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Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

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OH<br />

_<br />

O<br />

+<br />

N<br />

R 1<br />

a: R 1 = Me, b: R 1 = H<br />

hn/N 2<br />

MeCN<br />

Scheme 2.91<br />

NITRONE REACTIONS 209<br />

produced by thermal vacuum evaporation) (465). Analysis of the effects of substituents<br />

<strong>and</strong> solvents has revealed an important role of <strong>in</strong>tramolecular hydrogen<br />

bond<strong>in</strong>g to the nitrogen of the oxazirid<strong>in</strong>e r<strong>in</strong>g <strong>in</strong> a novel photochemical transformation<br />

of hydroxyl substituted aromatic nitrones to N,N -diarylformamide<br />

(Scheme 2.91) (466).<br />

Explanation of the observed de-oxygenation with the formation of the correspond<strong>in</strong>g<br />

im<strong>in</strong>es <strong>in</strong> the photochemical reaction of α-aryl-N-methylaldonitrones,<br />

conÞrms the <strong>in</strong>termediate formation of radical cations result<strong>in</strong>g from PET<br />

(441, 467).<br />

Photo transformations <strong>in</strong> Þlms of copolymers of α-(2-hydroxy-4-methacryloyloxyphenyl)—(2,6-dimethylphenyl)nitrone<br />

(HMDN) with methyl methacrylate<br />

(MMA) showed that, on radiation at 366 nm, an <strong>in</strong>crease of the steric<br />

effect <strong>in</strong> the N -aryl group leads to a decrease of the refractive <strong>in</strong>dex (468, 469).<br />

2.6.1.2. Oxygen Migration The reaction of pyrrol<strong>in</strong>e N -oxides (DMPO) with<br />

acid halides <strong>and</strong> acid anhydrides gives N-acyloxyim<strong>in</strong>es (251), which undergo a<br />

hetero-Cope rearrangement to β-acyloxyim<strong>in</strong>es (252) (Scheme 2.92) (470, 471).<br />

Mesylation of α-am<strong>in</strong>o nitrones (253) <strong>in</strong> dichloromethane gave amid<strong>in</strong>es (254)<br />

via a 3,3-sigmatropic rearrangement (Scheme 2.93) (351).<br />

On boil<strong>in</strong>g <strong>in</strong> benzene, C-(4-oxo-4H [1]benzopyran-3-yl)-N -phenylnitrone<br />

(255) rearranges to 2-(N -phenylam<strong>in</strong>o)-4-oxo-4H [1]-benzopyran-3-carboxaldehyde<br />

(256) (70%) <strong>and</strong> 3-(phenylim<strong>in</strong>omethylene)-chroman-2,4-dione (257) (25%)<br />

(Scheme 2.94) (472).<br />

2.6.1.3. Rearrangement of <strong>Nitrones</strong> Into Amides The most common nitrone<br />

rearrangement caused by the migration of the oxygen atom is the conversion to<br />

amides (Beckman rearrangement). It occurs under the <strong>in</strong>ßuence of electrophilic<br />

agents such as acid anhydrides, acid halides, <strong>and</strong> sulfonyl chlorides, as well as<br />

with nucleophilic agents such as alcoholate anions (6d).<br />

Pyrrol<strong>in</strong>e-N -oxide (258) is isomerized <strong>in</strong>to γ-lactam (259) <strong>in</strong> the presence of<br />

lithium diisopropylam<strong>in</strong>e (LDA) (470) <strong>and</strong> sodium trityl (471). In these reactions,<br />

deprotonation at C3 occurs, lead<strong>in</strong>g to carbanion (260). Then oxygen<br />

migration from N1 to C2 takes place via <strong>in</strong>termediate formation of oxazirid<strong>in</strong>e<br />

H<br />

O<br />

H<br />

N<br />

O<br />

R 1

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