Solvent recycling in solid-phase peptide synthesis (SPPS): combination of a swellable macroporous polystyrene (SMPS) resin and green binary solvents

Abstract

Solid-phase peptide synthesis (SPPS) relies heavily on hazardous solvents, such as N,N′-dimethylformamide (DMF), whose substitution by green solvents is proving more challenging than expected. Currently, binary mixtures of dimethyl sulfoxide (DMSO) or N-butylpyrrolidone (NBP) with other solvents are rendering the best results. These solvent mixtures are difficult to recycle due to the high boiling points of DMSO and NBP. Ideally, the solvent or solvent mixture used in SPPS should have a boiling point that allows its recycling without excessive energy use. Herein, we have developed a new swellable macroporous polystyrene (SMPS) resin that enables SPPS in greener, more recyclable solvent systems. The SMPS resin, characterized by a 6% divinylbenzene crosslinking degree and macroporous architecture, exhibited good swelling behavior across a broad range of single and binary solvents, including acetonitrile (MeCN), ethyl acetate (EtOAc), 2-methyltetrahydrofuran (2-MeTHF), and mixtures of EtOAc with MeCN, triethyl phosphate (TEP), and toluene (Tol). Its performance was assessed through the synthesis of three model peptides of increasing complexity: Leu-enkephalin, a decapeptide analogue, and an afamelanotide analogue. At 50 °C, several alternative solvent systems matched or outperformed DMF, with MeCN and EtOAc giving excellent results for shorter sequences, while binary mixtures proved superior for longer peptides. In particular, EtOAc–Tol (3 : 1) afforded the best crude purity for the afamelanotide analogue and, after optimization of deprotection and coupling conditions, reached 91% purity. Importantly, this solvent mixture could be recovered by distillation, reducing the process mass intensity (PMI) from 3206 to 1137, corresponding to an approximately 65% decrease in solvent contribution. These results demonstrate that combining SMPS resin with recyclable binary solvents is a promising strategy to improve the sustainability of SPPS without compromising synthetic efficiency.