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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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H2NC<br />

NH<br />

OH<br />

N O<br />

432<br />

APPLICATIONS OF NITRILE OXIDES 95<br />

NHCO2Bu<br />

CH2CONHCH2 CH CO2H<br />

Diastereoselective <strong>in</strong>termolecular nitrile oxide—oleÞn cycloaddition has been<br />

used <strong>in</strong> an enantioselective synthesis of the C(7)-C(24) segment 433 of the 24membered<br />

natural lactone, macrolact<strong>in</strong> A 434 (471, 472). Two (carbonyl)iron<br />

moieties are <strong>in</strong>strumental for the stereoselective preparation of the C(8)-C(11)<br />

E,Z-diene <strong>and</strong> the C(15) <strong>and</strong> C(24) sp 3 stereocenters. Also it is important to<br />

note that the (carbonyl)iron complexation serves to protect the C(8)-C(11) <strong>and</strong><br />

C(16)-C(19) diene groups dur<strong>in</strong>g the reductive hydrolysis of an isoxazol<strong>in</strong>e r<strong>in</strong>g.<br />

HO<br />

HO<br />

O<br />

OMe<br />

OSiMe2Bu-t<br />

Me<br />

HO<br />

HO<br />

OH<br />

433 434<br />

An expedient <strong>and</strong> fully stereocontrolled synthesis of epothilones A (435, R=<br />

H)<strong>and</strong>B(435, R= Me) has been realized (473, 474). The routes described,<br />

<strong>in</strong>volve an extensive study of nitrile oxide cycloadditions, as substitutes for aldol<br />

addition reactions, lead<strong>in</strong>g to the realization of a highly convergent synthesis,<br />

based on the Kanemasa hydroxyl-directed nitrile oxide cycloaddition.<br />

Me<br />

S<br />

N<br />

Me<br />

O<br />

O<br />

R<br />

Me<br />

Me Me<br />

O<br />

435<br />

OH O<br />

Two stereoselective aldol reactions, followed by a nitrile oxide cycloaddition<br />

<strong>and</strong> a stereoselective late-stage epoxidation are the key steps <strong>in</strong> the total synthesis<br />

of myriaporones 1, 3, <strong>and</strong> 4 (436, 437, <strong>and</strong>438). The synthesis allows<br />

OH<br />

Me<br />

O<br />

O<br />

Me

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