Mass spectrometry of trialkylsilyl and acyl derivatives of 2′‐deoxynucleosides

Abstract

AbstractThe electron impact mass spectra of monosilyl and mixed acyl‐silyl derivatives of 2′‐deoxynucleosides are described in detail. (Silyl = tert‐butyldimethylsilyl, cyclo‐tetramethylene‐isopropylsilyl, or cyclo‐tetramethylene‐tert‐butylsilyl; acyl = acetyl or trifluoroacetyl.) The interpretation of the fragmentation pathways was aided by metastable ion decomposition studies, precise mass and deuterium labelling measurements. Mass spectrally, the acyl substituents are mostly ‘passive’ and have (with possibly one exception) little fragmentation directing capability. In contrast, the silyl groups have powerful fragmentation directing properties. Elimination of the bulky alkyl radical R˙ (tert‐butyl or isopropyl) from the molecular ion produces the siliconium ion, [M–R]+, which is the precursor for most of the other prominent ions in the spectra. These arise from ‘siliconium ion rearrangements’ resulting from the interaction of the positively charged siliconium ion centre with the electron dense regions (i.e. oxygens) in the molecule, to form cyclic silyloxonium ions which subsequently decompose. Since the interacting oxygen and silicon must be sterically accessible, the fragment ion types and their abundances are very dependent upon structure. Consequently, [M–R]+ ions formed from 3′‐ or 5′‐O‐silyl groups give rise to different sets of daughter ions which, for the most part, are not found, or have very low abundances, in the mass spectra of underivatized or trimethylsilylated nucleosides. Detailed information on sugar and base moieties and isomeric substitution is readily obtained.