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An i-i+4 or i-i+3 bimane-containing linker was introduced into a peptide known to target Estrogen Receptor alpha (ERα), in order to stabilise an α-helical geometry. These macrocycles were studied by CD and NMR to reveal the i-i+4 constrained peptide adopts a 310 -helical structure in solution, and an α-helical conformation on interaction with the ERα coactivator recruitment surface in silico. An acyclic bimane-modified peptide is also helical, when it includes a tryptophan or tyrosine residue; but is significantly less helical with a phenylalanine or alanine residue, which indicates such a bimane modification influences peptide structure in a sequence dependent manner. The fluorescence intensity of the bimane appears influenced by peptide conformation, where helical peptides displayed a fluorescence increase when TFE was added to phosphate buffer, compared to a decrease for less helical peptides. This study presents the bimane as a useful modification to influence peptide structure as an acyclic peptide modification, or as a side-chain constraint to give a macrocycle.

A series of dipeptide aldehydes containing different N-terminal heterocycles was prepared and assayed in vitro against α-chymotrypsin to ascertain the importance of the heterocycle in maintaining a β-strand geometry while also providing a hydrogen bond donor equivalent to the backbone amide nitrogen of the surrogate amino acid. The dipeptide containing a pyrrole constraint (10) was the most potent inhibitor, with >30-fold improved activity over dipeptides which lacked a nitrogen hydrogen bond donor (namely thiophene 11, furan 12 and pyridine 13). Molecular docking studies of 10 bound to α-chymotrypsin demonstrates a hydrogen bond between the pyrrole nitrogen donor and the backbone carbonyl of Gly216 located in the S3 pocket which is proposed to be critical for overall binding.

Glycosylated human leukocyte elastase (HLE) was crystallized and structurally analysed in complex with a 1,3-thiazolidine-2,4-dione derivative that had been identified as an HLE inhibitor in preliminary studies. In contrast to previously described HLE structures with small-molecule inhibitors, in this structure the inhibitor does not bind to the S1 and S2 substrate-recognition sites; rather, this is the first HLE structure with a synthetic inhibitor in which the S2′ site is blocked that normally binds the second side chain at the C-terminal side of the scissile peptide bond in a substrate protein. The inhibitor also induces the formation of crystalline HLE dimers that block access to the active sites and that are also predicted to be stable in solution. Neither such HLE dimers nor the corresponding crystal packing have been observed in previous HLE crystal structures. This novel crystalline environment contributes to the observation that comparatively large parts of the N-glycan chains of HLE are defined by electron density. The final HLE structure contains the largest structurally defined carbohydrate trees among currently available HLE structures.

Electrochemical studies are reported on a series of peptides to determine the influence of different side-chains and backbone rigidity on electron transfer, to progress the field of molecular electronics. Specifically, these peptides share either a common helical or β-strand conformation to cover a range of secondary structures, to fully investigate the influence of backbone rigidity. Two types of side-chain tethers, either triazole-containing or alkene-containing, are also compared to investigate these effects on electron transfer. Our results showed that the observed formal potentials (Eo) and electron transfer rate constants (ket) fall into two distinct groups. The peptides constrained via a side-chain tether exhibited high formal potentials and low electron transfer rate constants, whereas the linear peptides displayed low formal potentials and high electron transfer rate constants. This was found to occur irrespective of the backbone conformation, or the nature of the side-chain constraint. The vast formal potential shifts (as much as 482 mV) and the large disparity in the electron transfer rate constants (as much as 97%) between the constrained and linear peptides, provides two distinct states (i.e. on/off) with a sizeable differential, which is ideal for the design of molecular switches.

Here we describe an NMR and X-ray crystallography-based characterisation of the mechanism by which a new class of macrocyclic peptidomimetic aldehyde inhibits α-chymotrypsin. In particular, a (13)C-labelled analogue of the inhibitor was prepared and used in NMR experiments to confirm formation of a hemiacetal intermediate on binding with α-chymotrypsin. Analysis of an X-ray crystallographic structure in complex with α-chymotrypsin reveals that the backbone adopts a stable β-strand conformation as per its design. Binding is further stabilised by interaction with the oxyanion hole near the S1 subsite and multiple hydrogen bonds.

Constrained α-helical peptides are showing potential as biological probes and therapeutic agents that target protein-protein interactions. However, the factors that determine the optimal constraint locations are still largely unknown. Using the β-integrin/talin protein interaction as a model system, we examine the effect of constraint location on helical conformation, as well as binding affinity, using circular dichroism and NMR spectroscopy. Stapling increased the overall helical content of each integrin-based peptide tested. However, NMR analysis revealed that different regions within the peptide are stabilised, depending on constraint location, and that these differences correlate with the changes observed in talin binding mode and affinity. In addition, we show that examination of the atomic structure of the parent peptide provides insight into the appropriate placement of helical constraints.

Gramicidin S is a naturally occurring antimicrobial cyclic peptide. Herein, we present a series of cyclic peptides based on gramicidin S that contain an azobenzene photoswitch to reversibly control secondary structure and, hence, antimicrobial activity. H-1 NMR spectroscopy and density functional theory calculations revealed a beta-sheet/beta-turn secondary structure for the cis configuration of each peptide, and an ill-defined conformation for all associated trans structures. The cis-enriched and trans-enriched photostationary states (PSSs) for peptides 1-3 were assayed against Staphylococcus aureus to reveal a clear relationship between well-defined secondary structure, amphiphilicity and optimal antimicrobial activity. Most notably, peptides 2 a and 2 b exhibited a fourfold difference in antimicrobial activity in the cis-enriched PSS over the trans-enriched equivalent. This photopharmacological approach allows antimicrobial activity to be regulated through photochemical control of the azobenzene photoswitch, thereby opening new avenues in the design and synthesis of future antibiotics. Refereed/Peer-reviewed

Here the synthesis and characterization of a new class of spiropyran-based protease inhibitor is reported that can be reversibly photoswitched between an active spiropyran (SP) isomer and a less active merocyanine (MC) isomer upon irradiation with UV and visible light, respectively, both in solution and on a surface of a microstructured optical fiber (MOF). The most potent inhibitor in the series (SP-3 b) has a C-terminal phenylalanyl-based α-ketoester group and inhibits α-chymotrypsin with a Ki of 115 nM. An analogue containing a C-terminal Weinreb amide (SP-2 d) demonstrated excellent stability and photoswitching in solution and was attached to the surface of a MOF. The SP isomer of Weinreb amide 2 d is a competitive reversible inhibitor in solution and also on fiber, while the corresponding MC isomer was significantly less active in both media. The ability of this new class of spiropyran-based protease inhibitor to modulate enzyme activity on a MOF paves the way for sensing applications.

Peroxisome proliferator-activated receptor γ (PPARγ) is a nuclear receptor central to fatty acid and glucose homeostasis. PPARγ is the molecular target for type 2 diabetes mellitus (T2DM) therapeutics TZDs (thiazolidinediones), full agonists of PPARγ with robust antidiabetic properties, which are confounded with significant side effects. Partial agonists of PPARγ, such as INT131 (1), have displayed similar insulin-sensitizing efficacy as TZDs, but lack many side effects. To probe the structure-activity relationship (SAR) of the scaffold 1, we synthesized 14 analogs of compound 1 which revealed compounds with higher transcriptional potency for PPARγ and identification of moieties of the scaffold 1 key to high transcriptional potency. The sulfonamide linker is critical to activity, substitutions at position 4 of the benzene ring A were associated with higher transcriptional activity, substitutions at position 2 aided in tighter packing and activity, and the ring type and size of ring A affected the degree of activity.

A series of azobenzene-containing peptidic boronate esters was prepared and the activity of the thermally adapted states (TAS), enriched in trans isomer, and the photostationary states (PSS), enriched in cis isomer, for each compound were evaluated against β5 and β1 proteasome subunits. Compounds with a sterically demanding phenyl-substituted azobenzene at P2 (4c), and a less sterically demanding unsubstituted azobenzene at the N-terminus (5a), showed the greatest difference in activity between the two states. In both cases, the more active trans-enriched TAS had activity comparable to bortezomib and delanzomib. Furthermore, cis-enriched 4c inhibited tumor growth in both breast and colorectal carcinoma cell lines. Significantly, the initial trans-enriched TAS of 4c was not cytotoxic against the non-malignant MCF-10A cells.

The onset of new multidrug-resistant strains of bacteria demands continuous development of antibacterial agents with new chemical scaffolds and mechanisms of action. We present the first structure–activity relationship (SAR) study of 16 derivatives of a structurally novel antibiotic merochlorin A that were designed using a biosynthetic blueprint. Our lead compounds are active against several Gram-positive bacteria such as Staphylococcus aureus (SA), methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus faecium (VRE) and Bacillus subtilis, inhibit intracellular growth of Mycobacterium bovis, and are relatively nontoxic to human cell lines. Furthermore, derivative 12 c {(±)-(3aR,4S,5R,10bS)-5-bromo-7,9-dimethoxy-4-methyl-4-(4-methylpent-3-en-1-yl)-2-(propan-2-ylidene)-1,2,3,3a,4,5-hexahydro-6H-5,10b-methanobenzo[e]azulene-6,11-dione} was found to inhibit the growth of Bacillus Calmette–Guérin (BCG)-infected cells at concentrations similar to rifampicin. These results outperform the natural product, underscoring the potential of merochlorin analogues as a new class of antibiotics. Refereed/Peer-reviewed

Inhibitors of Staphylococcus aureus biotin protein ligase (SaBPL) are generated by replacing the acyl phosphate group of biotinyl-5′-AMP with either a 1,2,3-triazole (see 5/10a/10b) or a 1,2,4-oxadiazole (see 7) bioisostere. Importantly, the inhibitors are inactive against the human BPL. The nature of the 5-substituent in the component benzoxazolone of the optimum 1,2,3-triazole series is critical to activity, where this group binds in the ATP binding pocket of the enzyme. Refereed/Peer-reviewed

Electrochemical studies are reported on a series of peptides constrained into either a 310-helix (1-6) or β-strand (7-9) conformation, with variable numbers of electron rich alkene containing side chains. Peptides (1 and 2) and (7 and 8) are further constrained into these geometries with a suitable side chain tether introduced by ring closing metathesis (RCM). Peptides 1, 4 and 5, each containing a single alkene side chain reveal a direct link between backbone rigidity and electron transfer, in isolation from any effects due to the electronic properties of the electron rich side-chains. Further studies on the linear peptides 3-6 confirm the ability of the alkene to facilitate electron transfer through the peptide. A comparison of the electrochemical data for the unsaturated tethered peptides (1 and 7) and saturated tethered peptides (2 and 8) reveals an interplay between backbone rigidity and effects arising from the electron rich alkene side-chains on electron transfer. Theoretical calculations on β-strand models analogous to 7, 8 and 9 provide further insights into the relative roles of backbone rigidity and electron rich side-chains on intramolecular electron transfer. Furthermore, electron population analysis confirms the role of the alkene as a "stepping stone" for electron transfer. These findings provide a new approach for fine-tuning the electronic properties of peptides by controlling backbone rigidity, and through the inclusion of electron rich side-chains. This allows for manipulation of energy barriers and hence conductance in peptides, a crucial step in the design and fabrication of molecular-based electronic devices.

There is a real need for simple structures that define a β-strand conformation, a secondary structure that is central to peptide-protein interactions. For example, protease substrates and inhibitors almost universally adopt this geometry on active site binding. A planar pyrrole is used to replace two amino acids of a peptide backbone to generate a simple macrocycle that retains the required geometry for active site binding. The resulting β-strand templates have reduced peptide character and provide potent protease inhibitors with the attachment of an appropriate amino aldehyde to the C-terminus. Picomolar inhibitors of cathepsin L and S are reported and the mode of binding of one example to the model protease chymotrypsin is defined by X-ray crystallography.

Calpain inhibitors are possible therapeutic agents in the treatment of cataracts. These covalent inhibitors contain an electrophilic anchor (“warhead”), an aldehyde that reacts with the active site cysteine. Whilst high throughput docking of such ligands into high resolution protein structures (e.g. calpain) is a standard computational approach in drug discovery, there is no docking program that consistently achieves low rates of both false positives (FPs) and negatives (FNs) for ligands that react covalently (via irreversible interactions) with the target protein. Schroedinger’s GLIDE score, widely used to screen ligand libraries, is known to give high false classification, however a two-level Self Organizing Map (SOM) artificial neural network (ANN) algorithm, with KM clustering proved that the addition of two structural components of the calpain molecule, number hydrogen bonds and warhead distance, combined with GLIDE score (or its partial energy subcomponents) provide a superior predictor set for classification of true molecular binding strength (IC50). SOM ANN/KM significantly reduced the number of FNs by 64 % and FPs by 26 %, compared to the glide score alone. FPs were shown to be mostly esters and amides plus alcohols and non-classical, and FNs mainly aldehydes and ketones, masked aldehydes and ketones and Michael.

Experimental and theoretical data indicate that, for α-fluoroamides, the F-C-C(O)-N(H) moiety adopts an antiperiplanar conformation. In addition, a gauche conformation is favoured between the vicinal C-F and C-N(CO) bonds in N-β-fluoroethylamides. This study details the synthesis of a series of fluorinated β-peptides (1-8) designed to use these stereoelectronic effects to control the conformation of β-peptide bonds. X-ray crystal structures of these compounds revealed the expected conformations: with fluorine β to a nitrogen adopting a gauche conformation, and fluorine α to a C=O group adopting an antiperiplanar conformation. Thus, the strategic placement of fluorine can control the conformation of a β-peptide bond, with the possibility of directing the secondary structures of β-peptides.

New peptidic templates constrained into a β-strand geometry by linking acetylene and azide containing P(1) and P(3) residues of a tripeptide by Huisgen cycloaddition are presented. The conformations of the macrocycles are defined by NMR studies and those that best define a β-strand are shown to be potent inhibitors of the protease calpain. The β-strand templates presented and defined here are prepared under optimized conditions that should be suitable for targeting a range of proteases and other applications requiring such a geometry.

The central nervous system glutamate receptors are an important target for drug discovery. Herein we report initial investigations into the synthesis and glutamate receptor activity of 1,2,3-triazolyl amino acids. Two compounds were found to be selective AMPA receptor ligands, which warrant further investigation.

Victoria Peddie, Markus Pietsch, Karen M. Bromfield, Robert N. Pike, Peter J. Duggan, Andrew D. Abell

The dimerization and trimerization of the Dmt-Tic, Dmt-Aia and Dmt-Aba pharmacophores provided multiple ligands which were evaluated in vitro for opioid receptor binding and functional activity. Whereas the Tic- and Aba multimers proved to be dual and balanced delta/mu antagonists, as determined by the functional [S(35)]GTPgammaS binding assay, the dimerization of potent Aia-based 'parent' ligands unexpectedly resulted in substantial less efficient receptor binding and non-active dimeric compounds.

The physiological roles of calpains are discussed, as are the associated pathological disorders that result from their over-activation. We also present practical information for establishing functional inhibition assays and an overview of X-ray crystal structures of calpain-inhibitor complexes to aid inhibitor design. These structures reveal the expected extended beta-strand conformation for the inhibitor backbone, a geometry that has been engineered into inhibitors with the introduction of either an N-terminal heterocycle or a macrocycle that links the P(1) and P(3) residues. The structure and function of all the main classes of inhibitors are reviewed, with most examples being classified according to the nature of the C-terminal reactive warhead group that reacts with the active site cysteine of calpains. These inhibitor classes include epoxysuccinate derivatives, aldehydes, aldehyde prodrugs (hemiacetals) and alpha-keto carbonyl compounds. Inhibitors derived from the endogenous inhibitor calpastatin and examples lacking a warhead, are now known and these are also discussed.

Bridging between (i)- and (i+3)-positions in a β3-peptide with a tether of appropriate length is expected to prevent the corresponding 314-helix from unfolding (Fig. 1). The β3-peptide H-β3hVal-β3hLys-β3hSer(All)-β3hPhe-β3hGlu-β3hSer(All)-β3hTyr-β3hIle-OH (1; with allylated βhSer residues in 3- and 6-position), and three tethered β-peptides 2–4 (related to 1 through ring-closing metathesis) have been synthesized (solid-phase coupling, Fmoc strategy, on chlorotrityl resin; Scheme). A comparative CD analysis of the tethered β-peptide 4 and its non-tethered analogue 1 suggests that helical propensity is significantly enhanced (threefold CD intensity) by a (CH2)4 linker between the β3hSer side chains (Fig. 2). This conclusion is based on the premise that the intensity of the negative Cotton effect near 215 nm in the CD spectra of β3-peptides represents a measure of ‘helical content’. An NMR analysis in CD3OH of the two β3-octapeptide derivatives without (i.e., 1) and with tether (i.e., 4; Tables 1–6, and Figs. 4 and 5) provided structures of a degree of precision (by including the complete set of side chain–side chain and side chain–backbone NOEs) which is unrivaled in β-peptide NMR-solution-structure determination. Comparison of the two structures (Fig. 5) reveals small differences in side-chain arrangements (separate bundles of the ten lowest-energy structures of 1 and 4, Fig. 5, A and B) with little deviation between the two backbones (superposition of all structures of 1 and 4, Fig. 5, C). Thus, the incorporation of a CH2O(CH2)4OCH2 linker between the backbone of the β3-amino acids in 3- and 6-position (as in 4) does accurately constrain the peptide into a 314-helix. The NMR analysis, however, does not suggest an increase in the population of a 314-helical backbone conformation by this linkage. Possible reasons for the discrepancy between the conclusion from the CD spectra and from the NMR analysis are discussed.

The electrochemistry of self-assembled monolayers (SAMs) on gold containing a lipoic acid linker, the beta-peptide sequence (beta(3)Val-beta(3)Ala-beta(3)Leu)(n) for n = 1, 2, and a terminal ferrocenyl group has been described for the first time. Circular dichroism (CD), NMR, and molecular modeling were used to evaluate the beta-peptide structure in solution, while the monolayer film organization and electron-transfer kinetics were evaluated by cyclic voltammetry, chronoamperometry (CA), and ellipsometry. The peptides were assembled from trifluoroethanol solutions, where they are linear (n = 1) or helical (n = 2) based on CD, NMR, ellipsometry, and modeling evidence. The structure of the SAMs is less well understood. There is evidence for noncompact layers that allow electrolyte ions to approach the interface. Electron-transfer rates for n = 1, 2 were found to be 2500 and 1200 s(-1), respectively, and CA evidence indicated that the transfer is based on the hopping mechanism.

A series of peptidomimetic trifluoromethylketones containing the photoisomerisable azobenzene group have been synthesised, photoisomerised and assayed against the serine protease alpha-chymotrypsin. All are inhibitors of the enzyme and exhibit up to a fivefold increase in activity on UV irradiation. Visible irradiation returns activity to close to that of the original sample. These results suggest the trifluoromethylketone group to be the best electrophilic enzyme binding group for use in photoswitch inhibitors of alpha-chymotrypsin.

Olefin tethers of variable length, introduced into a natural amino acid (side-chain of Ser, Cys; N-terminus of Arg; C-terminus of Phe and Tic; and in both the side-chain and either the N- or C-terminus of Ser, Cys and Tyr), undergo metathesis on treatment with Grubbs' second generation catalyst. Side-chain linked dimers of Ser, Cys and Tyr were obtained by cross-metathesis, while olefin installation at the N- and C-terminus led to dimers of Arg and Phe (or Tic), respectively. Ring-closing metathesis of the doubly alkenylated derivatives of Ser, Cys and Tyr gave 12-, 20- and 24-membered macrocycles.

Three new compounds (3-5) based on a dipeptide non-covalent inhibitor (1) of α-chymotrypsin have been synthesised and assayed against the serine protease α-chymotrypsin. The stability of the compounds to α-chymotrypsin catalysed hydrolysis has been measured. All three compounds were inactive with a retained stability to enzyme catalysed hydrolysis.

To further study the preference of the antiperiplanar (ap) conformation in α-fluoro-amide groups, two β-peptides, 1 and 2, containing a (2-F)-β3hAla and a (2-F)-β2hPhe residue, have been synthesized. Their NMR-solution structures in CD3OH were determined and compared with those of non-F-substituted analogs, 3 and 4a. While we have found in a previous investigation (Helv. Chim. Acta2005, 88, 266) that a stereospecifically introduced F-substituent in the central position of a β-heptapeptide is capable of ‘breaking’ the 314-helical structure by enforcing the FCCO ap-conformation, we could now demonstrate that the same procedure leads to a structure with the unfavorable ca. 90° FCCO dihedral angle, enforced by the 314-helical folding in a β-tridecapeptide (cf.1; Fig. 4). This is interpreted as a consequence of cooperative folding in the longer β-peptide. A F-substituent placed in the turn section of a β-peptidic hairpin turn was shown to be in an ap-arrangement with respect to the neighboring CO bond (cf.2; Fig. 7). Analysis of the non-F-substituted β-tetrapeptides (with helix-preventing configurations of the two central β2/β3-amino acid residues) provides unusually tight hairpin structural clusters (cf.3 and 4a; Figs. 8 and 9). The skeleton of the β-tetrapeptide H-(R)β3hVal-(R)β2hVal-(R)β3hAla-(S)β3hPhe-OH (4a) is proposed as a novel, very simple backbone structure for mimicking α-peptidic hairpin turns.

Filling the pocket: Biaryl inhibitors of type II dehydroquinase are described, designed to bind to both the active site and a subsidiary pocket. Compounds exhibited nanomolar inhibition of the enzyme, which is rationalised by molecular docking and protein crystallographic studies.

4′-Substituted bicyclic pyridones were prepared and evaluated as non-steroidal inhibitors of type 1 and 2 steroid 5α-reductase (SR). A range of 4′-substituents were incorporated into the bicyclic scaffold to investigate SAR within and across different classes of non-steroidal inhibitors of SR. Bicyclic pyridones containing a 4′-benzoyl or long carbon chain tether showed more potent inhibition against type 1 SR than inhibitors with N-substituted acetamide groups in the 4′-position. SAR derived from 4′-substituted bicyclic pyridones reported here do not correlate with SAR derived from known potent 4′-substituted biaryl acid SR inhibitors. A 4′-benzoyl group is favoured by the active site in both isozymes.

A versatile synthesis of hydroxylated and epoxy 1-azepin 2-ones substituted at N1, C-3 and C-4 or C-7 has been developed. The sequence involves ring-closing metathesis of an amino acid derived diene amide, followed by either epoxidation or dihydroxylation, of the resulting alkene. Assay of the product epoxides (10, 18, 25) and diols (9a, 17, 24) against HIV protease reveals micromolar inhibition.

A series of peptidomimetic boronate esters containing the photoisomerisable azobenzene group has been synthesised and assayed against the serine protease alpha-chymotrypsin. Compounds with borophenylalanine inhibition groups were found to be inhibitors with micromolar activity, while those with aryl boronate inhibition groups were inactive. Selected compounds were isomerised by UV or visible light to obtain enriched states of the (Z) or (E) isomers, respectively, and assayed. A change in activity on photoisomerisation was observed, however some decomposition of the boronate group on irradiation was also observed, limiting reversibility.

S,3S)-3-(N-Cbz-L-Leucinyl)amino-2-hydroxy-4-phenylbutanoic acid 5 dimerised on treatment with EDCI and HOBT to give the symmetrical (1,4)dioxane-2,5-dione-based peptidomimetic 6.

Structural analogues of the arylaminobenzoate 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB), prepared using a simple reductive amination sequence, were tested for their effects on cytoplasmic streaming rates in the alga Nitella hookeri. Cytoplasmic streaming was sensitive to NPPB, with an IC50 value of 24 µmol/L. Removal of the nitro group from the benzoate ring decreased the IC50 to 455 µmol/L. The introduction of an extra carbon or double bond into the aliphatic chain had no effect on activity. Loss of the phenyl group decreased potency, with an IC50 of 6.4 mmol/L. These data are the first documenting the relative inhibitory effects of structural changes to arylaminobenzoates in algae. Patch-clamp data and the effects of tetrapentyl ammonium chloride on streaming suggest that the nitro and phenyl groups may act by inhibiting both K+ and Cl– channels. This is likely, through changes in the membrane potential, to affect Ca2+ fluxes and action potentials, thereby slowing cytoplasmic streaming.Key words: arylaminobenzoate, NPPB, cytoplasmic streaming, characian algae, ion channel blocker, reductive amination.

Two vitamin K analogues bearing a carboxylic acid side chain (9a and its deuterated analogue 9b) were each synthesised in six steps from commercially available menadione. Analogue 9b was conjugated to lysozyme and bovine serum albumin (BSA) using EDCI/HOBT and by prior formation of its activated succinimidyl ester 11. Quantification of the thus formed conjugates by ESMS and LC-MS revealed that the number of equivalents of the analogue used in the couplings systematically controls the number of analogues that conjugate to the protein.

The synthesis of vitamin K and its analogues has been an important goal since the biochemical roles of the K vitamins were elucidated. This review presents a detailed account of syntheses of natural K vitamins and analogues that contain side chain functionality.

A diastereoseletvive synthesis of a peptidic tetrahydropyridazinone 10 from (S)-phenylalanine is reported. This derivative was then converted to the bicyclic peptidomimetic 11, an important class of β-strand mimetic.

A number of routes to 3-hydroxyiminodehydroquinate 4, one of the most potent inhibitors of type II dehydroquinase that is currently known, have been investigated. Methods based on the existing literature synthesis, i.e. oxime formation of a suitably C-4 and C-5 protected methyl 3-dehydroquinate derivative were initially studied. Benzoyl protection as in 11 did give the desired product but in low overall yield. An alternative BBA protection strategy starting with 7 was successful in generating a C-4/C-5 analogue of the desired oxime 4 in high yield. Further investigation revealed that it was unnecessary to protect the dehydroquinate precursor, hence the potassium salt corresponding to oxime 4 was simply synthesised as a single isomer from methyl dehydroquinate 10.

An α-methylene tetrazole-based dipeptidomimetic (11) has been prepared as a constrained and non-hydrolysable core for incorporation into peptides. A single crystal X-ray structure determination revealed that its solid-state conformation closely resembles that of the isosteric core of JG-365 bound to HIV protease. The α-methylene tetrazole isosteric unit was then incorporated into a number of peptide sequences and the resulting compounds 6–8 were assayed against HIV protease. The assay results suggest that the longer the C-terminal substitution the greater the potency, a result that reflects the interplay of the geometry of the tetrazole isostere and the C-terminal substituent.

The design, synthesis, photoisomerism and biological testing of two peptide-based photoswitchable inhibitors of α-chymotrypsin are presented. The use of a dipeptide recognition sequence gave a ‘slow-tight binding’ inhibitor, while the introduction of a carbamate linker to the azobenzene gave a modest enhancement in photoswitching of enzyme activity for the photostationary state enriched in the ( Z )-isomer over the ( E )-isomer.

— Here we report the synthesis of a cis -dipeptide mimic N -Boc-Phe-[COCN 4 ]-Gly-OBn, 7 , containing the non-hydrolysable -keto tetrazole isostere and an unusual 2,5-disubstituted -keto tetrazole-based peptidomimetic, 8 . The incorpora-tion of the novel cis -amide bond isostere was achieved via direct alkylation of a precursor five-substituted (1 H )-tetrazole. Theassignment of the resulting 1,5- and 2,5-tetrazoyl regiomers was based on the first reported X-ray structure analysis of an -ketotetrazole, compound 8 . © 2001 Elsevier Science Ltd. All rights reserved. We have embarked on a program to develop a range ofnovel conformationally restricted amide bond isosteresthat incorporate a 1,5-disubstituted tetrazole. 1 The tet-razole ring has been shown to be an excellent mimic ofthe cis -amide bond, and as such, this structural motifcan be used to pre-organise the amide bonds of pep-tides, enzyme substrates and inhibitors into a cis -con-formation. 2 We have now extended this work with thedesign and preparation of a novel conformationallyrestricted -keto tetrazole isostere [COCN

N-Acyl and N-sulfonylhydroxymetyhylpyrroles have been synthesised and shown to inhibit alpha-chymotrypsin. A hydrophobic group in the N-substituent has been shown to be required for activity.

Heterocyclic-based peptidomimetics possess properties that are dictated by the chemical and physical properties of the constituent heterocycle. Here we review two general classes of peptidomimetic, one in which the conformation of a peptide is confined into a particular geometry, and a second, where chemical reactivity is masked and released in a controlled manner. We have used the aromatic heterocycles, pyrrole and tetrazole, to mimic the geometry of cis-peptide bonds and peptide bond isosteres. A tetrazole-based hydroxyethylamine isostere, which mimics a bioactive cis-like geometry of JG365 as bound to HIV protease, has been developed and shown to provide the basis of conformationally constrained inhibitors of this enzyme. A number of other simple tetrazole-based molecular scaffolds are also discussed. The second section of this review discusses how the inherent reactivity of a hydroxymethylpyrrole can be masked with the introduction of an electron withdrawing group on nitrogen. The selective removal of this group then releases a highly reactive species. Compounds of this type have been shown to inhibit α-chymotrypsin.

Three photoswitchable, peptidomimetic inhibitors of α-chymotrypsin have been synthesised. The compounds comprise an azobenzene, an α-ketoester and l -phenylalanine. The compounds were photoisomerised to give enriched states of the (E) and (Z) isomers and these states were assayed against α-chymotrypsin. The inhibitors were shown to be moderately active with switching ability of between two- and three-fold between the two isomer-enriched states. The behaviour of the inhibitors in solution was examined; specifically, their hydration and configurational stability.

A concise route to a bicyclo[4.2.1] ring system, based on an intramolecular Diels–Alder reaction, has been developed. Preliminary results on the interplay between Lewis acids and high pressure to facilitate the reaction are described. Products with isomerized double bonds were obtained with 1 equivalent of Lewis acid at 0°C. The stereochemistry of these compounds has been determined by X-ray crystallography. Double bond isomerization is suppressed by the use of excess Lewis acid at −78°C or high pressure.

Pyrrolidinone-based peptidomimetics, in which the peptide bond is forced to adopt either a cis or trans geometry, have been prepared from readily available amino acids. Peptidomimetics based on amino acid side chain to side chain cyclization (17), side chain to N cyclization (18), and α-H to side chain cyclization (28) have been developed. A key step in the syntheses is the treatment of a peptide-based β-keto ester or amide with an amine. The absolute configuration of compounds 28 was established using Seebach oxazolidinone chemistry.

We present an efficient synthesis of N-Z-(2SR,3S)-3-amino-2-hydroxy-4-phenylbutanoic acids by hydroxylation of an enolate derived from methyl (3S)-N-Z-3-amino-4-phenylbutanoate with oxodiperoxymolybdenum (pyridine) (hexamethyl phosphoric triamide) complex (MoOPH).

The four stereoisomers of 3-(N-acetylleucylamino)-3-benzyl-1-[(methylsulfonyl)oxy]succinimid e have been prepared and shown to inhibit alpha-chymotrypsin and human neutrophil elastase. The structural and stereochemical features that affect the potency and selectivity of inhibition are discussed.

We report a general method for the synthesis of mycalazol 11 and some related 5-acyl-2-hydroxymethylpyrroles using a Stille coupling of 5-(tri-n-butylstannyl)pyrrole-2-carboxaldehyde with an acid chloride as the key step. The newly prepared 5-acyl-2-hydroxymethylpyrroles 5–7, together with the 5-carboxamido-2-hydroxymethylpyrrole 10, have been assayed for in vitro cytotoxicity against the P388 murine leukemia cell line.

A sequence, based on a Mitsunobu displacement at the hydroxymethyl position of deuterium-labeled, N-substituted 2-(hydroxymethyl)pyrroles, is reported as a general procedure to determine the ability of the N-substituent to deactivate the heterocycle. An SN2 mechanism, in this reaction, has been shown to be favored over reaction via an azafulvenium intermediate, by employing N-(trifluoromethyl)sulfonyl as the deactivating group. Deuterium-labeling studies have demonstrated that suppression of contributions to the reaction from an azafulvenium intermediate is less effective with other deactivating groups, and the order for deactivation is triflyl > mesyl > BOC ≈ acetyl. The attachment of an electron-withdrawing group onto the nitrogen of a 2-formyl[formyl-d]pyrrole has also been shown to allow reduction to give a 2-(hydroxymethyl[methylene-d1])pyrrole of high configurational purity. The utility of the N-triflyl group to deactivate a pyrrole was demonstrated with the preparation of deuterium-labeled porphobi...

The 4-nitrophenoxy-based epoxide 6a, conveniently synthesized using Sharpless epoxidation chemistry, has been shown to be an irreversible inhibitor of HIV-1 protease (Kinact 1.8 μM, pH 6.5, I = 0.1 M). Related analogues, differing in the mode of attachment to the epoxide core, have also been prepared and assayed against HIV-1 protease.