Spectral studies on 1-substituted-3-(1-oxo-3-hydroxy-2-cyclohexene-2-yl)-4-oxo-4, 5,6,7-tetrahydroindoles — an unexpected mass spectral fragmentation

Abstract

IR, UV, NMR and mass spectral data for the title compounds are discussed. The EI mass spectra of4. and the analogues5–9 and11–15 display major (M-Cn₂H₃O₂)⁺ fragment ions that correspond to the loss of a structural element not present in the parent molecules. These (M-59)⁺ ions have no equivalent in the model compound l-phenyl-2,6,6-trimethyl-4-oxo-4, 5,6,7-tetrahydroindole (27). The unusual fragmentation is thought to be initiated by α-cleavage within the alicyclic 1,3-diketone moiety (ring C) under concomitant formation of a benzylic radical site. Reclosure of this ‘open’ intermediate to a lactone-type molecular ion provides two O-atoms in the proximity required for an ejection of a CH₂COOH radical as the neutral species in question. The¹H NMR spectra of4 and its methyl ether16 reveal restricted rotation of substituents at positions 1 and 3 of the pyrrole ring, the eight methylene protons becoming fully anisochronous at 500 MHz. X-ray studies on single crystals of8 confirmed its structure.