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

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712 NITRONATES<br />

(539). However, α-methoxycyclopentene reacts with nitrosoalkene (530a) togive<br />

primarily the product of the ene reaction, viz., silylated v<strong>in</strong>ylhydroxylam<strong>in</strong>e (541),<br />

whereas the normal adduct of the diene synthesis (540) is generated only as an<br />

impurity. The reaction of (530a) with EtOCH=CHMe produces only ene adduct<br />

(543), which undergoes complete res<strong>in</strong>iÞcation <strong>in</strong> attempt<strong>in</strong>g to perform desilylation.<br />

Therefore, the behavior of nitrosoalkene (530a) <strong>in</strong> reactions with oleÞns<br />

is similar to that of nitroso derivatives > C(EWG)N=O, which are generally<br />

<strong>in</strong>volved <strong>in</strong> ene reactions with oleÞns (543). (The mechanistic <strong>in</strong>terpretations of<br />

ene reactions of nitrosoalkene (530a) are presented <strong>in</strong> the lower part of Scheme<br />

3.272.)<br />

At the same time, nitrosoalkene (530a) reacts with conjugated cyclic dienes<br />

exclusively as a heterodienophile (542) (see products (544–546) <strong>in</strong> Scheme<br />

3.272). These reactions are chemo- <strong>and</strong> stereoselective, <strong>and</strong> products (544) <strong>and</strong><br />

(545) were isolated as endo-(E) isomers. Prolonged storage of monoadduct (544)<br />

with an excess of cyclopentadiene leads to the addition of a second cyclopentadiene<br />

molecule to give bis -adduct (546) <strong>in</strong> low yields. Adduct (544) isof<br />

particular <strong>in</strong>terest because the preparation of this compound is reversible <strong>and</strong>,<br />

therefore, dihydrooxaz<strong>in</strong>e (544) can be considered as a “reservoir” for storage of<br />

unstable nitrosoalkene (530a). In any case, the reaction of (544) with an excess<br />

of EtOCH=CH2 affords oxaz<strong>in</strong>e (539) <strong>in</strong> good yield.<br />

Therefore, the ability of (530a) to be <strong>in</strong>volved <strong>in</strong> [4 + 2]-cycloaddition reactions<br />

with some oleÞns <strong>and</strong> dienes is very important <strong>in</strong> dihydrooxaz<strong>in</strong>es chemistry,<br />

although the ene reactions limit the use of this nitrosoalkene <strong>in</strong> organic<br />

synthesis.<br />

3.5.5.3. Development of the Chemistry of 5,6-Dihydro-4H-Oxaz<strong>in</strong>es Ma<strong>in</strong><br />

approaches to the development of the chemistry of 5,6-dihydro-4H -oxaz<strong>in</strong>es with<br />

the use of product (539) are presented <strong>in</strong> Scheme 3.273 (541).<br />

EtO<br />

E<br />

CO 2Me<br />

O N<br />

547<br />

CO 2Me<br />

1. Base<br />

2. E X<br />

60% – 90 %<br />

Δ<br />

Retro-[4+2]<br />

EtO<br />

CO2Me<br />

O N<br />

539<br />

CO 2Me<br />

[4+2]<br />

G<br />

O N<br />

EtO E<br />

O N<br />

548<br />

EtO<br />

+<br />

E<br />

50% – 89 %<br />

G<br />

E X = R-Hal, RCOCl;G = OR, Alk; Base = LiHMDS, Et3N or Na2CO3 Scheme 3.273<br />

1. Base<br />

2. E X<br />

70% – 90 %<br />

CO 2Me<br />

O N<br />

EtO<br />

548<br />

E<br />

N<br />

E<br />

549<br />

CO2Me

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