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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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274 NITRONES: NOVEL STRATEGIES IN SYNTHESIS<br />

O<br />

O<br />

O<br />

O<br />

RO N<br />

R = TMS<br />

R = H<br />

O<br />

RO N<br />

R = TMS<br />

R = H<br />

S<br />

S R<br />

+<br />

R<br />

S S<br />

O<br />

−<br />

O N +<br />

OTBS<br />

Bn<br />

OTBS<br />

Bn<br />

292<br />

CO 2Me<br />

Citric acid<br />

MeOH<br />

CO2Me<br />

Citric acid<br />

MeOH<br />

Ph<br />

+<br />

TMSO<br />

Lewis acid<br />

O<br />

O<br />

Scheme 2.175<br />

O<br />

RO N<br />

O<br />

OTBS<br />

OMe<br />

S S S<br />

+<br />

RO N<br />

OTBS<br />

Bn<br />

S R R<br />

OTBS<br />

Bn<br />

CO2Me<br />

CO 2Me<br />

2-[(N-benzyl-N -hydroxylam<strong>in</strong>e)phenylmethyl]-3-hydroxybutanoate (398) (637).<br />

The absolute conÞguration (398) was determ<strong>in</strong>ed as (αR,βS ,γR); thus, diastereoselective<br />

addition of ketene silyl acetals (397) to nitrone proceeds as anti-α,<br />

β-anti-β, γ (Scheme 2.176).<br />

The reaction of O-methyl-O-tert-butyldimethylsilyl ketene acetal with N -<br />

benzyl- <strong>and</strong> N -methyl-2,3-O-isopropylidene D-glyceraldehyde nitrones (292), <strong>in</strong><br />

the presence of boron trißuoride etherate, affords the correspond<strong>in</strong>g isoxazolid<strong>in</strong>e-<br />

5-ones <strong>in</strong> high yields. These compounds were successfully applied as key <strong>in</strong>termediates<br />

<strong>in</strong> the synthesis of isoxazolid<strong>in</strong>yl nucleosides of the L-series (Scheme<br />

2.177) (638).<br />

Addition of ketene silyl acetals to α,N-diarylnitrones, catalyzed by lanthanum<br />

trißoromethanesulfonate [lanthanide trißate, La(OTf)3], affords addition

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