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

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

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Ph<br />

REACTIONS OF DIPOLAR 1,3-CYCLOADDITION ([3 + 2] CYCLOADDITION) 381<br />

C<br />

Cl Cl<br />

N<br />

Pt<br />

N<br />

cis - 752<br />

C Ph<br />

H<br />

C<br />

R1 + Me<br />

N<br />

O<br />

−<br />

753 a<br />

753 b<br />

O<br />

Ph<br />

Cl Cl<br />

C Pt<br />

N N<br />

Me<br />

N<br />

R2<br />

H<br />

756 a<br />

756 b<br />

a: R 1 , R 2 = Ph; b: R 1 , R 2 = p-MeC6H4<br />

Scheme 2.311<br />

C Ph<br />

2.8.4. Cycloaddition to Isocyanates, Isothiocyanates, <strong>and</strong> Ketenes<br />

H<br />

C<br />

R2 + Me<br />

N<br />

O<br />

−<br />

753 a<br />

753 b<br />

Cycloaddition of nitrones with electron-donat<strong>in</strong>g substituents R (757a,c,f) to<br />

phenyl isocyanate proceeds readily, <strong>in</strong> high yields, to give the correspond<strong>in</strong>g<br />

1,2,4-oxadiazolid<strong>in</strong>one (758a,c,f). <strong>Nitrones</strong> with electron-withdraw<strong>in</strong>g substituents<br />

(757b,d,e,g) give the correspond<strong>in</strong>g oxadiazolid<strong>in</strong>one (758b,d,e,g) <strong>in</strong><br />

moderate yields after prolonged heat<strong>in</strong>g (Scheme 2.312) (834).<br />

The <strong>in</strong>teraction of imidazol<strong>in</strong>e nitroxide (759) with isocyanates leads to oxadiazolid<strong>in</strong>es<br />

(760), which can be easily transformed <strong>in</strong>to 4R-am<strong>in</strong>o-3-imidazol<strong>in</strong>e-<br />

1-oxyls (761) on treatment with nucleophilic reagents (Nu = NaI, NaN3,NH4OH,<br />

AcONa, NaH) (Scheme 2.313) (835).<br />

Cyclic α-methoxynitrones (225f) <strong>and</strong> (223 g) react respectively with isocyanates<br />

<strong>and</strong> isothiocyanates at ambient temperature, giv<strong>in</strong>g 1,3-dipolar cycloaddition<br />

products (763) <strong>and</strong> (764) (Scheme 2.314). Under similar conditions, the<br />

reaction of aldonitrone (223a) proceeds much more slowly to give cycloadducts<br />

H R<br />

+<br />

R<br />

N<br />

1H2C O<br />

757<br />

−<br />

PhNCO<br />

Acetonitrile<br />

reflux<br />

Ph<br />

N<br />

R<br />

N<br />

O O<br />

1 R<br />

H<br />

H2C<br />

758<br />

a: R = 3,4-(MeO)2C6H3; R 1 = H<br />

b: R = 2-NO 2C6H4; R 1 = H<br />

c: R = Ph, R 1 = Ph<br />

d: R = 2-NO2C6H4; R 1 = 2,3-(MeO)2C6H3<br />

e: R = 2-NO 2C 6H 4; R 1 = Ph<br />

f: R = 2,3-(MeO) 2C5H3; R 1 = Ph<br />

g: R = 3-NO 2C 6H 4; R 1 = Ph<br />

Scheme 2.312

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