Alkylidene malonates, cycloaddition

As shown in Equation (42), the cycloadditions of alkylidene malonates and their derivatives enable selective synthesis of both [2 + 2]- and [2 + 3]-adducts by a proper choice of reaction temperature.167,168 Methyl propiolate undergoes [2 + 2]-cycloaddition predominantly. Use of excess allyltriisopropylsilane induces double [2 + 21-cycloaddition to methyl propiolate, forming a bicyclo[2.2.0]hexane.1 7 The [2 + 2]-cycloadditions of (E)- and (Z)-crotylsilanes proceed stereospecifically. 

Suitably substituted isoxazolidines can undergo a thermally induced cycloreversion to nitrone and alkene. For example, the adducts of nitrones with alkylidene malonates or tolylsulfinylfuran-2(577)-ones suffer an easy cycloreversion, which determines significant changes in the diastereomeric composition of the cycloaddition mixtures depending on the reaction conditions (kinetic vs. thermodynamic control) <20040L1677, 2005JOC8825>. 

Enantioselective cycloadditions of nitrones with alkylidene malonates were catalyzed by the complex of Go(ll) with trisoxazoline 528. The cycloaddition was reversible and the diastereoselectivity could be controlled by reaction temperature. For example, A, C-diphenyl nitrone and diethyl 2-benzylidenemalonate reacted at —40°C under kinetic control, affording mainly the m-adduct 530, but at 0°C the thermodynamically more stable /ra r-isomer 529 was the major product (Equation 85) <20040L1677>. 

Reduction of alkylidene malonates (60) in MeOH in an undivided cell using alkali metal halides as supporting electrolytes results in the unusual formation of 3,4-disubsti-tuted 1,1,2,2-cyclobutanetetracarboxylates, 61 [141]. Cyclobutane formation requires 4-7 F and is not a radical anion-catalyzed cycloaddition. The process was explained by the mechanism in Scheme 11, where the cyclization takes place by chemical oxidation of the... 

The complex of Co(II) with trisoxazoline 76 catalysed the 1,3-DC between a variety of nitrones and alkylidene malonates to give the corresponding isoxazolidines with both high enantio- and diastereoselectivity. The cycloaddition was reversibile and the endo/exo selectivity could be effectively controlled by reaction temperature. For example, 66 and 73 reacted in the presence of catalytic amounts of Co(C104)2 6H20 (5 mol%) and 76 (3.3 mol%) at -40 °C under kinetic control affording mainly the cis isoxazolidine 74, but at 0 °C the thermodinamically more stable trans isomer 75 was the major product <04OL1677>. 

Related [4+2] cycloadditions involving substrates that incorporate heteroatoms into the formed six-membered ring are also known. In representative examples from Matsubara and Ogoshi, the cycloaddition of alkylidene malonates with alkynes provide dihydropyran products, whereas cycloadditions of 1,3-dienes with nitriles provide pyridine products (Scheme 3-22). The formation of pyridines arises from oxidation of initially formed dihydropyridines. 

The chiral nickel(II)/Al,A -dioxide complex-catalysed asymmetric 3 + 2-cycloaddition of nitrones to alkylidene malonates produced isoxazolidine derivatives in good yields and excellent diastereo- and enantio-selectivities. (94 6 dr and 99% ee). Asymmetric organocatalysts obtained from (X)-BINAM and L-Q -amino 0 acids catalysed the 3 + 2-cycloaddition of alkyl glyoxylate-derived nitrones with (g)... 

The competition between Michael addition and 1,3-dipolar cycloaddition has been investigated for the reaction of alkylidene bisphosphates and alkylidene malonates PhCH=CX2 [X = P(0)(0R)2 or CO2R] with azomethine ylides (generated from PhCH=NCH2C02Me) catalysed by the CuOTf-BiphamPhos complex. ... 

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