Reactions of a sterically hindered tetrasubstituted thiocarbonyl ylide with acceptor-substituted ethylenes; regioselectivity and stereochemistry

The 1,3-cycloadditions of the tetra-substituted thiocarbonyl ylide , set free by N2 extrusion from thiadiazoline , with methyl acrylate and acrylonitrile furnish 3′-and 4′-substituted thiolanes, probably by a concerted pathway. In the reactions of with dimethyl 2,3-dicyanofumarate () and dimethyl 2,3-dicyanomaleate (), zwitterionic intermediates, which are capable of conformational rotation, sit at the branching point of two irreversible reactions: cyclization to thiolanes / and fragmentation to cyclopropanes / plus thione . Both reactions are accompanied by some loss of stereochemical purity. Two mechanisms for the cyclopropane formation are discussed: intramolecular nucleophilic substitution in anti-zwitterions / or unassisted heterolysis leading to a tert-carbenium zwitterion as further intermediate.