Hybrid receptor structure/ligand-based docking and activity prediction in ICM: development and evaluation in D3R Grand Challenge 3

In context of D3R Grand Challenge 3 we have investigated several ligand activity prediction protocols that combined elements of a physics-based energy function (ICM VLS score) and the knowledge-based Atomic Property Field 3D QSAR approach. Activity prediction models utilized poses produced by ICM-Dock with ligand bias and 4D receptor conformational ensembles (LigBEnD). Hybrid APF/P (APF/Physics) models were superior to pure physics- or knowledge-based models in our preliminary tests using rigorous three-fold clustered cross-validation and later proved successful in the blind prediction for D3R GC3 sets, consistently performing well across four different targets. The results demonstrate that knowledge-based and physics-based inputs into the machine-learning activity model can be non-redundant and synergistic.

Ligand-biased ensemble receptor docking (LigBEnD): a hybrid ligand/receptor structure-based approach

Ligand docking to flexible protein molecules can be efficiently carried out through ensemble docking to multiple protein conformations, either from experimental X-ray structures or from in silico simulations. The success of ensemble docking often requires the careful selection of complementary protein conformations, through docking and scoring of known co-crystallized ligands. False positives, in which a ligand in a wrong pose achieves a better docking score than that of native pose, arise as additional protein conformations are added. In the current study, we developed a new ligand-biased ensemble receptor docking method and composite scoring function which combine the use of ligand-based atomic property field (APF) method with receptor structure-based docking. This method helps us to correctly dock 30 out of 36 ligands presented by the D3R docking challenge. For the six mis-docked ligands, the cognate receptor structures prove to be too different from the 40 available experimental Pocketome conformations used for docking and could be identified only by receptor sampling beyond experimentally explored conformational subspace.

Use of Cysteine Trapping to Map Spatial Approximations between Residues Contributing to the Helix N-capping Motif of Secretin and Distinct Residues within Each of the Extracellular Loops of Its Receptor

Amino-terminal regions of secretin-family peptides contain key determinants for biological activity and binding specificity, although the nature of interactions with receptors is unclear. A helix N-capping motif within this region has been postulated to directly contribute to agonist activity while also stabilizing formation of a helix extending toward the peptide carboxyl terminus and docking within the receptor amino terminus. We used cysteine trapping to systematically explore spatial approximations between cysteines replacing each residue in this motif of secretin (sec), Phe(6), Thr(7), and Leu(10), and cysteines incorporated into the extracellular face of the receptor. Each peptide was a full agonist for cAMP, but had a lower binding affinity than natural hormone. These bound to COS cells expressing 61 receptor constructs incorporating cysteines in every position along each extracellular loop (ECL) and adjacent parts of transmembrane (TM) segments. Patterns of covalent labeling were distinct for each probe, with Cys(6)-sec labeling multiple residues in the carboxyl-terminal half of ECL2 and throughout ECL3, Cys(7)-sec predominantly labeling only single residues in the carboxyl-terminal end of ECL2 and the amino-terminal end of ECL3, and Cys(10)-sec not efficiently labeling any of these residues. These spatial constraints were used to refine our model of secretin bound to its receptor, now bringing ECL3 above the amino terminus of the ligand and revealing possible charge-charge interactions between this part of secretin and receptor residues in TM5, TM6, ECL2, and ECL3, which can orient and stabilize the peptide-receptor complex. This was validated by testing predicted approximations by mutagenesis and residue-residue complementation studies.

3-Oxoisoxazole-2(3H)-carboxamides and isoxazol-3-yl carbamates: Resistance-breaking acetylcholinesterase inhibitors targeting the malaria mosquito, Anopheles gambiae

To identify potential selective and resistance-breaking mosquitocides against the African malaria vector Anopheles gambiae, we investigated the acetylcholinesterase (AChE) inhibitory and mosquitocidal properties of isoxazol-3-yl dimethylcarbamates (15), and the corresponding 3-oxoisoxazole-2(3H)-dimethylcarboxamide isomers (14). In both series, compounds were found with excellent contact toxicity to wild-type susceptible (G3) strain and multiply resistant (Akron) strain mosquitoes that carry the G119S resistance mutation of AChE. Compounds possessing good to excellent toxicity to Akron strain mosquitoes inhibit the G119S mutant of An. gambiae AChE (AgAChE) with ki values at least 10- to 600-fold higher than that of propoxur, a compound that does not kill Akron mosquitoes at the highest concentration tested. On average, inactivation of WT AgAChE by dimethylcarboxamides 14 was 10-20 fold faster than that of the corresponding isoxazol-3-yl dimethylcarbamates 15. X-ray crystallography of dimethylcarboxamide 14d provided insight into that reactivity, a finding that may explain the inhibitory power of structurally-related inhibitors of hormone-sensitive lipase. Finally, human/An. gambiae AChE inhibition selectivities of these compounds were low, suggesting the need for additional structural modification.

Development of a highly selective allosteric antagonist radioligand for the type 1 cholecystokinin receptor and elucidation of its molecular basis of binding

Understanding the molecular basis of ligand binding to receptors provides insights useful for rational drug design. This work describes development of a new antagonist radioligand of the type 1 cholecystokinin receptor (CCK1R), (2-fluorophenyl)-2,3-dihydro-3-[(3-isoquinolinylcarbonyl)amino]-6-methoxy-2-oxo-l-H-indole-3-propanoate (T-0632), and exploration of the molecular basis of its binding. This radioligand bound specifically with high affinity within an allosteric pocket of CCK1R. T-0632 fully inhibited binding and action of CCK at this receptor, while exhibiting no saturable binding to the closely related type 2 cholecystokinin receptor (CCK2R). Chimeric CCK1R/CCK2R constructs were used to explore the molecular basis of T-0632 binding. Exchanging exonic regions revealed the functional importance of CCK1R exon 3, extending from the bottom of transmembrane segment (TM) 3 to the top of TM5, including portions of the intramembranous pocket as well as the second extracellular loop region (ECL2). However, CCK1R mutants in which each residue facing the pocket was changed to that present in CCK2R had no negative impact on T-0632 binding. Extending the chimeric approach to ECL2 established the importance of its C-terminal region, and site-directed mutagenesis of each nonconserved residue in this region revealed the importance of Ser(208) at the top of TM5. A molecular model of T-0632-occupied CCK1R was consistent with these experimental determinants, also identifying Met(121) in TM3 and Arg(336) in TM6 as important. Although these residues are conserved in CCK2R, mutating them had a distinct impact on the two closely related receptors, suggesting differential orientation. This establishes the molecular basis of binding of a highly selective nonpeptidyl allosteric antagonist of CCK1R, illustrating differences in docking that extend beyond determinants attributable to distinct residues lining the intramembranous pocket in the two receptor subtypes.

Neurotoxicology of bis(n)-tacrines on Blattella germanica and Drosophila melanogaster acetylcholinesterase

A series of bis(n)-tacrines were used as pharmacological probes of the acetylcholinesterase (AChE) catalytic and peripheral sites of Blattella germanica and Drosophila melanogaster, which express AChE-1 and AChE-2 isoforms, respectively. In general, the potency of bis(n)-tacrines was greater in D. melanogaster AChE (DmAChE) than in B. germanica AChE (BgAChE). The change in potency with tether length was high in DmAChE and low in BgAChE, associated with 90-fold and 5.2-fold maximal potency gain, respectively, compared to the tacrine monomer. The optimal tether length for Blattella was 8 carbons and for Drosophila was 10 carbons. The two species differed by only about twofold in their sensitivity to tacrine monomer, indicating that differential potency occurred among dimeric bis(n)-tacrines due to structural differences in the peripheral site. Multiple sequence alignment and in silico homology modeling suggest that aromatic residues of DmAChE confer higher affinity binding, and the lack of same at the BgAChE peripheral site may account, at least in part, to the greater overall sensitivity of DmAChE to bis(n)-tacrines, as reflected by in vitro assay data. Topical and injection assays in cockroaches found minimal toxicity of bis(n)-tacrines. Electrophysiological studies on D. melanogaster central nervous system showed that dimeric tacrines do not readily cross the blood brain barrier, explaining the observed nonlethality to insects. Although the bis(n)-tacrines were not good insecticide candidates, the information obtained in this study should aid in the design of selective bivalent ligands targeting insect, pests, and disease vectors.

Mapping spatial approximations between the amino terminus of secretin and each of the extracellular loops of its receptor using cysteine trapping

While it is evident that the carboxyl-terminal region of natural peptide ligands bind to the amino-terminal domain of class B GPCRs, how their biologically critical amino-terminal regions dock to the receptor is unclear. We utilize cysteine trapping to systematically explore spatial approximations among residues in the first five positions of secretin and in every position within the receptor extracellular loops (ECLs). Only Cys(2) and Cys(5) secretin analogues exhibited full activity and retained moderate binding affinity (IC(50): 92±4 and 83±1 nM, respectively). When these peptides probed 61 human secretin receptor cysteine-replacement mutants, a broad network of receptor residues could form disulfide bonds consistent with a dynamic ligand-receptor interface. Two distinct patterns of disulfide bond formation were observed: Cys(2) predominantly labeled residues in the amino terminus of ECL2 and ECL3 (relative labeling intensity: Ser(340), 94±7%; Pro(341), 84±9%; Phe(258), 73±5%; Trp(274) 62±8%), and Cys(5) labeled those in the carboxyl terminus of ECL2 and ECL3 (Gln(348), 100%; Ile(347), 73±12%; Glu(342), 59±10%; Phe(351), 58±11%). These constraints were utilized in molecular modeling, providing improved understanding of the structure of the transmembrane bundle and interconnecting loops, the orientation between receptor domains, and the molecular basis of ligand docking. Key spatial approximations between peptide and receptor predicted by this model (H(1)-W(274), D(3)-N(268), G(4)-F(258)) were supported by mutagenesis and residue-residue complementation studies.

Molecular basis of secretin docking to its intact receptor using multiple photolabile probes distributed throughout the pharmacophore

The molecular basis of ligand binding and activation of family B G protein-coupled receptors is not yet clear due to the lack of insight into the structure of intact receptors. Although NMR and crystal structures of amino-terminal domains of several family members support consistency in general structural motifs that include a peptide-binding cleft, there are variations in the details of docking of the carboxyl terminus of peptide ligands within this cleft, and there is no information about siting of the amino terminus of these peptides. There are also no empirical data to orient the receptor amino terminus relative to the core helical bundle domain. Here, we prepared a series of five new probes, incorporating photolabile moieties into positions 2, 15, 20, 24, and 25 of full agonist secretin analogues. Each bound specifically to the receptor and covalently labeled single distinct receptor residues. Peptide mapping of labeled wild-type and mutant receptors identified that the position 15, 20, and 25 probes labeled residues within the distal amino terminus of the receptor, whereas the position 24 probe labeled the amino terminus adjacent to TM1. Of note, the position 2 probe labeled a residue within the first extracellular loop of the receptor, a region not previously labeled, providing an important new constraint for docking the amino-terminal region of secretin to its receptor core. These additional experimentally derived constraints help to refine our understanding of the structure of the secretin-intact receptor complex and provide new insights into understanding the molecular mechanism for activation of family B G protein-coupled receptors.

Triazole-linked reduced amide isosteres: an approach for the fragment-based drug discovery of anti-Alzheimer's BACE1 inhibitors

In the course of a β-site APP-cleaving enzyme 1 (BACE1) inhibitor discovery project an in situ synthesis/screening protocol was employed to prepare 120 triazole-linked reduced amide isostere inhibitors. Among these compounds, four showed modest (single digit micromolar) BACE1 inhibition. Our ligand design was based on a potent reduced amide isostere 1, wherein the P(2) amide moiety was replaced with an anti-1,2,3-triazole unit. Unfortunately, this replacement resulted in a 1000-fold decrease in potency. Docking studies of triazole-linked reduced amide isostere A3Z10 and potent oxadiazole-linked tertiary carbinamine 2a with BACE1 suggests that the docking poses of A3Z10 and 2a in the active sites are quite similar, with one exception. In the docked structures the placement of the protonated amine that engages D228 differs considerably between 2a and A3Z10. This difference could account for the lower BACE1 inhibition potency of A3Z10 and related compounds relative to 2a.

Refinement of glucagon-like peptide 1 docking to its intact receptor using mid-region photolabile probes and molecular modeling

The glucagon-like peptide 1 (GLP1) receptor is an important drug target within the B family of G protein-coupled receptors. Its natural agonist ligand, GLP1, has incretin-like actions and the receptor is a recognized target for management of type 2 diabetes mellitus. Despite recent solution of the structure of the amino terminus of the GLP1 receptor and several close family members, the molecular basis for GLP1 binding to and activation of the intact receptor remains unclear. We previously demonstrated molecular approximations between amino- and carboxyl-terminal residues of GLP1 and its receptor. In this work, we study spatial approximations with the mid-region of this peptide to gain insights into the orientation of the intact receptor and the ligand-receptor complex. We have prepared two new photolabile probes incorporating a p-benzoyl-l-phenylalanine into positions 16 and 20 of GLP1(7-36). Both probes bound to the GLP1 receptor specifically and with high affinity. These were each fully efficacious agonists, stimulating cAMP accumulation in receptor-bearing CHO cells in a concentration-dependent manner. Each probe specifically labeled a single receptor site. Protease cleavage and radiochemical sequencing identified receptor residue Leu(141) above transmembrane segment one as its site of labeling for the position 16 probe, whereas the position 20 probe labeled receptor residue Trp(297) within the second extracellular loop. Establishing ligand residue approximation with this loop region is unique among family members and may help to orient the receptor amino-terminal domain relative to its helical bundle region.

Secretin occupies a single protomer of the homodimeric secretin receptor complex: insights from photoaffinity labeling studies using dual sites of covalent attachment

The secretin receptor, a prototypic family B G protein-coupled receptor, forms a constitutive homodimeric complex that is stable even in the presence of hormone. Recently, a model of this agonist-bound receptor was built based on high resolution structures reported for amino-terminal domains of other family members. Although this model provided the best solution for all extant data, including 10 photoaffinity labeling constraints, a new such constraint now obtained with a position 16 photolabile probe was inconsistent with this model. As the secretin receptor forms constitutive homodimers, we explored whether secretin might dock across both protomers of the complex, an observation that could also contribute to the negative cooperativity observed. To directly explore this, we prepared six secretin analogue probes that simultaneously incorporated two photolabile benzoylphenylalanines as sites of covalent attachment, in positions known to label distinct receptor subdomains. Each bifunctional probe was a full agonist that labeled the receptor specifically and saturably, with electrophoretic migration consistent with labeling a single protomer of the homodimeric secretin receptor. No band representing radiolabeled receptor dimer was observed with any bifunctional probe. The labeled monomeric receptor bands were cleaved with cyanogen bromide to demonstrate that both of the photolabile benzoylphenylalanines within a single probe had established covalent adducts with a single receptor in the complex. These data are consistent with a model of secretin occupying a single secretin receptor protomer within the homodimeric receptor complex. A new molecular model accommodating all constraints is now proposed.

Complete mapping of the stereochemical course of retentive deprotonation/alkylation of 1H-benzo[e][1,4]diazepin-2(3H)-ones

Enantiopure amino-acid derived 1H-benzo[e][1,4]diazepin-2(3H)-ones (BZDs) undergo highly retentive deprotonation/alkylation reactions. To confirm the role of stereolabile, axially chiral intermediates in these reactions and to determine the precise stereochemical course of deprotonation and alkylation, a new alanine-derived BZD (S)-1d was prepared. Because of slow diazepine ring inversion of the C3-alkylated derivatives of 1d, it proved possible to determine that electrophiles react at the concave face of the enolate derived from 1d. Furthermore, an enantiopure silyl enol ether derivative of (S)-1d was prepared and characterized by X-ray crystallography, confirming that deprotonation resulted in an (M)-configured axially chiral enolate. Activation parameters for diazepine ring inversion in the potassium enolate of 1d were determined experimentally and are well-matched by density functional calculations. Finally, the factors leading to concave-face alkylation of the enolate derived from 1d are analyzed based on calculated alkylation transition structures. Minimization of torsional effects at the BZD ring fusion and maximization of imine and amide resonance are proposed to favor concave-face alkylation.

Elucidation of the molecular basis of cholecystokinin Peptide docking to its receptor using site-specific intrinsic photoaffinity labeling and molecular modeling

G protein-coupled receptors represent the largest family of receptors and the major target of current drug development efforts. Understanding of the mechanisms of ligand binding and activation of these receptors remains limited, despite recent advances in structural determination of family members. This work focuses on the use of photoaffinity labeling and molecular modeling to elucidate the structural basis of binding a natural peptide ligand to a family A G protein-coupled receptor, the type 1 cholecystokinin receptor. Two photolabile cholecystokinin analogues were developed and characterized as representing high-affinity, fully biologically active probes with sites of covalent attachment at positions 28 and 31. The sites of receptor labeling were identified by purification, proteolytic peptide mapping, and radiochemical sequencing of labeled wild-type and mutant cholecystokinin receptors. The position 28 probe labeled second extracellular loop residue Leu(199), while the position 31 probe labeled first extracellular loop residue Phe(107). Along with five additional spatial approximation constraints coming from previous photoaffinity labeling studies and 12 distance restraints from fluorescence resonance energy transfer studies, these were built into two homology models of the cholecystokinin receptor, based on the recent crystal structures of the beta2-adrenergic receptor and A2a-adenosine receptor. The resultant agonist ligand-occupied receptor models fully accommodate all existing experimental data and represent the best refined models of a peptide hormone receptor in this important family.

Functional importance of a structurally distinct homodimeric complex of the family B G protein-coupled secretin receptor

Oligomerization of G protein-coupled receptors has been described, but its structural basis and functional importance have been inconsistent. Here, we demonstrate that the agonist occupied wild-type secretin receptor is predominantly in a guanine nucleotide-sensitive high-affinity state and exhibits negative cooperativity, whereas the monomeric receptor is primarily in a guanine nucleotide-insensitive lower affinity state. We previously demonstrated constitutive homodimerization of this receptor through the lipid-exposed face of transmembrane (TM) IV. We now use cysteine-scanning mutagenesis of 14 TM IV residues, bioluminescence resonance energy transfer (BRET), and functional analysis to map spatial approximations and functional importance of specific residues in this complex. All, except for three helix-facing mutants, trafficked to the cell surface, where secretin was shown to bind and elicit cAMP production. Cells expressing complementary-tagged receptors were treated with cuprous phenanthroline to establish disulfide bonds between spatially approximated cysteines. BRET was measured as an indication of receptor oligomerization and was repeated after competitive disruption of oligomers with TM IV peptide to distinguish covalent from noncovalent associations. Although all constructs generated a significant BRET signal, this was disrupted by peptide in all except for single-site mutants replacing five residues with cysteine. Of these, covalent stabilization of receptor homodimers through positions of Gly(243), Ile(247), and Ala(250) resulted in a GTP-sensitive high-affinity state of the receptor, whereas the same procedure with Ala(246) and Phe(240) mutants resulted in a GTP-insensitive lower affinity state. We propose the existence of a functionally important, structurally specific high-affinity dimeric state of the secretin receptor, which may be typical of family B G protein-coupled receptors.

Spatial approximation between secretin residue five and the third extracellular loop of its receptor provides new insight into the molecular basis of natural agonist binding

The amino terminus of class II G protein-coupled receptors plays an important role in ligand binding and receptor activation. Understanding of the conformation of the amino-terminal domain of these receptors has been substantially advanced with the solution of nuclear magnetic resonance and crystal structures of this region of receptors for corticotrophin-releasing factor, pituitary adenylate cyclase-activating polypeptide, and gastric inhibitory polypeptide. However, the orientation of the amino terminus relative to the receptor core and how the receptor gets activated upon ligand binding remain unclear. In this work, we have used photoaffinity labeling to identify a critical spatial approximation between residue five of secretin and a residue within the proposed third extracellular loop of the secretin receptor. This was achieved by purification, deglycosylation, cyanogen bromide cleavage, and sequencing of labeled wild-type and mutant secretin receptors. This constraint has been used to refine our evolving molecular model of secretin docked at the intact receptor, which for the first time includes refined helical bundle and loop regions and reflects a peptide-binding groove within the receptor amino terminus that directs the amino terminus of the peptide toward the receptor body. This model is fully consistent with the endogenous agonist mechanism for class II G protein-coupled receptor activation, where ligand binding promotes the interaction of a portion of the receptor amino terminus with the receptor body to activate it.

Towards a species-selective acetylcholinesterase inhibitor to control the mosquito vector of malaria, Anopheles gambiae

Anopheles gambiae is the major mosquito vector of malaria in sub-Saharan Africa. At present, insecticide-treated nets (ITNs) impregnated with pyrethroid insecticides are widely used in malaria-endemic regions to reduce infection; however the emergence of pyrethroid-resistant mosquitoes has significantly reduced the effectiveness of the pyrethroid ITNs. An acetylcholinesterase (AChE) inhibitor that is potent for An. gambiae but weakly potent for the human enzyme could potentially be safely deployed on a new class of ITNs. In this paper we provide a preliminary pharmacological characterization of An. gambiae AChE, discuss structural features of An. gambiae and human AChE that could lead to selective inhibition, and describe compounds with 130-fold selectivity for inhibition of An. gambiae AChE relative to human AChE.

Fluorescence Resonance Energy Transfer Analysis of Secretin Docking to Its Receptor

Full structural characterization of G protein-coupled receptors has been limited to rhodopsin, with its uniquely stable structure and ability to be crystallized. For other members of this important superfamily, direct structural insights have been limited to NMR structures of soluble domains. Two members of the Class II family have recently had the structures of their isolated amino-terminal regions solved by NMR, yet it remains unclear how that domain is aligned with the heptahelical transmembrane bundle domain of those receptors. Indeed, three distinct orientations have been suggested for different members of this family. In the current work, we have utilized fluorescence resonance energy transfer to establish the distances between four residues distributed throughout fully biologically active, high affinity analogues of secretin and distinct residues in each of four extracellular regions of the intact secretin receptor. These 16 distance constraints were utilized along with nine photoaffinity labeling spatial approximation constraints to study the three proposed orientations of the peptide-binding amino terminus and helical bundle domains of this receptor. In the best model, the carboxyl terminus of secretin was found to bind in a groove above the beta-hairpin region of the receptor amino terminus, with its amino-terminal end adjacent to the third extracellular loop and top of transmembrane segment VI. This refined model of the intact receptor was also fully consistent with the spatial approximation of the Trp(48)-Asp(49)-Asn(50) endogenous agonist segment with the third extracellular loop region that it has been shown to photolabel. This provides strong evidence for the orientation of peptide-binding and signaling domains of a prototypic Class II G protein-coupled receptor.

Molecular Approximations between Residues 21 and 23 of Secretin and Its Receptor: Development of a Model for Peptide Docking with the Amino Terminus of the Secretin Receptor

The structurally unique amino-terminal domain of class II G protein-coupled receptors is critically important for ligand binding and receptor activation. Understanding the precise role it plays requires detailed insights into the molecular basis of its ligand interactions and the conformation of the ligand-receptor complex. In this work, we used two high-affinity, full-agonist, secretin-like photolabile probes having sites for covalent attachment in positions 21 and 23 and used sequential proteolysis and sequencing of the labeled region of the receptor to identify two new spatial approximation constraints. The position 21 probe labeled receptor residue Arg(15), whereas the position 23 probe labeled receptor residue Arg(21). A homology model of the amino-terminal domain of the secretin receptor was developed using the NMR structure of the analogous domain of the corticotropin-releasing factor receptor. This was attached to a homology model of the secretin receptor transmembrane bundle, with the two domains oriented relative to each other based on continuity of the peptide backbone and by imposing a distance restraint recently identified between the amino-terminal WDN sequence and the region of the helical bundle above transmembrane segment six. Secretin was docked to this model using seven sets of spatial approximation constraints identified in previous photoaffinity labeling studies. This model was found to fully accommodate all existing constraints, as well as the two new approximations identified in this work.

Experimental and Computational Studies of Ring Inversion of 1,4-Benzodiazepin-2-ones: Implications for Memory of Chirality Transformations

We recently reported the enantioselective syntheses of quaternary 1,4-benzodiazepin-2-ones via memory of chirality. The success of this method depends on formation of conformationally chiral enolates that racemize very slowly under the reaction conditions. As a prelude to undertaking experimental and computational studies on the racemization of these enolates, we have studied the ring-inversion process of the parent 1,4-benzodiazepin-2-ones. In this paper, we use dynamic and 2D-EXSY NMR to characterize inversion barriers. Using DFT calculations, we reproduce the experimental results with high accuracy (within 1-2 kcal/mol). Structural parameters obtained from DFT calculations provide valuable insights into the important effect of the N1 substituent on the ring-inversion barrier and shed light on the mechanism of the memory of chirality method. These measurements and calculations provide a foundation for future studies of benzodiazepine enolates and will be valuable in the design of new memory of chirality reactions.

Enantioselective synthesis of "quaternary" 1,4-benzodiazepin-2-one scaffolds via memory of chirality

Glycine-derived 1,4-benzodiazepine-2-ones such as diazepam are chiral by virtue of the boat-shaped conformation of the diazepine ring and exist as a racemic mixture of conformational enantiomers. However, the presence of a chiral center at C-3 of the benzodiazepine perturbs this equilibrium and preferentially stabilizes one ring conformer. We report that N-i-Pr 1,4-benzodiazepine-2-ones derived from (S)-Ala and (S)-Phe can be deprotonated and alkylated in 86-99% ee, despite the fact that the original chiral center is destroyed in the deprotonation step. We attribute this highly enantioselective alkylation to the chiral memory of the benzodiazepine ring. This protocol provides easy access to the previously unexplored "quaternary" 1,4-benzodiazepine-2-ones.

Catalytic asymmetric synthesis of protected tryptophan regioisomers

Tryptophan 1 (Trp) is superior to all other naturally occurring peptide residues in its ability to bind cations (the cation-pi interaction). In an effort to expand the toolbox of Trp-like amino acids, in this note we report catalytic asymmetric syntheses of Trp regioisomers 2a-e, where the alanine unit is attached not to C-3 of indole but to C-2, C-4, C-5, C-6, or C-7. Excellent asymmetric induction is obtained in each case (generally >97% ee). Ab initio calculations suggest that the indole nuclei of 2a-e will bind Na(+) with the same affinity as that of Trp.