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

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716 NITRONATES<br />

However, if AN (562) conta<strong>in</strong> electron-withdraw<strong>in</strong>g groups at the β-C atom,<br />

SENA (563) generated after the Þrst silylation can elim<strong>in</strong>ate trimethylsilanol<br />

to give unstable α-nitrosoalkenes (565) rather than the correspond<strong>in</strong>g BENA<br />

(pathway 2). It is known (503) that conjugated nitrosoalkenes (565) can undergo<br />

rearrangements <strong>in</strong>to the correspond<strong>in</strong>g enoximes (567). The presence of an EWG<br />

group <strong>in</strong> the γ-position of nitrosoalkene, as well as the presence of bases <strong>in</strong> the<br />

reaction mixture, accelerate elim<strong>in</strong>ation (496, 545). In the presence of silylat<strong>in</strong>g<br />

agents, free enoximes (567) are transformed <strong>in</strong>to silyl derivatives (566). (Analogous<br />

isomerization reactions are suggested for the <strong>in</strong>termediate D <strong>in</strong> the synthesis<br />

of dioxime (534) (497); see Scheme 3.271).<br />

Unfortunately, the stereoselectivity of the generation of enoximes (567) toward<br />

the newly formed C,C bond is often low, <strong>and</strong>, <strong>in</strong> some cases, this reaction is complicated<br />

by the formation of by-products due the to reactivity of bis-oxyim<strong>in</strong>ium<br />

cationic <strong>in</strong>termediates (for more details, see Section 3.5.2).<br />

3.5.6.2. Method for the <strong>Synthesis</strong> of Conjugated Enoximes with a Remote<br />

Functional Group In Section 3.5.2.5, where the r<strong>in</strong>g-cha<strong>in</strong> tautomerism of<br />

bicyclic nitronates was considered (Scheme 3.218), the transformation of these<br />

nitronates <strong>in</strong>to conjugated enoximes, conta<strong>in</strong><strong>in</strong>g a remote carbonyl-conta<strong>in</strong><strong>in</strong>g<br />

group, (153, 293, 547) was mentioned. This transformation is based on the above<br />

considered [4 + 2]-cycloreversion of 5,6-dihydro-2H -N-siloxyoxaz<strong>in</strong>es. This reaction<br />

is presented <strong>in</strong> detail <strong>in</strong> Scheme 3.277.<br />

At Þrst glance, this transformation is very attractive because it enables the<br />

synthesis of conjugated enoximes (571) from very simple precursors <strong>in</strong> several<br />

R'<br />

O<br />

R 1<br />

R<br />

NO2<br />

SiBr<br />

X<br />

( )n<br />

R 1<br />

Base<br />

R1<br />

( )n<br />

X O<br />

O N<br />

X<br />

O<br />

O N<br />

X<br />

OSi<br />

O N<br />

( )n<br />

step 1<br />

( )n<br />

step 2<br />

( )n<br />

X<br />

A<br />

OSi<br />

step 3<br />

X<br />

( )n<br />

O<br />

N<br />

OSi<br />

568 569<br />

570<br />

R1 +<br />

_<br />

+<br />

=C6H5; X = OSiMe3, OMe; n = 1,2; base – Et3N; Si – SiMe3; Yield of (571) 15%–95%<br />

Scheme 3.277<br />

R'<br />

N<br />

571<br />

R<br />

OH<br />

−<br />

step 4 H2O, F<br />

R 1

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