6-endo Versus 5-exo radical cyclization: streamlined syntheses of carbahexopyranoses and derivatives by 6-endo-trig radical cyclization

Three factors that can direct 6-endo radical cyclization over 5-exo ring closure: substitution at C-5, vinyl radical cyclization and ring strain, have been considered in the context of the preparation of carbapyranoses from carbohydrate derivatives. As a result, alkyl radicals in substrates containing a strain inducing 2,3-O-isopropylidene ring, and vinyl radical in non-strained compounds undergo a completely regioselective 6-endo-trig ring closure leading to carbasugar derivatives.     

Lycopladine H, a novel alkaloid with fused-tetracyclic skeleton from Lycopodium complanatum

A novel Lycopodium alkaloid, lycopladine H (1), with a fused-tetracyclic ring system consisting of an azocane ring connected to a [2,2,2]-bicyclooctane ring and a 3-piperidone ring, was isolated from the club moss Lycopodium complanatum. The structure and relative stereochemistry of 1 were elucidated on the basis of spectroscopic data.     

Synthesis of new benzo[b]thieno fused ring systems via transition metal-mediated cyclisations

The compact synthesis of a new ring fused benzo[b]thieno derivative with an embedded nine-membered ring system via ring closing metathesis methodology is described. The preparation of the novel 11H-benzo[b]thieno[2,3-c]pyrrolo[2,3-a]indol-11-one via palladium-mediated oxidative cyclisation of benzo[b]thien-2-oyl indole derivatives is also reported.     

Synthesis and rotational barriers of atropisomers of 1,2-bis[5-(11H-benzo[b]fluorenyl)]benzenes and related compounds

Several 1,2-bis[5-(11H-benzo[b]fluorenyl)]benzenes and related compounds were synthesized via a cascade reaction sequence of the corresponding benzannulated enyne-allene precursors. The X-ray structures showed that the two benzo[b]fluorenyl moieties attached via the C5 carbons to the adjacent carbon atoms of the central benzene ring are oriented essentially perpendicular to the central benzene ring. The rates of rotation around the carbon-carbon single bonds attaching the benzo[b]fluorenyl moieties to the central benzene ring are relatively slow, allowing several anti and syn atropisomers to be separated at ambient temperature.     

One-pot synthesis of furo[2,3-h]quinoline and furo[2,3-h]isoquinoline derivatives using Fischer carbene complex

A new route for the synthesis of furo[2,3-h]quinoline and furo[2,3-h]isoquinoline derivatives have been explored through the coupling of carbene complexes with pyridine-bridged enynes. This reaction process, involving the annulation of both furan and benzene ring on to a pre-existing pyridine ring is highly efficient in the presence of triphenylphosphine as ligand additive.     

Mild stereoselective formation of tri- and tetrasubstituted olefins by regioselective ring opening of 1,1-disubstituted vinyl oxiranes with dialkyl dithiophosphates

Presented in this Letter is a mild new method to stereoselectively ring open 1,1-disubstituted vinyl oxiranes using dialkyl dithiophosphate nucleophiles. This new reaction proceeds in toluene at room temperature to afford tri- and tetrasubstituted allylic alcohols with high Z-selectivity. The syn relationship between the alcohol and the newly incorporated dithiophosphate moiety presented an opportunity to evaluate the anionic hopping properties of the ring opening product. Treatment of the ring opening product with base results in a facile phosphate hopping followed by a selective thiolate 3-exo-trig instead of a 5-exo-tet cyclization.     

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.     

Iterative synthesis of the ABCDEF-ring system of yessotoxin and adriatoxin

A stereocontrolled linear synthesis of the ABCDEF-ring system of yessotoxin and adriatoxin, diarrhetic shellfish toxins, is described. Iterative application of a tetrahydropyran synthesis by reaction of the alkylation of a sulfonyl-stabilized oxiranyl anion followed by 6-endo cyclization of a 4,5-epoxy alcohol led to the synthesis of the trans-fused hexacyclic ether system, and the seven-membered E ring was constructed by ring expansion reaction.     

An efficient non-catalytic, regioselective approach to the synthesis of angularly fused polycyclic systems

A novel approach to the synthesis of partially reduced different ring sizes of PAH analogs with sec.amino and nitrile functionalities is delineated through base-induced ring transformation of 4-sec.amino-2-oxo-5,6-dihydro-2H-benzo[h]chromene-3-carbonitriles by a carbanion, generated in situ from cyclopentanone, cyclohexanone, cycloheptanone, and cyclooctanone separately in good yields. An increase in the size of cycloalkanone ring beyond cyclooctanone restricts the ring transformation under analogous reaction conditions possibly due to bulky conformation of higher homologs. The synthetic method provides an efficient general route for the construction of angularly fused partially reduced polycyclic aromatic hydrocarbons: 5-sec.amino-2,3,6,7-tetrahydro-1H-cyclopenta[c]phenanthrene-4-carbonitriles, 6-sec.amino-2,3,4,7,8-pentahydro-1H-benzo[c]phenanthrene-5-carbonitriles, 7-sec.amino-2,3,4,5,8,9-hexahydro-1H-cyclohepta[c]phenanthrene-6-carbonitriles, and 8-sec.amino-2,3,4,5,6,9,10-heptahydro-1H-cycloocta[c]phenanthrene-7-carbonitriles.     

Synthesis of novel fused azecine ring systems through application of the tert-amino effect

Novel fused azecine ring systems were synthesized via the microwave-assisted thermal isomerization of terphenyl or biphenyl-pyridazine compounds possessing a vinyl and a tert-amino group, through application of a new extension of the tert-amino effect. Substrates for the ring closure were prepared from ortho-dihalobenzene or pyridazinone by consecutive Suzuki couplings with ortho-sec-amino- and formylphenylboronic acids, followed by Knoevenagel condensation of the aldehydes obtained.     

Design and synthesis of novel benzopyrazolodiazepinones via intra-molecular alkylation of α-alkylcarbonyl radicals mediated by dilauroylperoxide

Using a tin-free strategy, novel 4H-benzo[f]pyrazolo[1,5-a][1,3]diazepin-5(6H)-ones were synthesized in acceptable yields via intra-molecular alkylation over a benzene ring, of α-alkylcarbonyl radicals generated from ethyl pyrazolylbenzylaminoxanthates, using dilauroyl peroxide (DLP) as the radical initiator.     

Transformation of methyl N-methyl-N-(6-substituted-5-nitro-4-pyrimidinyl)aminoacetates into 4-methylamino-5-nitrosopyrimidines and 9-methylpurin-8-ones

N-Methyl-N-(6-substituted-5-nitro-4-pyrimidinyl)aminoacetic acid methyl esters under the treatment of sodium alkoxides, depending on the nature of substituents in 6 position of the pyrimidine ring, undergo ring closure and rearrangement to give 6-substituted-4-methylamino-5-nitrosopyrimidines or 9-methylpurin-8-ones.     

Novel generation and ring closure of 1,3-thiaza-1,3-butadiene S-oxides through thermal cycloreversion of 6H-1,3,5-oxathiazine S-oxides

Heating of 6H-1,3,5-oxathiazine S-oxides efficiently afforded 1,2,4-oxathiazoles through the mechanism involving thermal cycloreversion of the substrates and the subsequent ring closure of the intermediary 1,3-thiaza-1,3-butadiene S-oxides.     

Substituent control in the diastereoselectivity of dipolar cycloadditions of nitrones and their Zn(II) complexes with N-arylmaleimides

The π-π stacking interactions between maleimide's and nitrone's aromatic rings during the 1,3-dipolar cycloaddition were assumed to control the exo-endo selectivity of the reaction. The exo-endo ratios change during the reactions until they reach a constant value, which depends on the substituent. Electron-withdrawing groups favour the exo adduct while electron-donating groups favour the endo adduct. The nitrone ZnBr₂ complexes react much more slowly than the free nitrone and the cycloaddition is exo selective in all cases independent of the substituents on the maleimide's aromatic ring. Thermal retrocycloaddition of the cycloadducts produce the corresponding nitrones. The ring opening in the presence of secondary amines did not induce imine formation. endo Adducts were shown for the first time to be the stable paramagnetic compounds.     

Synthesis of trans-fused polycyclic ethers with angular methyl groups using sulfonyl-stabilized oxiranyl anions

An efficient method has been developed for the synthesis of trans-fused tetrahydropyrans with angular methyl groups adjacent to the ring oxygen. The procedure involves the coupling reaction of a sulfonyl-stabilized oxiranyl anion with an appropriate triflate followed by 6-endo cyclization, and is effective for the construction of 6/6- and 6/7/6-cyclic ether ring systems with sterically congested 1,3-diaxial methyl groups.     

An efficient synthesis of 4,5-dihydronaphtho[2,1-b]furan through a novel ring transformation of 2H-pyran-2-one

An innovative route for the synthesis of substituted naphtho[2,1-b]furan has been delineated through a ring transformation reaction of suitably functionalized 2H-pyran-2-ones by reaction with 6,7-dihydro-5H-benzofuran-4-one, in good yield.     

Concise total synthesis of flavone C-glycoside having potent anti-inflammatory activity

The total synthesis of anti-inflammatory active flavone C-glycoside isolated from oolong tea extract is achieved. Introducing a C-glucosyl moiety to an aryl system and constructing a fused tetracyclic ring characteristic to this natural product were conducted based on the O-to-C rearrangement of sugar moiety and the successive intramolecular Mitsunobu reaction, respectively. This concise and efficient synthetic pathway is applicable to the large-scale synthesis of target flavone and for constructing a large library of related compounds.     

Divergent synthesis of 2-C-branched pyranosides and oxepines from 1,2-gem-dibromocyclopropyl carbohydrates

The ring opening of 1,2-(gem-dibromo)cyclopropyl carbohydrates by two different modes leads to either 2-C-(bromomethylene)pyranosides (using base) or 2-bromooxepines (using silver salts), as shown previously by us for a d-glucal-derived cyclopropane. The base-promoted ring opening is extended to encompass additional alcohol, thiol and amine nucleophiles, and diastereoisomeric cyclopropane precursors. Cross-coupling of the 2-C-(bromomethylene)pyranosides leads to extended 2-C-branched pyranosides. Silver-promoted ring expansion of the cyclopropyl carbohydrates in the presence of various alcohols is described. Cross-coupling of the resulting benzyl 2-bromooxepines affords 2-C-substituted oxepines.     

Single- and double-chained truncated jaspine B analogues: asymmetric synthesis, biological evaluation and theoretical study of an unexpected 5-endo-dig process

An optimized synthesis of jaspine B analogues bearing an n-octyl or a p-fluorophenethyl lipophilic appendage was developed. Key to the approach was the use of acetylenic nucleophiles for the stereocontrolled introduction of the side chain and the implementation of a novel cyclization procedure to build the tetrahydrofuran ring. Three N-substituted amine or amide derivatives were also accessed. The biological activity of these four jaspine B analogues was shown to strongly depend on the nature of both the N-substituent and the aliphatic moiety connected to the tetrahydrofuran ring. Gratifyingly, the truncated jaspine B derivative proved to be a pro-apoptotic inhibitor of the conversion of ceramide into sphingomyelin. Finally, the efficient formation of a fused bis-furan derivative according to a 5-endo-dig process was observed under saponification conditions. On the basis of a theoretical study, a mechanistic pathway was delineated highlighting the Lewis acidity of the K⁺ ion as the driving force for this transformation in a strongly alkaline medium.     

Synthetic entry to the ABCD ring fragment of gymnocin-A, a cytotoxic marine polyether

Synthetic entry to the ABCD ring fragment of gymnocin-A, a cytotoxic marine polyether, has been achieved based on the B-alkyl Suzuki-Miyaura coupling-based strategy and radical cyclization for the construction of the tetrahydrofuran ring A.