Mass spectrometric investigation of stereosomeric 1,2‐dimethyl‐4‐substituted decahydroquinol‐4‐ol N‐oxides. Concerning the configuration of the asymmetric nitrogen atom

Abstract

The main fragmentation pathways of the N-1, C-2 and C-4 stereoisomers of the 1,2-dimethyl-4-R-transdecahydroquinoline-4-01 N-oxides (R=C%CH, CH=CH2 and C,H,) under electron impact are discussed. The correlation between the mass spectrometric chromatographic behaviour and the configuration of polar groups in the N-oxides examined is discussed. The mass spectra of the N-1 stereoisomers may be subdivided into two groups, depending only on the orientation of N+ 0 group and not of the 4-OH group. The spectra of N-oxides with the axial N-oxide group reveal less intense [MI? ions and much more intense [M-CH,]+, [M-O]', [M-OH]+ and [M-H,O]? ions, whereas in the spectra of their equatorial epimers the abundance of the [MIt ions exceeds the intensities of the latter ions.The electron impact promoted fragmentation of numerous heterocyclic N-oxides has been studied. The elimination of an ox en atom from the molecular ions has been found to be the most characteristic fragmentation pathway of the heteroaromatic Noxides. Only the presence of an alkyl substituent, ortho to the N-oxide group, stimulates the loss of an OH In the mass spectra of heteroalicyclic Noxides, however, both [M -161' and [M -171' ions were ob~erved."~ There has been practically no work on the mass spectrometry of the epimeric N-oxides formed by the oxidation of N-heteroalicyclic compounds. The present paper reports such studies and the application of these to the determination of the configuration of the asymmetric nitrogen atom in 1,2-dirnethyl-4-R-trans-decahydroquinoline-4-01 Noxides. On oxidation with 10% HzOz, each of the bases la-lc and 2a-2c, whose configurations have been proved earlier,' gave the corresponding pair of epimeric N-oxides-4a-4c and 5a-5c, and 6a-6c and 7~-7 c .~ From each of the alcohols 3a-3c with an axial 2-CH3 group, however, only one epimer 8a-8c was formed under these conditions. The N-oxides 4-8 retain the same configuration at C-2 and C-4 and ring fusion as the parent bases 1-3, since the latter are