Lewis acid mediated [2,3]-sigmatropic rearrangement of allylic ammonium ylides

An investigation of the Lewis acid mediated [2,3]-sigmatropic rearrangement of allylic ammonium ylides has been conducted by employing various bases and boron Lewis acids. Using BBr₃ together with the phosphazene base 6, various allylic amines could be rearranged in good yields with low to excellent diastereoselectivity.     

Synthesis of E-deoxypukalide, and its biomimetic conversion into deoxypseudopterolide by photochemical ring contraction involving a 1,3-allylic shift

Irradiation of a solution of synthetic Z-deoxypukalide 10 in acetonitrile (Pyrex; 400 W Hg lamp) resulted in isomerisation, leading to E-deoxypukalide 12, which was isolated recently from the octocoral Leptogorgia spp. Further irradiation of 12, or prolonged irradiation of 10, then gave the ring-contracted product, deoxypseudopterolide 1 (>90%) found in octocorals of the genera Pseudopterogorgia and Leptogorgia. The contraction of the 14-membered rings in 10 and 12 to the 12-membered ring as 1, occurs by way of photochemical [1,3]-sigmatropic rearrangement.     

Intramolecular cycloaddition of N-phthalimidoaziridines to double and triple carbon-carbon bonds

Thermolysis of 3-(2-allyloxyphenyl)- or 3-(2-propargyloxyphenyl)-1-phthalimidoaziridine-2-carboxylic acid derivatives results in stereospecific intramolecular cycloaddition of intermediate N-phthalimidoazomethine ylides to double or triple carbon-carbon bonds. This leads to condensed N-phthalimidopyrrolidines, N-phthalimidopyrrolines, or products of their subsequent transformations. On the other hand, thermolysis of similar dimethyl 3-aryl-1-phthalimidoaziridine-2,2-dicarboxylates gives exclusively 5-methoxyoxazoles, the products of a competitive 1,5-dipolar electrocyclization.     

Lewis acid catalyzed reactions of donor-acceptor cyclopropanes with 1- and 2-pyrazolines: formation of substituted 2-pyrazolines and 1,2-diazabicyclo[3.3.0]octanes

The reaction of 2-substituted cyclopropane-1,1-dicarboxylates with 1- and 2-pyrazolines is efficiently catalyzed by scandium or ytterbium triflates to give N-substituted 2-pyrazolines or 1,2-diazabicyclo[3.3.0]octanes. The reactions of 2-pyrazolines give diazabicyclooctanes as the major products. In contrast, the reactions starting from 1-pyrazolines predominantly give N-substituted 2-pyrazolines, which become the major compounds obtained if an equimolar amount of GaCl₃ is used. A possible reaction mechanism is suggested.     

Lewis acid-mediated [3+2] cycloaddition between hydrazones and olefins

[3+2] Cycloaddition between hydrazones and olefins was accelerated in the presence of a stoichiometric amount of BF₃·OEt₂ or a catalytic amount of Zr(OTf)₄, Hf(OTf)₄, or Sc(OTf)₃ under mild conditions. The corresponding pyrazolidine derivatives were obtained in moderate to high yields using this novel [3+2] Lewis acid catalysis.     

Fast ring opening of unstable mesoionic 1,3-dioxolylium-4-olates to acyloxyketenes: formation of [2+2] cycloadducts of acyloxyketene with several ketenophiles

Fast ring opening of mesoionic 1,3-dioxolylium-4-olate 5, generated by Rh₂(OAc)₄-catalyzed decomposition of a phenyldiazoacetic anhydride 6, to an acyloxyketene 10 was demonstrated by trapping the ketene 10. Rh₂(OAc)₄-catalyzed decomposition of p-nitrophenyldiazoacetic p-chlorobenzoic anhydride 6a in the presence of ketenophiles such as dihydrofuran, carbodiimides, and imines did not give 1,3-dipolar cycloadducts with the 1,3-dioxolylium-4-olates 5a, but their [2+2]-cycloadducts with the acyloxyketene 10a. PM3 calculation of heats of formation of a 1,3-dioxolylium-4-olate 5 and an acyloxyketene 10 indicates that the acyloxyketene 10 is 9 kcal/mol more stable than the 1,3-dioxolylium-4-olates 5.     

Reactions of diazoalkanes with unsaturated compounds 16. Catalytic reactions of diazomethane with 2-alkenyl-1,3-oxazolidines and 2-alkenyl-1,3-oxathiolanes

The influence of 1,3-oxazolidine and 1,3-oxathiolane fragments in substituted alkenes on the direction of their catalytic reaction with diazomethane has been investigated. The olefins bearing an oxazolidine substituent in the α- or γ-position and an oxathiolane substituent in the γ-position relative to the C=C bond react with diazomethane in the presence of Pd(acac)₂ selectively resulting in cyclopropanation products. The use of Cu(OTf)₂ does not result in cyclopropanation; however Cu(OTf)₂ catalyzes the reaction of diazomethane with 2-(alk-1-enyl)-1,3-oxathiolanes yielding 2,3,5,6-tetrahydro-1,4-oxathiocines formed through the [2,3]-sigmatropic rearrangement of the intermediate sulfonium ylides. For Part 15 see Ref. 1. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 604–608, March, 2008.     

Calix[4]arenes with a Lid in their Upper Rims: 1,3-Dipolar Cycloaddition Reactions of Benzonitrile Oxides with 5-Allyl-, 5,11-Dially- and 5,17-Diallylcalix[4]arenes

The upper rim substituted mono- and diisoxazolinomethylcalix[4]arenes 9-23 are synthesized in good yields from 1,3-dipolar cycloaddition reactions of papa-substituted phenyl nitrileoxides with 5-allyl-, 5,11-diallyl-and 5,17-diallycalix[4]arenes 5-7 ; the structures of 9-23 are consistent with a ‘cone’ conformation and the ‘vase-with-a-lid’ structure of 11 was confirmed by a single-crystal x-ray analysis. Preliminary results show that the monoisoxazolinomethylcalix[4] arene can form an inclusion complex with ammonium cation.     

New approaches to synthesis of tris[1,2,4]triazolo[1,3,5]triazines

Thermal cyclization of 3-R-5-chloro-1,2,4-triazoles (R = Cl, Ph) afforded 2,6,10-tri-R- tris[1,2,4]triazolo[1,5-a:1′,5′c:1″,5″-e][1,3,5]triazines 5 (R = Ph) and 7 (R = Cl). These compounds are first representatives of this class of heterocycles, whose structures were unambiguously established. Treatment of these compounds with nucleophiles (H₂O/NaOH, NH₃) results in the triazine ring opening to give compounds consisting of three 1,2,4-triazole rings linked in a chain. For example, treatment of cyclic compound 5 with aqueous alkali affords 3-phenyl-1-3-phenyl-1-(3-phenyl-1H-1,2,4-triazol-5-yl)-1,2,4-triazol-5-yl-1H-1,2,4-triazol-5-one. Treatment of 3,7,11-triphenyltris[1,2,4]triazolo[4,3-a:4′,3′c:4″,3″-e][1,3,5]triazine (2) with HCl/SbCl₅ leads to the triazine ring opening giving rise to 5-(3-chloro-5-phenyl-1,2,4-triazol-4-yl)-3-phenyl-4-(5-phenyl-1H-1,2,4-triazol-3-yl)-1,2,4-triazole. Thermal cyclization of the latter produces 3,7,10-triphenyltris[1,2,4]triazolo[1,5-a:4′,3′c:4″,3″-e][1,3,5]triazine (13). Thermolysis of both cyclic compound 2 and cyclic compound 13 is accompanied by the Dimroth rearrangement to yield 3,6,10-triphenyl-tris[1,2,4]triazolo[1,5-a:1′, 5′-c:4″,3″-e][1,3,5]triazine (14). Compounds 13 and 14 are the first representatives of cyclic compounds with this skeleton. ¹³C NMR spectroscopy allows the determination of the isomer type in a series of tris[1,2,4]triazolo[1,3,5]triazines.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 706–712, March, 2005.     

An efficient intermolecular [Pd]-catalyzed C-C and intramolecular [Cu]-catalyzed C-O bonds formation: synthesis of functionalized flavans and benzoxepine

An efficient three-step strategy for the synthesis of functionalized flavans, starting from readily available 2-bromoiodobenzenes and aryl vinyl alcohols, is presented and successfully extended to benzoxepine. An intermolecular [Pd]-catalyzed C-C and an intramolecular [Cu]-catalyzed C-O bond formations have been employed as key transformations of the strategy.     

Synthesis of Bridged Tetrahydrobenzo[b]azepines and Derivatives through an Aza‐Piancatelli Cyclization/Michael Addition Sequence

Herein, we report the preparation of bridged tetrahydrobenzo[b]azepines, which was accomplished through an aza‐Piancatelli cyclization/Michael addition sequence in a one‐pot fashion from readily available precursors. It is noteworthy that a general method to access these scaffolds was hitherto unprecedented. Additionally, the multifaceted aspects of this process have been exemplified through its application to the synthesis of 2‐azabicyclo[3.2.1]octanes and bridged tetrahydrobenzo[b]oxepines, along with post‐derivatizations.     

Lewis acid-catalyzed formation of Ugi four-component reaction product from Passerini three-component reaction system without an added amine

In the presence of a Lewis acid the phenol-Passerini three-component reaction (phenol-P-3CR) system is found to deliver a product of the phenol-Ugi four-component reaction (phenol-U-4CR). It is the first demonstration of an isocyanide as an amine equivalent in isocyanide-based multicomponent reactions (IMCRs). In general, by using Ti(O-i-Pr)₄ in MeOH both phenol-U-4CR and U-4CR products are synthesized from an aromatic aldehyde, a phenolic or carboxylic acid, and an isocyanide. Moreover, by using MeCN as the solvent, the phenol-P-3CR products can be obtained in good yields without contamination of the phenol-U-4CR products.     

新型三环系稠杂环［1,2,4］-三唑并［1,5-a］［1］-苯并氮杂及［1,2,4］-三唑并［1,5-a］-喹啉的合成

α-四氢萘酮的乙氧羰基腙(1)经LTA氧化, 得到α-偶氮-α-乙酰氧基化合物2. 在AlCl3作用下, 化合物2脱去乙酰氧基产生重氮正离子中间体3, 再经与腈的1,3-偶极环加成、 ［1,2］-迁移扩环、 碱性水解和与苦味酸作用, 得到新型［1,2,4］-三唑并［1,5 a］［1］苯并氮杂苦味酸盐6a~6c. 以2,3-二氢-1-茚酮为底物, 采用相同的合成路线, 合成了1,2,4-三唑并［1,5-a］-二氢喹啉苦味酸盐12a~12c.     

Kinetic studies on the formation of sulfonyl radicals and their addition to carbon-carbon multiple bonds

The reactions of α-hydroxyl and α-alkoxyl alkyl radicals with methanesulfonyl chloride (MeSO(2)Cl) have been studied by pulse radiolysis at room temperature. The alkyl radicals were produced by ionizing radiation of N(2)O-saturated aqueous solution containing methanol, ethanol, isopropanol, or tetrahydrofuran. The transient optical absorption spectrum consisted of a broad band in the region 280-380 nm with a maximum at 320 nm typical of the MeSO(2)(?) radical. The rate constants in the interval of 1.7 × 10(7)-2.2 × 10(8) M(-1) s(-1) were assigned to an electron-transfer process that leads to MeSO(2)Cl(?-), subsequently decaying into MeSO(2)(?) radical and Cl(-). The rate constants for the addition of CH(3)SO(2)(?) to acrolein and propiolic acid were found to be 4.9 × 10(9) M(-1) s(-1) and 5.9 × 10(7) M(-1) s(-1), respectively, in aqueous solutions and reversible. The reactivity of tosyl radical (p-CH(3)C(6)H(4)SO(2)(?)) toward a series of alkenes bearing various functional groups was also determined by competition kinetics in benzene. The rate constants for the addition of tosyl radical to alkenes vary in a much narrower range than the rate constants for the reverse reaction. The stabilization of the adduct radical substantially contributes to the increase of the rate constant for the addition of tosyl radical to alkenes and, conversely, retards the β-elimination of tosyl radical.     

Grob-type fragmentation of 5-oxabicyclo[2.1.1]hexane system: a strategy for synthesis of annulated and 2,2,5-trisubstituted tetrahydrofurans

Acid mediated, efficient Grob-type fragmentation reaction facilitated by vicinal ketal and ester moieties in variety of 5-oxabicyclo[2.1.1]hexanes leading to the corresponding annulated and 2,2,5-trisubstituted tetrahydrofurans is reported. Among the Brønsted and Lewis acids tested, BF₃·OEt₂ appears to give the best results, furnishing near quantitative yield (>99%) of tetrahydrofuran tricarboxylate derivatives under mild reaction condition. In case of unsymmetrical monosubstituted 5-oxabicyclo[2.1.1]hexanes two regioisomeric products are obtained. A strategy to transform one of the ester groups of the title compounds to protected hydroxymethyl moiety was evolved, which allows access to differentially protected 2-hydroxymethyl THF derivatives upon fragmentation. Employing TiCl₄/R or S-BINOL as chiral Lewis acid, an enantioselective fragmentation (up to 66% ee) was described for the meso bis-furan derivative.     

Synthesis and Ring Transformation of Pyrrolo[2,3-d][1,3]oxazine to Pyrrolo[2,3-d]Pyrimidines

A convenient route is reported for the synthesis of fused pyrrolo[2,3-d][1,3]oxazine and pyrrolo[2,3-d]-pyrimidine derivatives from 2-amino-1-benzyl-3-t-butoxycarbonyl]-4,5-dimethylpyrrole.     

Mechanistic insights into the rearrangement of azetidine N-oxides to isoxazolidines

Various functionalized azetidines were oxidized with mCPBA or hydrogen peroxide, to produce the corresponding N-oxide and study its fate in the [1,2] Meisenheimer rearrangement. This ring expansion leading to isoxazolidines occurs readily, without trapping of the transient N-oxide. Starting with azetidines bearing a nitrile or an ester group at C-2, the rearrangement is regioselective. However, a varying amount of epimerization on the migrating radical is observed, which can also be observed with the related [1,2] Stevens rearrangement.     

Thiophenol-catalyzed Claisen rearrangement and radical cyclization: formation of furo- and pyrano-coumarin derivatives

Regioselective synthesis of dihydrofurocoumarins and dihydropyranocoumarins in excellent yields from 4-prop-2-ynyloxy coumarin via a thiol mediated radical reaction is described. Alkenyl radicals are generated from easily available terminal alkynes and thiophenol. Thiophenol catalyzed the Claisen rearrangement of the 4-prop-2-ynyloxycoumarin ethers.