Thermal [2þ 3]-Cycloadditions of trans-1-Methyl-2,3-diphenylaziridine with C1⁄4S and C1⁄4C Dipolarophiles: An Unexpected Course with Dimethyl Dicyanofumarate

The thermal reaction of trans-1-methyl-2,3-diphenylaziridine (trans-1a) with aromatic and cycloaliphatic thioketones 2 in boiling toluene yielded the corresponding cis-2,4-diphenyl-1,3-thiazolidines cis4 via conrotatory ring opening of trans-1a and a concerted [2þ 3]-cycloaddition of the intermediate (E,E)-configured azomethine ylide 3a (Scheme 1). The analogous reaction of cis-1a with dimethyl acetylenedicarboxylate (5) gave dimethyl trans-2,5-dihydro-1-methyl-2,5-diphenylpyrrole-3,4-dicarboxylate (trans-6) in accord with orbital-symmetry-controlled reactions (Scheme 2). On the other hand, the reactions of cis-1a and trans-1awith dimethyl dicyanofumarate (7a), as well as that of cis-1a and dimethyl dicyanomaleate (7b), led to mixtures of the same two stereoisomeric dimethyl 3,4-dicyano-1-methyl-2,5diphenylpyrrolidine-3,4-dicarboxylates 8a and 8b (Scheme 3). This result has to be explained via a stepwise reaction mechanism, in which the intermediate zwitterions 11a and 11b equilibrate (Scheme 6). In contrast, cis-1,2,3-triphenylaziridine (cis-1b) and 7a gave only one stereoisomeric pyrrolidine-3,4dicarboxylate 10, with the configuration expected on the basis of orbital-symmetry control, i.e., via concerted reaction steps (Scheme 10). The configuration of 8a and 10, as well as that of a derivative of 8b, were established by X-ray crystallography.