Brønsted Acid-Lewis Acid (BA-LA) Induced Final Deprotection/Peptide Resin Cleavage in Fmoc/t-Bu Solid-Phase Peptide Synthesis: HCl/FeCl3 and AcOH/FeCl3 as Viable PFAS-Free Alternatives for TFA.

The widely used Fmoc/t-Bu solid-phase peptide synthesis (SPPS) is hampered by relying on corrosive, per/polyfluoroalkyl substance (PFAS) classified trifluoroacetic acid (TFA) as a universal protecting group (PG) removal/resin cleavage reagent. We report that suitable combinations of Brønsted acids (BAs) and Lewis acids (LAs) such as HCl/FeCl3 and AcOH/FeCl3 constitute viable alternatives for TFA as PFAS-free cleavage agents. Using water miscible dimethyl carbonate (DMC) and acetonitrile (MeCN) as solvents enabled diluting cleavage mixtures with suitable aqueous solutions, allowing for direct use in purification in which removal of >99.99% iron from an HCl/FeCl3 induced cleavage was demonstrated.

Environmentally Sensible Organocatalytic Fmoc/t-Bu Solid-Phase Peptide Synthesis.

Despite numerous reports on catalytic amide bond formation (ABF), these methods have thus far had a minimal impact on the universal fluorenylmethoxycarbonyl (Fmoc)/t-Bu solid-phase peptide synthesis (SPPS) methodology. We now report a proof-of-principle Fmoc/t-Bu SPPS in which both couplings and Fmoc deprotections were catalyzed by readily available reagents in an inexpensive green solvent. Couplings were carried out with >99% stereoselectivity, employing 1.1 equiv of Fmoc amino acids (AAs), using diisopropylcarbodiimide (DIC) as a coupling agent and 1-hydroxy-1,2,3-triazole-5-carboxylic acid ethyl ester (HOCt) (TON ∼ 30) as a catalyst, while Fmoc deprotections were carried out using 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) (TON ∼ 7), facilitating synthesis of a model pentapeptide in 95% HPLC purity while also enabling minimization of solvent washing.

Circular Aqueous Fmoc/t-Bu Solid-Phase Peptide Synthesis.

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.

1,4-Benzenedimethanethiol (1,4-BDMT) as a scavenger for greener peptide resin cleavages.

Aiming at elevating the environmental profile of the solid-phase peptide synthesis (SPPS) methodology by improving the quality of crude peptides that SPPS provides we assessed a series of benzylthiols (BTs) as scavengers for global deprotection/TFA cleavage of exenatide peptide resin accessed by Fmoc SPPS. In these studies we identified 1,4-BDMT as a scavenger that affords the peptide in higher quality than the standard aliphatic thiol reagents, not least in terms of the content of critical peptide impurities in the crude material. Further, 1,4-BDMT exhibited favorable UV detectability as well as stability and solubility in TFA. Finally, based on the MS assessment of the crude exenatide products herein we propose that thiol scavengers in the cleavage of Trp containing peptide resins do not minimize the content of Trp oxidants by means of inhibiting Trp oxidation but rather by forming a peptide-thiol adduct via a mechanism involving an attack of a thiol on an oxindolylalanine (Oia) impurity present in the crude material.