Reaction of dichlorocarbene with diazines in the presence of dimethyl maleate and dimethyl acetylenedicaroxylate involves intermediate formation of ylides, namely cycloimmoniodichloromethanides, followed by 1,3-dipolar cycloaddition of the latter to dipolarophiles. This carbene-ylide sequence was used to synthesize chloro-substituted pyrrolo[1,2-6]pyridazines, pyrrolo[1,2-c]- and pyrrolo[1,2-a]pyrimidines, pyrrolo[1,2-a]phthalazine, pyrrolo[1,2-a]-, pyrrolo[1,2-c]-, and pyrrolo[2,1-b]quinazolines, pyrrolo[1,2-a]-quinoxalines, and pyrrolo[1,2-a]naphthiridines. The cycloaddition is stereo- and regioselective, which suggests a concerted reaction mechanism. The regioselectivity of the cycloaddition is correctly predicted in terms of the PMO theory on the basis of MNDO electronic parameters of halogenated cycloimmonium ylides.