Classical method of peptide synthesis

The first total synthesis of a phallotoxin was that of norphalloin, the norvaline analogue of phalloidin.16 This synthesis, outlined in Scheme 2, initially involves the synthesis of two building blocks followed by their covalent binding via a tryptathionine moiety. Subsequent deprotection and coupling steps using classical methods of peptide synthesis results in the formation of norphalloin, which has proved to be even more toxic than the natural toxins. Since the synthesis of norphalloin, a number of synthetic analogues have been synthesized in order to obtain information on structure-activity relationships.[7 8]... 

The four-component condensation of isocyanides (also called isonitriles), discovered by Ugi in the late 1950s (Ugi four component reaction, Ugi-4CR) [176], provides another alternative to the classical methods of peptide synthesis in solution [177-180], The Ugi-4CR has been extensively studied in every conceivable aspect and several variants of the general scheme give different products. In one variant that is particularly relevant to the topic of this article, an N-protected peptide or amino acid, a chiral primary amine, an aldehyde and an isonitrile peptide fragment are combined to yield, via a spontaneous rearrangement of an unstable a-adduct, a compound 74 in which a new peptide segment has formed (Scheme 5.38, see also Section 5.4.1 above for an application of Ugi-4CRto PNA synthesis). 

It is not quite obvious why the simple, classical method of acylation with acid anhydrides, for instance acetylation with acetic anhydride, was adopted rather late in peptide synthesis. We believe that the economy of the process in which only half of the valuable blocked amino acid is utilized in peptide bond formation while the other half is regenerated appeared unsatisfactory in the eyes of the... 

A technique of peptide synthesis different from Merrifield s method has been introduced (Fridkin et aL, 1965a, 1965b, 1966 Wieland and Birr, 1966a, 1966b). This method is based on the use of polymer-supported amino acid active esters. Wunsch (1971) refers to this technique as polymeric reagent synthesis. The method is claimed to be free from certain limitations of the classical solid-phase peptide synthesis e.g., it offers the advantage of isolation and purification of the intermediate peptides, and the synthesis does not go unchecked. However, the method has certain limitations. These will be discussed later. 

The major disadvantage of solid-phase peptide synthesis is the fact that ail the by-products attached to the resin can only be removed at the final stages of synthesis. Another problem is the relatively low local concentration of peptide which can be obtained on the polymer, and this limits the turnover of all other educts. Preparation of large quantities (> 1 g) is therefore difficult. Thirdly, the racemization-safe methods for acid activation, e.g. with azides, are too mild (= slow) for solid-phase synthesis. For these reasons the convenient Menifield procedures are quite generally used for syntheses of small peptides, whereas for larger polypeptides many research groups adhere to classic solution methods and purification after each condensation step (F.M. Finn, 1976). 

Initially, the term Hquid-phase synthesis was used to contrast the differences between soHd-phase peptide synthesis and a method of synthesis on soluble polyethylene glycol (PEG) [5, 6]. Although soluble polymer-supported synthesis is less ambiguous than Hquid-phase synthesis, the latter term is more prevalent in the Hterature. In-keeping with previous reviews [7-12], the phrases classical or solution synthesis will be used to describe homogeneous reaction schemes that do not employ polymer supports while liquid-phase synthesis will be reserved... 

Seven years later, in a critical review on the synthesis of peptides, the following statement was made "Chemists in particular should respect the classical criteria of what constitutes synthesis of a natural product, i.e., that synthesis of a natural product has been achieved, when the physical, chemical and biological properties of the synthetic compound match those of the natural prototype. Unfortunately not a single one of the "synthetic proteins" satisfies these criteria. It is frequently argued that these criteria are not applicable to more complex situations, but lowering standards of purity is not likely to advance the field. Presently available analytical methods cannot adequately detect inhomogeneity in a high molecular peptide that is produced by stepwise synthesis. Consequently, the synthetic method must be chosen so that the product can be purified and critically evaluated by the available analytical techniques. F.M. Finn, 1976). 

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