Peptides, resin-bound, cleavage

FIGURE 5.16 Production of amides by cleavage of benzhydryl amides. Recognition that removal by acidolysis of benzhydryl protectors from carboxamides gave the amides (B) led to development of benzhydrylamine (BHA) resin (C).33 Treatment with HF of a peptide amide that has been assembled on a BHA resin using Boc/Bzl chemistry gives the peptide amide (D). Peptide amide is also obtainable by ammonolysis of the resin-bound benzyl ester (A), a reaction that is more efficient if gaseous NH3 is employed (see Section 8.3). 

The cleavage of resin-bound peptides, produced in the Merrifield synthesis, is aided by the addition of quaternary ammonium salts to the basic medium [17, 18]. In many cases, cleavage is 100% effective and rarely less than 70%. 

Lowe et al. investigated the substrate selectivity of the proteases papain and chy-motrypsin using PEGA-bound combinatorial peptide hbraries (32) (Fig. 10.3) [24]. The quahtative extent of enzymatic cleavage of the resin-bound peptide in case of an accepted substrate strand was rapidly visualized by a significant reduction in the fluorescence of the beads visuahzed with a fluorescence microscope. Furthermore it was proven that substrate selectivity of the enzyme remain the same on the solid phase and in solution. 

Boc-group removal usually requires treatment of the resin-bound carbamate with 50% TFA in DCM for 0.5 h (Section 10.1.10.1). Under these conditions, neither cleavage of the peptide from the support nor hydrolysis of side-chain protective groups... 

The free amide peptide was synthesized on 4-methylbenzhydrylamine-substituted polystyrene resin, and the resin-bound peptide for antibody production was prepar on aminomethyl polystyrene. Cleavage/deprotection was done with hydrogen fluoride/anisole at 0 C. The free amide was not active in either bioassay. However, polyclonal antibody prepared against the resin-bound (22 24) sequence was immunologically reactive to EDNH reverse phase column fractions. We are now attempting to determine if the sequence represents an inactive fragment of authentic EDNH. 

Rosse et al. described the oxidation of the resin-bound /3-sulfenyl hydroxamic acids A -sulfonyloxaziridines <2001SL538>. For example, oxidation of 143 with 33 afforded the corresponding sulfoxide 144 in 71% yield after cleavage of the resin with TFA. Apfel et al. in the preparation of some potent peptide deformylase inhibitors and antibacterial agents employed a similar transformation<2000JME2324>. 

N -Protection as allyl carbamate is used generally only for specific synthetic purposes where acid- or base-mediated deprotection procedures must be avoided or reduced to a minimal extent as in the case in the synthesis of glyco- and phosphopeptides (see Sections 6.3 and 6.5). Otherwise the orthogonality of this type of amino protection has been mainly exploited in side-chain protection to allow for selective chemistry such as selective acylations, e.g. cyclizations, to be performed on protected peptides in solution or in the resin-bound state (see Section 4.3.2.3.4). An additional interesting application of the Aloe group is the one-pot deprotection and acylation with carboxy components resistant to the paUadium(0)-catalyzed allyl cleavage by hydrostannolysis as reported in Section 4.3.2.3.4.P 1... 

The best way to prepare peptide aldehydes from the corresponding N -protected amino acids is by using a handle based on the Weinreb amide.f This commercial handle allows classical solid-phase elongation of peptides using protected Boc or Fmoc amino adds and, at the end of the synthesis, the peptide aldehyde is formed by reduction and concomitant cleavage from the resin with lithium aluminum hydride. Although the 4-hydro-xybenzoic acid handle also allows the preparation of peptide aldehydes by reduction of the resin-bound phenyl ester with lithium tri-tert-butoxyaluminum hydride, a noixture of the aldehyde and the alcohol is always formed. 

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