Direct intramolecular gas-phase transfer reactions during fragmentation of sildenafil and thiosildenafil analogs in electrospray ionization mass spectrometry.

A study is made of the mass spectral fragmentation pathways of sildenafil, thiosildenafil, and analogous compounds. A prominent gas-phase reaction that occurs during collision-induced dissociation (CID) of thiosildenafil compounds is the transfer of an alkyl group from the piperazine nitrogen atom to the sulfur atom of the thiocarbonyl group. This phenomenon is clearly demonstrated through a comparison of electrospray ionization mass spectral fragmentation patterns of four sildenafil-type compounds and three related thiosildenafil derivatives. Molecular modeling and fragmentation patterns support a direct intramolecular alkyl transfer mechanism rather than an ion-neutral complex mechanism. CID of thiohydroxyhomosildenafil results in a facile hydroxyethyl migration to the sulfur atom followed by a second intramolecular reaction to form a spiro-1,3-oxathiolane ring, which fragments in two directions to generate both carbonyl and thiocarbonyl product ions from this thiocarbonyl compound. While methyl migration to the thiocarbonyl sulfur atom of thiosildenafil is dominant, methyl migration to the carbonyl oxygen atom of sildenafil may occur to a small extent. Published in 2009 by John Wiley & Sons, Ltd.