- Decarboxylative alkenylation
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Olefin chemistry, through pericyclic reactions, polymerizations, oxidations, or reductions, has an essential role in the manipulation of organic matter. Despite its importance, olefin synthesis still relies largely on chemistry introduced more than three decades ago, with metathesis being the most recent addition. Here we describe a simple method of accessing olefins with any substitution pattern or geometry from one of the most ubiquitous and variegated building blocks of chemistry: alkyl carboxylic acids. The activating principles used in amide-bond synthesis can therefore be used, with nickel- or iron-based catalysis, to extract carbon dioxide from a carboxylic acid and economically replace it with an organozinc-derived olefin on a molar scale. We prepare more than 60 olefins across a range of substrate classes, and the ability to simplify retrosynthetic analysis is exemplified with the preparation of 16 different natural products across 10 different families.
- Edwards, Jacob T.,Merchant, Rohan R.,McClymont, Kyle S.,Knouse, Kyle W.,Qin, Tian,Malins, Lara R.,Vokits, Benjamin,Shaw, Scott A.,Bao, Deng-Hui,Wei, Fu-Liang,Zhou, Ting,Eastgate, Martin D.,Baran, Phil S.
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p. 213 - 218
(2017/05/19)
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- Preparative method for the quantitative separation of methylenic sterols from unsaturated sterol mixtures by chemoselective hydroxymercuration- deoxymercuration
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An unprecedented selective and quantitative method allowing the separation of the methylenic sterols from unsaturated sterol mixtures is reported. We have extended the hydroxymercuration reaction under modified Brown's conditions to a systematic study on thirty unsaturated sterols as various sterol mixtures. A high-yielding directed chemoselective hydroxymercuration was achieved on methylenic side chain of sterols. The resulting organomercurial intermediates were easily purified by flash chromatography on silica gel column and the reactive alkenes were regenerated by deoxymercuration in a biphasic system (ethyl acetate/l M HCl, 2:1 ratio). These conditions leave the ring double bonds of steroids intact. No isomerization of methylene double bonds was observed. Georg Thieme Verlag Stuttgart.
- El Kihel, Laila,Charles, Georges
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p. 2753 - 2759
(2008/02/05)
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- 24-METHYL-23-DEHYDROCHOLESTEROL: A NEW STEROL INTERMEDIATE IN C-24 DEMETHYLATION FROM THE NEMATODES PANAGRELLUS REDIVIVUS AND CAENORHABDITIS ELEGANS?
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Panagrellus redivivus produced 24-methyl-23-dehydrocholesterol as 4.0percent of the 4-desmethylsterols when propagated in a medium containing campesterol as the dietary sterol.The re-examination of previous data revealed that Caenorhabditis elegans produced 1.8percent 24-methyl-23-dehydrocholesterol when propagated in medium containing campesterol. 24-Methyl-23-dehydrocholesterol was not detected when the nematodes were propagated in medium containing 22-dihydrobrassicasterol or 24-methylenecholesterol.This may be a result of the greater efficiency of dealkylation of the latter two sterols.This is the first report of the natural occurrence of this sterol in a non-photosynthetic organism, and the first report in organisms that dealkylate 24-alkylsterols.
- Salt, Thomas A.,Lozano, Ruben,Lusby, William R.,Chitwood, David J.,Thompson, Malcolm J.
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p. 451 - 460
(2007/10/02)
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- INVESTIGATIONS ON THE Δ23-, Δ24(28)- and Δ25-STEROLS OF ZEA MAYS
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The sterols of Zea mays shoots were isolated and characterized by TLC, HPLC, GC/MS and 1H NMR techniques.In all, 22 4-demethyl sterols were identified and they included trace amounts of the Δ23-, Δ24- and Δ25-sterols. 24-methylcholesta-5,E-23-dien-3β-ol, 24-methylcholesta-5,Z-23-dien-3β-ol, 24-methylcholesta-5,25-dien-3β-ol 24-ethylcholesta-5,25-dien-3β-ol and 24-ethylcholesta-5,24-dien-3β-ol.In the 4,4-dimethyl sterol fraction, cycloartenol and 24-methylenecycloartanol were the major sterol components but small amounts of the Δ23-compound, cyclosadol, and the Δ25-compound, cyclolaudenol, were recognized.These various Δ23- and Δ25-sterols may have some importance in alternative biosynthetic routes to the major sterols, particularly the 24β-methylcholest-5-en-3β-ol component of the C28-sterols.Radioactivity from both MVA and methionine was incorporated by Z. mays shoots into the sterol mixture.Although 24-methylene and 24-ethylidene sterols were relatively highly labelled, the various Δ23- and Δ25-sterols contained much lower levels of radioactivity, which is possibly indicative of their participation in alternative sterol biosynthetic routes. (24R)-24-Ethylcholest-5-en-3β-ol (sitosterol) had a significantly higher specific activity than the 24-methylcholest-5-en-3β-ol indicating that the former is synthesized at a faster rate.Key Word Index -- Zea mays; Gramineae; sterols; sterol biosynthesis; cyclolaudenol; cyclosadol; 24-methylene-cycloartanol; 24-methylcholesta-5,E-23-dien-3β-ol; 24-methylcholesta-5,24(28)-dien-3β-ol; 24-methylcholesta-5,25-dien-3β-ol; 24-methylcholest-5-en-3β-ol; 24-ethylcholest-5-en-3β-ol.
- Misso, Neil L. A.,Goad, L. John
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