The application of anisole in greener solid-phase peptide synthesis protocols – Compatibility with green bases in Fmoc removal and new green binary mixture for coupling

Abstract

As a step toward green Fmoc solid-phase peptide synthesis (SPPS), we explored the compatibility of the green bases morpholine, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and NaOH with the green solvent anisole as a replacement for the traditionally used hazardous piperidine in N,N-dimethylformamide (DMF) for the Fmoc removal step. Green Fmoc cleavage protocols were optimized for the Fmoc-Ala-NH-Rink resin model and subsequently verified for other Fmoc-amino acids. Only Gly required a modified NaOH-based protocol with the addition of 5 % water. Furthermore, we evaluated the effects of basic treatment on the notorious aspartimide formation as an undesired side product of SPPS. We also introduced a new green solvent mixture, anisole/dimethyl sulfoxide (DMSO) (4:1) for acylation step, which did not cause racemization. The applicability of these new green protocols was shown during SPPS of the pentapeptides Leu-enkephalin and Aib-enkephalin using polystyrene (PS)-based resins. Additionally, we identified the new sequence-dependent side products that formed after treatment with NaOH in anisole/ethanol (EtOH) (1:1).