The extent to which conformational preferences of fused heterocyclic five-membered rings change with the nature of the heteroatom (O and S) was investigated in furo- (1, 2) and thieno[2,3-b]indolines (3, 4) by the combined use of 1H NMR spectroscopy and density functional theory (DFT) calculations. In contrast to the behavior observed for pyrroloindolines, the furo- and thienoindolines exist in solution in only one conformer, with structures in the 2E-2T3 (1,2) and 2T3-E (3,4) North/West region of the pseudorotational wheel, and with pseudorotation phase angles (P) of 315.8, 311.6, 337.2 and 331.6 degrees, respectively.

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The electron impact (EI)-induced fragmentations of 18 indolenines were studied using both double-focusing and ion trap mass spectrometers. The compounds used in this study were synthesized to provide correlations of characteristic fragment ions with specific structural differences. In 2-hydroxyindolenines the hydroxy group was involved in a major fragmentation process by interacting with the ester side chain to generate an alpha,beta-unsaturated gamma-lactone structure, with concomitant loss of the corresponding alcohol. In contrast, loss of an alkyl radical, derived solely from the 2-alkoxy group, is a major primary decomposition process for 2-alkoxyindolenines. EI-MS analyses using sector and ion trap spectrometers resulted in similar fragmentation patterns.