Intramolecular cyclization of acyl radicals onto the azido group: a new radical approach to cyclized lactams

[reaction: see text] Aryl- and alkyl-derived azidoacyl radicals, generated from thiolesters by intramolecular homolytic substitution at the sulfur, can undergo five- and six-membered cyclization onto the azido moiety to give cyclized lactams.     

Intramolecular cyclization of pendant phenylethynyl groups as a route to solvent resistance in polyphenylquinoxalines

Biphenyl-based bisbenzil monomers which contain pendant phenylethynyl groups in the 2,2′-positions were condensed with 3,3′-diaminobenzidine to make polyphenylquinoxalines (PPQs) of high molecular weight. Thermal cure of these polymers at 193°C caused an intramolecular cyclization (IMC) of the phenylethynyl groups to give the rigid 9-phenyldibenzanthracene system in the backbone of the polymer. The initial, uncured polymers formed though films which were soluble in m-cresol and chlorinated solvents, but after thermal cure the films became insoluble in all common organic solvents and acids while maintaining their toughness. DSC scans of the cured materials showed small residual exotherms indicating that after vitrification even the intramolecular rotation required for the IMC reaction became restricted.     

Palladium-catalyzed stereocontrolled cyclization of 1,3-diene monoepoxide: A route to a new synthetic intermediate for de-ab-cholestane derivative.

The optically active (2R, 3R)-3-[(3R)-(E)-Benzyloxymethoxy-5-methyl-1-hexenyl]-2-(3-butenyl)-2-methyl-1-cyclopentanone (8) was synthesized as a precursor of vitamin D₃(5) by palladium-catalyzed syn-S_N2′ cyclization of [Z,Z(22S, 23R)]-ene oxide 6 as a key reaction.     

Base-catalyzed cyclization of monofluorodienynes: a new route to substituted fluorobenzene derivatives

The Sonogashira reaction of 1-bromo-1-fluoro-4-phenyl-1,3-butadienes and terminal alkynes, followed by cyclization in the presence of DABCO in refluxing NMP affords fluorinated benzene derivatives site-specifically in good yields.     

A new synthetic route to substituted 1-fluoro-1-cyclopropanecarboxylates

Substituted 6-fluoro-2,4-dioxo-3-azabicyclo[3.1.0]hexane-6-carboxylates were obtained as a mixture of the exo- and endo-isomers by reaction of substituted 6,8-dioxo-2,3,7-triazabicyclo[3.3.0]oct-3-en-4-carboxylates with N-fluoropyridinium tetrafluoroborate.     

A synthetic route to the hexahydrobenzofuran nucleus of avermectins via diazoketone cyclization

A hexahydrobenzo[AB]furan characteristic of the avermectins and certain milbemycins was synthesized via acid-catalyzed intramolecular addition of a diazoketone to a γ-lactone; subsequent transformations led to structures containing much of the functionality and stereochemistry present in the corresponding segments of these macrolides.     

A new synthetic route to polycarbonate

A detailed study has been carried out on the new synthetic reaction of poly(p-xylylene carbonate) from potassium carbonate and p-xylylene dibromide by using a variety of crown ethers as a catalyst, which was recently found by the present authors. Crown ethers having 18-member ring showed the best catalytic property of the various crown ethers, and the reaction was conducted in various solvents at 50–160°C by using 18-crown-6-ether. Both the polymer yield and the molecular weight of the polymer increased in proportion to the amount of potassium carbonate, and they increased rapidly and reached constant values with increasing the concentration of 18-crown-6-ether. They also depended significantly upon the reaction temperature as well as the solvent used. A maximum yield with the highest molecular weight was obtained from the reaction at 100–120°C in diglyme solvent. The spectroscopic analysis of the polymer indicated that all the end groups of the resulting polymer had the structure of benzyl bromide. From these results, a plausible mechanism was proposed for the reaction. Similar reactions were also conducted by using several aliphatic dibromides, Br? (CH₂)_x? Br, in place of p-xylylene dibromide. The products were strongly dependent of the value of x: polycarbonate was obtained from dibromides with ≧4, and cyclic carbonates from dibromides with ≦3.     

A new synthetic route to aminothiazolinones

Novel 2-(5-R-1,3,4-thiadiazol-2-yl)aminothiazolin-4-ones 6a—h and 2-imino-3-(5-R-1,3,4-thiadiazol-2-yl)thiazolidin-4-ones 7a—h were prepared by treating N-(5-R-1,3,4-thiadiazol-2-yl)thioureas 4a—h with chloroacetic acid on various solid supports under microwave irradiation. Tautomeric mixtures of compounds 6a—h and 7a—h were obtained in all cases. In alkaline and neutral media, compounds 6a—h were the major products, while in acid media, 7a—h were the major products.     

Intramolecular cyclization of delta-iminoacetylenes: a new entry to pyrazino[1,2-a]indoles

[reaction: see text] The synthesis of the pyrazino[1,2-a]indole nucleus was achieved by intramolecular cyclization of several 2-carbonyl-1-propargylindoles in the presence of ammonia. The reaction conditions were optimized using microwave heating and a pool of catalysts. Cyclization of 1-alkynylindole-2-carbaldehydes was easily accomplished under standard heating conditions, whereas microwave heating contributed to reduced reaction times and improved overall yields. Moreover, a fine-tuning of the microwave irradiation time made possible the selective synthesis of both pyrazino[1,2-a]indole isomers. TiCl4 proved the catalytic system of choice to achieve pyrazinoindoles in satisfactory yields starting from 1-alkynyl-2-acetylindoles and 1-alkynyl-2-benzoylindole derivatives. Also in these cases, microwave heating contributed to faster reactions and improved yields. The uncatalyzed versus catalyzed reaction mechanism is discussed.     

A new synthetic route to trimethylgallium

A new synthetic route to trimethylgallium was developed. It is based on preparation of gallium methyl derivatives by the Green reaction, followed by their alkylation with methyl Grignard reagent. The suggested procedure is well reproducible, with the yield of pure trimethylgallium exceeding 90%.     

A new synthetic route to aminophosphines

The reactions of R₂PPR₂ (R = Me, Et, Ph) and (MeP)₅ with Me_3−nAs(NMe₂)_n (n = 1, 2, 3) and of Me₂PPMe₂ with Me₂AsNR′₂ (R′ = Et, Prⁿ, and Prⁱ) were investigated as a function of time at room temperature using ¹H and ³¹P NMR spectroscopy. For the diphosphine/Me₂AsNR′₂ reactions, the NMR spectral data suggest a reaction pathway involving the initial formation of R₂PAsMe₂ and the respective acyclic dialkylaminophosphine, R₂PNR′₂. The P---As intermediate then symmetrizes to R₂PPR₂ and Me₂AsAsMe₂, the parent aminoarsine is completely consumed, and additional R₂PNR′ is formed. The relative rate of aminophosphine production is dependent upon the nature of the substituent on the phosphorus and nitrogen atoms. For systems involving MeAs(NMe₂)₂ and As(NMe₂)₃ as reactants, the intermediates could not be characterized, but the products were the expected aminophosphine and (MeAs)₅ or elemental arsenic, respectively. (MeP)₅ reacts to give MeP(NMe₂)₂ and the expected As---As bonded species. A comparison of the reactivity of these systems with analogous diarsine/aminoarsine systems is discussed. The results of the NMR study were utilized in designing a convenient, high yield, synthetic route to acyclic aminophosphines.     

Addition of difluoromethyl radicals to glycals: a new route to alpha-CF2-D-glycosides

The synthesis of synthetically useful alpha-CF2-glycosides by radical addition of ethyl bromodifluoroacetate onto 2-benzyloxyglycals is described. The methodology provides an access to alpha-O-glycoside mimics and, potentially, to valuable alpha-O-glycoconjugate analogues.     

The intramolecular sila-Pummerer cyclization: A new route to sulfur heterocycles

The generality of the intramolecular cyclization of suitable nucleophilic sites to a  S⁺CH₂ center created by a sila-Pummerer rearrangement has been investigated. Successful nucleophilic sites included the OH group (in alcohols, carboxylic acids, and hydroxylamines) and the NH group (in amines and carbamates): attempts to produce carbon-based nucleophilic sites were not effective. Successful cyclizations were achieved to produce sulfur heterocycles with 5-, 6-, and 7-membered rings.