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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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NH 3<br />

N<br />

H<br />

*<br />

+<br />

H 2C<br />

CO 2Et<br />

R 1 = H or CO 2Me<br />

Si – SiMe3<br />

MeO2C<br />

C6H5<br />

*<br />

NH2<br />

SILYLATION OF NITRO COMPOUNDS AS A PROCESS 683<br />

N(OSiMe 3) 2<br />

R<br />

R 1<br />

N(OSi)2<br />

N<br />

CH 2Cl 2, 20°C<br />

R<br />

Scheme 3.246<br />

*<br />

N<br />

*<br />

CO2Et<br />

R 1<br />

OH<br />

dr ~ 1:1 464a<br />

42%<br />

CO2Me<br />

N(OSi)2<br />

CO 2Et<br />

N<br />

MeO 2C<br />

OSi<br />

N<br />

dr ~ 1:1<br />

E/Z ~ 5:1<br />

NaBH 4<br />

*<br />

N<br />

H<br />

Scheme 3.247<br />

C 6H 5<br />

OH<br />

N<br />

R<br />

R<br />

461<br />

N<br />

N<br />

*<br />

N<br />

*<br />

dr ~ 1:1<br />

E/Z ~ 5:1<br />

OH<br />

OH<br />

R 1<br />

R = Me: 85%<br />

Ph : 79%<br />

Bn : 72%<br />

CO 2Et<br />

N OH<br />

462a,b<br />

a: R 1 = H (65%)<br />

b: R 1 = CO2Me (75%)<br />

for R 1 =H HCl/CH2O<br />

HCl N<br />

N<br />

CO 2Me<br />

OH<br />

CO2Et<br />

OH<br />

463a<br />

OH<br />

47%<br />

dr ~ 1:1<br />

<strong>in</strong>ternal BENAs MeCH=CR’N(OSiMe3)2 (R ′ =H, CO2Me) (521, 522) (Scheme<br />

3.247). However, not only the low diastereoselectivity of N,C -coupl<strong>in</strong>g reactions<br />

but also low reaction rates of less nucleophilic α-am<strong>in</strong>o acid derivatives,<br />

compared to am<strong>in</strong>es, decrease the successful use of this approach.<br />

This reaction was applied to L-prol<strong>in</strong>e ethyl ester (R ′ = H), <strong>and</strong> two diastereomers<br />

were isolated, each diastereomer be<strong>in</strong>g a mixture of stereoisomers (E/Z )<br />

related to the oxim<strong>in</strong>o group. Attempts to completely separate these isomers<br />

failed. The oxim<strong>in</strong>o group of the product (462a) was removed by a CH2O/HCl<br />

mixture. As a result, hydrated aldehyde hydrochloride (463a) was obta<strong>in</strong>ed. It

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