Unusually high α-proton acidity of prolyl residues in cyclic peptides.

The acidity of the α-proton in peptides has an essential role in numerous biochemical reactions and underpins their stereochemical integrity, which is critical to their biological function. We report a detailed kinetic and computational study of the acidity of the α-proton in two cyclic peptide systems: diketopiperazine (DKP) and triketopiperazine (TKP). The kinetic acidity (protofugality) of the α-protons were determined though hydrogen deuterium exchange studies in aqueous solutions. The acidities of the α-proton in prolyl residues were increased by 3-89 fold relative to other amino acid residues (prolyl > glycyl ≫ alanyl > tyrosyl). Experimental and computational evidence for the stereoelectronic origins of this enhanced prolyl reactivity is presented. TKPs were 106-fold more reactive than their DKP analogues towards deprotonation, which we attribute to the advanced development of aromaticity in the earlier transition state for proton transfer in these cases. A Brønsted linear free energy analysis of the reaction data was conducted to provide estimates of α-proton pK as.

2,7-Diazabicyclo[2.2.1]heptanes: novel asymmetric access and controlled bridge-opening.

Organocatalysed asymmetric Michael additions of substituted triketopiperazines to enones afford products in high yield and enantiomeric ratio (er). Further modification delivers products possessing natural product (NP) scaffolds including diazabicyclo[2.2.1]heptane, prolinamide and harmicine.

Organocatalytic Stereoconvergent Synthesis of α-CF3 Amides: Triketopiperazines and Their Heterocyclic Metamorphosis.

The highly enantioselective alkylation of α-CF3 enolates, generated from triketopiperazines, has been accomplished through use of a bifunctional thiourea organocatalyst to facilitate 1,4-addition to varied enone acceptors. On treatment with appropriate nitrogen nucleophiles, the chiral triketopiperazine products undergo a metamorphosis, to provide novel fused heterocyclic lactams such as extended pyrazolopyrimidines.

First total synthesis of concavine.

The synthesis of the unusual alkaloid concavine, isolated from Clitocybe concava (Basidiomycetae), has been accomplished. The synthetic route features regio- and stereoselective manipulation of polycyclic imide intermediates via enolate substitution and Grignard addition, along with a key bridge-forming step involving a new method for sulfenylative radical cyclisation. The NMR data for synthetic concavine demonstrate that the original data reported for the natural product refer to the derived acetic acid salt, probably formed as an artefact of isolation or purification.

Rafoxanide and Closantel Inhibit SPAK and OSR1 Kinases by Binding to a Highly Conserved Allosteric Site on Their C-terminal Domains.

SPAK and OSR1 are two protein kinases that have emerged as attractive targets in the discovery of novel antihypertensive agents due to their role in regulating electrolyte balance in vivo. Herein we report the identification of an allosteric pocket on the highly conserved C-terminal domains of these two kinases, which influences their activity. We also show that some known WNK signaling inhibitors bind to this allosteric site. Using in silico screening, we identified the antiparasitic agent rafoxanide as a novel allosteric inhibitor of SPAK and OSR1. Collectively, this work will facilitate the rational design of novel SPAK and OSR1 kinase inhibitors that could be useful antihypertensive agents.

An Asymmetric Organocatalysis Approach to the Prenylated Alkaloid Family.

Michael addition of a proline-derived triketopiperazine (TKP) to ß-substituted enones and acrylamides, mediated by a cinchona alkaloid catalyst, delivers products possessing a bicyclo[2.2.2]diazaoctane structure in high yield and enantiomeric ratio (er). Further modification of the amide products toward polycyclic scaffolds resembling members of the prenylated alkaloid family is also demonstrated.

Towards the Development of Small-Molecule MO25 Binders as Potential Indirect SPAK/OSR1 Kinase Inhibitors.

The binding of the scaffolding protein MO25 to SPAK and OSR1 protein kinases, which regulate ion homeostasis, causes increases of up to 100-fold in their catalytic activity. Various animal models have shown that the inhibition of SPAK and OSR1 lowers blood pressure, and so here we present a new indirect approach to inhibiting SPAK and OSR1 kinases by targeting their protein partner MO25. To explore this approach, we developed a fluorescent polarisation assay and used it in screening of a small in-house library of ≈4000 compounds. This led to the identification of one compound-HK01-as the first small-molecule inhibitor of the MO25-dependent activation of SPAK and OSR1 in vitro. Our data confirm the feasibility of targeting this protein-protein interaction by small-molecule compounds and highlights their potential to modulate ion co-transporters and thus cellular electrolyte balance.

Highly enantioselective access to diketopiperazines via cinchona alkaloid catalyzed Michael additions.

Michael addition reactions of triketopiperazine (TKP) derivatives to enones, mediated by a cinchona alkaloid-derived catalyst, deliver products in high yield and enantiomeric ratio (er). Use of unsaturated ester, nitrile or sulfone partners gives bridged hydroxy-diketopiperazine (DKP) products resulting from a novel Michael addition-ring closure.

Bridgehead enolate or bridgehead organolithium? DFT calculations provide insights into a difficult bridgehead substitution reaction in the synthesis of the polycyclic polyprenylated acylphloroglucinol (PPAP) nemorosone.

A computational study (B3LYP), of the metallation of a bridged ketone, an important step in the synthesis of a polycyclic polyprenylated acylphloroglucinol (PPAP), nemorosone, shows three energetically distinct structural possibilities for the lithiated intermediate. These findings, along with observations of the reactivity of the intermediates in bridgehead substitutions, suggest that different intermediates may be formed depending upon the type of process used for lithiation.

The cascade radical cyclisation approach to prenylated alkaloids: synthesis of stephacidin A and notoamide B.

A strategy for the synthesis of members of the prenylated indole alkaloid family is described, which involves a radical cascade process of an appropriately substituted diketopiperazine (DKP) core structure. Several approaches to the generation of the initial radical were explored, with the most successful involving treatment of a sulfenyl substituted DKP under classical reductive conditions by heating with Bu3SnH and a radical initiator. The required, fully substituted, radical precursor DKP structures were prepared using regio- and stereocontrolled enolate chemistry of simpler proline-tryptophan derived DKPs. The new approach allowed rapid access to a key polycyclic indoline structure, which was converted into either of the natural products stephacidin A or notoamide B.

Synthesis of fumaramide derived [3]rotaxanes as potential precursors for molecular boxes.

Three new fumaramide-derived [3]rotaxanes have been synthesized, with the aim of macrocycle linking to form a molecular box. In one case, a nine-component templated [3]rotaxane synthesis was accomplished in 40% yield. Rotaxane reduction resulted in an unexpectedly facile de-slipping process.

Adventures in bridgehead substitution chemistry: synthesis of polycyclic polyprenylated acylphloroglucinols (PPAPs).

The polycyclic polyprenylated acylphloroglucinol (PPAP) family of natural products includes important compounds with notable biological activities, such as garsubellin A, hyperforin and clusianone. The synthesis of these complex, bridged, highly oxidized and substituted systems presents a formidable challenge to synthetic chemists. This feature article describes how the use of unconventional bridgehead substitution chemistry has enabled the synthesis of these natural products and their analogues.

Comparison of the cytotoxic effects of enantiopure PPAPs, including nemorosone and clusianone.

The synthesis of an unnatural polyprenylated acylphloroglucinol (PPAP), regioisomeric with nemorosone and clusianone, has been accomplished. The separated enantiomers of this new PPAP, along with those of nemorosone and clusianone, have been screened for activity against HeLa (cervix carcinoma), MIA-PaCa-2 (pancreatic carcinoma), and MCF7 (mamma carcinoma) cancer cell lines. All of the isomers examined gave surprisingly similar results in the screens.

Synthesis of the asperparaline core by a radical cascade.

A concise access to the pentacyclic core structure of the asperparalines is described. The key step is a radical cascade sequence comprised of a 1,6-hydrogen atom transfer followed by 6-exo-trig and 5-exo-trig cyclizations.

New approaches for the synthesis of erythrinan alkaloids.

A concise asymmetric total synthesis of (+)-erysotramidine is described, using chiral base desymmetrisation of a meso-imide, N-acyliminium addition, retro-Diels-Alder cycloaddition and radical cyclisation as the key steps. A related route, starting from a cyclobutene-fused imide, was explored, and established a novel construction of the Erythrina alkaloid skeleton using a key ring-opening/ring-closing metathesis step. Completion of this synthesis was thwarted by problems with the removal of an unwanted vinylic side-chain. Complementary enantiospecific routes to Erythrina systems were explored, starting from (L)-malic acid. Some unexpected observations were made concerning the diastereocontrol in malic acid-derived N-acyliminium ion cyclisations, where changing the protecting group of the alcohol function from acetate to OTIPS resulted in a dramatic change in diastereocontrol. Products from these reactions could be transformed into known intermediates for natural alkaloids, and into (+)-demethoxyerythratidinone itself, by means of radical cyclisations or intramolecular aldol reactions as the key steps.

Bridgehead lithiation-substitution of bridged ketones, lactones, lactams, and imides: experimental observations and computational insights.

The viability of bridgehead lithiation-substitution of bridged carbonyl compounds has been tested in the laboratory, and the results were rationalized with the aid of a computational study. Lithiation-substitution was found to be possible for ketones, lactones, lactams, and imides having small bridges, including examples having [3.2.1], [3.2.2], [3.3.1], [4.2.1], and [4.3.1] skeletons. Smaller systems, where the sum of the bridging atoms (S) was 5, for example [2.2.1] or [3.1.1] ketones or [2.2.1] lactams, did not undergo controlled bridgehead substitution. Ketones or lactams having a [2.2.2] structure also did not give bridgehead substitution. B3LYP calculations accurately predict this behavior with negative DeltaE(rxn) values being calculated for the successful deprotonations and positive DeltaE(rxn) values being calculated for the unsuccessful ones. NBO calculations were also performed on the anionic deprotonated species, and these show that some structures are best represented as bridgehead enolates and some are best represented as alpha-keto carbanions.

Bridgehead enolates and bridgehead alkenes in a welwistatin model series.

Bridgehead metallation is possible in a ketone having the welwistatin skeleton, and this facilitates installation of the isothiocyanate function present in the natural product, and also enables synthesis of remarkable bridgehead alkenes.

Concise enantioselective synthesis of ent-malbrancheamide B.

A concise enantioselective synthesis of the fungal metabolite ent-malbrancheamide B was accomplished through the union of a C-prenylated proline derivative and a substituted indole pyruvic acid SEM enol ether, followed by a cationic double cyclization as the key step.

Enantioselective synthesis of cyclopropylcarboxamides using s-BuLi-sparteine-mediated metallation.

Enantioselective synthesis of cyclopropylcarboxamides is possible by asymmetric metallation of prochiral starting cyclopropanes using s-BuLi-sparteine.