Fmoc-4-aminomethylcyclohexane carboxylic acid

Fmoc-Cys(t-Bu)-OH, Fmoc-Cys(Acm)-OH, and Fmoc-Cys(Trt)-OH exhibit excellent synthesis characteristics when used in Fmoc solid phase peptide synthesis on the Applied Biosystems Model 431A peptide synthesizer. The actual 5% scavenger mixture will vary according to the particular amino acid residues present. As was previously mentioned, an anisole/ethanedithiol/ethylmethylsulfide mixture (3:1:1) works well as a general scavenger solution for TFA cleavage of Fmoc synthesized peptide resins. It also may be possible to use lower acid (TFA) concentrations. The syntheses and workups of the peptide Somatostatin utilizing these derivatives demonstrate the ease of using these cysteine derivatives with the Fmoc chemistry approach. The use of either the t-Bu or the Acm moiety produces a peptide containing protected thiol groups after cleavage with 95% TFA. The Fmoc-Cys(Trt)-OH derivative is efficiently deprotected using 95% TFA. This investigation should provide further insight into synthesis options and cleavage protocols when working with cysteine-containing peptides.

A newly developed Fmoc-Asp derivative, Fmoc-Asp beta-(2,3,4-trimethyl-pent-3-yl) ester, has been tried in the Fmoc-based SPPS of H-Val-Lys-Asp-Xaa-Tyr-Ile-OH, a well-established peptide model for studying base-catalysed aspartimide formation. When synthesizing the hexapeptide incorporating Gly, Arg(Pbf), Asn(Mtt), Asp(OtBu) or Cys(Acm) for Xaa, considerable amounts of aspartimide-related by-products were to be expected. The Asp(3) beta-carboxy protecting group and the duration of exposure to bases were varied. By-product formation could be reduced by incorporation of the new Asp derivative more efficiently than by introducing the less bulky Asp(OMpe). Significant improvements were observed in cases of prolonged contact with piperidine or DBU. Both beta-carboxy protecting groups were superior to the standard Asp(OtBu) which was also included in this study, but the additional stabilization gained by our new protecting group was valuable especially in syntheses of long peptides or difficult sequences.

The sequence dependence of base-catalysed aspartmide formation during Fmoc-based SPPS was systematically studied employing the peptide models H-Val-Lys-Asp-Xaa-Tyr-Ile-OH. The extent of formation of aspartimide and related by-products was determined by RP-HPLC. Considerable amounts of by-products were formed in the case of Xaa = Asp(OtBu), Arg(Pbf), Asn(Mtt), Cys(Acm) and unprotected Thr. Aspartimide formation could be diminished by incorporation of Asp(OMpe) or by employing milder methods for Fmoc cleavage, e.g. hexamethyleneimine/N-methylpyrrolidine/HOBt/NMP/DMSO 4:50:4:71:71 (v/v/w/v/v).