Regioselectivity of the Claisen rearrangement in meta-allyloxy aryl ketones: an experimental and computational study, and application in the synthesis of (R)-(-)-pestalotheol D

A study of the regioselectivity of the Claisen rearrangement of meta-allyloxy aryl ketones showed that the electron-withdrawing carbonyl group has a major influence and strongly directs rearrangement to the more hindered ortho position. However, when the ketone is part of a ring structure, its electronic effect can be negated by conversion into its triisopropylsilyl enol ether, which dramatically reverses the regiochemistry of the Claisen rearrangement. DFT calculations suggest that the effect is electronic although there is also a steric effect of the bulky silyl group. This strategy for influencing the regiochemical outcome of the Claisen rearrangement was then employed in a short synthesis of the furo[2,3-g]chromene, (-)-pestalotheol D, that confirms the absolute stereochemistry of the natural product.     

Telomestatin: formal total synthesis and cation-mediated interaction of its seco-derivatives with G-quadruplexes

The structurally unique natural product telomestatin incorporates seven oxazole rings and one sulfur-containing thiazoline in a macrocyclic arrangement. The compound is a potent inhibitor of the enzyme telomerase and therefore provides a structural framework for developing new potential therapeutic agents for cancer. An efficient formal total synthesis of telomestatin is reported in which the key steps are the use of dirhodium(II)-catalyzed reactions of diazocarbonyl compounds to generate six oxazole rings, demonstrating the power of rhodium carbene methodology in organic chemical synthesis. CD spectroscopy establishes that seco-derivatives of telomestatin are potent stabilizers of G-quadruplex structures derived from the human telomeric repeat sequence. Mass spectrometry studies, confirmed by molecular dynamics simulations, provide the first evidence that high affinity binding to terminal G-tetrads in both 1:1 and 2:1 ligand complexes is mediated through the macrocycle coordinating a monovalent cation, with selectivity for the antiparallel structure.     

Synthesis of amino-1,4-benzoquinones and their use in Diels-Alder approaches to the aminonaphthoquinone antibiotics

A new protocol for the synthesis of protected amino-1,4-benzoquinones by oxidation of the corresponding 2,5-dimethoxyaniline derivatives using PhI(OAc)(2) or PhI(OCOCF(3))(2) in water containing 2.5% methanol is reported. The process represents an improvement over previously reported methods, both in terms of yield and number of steps, and in the range of nitrogen protecting groups that it tolerates. A number of novel aminobenzoquinones were prepared and subsequently used as dienophiles in Diels-Alder reactions to form building blocks for the synthesis of the aminonaphthoquinone antibiotics such as salinisporamycin.     

New nuclides: Neptunium-243 and neptunium-244

We have observedγ-rays following the β^?-decay of²⁴³Np and²⁴⁴Np after chemical isolation of neptunium isotopes from the products of the reaction of 835 MeV¹³⁶Xe with²⁴⁴Pu. The ground-state of 1.85-min²⁴³Np hasJ=5/2. The decay of 2.29-min²⁴⁴Np (probableJ ^π=7^?) populates high-spin members of the ground state rotational band in²⁴⁴Pu.     

Tandem mass spectrometry of aminated fullerene derivatives

The products of the reaction between fullerenes (C₆₀/C₇₀) and dimethylamine were investigated by fast atom bombardment (FAB) mass spectrometry and tandem mass spectrometry (MS/MS). The FAB mass spectrum shows peaks corresponding to the addition of up to eight dimethylamine species, exclusively to C₇₀. MS/MS reveals an unusual fragmentation pattern. The mass spectrum of the reaction products, together with a number of tandem mass spectra, are shown.     

Separation of group five elements by reversed-phase chromatography

A chemical separation method based on reversed-phase chromatography has been developed to separate the group five elements from the reaction products produced in the bombardment of ²⁴³Am with ⁴⁸Ca ions. Decontamination factors on the order of 10⁶ were achieved for group-three elements such as americium, and 10⁷ or more for selected reaction products as measured by gamma-ray spectrometry pre-and post-chemistry. Details of the chemical separation scheme are discussed and compared to previously reported results.     

A convenient precursor to 3,4-didehydropyridine

The preparation of 3-(3,3-dimethyltriazen-l-yl)pyridine-4-carboxylic acid (5), a convenient shelf-stable precursor to 3,4-didehydropyridine, is described.