Novel deprotection method of Fmoc group under neutral hydrogenation conditions

Catalytic Cleavage of the 9-Fluorenylmethoxycarbonyl (Fmoc) Protecting Group under Neat Conditions

This work reports the first solvent-free catalytic approach for the cleavage of the fluorenylmethoxycarbonyl (Fmoc) protecting group from amine and alcohol functionalities. Various saccharide, peptide, and glyco-amino acid substrates were efficiently deprotected by simple treatment with 20 mol % neat 4-dimethylaminopyridine (DMAP) (one of the effective base catalysts found), without any solvent or stoichiometric additives. Small model structures were finally assembled through one-pot, base-catalyzed, solvent-free multistep sequences combining the Fmoc cleavage with esterification, amidation, and/or glycosylation steps.

First Total Syntheses of Beauvericin A and allo-Beauvericin A

The first total syntheses of beauvericin A and allo-beauvericin A were achieved. N-Methyl-l-phenylalanine, (2R)-hydroxylvaleric acid, and (2R,3S)- or (2R,3R)-2-hydroxy-2-methylpentanoic acid were linked and cyclized to form the target natural products. The structure of synthetic beauvericin A was confirmed by X-ray crystallographic analysis. NMR data of the synthetic beauvericins were identical with those of the reported natural products. These results secure the structures of natural products, as originally proposed in the isolation studies.

Orthogonal Deprotection Strategy of Fmoc Provides Improved Synthesis of Sensitive Peptides: Application to Z-Arg-Lys-AOMK

Protecting groups (PGs) in peptide synthesis have inspired advanced design principles that incorporate "orthogonality" for selective C- and N-terminus and side-chain deprotections. The conventionally acid-stable 9-fluorenylmethoxycarbonyl (Fmoc) group is one of the most widely used N-protection groups in solid- and solution-phase synthesis. Despite the versatility of Fmoc, deprotection by the removal of the Fmoc group to unmask primary amines requires the use of a basic secondary amine nucleophile, but this stratagem poses challenges in sensitive molecules that bear reactive electrophilic groups. An expansion of PG versatility, a tunable orthogonality, in the late-stage synthesis of peptides would add flexibility to the synthetic design and implementation. Here, we report a novel Fmoc deprotection method using hydrogenolysis under mildly acidic conditions for the synthesis of Z-Arg-Lys-acyloxymethyl ketone (Z-R-K-AOMK). This new method is not only valuable for Fmoc deprotection in the synthesis of complex peptides that contain highly reactive electrophiles, or other similar sensitive functional groups, that are incompatible with traditional Fmoc deprotection conditions but also tolerant of N-Boc groups present in the substrate.

Improved Triplex-Forming Isoorotamide PNA Nucleobases for A-U Recognition of RNA Duplexes

Four new isoorotamide (Io)-containing PNA nucleobases have been designed for A-U recognition of double helical RNA. New PNA monomers were prepared efficiently and incorporated into PNA nonamers for binding A-U in a PNA:RNA2 triplex. Isothermal titration calorimetry and UV thermal melting experiments revealed slightly improved binding affinity for singly modified PNA compared to known A-binding nucleobases. Molecular dynamics simulations provided further insights into binding of Io bases in the triple helix. Together, the data revealed interesting insights into binding modes including the notion that three Hoogsteen hydrogen bonds are unnecessary for strong selective binding of an extended nucleobase. Cationic monomer Io8 additionally gave the highest affinity observed for an A-binding nucleobase to date. These results will help inform future nucleobase design toward the goal of recognizing any sequence of double helical RNA.

Aptamers with Self-Loading Drug Payload and pH-Controlled Drug Release for Targeted Chemotherapy

Doxorubicin (DOX) is a common anti-tumor drug that binds to DNA or RNA via non-covalent intercalation between G-C sequences. As a therapeutic agent, DOX has been used to form aptamer–drug conjugates for targeted cancer therapy in vitro and in vivo. To improve the therapeutic potential of aptamer–DOX conjugates, we synthesized trifurcated Newkome-type monomer (TNM) structures with three DOX molecules bound through pH-sensitive hydrazone bonds to formulate TNM-DOX. The aptamer–TNM–DOX conjugate (Apt–TNM-DOX) was produced through a simple self-loading process. Chemical validation revealed that Apt–TNM-DOX stably carried high drug payloads of 15 DOX molecules per aptamer sequence. Functional characterization showed that DOX payload release from Apt–TNM-DOX was pH-dependent and occurred at pH 5.0, which reflects the microenvironment of tumor cell lysosomes. Further, Apt–TNM-DOX specifically targeted lymphoma cells without affecting off-target control cells. Aptamer-mediated cell binding resulted in the uptake of Apt–TNM-DOX into targeted cells and the release of DOX payload within cell lysosomes to inhibit growth of targeted lymphoma cells. The Apt–TNM-DOX provides a simple, non-toxic approach to develop aptamer-based targeted therapeutics and may reduce the non-specific side effects associated with traditional chemotherapy.

Mechanochemical Catalytic Transfer Hydrogenation of Aromatic Nitro Derivatives

Mechanochemical ball milling catalytic transfer hydrogenation (CTH) of aromatic nitro compounds using readily available and cheap ammonium formate as the hydrogen source is demonstrated as a simple, facile and clean approach for the synthesis of substituted anilines and selected pharmaceutically relevant compounds. The scope of mechanochemical CTH is broad, as the reduction conditions tolerate various functionalities, for example nitro, amino, hydroxy, carbonyl, amide, urea, amino acid and heterocyclic. The presented methodology was also successfully integrated with other types of chemical reactions previously carried out mechanochemically, such as amide bond formation by coupling amines with acyl chlorides or anhydrides and click-type coupling reactions between amines and iso(thio)cyanates. In this way, we showed that active pharmaceutical ingredients Procainamide and Paracetamol could be synthesized from the respective nitro-precursors on milligram and gram scale in excellent isolated yields.

Synthesis of Fmoc-Protected ( S, S)- trans-Cyclopentane Diamine Monomers Enables the Preparation and Study of Conformationally Restricted Peptide Nucleic Acids

An efficient synthesis of Fmoc-protected ( S, S)- trans-cyclopentane PNA ( tcypPNA) monomers starting from mono-Boc-protected ( S, S)-1,2-cyclopentanediamine is reported. A general synthetic strategy was developed so that tcypPNA monomers with each nucleobase can be made in sufficient quantity and purity for use in solid-phase peptide synthesis (SPPS). The newly synthesized monomers were then successfully incorporated into 10-residue PNA oligomers using standard Fmoc chemistry for SPPS. The different tcypPNAs allow different positions in the sequence to be conformationally constrained with ( S, S)- trans-cyclopentane to determine the effects on binding to complementary DNA.

Scale-Up Synthesis and Identification of GLYX-13, a NMDAR Glycine-Site Partial Agonist for the Treatment of Major Depressive Disorder

GLYX-13, a NMDAR glycine-site partial agonist, was discovered as a promising antidepressant with rapidly acting effects but no ketamine-like side effects. However, the reported synthetic process route had deficiencies of low yield and the use of unfriendly reagents. Here, we report a scaled-up synthesis of GLYX-13 with an overall yield of 30% on the hectogram scale with a column chromatography-free strategy, where the coupling and deprotection reaction conditions were systematically optimized. Meanwhile, the absolute configuration of precursor compound of GLYX-13 was identified by X-ray single crystal diffraction. Finally, the activity of GLYX-13 was verified in the cortical neurons of mice through whole-cell voltage-clamp technique.

Exploring the impact of the side-chain length on peptide/RNA binding events

The impact of the amino-acid side-chain length on peptide-RNA binding events has been investigated using HIV-1 Tat derived peptides as ligands and the HIV-1 TAR RNA element as an RNA model. Our studies demonstrate that increasing the length of all peptide side-chains improves unexpectedly the binding affinity (K_D) but reduces the degree of compactness of the peptide-RNA complex. Overall, the side-chain length appears to modulate in an unpredictable way the ability of the peptide to compete with the cognate TAR RNA partner. Beyond the establishment of non-intuitive fundamental relationships, our results open up new perspectives in the design of effective RNA ligand competitors, since a large number of them have already been identified but few studies report on the modulation of the biological activity by modifying in the same way the length of all chains connecting RNA recognition motives to the central scaffold of a ligand.

Synthesis of Norfijimycin A with Activity against Mycobacterium tuberculosis

The total synthesis of norfijimycin A, a simplified analogue of the marine natural product fijimycin A, is described. Fijimycin A is a cyclic depsipeptide that has been shown to possess activity against methicillin-resistant Staphylococcus aureus. The natural product contains a rare N,β-dimethyl leucine unit with unknown stereochemistry at the β-carbon. To evaluate the importance of the β-methyl group for antimicrobial activity, we introduced N-methyl leucine into the natural product scaffold. The resulting norfijimycin A was shown to possess significant activity against Mycobacterium tuberculosis, the etiological agent of tuberculosis.

Solventless mechanosynthesis of N-protected amino esters

Mechanochemical derivatizations of N- or C-protected amino acids were performed in a ball mill under solvent-free conditions. A vibrational ball mill was used for the preparation of N-protected α- and β-amino esters starting from the corresponding N-unmasked precursors via a carbamoylation reaction in the presence of di-tert-butyl dicarbonate (Boc2O), benzyl chloroformate (Z-Cl) or 9-fluorenylmethoxycarbonyl chloroformate (Fmoc-Cl). A planetary ball mill proved to be more suitable for the synthesis of amino esters from N-protected amino acids via a one-pot activation/esterification reaction in the presence of various dialkyl dicarbonates or chloroformates. The spot-to-spot reactions were straightforward, leading to the final products in reduced reaction times with improved yields and simplified work-up procedures.

A mild removal of Fmoc group using sodium azide

A mild method for effectively removing the fluorenylmethoxycarbonyl (Fmoc) group using sodium azide was developed. Without base, sodium azide completely deprotected N (α)-Fmoc-amino acids in hours. The solvent-dependent conditions were carefully studied and then optimized by screening different sodium azide amounts and reaction temperatures. A variety of Fmoc-protected amino acids containing residues masked with different protecting groups were efficiently and selectively deprotected by the optimized reaction. Finally, a biologically significant hexapeptide, angiotensin IV, was successfully synthesized by solid phase peptide synthesis using the developed sodium azide method for all Fmoc removals. The base-free condition provides a complement method for Fmoc deprotection in peptide chemistry and modern organic synthesis.