Adventures with heterocycles

Elucidation of mechanisms and discovery of novel reactions are interconnect ed, as four research chapters from the Munich laboratory demonstrate. 1) In a new opening of the furan ring, 2-methoxyfuran and tetra-acceptor-substit uted ethylenes furnish methyl acrylates, which are cis-3-substituted by a c yclopropyl ring that bears the four acceptor groups. Experiments with trans - and cis-1,2-bis(trifluoromethyl)-1,2-dicyanoethylenes (BTE) clarify the r ole of zwitterionic intermediates. 2) The concerted nature of 1,3-dipolar c ycloaddition is well established. On reacting thiocarbonyl S-ylides as elec tron-rich 1,3-dipoles with tetra-acceptor-substituted ethylenes, the switch ing to a two-step pathway via zwitterion was diagnosed from a loss of stere ospecificity and the formation of strained 7-membered cyclic ketene imines. 3) Stable as crystals, these ketene imines rearrange to 5-membered thiolan es in solution. In contrast to open-chain ketene imines, the cyclic represe ntatives add vinyl ether at the C=N bond and show a novel pathway of dimeri zation. 4) Cycloadducts of isoquinolinium N-phenylimide with ethylenic dipo larophiles undergo an acid-catalyzed [3.3] sigmatropic hydrate rearrangemen t. An exception is the conversion of the dimethyl maleate adduct (C21H20N2O 4) by acid into a yellow compound C24H22N2O6; the structure and the astound ing mechanism were clarified.