Nitrone, Alkoxycarbonyl

Nitrone 1,3-DC reactions are still the most general approach to isoxazolidines. The stereocontrol is usually achieved by the use of chiral nitrones and/or dipolarophiles, but new interesting achievements on Lewis acid catalyzed cycloadditions are also frequently reported. Tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanatedionate) europium(III) [Eu(fod)3] selectively activated the Z-isomer of C-alkoxycarbonyl nitrone 75 existing as an E,Z-equilibrium mixture by forming the (Z)-75-Eu(fod)3 complex. (Z)-75-Eu(fod)3 reacted with electron-rich dipolarophiles such as vinyl ethers to give the trans-adducts with excellent diastereoselectivity <06T12227>. 

The formation of compound (16) is proposed to proceed through the nitrile oxide (15) as an intermediate to undergo intramolecular nitrile oxide-olefin cycloaddition (INOC). On the contrary, the generation of (18) is proposed to proceed through intramolecular alkoxycarbonyl nitronate-olehn cycloaddition (lAOC) step because only the trans isomers are formed. 

Under catalytic hydrogenation conditions, the adducts of nitrones and acrylates, 5-alkoxycarbonyl and 5-aminocarbo-nyl isoxazolidines, give 3-hydroxypyrrolidin-2-ones by intramolecular N-acylation of the amino group. 3,5-Dicarboxylic... 

C-alkoxycarbonyl nitrone with a vinyl nucleobase followed by reduction with NaBH4. For example, nitrone 584 reacted with vinylthymine with complete regioselectivity and moderate diastereoselectivity. The isolated major fra t-disubstituted isoxazolidine 585 was reduced to the corresponding enantiopure 3-hydroxymethyl N-O-nucleo-side 586 in 62% yield (Scheme 143). 

In a similar manner, nitrones 32c,133 32d,129 and 32e112 with various monosubstituted olefins afford the corresponding isoxazolizidines (33c-e). 4-Hydroxy-3-nitroisoxazoline-A(-oxide (321) is thoroughly investigated. With olefins 3, such as propylene, styrene, vinyl acetate, methyl acrylate and acrylonitrile, adducts 33f are formed.134 Reactions of alkoxy or alkoxycarbonyl derivatives of 32 are also known these include 32g,133 3 2h,13 32i,135 and 3 j135 32j is reactive enough to cyclize... 

C-Alkoxycarbonyl nitrones as building blocks for the s5mthesis of buteno-lides, lactams, and modified nucleosides 05MRO59. 

GEOMETRY-CONTROLLED CYCLOADDITION OF C-ALKOXYCARBONYL NITRONES SYNTHETIC STUDIES ON NONPROTEINOGENIC AMINO ACIDS... 

Mechanistically, the nitrone cycloaddition is a [4jt -T 2jt]-type concerted process similar to the Diels-Alder reaction [lb]. One of the problems in nitrone cycloaddition compared to the Diels-Alder reaction is the possibiUty of geometrical isomerization of the nitrone moiety during the cycloaddition reaction. In particular, C-alkoxycarbonyl nitrones 1 having electron-withdrawing ester groups are known to exist as equilibrium mixtures of ( )-l and (Z)-l in solution even at room temperature (Eq. 6.2) [2]. 

GEOMETRY-CONTROU D CYCLOADDITION OE C-ALKOXYCARBONYL NITRONES TABLE 6.1 The E and Z Ratios of Nitrones la-d... 

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