1,2-Bismethylenecyclobutanes and Pyrano-pyrans: Dimers from Differently Substituted Donor/Acceptor Allenes Depending on the substituents donor/acceptor-substituted allenes dimerize to give two different classes of compounds: 1,1-diethoxy-3,3-bis(trifluoromethyl)allene ( 4 ) in a [2 + 2]-cyclo...