A facile and rapid one-step synthesis of 8-substituted xanthine derivatives via tandem ring closure at room temperature

N-Bromo succinimide (NBS) was found to be an efficient and regioselective reagent in combination with AIBN for an unprecedented, facile and rapid one-step synthesis of 8-substituted xanthine derivatives at room temperature. The inexpensive, nontoxic and readily available NBS efficiently promoted the condensation of several aryl/cycloaryl/heteroaryl aldehydes with 5,6-diamino-1,3-dimethyluracils in presence of catalytic amount of AIBN in a single step via radical chain reaction. The notable advantages of this protocol were short reaction time, milder reaction conditions, simple work-up procedure, no requirement for chromatographic separation, use of non-hazardous reagent/solvent and applicable to a variety of substrates.     

Preparation of substituted pyridin-2-ones by ring closure of acylated aminoazabutadienes

A facile and regioselective synthesis of pyridin-2-ones from acylated aminoazabutadienes is described.     

Synthesis of 5-substituted pyrimidines. ortho-directed lithiation of pyrimidine derivatives

ortho-Directed lithiation of some pyrimidines has been investigated. Treatment of 2- and/or 4-alkoxy or acylaminopyrimidine with lithium 2,2,6,6-tetramethylpiperidide in ether at 0°, followed by quenching with various electrophiles afforded the corresponding 5-substituted pyrimidines.     

Preparation of phthalamic acid derivatives and ring closure to phthalimidobarbituric acids

Phthalamic acid derivatives with a barbiturate moiety were prepared from 5‐amino‐5‐ethylbarbituric acids. To circumvent an undesired acetylation in glacial acetic acid during the preparation of phthalimidobarbituric acids, two routes were proven exemplarily. On the one hand, a phthalamic acid ( 6a ) was isolated and subsequently cyclized with acetic anhydride to the corresponding phthalimide 2a . On the other hand, a phthalamic acid tert‐butyl ester ( 5b ) was successively treated with trifluoroacetic acid and trifluoroacetic anhydride to achieve heterocyclization to the phthalimide 2b . These routes might be useful for the preparation of other phthalimides derived from sterically hindered primary amines.     

Epoxide formation by ring closure of the cinnamyloxy radical

[formula: see text] The cinnamyloxy and oxiranyl benzyl radicals were generated by photolysis of alkyl 4-nitrobenzenesulfenates. The yet unprecedented epoxide ring formation from a primary alkoxy radical was observed. Experimental evidence supports the fact that the mode of ring opening of the oxiranyl carbinyl radical system is thermodynamically driven. B3LYP/6-31G* calculations indicate that the closed form of the radical is approximately 5 kcal/mol more stable than the open one.     

Enynones in organic synthesis. I. Spiroannulation by tandem oxy-cope rearrangement-electrocyclic ring closure

Bis-acetylenic alcohols of proper design undergo a facile oxy-Cope rearrangement to afford mixtures of E- and Z-enynones. These latter compounds afford methylenecyclopentenones upon enolization and electrocyclic ring closure.     

Preparation and thermal properties of polyquinazolone containing 4-substituted phenyl groups on the quinazolone ring

Polyquinazolones containing 4-substituted phenyl groups (H, Cl, CH₃, CH₃O, NO₂) on the quinazolone ring were synthesized in m-cresol and their thermal properties were studied by using dynamic thermogravimetry and isothermal weight loss in nitrogen and air. Polyquinazolones were synthesized with intrinsic viscosities in the range of 0.2–0.7 dL/g when phosphorus pentoxide was used as a catalyst. In TGA studies, there was a difference in the initial decomposition temperature of polyquinazolones with varying substituted groups. The introduction of substituted groups reduced oxidative thermal stability of the polymers. Furthermore, the thermal degradation mechanism was investigated by infrared spectrophotometry.     

Construction of 3,5-substituted 1,2,4-oxadiazole rings triggered by tetrabutylammonium hydroxide: a highly efficient and fluoride-free ring closure reaction of O-acylamidoximes

Tetrabutylammonium hydroxide (TBAH) is an efficient and mild alternative to tetrabutylammonium fluoride (TBAF) for base catalyzed cyclizations of 1,2,4-oxadiazoles from O-acylamidoximes. For most 3,5-substituted 1,2,4-oxadiazoles the reactions were dramatically accelerated by addition of 0.1 equiv of TBAH at room temperature. This method was also more generally applicable allowing for a wider range of substrates. Additionally, due to the absence of fluoride, TBAH will not result in corrosion of reactor vessels and therefore is better suited for large-scale synthesis.     

First synthesis of pyrrolothiadiazinones. An alternative core ring for xanthine based structures

The first synthesis of pyrrolothiadiazine core ring bearing substituents at positions 1, 3 and 6 is described, using a straightforward synthesis from diversely substituted 5-methylthiadiazines. These structures are of interest not only because they can be considered as useful building blocks but also because they can potentially be used as alternative core rings for biologically important molecules bearing xanthine as a central core.     

Synthesis of isoquinolines and naphthyridines by electrophilic ring closure of iminoalkynes

Substituted isoquinolines and naphthyridines have been prepared in good to excellent yields by the reaction of iminoalkynes with a variety of electrophiles under mild reaction conditions. Reaction: see text.     

Unusual ring cleavage reactions of 2-substituted 1,3-dithianes. Access to 2-alkylthiovinylsilanes.

Tuo new cleavage reactions, under carbanion forming conditions, of the 1,3-dithiane ring system are reported.     

The vilsmeier reaction in the synthesis of 3-substituted [1]benzopyrano[4,3-b]pyridin-5-ones. An unusual pyridine ring closure

Starting from 4-chlorocoumarin-3-carbaldehyde (1) and Wittig phosphoranes 2a-d the title compounds 6a-c have been synthesized via a four-step sequence. The intermediate 4-alkylamino-3-vinylcoumarins 5a-k have been prepared by the reaction of 4-chloro-3-vinylcoumarins 3a-d with primary amines 4a-h . The coumarin derivatives 5 (except 5k ) underwent an unusual pyridine ring closure under Vilsmeier conditions to form the benzopyrano[4,3-b]pyridines 6 . When the aminoaldehydes 7 were treated with the Wittig reagent 2b the fused N-alkyl-2 (1H) -pyridinones 8 have been obtained as expected.