NMR-Spectroscopic, computational and mass-spectrometric investigations on the cis/trans analogues of 2,3,4-trihydronaphthalene-1-one

Abstract—NMR-Spectroscopic, computational and mass-spectrometric studies of the cis/trans isomers of N-[8-(acetylamino)-4-(2,2- dimethyl-1,1-diphenyl-silapropoxy)-6-fluoro-5-methyl-1-one-2,3,4-trihydronaphthyl]acetamide (1a and 1b), obtained as intermediates in the synthesis of an important class of alkaloid molecules, are reported. 1H and 13C NMR analyses show an unusual axial preference of the TBDPSi– (tert-butyldiphenylsilyl) group in position 4 in both the isomers. Mass spectrometric evidence demonstrates that trans isomer has a higher affinity for ammonium ions than the cis isomer and that only the ammonium adduct [1bþNH4]þ and the protonated molecule [1bþH]þ show the fragmentation in which loss of benzene is observed. Moreover, molecular mechanics and semi-empirical calculations indicate that a group of trans conformers tend to place one of the phenyl rings of the TBDPSiO– group in a offset p-stacked geometry with the compound’s aromatic ring. The combination and the detailed analyses of these experimental and theoretical results could support the p–p interaction obtained as a conformational preference in the trans isomer.