09.02.2013 Views

Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

REACTIVITY OF NITRONATES 555<br />

O<br />

N<br />

O<br />

R<br />

O<br />

N<br />

O<br />

R<br />

CO2Me<br />

COR<br />

CO2Me<br />

167<br />

CO2Me<br />

N<br />

O<br />

H<br />

168<br />

NO 2<br />

O N<br />

O<br />

169<br />

MeO<br />

H<br />

+<br />

R<br />

O N<br />

CO2Me H OMe<br />

171 78%<br />

R–CO2Me<br />

COMe<br />

OEt<br />

CH 2Cl<br />

(92%)<br />

(57%)<br />

(64%)<br />

(93%)<br />

O N NO2<br />

O<br />

R<br />

O N<br />

NO2 COR<br />

H −NO2 −<br />

O N<br />

A B<br />

+<br />

C<br />

MeOH<br />

for R = CO2Me<br />

MeO<br />

O N<br />

170<br />

R<br />

Scheme 3.133<br />

H<br />

O<br />

MeOH<br />

R –OMe (68%)<br />

Ph (64%)<br />

O N<br />

D<br />

H<br />

COR<br />

H<br />

is absent among the reaction products as well as by the fact that isoxazoles cannot<br />

be detected among products of trapp<strong>in</strong>g of nitronate (172) by other acetylenes.<br />

All the same, it is rather surpris<strong>in</strong>g that acyl nitronate (172), which smoothly<br />

reacts with DMAD, does not form adducts with any other compound conta<strong>in</strong><strong>in</strong>g<br />

a multiple bond, <strong>in</strong>clud<strong>in</strong>g maleic anhydride.<br />

Interest<strong>in</strong>gly, the reaction of nitronate MeCBr = N(O)OSiMe2Bu t with DMAD<br />

also produces substituted dihydroisoxazole (175) <strong>in</strong> good yield rather than the<br />

correspond<strong>in</strong>g isomeric azirid<strong>in</strong>e (176) (Scheme 3.136), which was conÞrmed by<br />

IR spectroscopic data (87).<br />

Presumably, isoxazol<strong>in</strong>e (175) is thermodynamically more favorable than overcrowded<br />

azirid<strong>in</strong>e (176) due to π,π conjugation. Elim<strong>in</strong>ation of the bromide anion<br />

from <strong>in</strong>termediate (175) is also h<strong>in</strong>dered due to <strong>in</strong>stability of the carbocation that<br />

formed.<br />

3.4.3.2.2. Intermolecular [3 + 2]-Addition to other Dipolarophiles Data on this<br />

problem are scarce <strong>and</strong> concern primarily <strong>in</strong>teractions of SENAs with the C,S<br />

double bond. (383–386)<br />

Dipolarophiles (178) conta<strong>in</strong><strong>in</strong>g this fragment can easily be generated by<br />

irradiation of thio derivatives of acetophenone 177 (Scheme 3.137). (Special<br />

+<br />

O<br />

R

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!