Synthetic studies on the taxane skeleton: construction of eight-membered carbocyclic rings by the intramolecular B-alkyl Suzuki-Miyaura cross-coupling reaction

Construction of eight-membered carbocyclic rings via the intramolecular B-alkyl Suzuki-Miyaura cross-coupling reaction has been studied. This protocol proved its potency through the formation of the eight-membered ring possessing a quaternary carbon on its ring in high yield, affording promise of a new access to the eight-membered ring of Taxol. [reaction: see text]     

Synthetic studies on the taxane skeleton: effective construction of eight-membered carbocyclic ring by palladium-catalyzed intramolecular α-alkenylation of a methyl ketone

A new method for the construction of the eight-membered carbocyclic ring in the taxane skeleton is described. The palladium-catalyzed intramolecular α-alkenylation of a methyl ketone effectively constructed the eight-membered carbocyclic ring in the taxol model compound. To the best of our knowledge, this reaction is the first example of forming an eight-membered carbocyclic ring by the palladium-catalyzed intramolecular α-alkenylation of a ketone.     

First organophosphorus radical-mediated cyclisations to afford medium-sized rings: eight-membered lactones and seven- and eight-membered lactams

The phosphorus based radical precursors N-ethylpiperidine hypophosphite (EPHP) and diethylphosphine oxide (DEPO) are efficient reagents for carrying out the formation of seven- and eight-membered rings. Esters and amides were successfully converted into the corresponding eight-membered lactones and seven- and eight-membered lactams in good to excellent yields.     

A synthetic entry to 2,3-fused ring indole derivatives by ring-closing metathesis reactions

Six to eight-membered carbocycles and eight-membered azacycles fused to the 2,3-position of the indole ring, representing tricyclic substructures of several indole alkaloids, can be synthesized by RCM reactions from easily accessible dienic precursors.     

Palladium-catalyzed formal cycloaddition of silacyclobutanes with enones: synthesis of eight-membered cyclic silyl enolates

Treatment of silacyclobutanes with enones under palladium catalysis resulted in formal cycloaddition to yield the corresponding eight-membered ring. The reaction provides a facile and convergent access to synthetically useful eight-membered cyclic silyl enolates.     

Stereoselective construction of eight-membered carbocycles by brook rearrangement-mediated [3 + 4] annulation

[reaction: see text] A newly developed strategy for eight-membered carbocycles via [3 + 4] annulation that involves the combination of beta-substituted acryloylsilanes and enolates of cycloheptenone is described. A unique feature of this annulative approach is its capacity to generate, in two steps, eight-membered ring systems containing useful functionalities for further synthetic elaboration from readily available three- and four-carbon components.     

Zur struktur des diethylgermanium-diphenylsiliciumoxids und zur konformation von achtringen mit silicium und germanium

Diethylgermanium-diphenylsilicon oxide [(Et₂Ge)O(Ph₂Si)O]₂ has been prepared by reaction of diethylgermanium dichloride and diphenyldihydroxysilane. Spectra and X-ray analysis showed the presence of a puckered eight-membered silicon—germanium—oxygen ring. The ethyl groups of the germanium atoms are disordered. A procedure is proposed for eight-membered rings with low symmetry to obtain a classification into clearly explaining ring forms based on exact or approximated planes put through as many ring atoms as possible. All of the 42 published eight-membered rings with silicon or germanium and oxygen, nitrogen or carbon could be classified by this method.     

Solid-state conformation of a hybrid tripeptide between beta-amino acid; 8-aminocyclooct-4-enecarboxylic acid and 2-aminoisobutyric acid

An eight-membered cyclic beta-amino acid, 8-aminocyclooct-4-enecarboxylic acid, was designed as a conformationally restricted non-proteinogenic amino acid. A hybrid tripeptide containing this eight-membered cyclic beta-amino acid and 2-aminoisobutyric acids was synthesized by conventional solution methods. The conformation of the tripeptide was studied using X-ray analysis and was shown to form an eleven-membered hydrogen-bonded turn (3(11)-helical structure) in the solid state.     

Ligand effects in the synthesis of N-heterocycles by intramolecular Heck reactions

Chemo- and regioselectivity of intramolecular Heck reactions are dependent on the type of ligand employed. Six- to eight-membered benzolactams are produced in good yields using PPh3 as ligand. In contrast, a biaryl coupling occurred preferentially under ligandless conditions to form a dihydrophenanthridine product. Conformations of the seven- and eight-membered benzolactams in the solid state were examined by X-ray crystallography.     

Synthesis of oxacyclic dienes via ring-closing enyne metathesis: difference in construction of eight-membered rings

A series of oxacyclic diene compounds, especially eight-membered products bearing a single oxygen atom which have not been reported previously, were successfully synthesized via ring-closing enyne metathesis using the second-generation Grubbs catalyst. In contrast to the construction of the five-membered rings, completely opposite substrate selectivity that methyl substituted internal alkyne showed much higher reactivity than terminal alkyne was observed in building eight-membered ring derivatives.     

A new strategy for construction of eight-membered carbocycles by Brook rearrangement mediated [6 + 2] annulation

[reaction: see text] A newly developed strategy for construction of eight-membered carbocycles via [6 + 2] annulation that involves the combination of beta-alkenoyl acylsilanes and a vinyllithium derivative is described. A unique feature of this annulative approach is that it enables in one operation and a stereoselective manner construction of eight-membered ring systems containing useful functionalities for further synthetic elaboration from readily available six- and two-carbon components.     

Surprising obtention of an enantiopure eight-membered cyclic ether from camphor

A highly-functionalized eight-membered cyclic ether, with an additional interesting trans-fusion to a cyclobutane ring, is enantiospecifically obtained in high yield from a camphor-derived di(spiroepoxide)-substituted 1-norbornyl triflate, via a regio- and stereocontrolled domino process. The described process could constitute a novel model procedure for the preparation of eight-membered cyclic ethers from α,α′-bis(spiroepoxide) cyclopentyl derivatives.     

Stereoselective construction of eight-membered oxygen heterocycles by Brook rearrangement-mediated [3 + 4] annulation

[reaction: see text] A newly developed strategy for eight-membered oxygen heterocycle construction via [3 + 4] annulation is described. The method involves the combination of beta-substituted acryloylsilanes and enolates of 6-oxacyclohept-2-en-1-one. A unique feature of this annulative approach is its capacity to generate eight-membered ring systems containing useful functionalities for further synthetic elaboration from readily available three- and four-carbon components.     

Synthesis of eight-membered ring compounds using enyne metathesis

[reaction: see text] A novel procedure for synthesizing eight-membered ring compounds was developed using ruthenium-catalyzed enyne metathesis. When a CH2Cl2 solution of enyne connected with catechol, o-amino phenol, or o-phenylenediamine was stirred in the presence of benzylidene ruthenium carbene complex (10 mol %) at room temperature overnight, an eight-membered ring compound was obtained in high yield. In a similar manner, monocyclic 1,4-diaza- or 1-oxa-4-azacyclooctene derivative was obtained in high yield.     

Synthesis of sultam scaffolds via intramolecular oxa-Michael and diastereoselective Baylis-Hillman reactions

A divergent synthetic approach to new sultams utilizing intramolecular oxa-Michael and Baylis-Hillman reactions of readily prepared vinyl sulfonamides and suitably protected amino alcohols, is reported. A variety of seven- and eight-membered ring sultam scaffolds were synthesized using oxa-Michael pathways, whereas both five- and six-membered rings were synthesized using Baylis-Hillman methods. Baylis-Hillman reactions proceed with good to excellent levels of diastereoselectivity, and oxa-Michael reactions leading to eight-membered ring sultams provide empirical evidence validating 8- endo-trig cyclization pathways.     

Using nucleophilic substitution reactions to understand how a remote alkyl or alkoxy substituent influences the conformation of eight-membered ring oxocarbenium ions

[reaction: see text] A remote alkoxy substituent strongly stabilizes one particular conformer of an eight-membered ring oxocarbenium ion by a through-space electrostatic effect. X-ray crystallographic analysis of a crystalline derivative proves that kinetically controlled nucleophilic substitution favors the 1,4-trans product. Nucleophilic substitution of the corresponding alkyl-substituted acetate, however, is unselective. A computational model has been developed and experimentally validated to predict the low-energy conformers of C3-, C4-, or C5-alkyl- or alkoxy-substituted eight-membered ring oxocarbenium ions.     

Cyclische Phenyl-carbamate des Miotin-Typs und ihre Wirkung auf die Acetylcholinesterase

Cyclic Phenyl Carbamates and Their Action on Acetylcholinesterase Several six-, seven-, and eight-membered cyclic phenyl carbamates of the miotin type have been synthesised and their in vitro potency as inhibitors of acetylcholinesterase determined. The eight-membered rings were found to be the most potent and are comparable with physostigrnin or miotin. It was concluded that, for maximum potency, the orientation of the carbamate group relative to the aromatic plane has to be close to orthogonal.     

Construction of Benzosiloles, Six- and Eight-Membered Silacyclic Skeletons, via a Pd-Catalyzed Intramolecular Mizoroki-Heck Reaction of Vinylsilanes

A variety of silacycles including benzosiloles, six- and eight-membered silacyclic skeletons, were efficiently synthesized via a Pd-catalyzed intramolecular Mizoroki-Heck reaction of vinylsilanes.     

Novel water clusters in the crystalline state: structures of a symmetrical,cyclic hexamer and an 'opened-cube' octamer

Six- and eight-membered hydrogen-bonded water clusters of novel structure types have been found in crystalline hydrates.