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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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396 NITRONES: NOVEL STRATEGIES IN SYNTHESIS<br />

I<br />

NHOH<br />

O<br />

N<br />

O<br />

(53% overall)<br />

R<br />

I<br />

N O<br />

a b<br />

CO2Me<br />

O<br />

CO 2Me<br />

O<br />

N<br />

CO2Me CO2Me O<br />

N<br />

(53% overall)<br />

O<br />

(71% overall)<br />

CO2Me<br />

CO2Me<br />

CO 2Me<br />

CO 2Me<br />

O<br />

N<br />

O<br />

N<br />

R<br />

S<br />

(42% overall)<br />

O<br />

N<br />

OBn<br />

(48% overall)<br />

CO2Me<br />

CO2Me<br />

CO2M e<br />

CO2Me CO 2Me<br />

CO 2Me<br />

a: 794 (a-c, g or h), 10 mol% Yb(OTf) 3, 4Å MS, toluene, 25°C, 30 m<strong>in</strong>, then 791 (b) 18 h<br />

b: 20 mol% Pd(PPh 3)4, Et3N, CH3CN,80°C, 18 h<br />

Scheme 2.330<br />

This coupl<strong>in</strong>g method is useful for the preparation of <strong>in</strong>terest<strong>in</strong>g new molecular<br />

<strong>in</strong>termediates as well as for the synthesis of “FR900482” skeletal congeners<br />

(Scheme 2.330)<br />

Anhydrous magnesium iodide (MgI2) has been shown to be an effective promoter<br />

of homo 3 + 2 dipolar cycloadditions of nitrones with 1,1-cyclopropane<br />

diesters. In almost all cases the products, tetrahydro-1,2-oxaz<strong>in</strong>es, are formed <strong>in</strong><br />

excellent yields (854). Lewis acids assist r<strong>in</strong>g open<strong>in</strong>g by stabiliz<strong>in</strong>g the malonate<br />

anion. The low cis/trans diastereoselectivity (but high enantioselectivity)<br />

is <strong>in</strong> contrast to the cis-selectivity on us<strong>in</strong>g achiral Yb(OTf)3. (852), It suggests

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