Synthesis, stereochemistry, and rearrangement of thioxanthen-10-io-(bismethoxycarbonyl)methanides and their 9-alkyl derivatives

Abstract

Thioxanthen-10-io- and 2,4-dimethylthioxanthen-10-io-(bismethoxycarbonyl)methanides and their 9-alkyl derivatives were synthesised by the reaction of the corresponding thioxanthens with bismethoxycarbonylcarbene generated thermally from dimethyl diazomalonate in the presence of copper(II) sulphate. The stereochemistry of the ylides was determined by examination of the n.m.r. spectra and by X-ray analysis of trans-9-ethyl-2,4-dimethylthioxanthen-10-io(bismethoxycarbonyl)methanide. Refluxing the sulphonium ylides in toluene in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) caused rearrangement to the corresponding 9-(bismethoxycarbonylmethyl)thioxanthens with the exception of the 9-isopropyl derivative, which was stable even in refluxing xylene. The mechanism of the rearrangement is discussed.