Fmoc Wang resin

Scheme 15 Synthesis of P-lactam using Fmoc-Wang resin...

The details of the solid-phase technique have been improved substantially over the years, but the fundamental idea remains the same. The most commonly used resins at present are either the Wang resin or the PAM (phenyl-acetamidomethyl) resin, and the most commonly used N-protecting group is the fluorenylmethyloxycarbonyl, or Fmoc group, rather than Boc. 

The investigation of minor groove-binding polyamides was greatly accelerated by the implementation of solid-phase synthesis [48]. Originally demonstrated on Boc-y9-Ala-PAM resin with Boc-protected monomers, it was also shown that Fmoc chemistry could be employed with suitably protected monomers and Fmoc-y9-Ala-Wang resin (Fig. 3.8) [49]. Recently, Pessi and coworkers used a sulfonamide-based safety-catch resin to prepare derivatives of hairpin polyamides [50]. Upon activation of the linker, resin-bound polyamides were readily cleaved with stoichiometric quantities of nucleophile to provide thioesters or peptide conjugates. 

Fig. 3.8 Variations to solid phase synthesis of polyamides. Use of Fmoc monomers on jS-Ala-Wang resin (left) provides polyamides containing a jS-alanine residue near the C-ter-mini. Polyamides synthesized on the Kaiser...

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). 

The earliest published example of microwave-assisted SPOS involved diisopropyl-carbodiimide (DlC)-mediated solid-phase peptide couplings [24], Numerous Fmoc-protected amino acids and peptide fragments were coupled with glycine-preloaded polystyrene Wang resin (PS-Wang) in DMF, using either the symmetric anhydride or preformed N-hydroxybenzotriazole active esters (HOBt) as precursors (Scheme 12.1). 

Solid-phase synthesis was performed using Wang resin (0.57 mmol-g 1 0.35 g, 0.2 mmol) with the coupling of Fmoc amino acids and Fmoc-Hse[PO(OPh)2]-OH performed using PyBOP (3 mol equiv) and NMM (5 mol equiv) in DMF. The peptide-resin was treated with TFA/H20/TIS (95 2.5 2.5 5 mL) at rt for 2h, the resin filtered under N2, and washed with TFA (4mL). The solvent was removed under reduced pressure, the peptide precipitated by the addition of Et20 (20 mL), triturated with Et20 (2 x 10 mL), and dried under high vacuum. 

Fig. 5. 500 MHz HRMAS H NMR spectra of FMOC-isoleucine on Wang resin swollen in DMF- 7 and spun at 4 kHz. Spectrum A shows the presence of dissolved phenylalanine peaks, particularly around 3.0 ppm from its methylene protons, and residual protons of the solvent. Application of a diffusion filter using gradients selectively removes these signals in spectrum B. Reproduced with permission from Ref. 50. Copyright 1999 Elsevier.

As illustrated by the examples in Table 3.9, resin-bound 4-alkoxybenzylamides often require higher concentrations of TFA and longer reaction times than carboxylic acids esterified to Wang resin. For this reason, the more acid-sensitive di- or (trialkoxy-benzyl)amines [208] are generally preferred as backbone amide linkers. The required resin-bound, secondary benzylamines can readily be prepared by reductive amination of resin-bound benzaldehydes (Section 10.1.4 and Figure 3.17 [209]) or by A-alkyla-tion of primary amines with resin-bound benzyl halides or sulfonates (Section 10.1.1.1). Sufficiently acidic amides can also be A-alkylated by resin-bound benzyl alcohols under Mitsunobu conditions (see, e.g., [210] attachment to Sasrin of Fmoc cycloserine, an O-alkyl hydroxamic acid). 

Standard solid-phase peptide synthesis requires the first (C-terminal) amino acid to be esterified with a polymeric alcohol. Partial racemization can occur during the esterification of N-protected amino acids with Wang resin or hydroxymethyl polystyrene [200,201]. /V-Fmoc amino acids are particularly problematic because the bases required to catalyze the acylation of alcohols can also lead to deprotection. A comparative study of various esterification methods for the attachment of Fmoc amino acids to Wang resin [202] showed that the highest loadings with minimal racemization can be achieved under Mitsunobu conditions or by activation with 2,6-dichloroben-zoyl chloride (Experimental Procedure 13.5). iV-Fmoc amino acid fluorides in the presence of DMAP also proved suitable for the racemization-free esterification of Wang resin (Entry 1, Table 13.13). The most extensive racemization was observed when DMF or THF was used as solvent, whereas little or no racemization occurred in toluene or DCM [203]. 

Because no treatment with acid is required during peptide assembly, peptide synthesis with Fmoc amino acids can be conducted on acid-sensitive supports (e.g. Tenta-gel) and with acid-labile linkers. Wang resin is suitable for most purposes, but other supports, such as Sasrin or 2-chlorotrityl resin, can also be used. CPG, macroporous... 

Solid-phase preparation of trans 3-alkyl (3-lactams was reported by Mata et al. recently (Scheme 14) [103]. The synthetic sequence involved the start from Fmoc-glycine tethered to Wang resin, followed by addition of a controlled excess of 43 (4 equiv) and triethylamine (8 equiv) to the resin bound imine 42 in refluxing toluene. Cleavage form resin surface and esterification afforded the 3-alkyl (3-lactams, 45, as a single product with excellent trans-selectivity. 

Solid-phase synthesis of 3,4-disubstituted (3-lactams was accomplished [104] via reaction of in situ generated ketenes with immobilized aldimines under mild conditions. Initially Fmoc-protected Wang resin strategy was followed (Scheme 15) and the [2+2] cycloaddition was performed by adding phenox-ylacetyl chloride (49, R1 = phenoxy) and triethylamine in excess to a suspension of 48 in dichloromethane. The yields ranged from good to very good for the five-step synthetic sequence and exclusive formation of the cis isomer was detected in all cases. 

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