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Abstract: Intramolecular 1,3-dipolar cycloadditions of two N-alkenylnitrones are studied by means of density functional theory calculations. Cycloaddition of an acyclic 4-hexenylnitrone led to the expected isoxazolidine in 46% yield, but a 4-cycloheptenylnitrone did not react. Calculations of the transition states for cycloaddition indicate that although the cycloheptenyl nitrone has a more favorable activation entropy, the strain associated with distortion of the tethering groups into the required boat conformation disfavors the reaction of the cyclic substrate over the acyclic substrate by 8.7kcal/mol. [Copyright &y& Elsevier]

The intramolecular Diels-Alder reaction has been used as a powerful method to access the tricyclic core of the eunicellin natural products from a number of 9-membered-ring precursors. The endo/exo selectivity of this reaction can be controlled through a remarkable organocatalytic approach, employing MacMillan's imidazolidinone catalysts, although the mechanistic origin of this selectivity remains unclear. We present a combined experimental and density functional theory investigation, providing insight into the effects of medium-ring constraints on the organocatalyzed intramolecular Diels-Alder reaction to form the isobenzofuran core of the eunicellins. [ABSTRACT FROM AUTHOR]

The factors controlling chemo-, regio-, and stereoselectivity in a cascade of reactions starting from a bis(cyanoalkenyl)oxime and proceeding via nitrone cyclo- additions have been unraveled through a series of density functional theory calculations with several different functionals. Both kinetic and thermodynamic control of the reaction cascade are important, depending upon the conditions. Kinetic control was analyzed by the distortion/interaction model and found to be dictated by differences in distortions of the cycloaddends in the transition states. A new mechanism competing with that originally proposed in the application of these reactions to the histrionicotoxin synthesis was discovered in these studies. [ABSTRACT FROM AUTHOR]

Various fullerene-based electron acceptor materials for organic photovoltaic applications were prepared via [3 + 2] and [4 + 2] cycloadditions using a continuous flow approach. The 1,3-dipolar cycloaddition of the tosylhydrazone precursor and the Diels-Alder cycloaddition of indene to either C60 or C70 under conventional batch reaction conditions were translated to the continuous flow process. By varying the residence time, temperature, and equivalents of cycloaddition reagent, significant improvements in yields and reaction times were achieved over conventional batch processes. [ABSTRACT FROM AUTHOR]

Abstract: A domino cyclization reaction of the bis-unsaturated ketone 3 with hydroxylamine proceeds with good yield and high stereoselectivity, in a flow reactor system. The tricyclic spiropiperidine products 5 and 2 obtained are valuable building blocks for the synthesis of analogues of the histrionicotoxin alkaloids. [ABSTRACT FROM AUTHOR]

Abstract: The tricyclic-isoxazolidine analogues tetrahydrothiochromenoisoxazoles, hexahydroisoxazolequinolines and tetrahydroisoxazolepyranopyridines were prepared by an intramolecular 1,3-dipolar cycloaddition reaction of a nitrone with an alkene. For N-alkylated hexahydroisoxazolequinolines, reduction of the reaction time from two days to 40min was achieved using microwave heating. The cyclization to form tetrahydroisoxazolepyranopyridines only proceeded when the alkene was substituted with an electron withdrawing group. [Copyright &y& Elsevier]

The conversion of vinyl-substituted 6- and 7-membered cyclic carbonates into 8- and 9-membered medium-ring lactones has been achieved in good yield using dimethyltitanocene in toluene at reflux. The reaction proceeds by initial formation of a ketene acetal which undergoes subsequent in situ Claisen rearrangement to provide the corresponding lactones. The preparation of the cyclic carbonates is carried out under basic conditions and hence this methodology complements our existing selenium-based methodology for the synthesis of medium-ring lactones. [Copyright &y& Elsevier]