Total synthesis of the thiopeptide antibiotic amythiamicin D

The thiopeptide (or thiostrepton) antibiotics are a class of sulfur containing highly modified cyclic peptides with interesting biological properties, including reported activity against MRSA and malaria. Described herein is the total synthesis of the thiopeptide natural product amythiamicin D, which utilizes a biosynthesis-inspired hetero-Diels-Alder route to the pyridine core of the antibiotic as a key step. Preliminary studies using a range of serine-derived 1-ethoxy-2-azadienes established that hetero-Diels-Alder reaction with N-acetylenamines proceeded efficiently under microwave irradiation to give 2,3,6-trisubstituted pyridines. The thiazole building blocks of the antibiotic were obtained by either classical Hantzsch reactions or by dirhodium(II)-catalyzed chemoselective carbene N-H insertion followed by thionation, and were combined to give the bis-thiazole that forms the left-hand fragment of the antibiotic. The key Diels-Alder reaction of a tris-thiazolyl azadiene with benzyl 2-(1-acetylaminoethenyl)thiazole-4-carboxylate gave the core tetrathiazolyl pyridine, which was elaborated into the natural product by successive incorporation of glycine and bis-thiazole fragments followed by macrocyclization.     

Effects of changes in absolute signal level on psychophysical tuning curves in quiet and noise in patas monkeys

Forward masking psychophysical tuning curves (PTCs) were measured in patas monkeys (Erythrocebus patas) at 2, 4, and 8 kHz at signal levels of 10, 30, and 60 dB SL in quiet, and at 10 dB above masked threshold in two levels of wideband noise. Absolute signal levels with masking approximated those at 30 and 60 dB SL in quiet. Results in quiet agree with those reported in the literature, demonstrating broadening of the PTC as signal level is increased. The PTCs measured in noise also demonstrated a similar broadening, or loss of selectivity, at higher SPLs. These later findings differ from those of a previous study [D.M. Green, B.R. Shelton, M.C. Picardi, and E.R. Hafter, J. Acoust. Soc. Am. 69, 1758-1762 (1981)] which used maskers to control the broadened excitation pattern in humans at levels of up to 34 dB above threshold. Differences in findings might be attributed to higher SPLs used in the present study. The data taken in noise backgrounds are not consistent with explanations for broadening based on an increase in the width of excitation patterns, but instead support the suggestion that the filter itself is nonlinear. Moreover, comparisons of PTCs in quiet and noise suggest that "off-frequency" listening acts at any given measurement level to artificially sharpen PTCs.     

Biogeological Raman spectroscopic studies of Antarctic lacustrine sediments

Analysis of lacustrine sediments is an accepted method for deciphering the palaeoenvironment of a lake's catchment area, as each strata of the sediment gives information about the rock type it was eroded from and also the state of the lake, i.e. oxic or anoxic. Antarctica has long been accepted as a putative analogue for Mars, so the analysis of Antarctic material may give results that can be compared to sediments on Mars. Raman spectroscopy has been selected as the method of analysis as it does not destroy the sample, can be used in situ and requires very little sample preparation. It is a suitable method for analysing both inorganic and organic matter and a miniature spectrometer is currently being developed for use in the field. The results from the spectrometers can serve as a guide for analysing sediments on Mars. It has been shown that Raman spectroscopy can detect and differentiate between oxic and anoxic sediments. Both 1064 and 785 nm wavelengths are suitable for laser excitation of organic and inorganic matter.     

Dirhodium(II) tetraacetate catalysed reactions of diazo thioamides: isolation and cycloaddition of anhydro-4-hydroxy-1,3-thiazolium hydroxides (thioisomünchnones), an approach to analogues of dehydrogliotoxin

Dirhodium(II) tetraacetate catalysed reaction of the indoline diazo thioamide 8 gives the thioisomünchnone 9, a stable characterisable solid. This masked thiocarbonyl ylide undergoes 1,3-dipolar cycloaddition with N-methylmaleimide and maleic anhydride to give the exo-cycloadducts 10 and 11, characterised by X-ray crystallography. The thioisomünchnone 18, derived from diazo thioamide 17, is an extremely stable crystalline mesoionic system which can be characterised by X-ray crystallography but fails to undergo intramolecular cycloaddition. The related thioisomünchnone 19 can be generated by reaction of indoline-2-thione 7 with bromoacetyl chloride in the presence of triethylamine, and undergoes cycloaddition to give adducts 20 and 21.     

Simple one-pot process for the bioresolution of tertiary amino ester protic ionic liquids using subtilisin

An efficient hydrolase-catalyzed bioresolution of tertiary amino ester protic ionic liquids has been demonstrated. Protic ionic liquids have been prepared in one step from the corresponding tertiary amino alcohols by treatment with butyric anhydride. After bioresolution, unreacted esters can be easily separated from the corresponding alcohols by extraction with hexane. Bioresolution of quinuclidin-3-yl butyrate has been performed with excellent selectivity.     

Peroxidase enzyme sensor for on-line monitoring of disinfection processes in the food industry.

For desirable environmental reasons, peroxides have replaced halogenated substances for disinfection purposes in the food and beverage industry. However, cost issues and the requirement to remove these agents completely after disinfection necessitate simple, low-cost and sensitive test methods with a wide dynamic range and on-line capability. The development and performance of such a method is detailed here. Low-cost peroxide sensors were fabricated using a single deposition procedure, in which horseradish peroxidase enzyme and dimethylferrocene mediator were entrapped within a cellulose acetate membrane, over the working electrode area of a screen-printed three-electrode assembly. Optimum performance was obtained using HRP and DMFc loadings of 25 U and 0.03 micromol per electrode, respectively, and a mean cellulose acetate molecular weight of 37,000. The device had a detection limit of 49.5 microM hydrogen peroxide and mean RSD values of 21% across the concentration range 49.5-368 microM. In laboratory studies the sensor was shown to have a stability of > or = 4 d in continuous flow-mode maintaining an accuracy of +/- 16% that was considered acceptable for the intended on-line monitoring of the disinfection process. In a field study, it was successfully used on-line within a flow-cell to measure peroxide levels during disinfection of an industrial fermentation vessel.     

Inhibition of Hsp90 with Resorcylic Acid Macrolactones: Synthesis and Binding Studies

A series of resorcylic acid macrolactones, analogues of the natural product radicicol has been prepared by chemical synthesis, and evaluated as inhibitors of heat shock protein 90 (Hsp90), an emerging attractive target for novel cancer therapeutic agents. The synthesis involves acylation of an ortho‐toluic acid dianion, esterification, followed by a ring‐closing metathesis to form the macrocycle. Subsequent manipulation of the protected hydroxymethyl side chain allows access to a range of new analogues following deprotection of the two phenolic groups. Co‐crystallization of one of the new macrolactones with the N‐terminal domain of yeast Hsp90 confirms that it binds in a similar way to the natural product radicicol and to our previous synthetic analogues, but that the introduction of the additional hydroxymethyl substituent appears to result in an unexpected change in conformation of the macrocyclic ring. As a result of this conformational change, the compounds bound less favorably to Hsp90.     

Total synthesis of enantiopure beta-D-mannosyl phosphomycoketides from Mycobacterium tuberculosis

The first stereoselective total synthesis of a beta-d-mannosyl phosphomycoketide is reported. To introduce the stereogenic centers in the chain, three linear chiral building blocks were prepared using two different asymmetric catalytic conjugate addition protocols. Coupling of the various linear fragments was affected using a Julia-Kocienski sequence. This approach constitutes a general and convergent method for the construction of saturated oligoisoprenoid chains of any length and stereochemistry. In addition, an alternative approach for the formation of the difficult beta-mannosyl phosphate linkage was shown to be successful. Biological evalutation of the all-S compound revealed that its antigenic potency for T cells is identical to that of the natural product. This result implies that the fine structure of the lipid part has a strong influence on biological activity and that the T cell response is discriminating between different stereoisomers.