Fmoc-N-Me-Asp(OtBu)-OH

Fmoc-N-Me-Asp(OtBu)-OH
Nα-Fmoc-Nα-methyl-L-aspartic acid β-tert-butyl ester; (2S)-2-[9H-fluoren-9-ylmethoxycarbonyl(methyl)amino]-4-[(2-methylpropan-2-yl)oxy]-4-oxobutanoic acid

An aspartic acid derivative carrying both a methyl group on the backbone nitrogen and a tert-butyl ester on the side-chain (β) carboxyl. The α-carboxyl is free, so the residue is coupled into the backbone in the normal way, and the final TFA cleavage removes the tert-butyl ester along with the other acid-labile groups; include scavengers such as TIS and water to capture the tert-butyl cations released. Methylating the backbone nitrogen makes the amide it forms a tertiary amide, which removes a hydrogen-bond donor and restricts the local backbone. N-methyl scanning uses that to find which backbone NH groups a bioactive conformation depends on, and a library of N-methylated cyclic alanine pentapeptides was built to describe how multiple N-methylation modulates the conformation of cyclic peptides.

Coupling can be difficult at this residue, especially between consecutive N-methyl residues, so monitor it and optimize the conditions — PyAOP or PyBOP/HOAt performed best in the reported N-methyl-rich system — and repeat an incomplete coupling.

Repeated piperidine treatment can still cyclize the aspartate to an aspartimide. The ring closes onto the backbone nitrogen of the residue that follows, so the methyl on this residue’s own nitrogen does not by itself prevent it; how much forms depends strongly on the sequence and the conditions, Asp-Gly being the classic high-risk motif, and adding 0.1 M HOBt to the piperidine solution reduces it.

Applications

• N-methyl scans to map the backbone NH groups a bioactive sequence needs
• Cyclic and conformationally constrained peptides
• Analogues of naturally N-methylated peptides
• N-methylated aspartate positions in cyclic and side-chain-linked peptides

Storage
Store refrigerated (2–8 °C) in a tightly closed container, and let it reach room temperature before opening.

References

1. Chatterjee J et al. N-Methylation of Peptides: A New Perspective in Medicinal Chemistry. Accounts of Chemical Research 2008, 41, 1331–1342. doi:10.1021/ar8000603
2. Teixidó M et al. Solid-phase synthesis and characterization of N-methyl-rich peptides. The Journal of Peptide Research 2005, 65, 153–166. doi:10.1111/j.1399-3011.2004.00213.x
3. Mergler M et al. The aspartimide problem in Fmoc‐based SPPS. Part I. Journal of Peptide Science 2003, 9, 36–46. doi:10.1002/psc.430
4. Lauer JL et al. Sequence dependence of aspartimide formation during 9-fluorenylmethoxycarbonyl solid-phase peptide synthesis. Letters in Peptide Science 1995, 1, 197–205. doi:10.1007/bf00117955

For research use only. Not for human or veterinary use.