Negative ion mass spectra of dihydropyridine calcium-channel blockers

The negative ion mass spectra of some dihydropyridine analogues of nifedipine are studied; they show a fragmentation which is highly dependent on the position of the nitro group on the phenyl ring: 3'-nitro derivatives give essentially the molecular anion, whereas 2'-nitro derivatives lose successively H2O, RO and O. In addition, (2,1,3-benzoxadiazol-4-yl) derivatives show essentially a [M-ROH]- peak. Possible pathways for these fragmentations are given.