Coupling efficiency and side reaction control of AEEA-AEEA

Solid phase peptide synthesis (SPPS) is the mainstream method for preparing peptides or peptide like molecules containing AEEA-AEEA sequences. Due to the fact that AEEA-AEEA is a non natural dipeptide unit, its coupling behavior and side reaction profile are different from natural amino acids and require specialized optimization.
Coupling efficiency optimization: In standard Fmoc SPS, AEEA-AEEA is added in the form of Fmoc AEEA-AEA-OH. Recommended coupling conditions: 4-fold excess, HATU (4-fold)/DIEA (8-fold) activation, DMF solvent, reaction at room temperature for 30-60 minutes. Kaiser testing shows that the first round coupling rate is usually>99%. However, it should be noted that when AEEA-AEEA is attached to the resin, the amino group of the second AEEA may exhibit reduced reactivity due to the formation of a hexagonal ring structure through intramolecular hydrogen bonding. Solution strategy: ① Extend the coupling time to 1 hour; ② Use double equivalent (8 times) of activated ester; ③ Add 0.1 M LiCl to break hydrogen bonds. Microwave assisted SPPS (50 ℃, 25 W, 10 minutes) can further improve efficiency to>99.5%.
Side reaction control: The main side reactions include: ① Formation of diketopiperazine (DKP): When the C-terminus of AEEA-AEEA is Pro or Gly, the amino group undergoes cyclization with the carbonyl group of the previous residue after Fmoc removal. Prevention: Immediately introduce amino acids with high steric hindrance (such as Ile and Phe) after AEEA-AEEA coupling, or deprotection with low alkaline concentration (2 eq DIEA). ② Aspartame rearrangement: If Asn or Asp is attached after AEEA-AEEA, rearrangement may occur under alkaline conditions. Countermeasure: Shorten the deprotection time (3 × 1 minutes) and add 0.1 M HOBt as a scavenger. ③ Ether bond cleavage: PEG containing segments may undergo ether bond cleavage during TFA cleavage, especially in the presence of strong acid scavengers such as TMSBr. Recommended cracking solution: TFA/TIS/H ₂ O=95:2.5:2.5, 2 hours, ice bath. By correctly controlling the above parameters, the crude purity of the target product can reach 85-90%, and after purification by preparative HPLC, the final purity is>98%.