A straightforward double intramolecular cyclization of dibenzyl dichalcogenols into a triple bond

The intramolecular cyclizations of four types of dibenzyl chalcogenols, which contained one or two ethynyl groups, were carried out. Either the double 5-exo or 6-endo-dig mode ring closure reaction regio- and stereoselectively proceeded to give the corresponding symmetrical bis(benzo[c]chalcogenophene) or bis(isochromene) derivatives from the dibenzyl chalcogenols having a diyne moiety. In contrast, the regioselective tandem 5-exo-dig mode intramolecular ring closure reactions of dibenzyl thiol and selenol having a mono-yne into a triple bond gave the trans-biisobenzothiophene and selenophene as the sole product. However, the similar cyclization of dibenzyl tellurol proceeded in both tandem 5-exo and 6-endo-dig modes to afford the trans-biisobenzotellurophene and ditellurachrysene in good yields with the ratio of 1:1. The X-ray structural analysis of these novel compounds is also presented.     

A density functional theory study of the 5-exo cyclization reactions of alpha-substituted 6,6-diphenyl-5-hexenyl radicals

Density functional theory computations were done to study the 5-exo radical cyclization reactions of alpha-substituted 6,6-diphenyl-5-hexenyl radicals. The methoxy electron donor group substitution reduced the barrier to reaction by about 0.5 kcal/mol. On the other hand, the electron acceptor group substitutions (ethoxycarbonyl, carboxylic acid, carboxylate, and cyano) raised the barrier to reaction by varying amounts (0.5-2.1 kcal /mol). The entropic terms of these cyclization reactions are briefly discussed. Solvent effects on these reactions were explored by calculations that included a polarizable continuum model for the solvent. The density functional theory calculated results were found to be in good agreement with the experimental data available in the literature and help to explain some of the observed variation in these types of cyclization reactions with various substitutions. Our results also provide an explanation for why the rate constant for the carboxylate group substituted radical was found to be an order of magnitude smaller than the rate constant for those radicals with carboxylic acid and ethoxycarbonyl substitutions.     

Synthesis of 6-Aryl-3,3-dimethyl-5-phenyl-2,3,5,6-tetrahydropyran-2,4-diones

Zinc enolates obtained from methyl 4-bromo-2,2-dimethyl-3-oxo-4-phenylbutanoates and zinc react with aromatic aldehydes to form 6-aryl-3,3-dimethyl-5-phenyl-2,3,5,6-tetrahydropyran-2,4-diones as a single geometric isomer.     

Unexpected intramolecular nucleophilic cyclization of ureidoacetamide: A novel route towards the synthesis of 2,4,5-trisubsituted oxazoles

A variety of N, 2-diaryl-2-ureidoacetamide prepared from the condensation of N-aryl-α-aminoamide with potassium cyanate (KOCN) undergo efficient Hendrickson’s reagent-mediated nucleophilic cyclization to afford 2,5-diamine-4-aryloxazoles. The two-step synthesis provides seven target products in yields of 61–78% under mild conditions. This reaction involves an unusual pathway in which the electrophilic amide carbonyl carbon is activated by Hendrickson’s reagent and attacked by a nucleophilic ureido oxygen in a 5-exo-trig O-cycloisomerization.     

Electrophilic cyclization of 2-geranylthiophene and its 5-carboxy derivative

Conclusions An allylic thienyl group can serve as a terminator of electrophilic cyclization, which in this case proceeds by a stepwise scheme.See [1] for preliminary communication.Deceased.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2578–2583, November, 1980.     

Unexpected intramolecular cyclization of some 2′-aminochalcones to indolin-3-ones mediated by Amberlyst-15

The intramolecular cyclization of 2′-aminochalcones derived from 2- and 4-pyridinecarboxaldehydes was carried out in the presence of Amberlyst^®-15/AcOH media. Unexpectedly, the reaction proceeded through a 5-exo process turning into an alternative approach for the synthesis of 2-(pyridinylmethylene)indolin-3-ones.     

Transition metal salts dependent intramolecular cyclization of a diimine

1,4-Bis(N-tosylethylamino)-2,3-diphenyl-1,4-diaza-1,3-butadiene (L¹) was separated from a condensation reaction of benzil with N-tosyl-ethylenediamine. It was shown that the intramolecular cyclization of L¹ has much dependence on the transition metal salts. The transformation of L¹ in the absence of any transition metal salts gave product 1-tosylaminoethyl-2-tosylaminomethyl-4,5-diphenyl-1H- imidazole (L²) at a temperature above 100 °C. Similar reactions assisted by Ni(CH₃COO)₂·4H₂O, MnCl₂·4H₂O, Mn(CH₃COO)₂·4H₂O, NiCl₂·6H₂O or NiSO₄·7H₂O could also give L² at a much lower temperature and a much higher yield. However, the presence of Cu(CH₃COO)₂·H₂O in a similar transformation reaction led to the formation of a bicyclic derivative 1-tosyl-5,6-diphenyl-1H-2,3-dihydroimidazo[1,2-a]imidazole (L³). But both L² and L³ were not detected when Co(CH₃COO)₂·4H₂O was used in a similar reaction and all attempts to transform L² into L³ failed.     

The cyclization reaction of 5-hexenoyl chloride with tri-n-butyltin hydride

The title reaction under various conditions of solvent, temperature and concentration was found to give 1,5 cyclization product and the typical acyclic reduction products.     

On the origin of cis selectivity in the cyclization of N-protected 2-substituted 3-aza-5-hexenyl radicals: a density functional study

Cyclization of the N-dimethylphosphinoyl-2-methyl-3-aza-5-hexenyl radical has been studied at the UB3LYP/6-31+G(d)//UB3LYP/6-31G(d) hybrid density functional level. The corresponding radical precursor has been synthesized and found to give cis/trans ratios of up to 10/1 in reductive radical cyclizations. The relative energies of reactant and transition state conformers were determined. In discord with the Beckwith-Houk model, it has been found that chair-axial transition states, which lead to cis products, are lowest in energy, rationalizing the observed experimental diastereoselectivity.     

Potassium carbonate-promoted stereospecific 5-endo-trig cyclization of unactivated allenes in the absence of any transition metals

[STRUCTURE: SEE TEXT] Formation of 3-pyrrolines from simple unactivated allenes bearing a protected amino group under basic conditions is described. Treatment of alpha-amino allenes with potassium carbonate in DMF under reflux in the absence of any transition-metal catalysts gave the corresponding 3-pyrrolines in good to excellent yields, by 5-endo-trig mode cycloisomerization. The reaction of internal allenes with an axial chirality afforded the corresponding 3-pyrrolines in a stereoselective manner.