Mechanistic study of reversible solid-state melt isomerization of 2-oxindoles to 2-quinolinones and its occurrence in a mass spectrometer

An eco-friendly equilibrated rearrangement of a series of 2-oxo-3-indolyl acetic acids (1) with 2-oxo-1,2,3,4-tetrahydroquinoline-4-carboxylic acid derivatives (2) was investigated through a solid state melt reaction (SSMR). Mechanistic insight into the thermal rearrangement is provided by 13C-isotopic labeling. The standard mass spectra of 1 and 2 were virtually identical preventing their reliable identification. Reversible interconversion of 1 and 2 was evidenced to occur in the inlet system of a mass spectrometer under electron impact conditions. Relative abundances of fragment ions were found to be a function of temperature.