A study of the electrospray ionisation and ion-trap fragmentation of [M  H]  ions of new 3,5-disubstituted tetrahydro-2H-1,3,5-thiadiazin-2-thiones Roberto Martı ´nez-Alvarez, Nazario Martı ´n, Carlos Seoane, Margarita Sua ´ rez, Rolando Pe ´ rez, Hortensia Rodrı ´guez Nour Kayali 1 Departamento de Quı ´mica Orga ´nica, Facultad de Ciencias Quı ´micas, Universidad Complutense, E-28040 Madrid, Spain 2 Laboratorio de Sı ´ntesis Orga ´nica, Facultad de Quı ´mica, Universidad de La Habana, 10400-Ciudad Habana, Cuba 3 Servicio de Espectrometrı ´a de Masas, Universidad Complutense, E-28040 Madrid, Spain Received 12 February 2001; Revised 14 March 2001; Accepted 15 March 2001 The electrospray ionisation ESI) in negative mode of the pharmacologically significant 3,5- disubstituted tetrahydro-2H-1,3,5-thiadiazin-2-thiones, and their subsequent fragmentations using an ion-trap mass spectrometer, have been investigated. Experiments on sequential product ion fragmentations MS n ) were performed in order to elucidate the degradation pathways for these compounds. The data presented show that the fragmentation of the even-electron [M  H]  ions could proceed through an internal nucleophilic substitution displacement. Decarboxylation and extrusion of carbon disulfide are other fragmentations observed. Copyright # 2001 John Wiley & Sons, Ltd. Tetrahydro-2H-1,3,5-thiadiazin-2-thione TTT) 1) Scheme 1) and related compounds display important pharmaco- logical activity. Numerous studies have been published on their antibacterial, 1,2 antifungal, 3,4 antihelmintic 5 and anti- tuberculous 6 activity as prodrugs. In this regard, we have described the synthesis and antiprotozoan properties against Trypanosoma cruzi and Trichomonas vaginalis protozoa of new 2,5-disubstituted tetrahydro-2H-thiadiazin-2-thione deriva- tives, 7 as well as a study of their decomposition products and anticancer properties. 8 Recently, the preparation of different 5-substituted 3-5'-carboxypentyl)-tetrahydro-2H-1,3,5-thia- diazin-2-thiones, by using a solid phase synthesis from readily available starting materials, has been reported. 9 Relatively few thiadiazines have been examined by mass spectrometry and any conclusions on fragmentation mode must be tentative. 10 Moreover, the reported data were obtained using electron-impact ionisation only in order to confirm the relative molecular mass, and no examples of the electrospray ionisation and ion-trap fragmentation of this class of compounds are known. We wish to report herein a mass spectrometric study of some pharmacologically interesting, 5-substituted tetrahydro-2H-1,3,5-thiadiazin-2- thiones in order to determine their behaviour under collisionally induced dissociation CID) conditions. EXPERIMENTAL 1,3,5-Thiadiazin-2-thiones were prepared by following the general procedure previously reported from amines and carbon disulfide under basic conditions, to lead to the intermediate dithiocarbamate potassium salt which was not isolated), and further cyclocondensation with formaldehyde and the respective amino acid to afford the thiadiazin-2- thiones in good yields. 7 The expected structures were confirmed by the usual spectroscopic methods. We have reported a complete 1 H and 13 C NMR study of a series of different substituted thiadiazin-2-thiones. 11 The ESI spectra were recorded using a ESQUIRE-LC 2 ion- trap Bruker Daltonik, Bremen, Germany) using 4000 V capillary voltage, nitrogen as nebuliser gas with a flow of 7 L min 1 nebuliser pressure of 18 psi) at 300 °C. The samples were introduced as solutions in a mixture of methanol/ acetonitrile 1:1), using a syringe-pump with a flow of 3 mL min 1 . The MS n spectra were obtained by CID with helium after isolation of the appropriate precursor ions. An isolation width of 0.5 Th was used to isolate the selected peaks. The collision conditions were maintained at 0.90 V of fragmenta- tion voltage amplitude, and 40 ms of fragmentation time. The 70 eV mass spectra were recorded using a HP 5989A Scheme 1. Tetrahydro-2H-1,3,5-thiadiazin-2-thione (TTT) (1). *Correspondence to : R. Martõ Ânez-Alvarez, Departamento de Quõ Âmica Orga Ânica, Facultad de Ciencias Quõ Âmicas, Universidad Complutense, E-28040 Madrid, Spain. Email: rma@eucmos.sim.ucm.es Contract/grant sponsor: Proyectos Alma Mater, Cuba. Contract/grant sponsor: DGESIC; Contract/grant number: PB98-0018; Contract/grant number: PB98-0803. DOI:10.1002/rcm.294 Copyright # 2001 John Wiley & Sons, Ltd. RAPID COMMUNICATIONS IN MASS SPECTROMETRY Rapid Commun. Mass Spectrom. 2001; 15: 758±762