A series of proposed polyketide chain elongation intermediates has been synthesized as the /V-acetylcysteamine (NAC) thioesters and incorporated into nargenicin (1) in l3C-labeled form by administration to both actively fermenting and resting cultures of Nocardia argentinensis. Thus, feeding of (25,3/?)-[2,3-' 123C2] -2-methyl-3-hydroxypentanoyl NAC thioester 5a gave nargenicin bearing contiguous labels at C-l6 and C-17, as established by l3C NMR. In a complementary experiment, (25,3/?)-[3-2H,3-l3C]-2-methyl-3-hydroxypentanoyl NAC thioester 5b was incorporated into nargenicin without loss of deuterium. Incorporation of the triketide substrate (2£,4/?,5/?)-[2,3-13C2]-2,4-dimethyl-5-hydroxy-2-heptenoyl NAC thioester 8c gave nargenicin labeled at C-14 and C-l5. Finally, feeding of the tetraketide dienoic ester, (2£,4£,6/?,7/?)-[2,3-13C2]-4,6-dimethyl-7-hydroxy-2,4-nonadienoic acid NAC thioester 9d, gave nargenicin labeled, as expected, at C-l2 and C-l3. These results provide further confirmation for a processive model of polyketide chain elongation in which the stereochemistry and oxidation level are adjusted prior to each successive round of condensation-chain extension. The incorporation of the tetraketide precursor also lends support to a proposed intramolecular Diels-Alder mechanism for formation of the characteristic octalin ring system of 1.Nargenicin (1) is a member of a group of naturally occurring reduced alicyclic polyketides containing a characteristic cis-fused octalin ring system. The antibiotic, which is active against
Some philosophers think physical explanations stand on their own: what happens, happens because things have the properties they do. Others think that any such explanation is incomplete: what happens in the physical world must be partly due to the laws of nature. This book follows the debate between these views from Descartes to Hume. The book argues that the competing models of causation in the period grow out of the scholastic notion of power. On this Aristotelian view, the connection between cause and effect is logically necessary. Causes are “intrinsically directed” at what they produce. But when the Aristotelian view is faced with the challenge of mechanism, the core notion of a power splits into two distinct models, each of which persists throughout the early modern period. It is only when seen in this light that the key arguments of the period can reveal their true virtues and flaws. To make this case, the book explores such central topics as intentionality, the varieties of necessity, and the nature of relations. Arguing for controversial readings of many of the canonical figures, the book also focuses on lesser‐known writers such as Pierre‐Sylvain Régis, Nicolas Malebranche, and Robert Boyle.
Solid solutions of (KI)i-t(NH4l)x with concentrations x=0.14, 0.43, and 1 were investigated using dielectric spectroscopy. The high-temperature rocksalt structure of these compounds exhibits a dipole moment of 1.4 D. For x-0.14 and 0.43 the NaCl structure is stable down to the lowest temperatures and the reorienting dipoles undergo a cooperative freezing transition devoid of long-range orientational order.
tains isolated In216 units each consisting of two In1212/2 tetrahedra linked via a common edge [',21. Prolonged storage at or below room temperature can lead-irreproduciblyto the transformation of individual domains of the yellow B-phase into a red a-phase; a concomitant change in the crystal structure has already been demonstrated by X-ray powder and Raman investigation~ [~.~]. A sample of the aphase suitable enough for single crystal investigations cannot be obtained via this solid-solid transformation. According to our differential thermoanalytical investigations, a-InI, transforms endothermically into the (3-phase at + 57 "C on heating (AH = 6.70 kJ/mol). The transformation P-+a is apparently kinetically inhibited; at +5"C, however, (3-In13 could be converted quantitatively into the a-phase within ca. three weeks. Figure 1 shows the phase diagram of Id3-InBr,. Starting from InI,, the Id3-InBr3 system has an extensive mixed-crystal phase, a -I d , -xBrx, adjacent to a two-phase domain between 184 and 194°C in which a-mixed crystals ( x s 1) are in equilibrium with an Id,-rich melt. In this region it is possible to cultivate mixed crystals of the a-phase from the melt by cooling, albeit only in small yields. Information about the structures of the additional intermediary y-, 6-, and &-phases occurring at higher temperatures and increasing InBr, contents are not available[']; InBr, crystallizes in the high-temperature CrCI,(YCl,)-type structure[61 with almost cubic close packing of the anions[71. T PC 208 57 I I l I l I 1 I I I 10 20 30 LO 50 60 70 80 90 PCI L2 1 0 In13 [Mol-% InBr31 InBr3 Fig. 1. Phase relationships at the quasi-binary section In13-lnBr3.According to a crystal structure analysis[*] on a tabular pseudohexagonal single crystal of a-lnI, -xBrx (x = l), the phase crystallizes in the low-temperature CrC13-type structure"] with almost hexagonal close packing of the halogen atoms and ordered 2/3 occupation of the octahedral holes of every second octahedral layer by indium. The structural model was refined to convergence in the space groups R3 ( R = 0.046) and R3 ( R = 0.050); according to the Hamilton significance test[Io1 the space group R3 is preferred with a probability of > 99.5%. The temperature factors of the halogen positions also indicate negligible anisotropy by hexagonal treatment. For the "mixed atom" 12,,Br1/,, the atomic scattering factor of iodine[lol was applied with release of the occupation factor. Split positions, which would have taken into account the differences in the bond lengths In-I and In-Br, could not be consistently refined.The lattice constants of a-InI, were ascertained by indexing the Guinier diagram (ar,, =S.Ol l(5) A, a = 53.64(4)"; ah,,=7.229(1), ch,,=20.514(7) A); a-InI, proves to have a substantially higher density (5.32 g/cm3) than P-InI, (4.70 g/cm3)I2].Further examples of dimorphic aluminum, gallium, and indium trihalides have so far not been reported in the literature. However, in the framework of our current studies we recently found indications of AlBr, also ...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.