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

• Published in: Tetrahedron Vol. 58; no. 21; pp. 4185 - 4193
• Main Authors: Mloston, Grzegorz, Huisgen, Rolf, Giera, Henry
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 20.05.2002
• Subjects: 1,3-dipolar cycloadditions; cyclopropanes; steric and strain effects; thiocarbonyl ylides; zwitterionic intermediates; zwitterionic intermediates; cyclopropanes; steric and strain effects; 1,3-dipolar cycloadditions; thiocarbonyl ylides
• ISSN: 0040-4020; 1464-5416
• DOI: 10.1016/S0040-4020(02)00384-8

The 1,3-cycloadditions of the tetra-substituted thiocarbonyl ylide 8 , set free by N 2 extrusion from thiadiazoline 7 , with methyl acrylate and acrylonitrile furnish 3′-and 4′-substituted thiolanes, probably by a concerted pathway. In the reactions of 8 with dimethyl 2,3-dicyanofumarate ( 27 ) and dimethyl 2,3-dicyanomaleate ( 28 ), zwitterionic intermediates, which are capable of conformational rotation, sit at the branching point of two irreversible reactions: cyclization to thiolanes 23 / 24 and fragmentation to cyclopropanes 31 / 32 plus thione 12 . Both reactions are accompanied by some loss of stereochemical purity. Two mechanisms for the cyclopropane formation are discussed: intramolecular nucleophilic substitution in anti-zwitterions 29 / 30 or unassisted heterolysis leading to a tert-carbenium zwitterion as further intermediate. Graphic