Comparison of the mass spectra of 6‐thiotheophyllines and 6‐sulfinytheophyllines

Under electron impact, 6‐thiotheophyllines eliminate various fragments from the pyrimidine moiety. In a retro Diels‐Alder reaction, they lose the fragment XCNCH₃ from positions 1 and 2 of the pyrimidine ring. In 6‐sulfinyltheophyllines, the sulfinyl group is the main target for fragmentation; it can lose either oxygen or sulfur, and the abundance of [M—16]⁺ and [M—32]⁺ is much higher than the abundance of the molecular ion. Elimination of the sulfur atom of the 6‐sulfinyl substituent, with retention of its oxygen, may be explained by intermediate formation of a ring. All further fragmentations of the 6‐sulfinyl derivatives proceed by a primary loss of oxygen or sulfur, followed by elimination of fragments from the pyrimidine moiety, similar to the primary processes, observed in the mass spectra of the 6‐thiotheophyllines.