1,3-DIPOLAR CYCLOADDITION REACTIONS OF MONOCYCLIC AZIRIDINES WITH DIMETHYL 1-CYCLOBUTENE-1,2-DICARBOXYLATE

The thermally induced [2+3] cycloadditions of certain monocyclic aziri dines to dimethyl 1-cyclobutene-1,2-dicarboxylate are described. The s tructures of the resulting bicyclic adducts were established unambiguo usly by X-ray analyses. The stereochemistries of the [2+3] adducts wer e interpreted in terms of dipole-dipole interactions and secondary orb ital interactions. In certain cases an initial thermal conrotatory rin g opening of the cyclobutene to 2,3-di(methoxycarbonyl)-1,3-butadiene precedes the [2+3] cycloaddition, affording the unexpected pyrrolidine adducts 12a and 12b. The structures and stereochemistries of the latt er established by X-ray analysis strongly suggest that secondary orbit al interactions between the phenyl and eater groups control and stereo chemistry of the reactions.