Intramolecular hydrogen atom abstraction with an eight-membered cyclic transition state in open-chain aliphatic aminium radicals

Hydrogen abstraction by the nitrogen in long-chain aliphatic gas-phase aminium radicals (amine molecular ions) takes place via six-, seven-, and eight-membered cyclic transition states. The subsequent fragmentation is particularly facile after eight-membered ring hydrogen transfer.     

From galactose to highly functionalized seven- and eight-membered carbocyclic rings by ring-closing metathesis

[reaction: see text]A short synthesis of highly functionalized seven- and eight-membered carbocyclic rings from galactose derivatives has been achieved. The key steps are a zinc-mediated reductive ring opening of 6-iodogalactopyranoses and a subsequent ring-closing olefin metathesis using Grubbs' ruthenium catalyst.     

Ionization constants and structure of seven- and eight-membered cyclic phosphorous acid derivatives

Conclusions The spectral characteristics and ionization constants of some five-, seven-, and eight-membered cyclic arylenephosphorous acids were determined, and a conclusion regarding the effect of the size of the ring on the tautomerism of these compounds was made on their basis.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 689–691, March, 1978.     

Synthetic studies on the seven- and eight-membered rings by the intramolecular Nozaki-Hiyama reaction of the allylic phosphates

Synthetic studies on the seven- and eight-membered rings by the intramolecular Nozaki-Hiyama reaction of the allylic phosphates are described. The yield greatly depends on the structure of substrate; however, some complex substrates afforded desired products in high to excellent yield.     

Seven- and eight-membered heterocyclic biaryl synthesis through a metal-free oxidative coupling reaction

A metal-free intramolecular biaryl coupling mediated by DDQ and BF₃·OEt₂ was developed. The reaction proceeds smoothly at room temperature to give seven- and eight-membered heterocyclic biaryl compounds in up to 95% yields. Only 1 equiv of DDQ sufficed for the reaction, obviating the use of a large excess of oxidants and undesired chlorination pathway in metal-mediated process.     

Formation of seven- and eight-membered rings by manganese (III)-based oxidative free radical cyclization.

Oxidative free radical cyclizations of β-ketoesters 1, 5, 11, and 14 with Mn(OAc)₃·2H₂O and Cu(OAc)₂·H₂O proceed in moderate yield to give seven- and eight-membered rings. The regiochemistry of the cyclization and oxidative termination are described.     

Synthesis of 2H-1,5-benzodioxepin and 2,5-dihydro-1,6-benzodioxocin derivatives via ring-closing metathesis reaction

The synthesis of various 2H-1,5-benzodioxepin and 2,5-dihydro-1,6-benzodioxocin derivatives is described. The key step involves the construction of seven- and eight-membered rings via ring-closing metathesis reaction.     

Unique charge-separated intermolecular and eight-membered intramolecular H-bonds in bis-(thio)barbiturates

Reaction of symmetrical and unsymmetrical (thio)barbituric acids with aldehydes in the presence of triethylamine afforded a new form of bis-(thio)barbiturate containing charge-separated inter- and eight-membered intramolecular H-bonds. The reaction products were obtained as bis-(thio)barbiturates containing eight-membered intramolecular H-bond in the presence of l-(+)-tartaric acid (TA). The intramolecular H-bond strength (kcal/mol) and corresponding pKa value for 4ab′ were estimated to be 37 kcal/mol and ?1.3, respectively.     

Synthesis and biological evaluation of the first example of an eight-membered iminoalditol

The synthesis of the first examples of eight-membered iminoalditols has been achieved from 2,3,4,6-tetra-O-benzyl-d-glucopyranose by way of a ring-closing metathesis. The (2R,3R,4R,5S)-2-hydroxymethyl-azocane-3,4,5-triol (3a), which has the d-gluco configuration for C-2-C-5 and appears to exist predominantly in a boat-chair conformation, is a weak inhibitor of glycosidases.     

Stereoselective construction of an anti-β-alkoxy ether by Ireland-Claisen rearrangement for medium-ring ether synthesis

The asymmetric synthesis of an acyclic anti-β-alkoxy ether was achieved by the Ireland-Claisen rearrangement of Z-3-alkoxy-2-propenyl glycolate ester, prepared from Garner’s aldehyde, a glycolic acid derivative, and ethynyl N,N-diisopropylcarbamate. The resulting acyclic ether was facilely converted to seven- and eight-membered cyclic ethers via processes involving ring-closing olefin metatheses.     

Synthesis of fused-tricyclic indole derivatives through an acid-promoted skeletal rearrangement

We developed a novel method for synthesizing fused-tricyclic indole derivatives with a 3-aminomethyl indole motif through a reaction cascade involving ipso-Friedel–Crafts alkylation of phenols, rearomatization of the spirocyclohexadienone unit, and iso-Pictet–Spengler reaction. Using TFA as an acid promoter, six-, seven-, and eight-membered ring-fused indoles were obtained in 31–99% yield.     

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.     

Radical cyclization–fragmentation of ω-haloalkyl cyclobutanones: a modular approach to medium-sized carbocycles

A facile radical cyclization–fragmentation sequence of ω-haloalkyl-tethered spirocyclobutanones, which are readily available by the Kulinkovich cyclopropanation of cycloalkene carboxylates and subsequent electrophilic addition to haloalkyl acetals, provides a convenient method for appending seven- and eight-membered rings onto cycloalkene carboxylates. An enantioselective preparation of a medium-sized carbocycle is possible by the use of a non-racemic, C₂-symmetric acetal.     

Asymmetric synthesis of Sedum alkaloids via lithium amide conjugate addition

Conjugate addition of lithium (R)-N-allyl-N-(α-methylbenzyl)amide or lithium (R)-N-but-3-enyl-N-(α-methylbenzyl)amide to an alkyl hexa-2,4-dienoate or alkyl hepta-2,6-dienoate, followed by ring-closing metathesis of the olefin functionalities within the resultant β-amino ester, generates a range of diastereoisomerically pure azacycles in good yield. These homochiral templates are readily transformed to a range of piperidine alkaloids of the Sedum family, and the corresponding five-, seven- and eight-membered ring homologues.     

Investigation of oxacycle formation by base-promoted endo-mode ring-closing reaction of allenes

The base-promoted endo-mode ring closure of electron-withdrawing group-substituted allenes provided the following interesting results: (1) the endo-mode ring-closing reaction of 1-(benzyloxycarbonyl)-1-(ω-hydroxyalkyl)allenes smoothly proceeded during the formation of five-, seven-, and eight-membered rings; (2) base treatment of benzyloxycarbonylallene and sulfonylallene, having a 2-hydroxyethyl group at the C-1 position, in the presence of an aldehyde led to the ring closure and condensation with the aldehyde in one-pot; and (3) endo-mode ring closure of the sulfonylallenes by internal attack of the carboxylate anion afforded the six-membered lactone.     

Polarity and structure of eight-membered organosilicon compounds with planar fragments

For the first time is determined the polarity of eight-membered silocyns with planar fragments. By the methods of dipole moments and theoretical calculations (DFT B3LYP/6-31G*) of 1,3,2-dioxasilocyns is established that in these compounds occurs the conformational equilibrium of the forms bath-chair and distorted bath with the predominance of the first, in this case the bonds C(sp ³)-S and C(sp ³)-H of the exocyclic MeSCH₂ group are in the not eclipsed gauche orientation relative to each other.     

Pd-catalyzed arylation/ring-closing metathesis approach to azabicycles

Palladium-catalyzed arylation followed by Grignard addition to imines and ring-closing metathesis, using Grubbs’ catalysts, provides a route to six-, seven-, and eight-membered azabicycles.     

Dynamic NMR study on the trans-fused eight-membered ether ring model representing G ring of brevetoxin A

Conformational analysis of the trans-fused eight-membered ether ring model (2) representing G ring of brevetoxin A (1) was carried out by dynamic NMR study in combination with molecular mechanics calculations.     

Titanocene(III) mediated 8-endo radical cyclizations for the synthesis of eight-membered cyclic ethers

Titanocene(III) chloride (Cp₂TiCl) mediated 8-endo radical cyclizations towards the synthesis of eight-membered cyclic ethers have been described. Titanocene(III) chloride was prepared in situ from commercially available titanocene dichloride (Cp₂TiCl₂) and activated zinc dust in THF.