Presence Of Magnesium Bromide Research Articles

Methodological advances permit the stereocontrolled construction of diverse fully synthetic tetracyclines containing an all-carbon quaternary center at position C5a

Here we describe chemical innovations that enable the preparation of fully synthetic tetracyclines containing an all-carbon quaternary, stereogenic center at position C5a, a structurally novel class of compounds in this important family of therapeutic agents. In the key transformation and an important extension of the powerful Michael–Claisen cyclization (AB plus D) approach to the construction of fully synthetic tetracyclines, we show that the six-membered C ring comprising a C5a quaternary carbon center can be assembled by highly stereocontrolled coupling reactions of β-substituted AB enones and o-toluate ester anion D-ring precursors. Novel and versatile β-functionalization reaction sequences employing tris(methylthio)methyllithium and 2-lithio-1,3-dithiane have been developed to transform the AB enone 1 (the key precursor to fully synthetic tetracyclines) into a diverse range of β-substituted AB enone products, including a highly efficient, single-operation method for the synthesis of a β-methyl ester-substituted AB enone. A C5a–C11a-bridged cyclopropane tetracycline precursor was found to undergo efficient and regioselective ring-opening reactions with a range of nucleophiles in the presence of magnesium bromide, thus providing another avenue for the preparation of fully synthetic tetracyclines containing an all-carbon quaternary center at position C5a. Two compounds prepared from the bridged cyclopropane intermediate served as (further) diversifiable branch-points, allowing maximally expedient synthesis of C5a-substituted tetracyclines by final-step diversification.

ChemInform Abstract: Practical Synthesis of 1β-Methylcarbapenem Antibiotics: Side-Chain Substitution Reaction of 2-Arylsulfinyl and 2-Arylsulfonyl Carbapenems.

The C-2 side-chain substitution reaction of a sulfoxide and a sulfone derived from a p-nitrobenzyl (1R, 5S, 6S)-6-[(1R)-1-(tert-butyldimethylsilyloxy)ethyl]-2-[2-(diethylcarbamoyl)phenylthio]-1-methylcarbapen-2-em-3-carboxylate with various mercaptans progressed smoothly in the presence of magnesium bromide. Bromomagnesium thiolate prepared from mercaptans proved to be especially effective for the substitution reaction and gave precursors of 1β-methylcarbapenem antibiotics in high yield.

機能性ニトロンを用いたアミノ酸型天然物の合成研究

(5R)- [and (5S)]-5,6-Dihydro-5-phenyl-2H-1,4-oxazin-2-one N-oxides were designed and synthesized as chiral (E)-geometry-fixed α-alkoxycarbonylnitrones that exist as equilibrating mixture of (E)- and (Z)-isomers. The cyclic nitrone reacted with olefins under mild conditions to afford the main cycloadducts via the least sterically demanding exo modes. The nitrone also reacted with an allyl alcohol in the presence of magnesium bromide from the less hindered face via exo-mode to afford exclusively cycloadduct as a single stereoisomer. These reactions were applied to the syntheses of carbocyclic polyoxin C, anti-β-substituted aspartic acid, clavalanine, monatin, lycoperdic acid, neodysiherbaine A, and maremycins A and D1.

Synthesis and single-crystal characterization of novel 2-ferrocenyl-4 H-pyrazolo[5,1-c][1,4]oxazin-4-one derivatives

A series of novel 2-ferrocenyl-6-substituted-4 H-pyrazolo[5,1-c][1,4]oxazin-4-one derivatives were synthesized by the intra-esterification reaction of ethyl 1-(2-aryl-2-oxoethyl)-3-ferrocenyl-1 H-pyrazole-5-carboxylate in the presence of magnesium bromide and diisopropylethylamine in one-pot procedure. The structures of compounds were characterized by 1H NMR, 13C NMR, IR, HRMS and X-ray diffraction analysis.

Synthesis, characterization and enantioselective free radical reductions of (1 R,2 S,5 R)-menthyldiphenylgermane and its enantiomer

(1 R,2 S,5 R)-Menthyldiphenylgermane and its enantiomer have been prepared in a few steps from germanium tetrachloride. The initial step in this sequence, namely the reaction between germanium tetrachloride and menthylmagnesium chloride, produces menthylgermanium trichloride, which is the exclusive product of this Grignard reaction, presumably due to the bulk of the menthyl group. When used at a low temperature (−78 °C) and in conjunction with Lewis acids, such as magnesium salts, these chiral germanes are capable of reducing ester functionalized radicals in high enantioselectivity, but in low-moderate yield. For example, ( R)-naproxen ethyl ester was obtained in 15% yield and 99% ee by reaction in toluene of 2-bromonaproxen ethyl ester with (1 R,2 S,5 R)-menthyldiphenylgermane in toluene at −78 °C in the presence of magnesium bromide. At 80 °C, (1 R,2 S,5 R)-menthyldiphenylgermane reacted with primary alkyl radicals with a rate constant of 1.02 × 10 6 M −1 s −1. Kinetic studies reveal the Arrhenius expression for this reaction to be: log(k/M −1 s −1) = (11.1 ± 0.4) − (34.6 ± 3.1)/ θ where θ=2.3 RT kJ mol −1.

Radical polymerization of methyl methacrylate in the presence of magnesium bromide as the Lewis acid

We carried out the radical polymerization of methyl methacrylate (MMA) at 0–80°C in the presence of magnesium bromide. The polymerization rate and the molecular weight of the resulting polymers increased according to the amount of the added magnesium bromide as the Lewis acid. It was revealed that the microtacticity of the polymer depended on the amount of magnesium bromide. The effects of the solid surface of magnesium bromide, which is partly soluble in the polymerization systems, were also investigated. The results obtained in this work were compared with the results reported for the polymerization systems in the presence of the other Lewis acids. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 290–296, 1999

Radical polymerization of methyl methacrylate in the presence of magnesium bromide as the Lewis acid

We carried out the radical polymerization of methyl methacrylate (MMA) at 0–80°C in the presence of magnesium bromide. The polymerization rate and the molecular weight of the resulting polymers increased according to the amount of the added magnesium bromide as the Lewis acid. It was revealed that the microtacticity of the polymer depended on the amount of magnesium bromide. The effects of the solid surface of magnesium bromide, which is partly soluble in the polymerization systems, were also investigated. The results obtained in this work were compared with the results reported for the polymerization systems in the presence of the other Lewis acids. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 290–296, 1999

Chelation controlled 1,3-dipolar cycloaddition of 5,6-Dihydro-5-phenyl-1,4-oxazin-2-one N-oxide with allyl alcohols: A short-step synthesis of clavalanine intermediate

( R)-5,6-Dihydro-5-phenyl-1,4-oxazin-2-one N-oxide {( R)- 2} reacts with allyl alcohols 3a-c in the presence of magnesium bromide from the less hindered face via exo-mode to afford corresponding cycloadducts 4a-c with excellent stereoselection. Treatment of ( R)- 2 with three equivalents of racemic secondary allyl alcohols 3d-g under the same conditions causes partial kinetic resolution to give 4d-g as main products among eight possible stereoisomers. Cycloadduct ent- 4a from ( S)- 2 and 3a was converted directly to g-lactone 6, which is known as the key synthetic intermediate of antibiotic clavalanine.

Stereoselective synthesis of 1,4-dienes by chelation-controlled reduction of benzothiazole β-oxosulfides

Abstract Allylated β-oxosulfides of benzothiazole can replace β-oxophosphane oxides to provide a stereospecific alkene synthesis. These sulfides, by reduction with sodium borohydride afford predominantly syn β-hydroxy sulfides. DIBAL reduction in the presence of magnesium bromide improves the syn stereomer formation. Base treatment of these β-hydroxy sulfides affords (Z)-allyl thiiranes which are converted into (E, Z)-dienes.

Practical Synthesis of 1.BETA.-Methylcarbapenem Antibiotics: Side Chain Substitution Reaction of 2-Arylsulfinyl and 2-Arylsulfonyl Carbapenems.

The C-2 side-chain substitution reaction of a sulfoxide and a sulfone derived from a p-nitrobenzyl (1R, 5S, 6S)-6-[(1R)-1-(tert-butyldimethylsilyloxy)ethyl]-2-[2-(diethylcarbamoyl)phenylthio]-1-methylcarbapen-2-em-3-carboxylate with various mercaptans progressed smoothly in the presence of magnesium bromide. Bromomagnesium thiolate prepared from mercaptans proved to be especially effective for the substitution reaction and gave precursors of 1β-methylcarbapenem antibiotics in high yield.

Synthesis of optically active β‐lactams by the reaction of 2‐acyl‐3‐phenyl‐l‐menthopyrazoles with CN compounds

AbstractThe reaction of 1‐acyl‐3,5‐dimethylpyrazoles 1 with CN compounds was kinetically controlled with syn stereoselectivity through a lithium enolate intermediate using lithium diisopropylamide. In contrast, the anti stereoselective reaction of 1 was caused by the action of diisopropylethylamine in the presence of magnesium bromide under the thermodynamic control. Reaction of 2‐acyl‐3‐phenyl‐l‐menthopyrazoles 12 with CN compounds was observed in higher chemical and optical yields with the predominant 2′S configuration. An especially diastereomerically pure product was isolated in the reaction of 2‐propanoyl‐3‐phenyl‐l‐menthopyrazole (12a) with N‐benzylidene‐4‐toluenesulfonamide (2). The products from N‐acyl‐pyrazoles and CN compounds were further cyclized into β‐lactams directly or with short conversion steps.

Trimethylsilylcyanid als Umpolungsreagens, XXII. Nucleophile Acylierung α,β‐ungesättigter Aldehyde durch umgepolte α,β‐ungesättigte Aldehyde

Trimethylsilyl Cyanide — A Reagent for Umpolung, XXII. — Nucleophilic Acylation of α,β‐Unsaturated Aldehydes by Umpolung of α,β‐Unsaturated AldehydesAmbident anions 1aA → 1dA, obtained from 1a — d by LDA, are treated with α,β‐unsaturated aldehydes 2a — c. In all cases the kinetically controlled 1,2/α adducts 3 are formed which in contrast to 1,2/α adducts with α,β‐unsaturated ketones do not rearrange to the thermodynamically stable 1,4/α‐ or 1,4/γ products 4a and 6. Adducts 4 are smoothly formed if imines of 2 are applied. A thermal oxy‐Cope rearrangement transforms adducts 3 into 6. Substantial proportions of 1,2/γ adducts 5 are produced from 1A and 2 in the presence of magnesium bromide.

Reactions de cyclobutenes electrophiles avec les enamines : Facteurs determinant l'obtention de derives bicyclo[2.2.0]hexaniques

When reacted with different ketone enamines, in the presence of magnesium bromide, 1-methoxycarbonyl- and 1-cyano-cyclobutene lead to α-cyclobutyl ketones or to amino-bicyclo [2.2.0] hexane adducts. These products are not formed competitively, but apparently result from different reactions. The cyclobutene nitrile undergoes an exclusive cycloaddition reaction with morpholino enamines. All other reagent combinations lead solely to cyclobutyl ketones. Some reactions of the cycloadducts, among them the thermolysis, were investigated.

ChemInform Abstract: Conformational Change Occasioned by Complexation: “Contra‐Anomeric‐Effect” Epimerization of 2‐Methoxy‐1,3‐dioxanes in the Presence of Magnesium Bromide.

MgBr 2 change la conformation des orthoesters du titre; on obtient un epimere complexe avec effet contra-anomere

Conformational change occasioned by complexation: "contra-anomeric-effect" epimerization of 2-methoxy-1,3-dioxanes in the presence of magnesium bromide

MgBr 2 change la conformation des orthoesters du titre; on obtient un epimere complexe avec effet contra-anomere

ChemInform Abstract: THE SYNTHESIS OF ALKENES FROM CARBONYL COMPOUNDS AND CARBANIONS α TO SILICON. STEREOSELECTIVE SYNTHESIS OF 1‐TRIMETHYLSILYLBUTA‐1,3‐DIENES

AbstractIn Gegenwart von Magnesiumbromid oder Trimethylborat reagiert die Organo‐Siliciumverbindung (II) mit den Aldehyden (I) in stereoselektiver Weise zu den Alkoholen (III).

ADDITION OF ORGANOMETALLICS ON α,β-UNSATURATED THIOCARBONYL COMPOUNDS II. MICHAEL ADDITION OF LITHIUM ENOLATES ON THIOAMIDE AND DITHIOCARBAMATE VINYLOGS AND THIOAROYLFORMAMIDINE

Abstract α,β-Unsaturated thiocarbonyl compounds substituted by a secondary amino group in position β, such as thioamide vinylogs, dithiocarbamate vinylogs or thioaroylformamidines react with lithium enolates of esters, ketones or amides to give 1,4-addition compounds. In the presence of magnesium bromide 1,4-adducts undergo an intramolecular cyclization to give 2H-thiopyranones. Methylation of the 1,4-adducts is followed by a stereospecific elimination of the amine and leads to (1-Z)-1,3-dienic carbonyl compounds: with thioamide vinylogs, δ-methylthio-α,β,γ,δ-diethylenic ketones, esters and amides are obtained. Dithiocarbamate vinylogs give the α-ethylenic ketene dithioacetals, with a carbonyl function in δ-position. Substituted dihydrothiophenes and thiazolines are prepared by the reaction of ethyl α-chloro-α-lithiopropionate with aminopropenethione and thioaroylformamidine respectively.

The synthesis of alkenes from carbonyl compounds and carbanions ? to silicon. Stereoselective synthesis of 1-trimethylsilylbuta-1,3-dienes

Reactions of aldehydes with the 1,3-bis(trimethylsilyl)propenyl anion in the presence of magnesium bromide or trimethyl borate give stereoselectively the alcohols (2) which can be transformed stereospecifically to either (1E,3E)- or (1E,3Z)-1-trimethylsilylbuta-1,3-dienes.

Influence of magnesium bromide on the regioselectivity of the cycloaddition of ynamines with cyclenones: difference in reactivity between cyclohexenones and cyclopentenones.

The cycloaddition of ynamines uith cyclohexenones occurs in the presence of magnesium bromide at the carbonyl but proceeds by attack on the carbon—carbon double bond without catalyst whereas, in contrast, with cyclopentenones, the reaction takes place at the carbon—carbon double bond with or without magnesium bromide.

The Reaction of 1-Alkynes with Organometallic Compounds. V.1 The Reaction of Diethylmagnesium with Hexyne-1 in the Presence of Magnesium Bromide

The Reaction of 1-Alkynes with Organometallic Compounds. V.¹ The Reaction of Diethylmagnesium with Hexyne-1 in the Presence of Magnesium Bromide