manufacture,increased costs. 

There are many examples in the literature describing safety-catch linkers (1). The basic principle, is well illustrated with linker 7 (Fig, 4) (33). The synthesis begins with the addition of an amino acid as the symmetrical anhydride... 

---