Engaging the pyridine-DMAD zwitterion in a novel strategy for the selective synthesis of highly substituted benzene and cyclopentenedione derivatives.

[reaction: see text] The pyridine-mediated reaction of dimethyl acetylenedicarboxylate and cyclobutene-1,2-diones affords selective access to either hexasubstituted benzene derivatives or cyclopentenedione derivatives depending on the concentration of pyridine.