Modular Photocatalytic Synthesis of α-Trialkyl-α-Tertiary Amines

Molecules displaying an α-trialkyl-α-tertiary amine motif provide access to an important and versatile area of biologically relevant chemical space but are challenging to access through existing synthetic methods. Here, we report an operationally straightforward, multicomponent protocol for the synthesis of a range of functionally and structurally diverse α-trialkyl-α-tertiary amines, which makes use of three readily available components: dialkyl ketones, benzylamines, and alkenes. The strategy relies on the of use visible-light-mediated photocatalysis with readily available Ir(III) complexes to bring about single-electron reduction of an all-alkyl ketimine species to an α-amino radical intermediate; the α-amino radical undergoes Giese-type addition with a variety of alkenes to forge the α-trialkyl-α-tertiary amine center. The mechanism of this process is believed to proceed through an overall redox neutral pathway that involves photocatalytic redox-relay of the imine, generated from the starting amine-ketone condensation, through to an imine-derived product. This is possible because the presence of a benzylic amine component in the intermediate scaffold drives a 1,5-hydrogen atom transfer step after the Giese addition to form a stable benzylic α-amino radical, which is able to close the photocatalytic cycle. These studies detail the evolution of the reaction platform, an extensive investigation of the substrate scope, and preliminary investigation of some of the mechanistic features of this distinct photocatalytic process. We believe this transformation will provide convenient access to previously unexplored α-trialkyl-α-tertiary amine scaffolds that should be of considerable interest to practitioners of synthetic and medicinal chemistry in academic and industrial institutions.

Recent Advances in Photoredox-Mediated Radical Conjugate Addition Reactions: An Expanding Toolkit for the Giese Reaction

Photomediated Giese reactions are at the forefront of radical chemistry, much like the classical tin-mediated Giese reactions were nearly forty years ago. With the global recognition of organometallic photocatalysts for the mild and tunable generation of carbon-centered radicals, chemists have developed a torrent of strategies to form previously inaccessible radical intermediates that are capable of engaging in intermolecular conjugate addition reactions. This Review summarizes advances in photoredox-mediated Giese reactions since 2013, with a focus on the breadth of methods that provide access to crucial carbon-centered radical intermediates that can engage in radical conjugate addition processes.

Decarboxylative Radical Addition to Methylideneoxazolidinones for Stereocontrolled Synthesis of Selectively Protected Diamino Diacids

Syntheses of stereochemically pure and selectively protected diamino diacids can be achieved by redox decarboxylation of distal N-hydroxyphthalimide esters of protected aspartic, glutamic or α-aminoadipic acids via radical addition to methylideneoxazolidinones. The products are useful for solid-supported syntheses of robust bioactive carbocyclic peptide analogs. Yields of reactive primary radical addition are superior to those of more stabilized radicals, and the reaction fails if the alkylideneoxazolidinone has a methyl substituent on its terminus (i.e., 13a/13b).

Visible-Light-Mediated Carbonyl Alkylative Amination to All-Alkyl α-Tertiary Amino Acid Derivatives

The all-alkyl α-tertiary amino acid scaffold represents an important structural feature in many biologically and pharmaceutically relevant molecules. Syntheses of this class of molecule, however, often involve multiple steps and require activating auxiliary groups on the nitrogen atom or tailored building blocks. Here, we report a straightforward, single-step, and modular methodology for the synthesis of all-alkyl α-tertiary amino esters. This new strategy uses visible light and a silane reductant to bring about a carbonyl alkylative amination reaction that combines a wide range of primary amines, α-ketoesters, and alkyl iodides to form functionally diverse all-alkyl α-tertiary amino esters. Brønsted acid-mediated in situ condensation of primary amine and α-ketoester delivers the corresponding ketiminium species, which undergoes rapid 1,2-addition of an alkyl radical (generated from an alkyl iodide by the action of visible light and silane reductant) to form an aminium radical cation. Upon a polarity-matched and irreversible hydrogen atom transfer from electron rich silane, the electrophilic aminium radical cation is converted to an all-alkyl α-tertiary amino ester product. The benign nature of this process allows for broad scope in all three components and generates structurally and functionally diverse suite of α-tertiary amino esters that will likely have widespread use in academic and industrial settings.

Sulfamyl Radicals Direct Photoredox-Mediated Giese Reactions at Unactivated C(3)-H Bonds

Alcohol-anchored sulfamate esters guide the alkylation of tertiary and secondary aliphatic C(3)-H bonds. The transformation proceeds directly from N-H bonds with a catalytic oxidant, a contrast to prior methods which have required preoxidation of the reactive nitrogen center, or employed stoichiometric amounts of strong oxidants to obtain the sulfamyl radical. These sulfamyl radicals template otherwise rare 1,6-hydrogen-atom transfer (HAT) processes via seven-membered ring transition states to enable C(3)-H functionalization during Giese reactions.

Combining Constrained Heptapeptide Cassettes with Computational Design To Create Coiled-Coil Targeting Helical Peptides

A total of 32 heptapeptides have been synthesized and characterized to establish the effect of K → D (i → i + 4) lactamization upon their ability to adopt a helical conformation. Because most parallel and dimeric coiled-coil sequences can be deconvoluted into gabcdef repeats, we have introduced fixed solvent exposed b → f (K → D) constraints into this design scaffold. Interfacial " a" hydrophobic (L/I/V/N) and " e/g" electrostatic (E/K) options (4 × 2 × 2 = 16 cassettes) were introduced as core drivers of coiled-coil stability and specificity. All present as random coils when linear but adopt a helical conformation upon lactamization. Helicity varied in magnitude from 34 to 68%, indicating different levels of constraint tolerance within the context of a sequence required to be helical for function. Using the oncogenic transcription factor cJun as an exemplar, we next utilized our bCIPA coiled-coil screening engine to select four cassettes of highest predicted affinity when paired with four gabcdef cassettes within the full-length cJun target counterpart (164 = 65 536 combinations). This information was coupled with observed helicity for each constrained cassette to select for the best balance of predicted affinity when linear and experimentally validated helicity when constrained. As a control, the same approach was taken using cassettes of high predicted target affinity but with lower experimentally validated helicity. The approach provides a novel platform of modular heptapeptide cassettes experimentally validated and separated by helical content. Appropriate cassettes can be selected and conjugated to produce longer peptides, in which constraints impart appropriate helicity such that a wide range of targets can be engaged with high affinity and selectivity.

A practical and scalable system for heteroaryl amino acid synthesis

Here, we describe a highly-effective catalytic method for the synthesis of heteroarene-containing unnatural amino acids.

A robust system for the preparation of β-heteroaryl α-amino acid derivatives has been developed using photoredox catalysis. This system operates via regiospecific activation of halogenated pyridines (or other heterocycles) and conjugate addition to dehydroalanine derivatives to deliver a wide range of unnatural amino acids. This process was conducted with good efficiency on large scale, the application of these conditions to amino ketone synthesis is shown, and a simple protocol is given for the preparation of enantioenriched amino acid synthesis, from a number of radical precursors.

A novel peptide stapling strategy enables the retention of ring-closing amino acid side chains for the Wnt/β-catenin signalling pathway

An alternative all-hydrocarbon stapling approach in which the amino acid side chains are retained at the stapled positions.

The all-hydrocarbon peptide stapling strategy has recently been extensively explored in drug discovery. There remains the potential for improvement regarding the retention of the amino acid side chains at the stapled positions. Herein, we describe a new series of amino acids that not only contain the native side chains, but also carry the alkenyl arms that are needed for the ring-closing stapling chemistry. We incorporate the new amino acids into a β-catenin-binding domain of Axin (469–482) and develop a new category of stapled peptides with the retention of the native side chains. These stapled peptides exhibit high α-helicity, strong proteolytic stability and good cell permeability. Biochemical experiments demonstrate that these stapled peptides can activate β-catenin more efficiently than canonical stapled peptides due to the presence of extra side chains. We expect that the new side-chain-retention stapling method would expand the scope of the all-hydrocarbon stapled peptide strategy by retaining some important peripheral residues for protein–protein interactions or preserving key hydrophilic side chains to improve solubility.

Design of cell-permeable stapled peptides as HIV-1 integrase inhibitors

HIV-1 integrase (IN) catalyzes the integration of viral DNA into the host genome, involving several interactions with the viral and cellular proteins. We have previously identified peptide IN inhibitors derived from the α-helical regions along the dimeric interface of HIV-1 IN. Herein, we show that appropriate hydrocarbon stapling of these peptides to stabilize their helical structure remarkably improves the cell permeability, thus allowing inhibition of the HIV-1 replication in cell culture. Furthermore, the stabilized peptides inhibit the interaction of IN with the cellular cofactor LEDGF/p75. Cellular uptake of the stapled peptide was confirmed in four different cell lines using a fluorescein-labeled analogue. Given their enhanced potency and cell permeability, these stapled peptides can serve as not only lead IN inhibitors but also prototypical biochemical probes or "nanoneedles" for the elucidation of HIV-1 IN dimerization and host cofactor interactions within their native cellular environment.

Tetrahydro-β-carboline-based spirocyclic lactam as type II' β-turn: application to the synthesis and biological evaluation of somatostatine mimetics

The synthesis of novel spirocyclic lactams, embodying D-tryptophan (Trp) amino acid as the central core and acting as peptidomimetics, is presented. It relies on the strategic combination of Seebach's self-reproduction of chirality chemistry and Pictet-Spengler condensation as key steps. Investigation of the conformational behavior by molecular modeling, X-ray crystallography, and NMR and IR spectroscopies suggests very stable and highly predictable type II' β-turn conformations for all compounds. Relying on this feature, we also pursued their application to two potential mimetics of the hormone somatostatin, a pharmaceutically relevant natural peptide, which contains a Trp-based type II' β-turn pharmacophore.

(2S,4S)-3-Benzoyl-4-benzyl-2-tert-but-yl-1,3-oxazolidin-5-one

In the title compound, C(21)H(23)NO(3), the central oxazolidinone ring is approximately planar, the maximum deviation from the plane through the central ring being 0.043 (1) Å. The tert-butyl and benzyl substituents are cis to each other and trans to the N-benzoyl group. The inter-planar angle between the aromatic rings of the C-benzyl and N-benzoyl groups is 81.10 (4)°.

Inhibition of caspase-9 by stabilized peptides targeting the dimerization interface

Caspases comprise a family of dimeric cysteine proteases that control apoptotic programmed cell death and are therefore critical in both organismal development and disease. Specific inhibition of individual caspases has been repeatedly attempted, but has not yet been attained. Caspase-9 is an upstream or initiator caspase that is regulated differently from all other caspases, as interaction with natural inhibitor X-linked inhibitor of apoptosis protein (XIAP)-baculovirus inhibitory repeat 3 (BIR3) occurs at the dimer interface maintaining caspase-9 in an inactive monomeric state. One route to caspase-9-specific inhibition is to mimic this interaction, which has been localized to the α5 helix of XIAP-BIR3. We have developed three types of stabilized peptides derived from the α5 helix, using incorporation of aminoisobutyric acid, the avian pancreatic polypeptide (aPP)-scaffold or aliphatic staples. The stabilized peptides are helical in solution and achieve up to 32 μM inhibition, indicating that this allosteric site at the caspase-9 dimerization interface is regulatable with low-molecular weight synthetic ligands and is thus a druggable site. The most potent peptides against caspase-9 activity are the aPP-scaffolded peptides. Other caspases, which are not regulated by dimerization, should not be inactivated by these peptides. Given that all of the peptides attain helical structures but cannot recapitulate the high-affinity inhibition of the intact BIR3 domain, it has become clear that interactions of caspase-9 with the BIR3 exosite are essential for high-affinity binding. These results explain why the full XIAP-BIR3 domain is required for maximal inhibition and suggest a path forward for achieving allosteric inhibition at the dimerization interface using peptides or small molecules.

Synthesis of enantiomerically enriched (R)-C-labelled 2-aminoisobutyric acid (Aib) by conformational memory in the alkylation of a derivative of L-alanine

The method of Kouklovsky and coworkers for the enantioselective alkylation of cyclic N-naphthoyl derivatives of amino acids was used to introduce a ¹³C label into one of the two enantiotopic methyl groups of 2-aminoisobutyric acid (Aib) by retentive alkylation of L-alanine with ¹³CH₃I. Conditions were identified for optimization of yield and enantiomeric purity, and the absolute configuration of the labelled product was established.

Pyrrolinone-based peptidomimetics. "Let the enzyme or receptor be the judge"

Peptides and proteins, evolved by nature to perform vital biological functions, would constitute ideal candidates for therapeutic intervention were it not for their generally poor pharmacokinetic profiles. Nonpeptide peptidomimetics have thus been pursued because they might overcome these limitations while maintaining both the potency and selectivity of the parent peptide or protein. Since the late 1980s, we have sought to design, synthesize, and evaluate a novel, proteolytically stable nonpeptide peptidomimetic scaffold consisting of a repeating structural unit amenable to iterative construction; a primary concern is maintaining both the appropriate peptide-like side-chains and requisite hydrogen bonding. In this Account, we detail how efforts in the Smith-Hirschmann laboratories culminated in the identification of the 3,5-linked polypyrrolinone scaffold. We developed effective synthetic protocols, both in solution and on solid supports, for iterative construction of diverse polypyrrolinones that present functionalized peptide-like side-chains. As a result of the rigid nature of the pyrrolinone scaffold, control over the backbone conformation could be exerted by modulation of the stereogenicity of the constituent monomers and the network of intramolecular hydrogen bonding. The extended conformation of the homochiral 3,5-linked polypyrrolinone scaffold proved to be an excellent mimic for β-strands and β-sheets. Application to enzyme inhibitor design and synthesis led not only to modest inhibitors of the aspartic acid protease renin and the matrix metalloprotease class of enzymes, but importantly to bioavailable HIV-1 protease inhibitors with subnanomolar binding constants. The design and synthesis of a competent peptide-pyrrolinone hybrid ligand for the class II major histocompatibility complex (MHC) antigen protein HLA-DR1 further demonstrated the utility of the 3,5-polypyrrolinone motif as a mimic for the extended polyproline type II peptide backbone. Equally important, we sought to define, by synthesis, the additional conformational space accessible to the polypyrrolinone structural motif, with the ultimate goal of accessing pyrrolinone-based turn and helix mimetics. Toward this end, a mono-N-methylated bispyrrolinone was found to adopt an extended helical array in the solid state. Subsequent synthesis of d,l-alternating (heterochiral) tetrapyrrolinones both validated the expected turn conformations in solution and led to a functionally active mimetic of a peptidal β-turn (similar to somatostatin). Finally, the design, synthesis, and structural evaluation of both acyclic and cyclic heterochiral (that is, d,l-alternating) hexapyrrolinones yielded nanotube-like assemblies in the solid state. Taken together, these results illustrate the remarkable potential of the 3,5-linked polypyrrolinone scaffold as β-strand, β-sheet, β-turn, and potentially helical peptidomimetics.

Design, synthesis, and structural analysis of D,L-mixed polypyrrolinones. 2. Macrocyclic hexapyrrolinones

The design, synthesis, and structural analysis of two macrocyclic D,L-alternating hexapyrrolinones have been achieved. These cyclic peptide mimics adopt a flat, hexagonal conformation, stabilized by intramolecular hydrogen bonding between adjacent pyrrolinone rings. Extensive NMR studies and X-ray analysis reveal, respectively, that the macrocyclic hexapyrrolinones aggregate in solution and in the solid state form staggered stacked nanotube-like assemblies.

Asymmetric autocatalysis induced by meteoritic amino acids with hydrogen isotope chirality

Achiral meteoritic amino acids, glycine and alpha-methylalanine, with hydrogen isotope (D/H) chirality, acted as the source of chirality in asymmetric autocatalysis with amplification of ee to afford highly enantioenriched 5-pyrimidyl alkanols.

Synthesis and catalytic properties of diverse chiral polyamines

Chiral polyamines can be utilized for a variety of potential applications, ranging from asymmetric catalysis to nonviral gene delivery systems for DNA and RNA. They can also be utilized to solubilize carbon nanotubes. Thus, methods for the straightforward synthesis of chiral polyamines are needed. We present herein two synthetic strategies for accessing chiral polyamines. The potential of these chiral amines to catalyze two organic reactions with a high degree of chiral induction was also explored.Text: Chiral polyamines have been utilized for a variety of applications. First, polyamines are polycationic at neutral pH; as such, they interact strongly with both DNA and RNA.1 They can therefore be utilized as effective nonviral gene delivery agents.2 Second, chiral polyamines are efficient catalysts for various organic transformations.3 Polyamines have also been used to solubilize carbon nanotubes.4 Finally, chiral polyamines are excellent ligands for many transition metals.5 Due to their numerous applications, high-yielding synthetic strategies for their preparation are in great demand. We present herein two synthetic strategies for accessing chiral polyamines, and the potential of these chiral amines to catalyze two organic reactions.

Use of Fluorinated Functionality in Enzyme Inhibitor Development: Mechanistic and Analytical Advantages

On the one hand, owing to its electronegativity, relatively small size, and notable leaving group ability from anionic intermediates, fluorine offers unique opportunities for mechanism-based enzyme inhibitor design. On the other, the "bio-orthogonal" and NMR-active 19-fluorine nucleus allows the bioorganic chemist to follow the mechanistic fate of fluorinated substrate analogues or inhibitors as they are enzymatically processed. This article takes an overview of the field, highlighting key developments along these lines. It begins by highlighting new screening methodologies for drug discovery that involve appropriate tagging of either substrate or the target protein itself with (19)F-markers, that then report back on turnover and binding, respectively, via an the NMR screen. Taking this one step further, substrate-tagging with fluorine can be done is such a manner as to provide stereochemical information on enzyme mechanism. For example, substitution of one of the terminal hydrogens in phosphoenolpyruvate, provides insight into the, otherwise latent, facial selectivity of C-C bond formation in KDO synthase. Perhaps, most importantly, from the point of view of this discussion, appropriately tailored fluorinated functionality can be used to form to stabilized "transition state analogue" complexes with a target enzymes. Thus, 5-fluorinated pyrimidines, alpha-fluorinated ketones, and 2-fluoro-2-deoxysugars each lead to covalent adduction of catalytic active site residues in thymidylate synthase, serine protease and glycosidase enzymes, respectively. In all such cases, (19)F NMR allows the bioorganic chemist to spectrally follow "transition state analogue" formation. Finally, the use of specific fluorinated functionality to engineer "suicide substrates" is highlighted in a discussion of the development of the alpha-(2'Z-fluoro)vinyl trigger for amino acid decarboxylase inactivation. Here (19)F NMR allows the bioorganic chemist to glean useful partition ratio data directly out of the NMR tube.

Synthesis of precursors of the agalacto (exo) fragment of the quartromicins via an auxiliary-controlled exo-selective Diels-Alder reaction

[reaction: see text] A direct synthesis of the alpha-hydroxyaldehyde exo-5, a precursor of the exo-spirotetronate subunit of the quartromicins, was achieved through an exo-selective Lewis acid-catalyzed Diels-Alder reaction of dienophile 12a and diene 1.

2,5-Disubstituted pyrrolidines: synthesis by enamine reduction and subsequent regioselective and diastereoselective alkylations

Methodology for the diastereoselective synthesis of 2,5-disubstituted pyrrolidines by reduction of enamines derived from pyroglutamic acid is reported; the nature of nitrogen protection was found to be critical for the stereochemical control of the reaction outcome. Regioselective manipulation of the C-2 and C-5 substituents is possible, providing access to differently substituted pyrrolidines for a limited number of cases.

Synthesis and modification of dibenzylglycine derivatives via the Suzuki-Miyaura cross-coupling reaction

The objective of this paper is to describe in details of various available methods to prepare C(alpha,alpha)-dibenzylglycine (Dbzg) and then include our work involving the synthesis of side chain Dbzg derivatives. alpha,alpha-Disubstituted amino acids (alpha,alphaAAs) are important members in the family of modified amino acids. Replacement of the alpha-hydrogens of glycine 1 by alkyl groups leads to alpha,alphaAAs. The steric hindrance of the quaternary centre of Aib 2 combined with the helix-forming capacity has attracted the attention of structural biologists and protein crystallographers. Dbzg 3 is a special structural variant of Aib. The presence of two benzyl groups at C(alpha)-position not only impart rigidity to the peptide backbone in which it is incorporated, but also acts as a useful vehicle for studying pi-pi interactions. Although several C(alpha,alpha)-disubstituted glycines such as C(alpha,alpha)-diethyl glycine (Deg), C(alpha,alpha)-dipropyl glycine (Dpg) etc. have been studied in detail, not much has been known about Dbzg because of limited availability of synthetic procedures. Various Dbzg derivatives 19a-f have been prepared using ethyl isocyanoacetate 14 as a glycine equivalent (eq.). A useful and simple methodology has been developed using the Suzuki-Miyaura cross-coupling reaction for the modification of Dbzg derivatives 17d, 19d, 22. Using this 'Building Block Approach' (Accounts of Chemical Research 36, 2003, 342) one can generate a variety of Dbzg derivatives 20a-f and 23a-e, which may find useful applications in combinatorial synthesis and QSAR studies.