Regioselective synthesis and electron impact mass spectral fragmentation differentiation of 2- or 3-isopropylthio, phenylthiosuccinic acid 4-methyl/isopropyl ethers

2‐ and 3‐Isopropylthio/phenylthiosuccinic acid 4‐methyl/isopropyl esters were synthesized regioselectively by Michael additions of isopropylthiol/thiophenol to maleic anhydride, followed by alcoholysis with methanol/isopropanol. Their mass spectrometric behavior has been studied with the aid of mass‐analyzed ion kinetic energy spectrometry and accurate mass measurements under electron impact ionization. All compounds show a tendency to eliminate an alkoxy group, an alcohol, or an alkoxycarbonyl group from the respective molecular ion. Some molecular ions also show a tendency to eliminate a molecule of water, propene or CO2. The [M ‐ MeCH = CH2]+ ions could further lose H2O to form substituted succinic anhydride ions, or lose CO2 to form 3‐substituted propionic acid ions. Both of these ions could further yield other small fragment ions by loss of CO2H, CO or other small fragments. It has been found that 2‐isopropylthio‐ and 3‐phenylthiosuccinic acid 4‐esters show more abundant [M − H2O]+ peaks than their 3‐isopropylthio and 2‐phenylthio isomers in their mass spectra. Copyright © 2000 John Wiley & Sons, Ltd.