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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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N<br />

O<br />

373<br />

N<br />

O<br />

a<br />

374, 376, 377<br />

R2<br />

376 2-Py<br />

377 2-Thienyl<br />

a<br />

O<br />

R 2<br />

374<br />

c<br />

d<br />

N<br />

O<br />

374<br />

N<br />

OAc<br />

375<br />

b<br />

374 a<br />

N<br />

O<br />

R 1<br />

378<br />

NITRONE REACTIONS 267<br />

374 R1 a H<br />

b Ph<br />

c<br />

d<br />

e<br />

f<br />

g<br />

h<br />

O<br />

Ph<br />

1-naphthyl<br />

C(=CH2)CH3 OH<br />

CHO<br />

C(=O)CH3<br />

C(=O)Ph<br />

a: R- -MgBr/THF/373, −10 °C to r.t., 2 h, then NH 4Cl/MnO 2/CHCl 3,<br />

r.t. or reflux, 30 m<strong>in</strong> (46–78%).<br />

b: Ascorbic acid (5 equiv)/dioxane/H 2O (2:1), 40 °C, 15 m<strong>in</strong>,<br />

then CHCl3/ Et3N/AcCl, 0 °C to r.t., 1 h (87%).<br />

c: EtMgBr (2 equiv)/THF/375, 50 °C, 30 m<strong>in</strong>, then aromatic aldehyde,<br />

−10 °C to r.t., 1 h, NH 4Cl, then MnO2/CHCl3, reflux, 15 m<strong>in</strong> (52–71%).<br />

d: SMEAH/THF, 5 h, r.t., then 10% NaOH then MnO2/CHCl3, reflux, 30 m<strong>in</strong> (74%).<br />

Scheme 2.164<br />

Interaction of pyrrol<strong>in</strong>e-N -oxides (373) with alkynylmagnesium bromide gives<br />

alkyne substituted nitroxyl radicals (374), (376), <strong>and</strong> (377) (Scheme 2.164)<br />

(620).<br />

Addition of the Grignard reagent, generated <strong>in</strong> situ from (375), to nitrone (373)<br />

or to 2,5-dimethyl-1-pyrrol<strong>in</strong>e-N -oxide, affords biradical (379) or nitrone conta<strong>in</strong><strong>in</strong>g<br />

monoradical (380). Furthermore, (380) can be transformed <strong>in</strong>to biradical<br />

(381) <strong>and</strong> triradical (382) (Scheme 2.165) (620).<br />

Nucleophilic addition of α-lithiated sulfoxides to α-PBN, followed by oxidation,<br />

gives β-sulÞnyl nitroxyl radicals (383) (Scheme 2.166) (621).<br />

•<br />

OH

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