USE OF HIGHLY STEREOSPECIFIC 1,3-DIPOLAR CYCLOADDITIONS OF CYCLIC NITRONES WITH ACETYLENES IN THE PREPARATION OF NOVEL HETEROCYCLIC RING-SYSTEMS

Cyclic nitrones with substituents a to the nitrogen atom undergo 1,3-d ipolar cycloadditions in a stereo- and regio-specific manner with acet ylenes. The nitrone 3 reacts with methyl propiolate, methyl phenylprop iolate and methyl acetylpropiolate to give the corresponding pyrrolo[1 ,2-b]isoxazoles 5, 8 and 13 respectively. Compound 13 reacts with 4-ch lorophenylhydrazine to afford the pyrazolo[4,3-a]-pyrrolizine 15 by a novel rearrangement. The nitrone 18 undergoes a 1,3-dipolar cycloaddit ion with methyl propiolate in a stereo- and regio-specific manner to g ive the isoxazolo[2,3-a]pyridine 19. The nitrone 18, when reacted with methyl acetylpropiolate, gives a mixture of regioisomers ina 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 bicycli c 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.