Circular Aqueous Fmoc/t‐Bu Solid‐Phase Peptide Synthesis

Abstract

Circular economy and aqueous synthesis are attractive concepts for sustainable chemistry. Here it is reported that the two can be combined in the universal method for peptide chemistry, fluorenylmethoxycarbonyl(Fmoc)/t‐Bu solid‐phase peptide synthesis (SPPS). It was demonstrated that Fmoc/t‐Bu SPPS could be performed under aqueous conditions using standard Fmoc amino acids (AAs) employing TentaGel S as resin and 4 : 1 mixture of water with inexpensive green solvent PolarClean. This resin/solvent combination played a crucial dual role by virtue of improving resin swelling and solubility of starting materials. In a model coupling, TCFH and 2,4,6‐collidine afforded a full conversion at only 1.3 equiv. AA, and these conditions were used in SPPS of Leu enkephaline amide affording the model peptide in 85 % yield and 86 % purity. A method to recycle the waste by filtration through a mixed ion exchange resin was developed, allowing reusing the waste without affecting quality of the peptide. The method herein obviates the use of unconventional or processed AAs in aqueous SPPS while using lower amounts of starting materials. By recycling/reusing SPPS waste the hazardous dipolar aprotic solvents used in SPPS were not only replaced with an aqueous medium, solvent use was also significantly reduced. This opens up a new direction in aqueous peptide chemistry in which efficient use of inexpensive starting materials and waste minimization is coupled with the universal Fmoc/t‐Bu SPPS.

Sustainability in peptide chemistry: current synthesis and purification technologies and future challenges

“…As mentioned previously, Rasmussen's group also reported the use of PolarClean as an efficient water co-solvent for the first reported Fmoc/ t Bu SPPS in aqueous conditions. 65 The solubility of standard Fmoc-AAs was achieved by the addition of 20% PolarClean. 84 In this way the full GSPPS of a model peptide was obtained with the same purity as with DMF (Table 3, entry 50), achieving a new aqueous Fmoc/ t Bu peptide synthesis method with minimized amounts of inexpensive and non-hazardous solvents.…”

“…68 Rasmussen et al successfully extended the use of 4-MP to green solvents mixtures (Table 5, entries 14, 15, 28 and 34). 29 c ,65,78 The Tolomelli and Cabri group described the use of TBA and DEAPA in green solvents like NOP and NOP/DMC (Table 5, entries 29 and 30). 29 b TBA was also able to avoid the formation of the hydantoin (Hyd) impurity as rearrangement of the dihydroorotic (Hor) moiety during degarelix synthesis (Table 5, entry 16), but it was less efficient as a general base for Fmoc removal.…”

“…However, TFFH and tetramethylchloroformamidinium hexafluorophosphate (TCFH) have been recently tested in the water/PolarClean-based SPPS proposed by Rasmussen, with TCFH performing the best. 65 However, T3P® is a cyclic reagent that maintains a discrete stability in organic solvents. It is well known for promoting amidations in liquid phase with high efficiency and no epimerization.…”

Advances in solid-phase peptide synthesis in aqueous media (ASPPS)

“…40,100 Recently, N , N , N ′, N ′-tetramethylchloroformamidinium hexafluorophosphate (TCFH) paired with collidine has been proven to be a more efficient coupling agent than COMU for SPPS when aqueous 20% PolarClean was used as a solvent and TentaGel S as a resin. 41…”

“…30–39 Fully switching to water – the biochemical solvent for the biosynthesis of biomolecules, including peptides and proteins – is one of the most fascinating and promising unsolved challenges in SPPS. 40,41 Thus, the ultimate goal in Green SPPS is the so-called Aqueous SPPS (ASPPS) to achieve sustainable peptide synthesis. 42–48…”