The Mitsunobu Reaction: Origin, Mechanism, Improvements, and Applications

Investigating the role of T7 and T12 residues on the biological properties of thrombin-binding aptamer: enhancement of anticoagulant activity by a single nucleobase modification

An acyclic pyrimidine analogue, containing a five-member cycle fused on the pyrimidine ring, was synthesized and introduced at position 7 or 12 of the 15-mer oligodeoxynucleotide GGTTGGTGTGGTTGG, known as thrombin-binding aptamer (TBA). Characterization by 1H NMR and CD spectroscopies of the resulting aptamers, TBA-T7b and TBA-T12b, showed their ability to fold into the typical antiparallel chairlike G-quadruplex structure formed by TBA. The apparent CD melting temperatures indicated that the introduction of the acyclic residue, mainly at position 7, improves the thermal stability of resulting G-quadruplexes with respect to TBA. The anticoagulant activity of the new molecules was then valued in PT assay, and it resulted that TBA-T7b is more potent than TBA in prolonging clotting time. On the other hand, in purified fibrinogen assay the thrombin inhibitory activity of both modified sequences was lower than that of TBA using human enzyme, whereas the potency trend was again reversed using bovine enzyme. Obtained structure-activity relationships were investigated by structural and computational studies. Taken together, these results reveal the active role of TBA residues T7 and T12 and the relevance of some amino acids located in the anion binding exosite I of the protein in aptamer-thrombin interaction.

C-H activation: a complementary tool in the total synthesis of complex natural products

The recent advent of transition-metal mediated C-H activation is revolutionizing the synthetic field and gradually infusing a "C-H activation mind-set" in both students and practitioners of organic synthesis. As a powerful testament of this emerging synthetic tool, applications of C-H activation in the context of total synthesis of complex natural products are beginning to blossom. Herein, recently completed total syntheses showcasing creative and ingenious incorporation of C-H activation as a strategic manoeuver are compared with their "non-C-H activation" counterparts, illuminating a new paradigm in strategic synthetic design.

Asymmetric Methods for the Synthesis of Flavanones, Chromanones, and Azaflavanones

Flavanones, chromanones, and related structures are privileged natural products that display a wide variety of biological activities. Although flavanoids are abundant in nature, there are a limited number of available general and efficient synthetic methods for accessing molecules of this class in a stereoselective manner. Their structurally simple architectures belie the difficulties involved in installation and maintenance of the stereogenic configuration at the C2 position, which can be sensitive and can undergo epimerization under mildly acidic, basic, and thermal reaction conditions. This review presents the methods currently used to access these related structures. The synthetic methods include manipulation of the flavone/flavanone core, carbon-carbon bond formation, and carbon-heteroatom bond formation.

Intramolecular monomer-on-monomer (MoM) Mitsunobu cyclization for the synthesis of benzofused thiadiazepine-dioxides

The utilization of a monomer-on-monomer (MoM) intramolecular Mitsunobu cyclization reaction employing norbornenyl-tagged (Nb-tagged) reagents is reported for the synthesis of benzofused thiadiazepine-dioxides. Facile purification was achieved via ring-opening metathesis (ROM) polymerization initiated by one of three metathesis catalyst methods: (i) free metathesis catalyst, (ii) surface-initiated catalyst-armed silica, or (iii) surface-initiated catalyst-armed Co/C magnetic nanoparticles.

Synthesis and antiviral activity of 2'-deoxy-2'-fluoro-2'-C-methyl-7-deazapurine nucleosides, their phosphoramidate prodrugs and 5'-triphosphates

Thirty novel α- and β-d-2'-deoxy-2'-fluoro-2'-C-methyl-7-deazapurine nucleoside analogs were synthesized and evaluated for in vitro antiviral activity. Several α- and β-7-deazapurine nucleoside analogs exhibited modest anti-HCV activity and cytotoxicity. Four synthesized 7-deazapurine nucleoside phosphoramidate prodrugs (18-21) showed no anti-HCV activity, whereas the nucleoside triphosphates (22-24) demonstrated potent inhibitory effects against both wild-type and S282T mutant HCV polymerases. Cellular pharmacology studies in Huh-7 cells revealed that the 5'-triphosphates were not formed at significant levels from either the nucleoside or the phosphoramidate prodrugs, indicating that insufficient phosphorylation was responsible for the lack of anti-HCV activity. Evaluation of anti-HIV-1 activity revealed that an unusual α-form of 7-carbomethoxyvinyl substituted nucleoside (10) had good anti-HIV-1 activity (EC(50)=0.71±0.25 μM; EC(90)=9.5±3.3 μM) with no observed cytotoxicity up to 100 μM in four different cell lines.

One-pot stereoselective synthesis of 1,2-amino alcohol derivatives

Common β-hydroxy amino acids (such as threonine) can be readily transformed into 1,2-amino alcohols with excellent stereoselectivity. This one-pot decarboxylation-alkylation process allows the replacement of the carboxyl group by alkyl, allyl, or aryl groups, generally in high yields. A variation of the process (decarboxylation-Diels-Alder) allows the formation of bi- and polycyclic systems, which are useful precursors of alkaloid cores or iminosugars.

Ring opening of pymisyl-protected aziridines with organocuprates

The pyrimidine-2-sulfonyl (pymisyl) group is introduced as a new protecting group that can be used to activate aziridines towards ring opening. It is readily introduced and removed under mild conditions. Regioselective ring opening of pymisyl-protected 2-methyl-aziridine with organocuprates gives the corresponding sulfonamides in high yields, and the pymisyl group can subsequently be removed upon treatment with a thiolate. The versatility of this new nitrogen protecting group is illustrated with a new synthesis of Selegiline, a monoamine oxidase-B inhibitor marketed for the treatment of Parkinson's disease.

Mirror helices and helicity switch at surfaces based on chiral triangular-shape oligo(phenylene ethynylenes)

The self-assembly of triangular-shaped oligo(phenylene ethynylenes) (OPEs), peripherally decorated with chiral and linear paraffinic chains, is investigated in bulk, onto surfaces and in solution. Whilst the X-ray diffraction data for the chiral studied systems display a broad reflection centered at 2θ ∼20° (λ=Cu(Kα)), the higher crystallinity of OPE 3, endowed with three linear decyl chains, results in a diffractrogram with a number of well-resolved reflections that can be accurately indexed as a columnar packing arranged in 2D oblique cells. Compounds (S)-1 a and (R)-1 b-endowed with (S)- and (R)-3,7-dimethyloctyloxy chains-transfer their chirality to the supramolecular structures formed upon their self-assembly, and give rise to helical nanostructures of opposite handedness. A helicity switch is noticeable for the case of chiral (S)-2 decorated with (S)-2-methylnonyloxy chains which forms right-handed helices despite it possesses the same stereoconfiguration for their stereogenic carbons as (S)-1 a that self-assembles into left-handed helices. The stability and the mechanism of the supramolecular polymerization in solution have been investigated by UV/Vis experiments in methylcyclohexane. These studies demonstrate that the larger the distance between the stereogenic carbon and the aromatic framework is, the more stable the aggregate is. Additionally, the self-assembly mechanism is conditioned by the peripheral substituents: whereas compounds (S)-1 a and (R)-1 b self-assemble in a cooperative manner with a low degree of cooperativity, the aggregation of (S)-2 and 3 is well described by an isodesmic model. Therefore, the interaction between the chiral coil chains conditions the handedness of the helical pitch, the stability of the supramolecular structure and the supramolecular polymerization mechanism of the studied OPEs.

A linchpin carbacyclization approach for the synthesis of carbanucleosides

A convenient synthesis of carbanucleosides, with both enantiomers equally accessible, is reported. The key step is a tandem linchpin cyclization process to give access to substituted carbafuranose derivatives having the correct relative stereochemistry for subsequent nucleobase introduction with inversion of configuration at C1. This was illustrated by the synthesis of 2',3'-dideoxycarbathymidine via a convergent nucleobase introduction and of 2',3'-dideoxy-6'-hydroxycarbauridine via a linear nucleobase introduction. Both methods relied on Mitsunobu chemistry, and the first example of the Mukaiyama modification of the Mitsunobu reaction involving nucleobases as nucleophiles is reported.

Simple strategy for synthesis of optically active allylic alcohols and amines by using enantioselective organocatalysis

A simple organocatalytic one-pot protocol for the construction of optically active allylic alcohols and amines using readily available reactants and catalyst is presented. The described reaction is enabled by an enantioselective enone epoxidation/aziridination-Wharton-reaction sequence affording two highly privileged and synthetically important classes of compounds in an easy and benign way. The advantages of the described sequence include easy generation of stereogenic allylic centers, also including quaternary stereocenters, with excellent enantio- and diastereomeric-control and high product diversity. Furthermore, using monosubstituted enones as substrates, having moderate enantiomeric excess, the one-pot reaction sequence proceeds with an enantioenrichment of the products and high diastereoselectivity was achieved.