Beyond 20 in the 21st Century: Prospects and Challenges of Non-canonical Amino Acids in Peptide Drug Discovery.

Life is constructed primarily using a toolbox of 20 canonical amino acids-relying upon these building blocks for the assembly of proteins and peptides that regulate nearly every cellular task, including cell structure, function, and maintenance. While Nature continues to be a source of inspiration for drug discovery, medicinal chemists are not beholden to only 20 canonical amino acids and have begun to explore non-canonical amino acids (ncAAs) for the construction of designer peptides with improved drug-like properties. However, as our toolbox of ncAAs expands, drug hunters are encountering new challenges in approaching the iterative peptide design-make-test-analyze cycle with a seemingly boundless set of building blocks. This Microperspective focuses on new technologies that are accelerating ncAA interrogation in peptide drug discovery (including HELM notation, late-stage functionalization, and biocatalysis) while shedding light on areas where further investment could not only accelerate the discovery of new medicines but also improve downstream development.

Diversifying Amino Acids and Peptides via Deaminative Reductive Cross-Couplings Leveraging High-Throughput Experimentation.

A deaminative reductive coupling of amino acid pyridinium salts with aryl bromides has been developed to enable efficient synthesis of noncanonical amino acids and diversification of peptides. This method transforms natural, commercially available lysine, ornithine, diaminobutanoic acid, and diaminopropanoic acid to aryl alanines and homologated derivatives with varying chain lengths. Attractive features include ability to transverse scales, tolerance of pharma-relevant (hetero)aryls and biorthogonal functional groups, and the applicability beyond monomeric amino acids to short and macrocyclic peptide substrates. The success of this work relied on high-throughput experimentation to identify complementary reaction conditions that proved critical for achieving the coupling of a broad scope of aryl bromides with a range of amino acid and peptide substrates including macrocyclic peptides.

Total Synthesis of Darobactin A.

The collaborative total synthesis of darobactin A, a recently isolated antibiotic that selectively targets Gram-negative bacteria, has been accomplished in a convergent fashion with a longest linear sequence of 16 steps from d-Garner's aldehyde and l-serine. Scalable routes toward three non-canonical amino acids were developed to enable the synthesis. The closure of the bismacrocycle was realized through sequential, halogen-selective Larock indole syntheses, where the proper order of cyclizations proved crucial for the formation of the desired atropisomer of the natural product.

Nucleophilic Fluorination of Heteroaryl Chlorides and Aryl Triflates Enabled by Cooperative Catalysis.

Aryl and heteroaryl fluorides are growing to be dominant motifs in pharmaceuticals and agrochemicals, yet they are rare in both nature and commodity chemicals. As a consequence, there is an increasingly urgent need to develop mild, cost-effective, and scalable methods for fluorination. The most straightforward route to synthesize aryl fluorides is through the halide exchange "halex" reaction, but conditions, cost, and atom economy preclude most available methods from large-scale manufacturing processes. We report a new approach that leverages the cooperative action of 18-crown-6 ether and tetramethylammonium chloride to catalytically access the reactivity of tetramethylammonium fluoride and achieve halex fluorinations under mild conditions with operational ease. The described methodology readily converts both heteroaryl chlorides and aryl triflates to their corresponding (hetero)aryl fluorides in high yields and purities.

Direct regioisomer analysis of crude reaction mixtures via molecular rotational resonance (MRR) spectroscopy.

Direct analyses of crude reaction mixtures have been carried out using molecular rotational resonance (MRR) spectroscopy. Two examples are presented, a demonstration application in photocatalytic CH-arylation as well as generation of an intermediate in a natural product synthesis. In both cases, the reaction can proceed at more than one site, leading to a mixture of regioisomers that can be challenging to distinguish. MRR structural parameters were calculated for the low lying conformers for the desired compounds, and then compared to the experimental spectra of the crude mixtures to confirm the presence of these species. Next, quantitation was performed by comparing experimentally measured line intensities with simulations based on computed values for the magnitude and direction of the molecular dipole moment of each species. This identification and quantification was performed without sample purification and without isolated standards of the compounds of interest. The values obtained for MRR quantitation were in good agreement with the chromatographic values. Finally, previously unknown impurities were discovered within the photocatalytic CH-arylation work. This paper demonstrates the utility of MRR as a reaction characterization tool to simplify analytical workflows.

High-Throughput Approach toward the Development of a Mizoroki-Heck Reaction To Access Tricyclic Spirolactones.

The ent-kaurenes represent a class of naturally occurring diterpenes of biological importance. Several members of the ent-kaurenes contain a common, tricyclic spirolactone core as a key structural motif. This study details a concise approach toward the development of a Mizoroki-Heck reaction to access this spirolactone core. The strategy described herein was enabled in microscale high-throughput experiments to allow for the rapid identification and optimization of superior reaction conditions.

The merger of decatungstate and copper catalysis to enable aliphatic C(sp3)-H trifluoromethylation.

The introduction of a trifluoromethyl (CF3) group can dramatically improve a compound's biological properties. Despite the well-established importance of trifluoromethylated compounds, general methods for the trifluoromethylation of alkyl C-H bonds remain elusive. Here we report the development of a dual-catalytic C(sp3)-H trifluoromethylation through the merger of light-driven, decatungstate-catalysed hydrogen atom transfer and copper catalysis. This metallaphotoredox methodology enables the direct conversion of both strong aliphatic and benzylic C-H bonds into the corresponding C(sp3)-CF3 products in a single step using a bench-stable, commercially available trifluoromethylation reagent. The reaction requires only a single equivalent of substrate and proceeds with excellent selectivity for positions distal to unprotected amines. To demonstrate the utility of this new methodology for late-stage functionalization, we have directly derivatized a broad range of approved drugs and natural products to generate valuable trifluoromethylated analogues. Preliminary mechanistic experiments reveal that a 'Cu-CF3' species is formed during this process and the critical C(sp3)-CF3 bond-forming step involves the copper catalyst.

Rhodium-Catalyzed Asymmetric Hydroamination of Allyl Amines.

A Rh-catalyzed enantioselective hydroamination of allylamines using a chiral BIPHEP-type ligand is reported. Enantioenriched 1,2-diamines are formed in good yields and with excellent enantioselectivities. A diverse array of nucleophiles and amine directing groups are demonstrated, including deprotectable motifs. Finally, the methodology was demonstrated toward the rapid synthesis of 2-methyl-moclobemide.

Palladium-Catalyzed Carbon Isotope Exchange on Aliphatic and Benzoic Acid Chlorides.

An operationally simple protocol for a palladium-catalyzed 13CO and 14CO exchange with activated aliphatic and benzoic carbonyls is presented. Several 13C and 14C building blocks, natural product derivatives, and pharmaceuticals have been prepared to showcase the method for late-stage carbon isotope incorporation and its functional group compatibility.

Direct arylation of strong aliphatic C-H bonds.

Despite the widespread success of transition-metal-catalysed cross-coupling methodologies, considerable limitations still exist in reactions at sp3-hybridized carbon atoms, with most approaches relying on prefunctionalized alkylmetal or bromide coupling partners1,2. Although the use of native functional groups (for example, carboxylic acids, alkenes and alcohols) has improved the overall efficiency of such transformations by expanding the range of potential feedstocks3-5, the direct functionalization of carbon-hydrogen (C-H) bonds-the most abundant moiety in organic molecules-represents a more ideal approach to molecular construction. In recent years, an impressive range of reactions that form C(sp3)-heteroatom bonds from strong C-H bonds has been reported6,7. Additionally, valuable technologies have been developed for the formation of carbon-carbon bonds from the corresponding C(sp3)-H bonds via substrate-directed transition-metal C-H insertion8, undirected C-H insertion by captodative rhodium carbenoid complexes9, or hydrogen atom transfer from weak, hydridic C-H bonds by electrophilic open-shell species10-14. Despite these advances, a mild and general platform for the coupling of strong, neutral C(sp3)-H bonds with aryl electrophiles has not been realized. Here we describe a protocol for the direct C(sp3) arylation of a diverse set of aliphatic, C-H bond-containing organic frameworks through the combination of light-driven, polyoxometalate-facilitated hydrogen atom transfer and nickel catalysis. This dual-catalytic manifold enables the generation of carbon-centred radicals from strong, neutral C-H bonds, which thereafter act as nucleophiles in nickel-mediated cross-coupling with aryl bromides to afford C(sp3)-C(sp2) cross-coupled products. This technology enables unprecedented, single-step access to a broad array of complex, medicinally relevant molecules directly from natural products and chemical feedstocks through functionalization at sites that are unreactive under traditional methods.

Oxyfunctionalization of the Remote C-H Bonds of Aliphatic Amines by Decatungstate Photocatalysis.

Aliphatic amines, oxygenated at remote positions within the molecule, represent an important class of synthetic building blocks to which there are currently no direct means of access. Reported herein is an efficient and scalable solution that relies upon decatungstate photocatalysis under acidic conditions using either H2 O2 or O2 as the terminal oxidant. By using these reaction conditions a series of simple and unbiased aliphatic amine starting materials can be oxidized to value-added ketone products. Lastly, NMR spectroscopy using in situ LED-irradiated samples was utilized to monitor the kinetics of the reaction, thus enabling direct translation of the reaction into flow.

An improved procedure for the preparation of Ru(bpz)3(PF6)2 via a high-yielding synthesis of 2,2'-bipyrazine.

There has been a recent surge of interest in the use of transition metal polypyridyl complexes as visible light-absorbing photocatalysts for synthetic applications. Among the most attractive features of this approach is the availability of many known complexes with well-characterized photophysical and electrochemical properties. In particular, Ru(bpz)3 (2+) is a powerful photooxidant that has proven to be uniquely suited for oxidatively induced photoredox transformations. We present here a straightforward and high-yielding route to Ru(bpz)3(PF6)2 that features an improved Pd-catalyzed synthesis of the 2,2'-bipyrazine ligand that is amenable to gram-scale preparations.

A dual-catalysis approach to enantioselective [2 + 2] photocycloadditions using visible light.

In contrast to the wealth of catalytic systems that are available to control the stereochemistry of thermally promoted cycloadditions, few similarly effective methods exist for the stereocontrol of photochemical cycloadditions. A major unsolved challenge in the design of enantioselective catalytic photocycloaddition reactions has been the difficulty of controlling racemic background reactions that occur by direct photoexcitation of substrates while unbound to catalyst. Here, we describe a strategy for eliminating the racemic background reaction in asymmetric [2 + 2] photocycloadditions of α,ß-unsaturated ketones to the corresponding cyclobutanes by using a dual-catalyst system consisting of a visible light-absorbing transition-metal photocatalyst and a stereocontrolling Lewis acid cocatalyst. The independence of these two catalysts enables broader scope, greater stereochemical flexibility, and better efficiency than previously reported methods for enantioselective photochemical cycloadditions.

Solar synthesis: prospects in visible light photocatalysis.

Chemists have long aspired to synthesize molecules the way that plants do-using sunlight to facilitate the construction of complex molecular architectures. Nevertheless, the use of visible light in photochemical synthesis is fundamentally challenging because organic molecules tend not to interact with the wavelengths of visible light that are most strongly emitted in the solar spectrum. Recent research has begun to leverage the ability of visible light-absorbing transition metal complexes to catalyze a broad range of synthetically valuable reactions. In this review, we highlight how an understanding of the mechanisms of photocatalytic activation available to these transition metal complexes, and of the general reactivity patterns of the intermediates accessible via visible light photocatalysis, has accelerated the development of this diverse suite of reactions.

Recent Developments in Pd-Catalyzed Alkene Aminoarylation Reactions for the Synthesis of Nitrogen Heterocycles.

This short review describes new developments in Pd-catalyzed aminoarylation reactions between aryl halides and alkenes bearing pendant nitrogen nucleophiles. These transformations provide a novel and powerful method for accessing numerous 3-, 5-, 6-, and 7-membered nitrogen heterocycles.

Intramolecular alkene carboamination reactions for the synthesis of enantiomerically enriched tropane derivatives.

The synthesis of tropane derivatives via intramolecular Pd-catalyzed alkene difunctionalization reactions is described. Enantiopure N-aryl-γ-aminoalkenes bearing an aryl or alkenyl halide adjacent to the amino group were converted to benzo- or cycloalkenyl-fused tropane products in good yield and with no loss of enantiopurity.

Synthesis of polycyclic nitrogen heterocycles via alkene aminopalladation/carbopalladation cascade reactions.

A new method for the synthesis of tricyclic nitrogen heterocycles from N,2-diallylaniline derivatives is described. These transformations proceed via sequential alkene aminopalladation of an intermediate L(n)Pd(Ar)(NRR') species followed by alkene carbopalladation of the resulting L(n)Pd(Ar)(R) complex. Both alkene insertion steps occur in preference to C-N or C-C bond-forming reductive elimination. An unusual 1,3-palladium shift occurs when 2-Allyl-N-(2-vinylphenyl)aniline is employed as substrate, which yields a tetracyclic molecule with three contiguous stereocenters.