Nitrones reaction with furan

The Mannich reaction is a very common process that occurs in many tandem reaction sequences. For example, the Overman Aza-Cope cascade sequence is terminated by a Mannich reaction (cf. Scheme 35). Several groups have used variants of the Mannich reaction to initiate cascades that lead to the formation of heterocyclic molecules. For example, the Lewis acid-catalyzed intermolecular vinylogous Mannich reaction (01T3221) of silyloxy furan 281 with nitrone 282 produced a diastereomeric mixture (49 3 42 6) of azabicycles 284a-d in 97% combined yield (Scheme 52) (96TA1059). These products arose from an intramolecular Michael addition of the initially formed oxonium ion 283. 

The first example of a nitrone reaction with pyrroles and furan in the presence of HC1 as an activating agent was recently reported (589). Depending on reaction conditions, these acid-catalyzed reactions make it possible to obtain both, 2-heteroaromatic N -benzylhydroxylamines and symmetric as well as asymmetric 2,2 -bis -(heteroaryl) alkanes (Scheme 2.155). 

The DFT study of the 3 + 2-cycloaddition between ketene and TV-silyl-, IV-germyl-, and TV-stannyl-imines shows that the TV-germylimine reaction is a two-step process the TV-stannylimine reaction is a competition between two- and three-step processes whereas the TV-silyl process follows a three-step process44 A new and convenient synthesis of functionalized furans and benzofurans based on 3 + 2-cycloaddition/oxidation has been reported. The cyclization of cyclic 1,3-bis-silyl enol ethers (48) with l-chloro-2,2-dimethoxyethane (49), via a dianion, produced 5,6-bicyclic 2-alkylidenetetrahydrofurans (50), which are readily oxidized with DDQ to 2,3-unsubstituted benzofurans (51) (Scheme 13)45 The Evans bis(oxazoline)-Cu(II) complex catalyses the asymmetric 1,3-dipolar cycloaddition of a -hydroxyenones with nitrones to produce isoxazolidines.46 The... 

As indicated from computational studies, the catalyst-activated iminium ion MM3-2 was expected to form with only the (E)-conformation to avoid nonbonding interactions between the substrate double bond and the gem-dimethyl substituents on the catalyst framework. In addition, the benzyl group of the imidazolidinone moiety should effectively shield the iminium-ion Si-face, leaving the Re-face exposed for enantioselective bond formation. The efficiency of chiral amine 1 in iminium catalysis was demonstrated by its successful application in several transformations such as enantioselective Diels-Alder reactions [6], nitrone additions [12], and Friedel-Crafts alkylations of pyrrole nucleophiles [13]. However, diminished reactivity was observed when indole and furan heteroaromatics where used for similar conjugate additions, causing the MacMillan group to embark upon studies to identify a more reactive and versatile amine catalyst. This led ultimately to the discovery of the second-generation imidazolidinone catalyst 3 (Fig. 3.1, bottom) [14],... 

The 1,3-dithiolane 1,3-dioxide 365 was also investigated in the Diels-Alder reaction with a range of simple dienes (cyclopentadiene, 1-methoxybutadiene, l-methoxy-3-trimethylsilyloxybutadiene, furan) <1995JOC4962, 1998J(P1)2771> and 1,3-dipolar cycloadditions with A - /t-butyl-(7-phenyl nitrone <1998JOC3481> or 3-oxidopyr-idinium betaines <20030BC1884>. 

When phenylglyoxylate esters derived from various carbohydrate alcohols Eire employed in the Patemo-Buchi reaction with furan, the products (107) can show up to 80% diastereomeric excess.The nitrone (108) underwent facially-selective addition to the appropriate alkene to give adduct (109) ... 

Diacetates of 1,4-butenediol derivatives are useful for double allylation to give cyclic compounds. l,4-Diacetoxy-2-butene (126) reacts with the cyclohexanone enamine 125 to give bicyclo[4.3.1]decenone (127) and vinylbicy-clo[3.2.1]octanone (128)[85,86]. The reaction of the 3-ketoglutarate 130 with cij-cyclopentene-3,5-diacetate (129) affords the furan derivative 131 [87]. The C- and 0-allylations of ambident lithium [(phenylsulfonyl)methylene]nitronate (132) with 129 give isoxazoline-2-oxide 133, which is converted into c -3-hydroxy-4-cyanocyclopentene (134)[S8]. Similarly, chiral m-3-amino-4-hyd-roxycyclopentene was prepared by the cyclization of yV-tosylcarbamate[89]. 

Trimethylsilyl perrhenate forms colorless crystals, which are very sensitive to hydrolysis and which must be handled in a dry atmosphere. Under anhydrous conditions, the compound is stable to light and oxidation. Contaminated products undergo slow decomposition at room temperature, blue reduction products of Re(VII) being formed. In an excess of water trimethylsilyl perrhenate is hydrolyzed quickly and quantitatively. The perrhenic acid formed in this reaction together with trimethylsilanol can be determined by acidimetric titration or precipitation as nitron perrhenate. Trimethylsilyl perrhenate is soluble without decomposition in all common anhydrous and aprotic solvents (benzene, cyclohexane, ethyl ether, tetrahydro-furan, chloroform, etc.). 

Benzyne can be trapped in 1,3-dipolar cycloaddition reactions provided that the 1,3-dipolar species is sufficiently stable under the conditions necessary for benzyne generation. As an illustration of this, benzyne reacts with the nitrile oxide group in preference to the furan ring of compound 19, whereby adduct 20 is obtained.27 Benzyne and the nitrone 21 give adduct 22,28 and... 

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