Elucidation of mechanisms and discovery of novel reactions are interconnected, as four research chapters from the Munich laboratory demonstrate. 1) In a new opening of the furan ring, 2-methoxyfuran and tetra-acceptor-substituted ethylenes furnish methyl acrylates, which are cis-3-substituted by a cyclopropyl ring that bears the four acceptor groups. Experiments with trans- and cis-1,2-bis(trifluoromethyl)-1,2-dicyanoethylenes (BTE) clarify the role of zwitterionic intermediates. 2) The concerted nature of 1,3-dipolar cycloaddition is well established. On reacting thiocarbonyl S-ylides as electron-rich 1,3-dipoles with tetra-acceptor-substituted ethylenes, the switching to a two-step pathway via zwitterion was diagnosed from a loss of stereospecificity and the formation of strained 7-membered cyclic ketene imines. 3) Stable as crystals, these ketene imines rearrange to 5-membered thiolanes in solution. In contrast to open-chain ketene imines, the cyclic representatives add vinyl ether at the C=N bond and show a novel pathway of dimerization. 4) Cycloadducts of isoquinolinium N-phenylimide with ethylenic dipolarophiles undergo an acid-catalyzed [3.3] sigmatropic hydrazo rearrangement. An exception is the conversion of the dimethyl maleate adduct (C21H20N2O4) by acid into a yellow compound C24H22N2O6; the structure and the astounding mechanism were clarified.