Phytosterol and γ-Oryzanol Conjugates: Synthesis and Evaluation of their Antioxidant, Antiproliferative, and Anticholesterol Activities.

Fifteen new multifunctional conjugates were designed and synthesized by chemically linking the steroidal framework of natural occurring γ-oryzanol and γ-oryzanol-derived phytosterols to a wide range of bioactive natural compounds (fatty acids, phenolic acids, amino acids, lipoic acid, retinoic acid, curcumin, and resveratrol). Starting from γ-oryzanol, which is the main component of rice bran oil, this study was aimed at assessing if the conjugation strategy might enhance some γ-oryzanol bioactivities. The antioxidant activity was evaluated through three different mechanisms, namely, DPPH-scavenging activity, metal-chelating activity, and ß-carotene-bleaching inhibition. Measurement of the in vitro cell growth inhibitory effects on three different human cancer cellular lines was also carried out, and the potential hypocholesterolemic effect was studied. Compounds 10 and 15 displayed an improved antioxidant activity, with respect to that of γ-oryzanol. Compounds 2, 6, and 12 exerted an antiproliferative activity in the low micromolar range against HeLa and DAOY cells (GI50 < 10 µM). As for the claimed hypocholesterolemic effect of γ-oryzanol, none of the synthesized compounds inhibited the 3-hydroxy-3-methylglutaryl-coenzyme A reductase, a key enzyme in cholesterol biosynthesis.

Organocatalytic Access to Enantioenriched Spirooxindole-Based 4-Methyleneazetidines.

This work describes the synthesis of enantioenriched spiro compounds, incorporating the azetidine and the oxindole motifs. The preparation relies on a formal [2 + 2] annulation reaction of isatin-derived N-tert-butylsulfonyl ketimines with allenoates. The asymmetric induction is secured by an organocatalytic strategy, exploiting a bifunctional cinchona-type ß-isocupridine-based catalyst. Some post-transformation products, including unexpected spiropyrroline and 3,3-disubstituted oxindole derivatives, are also presented.

Organocatalytic Asymmetric Biginelli-like Reaction Involving Isatin.

The first asymmetric, Brønsted acid catalyzed Biginelli-like reaction of a ketone has been developed, employing N-substituted isatins as carbonyl substrates, and urea and alkyl acetoacetates as further components. BINOL-derived phosphoric acid catalysts have been used to achieve the synthesis of a small library of chiral, enantioenriched spiro(indoline-pyrimidine)-diones derivatives. The absolute configuration of the new spiro stereocenter was assessed on diastereoisomeric derivatives through computer-assisted NMR spectroscopy. X-ray diffractometry allowed the disclosure of the overall molecular conformation in the solid state and the characterization of the crystal packing of a Br-substituted Biginelli-like derivative, while computational studies on the reaction transition state allowed us to rationalize the stereochemical outcome.

Organocatalytic vinylogous Mannich reaction of trimethylsiloxyfuran with isatin-derived benzhydryl-ketimines.

A family of chiral quaternary 3-aminooxindole butenolides has been synthesized by BINOL-derived phosphoric acid-catalyzed addition of trimethylsiloxyfuran to isatin-derived ketimines. Such a vinylogous Mannich-type reaction was found to produce diastereoisomeric butenolides in good yields and in most cases high enantiomeric excesses. The configurational assignment of the obtained products was safely performed by chemical correlation. A computational study of the transition state allowed rationalizing the obtained stereochemical outcome, highlighting the possible binding modes of the catalyst-imine-nucleophile transition complex.

Disrupting the PCSK9/LDLR protein-protein interaction by an imidazole-based minimalist peptidomimetic.

Herein we report on the multicomponent synthesis of a novel imidazole-based compound, able to act efficiently as a minimalist ß-strand mimic. Biological evaluation proved its ability to impair the LDLR-PCSK9 protein-protein interaction, disclosing it as the first small molecule exerting a PCSK9-mediated hypocholesterolemic effect.

Complementary isonitrile-based multicomponent reactions for the synthesis of diversified cytotoxic hemiasterlin analogues.

A small family of structural analogues of the antimitotic tripeptides, hemiasterlins, have been designed and synthesized as potential inhibitors of tubulin polymerization. The effectiveness of a multicomponent approach was fully demonstrated by applying complementary versions of the isocyanide-based Ugi reaction. Compounds strictly related to the lead natural products, as well as more extensively modified analogues, have been synthesized in a concise and convergent manner. In some cases, biological evaluation provided evidence for strong cytotoxic activity (six human tumor cell lines) and for potent inhibition of tubulin polymerization.

Application of the Ugi reaction with multiple amino acid-derived components: synthesis and conformational evaluation of piperazine-based minimalist peptidomimetics.

The concurrent employment of α-amino acid-derived chiral components such as aldehydes and α-isocyanoacetates, in a sequential Ugi reaction/cyclization two-step strategy, opens the door to the synthesis of three structurally distinct piperazine-based scaffolds, characterized by the presence of L-Ala and/or L-Phe-derived side chains and bearing appropriate functionalities to be easily applied in peptide chemistry. By means of computational studies, these scaffolds have been demonstrated to act as minimalist peptidomimetics, able to mimic a well defined range of peptide secondary structures and therefore potentially useful for the synthesis of small-molecule PPI modulators. Preliminary biological evaluation of two different resistant hepatocellular carcinoma cellular lines, for which differentiation versus resistance ability seem to be strongly correlated with well defined types of PPIs, has revealed a promising antiproliferative activity for selected compounds.

Hemiasterlin analogues incorporating an aromatic, and heterocyclic type C-terminus: design, synthesis and biological evaluation.

A representative series of structural analogs of the antimitotic tripeptides hemiasterlins have been designed and synthesized, as potential inhibitors of tubulin polymerization. Relying also on a computational approach, we aimed to explore unknown extensive changes at the C-fragment, by incorporating the conformationally required double bond into five- and six-membered rings. Key steps of the synthetic strategy are a dynamic resolution affording the A-fragment in 97 % ee and the preparation of six new cyclic C fragments, all potentially able to interact with tubulin by means of H bonds. Unexpectedly, biological evaluation of these analogs did not provide evidences neither for cytotoxic effect nor for inhibition of tubulin polymerization.

Asymmetric Ugi 3CR on isatin-derived ketimine: synthesis of chiral 3,3-disubstituted 3-aminooxindole derivatives.

An efficient Ugi three-component reaction of a preformed chiral ketimine derived from isatin with various isonitrile and acid components has been developed. The reactions proceeded smoothly and in a stereocontrolled manner with regard to the new center of the Ugi products due to the stereoinduction of the amine chiral residue. A wide variety of novel chiral 3,3-disubstituted 3-aminooxindoles were obtained, a selection of which were subjected to post-Ugi transformations, paving the way to application as peptidomimetics.

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

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

Quinazolinecarboline alkaloid evodiamine as scaffold for targeting topoisomerase I and sirtuins.

This paper reports the synthesis of a series of evodiamine derivatives. We assayed the ability to inhibit cell growth on three human tumour cell lines (H460, MCF-7 and HepG2) and we evaluated the capacity to interfere with the catalytic activity of topoisomerase I both by the relaxation assay and the occurrence of the cleavable complex. Moreover, whose effect on sirtuins 1, 2 and 3 was investigated. Finally, molecular docking analyses were performed in an attempt to rationalize the biological results.

Synthesis, pharmacological evaluation and conformational investigation of endomorphin-2 hybrid analogues.

This study reports on new pharmacologically active endomorphin-2 analogues, incorporating ß(2)-hPhe, ß(3)-hPhe and ß(3)-hTic unnatural amino acids in the place of the Phe(3)-Phe(4)residues. Such α, ß-hybrid analogues were designed to exploit the great potential of ß-amino acids in generating conformational variation at the key positions 3 and 4, with the aim of evaluating the effect on the opioid binding affinity. Ligand-stimulated binding assays indicated that some analogues retained a significant affinity, especially for the δ receptor. (1)H NMR and molecular modelling suggested the predominance of bent structures for all compounds. The molecular docking with the µ-opioid receptor model was also performed, highlighting a common binding mode for active compounds and helping to rationalize the observed structure-activity data.

The diketopiperazine-fused tetrahydro-ß-carboline scaffold as a model peptidomimetic with an unusual α-turn secondary structure.

Aiming at restricting the conformational freedom of tryptophan-containing peptide ligands, we designed a THBC (tetrahydro-ß-carboline)-DKP (diketopiperazine)-based peptidomimetic scaffold capable of arranging in an unusual α-turn conformation. The synthesis is based on a diastereoselective Pictet-Spengler condensation to give the THBC core, followed by an intramolecular lactamization to complete the tetracyclic THBC-DKP fused ring system. The presence of conformers bearing the intramolecular thirteen-membered hydrogen bond that characterizes the α-turn structure is confirmed by (1)H NMR conformational studies. To the best of our knowledge, this scaffold represents one of the rare examples of a designed constrained α-turn mimic.

Ugi 4-CR/Pictet-Spengler reaction as a short route to tryptophan-derived peptidomimetics.

We report here a two step efficient route for the synthesis of 1,2,3,4-tetrahydro-ß-carboline (THBC)-based tetracyclic peptidomimetics from a Ugi 4-CR/Pictet-Spengler reaction sequence. Suitably N-protected 2-aminoacetaldehyde was for the first time applied as the carbonyl component in a Ugi four-component reaction, opening the way to the employment of N-protected α-amino acid-derived aldehydes in the same role. The potential of the obtained scaffolds is related to the possibility of further derivatization with the desired pharmacophoric groups, on both the terminal acid and amine functional groups, for the development of conformationally constrained tryptophan-containing peptide ligands. Extensive molecular modeling and (1)H NMR studies highlighted a robust, folded, ß-turn-like conformation for one of these peptidomimetic compounds.

Phe-Ala-based diazaspirocyclic lactam as nucleator of type II' ß-turn.

The synthesis of a novel Phe-Ala dipeptide mimic, built up on a diazaspirocyclic lactam core, is presented. This new scaffold was evaluated for conformational mimicry of reverse turn by combining molecular modeling, IR, NMR, and X-ray diffraction experiments. All these tools agree on the presence of a strong intramolecular hydrogen bond, thus demonstrating the ability of this spiro compound to act as a type II' ß-turn inducer.

Addition of TMSCN to chiral ketimines derived from isatin. Synthesis of an oxindole-based peptidomimetic and a bioactive spirohydantoin.

We investigated the Strecker-type reaction of isatin derived chiral ketimines with TMSCN in the presence of a Lewis acid. The desired α-amino nitriles have been obtained in good yields with moderate diastereoselectivity. Further elaboration of the cyanide group allowed the preparation of a new oxindole-based peptidomimetic and a pharmaceutically relevant spirohydantoin.

Olefin metathesis based approach to diversely functionalized pyrrolizidines and indolizidines; total synthesis of (+)-monomorine.

New scaffolds for the stereoselective synthesis of diversely functionalized chiral enantiopure indolizidines and pyrrolizidines were synthesized from the cross and ring-closing metathesis reactions of appropriate intermediates, readily available from L-pyroglutamic acid. The versatility of this strategy was demonstrated by the synthesis of an indolizidine-based azasugar analogue and of the natural alkaloid (+)-monomorine.

Grignard addition to imines derived from isatine: a method for the asymmetric synthesis of quaternary 3-aminooxindoles.

Addition of Grignard reagents to chiral imines derived from isatine afforded chiral, optically enriched 3-substituted 3-aminooxindoles in satisfactory yields and diastereoisomeric ratios. A general protocol is described for the addition of alkyl, alkenyl, and aryl Grignard reagents. In one case, the absolute configuration at C3 was determined and the selective N-deprotection was described, enabling further synthetic transformations of the reaction product.

Tetrahydroisoquinoline-based spirocyclic lactam as a type II' beta-turn inducing peptide mimetic.

We present here spirocyclic lactam derivatives, embodying D-Phe and L-Ala amino acids as the central core and acting as tetrapeptide and hexapeptide mimetics. An efficient route for their synthesis is demonstrated by using the strategic combination of Seebach's self-reproduction of chirality chemistry and the Pictet-Spengler condensation as key steps. The conformational behavior of peptide mimetics was investigated by molecular modeling, X-ray crystallography, NMR (solvent and temperature dependence), IR spectroscopy, and circular dichroism. All data suggest very stable and highly predictable type II' beta-turn conformations.

Computationally Driven Structure Optimization, Synthesis, and Biological Evaluation of Imidazole-Based Proprotein Convertase Subtilisin/Kexin 9 (PCSK9) Inhibitors.

Proprotein convertase subtilisin/kexin 9 (PCSK9) is responsible for the degradation of the hepatic low-density lipoprotein receptor (LDLR), which in turn regulates the circulating low-density lipoprotein cholesterol (LDL-C) level. For this reason, the PCSK9 inhibition, by small molecules or peptides, is a validated therapeutic approach for fighting hypercholesterolemia and cardiovascular diseases. In this field, we have recently reported an imidazole-based peptidomimetic that has shown PCSK9 inhibitory activity in the micromolar range. Here, by applying advanced computational techniques, the binding mechanism of that imidazole peptidomimetic was predicted. Then, among a small set of poly-imidazole analogs, compounds showing the highest theoretical affinity were suitably synthesized, relying on a van Leusen reaction based multicomponent strategy. One compound (named RIm13) displayed a PCSK9 inhibitory activity 10-fold lower than the template compound, and, remarkably, at a concentration of 1 µM, it successfully prevented the LDLR degradation mediated by PCSK9 on HepG2 cells. As well as increasing the LDL uptake at the same concentration, RIm13 represents currently one of the most potent small molecules targeting the PCSK9/LDLR protein-protein interaction.