Total synthesis of hectochlorin

Hectochlorin (1) is a marine natural product with significant fungicidal activity. A synthesis effort was initiated to develop a flexible route to hectochlorin which would allow access to analogues with potentially improved activity and/or attributes relative to the natural product. A successful total synthesis of hectochlorin is described. [structure: see text]     

Nucleophilic-alkylation-reoxidation: a functionalization sequence for single-wall carbon nanotubes

A new reaction sequence for the chemical functionalization of single-wall carbon nanotubes (SWNTs) consisting of the nucleophilic addition of t-BuLi to the sidewalls of the tubes and the subsequent reoxidation of the intermediates t-Bu(n)SWNT(n-) leading to t-Bu(n)SWNT was developed. During the formation of the t-Bu(n)SWNT(n-), a homogeneous dispersion in benzene was formed due to the electrostatic repulsion of the negatively charged intermediates causing debundling. The entire reaction sequence can be repeated, and the degree of functionalization of the products (t-Bu(n))(m)SWNT (m = 1-3) increases with increasing m. Degrees of functionalization expressed as the carbon-to-addend ratio of up to 31 were reached. The reaction was studied in detail by photoelectron spectroscopy, Raman spectroscopy, and scanning tunneling microscopy (STM). The C 1s core level spectra reveal that the nucleophilic attack of the t-BuLi leads to negatively charged SWNTs. Upon oxidation, this negative charge is removed. The valence band spectra of the functionalized samples exhibit a significant reduction in the pi-derived density of states. In STM, the covalently bonded t-butyl groups attached to the sidewalls have been visualized. Raman spectroscopy reveals that addition of the nucleophile to metallic tubes is preferred over the addition to semiconducting tubes.     

Spinosyn g: proof of structure by semisynthesis

[reaction: see text] Spinosyn G was isolated in the late 1980s as a minor component from the broth of our potent, fermentation-derived insecticide spinosad. Its structure was then tentatively identified as 5' '-epispinosyn A (3) on the basis of (1)H and (13)C NMR data, but the 4' '-epi compound 4 could not be conclusively ruled out with the data available. Described herein are unambiguous syntheses of both 3 and 4, whereby 3 was proved identical to the natural product. Compound 4 was prepared from intact spinosyn A by a novel F-TEDA-promoted oxidative deamination to the 4' '-ketone 5, stereoselective reduction to the equatorial alcohol 6, and nitrogen incorporation via the axial azide 7. Compound 3 was obtained by coupling spinosyn A 17-pseudoaglycone (9) with the N-protected dihydropyran 16 derived from methyl l-ossaminide (14). This gave an approximately 2:1 mixture of anomeric products 17 with the desired equatorial glycoside predominating, which was then converted to 3 by N-deprotection and methylation.     

In-situ bioconjugation in stationary media and in liquid flow by femtosecond laser ablation

In-situ functionalization of gold nanoparticles with fluorophore-tagged oligonucleotides is studied by comparing femtosecond laser ablation in stationary liquid and in biomolecule flow. Femtosecond laser pulses induce significant degradation to sensitive biomolecules when ablating gold in a stationary solution of oligonucleotides. Contrary, in-situ conjugation of nanoparticles in biomolecule flow considerably reduces the degree of degradation studied by gel electrophoresis and UV–Vis spectrometry. Ablating gold with 100 μJ femtosecond laser pulses DNA sequence does not degrade, while the degree of fluorophore tag degradation was 84% in stationary solution compared to 5% for 1 mL/min liquid flow. It is concluded that femtosecond laser-induced degradation of biomolecules is triggered by absorption of nanoparticle conjugates suspended in the colloid and not by ablation of the target. Quenching of nanoparticle size appears from 0.5 μM biomolecule concentration for 0.3 μg/s nanoparticle productivity indicating the successful surface functionalization. Finally, increasing the liquid flow rate from stationary to 450 mL/min enhances nanoparticle productivity from 0.2 μg/s to 1.5 μg/s, as increasing liquid flow allows removal of light absorbing nanoparticles from the ablation zone, avoiding attenuation of subsequent laser photons.     

Functionalization of single-walled carbon nanotubes with (R-)oxycarbonyl nitrenes

Sidewall functionalization of single-walled carbon nanotubes (SWCNTs) via the addition of (R-)oxycarbonyl nitrenes allows for the covalent binding of a variety of different groups such as alkyl chains, aromatic groups, dendrimers, crown ethers, and oligoethylene glycol units. Such additions lead to a considerable increase in the solubility in organic solvents such as 1,1,2,2-tetrachloroethane (TCE), dimethyl sulfoxide (DMSO), and 1,2-dichlorobenzene (ODCB). The highest solubilities of 1.2 mg/mL were found for SWCNT adducts with nitrenes containing crown ether of oligoethylene glycol moieties in DMSO and TCE, respectively. The presence of chelating donor groups within the addends allowed for the complexation of Cu(2+) and Cd(2+). Atomic force microscopy (AFM) and transmission electron microscopy (TEM) revealed that the functionalized tubes form thin bundles with typical diameters of 10 nm. The presence of thin bundles in solution is supported by (1)H NMR spectroscopy. The elemental composition of the functionalized SWCNT was determined by X-ray photoelectron spectroscopy (XPS). The use of Raman and electron absorption spectroscopy (UV/Vis-nIR) showed that the electronic properties of the SWCNTs are mostly retained after functionalization, indicating a low degree of addition within this series of SWCNT derivatives.