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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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466 NITRONATES<br />

R 1<br />

Ph<br />

NO2<br />

R 2<br />

H<br />

+ Me<br />

N<br />

Ph<br />

42 43a<br />

NO2<br />

+<br />

R OMe<br />

R′<br />

43b<br />

OMe<br />

LA<br />

ZnCl2<br />

R 2<br />

R 1<br />

R 1<br />

O<br />

Me<br />

N O<br />

N +<br />

Ph<br />

_<br />

N O<br />

O<br />

B<br />

R 2<br />

35e<br />

Scheme 3.43<br />

Scheme 3.44<br />

LA<br />

60˚C<br />

Ph<br />

Me<br />

N<br />

R = Me; R′ = Η<br />

80˚C<br />

R = R′ = OMe<br />

O<br />

Ph<br />

N O<br />

R 2<br />

R 1<br />

Me<br />

−<br />

N +<br />

Ph<br />

N O<br />

O<br />

LA<br />

R 2 Ph<br />

R 1<br />

NO 2<br />

B'<br />

44a<br />

Me<br />

N<br />

(R 1 = H; R 2 = Ph)<br />

35f<br />

Me<br />

OMe<br />

OMe<br />

MeO OMe<br />

MeO<br />

OMe<br />

Ph NO2 44b<br />

(Scheme 3.45, Eq. 2) through the most stabilized zwitterion to give only one<br />

stereoisomer of target 5-methylene-substituted nitronate (35 h).<br />

In the synthesis of six-membered cyclic nitronates (35) bythe(42 + 43) cycloaddition,<br />

facial discrim<strong>in</strong>ation can be achieved by <strong>in</strong>troduc<strong>in</strong>g enantiomerically<br />

pure chiral fragments <strong>in</strong>to nitro oleÞn (42) (147, 157) enam<strong>in</strong>e (117), or enol<br />

(134). In addition, Prof. Seebach (96) <strong>and</strong> postgraduate students supervised by<br />

Prof. Denmark (158) successfully used chiral LA for facial discrim<strong>in</strong>ation.<br />

80%<br />

60%

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