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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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APPLICATIONS OF NITRILE OXIDES 99<br />

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New isoxazol<strong>in</strong>e derivatives of α-tocopherol, the ma<strong>in</strong> component of vitam<strong>in</strong><br />

E, have been synthesized <strong>in</strong> a facile, two-step sequence consist<strong>in</strong>g of nitration<br />

followed by 1,3-dipolar cycloaddition. 5-Nitromethyl-α-tocopheryl acetate,<br />

obta<strong>in</strong>ed from α-tocopheryl acetate by direct nitration <strong>in</strong> one step, act as the<br />

nitrile oxide precursor <strong>in</strong> the reaction with various alkenes. The facile conversion<br />

proceeds <strong>in</strong> the presence of equimolar amounts of PhNCO <strong>and</strong> catalytic<br />

amounts of triethylam<strong>in</strong>e to give isoxazol<strong>in</strong>es, 446 (489).<br />

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A novel class of activators for chloride conductance <strong>in</strong> the cystic Þbrosis transmembrane<br />

conductance regulator prote<strong>in</strong> has been identiÞed. These 3-(2-benzyloxyphenyl)isoxazoles<br />

<strong>and</strong> 3-(2-benzyloxyphenyl)isoxazol<strong>in</strong>es have been<br />

synthesized employ<strong>in</strong>g the 1,3-dipolar cycloaddition of nitrile oxides with various<br />

alkene <strong>and</strong> alkyne dipolarophiles (490).<br />

3,4-Diarylisoxazole analogs of valdecoxib [4-(5-methyl-3-phenylisoxazol-4-yl)<br />

benzensulfonamide], a selective cyclooxygenase-2 (COX-2) <strong>in</strong>hibitor, have been<br />

synthesized by the 1,3-dipolar cycloaddition of arencarbonitrile oxides to the<br />

enolate ion of phenylacetone, regioselectively prepared <strong>in</strong> situ with lithium diisopropylamide<br />

at 0 ◦ (491). The correspond<strong>in</strong>g 3-aryl-5-methyl-4-phenylisoxazoles<br />

are easily generated by a dehydration/aromatization reaction, under basic conditions,<br />

of 3-aryl-5-hydroxy-5-methyl-4-phenyl-2-isoxazol<strong>in</strong>es <strong>and</strong> are further transformed<br />

<strong>in</strong>to their benzenesulfonamide derivatives. The biochemical COX-1/<br />

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