Environmentally Friendly SPPS II: Scope of Green Fmoc Removal Protocol Using NaOH and Its Application for Synthesis of Commercial Drug Triptorelin

Alkyl Levulinates and 2-Methyltetrahydrofuran: Possible Biomass-Based Solvents in Palladium-Catalyzed Aminocarbonylation

In this research, ethyl levulinate, methyl levulinate, and 2-methyltetrahydrofuran as bio-derived hemicellulose-based solvents were applied as green alternatives in palladium-catalyzed aminocarbonylation reactions. Iodobenzene and morpholine were used in optimization reactions under different conditions, such as temperatures, pressures, and ligands. It was shown that the XantPhos ligand had a great influence on conversion (98%) and chemoselectivity (100% carboxamide), compared with the monodentate PPh₃. Following this study, the optimized conditions were used to extend the scope of substrates with nineteen candidates (various para-, ortho-, and meta-substituted iodobenzene derivatives and iodo-heteroarenes), as well as eight different amine nucleophiles.

An Optimized Protocol for the Synthesis of Peptides Containing trans-Cyclooctene and Bicyclononyne Dienophiles as Useful Multifunctional Bioorthogonal Probes

Despite the great advances in solid-phase peptide synthesis (SPPS), the incorporation of certain functional groups into peptide sequences is restricted by the compatibility of the building blocks with conditions used during SPPS. In particular, the introduction of highly reactive groups used in modern bioorthogonal reactions into peptides remains elusive. Here, we present an optimized synthetic protocol enabling installation of two strained dienophiles, trans-cyclooctene (TCO) and bicyclononyne (BCN), into different peptide sequences. The two groups enable fast and modular post-synthetic functionalization of peptides, as we demonstrate in preparation of peptide-peptide and peptide-drug conjugates. Due to the excellent biocompatibility, the click-functionalization of the peptides can be performed directly in live cells. We further show that the introduction of both clickable groups into peptides enables construction of smart, multifunctional probes that can streamline complex chemical biology experiments such as visualization and pull-down of metabolically labeled glycoconjugates. The presented strategy will find utility in construction of peptides for diverse applications, where high reactivity, efficiency and biocompatibility of the modification step is critical.

N-Butylpyrrolidinone for Solid-Phase Peptide Synthesis is Environmentally Friendlier and Synthetically Better than DMF

Solid-phase peptide synthesis (SPPS) is the method of choice for the preparation of peptides in both laboratory scale and large production. Although the methodology has been improved during the last decades allowing the achievement of long peptides and challenging sequences in good yields and purities, the process was not revised from an environmental point of view. One of the main problems in this regard is the large amount of solvents used, and therefore the tons of generated waste. Moreover, the solvent of choice for the SPPS is N,N-dimethylformamide (DMF), which is considered as reprotoxic; thus, there is an urgent necessity to replace it with safer solvents. The DMF substitution by a green solvent is not a trivial task, because it should solubilize all the reagents and byproducts involved in the process, and, in addition to facilitating the coupling of the different amino acids, it should not favor the formation of side-reactions compared with DMF. Herein, it was demonstrated that the use of the green solvent N-butylpyrrolidinone (NBP) as a replacement of DMF was beneficial in two well-documented side reactions in peptide synthesis, racemization and aspartimide formation. The use of NBP rendered a lower or equal level of racemization in the amino acids more prone to this side reaction than DMF, whilst the aspartimide formation was clearly lower when NBP was used as solvent. Our findings demonstrate that the use of a green solvent does not hamper the synthetic process and could even improve it, making it environmentally friendlier and synthetically better.

Greening Solid-Phase Organic Synthesis: Environmentally Conscious Synthesis of Pharmaceutically Relevant Privileged Structures 5,6-Dihydropyridin-2(1H)-ones and Quinolin-2(1H)-ones

Solid-phase organic synthesis (SPOS) is a very efficient methodology for the synthesis of diverse organic molecules, particularly exploited in drug discovery. Here, we present the transformation of the traditional SPOS to an eco-friendlier methodology on examples of pharmacologically relevant privileged structures 5,6-dihydropyridin-2(1H)-ones and quinolin-2(1H)-ones. The green approach is primarily based on the utilization of environmentally friendly solvent 2-MeTHF in all steps of the synthesis. Target heterocycles were synthesized by extending our previously published synthesis of five-membered tetramic acid analogues to six-membered cycles. The crucial step of the synthesis is cyclization via nonclassical Wittig olefination of resin-bound esters. Traditional and green protocols provided comparable results with respect to purity and yield of products, thus opening the way for greener access to a variety of diverse heterocycles.