Zinc-mediated ring-expansion and chain-extension reactions of beta-keto esters

[Chemical reaction: see text] The reaction of cyclic beta-keto esters with CF3CO2ZnCH2I provided the corresponding ring-expanded products in moderate to good yields. Although alpha-substituted acyclic beta-keto esters reacted with much less efficient, chain-extension reaction of simple beta-keto esters also proceeded effectively to generate gamma-keto esters in high yields.     

氧杂双环烯烃开环反应的研究及其应用

有机金属试剂(其中包括镍、钯、铑、铁、铜)以及路易斯酸能选择性地催化氧杂双环烯烃的亲核开环反应,并建立多个手性中心。这种方法已经被用在一些生物活性分子的全合成过程当中,如甾体酮类似物等。     

Ring-opening reactions of diarylvinylidenecyclopropanes by iodine and bromine

Diarylvinylidenecyclopropanes undergo a novel ring-opening reaction upon treatment with iodine or bromine at 0-25 °C in 1,2-dichloroethane to give the corresponding iodinated or brominated naphthalene derivatives in good to high yields within 3 h. The further transformation of the corresponding iodinated or brominated naphthalene derivatives has been disclosed.     

Ring-opening reactions of benzotriazoles with Wittig reagents

Nonafluorobutanesulfonyl-1H-benzotriazole affords phenylazomethylenetriphenylphosphoranes upon treatment with in situ generated alkyl triphenylphosphoranylidenes. Methylenetriphenylphosphorylidene yields the corresponding bis-phenylazomethylene-triphenylphosphorane.     

Ring-opening reactions of methylenecyclopropanes promoted by metal halides

The methylenecyclopropanes (MCPs) react with various metal chlorides or bromides to give the corresponding homoallylic chlorides or bromides in good yields. [reaction: see text]     

Medium-sized rings versus macrocycles through rhodium-catalyzed ring-expansion reactions of cyclic acetals

α-Diazo β-ketoesters and diketones react with cyclic acetals under Rh(II) catalysis to yield unprecedented polyoxygenated 8- and 9-membered rings in one pot. The reactions occur under mild conditions with yields up to 90%. A perfect regioselectivity is obtained, which can be rationalized through a mechanistic hypothesis that considers 1) the formation of an oxonium ylide, 2) its transformation into an unsaturated acyclic oxocarbenium electrophilic intermediate, and 3) an intramolecular nucleophilic attack in a direct application of Baldwin's rules.     

Ring-opening reactions of cyclic ethers with diiodo- and dibromodimethylsilane equivalents

Ring-opening halosilation of cyclic ethers with reagents of (Me₂N)₂SiMe₂/4MeI (1a) and (Me₂N)₂SiMe₂/4allylBr (1b) was studied. Tetrahydrofuran and cyclohexene oxide reacted with 1a and 1b to give ring-opened di(haloalkoxy)dimethylsilanes in good yield. With less strained tetrahydropyran, however, only reagent 1a gave the ring-opened product. Reactions of reagents 1a and 1b with propylene oxide also proceeded smoothly, although the regioselectivity was rather low. When similar reactions were carried out with (Me₂N)₂SiMe₂/2MeI (2a) and (Me₂N)₂SiMe₂/2allylBr (2b) in a ratio of cyclic ethers/2a or 2b = 1/1, the corresponding 1:1 adducts were obtained.     

An improved catalyst system for the iron-catalyzed intermolecular ring-expansion reactions of epoxides

A considerable improvement is reported in the iron-catalyzed ring-expansion reactions of epoxides generating tetrahydrofuran derivatives by formal insertion of an alkene. Optimization of the catalyst system revealed that a preformed [Fe(salen)] complex minimizes the formation of polymerization side-products so that increased yields of intermolecular reactions were obtained. However, more importantly, the scope of the reaction could also be enlarged considerably. The iron-catalyzed ring-expansion reaction can now be applied to some styrene oxide derivatives, acting as radical donors, as well as to a wide variety of acceptor-substituted acyclic alkenes and cyclic dienes that act as radical acceptors. The use of unsymmetrical radical acceptors led to interesting questions concerning the regiochemistry of the reactions. The conservation of the stereochemistry of the starting materials in the products was investigated through a study of the reactions of E- and Z-configured acceptor-substituted double bonds. The reactions of fumaric and maleic esters were performed and the ratios of diastereomeric and regioisomeric products were determined.     

Ring-opening reactions of 2-aryl-3,4-dihydropyrans with nucleophiles

Ring-opening reactions of 2-aryl-3,4-dihydropyrans with nucleophiles were reported for the first time. A possible mechanism was also proposed. Finally, this method was used in the synthesis of a novel tetrahydrocarbazole derivative that possesses a biologically active skeleton.     

Synthesis of carborane-fused cyclobutenes and cyclobutanes

Transmetalation of carborane-fused zirconacycles to Cu(Ⅱ) induces the C-C coupling reaction to form four-membered rings. This serves as a new efficient and general methodology for the generation of a series of carborane-fused cyclobutenes and cyclobutanes. A reaction mechanism involving transmetalation to Cu(Ⅱ) and reductive elimination is proposed.     

Ring-opening reactions of some phosphoryl-substituted cyclopropanes by -NH and -SH nucleophiles

Reactions of (1-cyanocyclopropyl)diphenylphosphine oxide ( 1a ) with -NH and -SH nucleophiles gave the anticipated ring-opened products, while those of (1-ethoxycarbonylcyclopropyl)diphenyl-phosphine oxide ( 1b ) or diethyl 1-cyano- and 1-ethoxycarbonylcyclopropylphosphonates ( 1c–d ) were often deflected by ester-exchange reactions. The ring-opened products of 1a by amines reacted with aromatic aldehydes under PTC conditions to produce homoallylic amines in good yields. © 1996 John Wiley & Sons, Inc.     

Ring-opening reactions of aromatic N-heterocycles by scandium and yttrium alkyl complexes

The C-N bond in aromatic N-heterocycles is a strong bond, its cleaving involving mostly examples of metal-element multiple bonds. We report on the C-C coupling of two molecules of an aromatic N-heterocycle mediated by scandium and yttrium benzyl complexes supported by a ferrocene 1,1'-diamide ligand. The reaction with 1-methylimidazole leads, ultimately, to the formation of a ring-opened imidazole coupled to a 1-methylimidazole fragment, a structure showing extended conjugation of double bonds. The experimental evidence agrees with involvement of only sigma bonds in these transformations.     

Ring-opening reactions of cyclic acetals and 1,3-oxazolidines with halosilane equivalents

Reactions of acetal and 1,3-oxazolidine rings were examined using two kinds of iodosilane equivalent reagents, a 1:2 mixture of Me3SiNEt2 and MeI (reagent 1a) and a 1:1 mixture of Et3SiH and MeI containing a catalytic amount of PdCl2 (reagent 1b). In the reactions of alkanone ethylene acetals with reagent 1a, a C-O bond in the acetal ring readily cleaved to give 2-(trimethylsiloxy)ethyl enol ethers. Similarly, the C-O bond of 1,3-oxazolidine rings cleaved to give ring-opened imine or enamine derivatives. The reactions of aromatic ketone ethylene acetals and cyclohexanone trimethylene acetal led to deprotection of the acetal unit to liberate free ketones. With reagent 1b, cycloalkanone ethylene acetal afforded a dimeric product with 2-iodoethyl alkenoate moieties, while aromatic ketone ethylene or trimethylene acetals produced deprotected ketones.     

Ring-opening reactions of triazolo- and tetrazolo-pyridopyrimidines or quinazolines with some carbon nucleophiles

Syntheses of some new pyridotriazolo- and pyridotetrazolopyrimidines are described. These tricyclic systems and tetrazoloquinazoline react with some carbon nucleophiles, generated from compounds with reactive methylene groups, exclusively at position 2 of the fused pyrimidine ring to give the corresponding open-chain enamines.

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Ringöffnungsreaktionen von Triazolo- und Tetrazolo-Pyridopyrimidinen oder Chinazolinen unter dem Einfluß einiger Carbanionen als Nucleophile

Zusammenfassung Die Synthesen von einigen neuen Pyridotriazolo- und Pyridotetrazolopyrimidine werden beschrieben. Diese tricyclischen Verbindungen sowie Tetrazolochinazolin reagieren mit einigen Carbanionen, die aus Verbindungen mit reaktiven Methylengruppen entstehen, ausschließlich in Stellung 2 des kondensierten Pyrimidinringes, wobei die entsprechenden offenkettige Enamine entstehen.

     

Ring-opening reactions of difluoro(methylene)cyclopropanes with halogens and amines

The distal and proximal bond of difluoro(methylene)cyclopropanes (F2MCPs) could be cleaved, respectively, under different conditions to give the corresponding ring-opening products. The reaction mechanisms are discussed.     

Ring-opening oligomerization reactions using aluminium complexes of schiff's bases as initiators

Bifunctional telechelics with defined structure can be prepared by oligomerization of oxiranes, β-butyrolactone and L-lactide using aluminium Schiff's base complexes as initiators. Chiral initiator (SALCENAlCl) shows a stereoelective character leading to preferential oligomerization of one enantiomer from a racemic monomer mixture. The reaction with β-butyrolactone proceeds through O-alkyl cleavage. Alkoxy Schiff's bases aluminium complexes are used for oligomerization of L-lactide. All the prepared oligomers were fully characterized by IR, elemental analysis, ¹H and ¹³C NMR and GPC.     

Hexamethylsilirane: II. Ring-opening reactions with some simple reagents

Ring-opening reactions formally of the type Me₂SiCMe₂CMe₂ ? X-H→ Me₂XSiCMe₂CMe₂H occur on reaction of hexamethylsilirane with water, alcohols, ammonia, amines, HCl and car?ylic acids and germanium and tin hydrides. More complicated ring opening reactions are observed with halogens, tetrahalomethanes and molecular oxygen. The oxidation and protolysis (H₂O, ROH, HCl, RCO₂H, NH₃, amines) reactions of hexamethylsilirane are exothermic at room temperature.