Intramolecular 1,3-dipolar nitrone and nitrile oxide cycloaddition of 2- and 4-O-allyl and propargyl glucose derivatives: a versatile approach to chiral cyclic ether fused isoxazolidines, isoxazolines and isoxazoles

2-O- and 4-O-Allyl and -propargyl glucose and the corresponding oxime derivatives were prepared from readily available glucose dithioacetals. Intramolecular 1,3-dipolar cycloaddition of the N-benzyl and N-methyl nitrones of the above acyclic 2-O-allyl glucose derivatives led to the diastereoselective formation of chiral isoxazolidines incorporating the tetrahydrofuran ring. The EI mass spectra revealed a characteristic cleavage of the C-alkyl group adjacent to the furan oxygen atom. An enantiopure trisubstituted tetrahydrofuran was obtained by the reductive cleavage of the isoxazolidine ring of one of the cycloadducts. In contrast, the nitrile oxide cycloaddition of the 2-O-allyl derivatives afforded diastereomeric mixtures of the corresponding dihydroisoxazolines, the stereochemistry of which was tentatively assigned on the basis of the principle of optical superposition. The exclusive formation of a tetrahydrofuran ring from pentaallyl nitrone or nitrile oxide demonstrated the preferred formation of a five-membered ring to that of six or seven-membered rings. The nitrile oxide generated from a 3,4,5,6,7-pentaallyloxy-1-nitroheptane derivative obtained from pentaallylglucose underwent diastereoselective cycloaddition to give an isoxazoline fused to a pyran ring. Enantiopure isoxazoles containing tetrahydrofuran and oxepane rings were also prepared in good yields by the nitrile oxide cycloaddition of the 2-O- and 4-O-propargyl derivatives.     

Cycloadditions of 1-iminylphosphirane complexes with allenes

A cascade carbonylative ring expansion and [2+2]/[4+2] cycloaddition of strained 1-iminylphosphirane complexes with aryl allenes were reported.The carbonylative ring expansion of 1-iminylphosphirane complexes provides an azaphosphacyclohexone complex intermediate with a C=P double bond.The following [2+2] or dearomatic [4+2] cycloaddition of this intermediate with allenes is modulated by the aryl substituents on the imino carbon.The regioselective [2+2] cycloaddition with 1,1-diarylallene provides an entry to bicyclo[4.2.0]octan-4-one skeletons featuring a four-membered phosphacyclobutane moiety.While dearomatic [4+2] cycloaddition was preferred with less aromatic naphthalene and yielded octahydrochrysene skeleton containing heteroatoms.     

1,3-Dipolar cycloaddition of exo-methylenesugars with nitrone: approach to new amino-C-ketosyl disaccharides

Stereoselective 1,3-dipolar cycloadditions of exo-methylenesugars 1a-c to a sugar nitrone 2 were carried out with refluxing a toluene solution and afforded the corresponding cycloadducts, ketosyl spiro-isoxazolidine disaccharides 3a and 4a-c. Followed by reductive cleavage of the N-O bond of the isoxazolidine ring with the treatment of Zn-AcOH-Ac₂O or with catalytic hydrogenation (Pd(OH)₂/C), the cycloadduct 3a could be readily converted into a novel amino-C-disaccharide possessing a ketose form, providing an access to a novel amino-C-ketosyl disaccharides.     

An efficient and stereoselective cycloaddition of C-aryl and C-amido nitrones to dimethyl 2-benzylidenecyclopropane-1,1-dicarboxylate

1,3-Dipolar cycloadditions of dimethyl 2-benzylidenecyclopropane-1,1-dicarboxylate and a number of C-aryl or C-amido nitrones proceed with high efficiency and selectivity with the formation of only one isomeric spiro[cyclopropane-1,4-isoxazolidine] cycloadduct.     

Phosphine-promoted [3 + 2] cycloaddition between nonsubstituted MBH carbonates and trifluoromethyl ketones

A phosphine-promoted [3 + 2] cycloaddition from readily accessible MBH(Morita–Baylis–Hillman)carbonate and aryl trifluoromethyl ketone is described. The use of methyl vinyl ketone-derived allylic carbonate rather than common acrylate-derived counterpart renders the reaction pathway exclusive for 5-endo process, which enables the expeditious preparation of a range of trifluoromethylated 2,3-dihydrofuran in a chemospecific manner.     

endo-Selective (3+2) cycloaddition polymerizations of nitrone monomers with olefins utilising high pressure conditions

A high pressure mediated (3+2) cycloaddition polymerization strategy has been employed to afford linear poly(isoxazolidine) architectures. Under these high pressure conditions this cycloaddition process was found to afford primarily endo-heterocycles which when translated to the polymerization should ultimately affect the tacticity and resultant properties of the polymer. The stereoselectivity occurred as a result of a lower volume of activation for the endo-transition state and the application of a ‘type-I’ regime (HOMO_Dipole-LUMO_{Dipolarophile}) cycloaddition process that features secondary orbital interactions within the extended molecular orbitals. A variety of linker segments were employed in an attempt to affect the physical properties of the polymeric cycloadducts such as T_g and solubility in order to tailor these materials for use in coating applications.     

Synthesis and functionalization of N-sulfinyl imines: Sonogashira reaction and copper-catalyzed azide-alkyne cycloaddition

A small library of novel molecules was generated using a rapid and efficient methodology for the synthesis of N-sulfinyl imine triazole compounds. The process involves a coupling step from the Sonogashira cross-coupling reaction and then, in a one-step reaction, deprotection of the trimethylsilyl group and triazole heterocyclic ring formation using a microwave reactor.     

The face selectivity of 1,3-dipolar cycloaddition reactions of 4-butyloxycarbonyl-3,4,5,6-tetrahydropyridine 1-oxide

A study of the stereo- and face selectivity of the cycloaddition reactions of a series of mono- and disubstituted alkenes with 4-butyloxycarbonyl-3,4,5,6-tetrahydropyridine 1-oxide has been carried out. Rate constants for the cycloaddition of the nitrone to methyl acrylate, styrene, and methyl methacrylate have been determined at various temperatures by ¹H NMR spectroscopy. The activation parameters indicate the concerted nature of the reaction. The 4-substituted nitrone is found to be more reactive than its unsubstituted counterpart 3,4,5,6-tetrahydropyridine 1-oxide. The addition reactions have displayed a very high degree of face selectivity (9:1), and those reactions in micellar media are found to be very efficient. Conformational analysis and peracid-induced ring opening of a cycloadduct has been carried out to give second-generation cyclic nitrones.     

Cation radical induced cycloaddition reaction between aryl imines and ethyl diazoacetate

The reaction of ethyl diazoacetate with aryl imines can be initiated by persistent cation radical salt tris(4-bromophenyl)aminium hexachloroantimonate, giving exclusively cis-aziridine carboxylates.     

The preparation of perfluoroalkenyl allenes (trienes) and pentafluorophenyl allenes

Trifluorovinyl- and (Z)-pentafluoropropenylcopper reagents readily react with propargylic halides or tosylates. With primary propargylic substrates, the major product is the alkyne. With secondary propargylic substrates, mixtures of alkyne/allene are obtained; the allene product is the major product. With tertiary propargylic substrates, the allene is regiospecifically and stereospecifically formed in good isolated yields. With pentafluorophenyl copper and primary propargylic substrates, the major product is pentafluorobenzene. With secondary and tertiary propargylic substrates, the allene is regiospecifically formed in good isolated yields.     

Synthesis of 1-acetyl-2-silyoxycycloheptane derivatives via highly stereoselective formal [5+2] cycloaddition reaction

A stereoselective [5+2] cycloaddition reaction using a new five-carbon unit, that has a dicobalt acetylene complex moiety and an enol silyl ether moiety, was developed. In the presence of a Lewis acid, the five-carbon unit reacted with an enol triisopropylsilyl ether to give a 1-acetyl-2-silyoxycycloheptane derivative, in which the three contiguous substituents on the seven-membered ring arrange cis to each other.     

A novel access to highly functionalised β-lactams by regio- and stereoselective 1,3-dipolar cycloaddition reaction

A highly regio- and stereoselective synthesis of novel spiro pyrrolidines/pyrrolizidines containing β-lactam and oxazolone moieties under two different conditions is achieved using [3+2] cycloaddition methodology in moderate to good yield.     

Intramolecular ketonitrone-olefin cycloaddition reaction: direct and stereocontrolled synthesis of nitrogenated quaternary centered aminocyclopentitols as galactosidase inhibitors

Synthesis of nitrogenated quaternary centered polyhydroxylated aminocyclopentanes by implementation of ketonitrone-olefin cycloaddition reaction as a key step has been accomplished in a stereocontrolled manner. The target molecules were found to be moderate but selective inhibitors of galactosidases.     

Transition-metal-catalyzed carbonylation of allenes with carbon monoxide and thiols

In the presence of a catalytic amount of tetrakis(triphenylphosphine)platinum(0), allenes undergo carbonylative thiolation with carbon monoxide and thiols to provide the corresponding α,β- and β,γ-unsaturated thioesters in good yields. In contrast, the use of rhodium(I) catalysts such as RhH(CO)(PPh₃)₃ in place of Pt(PPh₃)₄ leads to copolymerization of allenes and carbon monoxide without incorporation of thio groups.     

Efficient synthesis of isoxazolidine-substituted bisphosphonates by 1,3-dipolar cycloaddition reactions

Several bisphosphonates bearing a substituted isoxazolidine ring have been synthesized in good yield by direct 1,3-dipolar cyclization reaction, under microwaves catalysis, in the absence of solvent. The method allows the simultaneous incorporation, on the geminal position of the bisphosphonate framework, of a basic nitrogen and of an oxygen atom, as third hook. Hydrophobicity-hydrophilicity of BPs is discussed with the help of distribution coefficients.     

The stereochemistry of 1,3-dipolar cycloaddition of internally H-bonded chiral methylenenitrones

A study of diastereoselectivity in the cycloaddition reactions of a series of mono- and disubstituted alkenes with two chiral, internally H-bonded methylenenitrones has been carried out. The high degree of stereochemical control in the presence of anhydrous magnesium bromide has been explained in terms of a metal chelated transition state. Intramolecular cycloaddition involving a methylenenitrone containing an alkene moiety linked to a nitrogen gave a stereoselective addition product.     

Regio- and stereoselective synthesis of spiro-pyrrolidine/pyrrolizidine/thiazolidine-grafted macrocycles through intramolecular 1,3-dipolar cycloaddition reaction

Regioselective synthesis of spiropyrrolidine-grafted 11-membered macrocycle was accomplished through an intramolecular [3+2] cycloaddition of azomethine ylides. The key precursor alkenyl diketone (4a–b) was obtained from simple starting materials. The dipole generated from isatin tethered to O-alkyl enone (4a–b) was reacted intramolecularly to yield the spiropyrrolidine-grafted macrocycles (6a–b). The structures of the cycloadducts were assigned by 2D NMR and confirmed by single crystal analysis.     

Convenient preparation of 4-arylmethyl- and 4-hetarylmethyl thiazoles by regioselective cycloaddition reactions of 3-sulfanyl- and selanylpropargyl alcohols

Complete details of thiazole syntheses by scandium-catalyzed cycloaddition reactions of 1-aryl- and 1,1-bisaryl-3-phenylsulfanylpropargyl alcohols with thioamides are described. Reactions of 1,1-bisarylpropargyl alcohols with thioamides and selenamide in MeNO₂/H₂O resulted in 4-bisarylmethyl-1,3-thiazoles 4aa-ic and 4H-4,4-bisaryl-1,3-thiazines 5ea-ga in high yields. Reactions in MeNO₂/D₂O resulted in 4-bisaryldeuteriomethyl-1,3-thiazoles 10ca-ia with high deuterium purity. Reactions of dialkyl and alkyl aryl propargyl alcohols are also described.     

金属有机骨架材料应用于二氧化碳环加成反应的研究进展

化石燃料的燃烧产生大量二氧化碳,引起了包括温室效应在内的诸多生态环境问题。二氧化碳作为一种重要的碳资源,也可用于制备多种重要的化工原料。环氧化合物与二氧化碳环加成是二氧化碳资源化利用的重要方向,并且产物环状碳酸酯在工业上能得到广泛利用。但二氧化碳具有惰性,不易被活化,因此寻求高效且稳定的催化剂成为实现二氧化碳快速转化的关键。金属有机骨架(MOFs)因具有不饱和金属位点、多孔性等优点而被应用到各类催化反应中。又因其具有路易斯酸碱位点,对二氧化碳与环氧化物环加成反应有着突出的催化效果,所以在该反应体系中也有着出色的表现,但其反应条件比较苛刻。环氧化物的活化是在环加成反应中的重要环节,卤化物对环氧化物的活化有很好的效果,但是存在难回收的问题;卤化物阴离子还会引起含铁金属的腐蚀,在一定程度上限制了大规模工业使用。很多研究人员致力于寻找减少使用该类助剂的方法,改进催化体系,于是催生出了关于MOFs改性的各类方法。本文列举了在催化二氧化碳与环氧化物环加成反应过程中关于MOFs的利用以及改性方法,并展望了MOFs材料在催化领域的发展前景。     

Regio- and stereo-specific preparation of (E)-1-aryl-3,3,3-trifluoro-1-iodo-propenes and their palladium-catalyzed reaction with terminal alkynes

A new type of iodo-containing trifluoromethylated building blocks were synthesized. The reaction of 1-aryl-3,3,3-trifluoropropynes 1 with lithium iodide in acetic acid at 75 °C gave (E)-1-aryl-3,3,3-trifluoro-1-iodo-propenes 2 in high yield, which undergo the palladium-catalyzed Sonogashira reaction with terminal alkynes afforded trifluoromethyl-containing 1,3-enynes in high yield.