Use of highly stereospecific 1,3-dipolar cycloadditions of cyclic nitrones with acetylenes in the preparation of novel heterocyclic ring systems
1997
Cyclic nitrones with substituents α to the
nitrogen atom undergo 1,3-dipolar cycloadditions in a stereo- and
regio-specific manner with acetylenes. The nitrone 3 reacts with methyl
propiolate, methyl phenylpropiolate and methyl acetylpropiolate to give
the corresponding pyrrolo[1,2-b]isoxazoles 5, 8 and 13 respectively.
Compound 13 reacts with 4-chlorophenylhydrazine to afford the
pyrazolo[4,3-a]pyrrolizine 15 by a novel rearrangement. The
nitrone 18 undergoes a 1,3-dipolar cycloaddition with methyl propiolate
in a stereo- and regio-specific manner to give the
isoxazolo[2,3-a]pyridine 19. The nitrone 18, when reacted with
methyl acetylpropiolate, gives a mixture of regioisomers in a ratio of
3∶2. The major regioisomer 21 reacts with 4-chlorophenylhydrazine
to afford the novel pyrazolo[4,3-a]indolizine ring system 23.
The bicyclic nitrone 26 reacts with methyl propiolate and methyl
acetylpropiolate to give the cycloadducts 27 and 28 respectively; both
these compounds contain the novel isoxazolo[3,2-i]indole ring
system.
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