Shaping Chirality via Stereoselective, Organocatalytic [4+2] Cycloadditions involving Heterocyclic ortho-Quinodimethanes.

Polycyclic compounds bearing a complex heterocyclic core such as an aromatic heterocycle "fused" with one or more functionalized rings, are widespread leading molecules in the domain of synthetic organic chemistry and pharmaceuticals. Although many synthetic methodologies have been devised to access achiral, fused heteroaromatic scaffolds, or related chiral variants adorned with out-of-cycle stereogenic elements, equally efficient strategies to afford chiral heterocycles featuring in-cycle stereocenters, exist to a lesser extent and presently represent a growing field of investigation. The mild, organocatalytic generation of elusive ortho-quinodimethane intermediates (oQDMs), derived from suitable heteroaromatic carbonyl- or carbonyl-like pronucleophiles has recently proved successful in the synthesis of such peculiar chiral architectures via stereoselective [4+2] cycloadditions. This review provides an overview of the most important advances attained in this field over the last decade.

Turning the Imidazole Core into Three-Dimensional Ring Systems: Mild Organocatalytic Entry to Enantiopure 6,7-Dihydrobenzimidazoles.

Organocatalytic asymmetric transformation of common aromatic heterocycles via in situ formation of highly reactive dearomatized ortho-quinodimethane diene species and subsequent [4+2] cycloaddition with suitable dienophiles has become a powerful tool to enter cyclohexane-fused heterocycles. Most of these reactions were previously applied to benzo-fused heterocycles or poorly aromatic rings. Herein, we disclose how previously intractable aromatic imidazole rings, equipped with removable methylidene malononitrile activating handle, could be involved as competent cycloaddends with ß-aryl enals in efficient eliminative [4+2] cycloadditions under mild organocatalytic conditions. This method allowed the efficient and direct preparation of scantly represented 6,7-dihydrobenzo[d]imidazoles with optimal enantio- and regioselectivities. Post-cycloaddition chemical editing provided imidazole-based ring systems with diverse oxidation state and functional groups.

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems.

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" ß, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

Metabotypes of flavan-3-ol colonic metabolites after cranberry intake: elucidation and statistical approaches.

PURPOSE: Extensive inter-individual variability exists in the production of flavan-3-ol metabolites. Preliminary metabolic phenotypes (metabotypes) have been defined, but there is no consensus on the existence of metabotypes associated with the catabolism of catechins and proanthocyanidins. This study aims at elucidating the presence of different metabotypes in the urinary excretion of main flavan-3-ol colonic metabolites after consumption of cranberry products and at assessing the impact of the statistical technique used for metabotyping. METHODS: Data on urinary concentrations of phenyl-γ-valerolactones and 3-(hydroxyphenyl)propanoic acid derivatives from two human interventions has been used. Different multivariate statistics, principal component analysis (PCA), cluster analysis, and partial least square-discriminant analysis (PLS-DA), have been considered. RESULTS: Data pre-treatment plays a major role on resulting PCA models. Cluster analysis based on k-means and a final consensus algorithm lead to quantitative-based models, while the expectation-maximization algorithm and clustering according to principal component scores yield metabotypes characterized by quali-quantitative differences in the excretion of colonic metabolites. PLS-DA, together with univariate analyses, has served to validate the urinary metabotypes in the production of flavan-3-ol metabolites and to confirm the robustness of the methodological approach. CONCLUSIONS: This work proposes a methodological workflow for metabotype definition and highlights the importance of data pre-treatment and clustering methods on the final outcomes for a given dataset. It represents an additional step toward the understanding of the inter-individual variability in flavan-3-ol metabolism. TRIAL REGISTRATION: The acute study was registered at clinicaltrials.gov as NCT02517775, August 7, 2015; the chronic study was registered at clinicaltrials.gov as NCT02764749, May 6, 2016.

New 4-Aminoproline-Based Small Molecule Cyclopeptidomimetics as Potential Modulators of α4ß1 Integrin.

Integrin α4ß1 belongs to the leukocyte integrin family and represents a therapeutic target of relevant interest given its primary role in mediating inflammation, autoimmune pathologies and cancer-related diseases. The focus of the present work is the design, synthesis and characterization of new peptidomimetic compounds that are potentially able to recognize α4ß1 integrin and interfere with its function. To this aim, a collection of seven new cyclic peptidomimetics possessing both a 4-aminoproline (Amp) core scaffold grafted onto key α4ß1-recognizing sequences and the (2-methylphenyl)ureido-phenylacetyl (MPUPA) appendage, was designed, with the support of molecular modeling studies. The new compounds were synthesized through SPPS procedures followed by in-solution cyclization maneuvers. The biological evaluation of the new cyclic ligands in cell adhesion assays on Jurkat cells revealed promising submicromolar agonist activity in one compound, namely, the c[Amp(MPUPA)Val-Asp-Leu] cyclopeptide. Further investigations will be necessary to complete the characterization of this class of compounds.

Shifting Towards αV ß6 Integrin Ligands Using Novel Aminoproline-Based Cyclic Peptidomimetics.

In recognition of the key role played by integrins in several life-threatening dysfunctions, the search for novel small-molecule probes that selectively recognize these surface receptors is still open and widely pursued. Inspired by previously established aminoproline (Amp)-RGD based cyclopeptidomimetics with attracting αV ß3 integrin affinity and selectivity, the design and straightforward synthesis of 18 new AmpRGD chemotypes bearing additional structural variants were herein implemented, to shift toward peptide-like αV ß6 integrin targeted binders. The ligand competence of the synthesized products toward αV ß6 was evaluated in competitive binding assays on isolated receptors, and αV ß6 /αV ß3 selectivity was determined for a subgroup of compounds, resulting in the identification of four very promising candidates. SAR considerations and docking simulations allowed us to appreciate the key structural features responsible for the observed activity.

Unlocking Access to Enantiopure Fused Uracils by Chemodivergent [4+2] Cross-Cycloadditions: DFT-Supported Homo-Synergistic Organocatalytic Approach.

The discovery of chemical methods enabling the construction of carbocycle-fused uracils which embody a three-dimensional and functional-group-rich architecture is a useful tool in medicinal chemistry oriented synthesis. In this work, an unprecedented amine-catalyzed [4+2] cross-cycloaddition is documented; it involves remotely enolizable 6-methyluracil-5-carbaldehydes and ß-aryl enals, and chemoselectively produces two novel bicyclic and tricyclic fused uracil chemotypes in good yields with a maximum level of enantiocontrol. In-depth mechanistic investigations and control experiments support an intriguing homo-synergistic organocatalytic approach, where the same amine organocatalyst concomitantly engages both aldehyde partners in a stepwise eliminative [4+2] cycloaddition, whose vinylogous iminium ion intermediate product may diverge-depending upon conditions-to either bicyclic targets by hydrolysis or tricyclic products by a second homo-synergistic trienamine-mediated stepwise [4+2] cycloaddition.

Phenyl-γ-valerolactones and phenylvaleric acids, the main colonic metabolites of flavan-3-ols: synthesis, analysis, bioavailability, and bioactivity.

Covering: 1958 to June 2018 Phenyl-γ-valerolactones (PVLs) and their related phenylvaleric acids (PVAs) are the main metabolites of flavan-3-ols, the major class of flavonoids in the human diet. Despite their presumed importance, these gut microbiota-derived compounds have, to date, in terms of biological activity, been considered subordinate to their parent dietary compounds, the flavan-3-ol monomers and proanthocyanidins. In this review, the role and prospects of PVLs and PVAs as key metabolites in the understanding of the health features of flavan-3-ols have been critically assessed. Among the topics covered, are proposals for a standardised nomenclature for PVLs and PVAs. The formation, bioavailability and pharmacokinetics of PVLs and PVAs from different types of flavan-3-ols are discussed, taking into account in vitro and animal studies, as well as inter-individual differences and the existence of putative flavan-3-ol metabotypes. Synthetic strategies used for the preparation of PVLs are considered and the methodologies for their identification and quantification assessed. Metabolomic approaches unravelling the role of PVLs and PVAs as biomarkers of intake are also described. Finally, the biological activity of these microbial catabolites in different experimental models is summarised. Knowledge gaps and future research are considered in this key area of dietary (poly)phenol research.

Integrin-targeted AmpRGD sunitinib liposomes as integrated antiangiogenic tools.

We report here the preparation, physico-chemical characterization, and biological evaluation of a new liposome formulation as a tool for tumor angiogenesis inhibition. Liposomes are loaded with sunitinib, a tyrosine kinase inhibitor, and decorated with cyclo-aminoprolineRGD units (cAmpRGD), efficient and selective ligands for integrin αVß3. The RGD units play multiple roles since they target the nanovehicles at the integrin αVß3-overexpressing cells (e.g. activated endothelial cells), favor their active cell internalization, providing drug accumulation in the cytoplasm, and likely take part in the angiogenesis inhibition by interfering in the αVß3-VEGFR2 cross-talk. Both in vitro and in vivo studies show a better efficacy of this integrated antiangiogenic tool with respect to the free sunitinib and untargeted sunitinib-loaded liposomes. This system could allow a lower administration of the drug and, by increasing the vector specificity, reduce side-effects in a prolonged antiangiogenic therapy.

Exploiting the Distal Reactivity of Indolyl Methylenemalononitriles: An Asymmetric Organocatalyzed [4+2] Cycloaddition with Enals Enables the Assembly of Elusive Dihydrocarbazoles.

An unprecedented technique for the in situ generation of indolyl ortho-quinodimethanes from 2-methylindole-based methylenemalononitriles by amine-mediated remote C(sp(3) )-H deprotonation was developed. These intermediates were efficiently trapped by diverse enals to provide a rapid entry to 2,9-dihydro-1H-carbazole-3-carboxyaldehyde structures through a formal asymmetric [4+2] eliminative cycloaddition governed by a α,α-diphenylprolinol trimethylsilyl ether catalyst.

Pushing the boundaries of vinylogous reactivity: catalytic enantioselective mukaiyama aldol reactions of highly unsaturated 2-silyloxyindoles.

The first example of catalytic, enantioselective hypervinylogous Mukaiyama aldol reaction (HVMAR) involving multiply unsaturated 2-silyloxyindoles is reported. The reaction utilizes a chiral Lewis base-catalyzed Lewis acid-mediated technology to deliver homoallylic 3-polyenylidene 2-oxindoles with extraordinary levels of regio-, enantio-, and geometrical selectivity. This work highlights a subtle yet decisive influence of the indole N-substituents on the propagation of the vinylogous reactivity space of the donor substrates up to ten bonds away from the origin of the vinylogy effect. Analysis of the (13) C NMR chemical shifts of the C-ω remote site within homologous silyloxyindole donors enabled rationalization of the results and easy qualitative prediction of the HVMAR reactivity/inertia toward a given aldehyde acceptor.

Organocatalytic, Asymmetric Eliminative [4+2] Cycloaddition of Allylidene Malononitriles with Enals: Rapid Entry to Cyclohexadiene-Embedding Linear and Angular Polycycles.

A direct aminocatalytic synthesis has been developed for the chemo-, regio-, diastereo-, and enantioselective construction of densely substituted polycyclic carbaldehydes containing fused cyclohexadiene rings. The chemistry utilizes, for the first time, remotely enolizable π-extended allylidenemalononitriles as electron-rich 1,3-diene precursors in a direct eliminative [4+2] cycloaddition with both aromatic and aliphatic α,ß-unsaturated aldehydes. The generality of the process is demonstrated by approaching 6,6-, 5,6-, 7,6-, 6,6,6-, and 6,5,6-fused ring systems, as well as biorelevant steroid-like 6,6,6,6,5- and 6,6,6,5,6-rings. A stepwise reaction mechanism for the key [4+2] addition is proposed as a domino bis-vinylogous Michael/Michael/retro-Michael reaction cascade. The utility of the malononitrile moiety as traceless activating group of the dicyano nucleophilic substrates is demonstrated.

Exploring the vinylogous reactivity of cyclohexenylidene malononitriles: switchable regioselectivity in the organocatalytic asymmetric addition to enals giving highly enantioenriched carbabicyclic structures.

Modulation of the complex reactivity of cyclohexenylidene malononitriles using diverse ß-aryl-substituted enals and proper organocatalytic modalities resulted in divergent asymmetric reaction patterns to furnish angularly fused or bridged carbabicyclic frameworks. In particular, use of remotely enolizable dicyanodienes 1, under one-pot sequential amine/NHC catalysis, led to [3 + 2] cycloaddition to afford ε,δ-bonded spiro[4.5]decanone structures 5. Alternatively, modifying the standard amine catalysis by adding a suitable chemical stimulus (p-nitrophenol cocatalyst) switched the reactivity decidedly toward a domino [4 + 2] cycloaddition to afford γ',δ-bonded bicyclo[2.2.2]octane carbaldehydes 8. Products invariably formed in good yields, with rigorous chemo-, regio-, diastereo-, and enantiocontrol. Experimental evidence, including carbon isotope effects measured by (13)C NMR, were indicative of the rate (and stereochemistry) determining step of these transformations and suggested a stepwise mechanism for the [4 + 2] cycloadditive pathway.

Synthesis, structure and inhibitory activity of a stereoisomer of oseltamivir carboxylate.

A stereodivergent plan is presented leading to all eight stereoisomers of oseltamivir carboxylate (OC). Key chemical manoeuvers are (1) a three-component vinylogous Mukaiyama-Mannich reaction, which sets the whole carbon skeleton and heteroatom substituents, and (2) an intramolecular, silylative Mukaiyama aldol reaction, which creates the targeted carbocycle. The viability of the plan was demonstrated by the first total synthesis of 4-epi-oseltamivir carboxylate (6), accessed in 15 steps from glyceraldehyde, o-anisidine and pyrrole siloxydiene precursors. Compound 6 inhibits influenza A virus strains H1N1 and H3N2 at the µM level, about 150 000-fold less than the OC reference, testifying that the stereodisposition of the C4 acetamido function is key for enzyme recognition. Guided by in-depth structural evaluation including NMR solution studies, molecular mechanics simulations, docking analyses and X-ray crystallography, rationalization of the biological verdict was established.

Nintedanib-Containing Dual Conjugates Targeting αVß6 Integrin and Tyrosine Kinase Receptors as Potential Antifibrotic Agents.

αVß6 Integrin plays a fundamental role in the activation of transforming growth factor-ß (TGF-ß), the major profibrotic mediator; for this reason, αVß6 ligands have recently been forwarded to clinical phases for the therapy of fibrotic diseases. Herein, we report the synthesis and in vitro biological evaluation as antifibrotic agents of three new covalent conjugates, constituted by c(AmpLRGDL), an αVß6 integrin-recognizing small cyclopeptide, and nintedanib, a tyrosine kinase inhibitor approved for idiopathic pulmonary fibrosis (IPF) treatment. One of these conjugates recapitulates optimal in vitro antifibrotic properties of the two active units. The integrin ligand portion within the conjugate plays a role in inhibiting profibrotic stimuli, potentiating the nintedanib effect and favoring the selective uptake of the conjugate in cells overexpressing αVß6 integrin. These results may open a new perspective on the development of dual conjugates in the targeted therapy of IPF.

A Screening of Native (Poly)phenols and Gut-Related Metabolites on 3D HCT116 Spheroids Reveals Gut Health Benefits of a Flavan-3-ol Metabolite.

SCOPE: Epidemiological evidence suggests that a reduced risk of colorectal cancer (CRC) is correlated with high consumption of fruits and vegetables, which are major sources of fiber and phytochemicals, such as flavan-3-ols. However, it remains unknown how these phytochemicals and their specific gut-related metabolites may alter cancer cell behavior. METHODS AND RESULTS: A focused screening using native (poly)phenols and gut microbial metabolites (GMMs) on 3D HCT116 spheroids is carried out using a high-throughput imaging approach. Dose-responses, IC50 , and long-term exposure are calculated for the most promising native (poly)phenols and GMMs. As a result, this research shows that (poly)phenol catabolites may play a key role in preventing cancer propagation. Indeed, µM concentration levels of (4R)-5-(3',4'-dihydroxyphenyl)-γ-valerolactone significantly decrease spheroid size at early stages of spheroid aggregation and gene expression of matrix metalloproteinases. CONCLUSION: A chronic exposure to (4R)-5-(3',4'-dihydroxyphenyl)-γ-valerolactone may lead to a reduced CRC risk. Daily intake of monomeric, oligomeric, and polymeric flavan-3-ols may increase the colonic concentrations of this metabolite, and, in turn, this compound may act locally interacting with intestinal epithelial cells, precancerous and cancer cells.

anti-Selective, catalytic asymmetric vinylogous Mukaiyama Mannich reactions of pyrrole-based silyl dienolates with N-aryl aldimines.

Pyrrole-based silyl dienolates undergo asymmetric vinylogous Mukaiyama Mannich reactions with a series of N-aryl aldimines in the presence of the Hoveyda-Snapper amino acid-derived silver(I) catalysts. The Mannich products-α,ß-unsaturated δ-amino-γ-butyrolactams-are typically obtained in high yields, excellent γ-site selectivities and anti-diastereoselectivities, and up to 80% enantioselectivity.

Diastereo- and enantioselective catalytic vinylogous Mukaiyama-Mannich reactions of pyrrole-based silyl dienolates with alkyl-substituted aldehydes.

A reliable, catalytic asymmetric vinylogous Mukaiyama-Mannich reaction of pyrrole-based silyl dienolates is introduced that is particularly apt for alkyl- and α-alkoxyalkyl-substituted aldehydes. The reaction course is effectively orchestrated by the Hoveyda-Snapper amino acid-based chiral ligand/silver(I) catalyst combination to produce valuable vicinal diamino carbonyl compounds in high yields, with virtually complete γ-site- and anti-selectivity and significant catalyst-to-product chirality transfer. The utility of the Mannich products can be seen in the synthesis of an unprecedented perhydrofuro[3,2-b]pyrrolone product, an aza-analogue of naturally occurring (+)-goniofufurone.

Uncatalyzed, diastereoselective vinylogous Mukaiyama aldol reactions on aqueous media: pyrrole vs furan 2-silyloxy dienes.

The first uncatalyzed, diastereoselective vinylogous Mukaiyama aldol reaction is reported, between pyrrole/furan-based dienoxy silanes and aromatic aldehydes on salty water/methanol medium, at almost human body temperature, under ultrasonic irradiation. With pyrrole dienes the reaction is anti-selective, while that of furan dienes is syn-selective. The dual role of water as both reaction medium and promoter is highlighted.

Asymmetric total synthesis of 1-deoxy-7,8-di-epi-castanospermine.

An efficient, stereocontrolled synthesis of (6S,7S,8S,8aR)-6,7,8-trihydroxyindolizidine (alias 1-deoxy-7,8-di-epi-castanospermine) (14) has been developed, which exploits an asymmetric vinylogous Mukaiyama aldol reaction (VMAR) between N-(tert-butoxycarbonyl)-2-(tert-butyldimethylsilyloxy)pyrrole (1) and 2,3-O-isopropylidene-D-glyceraldehyde (2) to construct the initial pyrrolidine building block 3, and an ene-ene ring closing metathesis reaction (RCM) (9 to 10) to install the indolizidine skeleton. The synthetic sequence was 13 steps, proceeding in 19.5% overall yield. The configurational and conformational structure of 14 was ascertained unambiguously and confronted to previously published assignments of rac-14 and ent-14.