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

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REACTIONS OF DIPOLAR 1,3-CYCLOADDITION ([3 + 2] CYCLOADDITION) 371<br />

H2C<br />

C<br />

MeO 2C<br />

Me<br />

OHC<br />

MeO2C<br />

SO2Ph<br />

CH2<br />

715<br />

SO 2Ph<br />

718 (63%)<br />

CH2<br />

CO2Me<br />

CO2Me<br />

CH<br />

RNHOH<br />

R<br />

Scheme 2.298<br />

H<br />

O −<br />

N<br />

+<br />

Me<br />

H2C<br />

R Me<br />

N<br />

O<br />

SO2Ph<br />

CO2Me<br />

CO2Me<br />

716 a: R = Me (85%)<br />

b: R = PhCH 2 (69%)<br />

SO 2Ph<br />

CO 2Me<br />

CO 2Me<br />

717 a: R = Me (89%)<br />

b: R = PhCH2 (85%)<br />

nitrones (716a,b) were isolated. Catalytic hydrogenation of 4-isoxazol<strong>in</strong>e (717b)<br />

resulted <strong>in</strong> N–O bond cleavage. Then, spontaneous benzylam<strong>in</strong>e elim<strong>in</strong>ation gave<br />

ultimately the cyclohexenyl substituted aldehyde (718) <strong>in</strong> 63% yield (Scheme<br />

2.298).<br />

2.8.2.2. Intermolecular Reactions Dimethyl acetylenedicarboxylate (DMAD)<br />

is frequently used as an alkyne dipolarophile (23, 24, 126b, 152, 241, 333).<br />

Reactions of α-methoxynitrones (203) <strong>and</strong> (223 g, R= OCH3) with DMAD<br />

give imidazoisoxazoles (719) <strong>and</strong> (720) (Scheme 2.299).<br />

Unlike α-methoxynitrones (203) <strong>and</strong> (223 g), the reaction of the correspond<strong>in</strong>g<br />

aldonitrones (223a) (Scheme 2.300) <strong>and</strong> (167) (Scheme 2.301) proceeds beyond<br />

the cycloaddition step. It undergoes r<strong>in</strong>g open<strong>in</strong>g, afford<strong>in</strong>g enam<strong>in</strong>e ester (721).<br />

Compound (721) adds reversibly a molecule of H2O to give the hydrate (721a).<br />

In the reaction of compound (167) with DMAD, apart from enam<strong>in</strong>o ester (722),<br />

trimethyl 3,3-dimethyl-1-phenyl-3H-pyrrolo[1,2-c]imidazole-5,6,7-tricarboxylate<br />

(723) was isolated (816).<br />

Enam<strong>in</strong>o ester (722) does not react with DMAD under these reaction conditions,<br />

while the reaction of nitrone (167) with DMAD gives both products (722)<br />

<strong>and</strong> (723) <strong>in</strong> approximately equal amounts, irrespective of the amount of DMAD<br />

used. This implies that the two reaction pathways are <strong>in</strong>dependent of each other.<br />

The cycloadduct produced <strong>in</strong>itially undergoes either r<strong>in</strong>g open<strong>in</strong>g to give enam<strong>in</strong>o<br />

ester (722) or isomerizes to yield 1,3-diazabicyclo[3.1.0]hex-3-ene (724).

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