Electron ionisation induced fragmentation of ethyl 5(1H)-oxo- and 7(1H)-oxo-1-aryl-2,3-dihydroimidazo[1,2-a]-pyrimidine-6-carboxylates: evidence for an unusually regioselective rearrangement of M(+*) ions.

The 70-eV electron ionisation (EI) mass spectra of the title compounds show clear differences between the 5-oxo and 7-oxo isomers due to regioselective fragmentations involving the ester function. Exceptionally abundant metastable peaks due to molecular ions fragmenting to [M -CO2](+.) were observed exclusively for the 7-oxo isomers, suggesting that the sufficiently long-lived molecular ions undergo a slow rearrangement preceding this fragmentation reaction. The results are contrasted to the available literature data on the ester group fragmentations involving the loss of CO2 and the EI mass spectrometry of pyrimidone beta-oxo esters. A reaction mechanism is proposed for the elimination of CO2 following ethyl group migration to the pyrimidone carbonyl oxygen.     

Conformational analysis—VIII : 1H NMR conformational study of 5-methyl-, 2,5-dimethyl- and 2,2,5-trimethyl-4-oxo-1,3-dioxans

Substituent effects on ¹H chemical shifts of a large number of methyl-substituted 4-oxo-1,3-dioxans have been estimated and shown to be additive for compounds with a uniform half-chair conformation. The additivity rule fails, however, if the substitution pattern forces the ring into a non half-chair conformation. The ring conformations of 5-methyl-, cis- and trans-2,5-dimethyl- and 2,2,5- trimethyl-4-oxo-1,3-dioxans have been clarified by NMR and through acid-catalyzed equilibration of the epimeric 2,5-dimethyl derivatives.     

Conformational analysis—V : 2,6-Dialkyl- and 2,2,6-trialkyl-4-oxo-1,3-dioxans

Chemical equilibration and ¹H NMR spectra were used to determine the ring conformations of 2,6-dialkyl- and 2,2,6-trialkyl-4-oxo-1,3-dioxans. These compounds have no greatly favoured ring conformation but they may exist in half-chair or (twist-) boat forms the distribution between them depending mainly on the steric requirements of the alkyl substituents.     

Mass spectrometric study of some cycloalkane/ene-condensed 2-thioxo-2935556-pyrimidin-4(1H)-ones and cycloalkane/ene-condensed [l,3] thwzino [3,2-a]-pyrimidinones under electron impact

The mass spectra of four cycloalkane/ene cis- and trans-condensed 2-thioxo-2,3,5,6-tetrahydropyrimidin-4(1H)-ones and the corresponding cycloalkane/ene cis- and trans-condensed [l,3]thiazino[3,2-a]pyrimidinones were measured. Fragmentation pathways were confirmed by metastable ion analyses and exact mass measurements. The bicyclic isomers were easy to distinguish from each other, whereas the corresponding tricyclic isomeric adducts gave fairly similar mass spectra, owing to their favoured reiro-Diels- Alder fragmentations accompanied by the migration of one or two hydrogen atoms.     

Evidence for an aryl migration during the electron impact induced fragmentation of substituted aryloxymethylquinoxalines.

The electron impact (EI) mass spectra of 34 differently substituted 2-phenoxymethyl-, 2-naphthyloxymethyl-, 2-pyridinyloxymethyl- and 2-chinolinyloxymethylquinoxalines were recorded. The fragmentation patterns were examined by metastable ion analysis and exact mass measurements, employing finally also selective deuterium labelling. The inclusion of the substituted aryl ring moiety appears to be important for the fragmentation of the aryloxymethylquinoxalines. A molecular ion rearrangement is proposed for the observed loss of OH* and CHO* radicals. The influence of the different substituents on the aryl ring moiety on the rearrangement in the gas phase and on the resulting fragmentation was investigated.     

Conformational analysis XX—13C NMR studies of saturated heterocycles 5—substituent effects on the 13C chemical shifts of methyl substituted 1,3-dithiolanes

The ¹³C NMR chemical shifts for 1,3-dithiolane and 13 methyl substituted derivatives are reported. Substituent effects are derived and compared with those for cyclopentanes and 1,3-dioxolanes. The magnitude and variety of the substituent effects are best explained with the aid of a half-chair conformation where the S-1? C-2? S-3 plane passes between C-4 and C-5.     

Analysis of the 1H n.m.r. spectrum of 4-methyl-1,3-oxathiolane. An interestingly coupled ABXM3-case

The 220 MHz proton magnetic resonance spectrum of 4-methyl-1,3-oxathiolane was recorded and analysed. The methyl and ethanic protons form an ABXM₃ system, the analysis of which was carried out through several ‘trial-and-error’ attempts. This part of the spectrum is an interesting example of second-order effects which, in the past, have been mistakenly associated with ‘virtual coupling’. The results are compared with those presented earlier for several alkyl substituted 4-methyl-1,3-oxathiolanes.     

Book reviews

Please forward books for review to the Book Review Editor: Magdolna Hargittai, Structural Chemistry Research Group of the Hungarian Academy of Sciences, Eötvös University, H-1431 Budapest, Pf. 117, Hungary.     

Substituent effects on 13C chemical shifts of alkyl-substituted 4-oxo-1,3-dioxolanes and 5-oxo-1,3-oxathiolanes

The 13C NMR chemical shifts for 4-oxo-1,3-dioxolane (1) and its all methyl-substituted derivatives (2-10) as well as for 5-oxo-1,3-oxathiolane (11) and its nine alkyl-substituted derivatives (12-20) are reported. The magnitude and variety of the substituent effects are in accordance with the envelope conformations in which the oxygen or sulfur atom locates at the tip of the envelope as postulated on the basis of earlier data.