Iodine(III)-mediated bicyclic diazenium salt formation

The hypervalent iodine(III) reagent PhI(OTf)₂ has been shown to be an effective oxidant for the conversion of linear aryl-hydrazones bearing a pendant alkene into bicyclic diazenium salts. This oxidative cyclization presumably occurs by the iodine(III) mediated formation of a 1-aza-2-azoniaallene salt intermediate that undergoes a subsequent intramolecular 1,3-dipolar cycloaddition with the pendant alkene.     

Homolytic Cyclizations of N-Chloroalkenylamines

Many N-chloroalkylamines with double bond(s) dissociate homolytically to give chlorine atoms and aminyl radicals which undergo intramolecular cyclization reactions. Here, the nature of the aminyl radicals (neutral, protonated, or complexed to metal salts) appears to be important. The reactivity varies strongly from one species to another, and their selectivities can be different towards addition, substitution, abstraction, or dimerization reactions. A good synthetic tool appears to be the “complexed radical”, which is generated under mild conditions. A variety of β-functionalized, substituted, fused, or bridged azaheterocycles with alkaloid part structures thus become easily accessible in high yields, in a regio- and, possibly, stereoselective manner. Many examples are discussed illustrating the synthetic possibilities, limitations, and mechanistic features of this cyclization process.     

The R3O+···H+ hydrogen bond: toward a tetracoordinate oxadionium(2+) ion

Oxatriquinanes are tricyclic oxonium ions which are known to possess remarkable solvolytic stability compared to simple alkyl oxonium salts. Their rigid, hemispherical structure presents an oxygen at the apex of three fused five-membered rings. While trivalent oxygen species like these have been well described in the literature, the ability of oxygen to enter into a fourth covalent bonding relationship has been visited in theory and suggested by the outcome of certain reactions conducted in superacidic media, but has never been established by the characterization of a stable, persistent R(3)OH(2+) or R(4)O(2+) ion. In this study, the nucleophilicity of the oxatriquinane oxygen was evaluated first by a series of protonation studies using the Br?nsted superacid H(CHB(11)Cl(11)) both in the solid state and in liquid HCl solution. The interaction of the oxatriquinane oxygen with a bridging carbocation was also examined. A strong case could be made for the occurrence of hydrogen bonding between H(CHB(11)Cl(11)) and oxatriquinane using IR spectroscopy. Under the most forcing protonation conditions, the oxatriquinane ring is cleaved to give a bridged, dicationic, protonated tetrahydrofuran-carbenium ion.     

Heterolysis of Dihydrogen by Silver Alkoxides and Fluorides

Alkoxide‐bridged disilver cations react with dihydrogen to form hydride‐bridged cations, releasing free alcohol. Hydrogenolysis of neutral silver fluorides affords hydride‐bridged disilver cations as their bifluoride salts. These reactions proceed most efficiently when the supporting ligands are expanded N‐heterocyclic carbenes (NHCs) derived from 6‐ and 7‐membered cyclic amidinium salts. Kinetics studies show that silver fluoride hydrogenolysis is first‐order in both silver and dihydrogen.     

Synthesis and use of mono- or bisxylyl linked bis(benzimidazolium) bromides as carbene precursors for C-C bond formation reactions

Two benzimidazolium moieties linked by one or two xylyls (m- and p-) have been synthesized, characterized and then they were used for Heck coupling reactions as in situ formed catalysts. Mono bridged salts are more efficient as compared to bisbridged salts. In addition, mono bridged salts were converted to Rh-NHC complexes which were tested as catalysts for the arylation of aldehydes.     

Nitraza-/Oxa-Propylene- and Hydrazonemethylene-Bridged 1,2,4-Nitraminotriazoles and Selected Salts

Two new bridged nitraminotriazoles with bridging oxapropylene and nitrazapropylene moieties were synthesized, and converted into several salts, as well as from the hydrazonemethylene bridged nitraminotriazole. All compounds were fully characterized by NMR and IR spectroscopy, elemental analysis as well as differential thermal analysis. The sensitivity towards friction and impact were determined according to BAM standard technics and the energetic properties were calculated by using the EXPLO5 computer code. The neutral compounds as well as the various salts were examined in terms of their physicochemical properties and detonation performance to each other and compared to the commonly used secondary explosive RDX.     

Synthesis of Pyrimidine-Annelated Heterocycles: Regioselective Heterocyclization of 5-(Cyclohex-2-enyl)-1,3-dimethyl-6-hydroxyuracil [1]

 5-(Cyclohex-2-enyl)-1,3-dimethyl-6-hydroxyuracil undergoes regioselective heterocyclization to afford fused tricyclic heterocycles upon treatment with bromine and m-CPBA. However, the same substrate furnished bridged tricyclic heterocycles when treated with N-iodosuccinimide and conc. H₂SO₄ and a mixture of bridged tricyclic heterocycles and fused tricyclic heterocycles when treated with hexamine hydrotribromide or pyridine hydrotribromide.     

An intramolecular [2 + 2] cycloaddition of ketenimines via palladium-catalyzed rearrangements of N-allyl-ynamides

A cascade of Pd-catalyzed N-to-C allyl transfer-intramolecular ketenimine-[2 + 2] cycloadditions of N-allyl ynamides is described. This tandem sequence is highly stereoselective and the [2 + 2] cycloaddition could be rendered in a crossed or fused manner depending on alkene substitutions, leading to bridged and fused bicycloimines.     

Fragmentation de Quelques Hydrazines,Tétrahydrocinnolines et Pyrazolidines sous I'Impact Electronique

The mass spectra of various hydrazines, tetrahydrocinnolines and pyrazolidines, prepared by the condensation of a diazenium cation or its conjugated base with olefins, are discussed. The main fragmentation mode of trisubstituted hydrazines corresponds to a retroaddition process which produces the diazenium cation and the olefin. Another fragmentation mode involves breaking of the N? N bond which is due to a charge localization in the phenyl ring the latter being substituted on a nitrogen atom in all the hydrazines studied. This N? N bond split is dominant in the fragmentation of tetrahydrocinnolies. On the other hand, its does not appear in the spectra of pyrazolidines. The latter mainly undergo a dipolar-1, 3 retroaddition-like process giving the protonated form of the azomethine imine used in thier synthesis.     

An approach to skeletal diversity using functional group pairing of multifunctional scaffolds

Diversification of enantioenriched Michael adducts through functional group pairing to gain access to a range of five- to ten-membered complex fused and bridged ring systems is described.     

E versus Z Diazeniumdiolation of Acetoacetate-Derived Carbanions

Nitric oxide adds to methyl acetoacetate in the presence of KOH in methanol at room temperature to form potassium acetylsydnonate N-oxide (K1) with an (E)-diazeniumdiolation and potassium acetate diazenium diolate (K(2)2) from a (Z)-diazeniumdiolation. A study of the reaction with LiOH, NaOH, and NMe(4)OH and with ethyl acetate substrate reveals that the temperature of the reaction greatly influences the nitric oxide reactivity. At 23 °C, nitric oxide adds to give both E and Z products, whereas at -5 °C the gas reacts almost exclusively to give Z addition. The (Z)-diazeniumdiolation products, namely, the alkali metal and NMe(4)(+) salts of methyl and ethylbutenoate-2-diazeniumdiolate-3-hydroxylate (3(2-) and 4(2-)), are isolated in good yields. The alkali metal salts are not amenable for recrystallization because of their ready decomposition in aqueous solutions. However, [NMe(4)](2)[MeC(O)C(N(2)O(2))CO(2)Me] is readily recrystallized from a methanol/acetonitrile solvent mixture. The crystals are unambiguously characterized by X-ray crystallography. NMR spectra for all of the 3(2-) and 4(2-) salts reveal the presence of two isomers in aq solutions. But the structure of the NMe(4)(+) salt contains only one of the isomers. Our attempts to cyclize the isolated and purified butenoatediazeniumdiolates from the (Z)-diazeniumdiolation to the E-containing sydnonate products were unsuccessful. TGA/DSC data for all of the products demonstrate the thermal instability of the salts at high temperatures. The salts decompose exothermally possibly with the release of N(2)O among other gases.     

Bridged to Fused Ring Interchange. Methodology for the Construction of Fused Cycloheptanes and Cyclooctanes. Total Syntheses of Ledol, Ledene, and Compressanolide

The type two intramolecular Diels-Alder reaction (T2IMDA) is an efficient method for the formation of medium rings. The methodology is particularly effective for the construction of seven- and eight-membered rings. A strategy for the synthesis of functionalized cycloheptanes and cyclooctanes has been developed that involves a bridged to fused ring interchange. The T2IMDA provides a synthesis for rigid bridged bicyclic molecules that can be stereoselectively elaborated before ozonolysis of the bridgehead double bond. Following oxidative cleavage, aldol condensation provides fused bicyclic ring systems that otherwise are difficult to synthesize. This methodology is amenable to the synthesis of terpene natural products. This is demonstrated here through total syntheses of (+/-)-ledol and (+/-)-ledene and a formal synthesis of (+/-)-compressanolide.     

Asymmetric synthesis of 1-azaspiro[4.5]decanes via intramolecular dipolar cycloaddition of nitrones containing the bornane-10,2-sultam chiral auxiliary

The intramolecular 1,3-dipolar cycloaddition of a cyclic nitrone, prepared by an asymmetric electrophilic enolate hydroxyamination using the (2R)-bornane-10,2-sultam chiral auxiliary, proceeds to give bridged and fused cycloadducts with total diastereocontrol. Reduction of the fused isoxazolidine provides a 1-azaspiro[4.5]decane as a potential intermediate in the asymmetric synthesis of the cylindricine alkaloids.     

Synthesis of Pyrimidine-Annelated Heterocycles: Regioselective Heterocyclization of 5-(Cyclohex-2-enyl)-1,3-dimethyl-6-hydroxyuracil [1]

Summary.  5-(Cyclohex-2-enyl)-1,3-dimethyl-6-hydroxyuracil undergoes regioselective heterocyclization to afford fused tricyclic heterocycles upon treatment with bromine and m-CPBA. However, the same substrate furnished bridged tricyclic heterocycles when treated with N-iodosuccinimide and conc. H₂SO₄ and a mixture of bridged tricyclic heterocycles and fused tricyclic heterocycles when treated with hexamine hydrotribromide or pyridine hydrotribromide. Corresponding author. E-mail: kcm@klyuniv.ernet.in Received September 27, 2001. Accepted (revised) December 3, 2001     

Dialkylative Cyclization Reactions of 3-(Phenylthio)-3-Sulfolenes

3-(Phenylthio)-3-sulfolene (4) underwent bridged dialkylative cyclization with 2-methylene-1,3-diiodopropane to give the bridged bicyclic 3-sulfolene 23, and spirodialkylation with 1,4-diiodobutane and 1,5-diiodopentane to give the spiro bicyclic 3-sulfolenes 15 as the major product. The reaction of 4 with 1,3-diiodopropane led to the fused bicyclic 2-sulfolene 12. 3-(Phenylthio)-3-sulfolenes bearing an ω-iodoalkyl group at the C-2 position gave the fused bicyclic 2-sulfolenes 28 and/or the spiro bicyclic 3-sulfolene 29 depending on the chain length.     

Preparation, properties, and reduction of a novel TCNQ-type thienoquinoid

[Structure: see text] TCNQ-type bithienoquinoid condensed with a alpha-dicarbonyl group was synthesized. The compound revealed a reversible reduction wave at +0.32 V vs SCE. The radical anion species was cleanly generated by a reduction with Na-Hg and characterized: lambdamax=1027 nm in vis-NIR and a large spin density of the beta-carbon atoms on the fused ring by EPR spectroscopy. The radical anion salts were isolated by a reduction with iodide and showed moderate conductivity.     

A Cascade “Prins‐Pinacol‐Type Rearrangement and C4‐OBn Participation” on Carbohydrate Substrates: Synthesis of Bridged Tricyclic Ketals,Annulated Sugars and C2‐Branched Heptoses

A “Prins pinacol type rearrangement followed by C4‐OBn participation” in a cascade manner has been observed while probing the fate of carbocation in some carbohydrate derived homoallylic alcohols in the Prins reaction. This has led to an easy access to tetrahydrofuran‐fused bridged bicyclic ketals (or tetrahydrofuran‐fused 1,6‐anhydro‐heptopyranose frameworks) which are further converted into some annulated sugars and C2‐branched heptoses.     

M(CN)(2) Species (M = Be, Mg, Ca, Sr, Ba): Cyanides, Nitriles, or Neither?

The "cyanide" salts of the group 2 (alkaline earth) metals exhibit remarkable structural variations: CN(-) binds to the metals via the carbon, via the nitrogen, and via bridged arrangements. The most stable geometries of the beryllium and the magnesium salts are linear (CNBeNC and NCMgCN, respectively), but CaC(2)N(2), SrC(2)N(2), and BaC(2)N(2) prefer twisted, bridged structures. However, several stationary points of the bridged complexes are close in energy, and considerable fluxionality is to be expected. These theoretical predictions (MP4SDTQ/6-311+G(2d)//MP2(fu)/6-31+G, Ca, Sr, Ba: 5s5p3d1f//5s5p3d basis sets and 10 valence electron pseudopotentials) invite experimental verification.     

Total Synthesis of Calophyline A

Reported herein is the total synthesis of calophyline A, an indoline natural product possessing distinct ring connectivity which has not been synthesized previously. The synthetic route features several key transformations, including an aza‐pinacol rearrangement to construct the nitrogen‐containing bridged [3.2.2] bicycle, a Heck cyclization to assemble the fused 6/5/6/5 ring system, and a challenging late‐stage aldol reaction to generate both a neopentyl quaternary stereogenic center and an oxygen‐containing bridged [3.2.1] bicycle.     

Selectivity Switch in a Rhodium(II) Carbene Triggered Cyclopentannulation: Divergent Access to Three Polycyclic Indolines

A selectivity switch in a Rh^II/carbene‐triggered cyclopentannulation with catalytic InCl₃ is reported for the first time, affording both diastereomers of the fused spiroindolines and an unusual bridged tetracyclic indoline in high yields with excellent selectivities. Mechanistic studies indicate an intramolecular annulation of the indole with an in situ formed aminocyclopropane. The stepwise thermal conversions from the kinetic spiroindoline to the metastable bridged indoline, and then to the thermodynamic spiroindoline, involving a ring‐opening rearrangement of a cyclopentane, is crucial for selectivity control.