Mass spectra of sterically crowded trialkylsilyl derivatives of nucleosides

The electron impact mass spectra of tert-butyldimethylsilyl-, cyclo-tetramethylene-tert-butylsilyl and cyclo-tetramethylene-isopropylsilyl- ether derivatives of ribo- and 2′-deoxyribonucleosides are described in detail. The interpretation of fragmentation pathways of full and mixed derivatives was aided by metastable ion decomposition studies, precise mass and deuterium labelling measurements, and spectra of mixed derivatives containing the ‘passive’ (in these spectra) trimethylsilyl group. The sterically crowded silyl groups have a powerful fragmentation directing effect. Elimination of a bulky 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 center with 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 2′, 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.     

A sequential Claisen/ring-closing metathesis approach to the synthesis of spirocyclic cyclopentanes and cyclohexanes

A new method for the formation of spirocycles is described using a sequential Ireland-Claisen/Metathesis strategy to construct spirocyclic systems. Optimization of the Ireland-Claisen conditions provides the key metathesis substrates. The metathesis reactions were highly regioselective by virtue of steric hindrance in the substrate olefins.     

The Significance of the Δkl Parameters in the Vibration-Rotational Energies of Diatomic Molecules

We discuss the physical significance of the parameters Δ_kl that arise from mostly the adiabatic and nonadiabatic effects in the vibration-rotational energles of diatomic molecules for which data from isotopic variants are available. Examples are given of the use c f these parameters to derive radial functions for HC1 and LiH. Similar to those for the U_kl coefficients, relations among the Δ_kl coefficients have been developed, but caution is urged in their use.     

Evidence for demethylation of syringyl moieties in archaeological wood using pyrolysis-gas chromatography/mass spectrometry

Archaeological oak (Quercus sp.) wood samples, ranging from 16(th) C. AD to 6000 BP, were studied using flash pyrolysis-gas chromatography/mass spectrometry to obtain insight into angiosperm lignin degradation. The pyrolysates revealed evidence of a number of 3-methoxy-1,2-benzenediol derivatives, methoxycatechols, directly related to 2,6-dimethoxyphenol, syringyl, moieties which are characteristic building blocks of angiosperm lignin. Mass spectra and mass chromatograms of these compounds are reported. The finding of these characteristic pyrolysis products in well-preserved archaeological wood provides unequivocal evidence that demethylation of syringyl units occurs very early in wood degradation. It is highly likely that the absence of abundant 3-methoxy-1, 2-benzenediols in degrading plant materials containing angiosperm lignin relates to the lability of these newly formed moieties.     

Silylated derivatives of arabinonucleosides

Procedure have been developed which allow for the selective derivatization of arabinonucleosides at the 5′? or 2′? and 5′? or 3′? and 5′? positions.