Enantioselective Dearomatization of Pyridinium Salts by Copper-Catalyzed C4-Selective Addition of Silicon Nucleophiles.

A copper-catalyzed C4-selective addition of silicon nucleophiles released from an Si-B reagent to prochiral pyridinium triflates is reported. The dearomatization proceeds with excellent enantioselectivity using Cu(CH3CN)4PF6 as the precatalyst and (R,R)-Ph-BPE as the chiral ligand. A carbonyl group at C3 is required, likely acting a weak group to preorganize and direct the nucleophilc attack towards C4. The resulting 4-silylated 1,4-dihydropyridines can be further converted into functionalized piperidine derivatives.

Nucleoside Phosphorylases make N7-xanthosine.

Modern, highly evolved nucleoside-processing enzymes are known to exhibit perfect regioselectivity over the glycosylation of purine nucleobases at N9. We herein report an exception to this paradigm. Wild-type nucleoside phosphorylases also furnish N7-xanthosine, a "non-native" ribosylation regioisomer of xanthosine. This unusual nucleoside possesses several atypical physicochemical properties such as redshifted absorption spectra, a high equilibrium constant of phosphorolysis and low acidity. Ultimately, the biosynthesis of this previously unknown natural product illustrates how even highly evolved, essential enzymes from primary metabolism are imperfect catalysts.

Dehydrative Coupling of 1,1-Diarylalkenes and Cyclohexa-2,5-diene-1-carbaldehyde Derivatives Induced by a B(C6F5)3-Initiated [1,2]-Alkyl Migration.

A four-step formal ipso allylation of benzoic acid derivatives involving a B(C6F5)3-initiated and proton-catalyzed [1,2]-alkyl shift as part of a dehydrative coupling of cyclohexa-2,5-diene-1-carbaldehyde derivatives and 1,1-diarylalkenes is reported. By this, a series of allyl arenes can be regioselectively obtained from readily available benzoic acids in good yields.

Biased Borate Esterification during Nucleoside Phosphorylase-Catalyzed Reactions: Apparent Equilibrium Shifts and Kinetic Implications.

Biocatalytic nucleoside (trans-)glycosylations catalyzed by nucleoside phosphorylases have evolved into a practical and convenient approach to the preparation of modified nucleosides, which are important pharmaceuticals for the treatment of various cancers and viral infections. However, the obtained yields in these reactions are generally determined exclusively by the innate thermodynamic properties of the nucleosides involved, hampering the biocatalytic access to many sought-after target nucleosides. We herein report an additional means for reaction engineering of these systems. We show how apparent equilibrium shifts in phosphorolysis and glycosylation reactions can be effected through entropically driven, biased esterification of nucleosides and ribosyl phosphates with inorganic borate. Our multifaceted analysis further describes the kinetic implications of this in situ reactant esterification for a model phosphorylase.

An Isolable 2,5-Disila-3,4-Diphosphapyrrole and a Conjugated Si=P-Si=P-Si=N Chain Through Degradation of White Phosphorus with a N,N-Bis(Silylenyl)Aniline.

White phosphorus (P4 ) undergoes degradation to P2 moieties if exposed to the new N,N-bis(silylenyl)aniline PhNSi2 1 (Si=Si[N(tBu)]2 CPh), furnishing the first isolable 2,5-disila-3,4-diphosphapyrrole 2 and the two novel functionalized Si=P doubly bonded compounds 3 and 4. The pathways for the transformation of the non-aromatic 2,5-disila-3,4-diphosphapyrrole PhNSi2 P2 2 into 3 and 4 could be uncovered. It became evident that 2 reacts readily with both reactants P4 and 1 to afford either the polycyclic Si=P-containing product [PhNSi2 P2 ]2 P2 3 or the unprecedented conjugated Si=P-Si=P-Si=NPh chain-containing compound 4, depending on the employed molar ratio of 1 and P4 as well as the reaction conditions. Compounds 3 and 4 can be converted into each other by reactions with 1 and P4 , respectively. All new compounds 1-4 were unequivocally characterized including by single-crystal X-ray diffraction analysis. In addition, the electronic structures of 2-4 were established by Density Functional Theory (DFT) calculations.

Consecutive ß,ß'-Selective C(sp3 )-H Silylation of Tertiary Amines with Dihydrosilanes Catalyzed by B(C6 F5 )3.

Tris(pentafluorophenyl)borane has been found to catalyze the two-fold C(sp3 )-H silylation of various trialkylamine derivatives with dihydrosilanes, furnishing the corresponding 4-silapiperidines in decent yields. The multi-step reaction cascade involves amine-to-enamine dehydrogenation at two alkyl residues and two electrophilic silylation reactions of those enamines, one inter- and one intramolecular.

Intramolecular Friedel-Crafts alkylation with a silylium-ion-activated cyclopropyl group: formation of tricyclic ring systems from benzyl-substituted vinylcyclopropanes and hydrosilanes.

A trityl-cation-initiated annulation of benzyl-substituted vinylcyclopropanes (VCPs) with hydrosilanes is reported. Two Si-C(sp3) bonds and one C(sp2)-C(sp3) bond are formed in this process where an intramolecular 6-endo-tet Friedel-Crafts alkylation of a silylium-ion-activated cyclopropane ring is the rate-determining key step. The reaction mechanism is proposed based on computations and is in agreement with experimental observations. The new reaction leads to an unprecedented silicon-containing 6/6/5-fused ring system. A phenethyl-substituted VCP derivative yields another unknown tricycle having 6/6/6 ring fusion by reacting in a related but different way involving a 6-exo-tet ring closure.

Boron Lewis Acid-Catalyzed Hydroboration of Alkenes with Pinacolborane: BArF3 Does What B(C6 F5 )3 Cannot Do!

The transition-metal-free hydroboration of various alkenes with pinacolborane (HBpin) initiated by tris[3,5-bis(trifluoromethyl)phenyl]borane (BArF3 ) is reported. The choice of the boron Lewis acid is crucial as the more prominent boron Lewis acid tris(pentafluorophenyl)borane (B(C6 F5 )3 ) is reluctant to react. Unlike B(C6 F5 )3 , BArF3 is found to engage in substituent redistribution with HBpin, resulting in the formation of ArF Bpin and the electron-deficient diboranes [H2 BArF ]2 and [(ArF )(H)B(µ-H)2 BArF2 ]. These in situ-generated hydroboranes undergo regioselective hydroboration of styrene derivatives as well as aliphatic alkenes with cis diastereoselectivity. Another ligand metathesis of these adducts with HBpin subsequently affords the corresponding HBpin-derived anti-Markovnikov adducts. The reactive hydroboranes are regenerated in this step, thereby closing the catalytic cycle.

The gyrase inhibitor albicidin consists of p-aminobenzoic acids and cyanoalanine.

Albicidin is a potent DNA gyrase inhibitor produced by the sugarcane pathogenic bacterium Xanthomonas albilineans. Here we report the elucidation of the hitherto unknown structure of albicidin, revealing a unique polyaromatic oligopeptide mainly composed of p-aminobenzoic acids. In vitro studies provide further insights into the biosynthetic machinery of albicidin. These findings will enable structural investigations on the inhibition mechanism of albicidin and its assessment as a highly effective antibacterial drug.

Mechanism of the cooperative Si-H bond activation at Ru-S bonds.

The nature of the hydrosilane activation mediated by ruthenium(ii) thiolate complexes of type [(R3P)Ru(SDmp)]+[BArF4]- is elucidated by an in-depth experimental and theoretical study. The combination of various ruthenium(ii) thiolate complexes and tertiary hydrosilanes under variation of the phosphine ligand and the substitution pattern at the silicon atom is investigated, providing detailed insight into the activation mode. The mechanism of action involves reversible heterolytic splitting of the Si-H bond across the polar Ru-S bond without changing the oxidation state of the metal, generating a ruthenium(ii) hydride and sulfur-stabilized silicon cations, i.e. metallasilylsulfonium ions. These stable yet highly reactive adducts, which serve as potent silicon electrophiles in various catalytic transformations, are fully characterized by systematic multinuclear NMR spectroscopy. The structural assignment is further verified by successful isolation and crystallographic characterization of these key intermediates. Quantum-chemical analyses of diverse bonding scenarios are in excellent agreement with the experimental findings. Moreover, the calculations reveal that formation of the hydrosilane adducts proceeds via barrierless electrophilic activation of the hydrosilane by sterically controlled η1 (end-on) or η2 (side-on) coordination of the Si-H bond to the Lewis acidic metal center, followed by heterolytic cleavage of the Si-H bond through a concerted four-membered transition state. The Ru-S bond remains virtually intact during the Si-H bond activation event and also preserves appreciable bonding character in the hydrosilane adducts. The overall Si-H bond activation process is exergonic with ΔG0r ranging from -20 to -40 kJ mol-1, proceeding instantly already at low temperatures.

Isolation and structure elucidation of the nucleoside antibiotic strepturidin from Streptomyces albus DSM 40763.

The antibiotic strepturidin (1) was isolated from the microorganism Streptomyces albus DSM 40763, and its structure elucidated by spectroscopic methods and chemical degradation studies. The determination of the relative and absolute stereocenters was partially achieved using chiral GC/EI-MS analysis and microderivatization by acetal ring formation and subsequent 2D-NMR analysis of key (1)H,(1)H-NOESY NMR correlations and extraction of (1)H,(13)C coupling constants from (1)H,(13)C-HMBC NMR spectra. Based on these results, a biosynthesis model was proposed.

Chronic pain has a negative impact on sexuality in testis cancer survivors.

Testis cancer is a disease that directly affects a man's sense of masculinity and involves treatments compromising sexual function. The aim of this study was to investigate the prevalence of sexual dysfunction and the influence of chronic pain on sexuality in long-term testis cancer survivors. Thus, we examined 539 patients after they had one testis removed because of a testicular germ cell tumor. Having completed oncologic therapy, all patients received a detailed questionnaire asking about the occurrence and clinical presentation of testis pain before and after orchiectomy. In addition, items from the abridged International Index of Erectile Function and Brief Sexual Function Inventory were used to gain precise information on individual sexual function. Overall, 34.5% of our testicular cancer survivors complained of reduced sexual desire, and sexual activity was reduced in 41.6%. Erectile dysfunction was present in up to 31.5% of patients. In 24.4%, the ability to maintain an erection during intercourse was impaired. Ejaculatory disorders (premature, delayed, retrograde, or anejaculation) occurred in 84.9% of our testis cancer survivors. A total of 32.4% of our participants experienced a reduced intensity of orgasm, and 95.4% experienced reduced overall sexual satisfaction. There was a significant correlation between the occurrence of chronic pain symptoms and the relative frequency and intensity of erectile dysfunction, inability to maintain an erection, ejaculation disorders, and reduced intensity of orgasm. In conclusion, chronic pain has a negative impact on sexuality in testis cancer survivors.

Phantom testis syndrome: prevalence, phenomenology and putative mechanisms.

Chronic phantom pain has been found in up to 78% of limb amputees and is a major complication of limb amputation. Less is known about phantom phenomena after the amputation of other, i.e. visceral, parts of the body. In a retrospective design, we identified 539 patients in whom one testis was removed between 1995 and 2005. The operative technique was a unilateral standard radical inguinal orchiectomy. The underlying pathology in all cases was a testicular germ cell tumour. All patients received a detailed questionnaire asking about the occurrence of phantom testis pain (pain felt in the removed testis), phantom testis sensations (non-painful sensations as if the removed testis was still intact) and hallucinations (illusionary perceptions on the removed testis). Furthermore, we asked about the occurrence and clinical presentation of pain before and after surgery and about pre-operative testicular pain. Out of 238 respondents, 125 patients (53%) reported any kind of phantom experience. The prevalence of phantom testis pain was 25% (60/238), non-painful phantom sensations 16% (37/238) and male gonad hallucinations 12% (28/238). Patients with phantom symptoms reported pre-operative pain in the removed testis more often than patients without phantom symptoms. This study presents first data on the clinical characteristics and possible mechanisms of the phantom testis syndrome after surgical removal of one testis.

Group epitope mapping considering relaxation of the ligand (GEM-CRL): including longitudinal relaxation rates in the analysis of saturation transfer difference (STD) experiments.

In the application of saturation transfer difference (STD) experiments to the study of protein-ligand interactions, the relaxation of the ligand is one of the major influences on the experimentally observed STD factors, making interpretation of these difficult when attempting to define a group epitope map (GEM). In this paper, we describe a simplification of the relaxation matrix that may be applied under specified experimental conditions, which results in a simplified equation reflecting the directly transferred magnetisation rate from the protein onto the ligand, defined as the summation over the whole protein of the protein-ligand cross-relaxation multiplied by with the fractional saturation of the protein protons. In this, the relaxation of the ligand is accounted for implicitly by inclusion of the experimentally determined longitudinal relaxation rates. The conditions under which this "group epitope mapping considering relaxation of the ligand" (GEM-CRL) can be applied were tested on a theoretical model system, which demonstrated only minor deviations from that predicted by the full relaxation matrix calculations (CORCEMA-ST) [7]. Furthermore, CORCEMA-ST calculations of two protein-saccharide complexes (Jacalin and TreR) with known crystal structures were performed and compared with experimental GEM-CRL data. It could be shown that the GEM-CRL methodology is superior to the classical group epitope mapping approach currently used for defining ligand-protein proximities. GEM-CRL is also useful for the interpretation of CORCEMA-ST results, because the transferred magnetisation rate provides an additional parameter for the comparison between measured and calculated values. The independence of this parameter from the above mentioned factors can thereby enhance the value of CORCEMA-ST calculations.

Saturation transfer difference NMR reveals functionally essential kinetic differences for a sugar-binding repressor protein.

The binding kinetics of disaccharides trehalose and trehalose-6-phosphate to repressor protein TreR have been determined using STD NMR and shed light on the contrasting biological roles of these two sugars.

Jacobsen's catalyst for hydrolytic kinetic resolution: structure elucidation of paramagnetic Co(III) salen complexes in solution via combined NMR and quantum chemical studies.

NMR investigation of chiral Co(III) salen catalysts, important for enantioselective hydrolytic kinetic resolution (HKR), revealed the presence of a paramagnetic high-spin Co(III) species, which is in solvent- and temperature-dependent equilibrium with the known diamagnetic low-spin Co(III) complex. Combined with quantum chemical DFT calculations, the para- and diamagnetic chemical shifts were used to study the salen ligand conformation of the para- and diamagnetic complexes, resulting in a mechanistic proposal for the enantioselective step in catalysis.

A C-linked glycomimetic in the gas phase and in solution: synthesis and conformation of the disaccharide Manalpha(1,6)-C-ManalphaOPh.

The effect of carbon is subtle but sweet: The flexible C-linkage in the newly synthesised C-glycosyl mimetic, Manalpha(1,6)-C-ManalphaOPh allows OH--pi bonding, both in the gas phase and in aqueous solution. This interaction is absent in the O-linked disaccharide (see figure).The intrinsic conformational preference of a newly synthesised glycomimetic, the C-linked disaccharide Manalpha(1,6)-C-ManalphaOPh (1), has been determined in the gas phase at about 10 K by infrared ion dip spectroscopy coupled with density functional theory and ab initio calculations, and compared with its dynamical conformation in aqueous solution at 298 K by NMR spectroscopy. Comparisons are also made between these conformations and those of the corresponding O-linked disaccharide 2 in the gas phase and the C-linked disaccharide Manalpha(1,6)-C-ManalphaOMe (3) in the gas phase and in aqueous solution. The C- and O-linked disaccharides 1 and 2 present quite distinct conformational preferences in the gas phase: inter-glycosidic hydrogen bonding, seen in one of the two conformers populated in 2, is not seen in 1 which adopts a conformation (not populated in 2) with glycosidic dihedral angles (phi, psi, omega) of -72 degrees , 52 degrees and 66 degrees ; supported in part by an OH--pi hydrogen bond. This conformer is also strongly populated in an aqueous solution of 1 (and very weakly, of 3) together with a second conformer, with dihedral angles (phi, psi, omega) of about -60 degrees , 180 degrees and 60 degrees , not seen in the gas phase but by far the dominant conformer in an aqueous solution of 3. The C-disaccharide 1 was tested as a potential inhibitor, but displayed no significant inhibitory activity against Jack Bean alpha-mannosidase.