SYNTHESIS AND 1,3-DIPOLAR CYCLOADDITION REACTIONS OF NOVEL HETEROPENTALENE MESOMERIC BETAINES, PYRROLO[1,2-C]IMIDAZOLE MESOMERIC BETAINES

A series of novel heteropentalene mesomeric betaines, pyrrolo[l,2-c]imidazole mesomeric betaines (10a~i), were prepared by condensation of 2-formylpyrroles with aromatic imines. The mesomeric structures 10a~i are proposed on the basis of spectral and microanalytical data and the results of their participation in 1,3-dipolar cycloaddition reactions. Peri-, regio-, and stereoselectivity of cycloadditions of mesomeric betaines l0a-i with acetylenic (DMAD, ethyl propiolate, ethyl phenylpropiolate, benzyl phenylpropiolate, and phenylacetylene) and olefinic (dimethyl fumarate and dimethyl maleate) dipolarophiles have been studied. High periselectivity was observed in cycloadditions with both series of dipolarophiles, with the dipolarophile adding exclusively across the 1,3-azomethine ylide dipole (10A). The respective formation of 2,2'-bipyrroles and 2’,3'-dihydro-2,2'-bipyrroles in the cycloaddition of acetylenic and olefinic dipolarophiles could be rationalized by considering rearrangements of the expected bicyclic cycloadducts 16 and 19. © 1990, American Chemical Society. All rights reserved.