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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 />

N<br />

O<br />

48<br />

REACTIONS OF NITRILE OXIDES 27<br />

P(O)MePh<br />

An <strong>in</strong>terest<strong>in</strong>g antibody-catalyzed <strong>in</strong>termolecular asymmetric 1,3-dipolar<br />

cycloaddition reaction between 4-acetamidobenzonitrile N-oxide <strong>and</strong> N,Ndimethylacrylamide<br />

generat<strong>in</strong>g the correspond<strong>in</strong>g 5-acylisoxazol<strong>in</strong>e was observed<br />

(216). Reversed regioselectivity of nitrile oxide cycloaddition to a term<strong>in</strong>al alkene<br />

was reported <strong>in</strong> the reaction of 4-tert-butylbenzonitrile oxide with 6A-acrylamido-<br />

6A-deoxy-β-cyclodextr<strong>in</strong> <strong>in</strong> aqueous solution, lead<strong>in</strong>g to the formation of the<br />

4-substituted isoxazol<strong>in</strong>e, <strong>in</strong> contrast to the predom<strong>in</strong>ance of the 5-substituted<br />

regioisomer from reactions of monosubstituted alkenes (217).<br />

Baker’s yeast catalyzed the regioselective cycloaddition of stable aromatic<br />

nitrile oxides ArCNO [Ar = 2,6-Cl2C6H3, 2,4,6-Me3C6H2, 2,4,6-(MeO)3C6H2]<br />

to ethyl c<strong>in</strong>namate, ethyl 3-(p-tolyl)acrylate, <strong>and</strong> tert-butyl c<strong>in</strong>namates (218).<br />

Reactions of dichloro- <strong>and</strong> trimethoxybenzonitrile oxides with all three esters<br />

proceeded regio- <strong>and</strong> stereoselectively to form exclusively alkyl trans-3,5-diaryl-<br />

4,5-dihydrooxazole-4-carboxylates. However, mesitonitrile oxide gave an analogous<br />

result, only with tert-butyl c<strong>in</strong>namate, whereas from the two other esters<br />

mixtures of isomeric 3,4-diaryl-4,5-dihydrooxazole-5-carboxylate ( 65:35) were<br />

obta<strong>in</strong>ed. An attempt to improve regioselectivity of the reactions of ethyl c<strong>in</strong>namates<br />

with mesitonitrile oxide, by us<strong>in</strong>g β-cyclodextr<strong>in</strong> as an artiÞcial enzyme<br />

along with baker’s yeast resulted <strong>in</strong> the reversal of regioselectivity (218). Baker’s<br />

yeast also catalyzed the asymmetric cycloaddition reactions of above-mentioned<br />

nitrile oxides to 2- <strong>and</strong> 4-v<strong>in</strong>ylpyrid<strong>in</strong>es to afford optically active 3-aryl-5-pyridyl-<br />

4,5-dihydroisoxazoles. The stereoselectivity was enhanced by direct<strong>in</strong>g the geometry<br />

of both the dipole <strong>and</strong> dipolarophile, us<strong>in</strong>g β-cyclodextr<strong>in</strong> as an additional<br />

b<strong>in</strong>d<strong>in</strong>g cavity along with baker’s yeast (219).<br />

1.3.4.1.2. Alkadienes <strong>and</strong> -trienes 1,3-Dipolar cycloaddition of bis(styryl) sulfone<br />

(E,E)-PhCH=CHSO2CH=CHC6H4Me-4 with 4-MeOC6H4CH=NOH, <strong>in</strong><br />

the presence of chloram<strong>in</strong>e-T, gave a mixture of bis(isoxazol<strong>in</strong>yl) sulfone 49 <strong>and</strong><br />

(styrylsulfonyl)isoxazol<strong>in</strong>e 50 (220).<br />

Ar SO2<br />

N<br />

O Ph O N<br />

Tol<br />

N<br />

O Ph<br />

49<br />

Ar = 4-MeOC6H4 50<br />

Ar<br />

Ar SO2<br />

1,3-Dipolar cycloaddition of 2,6-dichlorobenzonitrile oxide to unsymmetrical<br />

1,5-hexadien-3-ol proceeds regioselectively to some extent due to the hydrogen<br />

Tol

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